INVESTIGATION OF DIFFERENT GEOMETRIC STRUCTURE PARAMETER FOR HONEYCOMB TEXTILE COMPOSITES ON THEIR MECHANICAL PERFORMANCE

Transcription

1 INVESTIGATION OF DIFFERENT GEOMETRIC STRUCTURE PARAMETER FOR HONEYCOMB TEXTILE COMPOSITES ON THEIR MECHANICAL PERFORMANCE A thesis submitted to the University of Manchester for the degree of Doctor of Phiosophy In the Facuty of Engineering and Physica Sciences By Xiaozhou Gong MAY

2 LIST OF CONTENTS LIST OF CONTENTS... 2 LIST OF PUBLICATIONS... 8 LIST OF FIGURES... 9 LIST OF TABLES DECLARATION COPYRIGHT STATEMENT ABSTRACT ACKNOWLEGEMENT CHAPTER 1 INTRODUCTION Description of the probem Research aim and objectives Thesis ayout CHAPTER 2 LITERATURE REVIEW Cassification of Ceuar Soids Honeycomb structure Foam structure Main features of ceuar soids Low density Stiffness and strength of ceuar soids Open porosity structure and its appication Therma insuation property Manufacturing of honeycomb structure Mechanica performances of ceuar soids Previous studies on ceuar soids mechanic performances

3 2.4.2 Dynamic impact with different veocities Energy absorption of ceuar soids Textie Honeycomb Composites D woven fabrics D honeycomb fabrics Structure parameters for textie honeycomb composite Appications of textie honeycomb composite on PPE Comments 68 CHAPTER 3 DESIGN OF 3D HONEYCOMB FABRICS Design of 3D honeycomb weaves Representation of woven honeycomb structures Layer connection methods Weave creation Design of 3D honeycomb fabrics D honeycomb fabrics Design detais for 3D honeycomb fabrics Ce opening ange, Different ce size at the same number of ayers Length ratio of ce was ( b f ) Simiar sampe thickness with different ce size Manufacturing of 3D honeycomb fabrics Weft density of the 3D honeycomb fabric Parameter specifications for 3D honeycomb fabric in the weaving process Honeycomb fabric production. 91 CHAPTER 4 CREATION OF HONEYCOMB COMPOSITES AND TEST SAMPLE PREPARATION. 95 3

4 4.1 Fabric opening and consoidation Fabric opening Fabric impregnation Textie honeycomb composite Fabrication of woven honeycomb composite The sampe groups Summaries CHAPTER 5 EXPERIMENTAL DATA ANALYSIS ON TEXTILE HONEYCOMB COMPOSITES Low veocity drop weight impact tests Basic principe of ow veocity drop weight impact The set-up of the ow veocity impact instrument Test procedure Preparation for test Specimens of textie honeycomb composites Impact test setting Impact test resuts Data processing Basics for ow-veocity impact test Force attenuation Acceeration of the impactor Characteristics of the transmitted force Energy absorption performance Experiment resuts Various experiment resuts during impact procedure Experiment resuts for energy absorption Experiment resuts for force attenuation factor (f att ) Structure and properties of textie honeycomb composites

5 5.5.1 Structure parameters and performance indices Grouped sampe experimenta performance Ce size and its experimenta performance (8L3P60, 8L4P60, 8L5P60, 8L6P60) Opening ange and its experimenta performance(8l6p30, 8L6P45, 8L6P60, 8L6P75, 8L6P90) Length ratio of ce was and its experiment performance b b ( 1 : 8L3P60,8L(4+3)P60, 8L(6+3)P60; 1 : 8L(3+6) f P60,8L(4+6)P60,8L6P60) Honeycomb composites with simiar thickness and their performance (4L6P60, 6L4P60 and 8L3P60) Discussions on composite density and composite thickness Composite voume density Composite thickness Concusions f CHAPTER 6 EXPERIMENTAL DATA ANALYSIS ON TEXTILE HONEYCOMB COMPOSITE IMPACTED WITH LARGER MASS AND LOWER VELOCITY Low veocity impact test setting by Instron Dynatup Mode 8200 drop weight impact testing instrument Assemby of Instron Dynatup Mode 8200 drop weight impact testing instrument Testing procedure Cassifications of textie honeycomb composites Impact setting for Instron Dynatup Mode 8200 system 6.2 Tested resuts and discussion Ce size and its experimenta performance (8L3P60, 8L5P60,

6 8L6P60) Opening ange and its experimenta performance (8L6P30, 8L6P45, 8L6P60 and 8L6P75) Different ength ratio of bonded and free wa and its experiment performance ( b 1: 8L(3+6)P60, 8L(4+6)P60, 8L6P60; f 164 b f 1: 8L3P60, 8L(4+3)P60, 8L(6+3)P60) Comparison of the resuts between two different oading conditions Sampes with different ce size (8L3P60, 8L5P60, 8L6P60) Sampes with different opening ange (8L6P30, 8L6P45, 8L6P60, 8L6P75) Sampes with different ength ratio of free and bonded wa ( b 1: 8L(3+6)P60, 8L(4+6)P60, 8L6P60; f b f 1: 8L3P60, 8L(4+3)P60, 8L(6+3)P60) Summaries 181 CHAPTER 7 FEA ON TEXTILE HONEYCOMB COMPOSITES FEA Based on 2D Honeycomb Composite Modes Creation of 2D modes for textie honeycomb composites Meshing the geometrica modes and the impactor Meshing the impactor Materias The tensie test of a singe ayer composite Materia properties Boundary conditions appied to the honeycomb composite modes 190 6

7 7.1.6 Impact setting for FEA of 2Dmodes Resuts and discussions of FEA based on 2D modes Introduction of performance indices Cassifications of the FE composite modes Simuated resuts Deformation area under cyinder impact History of dynamic contact force Energy absorption performance Comparison betwen ba and cyinder impact Vaidation of the simuation resuts with experiment resuts FEA of 3D Textie Honeycomb Composites Creation of the geometric modes Boundary conditions Set-up of 3D FE modes D FE resuts and discussions Summaries on FEA 232 CHAPTE 8 CONCLUSIONS AND FUTURE WORK Concusions Recommendations for Further Research Work 237 REFERENCE

8 LIST OF PUBLICATIONS (i) Xiaogang Chen, Ying Sun and Xiaozhou Gong. (2008). Design, Manufacture, and Experimenta Anaysis of 3D Honeycomb Textie Composite Part I: Design and Manufacture. Textie Research Journa, vo.78. no.9, pp (ii) Xiaogang Chen, Ying Sun and Xiaozhou Gong. (2008). Design, Manufacture, and Experimenta Anaysis of 3D Honeycomb Textie Composites, Part II: Experimenta Anaysis. Textie Research Journa, vo.78, no.11, pp (iii) Xiaogang Chen, Ada Gong, Ying Sun, Danie Yu. (2006). 3D Honeycomb Textie Composites for Impact Protection. In: Kang,T.J. ed. Internationa fibre conference, vo. A4-1, May-June, Korea (iv) X.Chen and X.Gong.(2008). Manufacture and Characterization of Exatraight 3D Hoow Textie Composite, ECCM13 Conference, Jun, Sweden (v) Xiaogang Chen, Xiaozhou Gong and Shijun Tang.(2008). Design, Manufacture, and Anaysis of 3D Honeycomb Textie Composites. ECCM13 Conference, Jun, Stockhom,Sweden 8

9 LIST OF FIGURES Figure 2-1 Exampes of ceuar soids (Gibson and Ashby, 1997) 30 Figure 2-2 Figure 2-3 Figure 2-4 Figure 2-5 Figure Schematic iustration of honeycombs structure wi different ce shape (Gibson and Ashby, 1997) The range of properties avaiabe to the engineer through foaming (Gibson and Ashby, 1997) A chart showing materia Young s moduus and density where each materia cass occupies a characteristic fied on the chart (Pfug and Vangrimde, 2003) Honeycomb structure with hexagona ces (Gibson and Ashby, 1997) Typica compressive stress-strain curves for ceuar soids under inpane compression (Gibson and Ashby, 1997) 37 Figure 2-7 Figure 2-8 Figure 2-9 Geometric parameters of the honeycomb ce from Gibson and Ashby (1997) A schematic diagram shows the way the stress-strain curve changes with t/ (Gibson and Ashby, 1997) Schematic detaied description of the honeycomb sandwich structure (Abbadi et a., 2009) Figure 2-10 Different Sandwich core types (Herrmann et a., 2005) 43 Figure 2-11 Exampes for sandwich appication A380 (Herrmann et a., 2005) 43 Figure 2-12 Expansion manufacturing process (Bitzer, 1997) 46 Figure 2-13 Corrugation manufacturing process (Bitzer, 1997) 46 Figure 2-14 The peak stresses generated in foam of three densities in absorbing the same energy, W (Gibson and Ashby, 1997) Figure 2-15 Typica time-oad puses from uniaxia crushing tests for Redwood specimens (Reid and Peng, 1997) Figure 2-16 Crushing of a honeycomb in the X 1 direction, where v is the initia crushing veocity (Ruan et a., 2005)

10 Figure 2-17 Sketch of spacer fabric construction 59 Figure D woven composites (a) cyinder and fange; (b) egg crate structure; (c) turbine rotors; and (d) various compex shapes woven 60 preforms (Mouritz et a., 1999) Figure 2-19 Woven architectures used in 3D woven composites (Yi and Ding, 2004) Figure 2-20 A schematic diagram of woven fabric with mutiayer (Takenata et a., 1991) Figure 2-21 Cross section view of the honeycomb fabric in 3D form (Yassar, 1999) Figure 2-22 Parameters of singe honeycomb ce (Tan and Chen, 2005) 65 Figure 2-23 Schematic diagram of a 6-ayer honeycomb structure (Sun, 2005) 66 Figure 3-1 Region division of a honeycomb structure 71 Figure 3-2 Seection of weave 73 Figure 3-3 Honeycomb structure 2L1P 75 Figure 3-4 Honeycomb structure 4L3P 76 Figure 3-5 8L6P with different opening ange 82 Figure 3-6 Different ce size for 8-ayer composites 83 Figure 3-7 b Honeycomb structure with ength ratio of ce was ( 1) f 85 Figure 3-8 b Honeycomb structures with ength ratio of ce was ( 1) f 85 Figure 3-9 Structures with same thickness and different ce size 86 Figure 3-10 Weave ifting pan for 8L3P, 8L4P, 8L5P and 8L6P 90 Figure 3-11 The dobby weaving machine 92 Figure 3-12 Card punching 92 Figure 3-13 Photograph of one sampe fabric weaved from oom 94 Figure 4-1 Honeycomb fabric opening devices 96 Figure 4-2 Iustration of the thickness (T) of the honeycomb structure 97 Figure 4-3 Iustration of a four-ayer honeycomb composite 98 10

11 Figure 4-4 Photos of textie honeycomb composite with different ce size 103 Figure 4-5 Specimens with different opening ange 105 Figure 4-6 Specimens with different ce sizes 105 Figure 4-7 b Specimens with different ength ratios ( 1) f 106 Figure 4-8 b Specimens with different ength ratio of ce was ( 1) f 107 Figure 4-9 Specimens with same thickness 107 Figure 5-1 Schematic diagram for in-pane ow veocity impact test 110 Figure 5-2 Dropping hammer system for impact test of specimens 111 Figure 5-3 The impactor and the anvi 112 Figure 5-4 Charge ampifier used in the tests 113 Figure 5-5 Snapshot of the resutant curves for force and acceeration dispayed in Nicoet Windows 113 Figure5-6 Data processing fow chart for experimenta data anaysis procedures 116 Figure 5-7 Measured acceeration curves for 8L6P Figure 5-8 Measured transmitted force curves for 8L6P Figure 5-9 The response of contact force against dispacement 123 Figure 5-10 Trapezoida method to cacuate the energy absorption 124 Figure 5-11 Evauation curves of veocity, dispacement and energy absorption for 8L6P Figure 5-12 Comparison of transmitted force-time diagram 130 Figure 5-13 Comparison of vaue of peak transmitted force diagram 130 Figure 5-14 Comparison of contact force-dispacement diagram 131 Figure 5-15 Comparison of energy absorption and structure dispacement diagram 131 Figure 5-16 Comparison of transmitted force-time diagram 136 Figure 5-17 Comparison of vaue of peak transmitted force diagram 136 Figure 5-18 Comparison of contact force-dispacement diagram 137 Figure 5-19 Comparison of energy absorption and structure dispacement

12 diagram Figure 5-20 Energy dissipation direction diagram 139 Figure 5-21 Comparison of transmitted force-time diagram 140 Figure 5-22 Comparison of contact force-dispacement diagram 145 Figure 5-23 Comparison of energy absorption diagram 145 Figure 5-24 Comparison of transmitted force-time diagram 148 Figure 5-25 Comparison of contact force-dispacement diagram 148 Figure 5-26 Infuence of voume density on honeycomb composites 151 Figure 5-27 Infuence of composite thickness on honeycomb composites 152 Figure 6-1 Instron Dynatup Mode 8200 drop weight impact testing machine 158 Figure 6-2 Contact force and energy absorption behaviour of sampes with different ce size 162 Figure 6-3 Contact force and energy absorption behaviour of sampes with different opening ange 165 Figure 6-4 Energy absorption of sampes with different ength ratio of free and bonded wa 170 Figure 6-5 Contact force-dispacement curves of composite with different ce sizes 171 Figure 6-6 Energy absorption under different impact situation (sampes with different ce size) 173 Figure 6-7 Contact force-dispacement curves for composites with different opening anges 175 Figure 6-8 Energy absorption under different oading conditions (sampes with different opening ange) 175 Figure 6-9 Contact force-dispacement curve and energy absorption diagram for the sampe with different ength ratio of bond and free wa ( b 1) f 178 Figure 6-10 Contact force-dispacement curve and energy absorption diagram for the sampe with different ength ratio of bond and free wa

13 ( b 1) f Figure 7-1 Meshing of a ce 187 Figure 7-2 Meshed impactors (a) the cyinder and (b) the sphere 188 Figure 7-3 Stress-strain behaviour of cotton/epoxy sheet and stee 190 Figure 7-4 Schematic iustration of boundary conditions for the FE impact mode 191 Figure 7-5 Estimation of deformed cross-section represented by a trapezoida shaped area (use 8L6P60 as an exampe) 193 Figure 7-6 Deformation of modes with different ce sizes under impact energy of 6J, 8J and 10J 204 Figure 7-7 Comparison of deformation area in modes with different opening anges 206 Figure 7-8 Comparison of deformation area ratio of modes with different ce b wa ratio ( 1). 207 f Figure 7-9 Comparison of deformation area in modes with different ce wa b ratio ( 1) 208 f Figure 7-10 Dynamic contact force of modes under the impact energy of 8J 212 Figure 7-11 Peak contact force from cyinder impact 213 Figure 7-12 Vaidation of energy absorption between FEA and experiment 214 resuts Figure 7-13 Comparison of contact force-time response of 8L3P60 under 8j by 215 cyinder and ba impact Figure 7-14 Comparison of structure deformation under dynamic impact for 220 mode 8L3P at 8J impact (a) by cyinder impact (b) by ba impact Figure 7-15 Dynamic contact force of modes under ba impact at 8J 221 Figure 7-16 Comparison between ba and cyinder energy absorption capabiity 222 Figure 7-17 Comparison of contact force between experiment and simuated

14 (2D) resuts for 8L3P60 Figure 7-18 Created honeycomb mode 225 Figure 7-19 FEA and experiment resuts from 3D scae 229 Figure 7-20 Reationship of input force and transmitted force (a) 8L3P60 and (b) 8L4P60 through FE simuation

15 LIST OF TABLES Tabe 2-1 Density of ceuar soids and soid materia (Gibson and Ashby, ) Tabe 3-1 List of fabric types with weaving quantity and design ange 79 Tabe 3-2 Experimenta design outine in groups 80 Tabe 4-1 Cacuated sampe height and distance between wire and other 99 design parameter Tabe 4-2 Ce geometric parameters for testing specimens 102 Tabe 5-1 Experiment resuts from impact test 126 Tabe 5-2 Experiment resuts for the energy protection 127 Tabe 5-3 Experiment resuts for force attenuation 128 Tabe 5-4 Experiment resuts of the energy dissipated aong vertica and 139 horizonta direction Tabe 5-5 Experiment resuts (ength ratio of ce was) 141 Tabe 5-6 Experiment resuts (sampes with simiar thickness) 146 Tabe 5-7 Voume density, sampe thickness, energy absorption and peak transmitted force of different composites 149 Tabe 6-1 Experiment resuts from impact (sampes with different ce size: 161 8L3P60, 8L5P60 and 8L6P60 Tabe 6-2 Experiment resuts from impact (sampes with opening ange: 164 8L6P30, 8L6P45, 8L6P60 and 8L6P75) Tabe 6-3 Experiment resuts from impact (sampes with different ength ratio 167 of bonded and free wa) Tabe 7-1 Schematic iustrations with structura parameters of 12 geometric 184 modes Tabe 7-2 Mechanica properties of materias 189 Tabe 7-3 Impactor mass, impact veocity and impact energy 191 Tabe 7-4 Detais of FE modes

16 Tabe 7-5 Effect of ce size on modes under cyinder impact 196 Tabe 7-6 Effect of ce opening ange in the modes under cyinder impact 198 Tabe 7-7 Tabe 7-8 Tabe 7-9 b Effect of ce wa ratio ( 1) on the modes under cyinder impact Effect of ce wa ratio ( f b f ) on the modes under impact Effect of ce size on its maximum dispacement and energy absorption for textie honeycomb composite modes under ba impact Tabe 7-10 Dimension of cyinder impactor 225 Tabe 7-11 Dimension and ce parameter for the modes

17 DECLARATION No portion of the work referred to in the thesis has been submitted in support of an appication for another degree or quaification this or quaification of this or any other university or other institute of earning. 17

18 COPYRIGHT STATEMENT 1. The author of this thesis (incuding any appendices and/or schedues to this thesis) owns certain copyright or reated right in it (the Copyright ) and s/he has given The university of Manchester certain rights to use Copyright, incuding for administrative purpose. 2. Copies of this thesis, either in fu or in extracts and whether in hard or eectronic copy, may be ony in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and reguations issued under it or, where appropriate, in accordance with icensing agreements which the University has from time to time. This page must form part of any such copies made. 3. The ownership of certain Copyright patents, designs, trade marks and any and a other inteectua property (the Inteectua Property ) and any reproductions of copyright works in the thesis, for exampe graphs and tabes ( Reproductions ), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Inteectua Property Rights and Reproductions cannot and must not be made avaiabe for use without the prior written permission of the owner(s) of the reevant Inteectua Property Rights and/or Reproducitons. 4. Further information on the conditions under which discosure, pubication and commerciaisation of this thesis, the Copyright and any Inteectua Property and/or Reproductions described in it may take pace is avaiabe in the University IP Poicy (see in any reevant Thesis restriction decarations deposited in the University Library, The University Library s reguations (see and in The University s poicy on presentation of Theses. 18

19 ABSTRACT Textie honeycomb composites, with an array of hexagona ces in the cross section, is a type of textie composites having the advantage of being ight weight and energy absorbent over the soid composite materias. The aim of this research is to investigate the infuence of the geometric parameters on textie honeycomb composites on their mechanica performances under ow veocity impact, which can be used to hep designer contro over the textie honeycomb composites. Four groups of textie honeycomb composites, invoving 14 varieties, have been systematicay created for the experimenta anaysis. The geometric parameters of the honeycomb composites, incuding the ce opening ange, ce size, ce wa ength ratio and structura parameters such as composite thickness, composite voume density are studied for their infuence on the honeycomb composites under ow-veocity impact. Foowed by experimenta work, honeycomb composites with 12 varieties are modeed by finite eement method (FEM) to further investigate the honeycomb structure performance under various oading condition incuding different impact energy (6J, 8.3J and 10J) and impactor shape (cyindrica and spherica). The 3D honeycomb fabrics are successfuy manufactured and converted into textie honeycomb composites. It was found through the experimenta and finite eement anaysis (FEA) that changes in geometric and structura parameters of the textie honeycomb composites have noted infuences on the energy absorption, force attenuation and damage process of the structure. The ength ratio of ce wa and the ce opening ange are the most effective parameters for controing the energy absorption of the composites and composites with medium ce sizes tend to have more reiabe mechanica performances under various oading conditions. And it is aso found in FEA that cyindrica impacts are more threatening to human beings than the ba shaped impact. The methodoogy has been estabished by using FEM to investigate the composites more systematicay in the current study. This heps to provide a faster and economic design cyce for the honeycomb composites, which can substantiay decease the time to take products from concept to the production. 19

20 ACKNOWLEDGMENT I woud ike to thank my supervisor, Dr.Chen, Xiaogang, for his guidance, hep and support throughout this research project. Many thanks aso go to Mr. Zadoroshnyj, A. from materia centre for his heping in producing resin/cotton composite and Mr. Steve from mechanica centre for setting equipment and assisting test the composites patienty. Financia support from schoo of science and physics (EPS) and Dr.Chen, Xiaogang s research budget is gratefuy acknowedged. My specia thanks go to Mr. Robson, M.T. for his encouragement and support initiay from the starting of this degree. I woud ike to acknowedge the heping of Mr.Yin s famiy for their heping personay during the study and Dr.Wei,H. & Dr.Wang,H.W. Mr.Lee,P., Dr. Smith,M. and Mrs.Tina for their assistants during ater thesis writing stage. A ot of thanks to my friends Mrs.Chen,L. and Mrs.Shen,J.L. in China for their endess ove and encouragements to hep me emotionay and finicay in the ate stage of the Phd study. Thanks aso go to Mrs.Zhen,J.C. and his parents in China for their great hep during my study. Thanks aso extend to my dear coeges incuding Dr.Yu, Danie, Mrs.Sun,Y., Mr.Wang,J.F., Dr.Ai, Dr.Chris, Dr.Sun D.M., Mr.Tang, S.J. Dr.Yang,D., Dr.Zhou,F.L. Dr.Wang,Y., Mrs. Zhao,L.R., Mr.Zhou,Y., Mr. Zhu,F.Y., Mr.Ako,J.A.F., Mr. Bia etc. in the Textie and Paper division in the Schoo of Materias for providing a peasure working atmosphere. I woud ike to express my deepest gratitude to my dearest parents for their endess ove, patience, encouragement, and huge supports during the study. 20

21 CHAPTER 1 INTRODUCTION Textie composites are made from two most important parts, the textie perform and the matrix. Various forms of texties have been used as performs and reinforcements for textie composites. The advantages of textie composites have been widey recognised and utiised for appropriate appications. In genera, unidirectiona and 2D woven texties are the main forms used in creating composite materias athough new composites have been created from using different types of 3D texties structures for improved performance. Such 3D composite reinforcements are advanced in that they possess structura integrity and fibre continuity, and for these reasons they have attracted much attention in research and in appications. 3D honeycomb structures, which can be found in nature ranging from the spines of a porcupine to the stem of a pant of reed, have many features that are important for many of the composite appications, as have been described by Gibson and Ashby (1997). Composites made from this type of 3D reinforcements can be super-ight, energy absorbent, vouminous as we as being strong. Lightweight materias with comparabe properties such as high energy absorbency compared to traditiona materias have aways been a favourabe choice for many appications in the aerospace and automotive sectors (Schmueser and Wiciffe, 1987; Tao et a. 1993). Exampes incude woven H-joint connectors for joining honeycomb sandwich wing panes on the Beech starship in the aircraft (Wong, 1992) and foor beams in trains and fast ferries, and so on (Mouritz et a., 1999). Lightweight materias are of interests for energy absorption and protection against trauma impact, where capabiities of the materias in impact energy absorption and in impact force attenuation become important. Theoretica anaysis on the 3D honeycomb composites were carried out (Tan and Chen, 2005; Tan et a. 2007; Yu and Chen, 2006) and the 21

22 resuts suggested that the honeycomb composites have advantages over other types of ceuar materias in energy absorption and force attenuation under in-pane direction. Exceent energy absorption is one of honeycomb composites important mechanica characteristics and investigations have been carried out in various fieds of engineering deaing with different materias. For instance, energy absorbing sub-foor structures for aircraft have been studied and arge composite sub-foor structures appying ceuar structures as box eement core have been deveoped for commuter and transport aircraft (Herrmann et a., 2005). Energy absorption of paper ceuar structures has been studied by dynamic and static compression tests and the resuts showed that increasing the oading speed and number of the ayers coud increase energy absorption accordingy (Kobayashi et a. 1998). A number of investigations into honeycomb structures have been carried out using auminum aoy materias and for instance the mechanisms governing in-pane crushing of hexagona auminum honeycombs have been investigated with finite size honeycomb specimens crushed quasi-staticay between parae rigid surfaces (Papka and Kyriakides, 1998). Increasing interest in textie composites has been deveoped on the account of their attractive properties of ight-weight and high energy absorption capabiity for a variety of appications (Mouritz et a., 1999). For many composite appications, such as those in automobie, aerospace and aircraft sectors, reduction in component weight is highy desirabe. A wide range of fabrics avaiabe for composite reinforcement in the fied of textie structura composites has been reviewed by Wang and Zhao(2006), Bibo and Hogg (1996). They summarised the different forms of texties which were used for composite reinforcement incuding different impact conditions and genera materia variabes such as fibre and resin type, and they pointed out that there was sufficient information avaiabe to indicate that contro of fibre organization by the use of texties might be an effective method of optimising composite properties for specific end use properties. Recenty, it was reported (Qiu et a., 2001) that 3D ceuar matrix composites were fabricated and their structura and mechanica properties were investigated and compared to the 3D reguar matrix composite. The former composite has higher specific tensie strength, greater specific tensie moduus, ower specific 22

23 fexure strength, and higher specific impact energy absorption. Composites reinforced by a new cass of knitted structures have been designed (Cox and Davis, 2001) to maximize the tota energy absorbed during tensie faiure. Leves of energy absorption achieved reach approximatey 40MJ/m 3 or 25J/g. With optimisation, eves of MJ/ m 3 or J/g seem feasibe. Textie composite panes reinforced with integray woven 3D fabric have been investigated at University of Manchester (Zic et a., 1990) and their mechanica properties were found comparabe to those of softwood, auminium aoy, and stee. 1.1 Description of the probem Numerous investigations have been carried out to investigate the impact energy absorption of the 3D honeycomb composites, but itte has been reported in the iterature in reation to the infuence of the structura parameters on impact energy absorption. However, woven honeycomb composites used for ow veocity impact energy absorption were deveoped and produced at the University of Manchester over the recent years (e.g. imb protection intended for the riot poice). Previous research has indicated that the mechanica properties of textie based honeycomb composites can be engineered and controed by seecting appropriate structura parameters. Indepth anaysis such as ce height and ce size in the woven honeycomb structures has been carried out and the resuts show that the thick pane with 60 expansion/opening ange eads to optima performance in energy absorption (Tan and Chen, 2005). Wang and Zhao (2006), and Bibo and Hogg (1996) have reviewed a wide range of fabrics avaiabe for composite reinforcement in the fied of textie structura composites. A systematic work has been reported to engineer and characterizes 3D honeycomb composites for impact appications at the University of Manchester (Wu, 2003). Chen and Wang (2006) worked on the mathematica modeing of integrated ceuar woven preforms and on a CAD too for designing ceuar fabrics with various structura parameters. Tan and Chen (2005) and Yu and Chen (2004) carried out FE anaysis adopting the quasi-static and dynamic approaches respectivey reporting on the infuence of structura parameters on the mechanica behaviour of 3D honeycomb composites. Besides these, itte experimenta and numerica studies on impact on 23

24 textie honeycomb composites were reported, which is an important gap in the study in order to gain comprehensive understanding on 3D honeycomb textie composites. There is a ack of systematic investigation to examine the effect of the parameters of the honeycomb reinforcement and the composite performance in energy absorption and force attenuation. 1.2 Research Aim and Objectives Textie technoogy is capabe of creating 3D honeycomb woven structures without the need for weaving machine modifications. Based on the research that has been carried out (Tan and Chen, 2005; Chen et a., 2007; Tan et a., 2007), this present research is set to investigate the geometric parameters of honeycomb textie composites such as the ce dimension, ce opening ange, ratio of ce was on the composite performance through experimenta and numerica study. This study wi start with the design and manufacture of 3D honeycomb fabrics as reinforcements and wi then move to characterise honeycomb composites in terms of the behaviours and performance. Evauation the honeycomb composites for their energy absorption, force attenuation under the infuence of structura parameters of the composites wi be foowed up by theoretica and experimenta study of the 3D honeycomb composites for their mechanica properties. The aim of this research is to investigate how geometric and structura parameters of honeycomb composites woud affect mechanica performances under ow veocity impact. The outcome of the investigation coud be usefu to hep design protection products against trauma impact, such as shieds for improved protection. This study wi focus on three aspects. The first aspect of the study is to manufacture the honeycomb composites from the 3D honeycomb fabrics with appropriate resin and hardener according to their designed ange and ce size. The objectives for this part of the research are described as foows. 1) to deveop a simpe yet effective method for making honeycomb textie 24

25 composite. In the current study, a set of apparatus [see Chapter 4] has been deveoped which wi aow fabrics to be stretched into 3D and by adjusting the height of the apparatus, the geometric parameters of the 3D textie honeycomb composite can be atered; 2) to design and optimize the geometric parameters for the 3D honeycomb structure and this incudes ce height, ce size, ce opening ange and ength ratio of ce was. Four groups of honeycomb composites, invoving 14 varieties, wi be systematicay created in order for the future experimenta anaysis of the honeycomb composites to be carried out; 3) to estabish a procedure for the sake of manufacturing 3D textie honeycomb composites and this procedure wi be abe to guide future practica production processes for simiar composites. This procedure is expected to aow easy reproduction in the future. The second aspect of this study is to conduct ow-veocity impact tests on the 3D honeycomb textie composite sampes in groups according to their ce size, ce opening ange, ce bonded and free wa ratio and ce voume density, and the acquired data wi be anaysed for the investigation of mechanica behaviour and energy absorption properties of the honeycomb composites, which wi ead to the optima geometries for honeycomb textie composites. The objectives for this part of the research are: 1) to set up the testing equipment for the ow veocity impact tests. The initia impact veocity and impact energy wi be targeted to divide the experiment into different groups according to the impact energy eve; 2) to conduct the ow-veocity impact tests on the 3D honeycomb textie composites in order to obtain the associated data for anayzing the mechanica properties and energy absorption behaviour of the composites; 25

26 3) to anayse the experimenta data to investigate how the structura and geometric parameters of the 3D honeycomb textie composite affects its mechanica and energy absorption behaviour that impact on the protection capabiity of the composites; and 4) to carry out tensie tests on the singe piece of she from the honeycomb composite to obtain materia properties for the future theoretica and numerica study. The third aspect of the research is to use the finite eement method (FEM) to investigate the impact performance of different geometricay optimised 3D honeycomb textie composites and to vaidate the numerica resuts with practica experimenta resuts. This part of the work wi ead to the estabishment of a design procedure for engineering 3D honeycomb composites. The objectives for the third part of the research are as foows: 1) the first objective is to estabish geometrica modes for the 3D honeycomb textie composites. Because the geometric parameters of singe ce honeycomb textie composite has been estabished by Tan and Chen (2005), the present study starts by creating the geometric modes based on the previous research findings and wi expand the singe ce to muti-ce matrix of the composite. Such geometric modes wi then be used for various FE anayses. 2) the second objective is to create modes of three sets of honeycomb textie composite structures with different ce size, opening anges, and ce bonded wa ength to free wa ength in order to examine their mechanica and energy absorption behaviour. The resuts are expected to provide comprehensive detais in mechanica performance between the modeed honeycomb composite and the physica honeycomb composite specimen. 3) the third objective is to vaidate the theoretica resut with the experimenta resut to seek out the simiarity from both modes. 26

27 1.3 Thesis Layout After this introductory chapter, Chapter 2 wi present a review of the iterature. genera introduction of honeycomb structure and its appications in the fied, review of mechanica behaviour incuding the energy absorption performance of the honeycomb structure under impact, theoretica equations reported by previous researchers on honeycomb textie composites and the design and manufacture of 3D honeycomb woven fabric in the weft direction. Chapter 3 presents works on designing and manufacturing 3D honeycomb fabrics; Chapter 4 describes the creation of honeycomb composites and test sampe preparation; Chapter 5 reports on the resuts and anayses of the ow veocity impact test under dropping hammer system; Chapter 6 reports on the experimenta data anaysis on textie honeycomb composites impacted by arger mass and ower veocity impactor with comparing the resut with the resuts from Chapter 5; Chapter 7 presents a finite eement anaysis (FEA) on honeycomb composites; Chapter 8 ends the thesis with concusions and future recommendations. 27

28 CHAPTER 2 LITERATURE REVIEW Ceuar soids such as sandwich panes have been used as advanced materias in aerospace, automobie and marine industries for decades (Torre and Kenny, 2000; Meo et a., 2003; Kim and Chung, 2007; Shin et a., 2008) due to their unique combination of properties derived from their ceuar structures. Scientists and engineers have paid more and more attentions to ceuar soids since new techniques for making ceramic and metaic foams have widened the range of man-made materias and the diversity of their appications (Gibson and Ashby, 1997). Textie reinforced honeycomb composite (Sun, 2005, Tan and Chen, 2005) can be regarded as a kind of ceuar soid due to its hoow core structure and as an innovative product, much interests have been drawn on it to find out its mechanica performance under various oading conditions(tan and Chen, 2005; Yu and Chen, 2006; Tan et a., 2007). This chapter presents a iterature review on ceuar soids incuding textie honeycomb composites in the foowing aspects, which are (1) cassification, appications, mechanica and non-mechanica features of ceuar soids (2) honeycomb structure manufacturing techniques (3) the mechanica performances of ceuar soids under various impact conditions (4) the energy absorption anaysis of ceuar soids (5) the basic concept of three-dimensiona (3D) fabrics and structura parameters for textie honeycomb composite (6) the appication of 3D honeycomb fabrics on persona protection equipment (PPE). 2.1 Cassification of Ceuar Soids Ceuar soids which are made from very diverse materias incuding wood, poymers, metas, ceramics, gasses and composites and they are used in a broad casses of appication. 28

29 For exampe, ceuar soids are notaby used as core materias for sandwich structures (Torre and Kenny, 200) and they are widey used as energy absorbers and shock protectors in packaging industry too (Wang, 2009; Pfug and Veopoest, 1999; Pfug et a., 2002). Besides those appications, ceuar soids are aso equipped as therma insuation for housing, for refrigeration and for high temperature equipment (Ashby and Meh, 1983) as we as foatation and buoyancy-aids in the ship (Gibson and Ashby, 1997) Athough there are a variety of ceuar soids exist in nature and man-made, according to their ce structures, coud be cassified as honeycomb structure and foam structure. It is noted that in the current study, if the ceuar soids exhibit a honeycomb structure, it is caed honeycomb and if it shows up as a foam structure, it wi be caed foam Honeycomb structure Gibson and Ashby (1997) defined honeycomb as a two-dimensiona array of poygons that pack to fi a pane ike the hexagona ces of a beehive as shown in Figure 2-1(a). 29

30 (a) Two-dimensiona honeycomb (b) Three-dimensiona foam with open ces (c) Three-dimensiona foam with cosed ces Figure 2-1 Exampes of ceuar soids (Gibson and Ashby, 1997) In nature such as basa wood, the honeycomb structure exists from the frequent deviation from reguarity caused by the way in which the individua ces nuceate and grow, and the rearrangement that take pace as they are deveoping. However, in many man-made honeycombs, the honeycomb structures come in many different shapes and sizes such as trianges, squares or hexagons and they are reguar in pattern which are shown in Figure 2-2 (Gibson and Ashby, 1997). 30

31 Figure 2-2 Schematic iustration of honeycombs structure wi different ce shape (Gibson and Ashby, 1997) Foam structure True honeycomb structures are reativey rare and the structures used mosty in sandwich panes are as core materia made by hexagona auminium (Abbadi et a., 2009). More commony, in ceuar soids, the ces are poyhedra which are packed in three-dimensions to fi the space and such ceuar soids are caed foam and Figure 2-1 (b) and (c) gives two photographs of man-made foams. The foam structure can be divided into open-ceed foam structure which contains the ce edge ony and cosedceed foam structure when the faces of the ce is soid and each ce is seaed off from its neighbors. Incidentay, some foam structure is party open and party cosed. 2.2 Main Features of Ceuar Soids Both honeycomb and foam exhibit a unique combination of properties which are derived from their ceuar structures. In Figure 2-3, Gibson and Ashby (1997) stated that comparing to the fuy dense soids; ceuar soids with hoows inside their structure provide some outstanding features such as ow density, unique stiffness and strength according to their oading direction, open porosity structure and ow therma 31

32 conductivity. These four enormous extensions of properties create appications for the ceuar soids which cannot be easiy fied by fuy dense soids. With a ow density and open pore structure, a ceuar soid can be used to design ight, stiff components such as sandwich panes and arge portabe structures (Torre and Kenny, 2000). The ceuar soids can aso exhibit a ow stiffness and strength behaviour depend on their oading direction (Mitz et a., 2003) and this unique feature makes them idea for cushioning and energy absorption appications (Shaw and Sata, 1966). Additionay, with ow therma conductivity, it aows ceuar soids to be used as disposabe coffee cups to refrigerated trucks for the modern buidings. Figure 2-3 The range of properties avaiabe to the engineer through foaming (Gibson and Ashby, 1997) 32

33 2.2.1 Low density Gibson and Ashby (1997) states that the reative density of the ceuar materia coud be cacuated by 1 2, that is, the density of the ceuar materia, 1 divided by that of the soid from which the ce was are made, 2. Poymeric foam used for cushioning, packaging and insuation have reative densities which are usuay between 0.05 and 0.2 and cork is about 0.14; most softwoods are between 0.15 and Specia utra-owdensity foam can be made with a reative density as ow as According to Figure 2-4: the materia property charts from Pfug and Vangrimde (2003), it can be seen that the density of foam and honeycomb ie near the bottom eft of the chart which is beow 0.4kg/dm 3 (that is 0.4g/cm 3 )whie metas positioned near the top right (over 100 times denser than ceuar soids), which indicates metas own a high density and high moduus; fine ceramics such as auminium or concrete are ess dense than meta but stiffer sti. Tabe 2-1 further examped the density of aumina ceramic honeycomb and rigid poyurethane foam, comparing with soid ceramics and soid stee. The resuts indicate that the ceuar soids can reduce the weight of the materia dramaticay. Takenata et a., (1991) specified that for honeycomb structured woven composite, the density of the composite materia is 0.03 to 0.2g/cm 3. If the density is ower than 0.03g/cm 3, sufficient high compression strength is difficut to attain. On the other hand, if the density is higher than 0.2g/cm 3, the mechanica performance of the composite materia can be significanty increased but the weight-reducing effect is decined. 33

34 Figure 2-4 A chart showing materia Young s moduus and density where each materia cass occupies a characteristic fied on the chart (Pfug and Vangrimde, 2003) Tabe 2-1 Density of ceuar soids and soid materia (Gibson and Ashby, 1997) Materia Density, (g/cm 3 ) Honeycomb (aumina ceramic) 1.4 Foam (rigid poyurethane) Soid ceramics (siicon carbide, SiC) 3.2 Soid Stees

35 Because of the ow density of ceuar soids, one of the eariest market for them is in marine buoyancy where the ight cosed-ce pastic foam are extensivey used as supports for foating structures and as foatation in boats. Because of their cosed ces, they can retain their buoyancy even when extensivey damaged and they are unaffected by extended immersion in water with their resistances to rust or corrode (Gibson and Ashby, 1997). Another major use of man-made ceuar soids is in packaging because ow density means the package is ight which can reduce handing and shipping costs. Currenty, the foam being frequenty used in packaging is poystyrene, poyurethane and poyethyene Stiffness and strength of ceuar soids It is important to understand the stiffness and strength performances of honeycombs when they are used in oad-bearing structure. Gibson and Ashby (1997) specified that generay, if a honeycomb is compressed in-pane that is the pane aong X 1 and X 2 direction in Figure 2-5, the ce wa at first bend, giving inear eastic deformation. Beyond a critica strain, the ces coapse by eastic bucking, pastic yieding, creep or britte fracture, depending on the nature of the ce wa materia. Ce coapse ends once the opposing ce was begin to touch each other and as the ces cosed up, the stiffness of the structure increases rapidy. When the oading is aong out-of-pane direction, which is aong X 3 direction in Figure 2-5, the stiffness and strength are much higher because they require extra axia extension or compression of the ce was. 35

36 Figure 2-5 Honeycomb structure with hexagona ces (Gibson and Ashby, 1997) Figure 2-6 exhibits the three regimes of behaviour of a ceuar soids when they are undergoing in-pane oading and it can been seen that at strains ess than about 5%, the materia is inear-eastic and with the increase of the oading, depending on the properties of ce was, the ces began to coapse by eastic bucking, pastic yieding or britte crushing (Barma et a., 1978; Ashby and Meh., 1983; Kurauchi et a., 1984; Maiti et a., 1984). Coapse progresses at a roughy constant oad unti the opposing was in the ces meet and touch, when densification causes the stresses to increase steepy (Shaw and Sata, 1966; Papka and Kyriakids,1998; Ruan et a, 2002). Regarding the out-of-pane oading, Gibson and Ashby (1997) aso said that when the honeycombs are compressed in out-of-pane direction, that is X 3 direction in Figure 2-5, its inear-eastic regime is truncated by bucking (eastic for an eastomer, pastic for a meta or rigid poymer) and fina faiure is by tearing or crushing respectivey. 36

37 (a) Eastomeric rubber (b) eastic-pastic meta (c) eastic-britte ceramic Figure 2-6 Typica compressive stress-strain curves for ceuar soids under in-pane compression (Gibson and Ashby, 1997) After the strain-stress curve has been generated by the researchers mentioned above, Ruan et a. (2002) discovered that pateau stress of auminium foam has a power aw reationship with the foam s reative density (Equation 2-1). Not ony Ruan et a., eary in 1982, Hiyard (1982) aready mentioned in his book that the strength of the ceuar soids incuding compressive and shear, can be described as a function of the density of formed materia, and Vura and Ravichandran (2003) aso found out that the stress of basa wood increases with its materia density. Moreover, other researchers (Kenny, 1996; Baumeister et a., 1997; Banhart and Baumeister, 1998) drew the same concusions in their papers. n A =A 0 [2-1] Where A is the property of foam, is the density of foam, A 0 is a factor which refects the properties of the soid ce wa materia and n is an exponent. It seems the materia density is one parameter which affects the stiffness and strength of ceuar soids; however, there are other parameters which have the same infuences too. Eary in 1978, Barma et a. (1978) mentioned in their paper that they found out that the moduus of the foam as we as the yied stress is proportiona to t/, where t is the thickness of foam struts and is the ength of foam struts, and this means the foam 37

38 materia property is reated to their ce thickness and ce size. Ashby and Meh (1983) further specified that the mechanica properties (eastic, pastic, creep and fracture) of ceuar soids or foam are affected by their ce geometry. Decades ater, Gibson and Ashby (1997) demonstrate in their book that when a honeycomb is oaded in-pane, the ce was bend and it deforms in a inear-eastic way firsty (Abd EI-Sayed et a., 1979; Gibson et a., 1982). The response can be described by five modui numericay: two Young s modui * E1 and compression or tension appied in-pane individuay; a shear moduus Poisson s ratios, * 12 and * 21 * E 2 which are moduus of * G12 and two. Gibson and Ashby (1997) further identified the reationship between honeycombs Young s moduus under inear-eastic deformation with their ce geometric parameters such as ce wa thickness, ce wa ength and ce opening ange in Figure 2-7 and Equation 2-2 to 2-6: Figure 2-7 Geometric parameters of the honeycomb ce from Gibson and Ashby (1997) 38

39 in Figure 2-7, t is the thickness of the ce wa; and h are the ength of the ce was. Therefore, in the foowing Equation 2-2 to 2-6, t/ is the ratio of ce wa thickness to its ength and h/ is the ength ratio of two was; θ is the opening ange which has been defined in Gibson and Ashby (1997) s work to describe the ange between ce was. E E * 1 s t 3 ( h cos 2 sin )sin [2-2] E E * 2 s t 3 ( h / sin ) 3 cos [2-3] * cos 2 ( h / sin )sin [2-4] 1 ( h / sin )sin cos * [2-5] G E * 12 s 3 t ( h / sin ) [2-6] 2 ( h / ) (1 2h / )cos where * E 1 and * E 2 are the Young s moduus of the honeycomb composite aong X 1 and X 2 directions respectivey and Es is the Young s moduus of the honeycomb composite * * wa materia; 12 and 21 are the Poisson s ratios of the materia in the X 1 and X 2 directions; * G 12 means the shear moduus of the honeycomb composite. Gibson and Ashby (1997) aso summarized the mechanisms for compressive deformation of honeycomb structure schematicay in Figure 2-8. This figure is a schematic diagram for a honeycomb oaded in compression in the X 1 -X 2 pane, showing the inear-eastic, coapse and densification regimes and the way the stress-strain curve changes with t/. From Figure 2-8, it shows how the stress-strain curves changes with 39

40 the increasing of the t/. It seems that the moduus of the structure goes up with an increasing in the t/ and the ce was touch sooner which reduce the strain at which densification begins. This means, assuming that the honeycomb ces are under the same thickness (t), if t is kept constant but making the ce wa ength () onger, in other words, to make the ce size arger, thus t/ woud get smaer which eads to ower stress but more strain. Figure 2-8 A schematic diagram shows the way the stress-strain curve changes with t/ (Gibson and Ashby, 1997) Not ony the stiffness and strength are infuenced by honeycomb geometric configuration, the energy absorption characteristics in impact crush of ceuar soids are strongy affected by their geometric configuration too. This can be traced back to the previous researchers (Wierzbicki, 1983; Wu and Wu, 1997; Yamashita and Gotoh, 2005) who have investigated the energy absorption performance or the compressive strength in the out-of-pane crush situation for the auminium honeycomb pane with different 40

41 ce geometric parameters such as ce wa thickness and ce size. They concuded that the use of a smaer ce size and core height with a stronger ce materia wi enhance energy absorbing capabiity of the honeycomb structure, at the same time that honeycomb structure with smaer size yieds a higher compressive strength respectivey. From the work mentioned above, it can be seen that the mechanica properties of the ceuar soids such as stiffness, compression/tensie strength, shear strength, atera expansion and energy absorption are determined by their materia density and ce geometric parameters such as: ce size; the ratio of ce wa thickness to ength (t/ ); and ratio of bonded to free wa ength (h/) Open pore structure and its appication Many natura structura materias with open pore structure are ceuar soids such as wood and canceous bone that can support arge static and cycic oads for a ong period of time (DeBonis and Bodig, 1975; Odgaard and Linde, 1991). Even today, wood is sti the word s most widey used structura materia and the understanding of the way in which wood s properties depends on the wood density and on the direction of oading and this can ead to improved design with wood. Interest in the mechanics of canceous bone stems from the need to understand bone diseases and attempts to devise materias to repace damaged bone. Both wood and canceous bones are good exampes of taking advantage of the open porosity structure of ceuar soids to benefit human beings. Not ony natura ceuar soids are used widey, there are more man-made foam and honeycombs which are used to perform a truy structura function. The most obvious exampe is their use in sandwich panes. The innovative design of the de Haviand Mosquito (a Word War II bomber) used sandwich panes make from thin pywood skins bonded to basa wood cores (Hoff, 1951) and in ater designs the basa wood was repaced by ceuose acetate foam. Man-made honeycomb sandwich panes are increasingy being used to repace traditiona materias in highy oaded appications (Kim and Chung, 2007; Shin et a., 2008). Figure 2-9 iustrated a typica honeycomb 41

42 sandwich structure which consist of a thick ayer (core) intercaated between thin-stiff ayers (skins) (Abbadi et a., 2009). Figure 2-9 Schematic detaied description of the honeycomb sandwich structure (Abbadi et a., 2009) There are a arge variety of sandwich panes that are being appied in structura engineering such as aerospace, transportation, marine and packaging due to their open porosity structure which separates the two thin ayers and aows for an outstanding weight specific bending stiffness and reduces the weight of the composite dramaticay (Pfug et a., 2002; Pfug and Vangrimde, 2003; Wang, 2009). Depending on the oading rate, the mechanica behaviour of sandwich structure coud be various. In fact, they can have a ductie behaviour in case of static oading, but may behave in a britte manner and fai catastrophicay when subjected to impact oads (Gibson and Ashby, 1997). Figure 2-10 isted a series of sandwich panes with different core types and among them honeycomb cores with hexagona ce are characterized by a considerabe rigidity in shear, high crushing stress, amost constant crushing force, ong stroke, ow weight and reative insensitivity to the overa oss of stabiity (Wierzbick, 1983). In the ast decades, sandwich panes have increasingy been adopted in numerous aircraft structures such as contro surfaces, fairings or in the cabin interior. One of the 42

43 exampes is its appication for arge structures at AIRBUS started in 1983 (Herrmann et a., 2005) when the A310 was the first aircraft in the AIRBUS feet to be equipped with a composite honeycomb sandwich rudder. Ever since, the experience with arge composite structures was extended and there is a broad range of composite sandwich structures appication in Airbus aircraft such as bey fairings, ing and traiing edge, engine cowing etc. and Figure 2-11 shows the detais of their appication in AIRBUS A 380. Figure 2-10 Different Sandwich core types (Herrmann et a., 2005) Figure 2-11 Exampes for sandwich appication A380 (Herrmann et a., 2005) 43

44 Due to their unique open pore structure, honeycomb sandwich panes have been excessivey used in packaging industry besides furniture and buiding industry for the sake of their favourabe cushioning properties (Shaw and Sata, 1966; Wang, 2009) and especiay paper honeycomb products are usuay used as cushioning materia in ogistic processing to withstand vibration and shock by means of absorb the energy so as to protect products from damage. Regarding the sandwich panes, there are a ot of researches have been conducted on the area of vaidating new cacuation methods and toos, better understanding of effects of defects, improved and more economic Non Destructive Testing (NDT) capabiities, advanced core materias, nove manufacturing methods and integration of structura and non-structura functions for the sandwich panes and their core structure (Keineber et a., 2002; Ley et a., 1999; Andersson and Van den). Athough ceuar soids are favourabe used as core materia for sandwich panes, there are some drawbacks in them and the significant one is that ces may suffer from accumuating and condensing water which are trapped to increase the weight and decrease the mechanica properties for the materia (Vaviov et a., 2003; Keineberg et a., 2002) Therma insuation property Foam is remarkabe for its good therma insuation and there is a considerabe iteratures regarding on this subject (Yee and Duardo, 1983; Gicksman et a., 1987; Micco and Adao, 2006). More foam is used for therma insuation than for any other purposes. The cosed-ce foam has the owest therma conductivity of any conventiona non-vacuum insuation and it is used, for exampe, in frozen food industry to fi the doube skins of refrigerated truck and raiway cars. Gibson and Ashby (1997) states that there are severa factors combine to imit heat fow in foams: the ow voume fraction of the soid phase; the sma ce size which virtuay suppresses convection and reduces radiation through 44

45 repeat absorption and refection at the ce was; and the poor conductivity of the encosed gas. 2.3 Manufacturing of honeycomb structure Gibson and Ashby (1997) summarized that the honeycomb structure can be made in at east four ways. The most common way is to press sheet materias into a haf-hexagona profie and gue the corrugated sheets together. More commony, gue is aid in parae strips on fat sheets, and the sheets are stacked so that the gue bonds them together aong the strips. The stack of sheets is pued apart to give a honeycomb. Paper-resin honeycombs are normay made ike this that the paper is gued and expanded, and then dipped into the resin to protect and stiffen it. Honeycombs can aso be cast into a moud and increasingy, honeycombs can be made by extrusion; the ceramic honeycombs used to support exhaust catayst in automobies are made in this way. Besides the methods to manufacture the honeycomb structure stated by Gibson and Ashby (1997), in Bitzer s (1997) book, more detaied manufacturing process to produce honeycomb core structure was described. According to him, there are two basic techniques used to convert the sheet materia into honeycomb: the expansion process and the corrugation process. Expansion process is a more efficient technique to produce the majority of the adhesive bonded cores and the whoe method is iustrated in Figure For metaic cores, a corrosive resistant coating is appied to the foi sheets, and adhesive ines are printed. The sheets are cut and stacked, and the adhesive is cured under pressure at eevated temperature. Then the sices are cut into the required thickness and expanded. When metaic cores are expanded, the sheets yied pasticay at the node-free wa joints and thereby retain their expanded geometric shape. 45

46 Figure 2-12 Expansion manufacturing process (Bitzer, 1997) The procedure for non-metaic honeycomb is sighty different. Here the honeycomb does not retain its shape after expansion and must be hed in a rack. The bock web materia contains a sma amount of resin that is heat-set in an oven. Most paper cores wi retain their expanded shape. Then honeycomb bock, sometimes as arge as 4ft by 8ft by 3ft thick, is dipped in iquid resin (usuay phenoic or poyimide) and oven cured. The dipping-curing cyce is repeated unti the bock is at the desired density. Bitzer (1997) aso described the corrugation method which is iustrated in Figure 2-13, and this method is the origina technique used to fabricate honeycomb core. Athough it is abour intensive, this method is sti used for making high density metaic and some non-metaic cores. Figure 2-13 Corrugation manufacturing process (Bitzer, 1997) 46

47 In the corrugation process the sheets are first corrugated, then adhesive is appied to the nodes and sheets are stacked and cured in an oven. Some non-metaic corrugated bocks must be brought up to fina density by resin dipping to achieve the optimum resin-to-reinforcement ratios. Takenaka et a. (1991) specified in his patent that in genera, conventiona honeycomb cores are obtained by coating an adhesive in stripes spaced equidistanty on a thin sheet such as paper, an auminum foi or a fim, aminating and bonding such adhesive-coated thin sheets, and expanding the bonded structure to form honeycomb-ike structure having a mutipicity of ces. Regarding to use the woven fabric as honeycomb core structure, he said that normay, a pane woven fabric composed of gass fibers or the ike is used as the sheet materia for forming a honeycomb core according to the above mentioned process and a thermosetting resin such as an epoxy resin is impregnated over the honeycomb core to form the composite materia. However, this kind of honeycomb does not have a sufficient tensie strength, pee strength and shear strength of the bonded surface and it is easy to deaminate at the bonding point. Therefore, he invented a type of woven fabric which is having a muti-ayer structure and comprises a puraity of woven fabric ayers that are integrated through combined portions. It is formed by interacing warps or wefts of one of adjacent woven fabric ayers to construct the textie honeycomb structure. One of the advantages of this type of integrated muti-ayer fabric is that it can sufficienty sove the deamination probem between the ayers for the honeycomb structure. 2.4 Mechanica performances of ceuar soids Ceuar soids are widey used in energy absorption appications against various oading conditions, and it is necessary that the mechanic performances of the ceuar soids are understood. The foowing sections have a good review on ceuar soids mechanica behaviours when they are under various oading conditions such as statistic and dynamic. 47

48 2.4.1 Previous studies on ceuar soids mechanic performances Pioneering works on the mechanic properties of honeycomb and foam materia, incuding compression, are those Gent and Thomas (1959), Shaw and Sata (1966) and Barma et a. (1978), whereas a book edited by Hiyard (1982) contains a series of artices which summarize the state of the art, at that time, on poymeric foam. McFarand et a. (1963), who have studied the crushing behaviour of honeycomb structure, then deveoped a semi-empirica mode to predict the crushing stress of hexagona ce structure subjected to axia oading. This mode was ater improved to incorporate both bending and extensiona deformation of such ceuar structure by Wierzbicki (1983). Gibson and Ashby are the two most famous researchers in the area of ceuar soids. They and their co-workers have focused their research attentions on ceuar soids since 1980s (Gibson and Ashby, 1982; Gibson et a., 1982; Ashby and Meh, 1983; Gibson et a., 1989). In 1997, they pubished their credited book named Ceuar soids: structure & properties and this book has a comprehensive coverage of this subject and considering both man-made and natura ceuar soids. They introduced the deformation mechanism of the ceuar soids incuding honeycomb and foam aong in-pane and outof-pane direction and an in-depth description of their materia mechanics were carried out on the aspect of inear-eastic deformation, eastic bucking, pastic coapse, britte faiure, viscoeastic deformation, creep and densification. They aso devoted a chapter in their book to introduce the seection of materias for ow speed impact appications. The area they investigated which are reating to the current study is that they verified that the stress and stiffness incuding atera deformation with shear performance of the honeycomb ce structure are affected by their geometric properties. They aso mentioned in their book that the energy absorption of the foam is proportiona to the density of the foam and it is shown in Figure From the figure, it can be seen that, assuming the foam is compressed unti they accumuate the same compressive strain (ε), with the increase of the foam density (ρ), the absorbed energy (W) and compressive stress (σ) increases too. 48

49 Figure 2-14 The peak stresses generated in foam of three densities in absorbing the same energy, W (Gibson and Ashby, 1997) Other researchers who studies the mechanica performance of natura ceuar soids such as basa wood in ongitudina and/or transverse direction are particuary associated with names of Knoe (1966), Soden and McLeish (1976), Eastering et a. (1982), Vura and Ravichandran (2003a; 2003b) and Reid and Peng (1997). Knoe (1966) investigated the effects of environmenta and physica variabes (temperature, moisture content and ambient pressure) on the mechanica response of basa wood. Soden and McLeish (1976) carried out an extensive investigation, which mainy concentrated on the variation of tensie strength with fiber aignment. They aso reported compressive strength data. Eastering et a. (1982) paid particuar attention to the micromechanics of deformation in their experiments, during which they performed in scanning eectron microscopy (SEM) observations and defined the end-cap coapse of grains as the dominant compressive faiure mechanism in ongitudina direction. Vura and Ravichandran (2003a, 2003b) documented the compressive strength; pateau stress and densification strain of basa wood in its entire density range, identified the variations in faiure mechanisms with density and described simpe anaytica modes to represent the observed experimenta strength data. They aso appied the oading force 49

50 dynamicay onto the basa wood and compared the dynamic data with quasi-static experiment resuts and concuded that the initia faiure stress is very sensitive to the rate of oading; pateau stress remains unaffected by the strain rate. Reid and Peng (1997) investigated the dynamic crushing behavior of severa wood species, incuding Basa wood, Yeow Pine, Redwood, American Oak and Ekki, through the impact of specimens aong/across grain direction. Their tests covered a wide range of impact veocities up to approximatey 300m/s, they tested the specimens at a certain density for each species and stress-strain curves were obtained. They found out that under dynamic oading especiay when the oading is aong in-pane direction, there is a significant enhancement of the initia crushing strength of the specimens if the veocity is increased and the corresponding time-oad curve is drawn in Figure That is to say, for the same ceuar soids and here is Redwood, if increases their impact veocity and here from 80m/s to 150m/s, their crushing strength wi sharp up dramaticay, which means the in-pane crushing strength of wood is highy sensitive to the its impact veocity. Figure 2-15 Typica time-oad puses from uniaxia crushing tests for Redwood specimens (Reid and Peng, 1997) 50

51 Besides natura ceuar soids, man-made ceuar honeycombs and foam were activey studied by Reid, Stronge, Shim and Wierzbick, etc. (Reid and Be, 1982; Reid and Reddy, 1983; Reid et a., 1983; Reid and Be, 1984; Su et a., 1994; Kintworth and Stronge, 1988; Stronge and Shim, 1988; Shim et a., 1992; Abramowicz and Wierzbicki, 1989). In the foowing section, Papks and Kyriakides (1998), Yamashita and Gotoh (2005) wi be taken as representatives and their work wi be described in detais. Papka and Kyriakides (1998) studied the response of hexagona auminium honeycomb under in-pane oading. They focus their work on the post-yied behaviour of auminium honeycomb structure. Experiment and numerica resuts agreed we with each other in their work. According to them, under uniaxia, quasi-static experiment compression, the force-dispacement response is initiay stiff and eastic but this terminated by certain oad instabiity. Locaized crushing invoving narrow zones of ces in initiated and subsequenty crushing spreads through the materia whie the oad remains reativey constant. When the whoe specimen is crushed and the response stiffens again. Both of their experimenta and finite eement anaysis confirmed the honeycomb s imit stress is depended on its reative density. Yamashita and Gotoh (2005) studied the quasi-static compression response of auminium honeycomb in the thickness direction. They investigated the effect of the ce shape and the foi thickness on the crush behaviour. The numerica investigation showed that the cycic bucking mode takes pace in every case and that the crush strength is higher for the smaer ce ange specimens. This information reveas that the ce ange affects the mechanic performance of honeycomb structure significanty. Dynamic impact tests were aso performed by Wu and Wu (1997) who have used a gas gun to study the out-of-pane properties of auminium honeycombs. In their study, honeycombs with different ce size, materia strength and core thickness were compared when they were under quasi-static and dynamic impact oading conditions. The ce size of the specimen they have chosen is 3.2mm and 4.7mm, and finay, they concuded that in order to make the best use of a honeycomb structure as an energy 51

52 absorber, honeycombs which is sma in ce size and core height, made by a highstrength materia is recommended. Simiar to his concusions, recenty, Tang et a. (2008) produced a 3D gass/poyester resin ceuar woven composite with vacuum aided resin transfer mouding (VARTM) technique and the fibre voume fraction in his composite is 40% which can significanty thicken the ce wa and increases the strength of the composite materia. Athough Wu and Wu (1997) studied the crushing behaviour of auminium honeycombs, in their study, after-test specimens were observed whie the whoe crushing processes of the honeycombs were not investigated. The deveopment of finite eement (FE) technoogy has made it possibe for the researchers to see ceary the whoe crushing process such as the stress concentration and distribution etc., to anayse and predict the composite structure behaviour against various oading conditions more accuratey. In 1998, Abrate (1998) summarized recent modeing techniques for ocaized impact of sandwich panes with aminated face sheets, incuding contact aws, composite sandwich beam and pate theories, and dynamic spring-mass modes. Such anaytica soutions are needed for designing sandwich panes against impact damage. They provide vauabe information for ocating damage and estabishing criteria for acceptance or repair of structura components. Since then, a ot of other researchers such as Ruan et a. (2003), Zheng et a. (2005), Yu and Chen (2006) have conducted impact researches on various ceuar composites by means of FE anaysis toos. Ruan et a. (2003) studied the infuences of ce wa thickness of honeycombs and the impact veocity on the mode of ocaised deformation and its pateau stress by means of FE anaysis too: ABAQUS (Hibbitt et a., 1998) aong in-pane direction. The hexagona ce is modeed with an opening ange equas to 60 and the ce wa ength () is 4.7mm with ce wa thickness (t) varies from 0.08mm to 0.5mm; the impact veocity (v) is increased from 3.5m/s to 280m/s. They reported three different patterns of deformation appeared during the impact oading: X, V and I, depending on the impact veocity of the honeycombs and the schematic iustrations can be seen in Figure When the impact veocity is 3.5m/s, the deformation pattern shows up as X 52

53 shape; at 14m/s, it is V shape and at 70m/s, the shape is I. This means the deformation of honeycombs with hexagona ce is highy sensitivity to its impact veocity. Furthermore, they aso figured out that both deformation patterns and impact pateau stresses of honeycombs are reated to the ce wa thickness and impact veocity and the power ow is shown in Equation 2-7. Supposing the thickness of the wa (t) are the same, when the impact veocities (v) is sufficienty high, the impact pateau stresses (σ/σ ys ) show a good correation to its impact veocities by a square aw. Ruan et a. s study again verified that impact veocity and ce wa ength are two factors which are worthwhie to be investigated regarding the mechanica performances of honeycombs. ys 3 2 t t t [2-7] In Equation 2-7, σ/σ ys is the ratio of impact pateau stress (σ) to its yied stress (σ ys ); t/ is the ratio of ce wa thickness (t) to its ce wa ength () and v is the impact veocity of the honeycombs. (a) Origina shape (b) v=3.5m/s (c) v=14m/s (d) v=70m/s Figure 2-16 Crushing of a honeycomb in the X 1 direction, where v is the initia crushing veocity (Ruan et a., 2005) Zheng et a. (2005) got simiar resuts regarding the deformation patterns by conducting 2D FE anaysis comparing to the resuts from Ruan et a. (2003) s work. Besides reguar hexagona ce shaped honeycombs, Zheng et a. (2005) aso studied the honeycombs with irreguar ce shape. Finay, they found out that the deformation in an irreguar 53

54 honeycomb is more compicated than that in a reguar honeycomb due to its ce irreguarity, nevertheess, the energy absorption of the honeycombs can be improved by increasing their ce irreguarity. Zheng et a. further investigated the inertia effect on the deformation of the honeycombs and this wi be described more in the next Section The current study is a continuous progress from part of Yu and Chen s work (Yu and Chen, 2006), when they did some numerica anaysis by FE anaysis toos (Mac Mentat, 2005) on investigating the shape and materia of the impactor. In their study, the materia of the modeed textie honeycombs are gass/epoxy sheet and the shape of the ce is reguar hexagona. There are a range of impact objects of spherica, rectanguar and cyinder which are modeed as impactor with item mass between 0.4kg to 0.6kg to represent hand thrown missies and the impact veocity is 5m/s, 10m/s and 15m/s respectivey. The materias for the impact objects are modeed as wood, iron, gass and concrete to represent a wooden ba, a short meta bar, a thick gass botte and a haf house brick. Eventuay, they concuded that textie reinforced ceuar structure can provide much better protection, such as energy absorption and force attenuation, than the current used foam-she structure imb protector for the poiceman. Athough Yu and Chen s work covers a wide range of the impact objects, they didn t consider the structure of the honeycombs themsef may bring effect on the protection capabiity too and this is one of the reasons to initiate the current research work Dynamic impact with different veocities The impact mechanic response to the impactor coud be divided into ow-veocity and high-veocity impact and they differ in their nature which is determined by their impact veocities and the mass of impactor. However, no ceary defined boundary exists between these two groups. For that purpose the structura response of the target is taken into account (Richardson and Wisheart, 1996). At ow-veocity impact, the composite structure has sufficient time to response to the dynamic oading, and the contact duration between the impactor and the target is reativey ong. Consequenty, more energy can be absorbed easticay. This is specificay the case for persona protective 54

55 equipment (PPE) where the mass of the impact threats is between 0.05kg to 1.0kg and the veocity are frequenty no more than 30m/s (Dionne et a., 2003). On the other hand, high-veocity impact responses can be characterised by the shockwave propagation through the materia, where the structure does not have time to respond (Fanagan et a., 1999). A typica exampe is a baistic impact on a miitary hemet, where the buet s mass may be ony a few grams and the veocity is around 360m/s or more (Aare and Keiven, 2007). When ceuar soids are impacted under high-veocity situation, the oading force, at a given instant, caused the concentration of deformation in one particuar area and it changes the ce shape which eading to a oca strain-rate much arger than the nomina strain-rate and this resuts in a much ocaised damage in the composite (Gibson and Ashby, 1997). Abrate (1998) aso specified that at higher impact veocities, a critica condition wi be reached when oca contact stress exceeds oca strength, eading to the structure faiure, core interface deaminating and core compression strength faiure too. Whatever the impact veocity is ow or high, Gibson and Ashby (1997) specified in their book that micro-inertia which can be determined by the thickness of the ce wa pays a significant roe in controing distribution of the crushing in ightweight open-ce foam and honeycombs. This is based on the concusions from Kinworth and Strong (1998) that micro-inertia is associated with rotation and atera motion of ce was when they bucke and tend to supress more compiant bucking modes in order to increase the crushing stress and diffuse the crushing wavefront respectivey. Zheng et a. (2005) further reported that at a ow impact veocity, the deformation of the honeycombs can be regarded as under quasi-static oading and the inertia effect can be negected whie under high impact veocity, inertia effects dominated the deformation of the honeycombs which wi cause the transverse band inside the honeycombs. Schube et a. (2005) studied experimentay the ow-veocity impact behaviour of sandwich panes consisting of woven carbon/epoxy face-sheets and a PVC foam core. 55

56 The impact veocity is set up in the range of 1.6m/s to 5.0m/s and the impact energy is around 7.8J to108j. They compared the test resuts with an equivaent static oading resut and they found out that ow veocity impact was generay quasi-static in nature except for ocaized damage. This kind of impact veocity eve is more simiar to the current study due to that this research work is deveoped from an existing project (Yu and Chen, 2006), which invoves repacing the shied foam core with textie honeycomb composites for UK poicemen in order to reduce the weight of the shied and improve the protection. Considering that in most trauma and sash cases, the projectie veocity can be categorised into ow-veocity impact, therefore, veocities under 30m/s wi be defined as impact veocity for the experiment and FE anaysis in this study Energy absorption of ceuar soids Honeycomb is one of the most commony used materias as core materia for energy absorption besides meta tube, conica she, tube array, foams and woods, etc. (Shih and Jang, 1989; Kim and Jun, 1992; Chun and Lam, 1997;Gibson and Ashby, 1997; Reddy and Reid, 1980; Reid and Be, 1982; Gupta et a., 1997; Tang et a., 2008). When design the structure for the core materia for the purpose of cushioning performance, the major concern is that this kind of materia shoud be capabe of accommodating arge permanent deformation without structure faiure and it shoud show reiabe and controed oad-deformation behaviour under dynamic oading conditions (Hernasteen and Lebois, 1976). Johnson et a. (1977) again summarised the requirements for the energy absorption of composites and they concuded that firsty the composites shoud utiize the pastic deformation rather than eastic deformation, as their major energy-dissipation mechanism. They aso said that whie providing sufficient energy absorption capacity, the peak force (thus the peak deceeration) must be kept beow the threshod that woud cause damage or injury and the structure deformation stoke shoud be ong, stabe, repeatabe and reiabe under impact condition. Finay, Johnson et a. (1977) pointed out that the composites shoud be ight themseves, possessing high specific energy 56

57 capacity (i.e. energy absorption capacity per unit weight) and thus textie honeycomb composite with voume density ess than 0.2g/cm 3 shoud meet the requirements. Regarding the energy absorption mechanisms, Gibson and Ashby (1997) said natura ceuar soids such as wood, bones, and eaves have ce was which are themseves composites. When these materias are deformed, the fibres were pued out and unrave in compicated ways which dissipate a great dea of energy. However, for man-made ceuar soids, there is a number of mechanisms are at work in absorbing energy (Schwaber, 1973) which reated to the eastic, pastic or britte deformation of the ce was. The energy coud be converted into ocaized pastic deformation, heat and a sma part coud be remained as kinetic energy as a resut bouncing occurs (James and Stephen, 2001). In genera, the absorbed energy (W), up to a strain (ε), can be expressed as foows: W = 0 ( ) d [2-8] where σ(ε,) is the stress up to a strain of the deformed structure (Gibson and Ashby, 1997). Athough there are a ot of researchers (Kobayashi et a., 1998; Yasui, 2000; Zhao and Gary, 1998; Wang and Wang, 2007; Wang, 2009) who have studies different types of honeycomb core as energy absorber made from various materias: poypropyene (PP), poyester (PET), paper and auminium, there is few papers have been pubished on the area of woven textie honeycomb cores. One of the pioneers who studied woven textie honeycomb composite is Chen and Tan (Tan and Chen, 2005; Tan et a., 2007) who did a ot of work on optimising the geometric parameters for the textie honeycomb composites theoreticay. They compared various essentia structura parameters which affect the energy absorption and deformation behaviour of the composites and they found that ce opening ange, ce wa ength and ce wa thickness significanty affects the energy absorption and 57

58 deformation behaviour of the composites. The strain energy density concentrations appear very seriousy around the ce corners during quasi-static impact. Further investigations about the honeycomb composites assembed with face sheet pies were conducted theoreticay and the resuts show that deformation and distributed strain energy density of both outer and inner surfaces of the appied structure are significanty affected by py assemby, outer py materia, outer py thickness, and oading area. Wu (2003) conducted imited mechanica experiments to vaidate above findings and found that the change of the structura parameters can resut in significant change of honeycomb composites mechanica performancse. Therefore, systematic experimenta and theoretica investigations are required and this initiaized this research project. 2.5 Textie Honeycomb Composites There are different types of textie composites incuding 3D woven, braided or knitted textie reinforced composites. These types of textie reinforced composite materias have drawn a wide research interests due to their capabiity of efficienty absorbing kinetic energy to weight ratio. And the cost is ow during manufacturing process and their damage toerance is exceent too. 3D woven composites were first deveoped neary 40 years ago in an attempt to repace expensive high temperature meta aoys in aircraft brakes by Muen and Roy (1972). In their work, a speciaised oom was deveoped to aow the weaving of hoow cyindrica preforms in which carbon fibres were aigned in the radia, circumferentia and axia directions. The produced composites dispay some specific strength and stiffness properties as we as exceent resistance to therma deterioration. Mouritz et a. (1999) state in their paper that: one of the advantages of 3D woven composites is that they have higher deamination resistance, baistic damage resistance and impact damage toerance. The 3D braided composites have the simiar performance as 3D woven composites besides they have a greater crashworthiness property (Mouritz et a., 1999). Generay, braided composites have a higher eves of conformabiity, drapabiity and structura integrity, which makes it possibe to produce composite structures with intricate geometric to the near-net-shape. (Ko and Hartman, 1986; Crane and Camponeschi, 1986; 58

59 Whitney et a., 1971; Gong and Sankar, 1991). However, one of the major imitations for 3D braided composites is that their maximum preform size is determined by the braiding machine size, and most industria machines are ony abe to braid preforms with a sma cross-section (Dexter, 1996) 3D knitted composites can be divided into 3D knitted sandwich composites, 3D warp knitted non-crimp composites and 3D near-net-shape knitted composites (Mouritz et a., 1999). The recenty innovation of 3D knitted composites is Spacer Fabric, which is produced on circuar knitting machines (Bartes, 2003) and its structure is by interacing the upper and ower ayers of the fabric with chain yarns through thickness direction and the schematic iustration is shown in Figure Sun et a. (2010) specified in their paper that spacer fabric has exceent air permeabiity under high area pressure and this kind of structure has high interaminar shear strength, and can prevent the side between upper and ower ayer. However, because a the yarns are in curves and not in straight ines, the tensie stiffness and strength of the spacer fabrics are reativey ower than those of woven fabrics in the in-pane direction. Figure 2-17 Sketch of spacer fabric construction. 59

60 D woven fabrics The current study is based on 3D woven fabrics; therefore, reviews wi be focused on this type of fabric and specificay, the reviews wi be drawn on 3D woven fabric by means of muti-ayer techniques with hoow structure in between. The appications of 3D woven structure as core materias for textie composites have been constanty growing, because they possess major advantages over conventiona materias. First of a, they are generay ight weight and show no heat degradation whie processing. And since their fibres are interaced in the cross-wise, engthwise and thickness direction they can withstand muti-axiay stresses, which is obviousy one of the key requirements for composites empoyed in industry and engineering (Mohamed, 1990; Mouritz et a., 1999). In Figure 2-18, a range of diverse compex 3D woven composites is dispayed, which coud be found in industries ike aircraft, automobie and civi infrastructure. Figure D woven composites (a) cyinder and fange; (b) egg crate structure; (c) turbine rotors; and (d) various compex shapes woven preforms (Mouritz et a., 1999) 60

61 Some of 3D woven composites are produced with speciay-deveoped weaving machinery (Buesqen, 1995; Dickson et a., 2000). However such specia machinery is expensive and can ony weave a imited range of 3D composites, so that producers are more interested in ways to produce 3D composites on conventiona ooms, which are more economica and versatie in usage. Popuar 3D hoow woven structures, which can be produced on standard weaving ooms, are pictured in Figure These structures are known as mutiayer woven fabrics, as they are composed of severa series of warp and weft yarns which form distinct ayers, one above the other (Ko, 1989). Obviousy the number of ayers and the way they interace contributes mainy to the through-thickness strength and the mechanica properties of such structures can be reativey easy atered by varying the density and types of weft, warp and binder yarn and use of different weave pattern (Chen et a., 1999). Figure 2-19 Woven architectures used in 3D woven composites (Yi and Ding, 2004) D honeycomb fabric A woven fabric made from one set of warp and one set of weft yarns is regarded as 2D, whereas fabric structures with obvious thickness due to addition of warp and/or weft yarns are referred to as 3D. 3D fabric structures may be made in the form of soid or hoow depending on its appications and 3D hoow structure are used in creating composites that are buky, ightweight and energy absorbent. 3D hoow fabric is 61

62 featured by one or more ayers of trianguar or trapezoida cross-sectiona shapes. The hoows or the tunnes are formed between the adjacent fabric sections and the structure is sef-opening. The use of CAD/CAM systems for woven structures has made the weaving process more efficient and more versatie and the current textie technoogy is capabe of creating 3D honeycomb woven fabric with no or itte need for machine modifications. A conventiona oom equipped with a dobby shedding mechanism wi be sufficient for making this kind of fabric. Computer representation of 3D woven fabric structure has been deveoped by eary researchers such as Hoskins (1983), Xu (1992) at UMIST and they have modeed and visuaized thread paths of fabrics in 3D. Then other workers deveoped the soid mode for fabrics and the most notaby is Keefe et a. (1992). Chen et a. (1996) worked on the mathematica representations of weaves for 2D and 3D structures in detai, and Chen and Potiyaraj (1999a; 1999b) impemented the mathematica modes and created a CAD package that covers 3D soid structure and backed fabric structures, as we as singe ayer structure. Tracing back in 1991, Takenaka et a. (1991) has invented a woven fabric having muti-ayer structure to form hexagona and tetragona ce shape to form the composite materia and he has hod a patent on this kind of structure, which is shown in Figure In his invention, the thickness of the muti-ayer woven fabric can be increased by increasing the number of woven fabric ayers to be superposed. 62

63 (a) woven fabric with mutiayer structure (hexagona shape) (b) woven fabric with mutiayer structure (tetragona shape) Figure 2-20 A schematic diagram of woven fabric with mutiayer (Takenata et a., 1991) Since Takenaka (1991) invented the mui-ayer fabric which can form the honeycomb structure, the automaticay generated weaves have been used to contro the shedding mechanisms of conventiona weaving machines at UMIST, eading to successfu production of 3D hoow structures and in 1999, Yassar (1999) managed to weave 3D 63

64 muti-ayer fabric by using poyester yarns with conventiona 2D weaving process and he concuded that an opening process is needed to provide thickness and to convert the fabrics from 2D form into 3D as shown in Figure Figure 2-21 Cross section view of the honeycomb fabric in 3D form (Yassar, 1999) By using more advanced IT technoogy, a significant progress was made by Chen et a. (2004) foowed by Chen and Wang (2006) who mathematicay modeed the 3D hoow woven structures and estabished an agorithm to create the weave diagrams and ifting pans for the design and manufacture woven honeycomb structure and their agorithms were impemented in a CAD/CAM software package speciay designed to weave this woven honeycomb fabric. Sun (2005) pursued on weaving 3D honeycomb fabric by using cotton fabric and 10 fabrics with different ayers and picks that were manufactured in the University of Manchester and these fabrics were used in the current research to convert them into textie honeycomb composites. The design of the honeycomb fabric wi be expained more in detais in Chapter Structure parameters for textie honeycomb composite In this section, reviews wi be focused on introducing the geometric structure of the honeycomb from its singe ce to the whoe structure to hep readers buiding up the 64

65 genera ideas of a the parameters describing the textie honeycomb composites in the current research. : opening ange b : bonded wa ength f : free wa ength t b : bonded wa thickness t f : free wa thickness h : height of ce Figure 2-22 Parameters of singe honeycomb ce (Tan and Chen, 2005) A honeycomb structure is composed of an array of hexagona ces and ce is the basic component of the honeycomb structure. A ce structure is formed by free and bonded was where free was refer to the ce sides that are free from other sides, whereas the bonded was are those having to be bonded together for the formation of the ceuar cross-section. Parameters such as the opening ange, wa thickness, and ength of each wa are used to describe a ce. Figure 2-22 iustrates the parameters of a hexagona ce (Tan and Chen, 2005). Generay, opening ange is between 0 and 90. Given the assumption that the thickness of the yarn does not change during weaving, with the weaving method used in this study, the thickness of free wa t f is approximatey twice the yarn diameter, and the thickness of bonded wa t b is approximatey three times of the yarn diameter. The height of the ce is cacuated as h = 2 f sin (Tan and Chen, 2005). A repeat of the cross-section of the honeycomb structure is made of two coumns, one being a ce onger than the other. The overa vertica dimension can be described by the number of ces in either onger or the shorter coumn and the horizonta dimension 65

66 are expressed by the number of repeats of the ce coumns. Figure 2-23 iustrates a 6- ayer honeycomb structure. From the top to the bottom, the ayers are numbered in sequence from ayer 1 to ayer 6. The first ayer produces a fat surface to form the topside of first coumn hexagon. Then, it owers and cross-inks with ayer 2 to form the second coumn hexagon, as indicated in Figure Layer 2 and ayer 3 are interconnected to form the topside of the second hexagon in the first coumn and then divided into two. Layer 3 is connected with ayer 4 and ayer 5 is connected with ayer 6 for the second coumn hexagons. The same ayer connect rue appies on the third coumn hexagons. The number of ayers of any honeycomb structure shoud be an integer arger than 2 (Sun, 2005). Figure 2-23 Schematic diagram of a 6-ayer honeycomb structure (Sun, 2005) Sun (2005) aso mentioned in the case of a textie honeycomb structure, the free was wi be made by a singe ayer fabric, and the bonded was wi contain the amount of yarns for making two ayers of fabrics. In weaving the honeycomb fabric, the structure wi be woven fattened, eaving the open ange a redundant parameter at the fabric stage. The opening ange wi be used during the consoidation stage when the 3D honeycomb fabric wi be open up. 2.6 Appications of textie honeycomb composite on PPE A poice officer in a pubic order emergency is in a high risk situation. There is a range of potentia injuries from sips, trips and fas to missie attack and assaut. Rioters wi 66

67 pick up anything to use as a missie from wood, meta, bricks to petro bombs. There is aso a risk through direct contact with suspects during foot pursuit or restraint. For this reason there is a range of PPE an officer wi wear in a pubic order situation. With shieds cover most of other parts of the body, most of times; the focus of PPE design is on how to avoid the attack to ower imbs which is not easy to be covered by shied and often prone to attacks. Therefore, a new materia must be deveoped in order to provide better protection and textie honeycomb composite is idea for this kind of appication because it has an extremey good energy absorption performance. Bajaj and Sengupta (1992) state that there are three main requirements as how a protector is protecting wearers against impact oading. Firsty, it needs to disperse the impact energy from impact point to a arge area by hard shes, such as armour, shied, etc. Secondy, it shoud deay the occurrence time of peak transmitted force to the wearer. By observing transmitted force-time diagram of the materia, fatter curves usuay are preferabe to avoid high peak transmitted force. Lasty, it needs to absorb the impact shock by energy absorption materia with deformation. Med-Eng Inc. (2001) describes the injury threshod force of ower imbs which can be used as reference vaues for PPE design (Dionne et a., 2003; Med-Eng System Inc, 2001). For shin, the threshod vaues is between 4.30KN and 8.93KN, and for knee, it is between 7.56KN and 10KN. Therefore, PPE shoud be designed to reduce the impact force significanty to be ower than the higher imit of such threshod vaues in a safer range. To choose a ight weight new materia with high energy absorption capabiity and to reduce force attenuation is drawing more and more attention in the scientific area and fabric composites with speciay designed honeycomb structure matches the criteria isted above and coud be a good candidate for such purpose. 67

68 2.7 Comments The iterature review has shown that the honeycomb structured composites is a type of textie woven composite providing exceent energy absorption and shock protection comparing to other conventiona materias, and the weight of this type of composite is extremey ight (Wang, 2009; Pfug and Veopoest, 1999; Pfug et a., 2002). Therefore, poiceman who are working in a high risk situation, are considering to repace the reativey heavy shied with new materias to reduce its weight and at the same time, to improve the shied s protection and energy absorption performance. Textie honeycomb composites are one of the options which can meet the requirements correspondingy However, the mechanica performances of honeycomb structured composites seem to be affected significanty by their geometric parameters. In more detais, the composites voume density, ce size, ce wa thickness, ce wa ength ratio and ce opening ange are the parameters which have been investigated frequenty by researchers (Barma et a., 1978; Ashby and Meh, 1983; Gibson and Ashby, 1997) to seek out these geometric parameter s reationship with the composites mechanica performances such as strength and stiffness, deformation pattern, damage toerance, fatigue performance, etc. However, most of the studies are based on meta and paper honeycomb structure composites, few iteratures have been reported on textie based honeycomb composite. It has to be noted that the currenty used expansion or corrugation techniques for making honeycomb structure core materias are not suitabe for making textie based honeycomb structure (Bitzer, 1997) because it cause deaminating between the adhesive ayers easiy, Therefore an aternative way, which can simpify and integrate the honeycomb structure to sove the deaminating probem, has to be found in order to get a more reiabe structure performance. The aim of this research is to investigate how geometric and structura parameters of textie honeycomb composites woud affect the mechanica performance and energy absorption capabiity under ow veocity impact. Athough a considerabe body of knowedge has been generated in the past years about textie honeycomb composites (Tan and Chen, 2006, Tan et a., 2007), more research is required to deveop design 68

69 guideines for optimizing materia performance by manipuating the honeycomb composite s architecture. In order to take advantage of the attractive features offered by textie structura composites, there is a need for the deveopment of a sound database and design methodoogies which are sensitive to manufacturing technoogy. An examination of the iterature indicated that ony a imited number of systematic experimenta studies have been carried out on 3D fabric reinforced composites. Nowadays, the usage of finite eement anaysis (FEA) method on the structure performance for the composites becomes more and more popuar due to that this method makes it possibe for the researchers to see ceary the whoe crushing process even at an instant time. FEA provides not ony vauabe information for composite ocaised damage but aso estabish criteria for acceptance or repair of structura components (Abrate, 1998). Whie design of the structura components tends to be very compex and time-consuming in practices, the deveopment of efficient anaytica preprocessors by FEA can decrease cost and make FE modeing an economic and easy-touse soution. 69

70 CHAPTER3 DESIGN OF 3D HONEYCOMB FABRICS In order to get the textie honeycomb composites, honeycomb fabrics need to be produced firsty. This chapter introduces the procedure of design and manufacture of 3D honeycomb fabrics as reinforcement materia. It must be mentioned that this part of work was carried out in coaboration with a feow researcher in the University of Manchester (Sun, 2005; Chen et a., 2008). It has to be noted that in this part of the experimenta work Sun (2005) has designed the weave structure for the muti-ayer honeycomb fabrics by using CAD software created by Chen and Wang (2006). She aso manufactured the honeycomb fabrics on a dobby weaving machine. The author s part of the experimenta work is on the determination of the geometric and structura parameters for the honeycomb fabrics for the experiment purpose, and the manufacture of honeycomb composites from these fabrics. An agorithm has been estabished to create weaves based on the specification of the 3D honeycomb composite parameters, and this agorithm has been impemented into a CAD programme, Hoow CAD made in the University of Manchester, which gives accurate soutions for making reinforcing fabrics of this type (Chen et a., 2004). The current weaving technoogy is capabe of creating the 3D honeycomb woven fabrics without carrying out machine modification and 3D honeycomb woven fabrics with various structura parameters can be engineered from commercia oom. Nevertheess, a good understanding of weaving process and woven structure is sti required in the 3D honeycomb fabric design in order to characterise the performance of the textie honeycomb composites in the current research work. 3.1 Design of 3D Honeycomb Weaves One repeat of a honeycomb fabric can be divided into four regions, and they are regions I, II, III and IV, as shown in Figure 3-1. Region I corresponds to the section of the 3D 70

71 honeycomb structure where the fabric ayers a separated from each other; region II is where the adjacent ayers join together at an aternate interva; region III is the same as region I; and region IV is again the joining section but the joining ayers are different from that in region II. (a) Honeycomb fabric woven before opening (b) Honeycomb structure after opening Figure 3-1 Region division of a honeycomb structure Because of the nature of weaving, the honeycomb fabric is woven with a ces fattened as indicated in Figure 3-1(a), and the honeycomb structure is achieved when the fabric is opened up after weaving and consoidated as shown in Figure 3-1(b). According to the definition of a ce, region II and IV correspond to the bonded was with ength b and region I and III the free was with ength f. Note that Figure 3-1(b) 71

72 shows a seected part of the honeycomb structure that woud open up to from Figure 3-1(a) Representation of woven honeycomb structures In this study, a woven honeycomb structure in weft direction can be defined by specifying the structura parameters. The foowing genera coding format is used to denote a particuar honeycomb structure: xl(y+z)pθ where, x is the number of fabric ayers used to form the honeycomb structure; y is the ength of the bonded wa measured in the number of picks; z is the ength of the free wa measured in the number of picks; θ is the opening ange of the hexagona ces which varies from 0 to 90 and opening ange is the structura parameter to investigate and compare in the group; L is used to denote the ayer ; P is used to denote the pick. There are situations when the engths of free and bonded was are the same, i.e. y=z. In such a case, the coding format can be reduced to: xlypθ In the above, x, y, z are integers and x>2, y>1, z>1. When θ is not shown, the opening ange assumes a defaut size of 60. According to the format discussed above, a 4L6P honeycomb structure stands for a structure comprising four ayers of fabric, where the ength of both the free and bonded was is six picks and the opening ange is 60. As another exampe 8L6P45 denotes a honeycomb structure made from eight ayers of fabric, where the ength of the free and bonded was is six picks with the opening ange being 45. On the hand, 8L(4+3)P refers to a honeycomb structure made from eight ayers of fabric, where the engths for the bonded and free was are four picks and three picks, respectivey, with the ce opening ange being

73 3.1.2 Layer connection methods In the honeycomb fabric, adjacent ayers are connected at given intervas in order to achieve the honeycomb effect. There may be many different ways for the adjacent ayers to be woven together. Firsty, the two ayers can be woven together as a singe ayer fabric where the warp density wi have to doube in this section of the fabric. Secondy, it can be woven as a doube ayer fabric connected together either by stitching or by ayer interchange. Thirdy, the orthogona or ange-interock 3D weave structures can be used for this section of fabric. There coud be further ways for joining the two ayers together. In the current work, however, a type of orthogona structure is used for connection two ayers together (see Figure 3-3(b)). In this particuar construction, haf of the warp ends are used for binding whist the other haf simpy embedded in the midde without interacing with the weft yarns. Figure 3-2 iustrates the weaves used for the individua ayers (free was) and the weave construction for the joined ayers (bonded was). Figure 3-2(b) shows that two singe fabric ayers are combined to form a condensed singe ayer as an exampe. (a) pain weave for singe ayers (b) ayer joining Figure 3-2 Seection of weaves 73

74 3.1.3 Weave creation After the honeycomb structure is specified, it is important that the weave is created accordingy. A procedure for creating waves for the honeycomb structure has been created and it has been impemented into software Hoow CAD (Chen and Wang, 2006). In this section, two exampes of 2L1P and 4L3P structure are used to expain the principe of the weave generation for honeycomb fabrics. In a exampes, the pain weave is used for singe ayer fabric section because of its advantages such as the simpicity, structura integrity, and good acceptance by the technica end users. The 2L1P structure The simpest honeycomb structure is with 2 ayers and 1 pick in was which has been named 2L1P. Figure 3-3(a) shows the honeycomb structure when opened, and (b) is the iustration of the interacement between warp and weft yarns for this honeycomb structure. It can be seen that there are atogether 4 warp ends, ends 1 and 2 being responsibe for weaving the top ayer and 3 and 4 for the bottom ayer. The four regions (I, II, III and IV) are a compete repeat aong the warp direction. Warp end 1 is taken to expain the weave creation. In region I, warp end 1 goes above the two picks and therefore in the weave diagram shown in Figure 3-3(c) the first warp end received two warp-up marks. When this warp end traves into region II, it goes under both picks and therefore in the weave diagram the first warp end receives two warp-down marks, which are indicated as bank grids. In region III, the warp ends traves above the 2 picks again, and correspondingy in the weave diagram there are two warp-up marks. In the fina region, this warp end is underneath the pick for the top ayer but above the one for the bottom ayer. Accordingy, in region IV in the weave diagram, there is firsty and a bank then foowed by a mark. In the same way, foowing the movement of warp ends 2, 3 and 4 wi compete the coumns 2, 3 and 4 in the weave diagram respectivey. 74

75 (a) 2L1P honeycomb structure (b) Cross-sectiona view of 2L1P (c) Weave diagram Figure 3-3 Honeycomb structure 2L1P 75

76 The 4L6P structure (a) 4L6P structure (b) 3D view of the 4L6P structure for a four regions (c) Weave diagram Figure 3-4 Honeycomb structure 4L3P 76

77 This structure comprises 4 ayers of pain woven fabric invoving eight warp ends and the interayer connections are iustrated in Figure 3-4(a). The interacing detais for each region are shown in Figure 3-4(b), where there are 12 picks invoved in each region. In region II, the four ayers of fabrics are organised into two bonded was, whereas in region IV the bonded wa is created by joining ayers 2 and 3 eaving ayer 1 and 4 to form the free was. Using the same principe expained aready, the weave diagram can be created where there are eight warp ends a together 48 picks, where is shown in Figure 3-4(c). In this work, it is assumed that the tunnes formed by the ces run in the weft direction, athough they can be arranged to go in warp direction too. The reason to make this arrangement is that ess heads are needed during the weaving process when the tunnes are run in the weft direction. Take 4L6P as an exampe, according to its weaving diagram in Figure 3-4(c), when the tunnes are run in the weft direction from eft to right in Figure 3-4(c), it is ceary that there are ony 8 different design patterns in one weave repeat, therefore, 8 heads wi be needed to ift the warp ends to form the fabric. However, if the tunnes are run in the warp direction from bottom to the top of the weaving diagram in Figure 3-4(c), there are 14 different design patterns shows up aong warp direction, therefore, 14 heads wi be requested to ift the warp ends for the fabric production. And the same rue appies to other honeycomb fabric with 6 and 8 ayers too. 3.2 Design of 3D Honeycomb Fabrics D honeycomb fabrics In this research, pain weave has been chosen for the first attempt as this is one of the easiest ways to induce mutiayer anaysis. It aso heps to simpify the manufacture process by using pain weave structure as it reduces the heads significanty. As shown in Figure 3-2(a), the anguar sides of each hexagon to the axis were created by the pain weave and most of the top and bottom sides of a hexagon were formed by interconnecting the two ayers, shown in Figure 3-2(b), except the topside of the first ayer and the bottom part of the ast ayer are just pain weave. 77

78 In order to investigate systematicay the 3D honeycomb composites, 10 honeycomb fabrics are designed and manufactured, which are 4L6P, 6L4P, 8L3P, 8L4P, 8L5P, 8L6P, 8L(4+6)P, 8L(4+3)P, 8L(3+6)P and 8L(6+3)P. This is based on the foowing geometric parameters of a honeycomb structure: 1) Ce opening ange, 2) Different ce size at the same number of ayers 3) Length ratio of ce was, b f 4) Simiar sampe thickness with different ce size According to the above four structura parameters and weaving capabiity in the aboratory, the fabric to be woven is categorized into four comparison groups. An outine of the designed fabric types and desired structura parameters are isted in Tabe 3-1 and Tabe 3-2. Tabe 3-1 aso shows the actua needs of quantities of a the fabric types to weave. 78

79 Tabe 3-1 List of fabric types with weaving quantity and designed ange Index Fabric (m) θ (º) Sampe Quantity 1 4L6P L4P L3P * 4 8L4P L5P L6P * L(4+6)P L(4+3)P L(3+6)P L(6+3)P Tota 10 types where, represents the tota ength of the fabric been produced in the unit of m; θ means the opening ange for the fabric in the future composite design; the fabric which is marked with * wi be used for comparison in three different group for the future experiments. The fabric types which are categorized into 4 groups for comparison are isted in Tabe

80 Tabe 3-2 Experimenta design outine in groups Group 1. Opening anges, θ(º) (5 sampes) Index Sampe θ (º) 1 8L6P L6P L6P L6P L6P 90 Group 2.Different ce size at the same number of ayers (4 sampes) Index Sampe θ (º) 1 8L3P L4P L5P L6P 60 b Group 3. The ratio of wa ength, 1(3 sampes) Index f f b Sampe θ (º) 1 1:2 8L(3+6)P :3 8L(4+6)P L6P 60 b Group4. The ratio of wa ength, 1 (3 sampes) f 1 1 8L3P :3 8L(4+3)P L(6+3)P 60 Group 5. Same sampe thickness with different ce size (3 sampes) Index Sampe θ (º) 1 4L6P L4P L3P 60 80

81 3.2.2 Design detais for 3D honeycomb fabrics Opening ange, ce size, ength ratio of ce was have been regarded as the potentia key geometric parameters to infuence the mechanica behaviour of textie honeycomb composite (Tan and Chen, 2005; Tan et a., 2007) and it is of interest the current research to further investigate them in detais. Before the textie honeycomb composites are made from honeycomb fabric, the design detais for various fabric types wi be expained in the foowing sections Ce opening ange, The opening ange of ce units is an important parameter for 3D honeycomb composite structure, which can change the structure thickness and materia use efficiency as we as mechanica properties. To show the comparabiity of this group, diagram in Figure 3-5 has shown five open 8L6P woven honeycomb structures in weft views with different anges of 30, 45, 60, 75 and 90. In Figure 3-5, it is worthwhie to note that the thicknesses of the specimens are different after the change of the ce opening anges if the number of ayers is kept constant (it is 8 in this group). Therefore, in this group, resuts from comparison may aso indicate the effect from different thicknesses of specimens. Additionay, to achieve a comparabe resut in this group, the wa engths for a sampes are a the same by fixing the pick numbers of a was to 6. 81

82 Figure 3-5 8L6P with different opening ange Different ce size at the same number of ayers Ce size is another significant parameter for honeycomb structure besides opening ange which aso infuences the energy absorption capacity effectivey. In this study, the change of ce size is made by changing the ength of the was, or in another word, changing the pick numbers of was in weaving. With the same density of each ayer, the ength of wa wi be increased or decreased by adding or reducing picks number. To show the comparabiity of this group, diagrams of open honeycomb structures in weft view for fabric types in this group (8L3P, 8L4P, 8L5P, 8L6P) are shown in Figure 3-6. Except the change of wa engths, the number of ayers is fixed to the same at 8 and a ces are with opening ange of 60. The bonded wa ength is the same as the free wa ength, b = f.. Again, in Figure 3-6, significant change of specimen s thickness may be observed after the change of the ce size whie the ayer number is fixed for a types. Same as the first 82

83 group, the effect from the change of the specimen s thickness might combine with the effect of ce size on the mechanica properties of the specimens in the mechanica tests. Another point to note out is, due to the imited yarn purchased, 8L7P were not woven in actua study. But it is beieved giving up 8L7P shoud not infuence the comparabiity of this group and discussions on the fina resuts of the group. Figure 3-6 different ce size for 8-ayer composites Length ratio of ce was ( b f ) In practica use, the bonded wa ength b and the free wa ength f do not have to be the same. Therefore, it is vauabe to investigate how such ratio of energy absorption capabiity of the honeycomb composites. b f might change the Seectivey, current study chose 6 sampe types with fact that pick numbers of the free wa for these three types are a 6 indicates there is no change of the free wa ength in this subgroup. Therefore there is no change of the 83 b f in two subgroups: b The first subgroup ( 1) incudes 3 sampe types: 8L(3+6)P, 8L(4+6)P, 8L6P. The f

84 structure thickness. However, by changing the pick number of bonded wa (the bonded wa engths), the ratios b f for these three types can be set as 2 1, 3 2 and 1 1 respectivey. Apparenty, the bonded wa ength is shorter than the free wa ength in this subgroup. b Simiary, the second subgroup ( 1) consists of 8L3P, 8L(4+3)P, 8L(6+3)P with f ratios of,, respectivey. This time, the bonded wa ength is onger than the free wa ength. This time, the pick number of free wa is specified at 3, but changing the pick numbers of bonded was to 3, 4 and 6. A fabric types in both subgroups are with 60 opening ange as shown in schematic diagrams in Figure 3-7 and Figure 3-8 for two subgroups separatey. Different from the previous two comparison groups, since there is no change of free wa engths and opening anges, there is no change of specimen s thickness within each subgroup. Therefore, by comparing the resuts of each subgroup, the difference of energy absorptions wi be from the different b f soey. 84

85 b Figure 3-7 Honeycomb structures with ength ratio of ce was ( 1) f b Figure 3-8 Honeycomb structures with ength ratio of ce was ( 1) Simiar sampe thickness with different ce size f To avoid the potentia effect from the thickness change, this group which incudes 4L6P, 6L4P, 8L3P with the same opening ange 60 is aso designed with same specimen s thickness. This is because 4 4 sin60=6 6 sin60=8 8 sin60 where 4, 6, 8 are with pick numbers of 6, 4 and 3 respectivey. In such circumstances, changing number of ayers is aso changing the ce size at the same time. Thus, the comparison resuts of this group are potentiay the combined effects from number of ayers and ce size, but without the effect from specimen s thickness. Figure 3-9 presents the above specifications in weft view of open honeycomb structures in diagrams. 85

86 Figure 3-9 Structures with same thickness and different ce size 3.3 Manufacturing of 3D honeycomb fabrics In order to investigate systematicay the 3D honeycomb composites, 10 honeycomb fabrics are designed and wi be manufactured, which are 4L6P, 6L4P, 8L3P, 8L4P, 8L5P, 8L6P, 8L(4+6)P, 8L(4+3)P, 8L(3+6)P and 8L(6+3)P. This part of thesis presents the weft density of the 3D honeycomb fabric and the detaied parameter specifications for fabrics in rea weaving ab. The ifting pans designed from Hoow CAD [Sun, 2005] for each fabric type are aso shown in this section Weft density of the 3D honeycomb fabric Before making the fabrics, the weft density of the overa fabric must be worked out based on the number of picks specified for each of the ce was in the fabric specification and the actua ength expected. Suppose that z is the number of picks specified for a wa of singe ayer fabric session (picks), is the required ength of the fabric session (cm), d i is the weft density for this singe ayer (picks/cm), then d i = z [3-1] If the honeycomb fabric is composed of m fabric ayers, then the weft density of the honeycomb fabric d, wi have to be set to 86

87 m d = di i 1 [3-2] In the case that a ayers have the same weft density, the weft density of the honeycomb fabric becomes d = md i [3-3] The honeycomb fabrics are designed to have four, six, and eight fabric ayers. In a cases, it was decided that each ayer of fabric wi have a warp and weft density of 20 picks/inch (7.87 picks/cm). The warp and weft density of the overa honeycomb fabric can be found by mutipying the number of ayers to the warp and weft density per ayer, as described in equation [3-3]. Therefore, the warp and weft densities of these three honeycomb fabrics are 80, 120, and 160 picks/inch (31.5, 47.2 and 63.0 picks/cm). In a these designs, the tunnes run in the weft direction, and the ength of the ce was can be found using the equation [3-1]. With the wa ength being 3, 4, 5 and 6 picks, the actua ength of the was wi be 3.8mm, 5.1mm, 6.4mm and 7.6mm respectivey. This research does not intend to examine the infuence of fiber type and yarn parameters, incuding yarn twist and yarn inear density, on composite properties because a fabrics are made from the same types of yarn (14.8tex/3, 3 40 s cotton yarns, with the py twists of 433turns/m) was used for making the 3D honeycomb fabrics Parameter specifications for 3D honeycomb fabric in the weaving process As part of weaving design, the detaied parameter specifications for fabrics and ifting pan are important for the honeycomb fabric manufacturing as they wi be directy used in the rea weaving oom. However, the opening anges are actuay formed in resinimpregnation process after the fabric been produced. Take one group of sampes with different ce size at the same number of ayers as an exampe (8L3P, 8L4P, 8L5P and 8L6P). To maximum capabiity of the weaving 87

88 machine, the density of each ayer is fixed as that for the reed, 20 picks/inch. Therefore the tota density for 8 ayers wi be 160 picks/inch. Given the desired width of 12 inch (304.8mm) for the fabrics, the tota number of warp ends can be cacuated as tota density per inch mutipy tota width, which is 160 picks/inch inch = This resut impies that 1920 warp yarns shoud be used for weaving eight ayers 12 inch wide fabrics in this study. For a weaving repeat which consists of four parts discussed in section 3.1.3, the tota pick numbers for a fu circe are cacuated as pick numbers at each part times four parts times number of ayers, which is =192. This indicates that 192 weft yarns wi be woven for a fu weaving repeat. The tota weft yarns can be determined from the desired ength of the fabric, which is isted in Tabe 3.1. Given the weaving weft density of 20 picks/inch and the pick numbers of was are 6 as an exampe, the engths of was b and f can be cacuated as 6 20/inch = 0.30 inch (7.62 mm). It is based on the assumption that the yarns woud stricty foow the density specifications in weaving. Among the four fabric types, the pick numbers in weft direction are different due to the different pick numbers in weft. For a fu weft weaving repeat, the picks are cacuated for four types as foows: 8L3P: 3 4 8=96 picks; 8L4P: 4 4 8=128 picks; 8L5P: 5 4 8=160 picks; 8L6P: 6 4 8=192 picks. 88

89 Consequenty from the difference of the pick numbers in ce was, the engths of bonded was and free was are different. Foowing the same cacuation methods in the first group, these are cacuated as: Given the weaving density of 20 picks/inch and the pick numbers of was are 6, the engths of was b and f can be cacuated as 6 20/inch = 0.30 inch (7.62 mm). It is based on the assumption that the yarn diameter bears no significant change during weaving. 8L3P = 3 20/inch = 0.15 inch (3.81 mm) 8L4P = 4 20/inch = 0.20 inch (5.08 mm) 8L5P = 5 20/inch = 0.25 inch (6.35 mm) 8L6P = 6 20/inch = 0.30 inch (7.62 mm) For comparison, a weaving designs from Hoow CAD (Sun, 2005) for 8L3P, 8L4P, 8L5P, 8L6P are shown as in Figure The weaving design of 8L6P is used for fabric types in the first group as we. 89

90 8L3P 8L4P 8L5P 8L6P Figure 3-10 Weave ifting pan for 8L3P, 8L4P, 8L5P and 8L6P 90

91 3.3.3 Honeycomb fabric production The fabric production was conducted in the weaving aboratory of the Schoo of Materias in the University of Manchester. A dobby oom with maximum 16 head frames was used for the fabric manufacture. It can be used for weaving up to 8 ayers of fabrics when the pain weave is used for a ayers. Three weaver s beams can be used for warp suppy, and each beam can contain a maximum of 1000 warp ends. Punched cards are used to contro the dobby shedding mechanism. Figure 3-11 shows the overa ook of the weaving machine. For warp ends and weft picks in fabric in this study, the white 100% cotton 40 s/3 (14.8tex/3, 3 40 s cotton yarns) yarns were seected due to its reativey higher tension toerance feature comparing to other fabric materias. Using the same materias for both warp ends and weft picks is aso for the convenience consideration in further resuts cacuations and comparisons. Considering the weaving capabiity of the dobby machine in the ab, 20 picks/inch at each ayer is used for the densities of both warp and weft. For mutiayer fabrics, the tota density for the fabric can be achieved by mutipying 20 picks/inch and number of ayers. In order to produce comparabe resuts, the fabric density was kept constant for a fabrics in this study. 91

92 Figure 3-11 The dobby weaving machine (a) card punching machine (b) card punching Figure 3-12 Card punching 92

93 A card is a pattern chain that contains the ifting information to contro the movement of the dobby shedding mechanism. A pattern card is done by punching a hoe in the card at the position where the unit square of weaving designs in Chapter 3 is in bue. Since straight drawing-in head frames are used for threading, each weaving design described in Chapter 3 was punched on cards using the card punching machine shown in Figure It is worth noting that the eft bottom unit in the designs shoud be used as the starting point of punching. After punching the whoe circe of the weaving design is finished, a few extra rows shoud be produced for card circe formation purpose. Normay the first two rows are re-punched for overapping. As there are 10 muti-ayer fabrics were designed for this study which woud make 14 honeycomb composites after opening, 10 cards were punched according to the weaving designs (ifting pans). One of the finished punched cards is shown in Figure In the present study, three beams are used for weaving. A warp density 20 picks/inch has been chosen for each ayer, therefore 4 to 8 ayer fabrics shoud be with the warp density from 80 picks/inch to 160 picks/inch. The width of the woven fabrics is set to be 12 inches (304.8 mm), therefore the tota number of warp ends can be counted as 960 to 1920 correspondingy. The desired ength of each fabric determines the ength of warp at each beam. Warping yarns on the beams wi be different if the number of warp ends is different. It depends on the number of ayers to be woven if the density is the same. In this study, warping is required at east for three times: one for 4-ayer structure, one for 6-ayer structure and the third one for 8-ayer structure. Take 8-ayer fabric types as an exampe, there are eight different fabrics to be woven, and 8L6P may be woven onger as it is required in five different opening anges when it is opened. Since a the 8-ayer fabrics share one warping of 1920 ends, ten meters for 8L6P, two meters for rest seven kinds of 8-ayer fabric types and a 8-ayer fabric types in tota give 24 metres for the usefu part of the fabrics. Considering the extra three metres in warping, there wi be 27 metres warping ength of yarns on each beam at east for weaving a 8-ayer fabrics. Figure 3-13 iustrates the cross section of an sampe fabric which has been produced from the oom. 93

94 For the whoe weaving aboratory work, reativey simper structures were woven, and 4L6P was the first fabric made with 8 head frames were used, foowed by 6L4P after adding four more head frames on the weaving machine. For a 8-ayer fabrics designed, 16 head frames were required. A 8-ayer structures woven incude 8L3P, 8L4P, 8L5P, 8L6P, 8L(4+3)P, 8L(6+3)P, 8L(3+6)P and 8L(4+6)P. The successfu weaving in the ab proved the correctness and effectiveness of the weaving design from Hoow CAD (Sun, 2005). This package, may be the first one of its type, is potentiay important and usefu in automatic manufacturing in textie industries. Figure 3-13 Photograph of one sampe fabric weaved from oom 94

95 CHAPTER 4 CREATION OF HONEYCOMB COMPOSITES AND TEST SAMPLE PREPARATION As mentioned previousy in Chapter 3, the honeycomb fabrics are in fat form when manufactured and need opening before impregnation. This chapter wi introduce the textie honeycomb composite production process. This incudes the use of an opening device which has been designed at the University of Manchester and resin impregnation of the fabric to form composites. Four groups of textie honeycomb composites with different geometric parameters wi be produced, and the division is done according to the ce size, ce opening ange, ength ratio of ce was and sampes with simiar thickness but different ce size. 4.1 Fabric Opening and Consoidation Fabric opening The fabrics were cut into the size of 20cm 20cm before opening. The schematic diagram of the opening device is shown in Figure 4-1(a). Two sets of meta wires, iustrated as A and B in Figure 4-1(a), were used to open the woven honeycomb fabric and they were aid on top of the surface of four meta bars. The fabric was hod tighty and then a set of stainess stee wires with 3mm diameter, coated with poytetrafuoroethene (PTFE,) were inserted into the top and bottom tunnes of the fabric respectivey. The top and bottom wires were pushed apart to the desired vertica distance by adjusting the screws at both ends of the opening device. The edges of the wires inserted into the top ayer of the honeycomb fabric were paced onto the surface of two up meta bars and the edges of the wires which have been inserted into the bottom ayer of the honeycomb fabric were aocated onto the surface of another two ower meta bars. The photograph of the opening device is captured in Figure 4-1(b). 95

96 A: wires inserted to the top hexagon in the fabric B: wires inserted to the bottom hexagon in the fabric d w : distance between the two meta wires m: adjustabe meta screw (a) Schematic diagram of the fabric opening device (b) Photograph of the fabric opening device Figure 4-1 Honeycomb fabric opening devices 96

97 Athough the honeycomb fabrics for composites coud be designed with either even or odd ayers, in the current study, the honeycomb fabrics are woven with even ayers. Therefore, in the foowing equations [4-1] and [4-2], the (x) is taken as an even integer. The distance between the ower and upper meta bars, d w, of the fabric opening device is adjustabe in order to open the fabric up to the required opening ange. For given engths of the ce was, the height of opened honeycomb structure reates to the ce opening ange. For an xlypθ sampe where z=y, if the free wa ength is f, and the free and bonded wa thicknesses are t f and t b respectivey, then the opening ange and the height of the opened honeycomb structure T can be expressed in Figure 4-2 as foows: Figure 4-2 Iustration of the thickness (T) of the honeycomb structure T x x f sin 2t f 1 tb [4-1] 2 where x is the ayer of fabrics invoved in the honeycomb structure, and it is an even integer and x >2. To achieve a given ce opening ange, the distance between the two sets of the meta wires, d w, shoud be d x xf sin 2t f 1 tb d [4-2] 2 w 2 97

98 where, d is the diameter of the meta wires and x is an even integer with x >2. In the above equations, t f is estimated to be the sum of the warp and weft yarn diameters, and t b is equa to 1.5 of t f. Figure 4-2 iustrates a honeycomb composite that is made from 4 ayers of fabrics. The distance between the top surface of the up stee wire and the bottom surface of the ower stee wires (indicated as d w in Figure 4-1) is the critica parameter that determines the opening ange. Since the wire diameter is known and the desired height of the honeycomb structure is aso decided, then this distance can be cacuated by subtracting the thicknesses of the top and bottom ayers of the honeycomb structure. The thickness of the top ayer (t f ) and the bottom ayer (t b ) of the honeycomb composite were measured by ruer and the cacuated heights of the various textie honeycomb composites and the distance between the up and ower wires were cacuated according to Equation [4-1] and [4-2] and isted in Tabe 4-1. The cacuation procedures are outined as foows, taking 4L6P with opening ange θ=60 as exampe (Figure 4-3). Figure 4-3 Iustration of a four-ayer honeycomb composite The ength of the free was ( f ) and the bonded was ( b ) of a ce can be cacuated from the weft density and pick numbers in the was. For 4L6P, the weft density is 7.87 picks/cm, and the free and bonded was both contain 6 picks in them. Therefore, the engths of the free wa ( f ) and bonded was ( b ) are both 6(picks) 7.87(picks/cm) =7.62mm. The number of fabric 98

99 ayers (x) of 4L6P is 4. The measured thickness of the free wa (t f ) is 0.78mm and that of the bonded wa (t f ) is 1.09mm. Accordingy, the height T of the 4L6P (opening ange θ=60 ) composite, according to Equation [4-1], is x 4 T= 4 f Sin θ +2 t f + 1 tb = Sin = mm. 2 2 The distance between the wires, as a resut of subtracting d ayer from T is mm. Based on the designed weft density of each of the fabric ayer (7.87picks/cm), a the reevant structura parameters are summarised in Tabe 4-1, taking d=3mm (where, d is the diameter of the meta wires). Tabe 4-1 Cacuated sampe heights and distance between wire and other design parameter θ b +2 f Sampe ( ) (mm) t f t b (mm) (mm) (mm) (mm) T d wire 4L6P L4P L3P L4P L5P L6P L6P L6P L6P L6P L(3+6)P L(4+6)P L(4+3)P L(6+3)P

100 Where in Tabe 4-1, θ the is the opening ange of the ce; b and f are the engths of the bonded and free was; t b and t f are the thickness of the bonded and free was; T is the specimen height; and d wire is the wire distance Fabric impregnation The soution for fabric consoidation was made as a mixture of resin and hardener. The criteria for seecting the resin and hardener are determined by the requirement that the soution shoud penetrate and wet a the ayers of the fabric in order to form a continuous rigidified composite structure, aso that the composite shoud cure within a reasonabe period of time, for exampe, within 24 hours. Hence, the viscosity of the soution is quite important in order for it to adhere to the specific fabric materias uniformy. It is worth noting that the strength of the resin and hardener is aso a factor in infuencing the capabiity in energy absorption of the textie honeycomb composite, a performance that is important to seek in anaysis (Miravete, 1999; Wu, 2003). Foowing the comparison of the three major resin systems by Wu (2003), the seection of the resin and hardener is as foows: Resin: LY5152 Epoxy pheno novoak resin (60-70%) Botanediodigycidy (34-42%) Hardener: HY5052 2,2-dimethy 1-4,4 methyenebis (cycohexyamine) (50-60%) Isophorone diamine (35-45%) 2,4,6-tris (dimethy-aminomethy) pheno (1-5%) The mixing ratio of the resin and the hardener is LY5152:HY5052=100:38. There are many methods to impregnate the reinforcing fabrics, such as the vacuum process, spraying, roing and brushing (Wu, 2003). Due to the compexity of the reinforcing fabrics in this work, the brushing method was chosen in the current study to convert the soft fabric into a honeycomb composite. 100

101 The detaied procedure is described as foows: the fabric was paced faty when the resin/hardener mixed soution was brushed onto both sides of the fabric. After a sections of the fabric had been wet through, the impregnated fabric was opened using the fabric opening device described in Figure 4-1. The impregnated fabric was eft for 30 minutes in the ambient atmosphere and after the 30 minutes interva, the impregnated fabric was turned over to try to achieve a uniform distribution of the resin/hardner soution inside the ces of the composites. During the curing procedure, the specimen was paced in the fume cupboard for quicker hardening. It took around 24 hours for the composite to be cured. Finay, reducing agents were used to reease the wires from sticking to the fabric during the curing process. Tapes of poytetrafuoroethene (PTFE), which has high resistance to adhesion, were aso wrapped onto the wires before being used to open the fabric to avoid the adhesion of the wires to the fabric in the consoidation process. However, it has to be noted that because the resin was hand painted onto the honeycomb fabric, and there are no faciities to stricty contro the resin being eveny distributed inside every ce of the composites, therefore, it caused the thickness of the ce was various to each other among a the honeycomb composites and the detaied thickness of ce free wa (t f ) and bonded wa (t b ) are isted in Tabe 4-1. Another index to show how many percentage of fabric and resin are contained inside the honeycomb composites is defined as foowing: M fabric R 100 (%) [4-3] M composite where the R means the fabric/resin ratio; M fabric is the weight of the fabric and M composite is the weight of the honeycomb composite. The cacuated fabric/resin ratios (R) are isted in Tabe 4-2, and it seems that the specimen of 4L6P60 and 8L3P60 contains more resin than the rest of the sampes. This can affect the performances of the resuting composites, and for exampe, if the sampe coated with a thicker resin provides a better force protection and it is hard to expain whether this is caused by the 101

102 composite s structure optimization or by its heavy coating. The issue shoud be considered when conducting the data anaysis in the ater sections. Tabe 4-2. Fabric/resin ratio for the honeycomb composites Sampe Weight of Honeycomb Weight of Honeyocmb Fabric/Resin Fabric (M fabric )(g) Composite (M composite )(g) Ratio (R) 4L6P % 6L4P % 8L3P % 8L4P % 8L5P % 8L6P % 8L6P % 8L6P % 8L6P % 8L6P % 8L(3+6)P % 8L(4+6)P % 8L(4+3)P % 8L(6+3)P % Textie honeycomb composite After the resin/hardener soution had cured thoroughy, the manufactured textie honeycomb composites were cut into sma specimens with the dimensions of 60mm 120mm for the future impact testing. For iustration purposes, textie honeycomb composites with different ce sizes (8L3P, 8L4P, 8L5P) are shown in Figure

103 8L3P 8L4P 8L5P Figure 4-4 Photos of textie honeycomb composite with different ce size 4.2 Fabrication of Woven Honeycomb Composite Fourteen groups of textie honeycomb composites with different parameters were manufactured from 10 types of honeycomb fabrics. Each group of composites has nine sampes and they were cut into size of 60mm 120mm. The ce structure geometric parameters for these testing specimens are isted in Tabe 4-2. However, the specimen dimensions are according to the rea sampes been made and some modifications were appied which restricted by the engineering equipment provided. It is noted that the rea composite height (T) in Tabe 4-3 is different from the cacuated vaues in Tabe 4-1. In opening the honeycomb structure, two meta wires of diameter of 3mm were used for each ce. The meta wires used to ift the top and bottom ces of the woven honeycomb fabric provide a width of 6mm. Therefore, for the top and bottom ayers of the textie honeycomb composite, the bonded wa ( b ) is 6mm in ength which causes the ength of free was ( f ) to change from the cacuated vaue. Take sampe 4L6P for an exampe. The ength of b +2 f is 22.86mm according to Tabe 4-1 and therefore, the ength of f shoud be 22.86mm 3=7.62mm. However, in the rea case, the f is ( ) 2=8.43mm. This is one reason that has caused the inaccuracy in the height (T) of the composites. Another reason contributing to this probem is that when the meta wires were handed to separate the surfaces of the woven honeycomb fabric, they may not be exacty centray aocated in the ce. Thus, a resut of f coud be even different from above cacuated 8.43mm. The third reason to cause this 103

104 probem is that manuay to measure the size of the composites by ruer coud bring the inaccuracy in the fina data acquired. Tabe 4-3. Honeycomb geometric parameters for testing specimens (by rea measurement) θ Sampe T (mm) ( ) d wire (mm) 4L6P L4P L3P L4P L5P L6P L6P L6P L6P L6P L(3+6)P L(4+6)P L(4+3)P L(6+3)P The Sampe Groups The above fourteen textie honeycomb composite with different parameters were categorized into four groups for the future anaysis. These groups and their features are described as foowing: 104

105 Group 1: Composites with different opening anges A the composites in this group were made from 8L6P honeycomb fabric, which are composed of 8 fabric ayers and 6 picks in each ce wa with different ce opening anges of 30, 45, 60, 75 and 90 respectivey. Consequenty the thickness of the sampe increases as the opening ange gets arger. 8L6P30 8L6P45 8L6P60 8L6P75 8L6P90 Figure 4-5 Specimens with different opening ange Group 2: Composites with different ce sizes 8L3P60 8L4P60 8L5P60 8L6P60 Figure 4-6 Specimens with different ce sizes A the composites in this group are based on eight-ayer structures but the engths of the hexagona ce was are changing from 3, 4, 5, to 6 picks which resuts in a change in ce size. The opening ange for this group of composites is 60 for a the sampes. When the ength of 105

106 the wa gets onger, the specimen gets thicker. Figure 4-6 ceary iustrates the change of the ce size. Group 3: Composites with different ength ratios of ce wa The third group of composites was made from an eight-ayer fabric, but with different ength ratios of free wa to bonded wa. The ce opening ange is 60 for a the sampes. In this b group, the composites are divided into two subgroups with 1 f b and 1 f respectivey. The first subgroup incudes three sampes: 8L(3+6)P60, 8L(4+6)P60, and 8L6P60. They share the same free wa ength of 6 picks, whereas the bounded wa engths change from 3 picks to 4 picks to 6 picks. These composites are shown in Figure 4-7. They are made to have the same thickness since the thickness, which is determined by the free wa ength and opening ange, as they are a made from 8 ayers of fabrics. It needs to mention that due to an inaccurate operation in using the fabric opening device, the thickness of 8L(3+6)P60 composite is sighty thinner than the other two in the group. This is evident in Figure 4-6 and may affect the test resut. 8L(3+6)P60 8L(4+6)P60 8L6P60 b Figure 4-7 Specimens with different ength ratios ( 1) f 106

107 The second subgroup incudes composites 8L3P60, 8L(4+3)P60 and 8L(6+3)P60, where the free wa ength in a is 3 picks and the bonded wa ength is 3, 4, and 6 picks respectivey. Figure 4-8 shows the photo of these three composites. 8L3P60 8L(4+3)P60 8L(6+3)P60 b Figure 4-8 Specimens with different ength ratio of ce was ( 1) f Group 4: Composites with the simiar thickness The ast group of composites was made from fabrics with different numbers of ayers and different ce wa engths, but they were opened to very simiar composite thickness, with the opening ange being 60 in a cases. They incude composites 4L6P60, 6L4P60, and 8L3P60, which are shown in Figure 4-9 with significant ce size variation. 4L6P60 6L4P60 8L3P60 Figure 4-9 Specimens with same thickness 107

108 4.4 Summaries In this chapter, a device which has been used to opening the woven honeycomb fabric has been designed and introduced at the University of Manchester for this research purpose. The resin was impregnation onto the fabric to form textie honeycomb composites. However, by hand painting the resin, it is not eveny distributed a over the fabric and this caused variation in the measured vaue of composite s height (T sampe ) and wire distance (d wire ). Therefore, more advanced technoogy such as vaccum-assistant-resin transfer-mouding can be used to consoidate the textie honeycomb composite in the future. Fourteen textie honeycomb composites with different geometric parameter are made from ten types of woven honeycomb fabric and they are divided into four groups with different ce size, opening ange, ength ratio of ce was and simiar thickness but different ce size. These composites wi be used in the next experiment stage to investigate their mechanica performance against ow veocity impact test. 108

109 CHAPTER 5 EXPERIMENTAL DATA ANALYSIS ON TEXTILE HONEYCOMB COMPOSITES As mentioned in the previous chapters, the three-dimensiona (3D) textie fabrics were consoidated into textie honeycomb composites with different geometric parameters and were designed in four groups, i.e. the ce size group, the opening ange group, the ength ratio group, and the simiar thickness group. It is of academic interest to investigate the infuence of these parameters on the impact performances on these honeycomb composites. This chapter aims to anayse and compare the impact performances between the groups of the textie honeycomb composites. This chapter wi start by describing the ow veocity impact instrument, aso known as the dropping hammer system, and foowed by the presentation and anaysis of the experimenta resuts from testing the textie honeycomb composites. A discussion is aso carried out to evauate the effect of the geometric parameters on the performance of the composites. 5.1 Low Veocity Drop Weight Impact Tests Basic principe of ow veocity drop weight impact For ow veocity drop weight impact, the assumption is that the friction between the impactor assemby and the rais it drops aong can be negected and there is no energy oss whie the potentia energy is converted to kinetic energy. According to the energy 1 2 conservation aw, the kinetic energy ( mv ) carried by the impactor assemby (known 2 as the externa energy) at the start of the impact shoud be the same as the sum of energy absorbed by honeycomb composites through deformation (known as interna energy), energy transmitted through the honeycomb composite, and other forms of energy during 109

110 the impact process. In an idea situation where there is no fracture, this can be mathematicay described as foows: s E F 0 oad ds [5-1] where, E is the change in the kinetic energy carried by the impactor assemby, F oad is the oading force or contact force appied to the composite at a given time during the impact, and s is the deformation depth. If the composites are fractured, the energy taken to fracture the composite ce was must aso be counted in this equation. In the present study, the ow veocity impact test was conducted aong the in-pane direction of the ces in the honeycomb composites. As indicated in Figure 5-1, x 1 is the warp direction, x 3 the weft direction, and x 2 the thickness direction. Athough it is possibe to impact the composite aong any of the principe axes to evauate the mechanica behaviour (Wierzbicki, 1983; Zhang and Ashby, 1992), in the current study, the impact comes in x 2 direction which represents an in-pane impact in reation to the honeycomb ces for an intended appication. (a) An overa view (b) Loading on a ce Figure 5-1 Schematic diagram for in-pane ow veocity impact test 110

111 5.1.2 The set-up of the ow veocity impact instrument A photograph of the drop weight impact instrument (the dropping hammer system) is shown in Figure 5-2 with the major components indicated. The reationship between the impactor and the anvi is shown in Figure 5-3. Figure 5-2 Dropping hammer system for impact test of specimens The major parts of this experimenta set-up pictured in Figure 5-2 incude: 111

112 a. Impactor and acceerometer: a hardwood tup (a bunt wood cyinder, 30mm diameter), a stee hoder which hods the hardwood tup and an acceerometer embedded in the impactor, which inks to the ampifier via a wire. The mass of the impactor is 0.55kg. b. Stee tube: to provide a guidance track for the impactor siding inside smoothy and ensure that the impactor strikes in the upright position.. c. Force transducer: to detect and measure the transmitted force during the experiment. It is embedded in the anvi under the specimen. The coected signa wi be send to the charge ampifier and then to the data recorder via another data wire. Figure 5-3 The impactor and the anvi d. Charge ampifier: two identica charge ampifiers were used to detect and ampify the experimenta signas in vots and within a proper range (Instrument mode: KISTLER, Type 5009). Signas from the charge ampifier were recorded and a reative interva of time (5μs) was given by the high speed data recorder (Nicoet 500) and a computer with the appropriate software (Nicoet Window) was set up to record and dispay the data (Figure 5-4). Channe 1 was used to acquire data for the transmitted force, and channe 2 was set up to coect the acceeration data. 112

113 Figure 5-4 Charge ampifier used in the tests e. High-speed data recorder (Nicoet 500) and computer: used together with software (Nicoet Windows) for data recording and processing, as we as dispaying. Figure 5-5 Snapshot of the resutant curves for force and acceeration dispayed in Nicoet Windows 113

114 5.1.3 Test procedure A kinds of specimens are trimmed into the size of approximate 60mm 120mm. This is determined by the dimension of the anvi and the diameter of the impactor, which is 30mm. The test procedures are summarised as foows: Pre-test and make sure that everything is ready: this incudes the settings of the charge ampifiers and Nicoet Windows; Pace the specimen correcty: the specimen shoud be kept tight on the anvi to keep the experimenta conditions of each specimen identica and comparabe with each other; Measure and mark the distance of tub faing height: the height of the tub is determined according to the desired veocity which is approximate 5.5m/s when the impactor hits the specimen surface. It is 1.62m from the reeasing position to the anvi surface during the experiment. Therefore, the faing height which represents the distance from reeasing position to specimen surface wi be around 1.54m subtracting the height of the specimen. Reease the thread: The raising/reeasing thread was bunded together with the signa wire for the acceeration. When reeasing the thread, specia care needs to be taken for the wire to avoid potentia damage, which transfers the detected signa of acceeration from the acceerometer to the charge ampifier. The wire traves at the same speed as the impactor, and its catching with the edge of the stee tube may cause damage to the wire. Check and save the recorded data: in two separate channes, the votage signas from two charge ampifiers are converted and dispayed on the computer screen using Nicoet Windows. Channe 1 was set for transmitted force and channe 2 was set for impactor acceeration. The votage vaues can be saved to computer separatey. The rea force and acceeration magnitudes shoud then be cacuated from these votage vaues, mutipying by the scae factors of the charge ampifiers previousy described. 114

115 5.2 Preparation for Test Specimens of textie honeycomb composites The production of textie honeycomb composites and their group division has been expained in the Chapter 4. Fourteen different textie honeycomb composites with various geometric parameters have been created in the present study. The composites are with the foowing variations: Different opening ange (8L6P30, 8L6P45, 8L6P60, 8L6P75, 8L6P90) Different ce size (8L3P, 8L4P, 8L5P, 8L6P) Different free wa to bonded wa ratio: b f 1: (8L3P, 8L(4+3)P, 8L(6+3)P) b f 1: (8L6P, 8L(4+6)P, 8L(3+6)P) Different ce size with same thickness in tota (4L6P, 6L4P, 8L3P) Impact setting for the dropping hammer system (v 0 =5.5m/s) The current research is deveoped from a previous project (Yu and Chen, 2006) on textie honeycomb composite materias for the riot poice as imb protectors. In this research, Yu and Chen specified that the projectie impact veocity is 5.5m/s and the projectie mass is 0.55kg, resuting in the impact energy of 8.3J. To match their work, in the present investigation, the impactor weighing 0.55kg was positioned on a rai at a height of 1.54m above the top surface of the specimens. The impact veocity of the impactor is therefore 5.5m/s, thus the impact energy is 8.3J. At the same time, the signas were set to be recorded in every 5μs interva. 115

116 5.3 Impact Test Resuts Data processing The data processing method was introduced in this section, taking 8L6P60 for exampe. To hep with a cear iustration, a data processing fow chart was drawn and shown in Figure 5-6. Raw Data incuding Transmitted Force Time Diagram and Acceeration Time Diagram: a F t t Zero Resetting: (Peak transmitted force and its arriva time can be retrieved from Transmitted Force Time diagram at this stage. The peak transmitted force attenuation factor can be cacuated.) F a : t t 116

117 Veocity Cacuation Based on Acceeration Integration: a v t Contact Force equas Acceeration mutipied by Mass: a Fc t t t Dispacement Cacuation Based on Veocity Integration: v s t t Cacuation of Energy Absorption in Vertica Deformation from Integration of Contact Force Dispacement Diagram: Fc E s t Figure 5-6. Data processing fow chart for experimenta data anaysis procedures 117

118 During the impact tests, there are a few experimenta factors which infuence the test accuracy such as the irreguar shape of honeycomb structure of some specimens, uneven resin coating on different sides of the specimens, the disturbance during the impactor reeasing and the friction of the tube track to the impactor. Such isted factors may resut in differences of initia impact veocity when the impactor hits the specimen top surface in magnitude and direction. Therefore, athough about 10 specimens were tested for each type of composite, to improve the data accuracy, ony resuts from three tests with the most repeatabiity were seected for further data processing. In the foowing detaied data processing procedures, introductions wi be put forwarded on basic principes for ow-veocity impact test, force attenuation, acceeration of the impactor, characteristics of the transmitted force and energy absorption performance of the textie honeycomb composites Basics for ow-veocity impact test For impact tests based on the drop-weight principe, the impact energy depends on the mass of the impactor assemby m and the height of the impactor assemby h over the specimen. The impact energy K is express as: K=mgh [5-2] where, g is the gravitationa acceeration of 9.8 m/s 2. The veocity of impact when impactor head first touches the specimen may be expressed as: v 0 = 2 gh [5-3] where, h is the height from which the impactor starts dropping onto the top surface of the specimen, v 0 is the initia impact veocity when the impactor first hits the top surface of the specimen, and g is the gravity acceeration of 9.8 m/s

119 It is assumed that there is no energy oss during the fa of the impactor assemby. The acceerometer detects the changes in acceeration during the impact process, which refects the changes in the norma contact force between the impactor and the specimen. It aso describes the movement of the impactor too. Energy absorption by the specimen can be obtained from the specimen deformation. The ratio of energy absorbed by the specimen to the impact energy can be used as a measure of the specimen s capabiity for energy absorption Force attenuation In a typica impact on a honeycomb composite, the impact energy is absorbed by the eastic and pastic deformation as we as the coapse of the ces (Gibson and Ashby, 1997). The impact energy that is not absorbed by the honeycomb composite may cause damage to materias and structures beneath the composite by exerting a transmitted force downwards. The transmitted force can be detected by a oad ce embedded in the anvi. Compare the transmitted force with the norma impact force acting on the anvi directy, without the invovement of the specimen, eads to the definition of impact force attenuation. The attenuation factor (f att ) is used to demonstrate the force-bocking effectiveness of the specimen, and it is defined as (Dionne et a., 2003): F 1 trans f att 100 (%) [5-4] F where, F trans is the transmitted force through the specimen and F is the impact force acting directy on the anvi. A vaue of 100% for the attenuation factor corresponds to no force being transmitted underneath and a vaue of 0% indicates that a the force has been transmitted. 119

120 Therefore according to the maximum transmitted force obtained by the impact testing with and without specimen, the force attenuation can be cacuated by using the maximum transmitted force as the F trans (with sampe) and F (without sampe). Research report from Med-Eng System Inc.(2001) shows that f att vaue of many commony used materia for bunt impactor are within 20%-30%, however, the vaue for a textie based honeycomb composites are above 90%. In the current dropping hammer system, the mechanica test without specimen with the same experimenta setup had obtained 17.5KN as F (without sampe) and it wi be used to cacuate f att for a textie honeycomb composites. Take 8L3P60 for exampe as isted in Tabe 5-3, f att is cacuated as: 0.95 f att (%) 17.5 This indicates that during the ow veocity impact in the current experiment, about 94.2% impact force was attenuated by the 8L3P60 whie ony 5.8% impact force has been transmitted to the anvi. This effectiveness shows the potentia of such textie honeycomb composite in PPE industries Acceeration of the impactor The impactor experiences a deceeration when it strikes the specimen; the deformation and coapse of the honeycomb ces contribute to the overa deformation of the honeycomb composites as a whoe. The deceeration curves from three impact tests on the 8L6P60 together with the averaged curve are shown in Figure

121 Figure 5-7 Measured acceeration curves for 8L6P60 It can be seen that the deceeration reached its peak about 7ms during the impact and the impact process competed within about 20ms. The fuctuation in the curves is beieved to have come from the deformation and coapse of the honeycomb ces Characteristics of the transmitted force The oad ce embedded in the anvi detects and picks up the force signa transmitted through the specimen. The magnitude of the transmitted force perceived from beneath the materia, in this case the honeycomb composites, is an important indicator for protective capabiity of the honeycomb composite. Figure 5-8 takes 8L6P60 as an exampe and retrieved the transmitted force curve from three impact tests. The transmitted force was measured at an interva of 5μs. 121

122 Figure 5-8 Measured transmitted force curves for 8L6P60 The measured transmitted force curve is important as it shows the peak transmitted force and the peak transmitted force strike time. To be considered as materia in PPE design, the peak transmitted force shoud be designed ower than the threshod force of chin and knees to avoid damage to wearer s bones. The peak transmitted force arriva time is another important factor as the ater the peak transmitted force strikes, the more is the reaction time for the wearer to escape from the attack Energy absorption performance Veocity v and the dispacement y for the impactor can be derived by integrating the acceeration curve once and twice, respectivey, eading to v 2 gh adt [5-6] and, y 2gh T0 adt) dt [5-7] ( where T 0 is the duration of the impact. 122

123 Contact Force(KN) The impact force, aso referred to as the contact force, can be cacuated using Newton s second aw of motion and it is expressed as: F contact M a [5-8] where M is the mass of the impactor (0.55kg in current study) and a is the measured deceeration. 0.4 C ontact Force Dispacement(mm) Figure 5-9 The response of contact force against dispacement Figure 5-9 shows the contact force-dispacement curve for a typica samped composite, where the area under the curve gives the energy absorption (E): E Fcontact dy [5-9] Energy absorption can be cacuated by integrating the cosing area of contact forcedispacement curve in Figure 5-9. In detais, the trapezoida method was used for the 123

124 numerica integration in cacuating the energy absorption and it can be schematicay interpreted as Figure 5-10: Figure 5-10 Trapezoida method to cacuate the energy absorption By dividing the cosed area between contact force - dispacement curve and the x-axis (Figure 5-10) to infinite sma trapezoida, every singe interva area (equas to every interva energy absorption) can be cacuated and the sum of a wi give the area of the cosed curve (that is aso the tota energy absorption deformed by the specimen). n 1 E = S i = ( Fi Fi 1)( yi 1 yi ) [5-10] 2 i 0 where E is the absorbed strain energy, S i is the trapezoida area, F i is the contact force appied on the specimen which can be cacuated from Equation 5-10 and y i is the dispacement increment at each time interva of 5μs caused by the impact force. The absorbed strain energy is cacuated as the integration of contact force mutipes dispacement increment at each time interva. 124

125 Veocity (m/s), Dispacement (cm), Energy (J) Adjusting the units appropriatey, the tempora evauation curves for veocity, dispacement and the energy absorption in vertica deformation for each composite type can be potted together as that for 8L6P60 in Figure As a genera feature description, athough the veocity of the impactor is reduced due to the resistance from the specimen, the impactor keeps going deeper into the specimen and the energy is therefore absorbed by the specimen deformed verticay Veocity Dispacement Energy Time (s) Figure 5-11 Evauation curves of veocity, dispacement and energy absorption for 8L6P60 125

126 5.4 Experiment Resuts Various experiment resuts during impact procedure According to the trave height of the specimen (h), the initia veocity (v o ) equas to 2 gh and the resuts are shown in the Tabe 5-1. By comparing the absorbed strain energy with the kinetic energy, the energy absorption ratio can aso be easiy cacuated. Tabe 5-1 Experiment resuts from impact test Sampe Density Thickness h (g/cm 3 ) (mm) (mm) v o K E E/K (m/s) (J)) (J) (%) 4L6P L4P L3P L4P L5P L6P L6P L6P L6P L6P L(4+3)P L(6+3)P L(3+6)P L(4+6)P In this tabe, E is the strain energy being absorbed because of the structure deformation, and E/K is the percentage of absorbed energy divided by kinetic energy, h is the trave height of the impactor and v o is the initia veocity when the impactor hits the sampe, K 1 2 means the kinetic energy of the impactor which can be cacuated by mv

127 5.4.2 Experiment resuts for energy absorption The energy absorption resuts compared to the potentia gravity energy which can be cacuated by using mgh (where g: gravity acceeration) without specimen are shown in Tabe 5-2. From the resuts, it can be seen that 8L6P30 (E/K 1 =96.89%), 8L6P45 (E/K 1 =95.65%), 8L(3+6)P60 (E/K 1 =94.58%) shows a good performance of energy absorption. Tabe 5-2 Experiment resuts for the energy absorption Sampe h 1 (mm) K 1 E E/K 1 (J) (J) (%) 4L6P L4P L3P L4P L5P L6P L6P L6P L6P L6P L(4+3)P L(6+3)P L(3+6)P L(4+6)P Where in the tabe, h 1 is the trave height of the impactor without sampe, K 1 is the kinetic energy when the impactor hit the anvi without sampe, E is the absorbed energy by the structure deformation and E/K 1 is the percentage of the absorbed energy divided by kinetic energy (without sampe). 127

128 5.4.3 Experiment resuts for force attenuation factor (f att ) From the force attenuation point of view in Tabe 5-3, 8L6P60 (f att =97.9%), 8L6P75 (f att =98.3%), 8L6P90 (f att =98.4%), 8L(4+6)P60 (f att = 97.6%) iustrates a very good force protection. A possibe reason for causing this phenomenon is that the thickness of above sampe being mentioned are reativey higher than the rest of the sampes (See Tabe 5-1), which decrease the transmitted force attenuate underneath the specimen and provide it a onger peak arriva time. Tabe 5-3 Experiment resuts for force attenuation Sampe t (ms) F trans (KN) (KN) (%) 4L6P L4P L3P L4P L5P L6P L6P L6P L6P L6P L(4+3)P L(6+3)P L(3+6)P L(4+6)P F f att Where in this tabe, F trans is the peak transmitted force which has been detected by the force transducer in the dropping hammer system, t is the peak transmitted force arriva 128

129 time, F is the peak transmitted force being captured during the impact without specimen, and f att is the attenuation factor cacuated by Equation Structure and Properties of Textie Honeycomb Composites Structure parameters and performance indices As iustrated in the four comparabe groups, the structura parameters investigated incude the ce size at the same number of ayers, the opening ange of ce, the ratio of was engths ( b f ) and the specimen with same thickness but different voume density. The parameters are used to categorize the composite types into four groups so that each group has ony one changing parameter. Two most important performance indices were seected to describe the effectiveness of the textie honeycomb composites in the foowing data anaysis and they are the peak transmitted force and the energy absorption performance. The peak transmitted force performance is defined as the maximum vaue of the transmitted force detected by the force transducer. The energy absorption performance is defined as the energy absorbed by honeycomb structure deformation. Due to the existence of the fuctuations in measured data profies, the two seected indices are with better reiabiity and more direct physica meaning, to describe the effectiveness of textie honeycomb composites Grouped sampe experimenta performance Ce size and its experimenta performance (8L3P60, 8L4P60, 8L5P60, 8L6P60) A honeycomb composites in this group are made from eight ayers of fabrics invoving four reguar hexagona ces, where the six was of each ce have the same ength. The opening ange of the ces in these composites is 60. With the same weft density for a reinforcing fabric sections, the ce wa ength changes from three, four, five, to six picks, resuting in ces with increasing sizes. These composites are 8L3P60, 8L4P60, 8L5P60, and 8L6P60 among which 8L3P60 is the thinnest and 8L6P60 is the thickest. 129

130 Peak Transmitted Force (KN) Transmitted Force (kn) The transmitted force performance and the energy absorption performance are iustrated in Figure 5-12 and Figure The peak transmitted force vaue and the energy absorption vaue have been demonstrated in Figure 5-13 and Figure 5-15 respectivey L3P60 8L4P60 8L5P60 8L6P Time (s) Figure 5-12 Comparison of transmitted force time diagram (sampes with different ce size) L3P60 8L4P60 8L5P60 8L6P60 Figure 5-13 Comparison of peak transmitted force vaue (sampes with different ce size) 130

131 Energy Absorption(J) Structure Dispacement(%) Contact Force (kn) L3P60 8L4P60 8L5P60 8L6P Dispacement (cm) Figure 5-14 Comparison of contact force dispacement diagram (sampes with different ce size) Energy Absorption Dispacement(%) L3P60 8L4P60 8L5P60 8L6P Figure 5-15 Comparison of energy absorption and structure dispacement diagram (sampes with different ce size) 131

132 Regarding the transmitted force-time curve of 8L3P60 and 8L6P60 in Figure 5-12 and Figure 5-13, it is noticed that 8L3P60 has a higher transmitted force that reaches 0.95KN, however, the whoe deformation time is quite short, ony about 8ms. It shows a high peak transmitted force instanty at 3.5ms. In contrast, 8L6P60 has a onger deformation time (21ms) but the peak transmitted force is quite ower about 0.35KN. Thus it shows much ower peak transmitted force and ce coapsed with a onger stroke. Impact force attenuation is dependent on the ce size of honeycomb composites as demonstrated in Figure It is cear that honeycomb composites with arger ces perform better; the atitude of the peak transmitted force is much reduced and the curve is smoother. It is aso evident that the maxima transmitted force occurs ater, in genera, as the ce size becomes arger. This is a favourabe property for materia intended for body and imb protection against trauma impact; composites with arger ces aow more time for the human body to react to impact, hence reducing the risk of more serious injuries. The resut in contact force-dispacement curve in Figure 5-14 iustrated that 8L3P60 experienced ess than haf coapses of the ce and a higher oading force comparing to sampes with big ce (8L5P and 8L6P). However, with the simiar initia impact veocity, 8L6P60 dispayed a ower oading force and stroke at a onger distance coapse. It can be expained that with the increase of the ce size, the maximum impact distance is increased too as the thickness of the specimen changed respectivey according to the ce size. Therefore, the arger the singe ce, the deeper the specimen can be impacted with. However, resuts from Figure 5-14 and Figure 5-15 shows the energy absorption among 8L3P60(E=7.46J), 8L4P60(E=7.78J),, 8L5P60(E=7.83J), 8L6P60(E=7.54J) are quite simiar. Hence, changes of the ce size are not a major factor that wi affect the energy absorption performance of the specimen. It is seen from the contact force-dispacement curves in Figure 5-14 that composites with smaer ce sizes have high impact moduus and conversey those with arger ce sizes have ow impact moduus thus it eads to harder and softer composite materias property. This information suggests that for honeycomb composites with the same 132

133 number of ces in a coumn, ce sizes can be used as the key parameter for atering the softness of the composite materia. Therefore, with the consideration of the materia property of 8L3P60 and 8L6P60, 8L3P60 has smaer ces that are very rigid on handing whereas 8L6P60 is much softer on handing but much bukier in voume. As the energy absorption among 8L3P60, 8L4P60, 8L5P60, 8L6P60 are simiar under the impact, the physica performance of transmitted force and deformation time became the major factor to judge the protection abiity of the specimen. It seems the softer the textie honeycomb sampe, the ess transmitted force wi be encountered (as 8L6P60 is very soft in handing), and it takes onger deformation time for the whoe process. Respectivey, rigid sampe (8L3P60) wi have a higher transmitted force and shorter deformation time, but it is ess buky. Figure 5-15 aso shows the atitude of energy absorption of sampes with different ce size and their structure dispacement. It seems that the energy absorption is simiar for 8L3P60, 8L4P60, 8L5P60 and 8L6P60. However, the structure dispacement for 8L6P60 is much deeper than the rest of the sampes. This indicates that there is a arger structure deformation for 8L6P60 whie the strain energy absorption has not been significanty increased. The reason to expain it might be that the thickness of 8L6P60 is higher than the rest of sampes, therefore, it provide more spaces for the impactor to strike through which eads to a deeper structure dispacement verticay. With reference to the transmitted force and energy absorption, composites 8L3P60 and 8L6P60 absorb simiar amount of energy, but their peak transmitted force and striking time are significanty different. 8L3P60 is associated with the higher transmitted forces and it has the smaer thickness and is more rigid. By contrast, 8L6P60 is much bukier and softer as a materia. Ideay, the honeycomb textie composites with softer handing and ess buk are sought for improving the protection abiity of the impact force. Structure such as 8L3P60 who hods smaer ce size therefore its bukiness is idea in the PPE appication. Even better is that if there is a simiar composite whose handing property is ess rigidity than that 133

134 of 8L3P60 which wi be more suitabe to be used for protection purpose because the softer the composite materia, the more striking time it wi take for the structure to react against outside attack. And this wi give more timing for the human being to be prepared against impact from outside Opening ange and its experimenta performance (8L6P30, 8L6P45, 8L6P60, 8L6P75, 8L6P90) 8L6P30, 8L6P45, 8L6P60, 8L6P75 and 8L6P90 are grouped to investigate the difference of opening ange which intend to affect the mechanica performance of the textie honeycomb composites. These composites are made from the same fabric as reinforcement but have different opening anges, as indicated by the ast two digits in the codes. Generay, the deformation timing of the specimens with different anges is quite simiar and this can be seen in the transmitted force-time curve in Figure However, the specimens with smaer opening anges (8L6P30, 8L6P45) own a higher transmitted force at 0.49KN (8L6P30) and 0.42KN (8L6P45) comparing to the rest of the specimens. It aso demonstrates that the trend of transmitted force-time curve for 8L6P60 and 8L6P75 are very simiar and the peak transmitted force vaue is cosey matched too. However, when the opening ange is more than 75º, the peak transmitted force is decreasing to 0.29KN (8L6P45) which has been shown in Figure Figure 5-16 aso show that whist the peak transmitted force decreases as the composite opening ange increases, it is cear that the peak forces tend to be smoothed when the composites get thicker, due to the enargement of opening ange. Referring to the maxima transmitted force in Figure 5-17 for the five honeycomb composites in this group, it demonstrates a cear correation between increasing opening ange and the decreasing maxima transmitted force. The thickness of the composites is beieved to have an important roe to pay in this together with the ce geometry in the crosssection of the composites. 134

135 From the data anaysis, it seems that if the specimen s opening ange is ess than 60, the ce wi have a much higher transmitted force eading to a poor protection performance. However, when the ce opening ange is designed between 60 and 75, there is a smaer transmitted force coming through and the height of the composite is medium heighted. Therefore, if using the honeycomb composites as PPE, ce opening ange between 60 and 75 wi be the first choice when good protection is required. According to the contact force-dispacement curve in Figure 5-18, it is iustrated that when the impactor reaches the maximum impact distance, 8L6P30 and 8L6P90 has a reduced coapse distance whie 8L6P45, 8L6P60 and 8L6P75 have a onger coapse distance. However, considering the thickness of specimen 8L6P30 (31.6mm) is ess than haf of that of 8L6P90 (66.2mm), the reative coapse distance for 8L6P90 is very sma. This is due to when ce-opening ange reaches 90, the ce free wa and bonded wa forms a shape of rectanguar, so that the ce wi be ess fexibe. Looking at the contact force-dispacement curve between a the specimen, 8L6P60 and 8L6P75 are much fatter and ower than rest of the sampes in the same group, which means opening ange between 60 and 75 causes a ower oading force during the impact testing. Generay, there is no obvious trend can be seen the contact force-time curves to indicate the opening ange as a major mechanism by which the impact energy was absorbed. Therefore, other structura parameters such as the thickness of the honeycomb composites must have payed important roes, too. 135

136 Peak Transmitted Force(KN) Transmitted Force (kn) L6P30 8L6P45 8L6P60 8L6P75 8L6P Time (s) Figure 5-16 Comparison of transmitted force time diagram (sampes with different opening ange) L6P30 8L6P45 8L6P60 8L6P75 8L6P90 Figure 5-17 Comparison of peak transmitted force vaue diagram (sampes with different opening ange) 136

137 Energy Absorption(J) Structure Dispacement(%) Contact Force (kn) L6P30 8L6P45 8L6P60 8L6P75 8L6P Dispacement (cm) Figure 5-18 Comparison of contact force-dispacement diagram (sampes with different opening ange) Energy Absorption Dispacement(%) L6P30 8L6P45 8L6P60 8L6P75 8L6P Figure 5-19 Comparison of energy absorption and structure dispacement diagram (sampes with different opening ange) 137

138 It is iustrated in Figure 5-19 that the amount of energy absorbed among specimens with different anges show the trend that with the increase of the ce opening ange, the energy dissipated inside the honeycomb structure start to reduce accordingy. This is beieved to be because the textie honeycomb composites with smaer opening anges present ower resistance to the bending of its anged ce was and therefore create arger deformation. In Figure 5-19, the structure dispacement ratio for the specimen with the smaest ange (8L6P30) is the argest, accordingy, the energy absorption for 8L6P30 is the highest too. Opening ange exceeding 45 can stops the impactor going further deeper with reasonabe a energy absorption behaviour. However, if the opening ange continues enarging and exceeding 75, the striking distance goes shaow, accordingy, the energy absorption reduces significanty. As mentioned in Section , the energy absorption was obtained by integrating the area underneath contact force-dispacement curve and this can be treated as the strain energy absorbed aong vertica direction. However, the tota kinetic energy shoud a dissipate during the impact process and therefore the rest of the energy was dissipated in other forms. Taking the vaue of energy dissipated in the vertica direction and in other forms in Tabe 5-4 and Figure 5-20 as reference, it is beieved that 8L6P30 can absorb the most energy in the vertica direction and ony 0.09J energy was dissipated in the other forms, whie 8L6P90 can ony absorb 7.05J energy in the direction of vertica and the rest of them (1.27J) were transmitted in the other forms. In another words, the format of the energy absorption changes when the ce opening ange changes and it brings the trend that more energy was dissipated in other forms rather than strain energy aong vertica direction when the ce opening ange is enarged. However, in term of using the honeycomb structured composites as PPE, it wi be better if more impact energy is absorbed as strain energy due to the deformation of the composite structure, 138

139 Absorbed Energy (J) therefore, if ess energy is absorption in other forms, it wi be better for the protection purpose. Tabe 5-4 Resuts of the energy dissipated aong vertica and in other forms Sampe K (J) E 1 E 2 (J) (J) 8L6P L6P L6P L6P L6P where, K is the impactor kinetic energy which is cacuated by mv 0, E1 is the strain 2 energy being absorbed aong the vertica direction and E 2 is the strain energy being absorbed in other forms. E 2 is cacuated using E 1 subtract by K. Energy Dissipation Format L6P30 8L6P45 8L6P60 8L6P75 8L6P90 Other forms of energy Vertica Figure 5-20 Energy dissipation direction diagram (sampes with different opening ange) 139

140 b Length ratio of ce was and its experiment performance ( 1: b 8L3P60,8L(4+3)P60, 8L(6+3)P60; 1: 8L(3+6) P60,8L(4+6)P60, 8L6P60) f f Two subgroups of textie honeycomb composites, based on the eight-ayer construction, have been created for the study of infuence of ce wa ength ratio on their impact performance. The first subgroup invoves 8L3P60, 8L(4+3)P60, and 8L(6+3)P60 where the free wa ength is kept at three picks and the bonded wa ength takes the vaues of three, four and six picks. In the second subgroup invoves 8L(3+6)P60, 8L(4+6)P60, and 8L6P60, the free wa ength is constant at six picks whie the ength of the bonded was changes from three, four, to six picks. The ce opening ange for a the composites is 60. The resuts from statistic data anaysis are summarized in Tabe 5-5. The change of the b b ce wa ength ratio between the case for 1 and 1 are potted according to their peak transmitted force in Figure 5-21 and energy absorption performance in Figure f f 140

141 Tabe 5-5 Resuts for sampes with different ce wa ength ratio Sampe θ b F trans f ( ) (KN) b f 1 f att E (%) (J) t (ms) 8L3P L(4+3)P 60 3: L(6+3)P b f 1 8L(3+6)P 60 1: L(4+6)P 60 2: L6P where, θ is the opening ange of the composite ce foowing that b f represent the ength ratio of bonded and free wa. F trans is the peak transmitted force accumuated during the dynamic impact and f att is the force attenuation factor which has been specified in Equation 5-4. E is the absorbed strain energy in vertica direction and t is the striking time when the peak transmitted force arrived. 141

142 Transmitted Force (kn) L(3+6)P60 8L(4+6)P60 8L6P60 8L3P60 8L(4+3)P60 8L(6+3)P Time (s) Figure 5-21 Comparison of transmitted force time diagram (sampes with different ength ratio of ce was) In Figure 5-21, considering the free wa ength ( f ) of the specimen, when it is constructed by 6 picks: 8L(3+6)P, 8L(4+6)P and 8L6P, the curve goes fat and ong which means the specimen has encountered a ow transmitted force and a ong stroke time, however, the difference of the peak transmitted force among these three sampes is not so significant. This kind of resuts reveaed that the free wa ength ( f ) has payed a significant roe in reducing transmitted force, and increasing the b f can aso eads to the reduction of transmitted force. Previous research work has expained this phenomenon with the reason that free was are mainy against the vertica oads on the top of the structure and it heps dissipating the force across the ce network more efficienty, thus much more deformation appears than that of the horizonta ce wa (Tan and Chen, 2005). 142

143 b However, in Figure 5-21 it is aso show that when 1, the specimen wi transfer a high transmitted force and a short stroke time with composite ce free wa ( f ) formed by 3 picks of 8L3P, 8L(4+3)P and 8L(6+3)P. It is aso demonstrated that more force has b been attenuated with a shorter bonded wa sampe when 1. It is indicated that under the circumstance of that bonded wa ( b ) onger than free wa ( f ), changes of the ength of b does infuence the mechanica performance of the textie honeycomb composite and to sum up that shorter bonded wa increases the transmitted force correspondingy. The reason to cause the variety of the transmitted force may be due to the bending behaviour of the bonded wa after being impacted aong in-pane direction. Practicay, this kind of phenomena coud be used when designing the textie honeycomb textie composite for the protection purpose that the onger the bonded wa (or the higher ratio of b f ) can generate ower transmitted force. f f Figure 5-22 presents the contact force-dispacement curves of the sampes under the impact veocity at about 5.5m/s. Figure 5-23 iustrates the comparison of the energy b absorption vaue and vertica dispacement ratio between subgroup 1 f and subgroup b f 1. The resuts from Figure 5-22 ceary show that the two subgroups perform very b differenty. When 1, the dispacement of the impactor goes shaow but the contact f b force runs very high. On the contrast that, when 1, the curves occupies more dispacement but the contact force is much ower. This ead to a concusion that the b b composite with 1 demonstrates a high impact moduus and composite with 1 f f f 143

144 b has a ductie behaviour. With the 1 f subgroup, it is cear that 8L3P whose ce wa ratio is 1:1 shows the highest impact moduus and 8L(6+3)P, with a ength ratio of 2:1, dispays the owest impact moduus in this subgroup. b For the subgroup with 1, a ductie behaviour is demonstrated for a three sampes f invoved, with the 8L6P60 being the most ductie. This coud ead to a good resistant to the impact oads as materias with a ductie performance normay exhibits a greater resistance to impact oads than do britte materia (James and Stephen, 2001). Figure 5-23 shows, however, that the tota amount of impact energy absorbed by the b b composites of 1is about the same whie for the composites with 1, there are f subte differences in their energy absorption. There is no significant reationship between energy absorption and their structure dispacement in each subgroup and the subte differences in the absorption of impact energy among the composites, especiay b in the 1 subgroup, require further investigation. f f The above resuts can be a good instruction for the design of PPE. Instead of changing the whoe ce size, by simpy modifying the ength ratio of ce bonded and free wa, it can avoid increasing the protector s weight significanty whie improve the mechanica performance of the honeycomb composites the same time. But it is beieved that there wi be a imitation of increasing this ratio ( b f ), and it means the designer cannot increase the ratio b f dramaticay to achieve higher energy absorption with the same weight. The current research work can ony prove that change of this ratio up to 2:1 is sti effective to improve the energy absorption capabiity of the textie honeycomb composite, however, to find out the critica ratio vaue higher than 2:1 with optima energy absorption requires more experimenta investigations. 144

145 Energy Absorption(J) Structure Dispacement(%) Contact Force (kn) L(3+6)P60 8L(4+6)P60 8L6P60 8L3P60 8L(4+3)P60 8L(6+3)P Dispacement (cm) Figure 5-22 Comparison of contact force-dispacement diagram (sampes with different ength ratio of ce was) 10 Energy Absorption Dispacement L3P60 8L(4+3)P60 8L(6+3)P60 8L(3+6)P60 8L(4+6)P60 8L6P60 0 Figure 5-23 Comparison of energy absorption diagram (sampes with different ength ratio of ce was) 145

146 Honeycomb composites with simiar thickness and their performance (4L6P60, 6L4P60 and 8L3P60) This group of 3D textie honeycomb composites incudes 4L6P60, 6L4P60 and 8L3P60, whose thickness is 29.2mm, 30.4mm and 31.6mm respectivey, and their thinness is very simiar in practica term. However, the voume density of the honeycomb composites are various as 4L6P<6L4P<8L3P, which are isted in Tabe 5-6. In this section, data anaysis wi be focus on these sampes who are simiar in thickness but different in their voume density. The resuts from experiments are summarized in Tabe 5-6. Tabe 5-6. Experiment resuts (sampes with simiar thickness) Sampe θ ( ) Density (g/cm 3 ) F trans f att E t (KN) (%) (J) (ms) 4L6P L4P L3P Where in this tabe, θ is the ce opening ange; F trans is the peak transmitted force and t is the peak transmitted force arriva time, f att is the attenuation factor and E is the absorbed energy. Figure 5-24 dispays the transmitted force against the impact time. It is shown that 4L6P60 was crushed by the impact eading to a arge transmitted force and the whoe sampe was totay crushed during the test visuay too. Composite 8L3P60 is associated to a higher transmitted force than 6L4P60 because the former has a higher impact moduus, which reates to the voume density of the composites. The behaviour of 4L6P60 is quite abnorma comparing to the rest of the sampes from their test resuts. 4L6P60 is very soft in handing and the whoe sampe is crushed after the impact, which aows the impactor to go through the sampe and touches the anvi 146

147 underneath during the impact, therefore, the transmitted force is reativey higher at 1.27KN and from Tabe 5-3 it can be seen that the transmitted force factor (f att ) is ony at 92.3% which is the owest among a the rest of 13 sampes. Besides the poor behaviour of transmitted force, the energy absorption capabiity of 4L6P60 is aso very imited at ony 6.31J in Tabe 5-6, and it is the owest among a the tested textie honeycomb composites. The energy absorption of uncrushed sampes of 6L4P60 and 8L3P60 are amost the same at 7.18J and 7.46J and this is about norma. From Tabe 5-7, it can be seen that the voume density of the three composites in this group is in the order of 4L6P60<6L4P60<8L3P60. In creating engineering materia, ow density is one of the features sought for the materia. The data anaysis here woud suggest that a ightweight honeycomb composite must be combined with strong ce was in order for the composite to be of engineering significance. For honeycomb with particuary ow densities such as sampe 4L6P60, it is more important for the ce was to have high strength. 147

148 Contact Force (kn) Transmitted Force (kn) L6P60 6L4P60 8L3P Figure 5-24 Comparison of transmitted force time diagram (sampes with simiar thickness) Time (s) L6P60 6L4P60 8L3P Dispacement (cm) Figure 5-25 Comparison of contact force - dispacement diagram (sampes with simiar thickness) 148

149 5.5.3 Discussions on composite voume density and composite thickness So far, anaysis has been carried out for the infuence of the structura parameters, incuding ce opening ange, ce size, ce wa ength ratio, and composite voume density, on the impact characteristics of the textie honeycomb composites. It is important to be abe to engineer the composites with required properties by manipuating the structura parameters. However, changes in structura parameters wi have to ead to ateration of the voume density and thickness of the honeycomb composites. Therefore, it is necessary to see how the voume density and the thickness of a composites woud affect the impact performance, particuary the impact energy absorption by the honeycomb composites and the transmitted force through the honeycomb composites. The voume density and the thickness of the sampes were measured and are isted in Tabe 5-7. Tabe 5-7. Voume density, thickness, energy absorption and peak transmitted force of different textie honeycomb composites Sampe Density Thickness E (g/cm 3 ) (mm) (J) F trans (KN) 4L6P L4P L3P L4P L5P L6P L6P L6P L6P L6P L(4+3)P L(6+3)P L(3+6)P L(4+6)P Where in the tabe, E is the absorbed energy and F trans means the peak transmitted force. 149

150 Composite voume density The impact energy absorption for a the composites against the composite voume density is iustrated in Figure 5-26(a), where the navy mark is for 4L6P60 which was crushed during the impact test. No cear trend can be found in the reationship between the composite voume density and their energy absorption. As ong as the honeycomb composites are not competey crushed, they demonstrate simiar capabiity for impact energy absorption. This suggests that for a given eve of impact energy, it is possibe to create ow density honeycomb composites for impact energy absorption. Work in this direction wi ead to materias of high energy absorption to density ratio, which is attractive to many engineering appications. Figure 5-26(b) shows the reationship between the transmitted force and the composite voume density. Apart from the composite that was crushed during the test, a trend is ceary shown that for the honeycomb composites the transmitted force increases as the voume density goes up. As ong as the composites are strong enough not to be crushed, ower density composites wi be more capabe to attenuate the impact force. This prompts more work in engineering design of honeycomb composites which are ightweight and mechanicay protective Composite thickness Figure 5-27(a) shows that the thickness of the composites which are not demonstrating obvious infuences on the energy absorption performance. Simiar to the case of composite voume density, it seems that a textie honeycomb composites are capabe of absorbing simiar amount of impact energy as ong as the composite is not competey crushed. However, if the energy absorption and transmitted force are considered separatey, it seems that the thickness of composites has an obvious affect on the transmitted force, for exampe, the thicker the composite, the smaer the transmitted force. It is noticed that in Figure 5-27, the navy square mark represents composite 4L4P60 which was crushed totay in the test. 150

151 Transmitted Force(KN) Energy Absorption(J) Composite Density(g.cm-3) (a) Energy absorption Composite Density(g.cm-3) (b) Transmitted force Figure 5-26 Infuence of voume density on honeycomb composites 151

152 Transmitted Force(KN) Energy Absorption(J) Composite Thickness(mm) (a) Energy absorption Composite Thickness(mm) (b) Transmitted force Figure 5-27 Infuence of composite thickness on honeycomb composites 152

153 5.6 Concusions In this chapter, experimenta study of 14 systematicay designed and manufactured 3D textie honeycomb composites was carried out with an emphasis on their impact behaviour. The 3D textie honeycomb composites were created from integra 3D textie reinforcements with the required materia continuity in the composites. For 3D textie honeycomb composites, the ce geometries are critica issues for achieving higher energy absorption capacity. For the ces designed for the experiment, ce size, ce opening ange, ength ratio of ce was, and aso the composite in the simiar thickness with different voume density were seected as variabes to expore the infuence of the ce geometry on resutant impact performances. Among a of ce geometrica parameters being investigated, ength ratio of ce wa or the ce opening ange of the ce structure is the most effective parameter for controing the energy absorption performance of the honeycomb textie composites. These concusions provide usefu information for engineering textie honeycomb composites against impact: 1). the opening ange of ces in honeycomb composites pays an important roe in determining their properties. For the same fabric, increasing the opening ange resuts in a ess energy absorbent and ess force attenuating honeycomb composite, and vice versa. 2). an increase in ce size of honeycomb composites makes them more efficient in impact force attenuation. It heps reducing the peak transmitted force and deays its arriva time. In addition, reducing the ce size can ead to the east buky engineering materia for simiar impact energy absorption. 3). Manipuation of the ength ratio of ce was eads to the creation of two damping materias with different impact behaviour in absorbing impact energy. Honeycomb composites with ength ratio of bonded wa to free wa ratio more than or equa to 1 153

154 b ( 1) is associated with high strength. The inverse design eads to composites that f have ow moduus and ow strength with the same eve of impact. However, the tota energy absorption is not affected by the ength ratio of ce was. 4). for the same voume, ow density honeycomb composite eads to ow contact force and ow transmitted force. However, composites with too ow density can be easiy crushed as in the case of composite 4L6P60. As ong as the composite is not destroyed by crushing, they are abe to absorb a simiar eve of impact energy. 154

155 CHAPTER 6 EXPERIMENTAL DATA ANALYSIS ON TEXTILE HONEYCOMB COMPOSITES IMPACTED WITH LARGER MASS AND LOWER VELOCITY Chapter 5 aims to estabish the understanding of the mechanica performances of the textie honeycomb composites under ow veocity impact (v 0 =5.5m/s and M=0.55kg). In this chapter, an experimenta investigation is reported on mechanica behaviour of the textie honeycomb composites under the simiar impact energy but with a ower veocity ess than 2.0m/s and a bigger mass at 4.52kg. The impact energy in these cases is quite simiar (8.31J versus 8.5J), which indicates that the impact energy is simiar whie the energy construction is different. The tested resuts wi be compared with the resuts obtained under dropping hammer system from chapter 5 to see how the construction of the impact kinetic energy infuences the fina mechanica performances for the textie honeycomb composites with different geometric parameters. The majority mechanica performances discussed in this chapter wi be focus on contact force and energy absorption performances. It has to be noted that as a reference, in terms of impact mechanism, it is reported by Prashant and Badri (1993) that a heavier impactor wi cause an overa 30-50% more damages to the item comparing to the ight weight impact with same impact energy. Both Defosse et a.,(1993) and Ujhashi (1993) expained this phenomenon by the fact that for heavier mass ow veocity impact, there are many sma superimposed osciations due to the pate vibrating against the impactor during contact, therefore more damages occurs accordingy. 155

156 6.1 Low Veocity Impact Test Setting by Instron Dynatup Mode 8200 Drop Weight Impact Testing Instrument In the present study another commercia ow veocity dropping weight impact instrument has been newy bought by university of Manchester and used to conduct the ow veocity impact test. This machine is caed the Instron Dynatup Mode 8200 drop weight impact test instrument and is designed for acquiring the tested data by capturing the impact veocity and oad signas which have been transmitted from the test machine to the data system for anaysis. It is a compete system consisting of a drop weight impact test machine and a data acquisition system which provides a comprehensive oad and energy record from each test. This machine is going to be used to conduct the impact test with a arge impactor mass (4.52kg) and ower veocity (ess than 2.0m/s) to investigate how the textie honeycomb composites structure response under different impact situations. The test resuts wi be compared with those from dropping hammer system which have been described in Section Assemby of Instron Dynatup Mode 8200 drop weight impact testing instrument The Instron Dynatup Mode 8200 drop weight impact test machine was used in this research work to conduct the impact tests (v 0 <2.0m/s and M=4.52kg) for the textie honeycomb composites, therefore it is important to introduce its correct assemby firsty. The Instron Dynatup Mode 8200 drop weight impact test machine is a gravity driven test instrument that is used to test the impact characteristics of an extensive variety of materias and components over a wide range of impact veocities. The instrument is capabe of testing at veocities up to 4.4m/s and in this research the impact veocity is about 2.0m/s. Figure 6-1 shows the assembed instrument. The basic assemby of the Instron Dynatup Mode 8200 drop weight impact testing machine coud be described as foowing and they are iustrated in Figure 6-1: 156

157 (a). drop weight and tup (impactor): drop weight and tup provide the mass for the impact testing. The drop weight is a framework with an empty weight of approximatey 3kg of which weights can then be added up to 13.6kg. In the current work, the impactor mass is 4.52kg. The tup is a device that measures the force appied to a specimen by the drop weight assemby. It consists of two parts: the tup itsef, which is a oad ce for measuring force and the tup insert, which is the too stee component that actuay strikes the specimen. It has to be noticed that the diameter for the tup insert is 20mm which is smaer than the one (30mm) used in the previous experiment (in Chapter5). This coud bring differences to the resuts as the thinner impactor intends to produce a more ocaised deformation into the honeycomb composite specimens than the thicker one. (b). veocity fag and detector: a mechanica ever is provided to manuay reease the drop weight from a pre-seected drop position. This position is set by moving the camp frame up and down and then camping it to the guide coumns using the camp knobs provided. The reease atch is protected against inadvertent reease by a guard over the atch. 157

158 Figure 6-1 Instron Dynatup Mode 8200 drop weight impact testing machine (c). drop tower framework: the framework of the test machine consists of two guide coumns and the back wedment. The guide coumns are 19mm diameter chrome pated stee rods and the drop weight assemby rides on the guide coumns via hoes in its upper and ower guide bocks. The back wedment is a painted section of C- channe and it provides rigidity and vertica stabiity to the drop tower. (d). anvis: anvis are fixtures that hod the specimens during testing. Many different styes of anvis are avaiabe to accommodate various test specifications and techniques. The anvis sit on the tabe and are secured in pace using standard bots. 158

159 6.1.2 Testing procedure A the specimens were trimmed into approximate size of 60mm 120 mm as those sampes which have been tested in the dropping hammer system (see Section 5.2.1). The testing procedure was divided into a few sections: 1. pre-test preparation: this action consists of adjusting the testing assemby incuding the drop height, veocity detector, stop bocks and drop weight mass. Position the specimen on the specimen support fixture to prepare for the test. The instrument first needs to be set up to conduct a pre-test in order to measure the initia kinetic energy of up to 9J. 2. performing a test: conduct the test by using the integrated software and reease the impactor in order to conduct the test. Each test needs to be done 3 times and new sampes were repaced each time. 3. capture the data and anaysis: tested data were passed to the computer through A/D digita convertor for future anaysis. The Instron Dynatup Mode 8200 drop weight impact testing machine is the hardware to conduct the testing and to ampify and capture the dynamic transducer output from a high speed impact event. A the raw data wi be interpreted through the associated software which is caed Instron- Dynatup impuse data acquisition software. After the test has been done, anayse the data and find out its reationship among different textie honeycomb composites and their mechanica performances Cassifications of textie honeycomb composites Same as the composites which have been tested in the previous experiment (see Section 5.2), the honeycomb textie composites are divided into four groups according to their ce size, opening ange, ength ratio of ce wa and sampes with simiar thickness. However, because of the shortage of the fabric, 8L4P60 and 8L6P90 has been eiminated from the composites which wi not affect the study on the sampes. 159

160 Therefore, there are tweve different textie honeycomb composites which have been created and they have been isted as foowing: 1. Different opening ange (8L6P30, 8L6P45, 8L6P60, 8L6P75), 2. Different ce size (8L3P, 8L5P, 8L6P), 3. Different free wa and bonded wa ength ratio: b f b f 1: (8L(3+6)P, 8L(4+6)P, 8L6P), 1: (8L(6+3)P, 8L(4+3)P, 8L3P), 4. Different ce size with simiar thickness (4L6P60, 6L4P60, 8L3P60). However, it is found that during the current impact tests, the sampes with 2 ayers and 3 ayers (4L6P and 6L4P) are crushed instanty. The energy absorption and contact force generated from crushed sampes wi not be comparabe to those sampes which haven t been crushed; therefore, sampes of 4L6P and 6L4P are obsoeted from the ate resuts discussion in Section Impact setting for Instron Dynatup Mode 8200 system The test was competed by using Instron Dynatup Mode 8200 drop weight impact testing machine and a of the specimens were impacted at the initia veocity around 2.0m/s with the impactor tup mass of 4.52kg to ensure the initia impact energy is around 8.5J. A set of data incuding time, contact force, veocity, dispacement etc., were captured after the impact test and finay, every singe contact force vaue were aigned to form the curve with the interva time as x-axis. Additionay, every singe contact force vaue was aso aigned with the dispacement to evauate the contact force and energy absorption capacity. 160

161 6.2 Tested Resut and Discussion Ce size and its experimenta performance (8L3P60, 8L5P60, 8L6P60) Sampes with different ce size from sma to big (8L3P60, 8L5P60 and 8L6P60) are constructed to investigate how the mechanica performance achieves under the simiar impact energy around 8.5J with a arger mass (M=4.52kg) and ower veocity (v 0 <2.0m/s) impact. Tabe 6-1 has isted some of the experiment resuts after the impact. Tabe 6-1 Experiment resuts from impact (sampes with different ce size) Composite Type θ ( ) F max (KN) E 1 (J) S max (mm) T peak (ms) v o (m/s) E 2 E 1 (%) (J) E 2 8L3P L5P L6P In Tabe 6-1,θ is the opening ange of the ce structure; F max means peak contact force and T peak is the arriva time for this contact force; E 1 is the absorbed energy and E 2 is the 1 2 impact kinetic energy cacuated by mv 0 (where m=4.52kg); v o is the initia impact 2 veocity; S max is maximum dispacement of the composites; and energy absorption ratio E is cacuated by 1 100(%). E 2 After the specimens were impacted by an impactor that has a ower veocity and a bigger mass, the contact force-dispacement curves of 8L3P, 8L5P and 8L6P are created which are demonstrated in Figure 6-2(a). It is noticed that both 8L3P and 8L6P have encountered high peak contact force. 8L3P is more rigid because the sma ce size makes this composite dense and hence having higher moduus, and this is the reason that 8L3P is associated to high contact force. In the case of 8L6P, the peak contact force 161

162 Energy Absorption(%) Max Dispacement(mm) Contact Force(KN) happens as a resut of crush of the composite. It is evident from Figure 6-2(a) that 8L6P is much softer than 8L3P, and this is due to the bigger size of the ces. Different Ce Size Dispacement(mm) (a) Contact force-dispacement curve 8L3P60 8L5P60 8L6P60 Energy Absorption(%) Dispacement(mm) 120% % 80% 95.0% 95.8% % 40% 20% % % 8L3P60 8L5P60 8L6P60 0 (b) Comparison of energy absorption Figure 6-2 Contact force and energy absorption behavior of sampes with different ce size 162

163 When the sampes are impacted under the current oading situation, the contact responses of 8L6P60 are totay different comparing to the resuts from sma mass and higher veocity impact in Section During previous impact tests under dropping hammer system in Figure 5-13, 8L6P shows a ductie performance comparing to the rest sampes. This indicates that sampes with big ce size are more sensitive to the oading conditions and tend to be easiy damaged if the impactor mass increases. Figure 6-2(b) shows the energy absorption for sampes (8L3P, 8L5P, 8L6P) with different ce sizes. As 8L6P tends to be damaged during the impact, it is obsoeted from the current discussions. For sampes of 8L3P and 8L5P, which are stronger enough to resist the incoming force, the energy absorption between them are very simiar. Figure 6-2(b) aso shows that the maximum dispacement ratio for 8L3P is ess than the rest sampes at ony 14.2%, in other words, 8L3P has encountered a smaer deformation than that of 8L5P. This coud be due to that the ces for 8L3P are very dense thus it makes 8L3P hard to be deformed. Therefore, under the same impact, the depth of deformation for 8L3P is reativey shaow correspondingy. With reference to the contact force, max dispacement ratio and energy absorption; it seems that athough the sampes of 8L3P and 8L5P absorb a simiar amount of energy, 8L3P absorbs the energy in a way of high contact force and ess deformation whie 8L5P absorbs the energy in a way of ow contact force and more deformation. Composite materias with a ductie behavior under impact are more attractive for the cushioning purposes; therefore, it further verified that sampes with medium ce size can perform better under various oading conditions. 163

164 6.2.2 Opening ange and its experimenta performance (8L6P30, 8L6P45, 8L6P60 and 8L6P75) The design of the composite with different opening ange from 30 to 90 aims to figure out the mechanica performances of the textie honeycomb composites under a arge mass and ower veocity impact (v 0 <2.0m/s & M=4.52kg). As discussed in the previous work in Section ), the composites with the opening ange between 60 and 75 own a better protection capabiity when it is stroke under the veocity of 5.5m/s with the impact mass of 0.55kg, the present research is targeted to find out is there any simiarity or variety when the composite is impacted under the current condition. Ideay five sampes with opening ange at 30, 45, 60, 75 and 90 shoud be prepared for the testing in order to compare them with the previous tests in Chapter 5, however, due to the fabric shortage, sampes with 90 opening ange is obsoeted in the current experiment. Four sampes with opening ange at 30, 45, 60 and 75 were impacted with different veocities around 2.0m/s and the tested resuts are isted in Tabe 6-2 for the data anaysis purpose. Tabe 6-2 Experiment resuts from impact (sampes with different opening ange) Composite Type θ F max (KN) E 1 (J) S max (mm) T peak (ms) v o (m/s) E 2 E 1 (%) (J) E2 8L6P L6P L6P L6P

165 Energy Absorption(%) Max Dispacement(mm) Contact Force(KN) Different Opening Ange L6P30 8L6P45 8L6P60 8L6P Dispacement(mm) (a) Contact force-dispacement curve Energy Absorption (%) Max Dispacement(mm) 90% 80% 82.9% 83.2% % 60% % 25 50% 20 40% 38.1% 15 30% 20% 10 10% 5 0% 8L6P30 8L6P45 8L6P60 8L6P75 (b) Comparison of energy absorption 0 Figure 6-3 Contact force and energy absorption behavior of sampes with different opening ange 165

166 Figure 6-3(a) shows the contact force-dispacement curves of sampes with different opening anges. It seems composites with 30, 45 and 60 opening anges were destroyed during the current impacts because significant sharp increases of the contact forces are observed in Figure 6-3(a) after the composites are deformed for a period time. The reason to cause this coud be the impactor touches the anvi thus increases the contact force suddeny. However, from Figure 6-3(a) and (b), it seems sampes with 75 opening anges have encountered a ower contact force and a smaer absorbed energy. This is against basic principe due to the fact that if the contact force of the composite is ower which means the composite is easier to be deformed and this wi ead to a arger structure deformation in the resut of absorbing more impact energy uness the impact stops instanty. However, in Figure 6-3(a), the dispacement of the 8L6P75 is ong compared to other composites. It seems that experiment errors might have occurred during the testing, thus, 8L6P75 wi not be incuded in the resut discussions in the current study Different ength ratio of bonded and free wa and its experiment performance ( b 1 : 8L(3+6)P60, 8L(4+6)P60, 8L6P60; b 1 : 8L3P60, 8L(4+3)P60, 8L(6+3)P60) f f 8L(3+6)P, 8L(4+6)P and 8L6P of which b 1as we as 8L3P, 8L(4+3)P, 8L(6+3)P of f which b 1 f are grouped to investigate the effect of ength ratio of bonded and free wa on the composite mechanica performance under current oading condition. The resuts from experiments are summarized in Tabe 6-3 and their contact forcedispacement curve and absorbed energy were iustrated in Figure

167 Figure 6-4(a) shows the contact force-dispacement curve of the sampes with b 1. It is evident from Figure 6-4(a) that sampes with onger free was such as 8L(3+6)P have shown a reativey ductie performance comparing to the rest sampes. As it has been mentioned in Section 6.2, sampe of 8L6P is destroyed during the current impacts; therefore, it seems to increase the free wa ength of the composite ce can hep to reduce the structure faiure sufficienty. f Nevertheess, the contact force responses for 8L6P is in a distincty different way comparing the resuts from dropping hammer system in Section where 8L6P shows the most ductie performance. This means, textie honeycomb composite with an even ength ratio of ce was ( b : f =1:1) tend to be more impactor weight sensitive. The reasons to cause this need further investigation in the future work. Tabe 6-3 Experiment resuts from impact (sampes with different ength ratio of ce was) Composite Type θ F max (KN) E 1 (J) S max (mm) T peak (ms) v o (m/s) E 2 E 1 (%) (J) E2 b f 1 8L(3+6)P L(4+6)P L6P b f 1 8L3P L(4+3)P L(6+3)P

168 Comparing to the sampes with b 1, Figure 6-4(b) indicates that the subgroup of f sampes with b 1 f (8L3P60, 8L(4+3)P60 and 8L(6+3)P60) not ony resist the contact force sufficienty without structure faiure but aso finish the impact with a bouncing process towards the end of the impact. This can be seen from the returning of the curves towards the end of the curving in Figure 6-4(b) and this is the resut of the impactor being bounced back as the striking distance has reached a certain depth then reduced back to a short distance. Normay, contact force-dispacement curve ike this means the sampes haven t been striking through during the impact process as the structure is fexibe and strong enough to accumuate the incoming force. It has to be mentioned that the bouncing process towards the end of the impact wi reease a sma amount of impact energy. However, compared to the impact energy which has been absorbed during the whoe impact process, the reeased impact energy can be negected (Sun, 2005; Yu and Chen, 2006). Considering the free wa ength of the composites, are a composed by 3 picks which makes the composites ce very sma in sizes. Therefore, 8L3P, 8L(4+3)P and 8L(6+3)P are very dense in their materia property which can provide more resistance to the incoming force accordingy. 168

169 Energy Absorption(%) Contact Force(KN) Contact Force(KN) Different Wa Ratio (b/f<=1) L(3+6)P60 8L(4+6)P60 8L6P Dispacement(mm) (a) Contact force-dispacement curve for subgroup b 1 f Different Wa Ratio (b/f>=1) Dispacement(mm) 8L3P60 8L(4+3)P60 8L(6+3)P60 (b) Contact force-dispacement curve for subgroup b 1 f Energy Absorption 120% 100% 90% 88.70% 95% 96.50% 93.50% 80% 60% 64.90% 40% 20% 0% 8L(3+6)P60 8L(4+6)P60 8L6P60 8L3P60 8L(4+3)P60 8L(6+3)P60 Composite (c) Comparison of energy absorption 169

170 Figure 6-4 Energy absorption of sampes with different ength ratio of free and bonded wa Figure 6-4(c) ceary demonstrated that subgroup with b 1 has a sighty better f capabiity for energy absorption comparing to subgroup with b 1 f under current impact situation The subte differences in the absorption of impact energy among the composites, especiay in the b 1 subgroup, require further investigation. f Comparison of the resuts between two different oading conditions The discussions in the foowing sections wi be focus on the mechanica performances of textie honeycomb composites between the current experiments and the test resuts from Chapter 5. It wi briefy compare the contact force and energy absorption behavior between these two different oading conditions Sampes with different ce size (8L3P60, 8L5P60, 8L6P60) Sampes with different ce sizes from sma to big have been impacted under two different oading conditions, Figure 6-5 (a);(b);(c) have compared their contact forcedispacement curves in individua cases. From Figure 6-5(a), it seems there is a significant curve returning towards the end of the impact when the composites is under bigger mass impact (M=4.52kg) with a ower impact veocity (v 0 =1.78m/s), this means, the impactor is bouncing back when the impact finishes. However, the red curve, which represents the contact responses under another oading condition doesn t show up this bounce process at the end of the impact. 170

171 Contact Force(KN) Contact Force(KN) Conatact Force(KN) 8L3P60 Impact Veocity=1.78m/s Impact Veocity=5.5m/s Dispacement(mm) (a) 8L3P 8L5P60 Impact Veocity=1.90m/s Impact Veocity=5.5m/s Dispacement(mm) (b) 8L5P 8L6P60 Impact Veocity=1.94m/s Impact Veocity=5.5m/s Dispacement(mm) (c) 8L6P Figure 6-5 Contact force-dispacement curves of composite with different ce sizes 171

172 Figure 6-5(b) shows the contact force responses of composites with medium ce size (8L5P) and it seems the composites under both oading conditions are capabe of resist the incoming forces. The depth of deformations for the sampes under current impacts is deeper than those under ight weight impacts. The can be expained by that the heavier impactor wi strike deeper into the composites than the ight weight impactor. From Figure 6-5(b), it can aso be seen that the trend of both contact force curves are simiar under two different oading conditions, this means, 8L5P performances stabe under both impacts. This is good for the purposes of PPE as reiabe performances of the composites are requested in practice. The sampe of 8L6P is destroyed under heavy weight impacts which have been expained in Section 6.2.1, and from Figure 6-5(c), it ceary shows that 8L6P is sufficient to resist the incoming oading when the impactor is ight weight. Regarding the energy absorption performances of the composites, the kinetic energy shoud be absorbed as much as possibe to prevent damage underneath. Generay, the more impact energy is absorbed by the composite the ower the acceeration and damage of the protected item. Figure 6-6 iustrates the infuences of the composite ce sizes on their energy absorption performances under two different impacts. Assuming the composites are strong enough to resist the impacts, 8L3P and 8L5P both absorb more kinetic energy under heavier weight impacts. Combined with composites contact force and energy absorption performances under both oading conditions, it seems sampes with sma and big ce sizes (8L3P and 8L6P) show different contact force performances obviousy. This means, the discrimination of performances under different impact situations for the composites with sma and big ce sizes are very significant and this is bad in their appication. When the designer 172

173 Energy Absorption (%) choose materias for protection purposes, it is vita the materias shoud have stabe mechanica performances under a kinds of exposed impact situations. 120% 100% 80% 60% Comparison of energy absorption under different impact situation 95% 85.96% 95.80% 90.17% 64.90% 86.90% 40% 20% 0% 8L3P60 8L5P60 8L6P60 v<2.0m/s v=5.5m/s Figure 6-6 Energy absorption under different impact situation (sampes with different ce size) Sampes with different opening ange (8L6P30, 8L6P45, 8L6P60, 8L6P75) Figure 6-8(a);(b);(c);(d) show the contact force-dispacement curves under two different oading conditions individuay. It is obviousy that the contact forces from these two experiments are very different. Sampes constructed by sma to medium opening ange at 30, 45 and 60 encountered structure faiure during the heavier weight impact and this has been mentioned in Section aready. The red curves from ight weight impact are much ductie and have generated ower contact force for a the composites. Athough composites at 75 opening ange seems to have a coser peak contact forces under both oading conditions and they are sufficient to resist the incoming forces too, however, the arge opening ange significanty increases the composites thickness which makes the composites very buky in practice. 173

174 Contact Force(KN) Contact Force(KN) It seems, whatever the opening anges of the composites are, if the impact weight increases dramaticay, and here from 0.55kg to 4.52kg, the honeycomb structures wi ost their efficiency to resist the incoming forces. In another word, by ony adjusting the opening ange of the ces to strengthen the honeycomb composite materias against the impact oadings is not sufficient enough to resist the incoming forces, others methods shoud be found out to enhance the composite structure performances against impacts too. 8L6P30 Impact Veocity=1.95m/s Impact Veocity=5.5m/s Dispacem ent(m m) (a) 8L6P30 8L6P45 Impact Veocity=1.97m/s Impact Veocity=5.5m/s Dispacement(mm) (b) 8L6P45 174

175 Energy Absorption (%) Contact Force(KN) Contact Force(KN) 8L6P60 Impact Veocity=1.94m/s Impact Veocity=5.5m/s Dispacement(mm) (c) 8L6P60 8L6P75 Impact Veocity=1.94m/s Impact Veocity=5.5m/s Dispacement(mm) (d) 8L6P75 Figure 6-7 Contact force-dispacement curves for composites with different opening anges Comparison of energy absorption under different impact situation 120% 100% 80% 60% 40% 82.9% 96.9% 95.7% 83.2% 64.9% 86.9% 38.1% 84.4% 20% 0% 8L6P30 8L6P45 8L6P60 8L6P75 v<2.0m/s v=5.5m/s Figure 6-8 Energy absorption under different oading conditions (sampes with different opening ange) 175

176 The test resuts under ight weight impacts in Section have mentioned that when the opening anges of the composites increase up to 90, the free wa and bonded wa of the ces form a shape of amost rectanguar and it reduces the fexibiity of the composite which restricts the deformation of the composites and causes ess strain energy been absorbed. It seems under current heavier weight impacts, the honeycomb structures don t deform much either. However, from Figure 6-8, it seems the energy absorption under heavier weight impacts for the sampes (8L6P75) are ess than those under ight weight impacts. The reasons to cause it are not cear at the moment and more tests wi be needed to expain it in the future Sampes with different ength ratio of free and bonded wa ( b 1: 8L(3+6)P60, 8L(4+6)P60, 8L6P60; b 1: 8L3P60, 8L(4+3)P60, 8L(6+3)P60) f f Figure 6-9(a);(b);(c) iustrates the contact force-dispacement curves for the sampes with b 1. It seems ony the sampes with the ongest free wa (8L(3+6)P) can resist f the heavy weight impact and haven t been destroyed. The contact force responses in Figure 6-9(b) and (c), are both showing that the rest two sampes are encountering structure faiure significanty. This indicates that increasing the ce free wa on the assumption of that the bonded wa is in fixed engths, can strengthen the honeycomb structure against the incoming force. The energy absorption performances in Figure 6-9(d) state that 8L(3+6)P absorbs simiar energy under two different oading conditions. 176

177 Contact Force(KN) Contact Force(KN) Contact Force(KN) 8L(3+6)P60 Impact Veocity=1.94m/s Impact Veocity=5.5m/s Dispacem ent(m m) (a) 8L(3+6)P 8L(4+6)P60 Impact Veocity=2.0m/s Imapct Veocity=5.5m/s Dispacement(mm) (b) 8L(4+6)P 8L6P60 Impact Veocity=1.94m/s Impact Veocity=5.5m/s Dispacement(mm) (c) 8L6P 177

178 Energy Absorption (%) Comparison of energy absorption under different impact situation 100% 80% 60% 94.6% 90.0% 88.7% 82.6% 64.9% 86.9% 40% 20% 0% 8L(3+6)P60 8L(4+6)P60 8L6P60 v<2.0m/s v=5.5m/s (d) Comparison of energy absorption Figure 6-9 Contact force-dispacement curve and energy absorption diagram for the sampe with different ength ratio of bond and free wa ( b 1) f Figure 6-10(a);(b);(c) ist the contact force-dispacement curves for the sampes with b f 1, which have been impacted under both oading conditions. It seems composites with a sighty difference in their wa ratio ( b : f =4:3) have a more simiar contact force responses under various oading conditions, this means, they are more reiabe in their mechanica performances when the impact situations change. 178

179 Contact Force(KN) Contact Force(KN) Conatact Force(KN) 8L3P60 Impact Veocity=1.78m/s Impact Veocity=5.5m/s Dispacem ent(m m) (a) 8L3P60 8L(4+3)P60 Impact Veocity=1.90m/s Impact Veocity=5.5m/s Dispacem ent(m m) (b) 8L(4+3)P60 8L(6+3)P60 Impact Veocity=1.87m/s Impact Veocity=5.5m/s Dispacement(mm) (c) 8L(6+3)P60 179

180 Energy Absorption (%) Comparison of energy absorption under different impact situation 120% 100% 80% 60% 40% 20% 95.0% 96.5% 86.0% 84.5% 93.5% 83.9% 0% 8L3P60 8L(4+3)P60 8L(6+3)P60 v<2.0m/s v=5.5m/s (d) Comparison of energy absorption (e) Figure 6-10 Contact force-dispacement curve and energy absorption diagram for the sampe with different ength ratio of bond and free wa ( b 1) However, from the shape of contact force-dispacement curve in Figure 6-11, it seems that the structure of 8L3P60 and 8L(4+3)P60 is capabe enough to bounce back the impactor when it is under arge mass and ower veocity impact (navy cure in Figure 6-11(a)(b)) and therefore the curves were returning backwards at the end of the impact Generay, the energy absorption for the composites in the subgroup of b 1 shows a higher vaue under ight weight impact in Figure 6-10(d). Further investigations wi be required to seek out the reasons to cause this phenomenon in the future. f f 180

181 6.3 Summaries The present chapter investigates the composites with different geometric parameters under heavy weight and ower veocity impact in order to compare their mechanica performances with another impact situation of ight weight and higher veocity in Chapter 5, using various impact indices, incuding contact force, energy absorption and maximum structure dispacement. The different performance indices are discussed as foows. Generay speaking, most of the textie honeycomb composites are impactor weight sensitive regarding their mechanica performances. Under heavy weight oading conditions, ony those composites with sma to medium ce sizes or arger opening anges are providing sufficient resists to the impact forces. Composites with medium ce sizes (8L5P) have more stabe mechanica performances under various exposed impact conditions. Athough sampes with smaer ce sizes are capabe of resist the impact, it encountered a higher oading force under heavy weight impacts and this wi acceerate the composites and cause more damages to the item underneath. It seems that if sighty increases the bonded wa ength ( b ), provided that the composites are strong enough to resist incoming forces, can bring more stabe performances to the composites when they are under various oading conditions. There are a few composites with free wa ength of 6 picks (8L6P, 8L6P30,8L6P45, 8L(4+6)P) are a destroyed during heavy weight impact, this indicates, big ce sized composites are very easy to be destroyed under heavy weight oading conditions. Therefore, under more critica oading conditions, it is necessary to find a way to enhance the big ce sized composites wa materia or structures to strengthen their structure performances correspondingy. 181

182 CHAPTER 7 FEA ON TEXTILE HONEYCOMB COMPOSITES The discussion on mechanica behaviour based on the resuts from experiments described in the previous chapters has reveaed compex interactions among the structura parameters of the honeycomb composites and the impact performances incuding the energy absorption, transmitted force, and impact force. In addition to the experimenta anaysis, finite eement (FE) method is used to mode the performance of the 3D textie honeycomb composites and to study the infuence of the geometrica parameters on the performance of the 3D honeycomb composites. This chapter reports on the FE simuations with the aim to examine (i) the composite performance under impact with different energy eves and (ii) the infuence of the impactor shape on the composite performances. Standard tensie test was conducted to obtain the materia data of the cotton/epoxy singe ayer sheet for the finite eement anaysis (FEA). Ideay, textie honeycomb modes in 3D shoud a be created for the FEA. However, the amount of cacuation is usuay too big for the computer hardware and software to hande, therefore, in the current work, 3D modes for imited composites are created and used for FEA in order to save cacuation time. As a simpified method, honeycomb modes in 2D are created as it is reported that the 2D modes are aso be abe to be used to evauate effectivey the performances of the modeed objects, though the accuracy is ower than in the case of 3D modes (Yu and Chen, 2006). The purpose of the FEA based on the 2D modes is to examine the mechanica performances of the composites under impact with different impactor shapes i.e., the cyindrica and the spherica and they are two of the most commony used projectie shapes (Yu and Chen, 2006). The impact energy (E) is assumed to be 6J, 8.3J and 10J, 182

183 of which E=8.3J is to simuate the impact energy in the experiments (E=8.3J-8.5J). Impact energy eves 6J and 10J are designed for FE anaysis ony. 3D geometrica modes are more compicated but coser to reaity. Three modes are created for 8L3P, 8L4P and 8L6P honeycomb composites ony. Experimenta resuts are used to vaidate the simuation resuts for both modes in 2D and 3D. 7.1 FEA Based on 2D Honeycomb Composite Modes Creation of 2D modes for textie honeycomb composites Stricty speaking, before modeing the textie honeycomb composites, there are a ot of variants which shoud be considered for the cotton/epoxy singe ayer composites in the micro-structura scae incuding fabric weave structure, fabric weave density, fibre and matrix materia, yarn pacement, yarn size and type etc. However, to mode the honeycomb composites in such a micro-structura detaied is extremey difficut, time consuming and amost impossibe to carry out in practice. Therefore, in the current study, these cotton/epoxy singe ayer composites are modeed as homogenous and isotropic sheet which is the most common method to conduct macro-mechanica anayses for woven composite materias (Antonio, 2000), without invoves the microstructure fabric and yarn. This makes the FEA on honeycomb composite modes to become simper. Aso, it has to be noted that for simpicity, the cotton/epoxy singe ayer composites was assumed to be homogeneous and isotropic in the FE mode, rather than anisotropic as it shoud be. Assumptions ike this have been widey used by other researchers in the FEA for textie woven composites (Xu et a., 1995; Yu and Chen, 2004; Tan and Chen, 2005; Tan et a., 2007). On the other hand, it is amost impractica to investigate experimentay the materia property of the singe cotton/epoxy sheet aong thickness direction due to the shortage of techniques to obtain the strain-stress behaviour for the singe ayer sheet. This assumption, however, woud ead to usefu resuts for 183

184 estabishing understanding of the impact performances of the honeycomb textie composites. The composite mode design aims to refect the fundamenta features of the textie honeycomb composites, such as overa dimension, materia of singe ayer composites and structure design. As mentioned in the previous chapter (see Section 4.1.1), the honeycomb composites were made by impregnating the 3D honeycomb fabrics with resin and there are a together 14 composites with different geometric parameters in the experimenta investigations. The average dimensions of the sampe composites are 120mm (L) 60mm (W) with varying thickness because of the opening anges and the increased ayers of the specimens. In this part of FEA, tweve 2D modes of 4-ayer honeycomb composites with different ce sizes, ce opening anges, and ce wa ratios ( b f ) are created. The modes of honeycomb composites with 2 ayer and 3-ayer (4L6P and 6L4P) are obsoete from the current FEA because there are errors in running these 2 modes and the software stops working. It coud be because the honeycomb structures with fewer ayers are too weak to resist the incoming force, in another words, the force appied on the impactor mode exceeds the toerance of honeycomb mode. The detaied schematic iustrations with structura parameters are isted in Tabe 7-1. Tabe 7-1 Schematic iustrations with structura parameters of 12 geometric modes Ce Structure θ ( ) b f h w (mm) (mm) (mm) (mm) 8L3P Modes 8L4P

185 8L5P L6P L6P L6P L6P L6P L(6+3)P L(4+3)P L(4+6)P L(3+6)P In Tabe 7-1, θ is the opening ange of the ce; b and f are the ength of the bonded wa and the ength of the free wa of the ce measured in mm. t b and t f represents the 185

186 thickness of the bonded and free was of the ce; h indicates the height of the honeycomb composite and w is the width of the composite, both measured in mm Meshing the geometrica modes and the impactor The creation of FE modes, aso known as meshing, is an important step in the FEA. The seection of eement type and meshing quaity woud infuence the accuracy of the FEA resuts. FEA are known to ose accuracy when the origina mesh becomes highy distorted. A refinement of the mesh which supports the capture of noninear materia effects is often required due to the fact that there is aways a potentiay arge reative deformation in the FE impact simuation. However, the refinement of the FE mesh is inked to an increase of the computationa costs. In order to retain the efficiency of the cacuation in the present work, the mesh size woud ony be refined in the impact vicinity of the modes, whereas the rest area uses a coarser mesh and this method has been used frequenty in 3D FE mode meshing as more compicate she refinement wi be needed and more meshed eement wi be invoved. The software (Marc.Mentat, 2005a) is used for the mode creation as we as the FEA. The individua ce was are created by defining the surfaces first then each surface is meshed individuay using the 4-node axisymmetric eements with fu integration. The eement type uses Type 10, which is regarded as suitabe for dynamic contact anaysis. It is a four-node, isoparametric arbitrary quadriatera eement for axisymmetric appications. The term node describes the edge points of an eement and the ocation of the eement in the 2D space. The nodes on the top or bottom surface of the ce wa usuay are used as reference points for the subsequent data coection. The meshing of a ce is shown in Figure

187 Figure 7-1 Meshing of a ce Meshing the impactor Two shapes are used to represent the impactor in the 2D FE modeing, which are cyindrica and spherica. The cyindrica impactor wi impact at the centre of the top surface of the honeycomb composite modes. The geometry of the cyinder impactor is 30mm (d) 99mm (L), the diameter of the cyindrica shaped impator are modeed as the same size as used in experiment. The materia of the impactor is assumed to be stee instead of wood which has been used in the experiments, thus, the ength of the cyindrica impactor was shortened in order to keep the same impactor mass as that used in the experiments for0.55kg. The mesh size is refined to match the fineness in the impact vicinity of the top surface of the honeycomb mode. The eement type used for the cyindrica impactor is the 4-node quadriatera eement because this eement suits dynamic contact anaysis. Figure 7-2(a) shows the 2D projection of the meshed cyinder impactor. A ba impactor is aso used for the FE impact anaysis. The diameter of the ba impactor is 32.4mm and the impactor materia is assumed to be stee. The mesh size is 187

188 refined to match the fineness in the impact vicinity of the impact mode. The eement type been is defined the same as that in the cyindrica impactor. The meshed spherica impactor is shown in Figure 7-2(b). (a) (b) Figure 7-2 Meshed impactors (a) the cyinder and (b) the sphere Materias FEA requires the materia properties, incuding the Young s Moduus, materia density and stress-strain behaviour, to be specified. As has been mentioned in the previous chapters, the honeycomb composites sampes were made from woven fabrics with cotton yarns and epoxy resin. Therefore, cotton/epoxy composite properties were used as the materia for ce was of the honeycomb composites in the FEA and they are assumed to be homogenous and isotropic. The materia properties for the cyinder and ba shaped impactors were seected to be that of stee (James, 2001) The tensie test of a singe ayer composite The mechanica properties of the cotton/epoxy singe ayer sheet required for the FE modeing were determined by using the grab-test procedure described in standard ASTM D (2004), which requires 6 rectanguar specimens of predefined dimensions. Grab-test is chosen to conduct the current tensie test instead of strip-test 188

189 because the testing conditions are simiar to the oad appication on the specimen in practica use (Bassett et a., 1999; Pan et a., 2003). To prepare the test sampes, a pain woven cotton fabric impregnated with epoxy resin (see section 4.1.1) were cut into a rectanguar shape with the size of 25mm 250mm according to the ASTM standard. INSTRON 4505 tensie tester was used for the tensie testing; the maximum oad ce capacity of the tensie tester is 50KN. For the test, the crosshead was set to move at a rate of 2mm/min. Each specimen was then camped with a grip width of 25mm, and the gauge ength between the camps was set to be 150mm. To ensure a secure grip an emery coth was foded around the ends of the specimen Materia properties The Young s moduus and stress-strain curve of the cotton/epoxy composite were obtained from the above mentioned tensie test and the specific density, ρ, of the cotton/epoxy singe ayer sheet was measured too. Tabe 7-2 ists the basic mechanica properties of the cotton/epoxy composite required for the FEA and the stee impactor (James, 2001). Tabe 7-2. Mechanica properties of materias Materia Properties Cotton/Epoxy Stee (James,2001) Specific Density, ρ (kg/m 3 ) Young s Moduus, E (GPa) The determined stress-strain curve of the cotton/epoxy materia is dispayed in Figure 7-3 together with the curve for stee (James, 2001). In the subsequent FEA, the ce was of the honeycomb composites wi be defined as the cotton/epoxy composite materia and the impactor wi be specified as stee. 189

190 Tensie Stress [GPa] Stress-Strain behaviour of Cotton/Epoxy composite and Stee Cotton/Epoxy Stee Tensie Strain [%] Figure 7-3 Stress-strain behaviour of cotton/epoxy sheet and stee Boundary conditions appied to the honeycomb composite modes In order to carry out FEA on the composites, boundary conditions have to be defined to simuate the camping and pacement of the rea test specimen. This wi provide certain degrees of freedom to aow a natura coision response to the impact. In the current case, the constraints are defined based on the conditions used in the impact test of textie honeycomb composites, where the specimen is paced and fixed by adhesive tapes on the anvi. The honeycomb composite mode can ony deform corresponding to the impact oad and wi not rotate. This boundary conditions are added by fixing the edges of the honeycomb composite modes with the constraints having their effects in x and y directions. An iustration of the FE mode and the constraints is shown in Figure 7-4.This constitution provides sufficient oca fexibiity of the honeycomb modes for the impact response. 190

191 Figure 7-4 Schematic iustration of boundary conditions for the FE impact mode Impact setting for FEA of 2D modes The anaysis is for ow-veocity impacts, which is the same oading condition as that in the experiments. The eve of impact energy has been seected to be 6J, 8.3J and 10J. The impactor shapes are cyindrica and spherica and the weight of the impactor is 0.55kg. It is observed from experiments that 25ms is sufficient enough for the honeycomb composites to finish an impact process, therefore, the simuation time in the current FEA is defined as 25ms and the time step is defined as 100 as an interva. The materia for the honeycomb composite is assumed to be isotropic, the Young s Moduus is 0.23GPa, and the specific density of the materia is (kg/m 3 ) as has specified in Section The eves of impact energy and veocity are dispayed in Tabe 7-3, and Tabe 7-4 gives detais of the FE modes. Tabe 7-3 Impactor mass, impact veocity and impact energy Impactor Mass(kg) Impact Veocity (m/s) Impact Energy (J)

192 Tabe 7-4 Detais of FE modes Group Ce Size Opening Ange Ratio of Bonded and Free Wa Modes 8L3P60, 8L4P60, 8L5P60, 8L6P60 8L6P30, 8L6P45,8L6P60, 8L6P75, 8L6P90 b 1 8L(3+6)P60,8L(4+6)P60, 8L6P60 f b f 1 8L(4+3)P60, 8L(6+3)P60, 8L3P60 Impactor Shape Cyinder and Ba Impact Energy (J) 6, 8.3 and Resuts and discussions of FEA based on 2D modes In this section, 2D FEA resuts are given and discussed regarding the mechanica performances for the modes incuding deformation area, the depth of deformation, dynamic contact force and the energy absorption of the composites. FEA resuts are aso compared to the experimenta resuts for vaidating the modes Introduction of performance indices Before introducing the FEA resuts, it is necessary to describe some of the performance indices that are to be used in this anaysis. Such indices incude composite deformation area, energy absorption density and the depth of deformation in the composite modes. Deformation area in the composite modes under cyinder impact In investigations for performances of the honeycomb modes under impact, the shape and dimension of the deformed voume bear important information that indicates the composite behaviour. In 2D FEA, a trapezoida shaped area, which represents the cross- 192

193 section of the deformed voume of the composite, is used as a performance index to represent approximatey the manner of deformation for the honeycomb modes. It is assumed that the abiity for energy absorption of a textie honeycomb composite is reated to the deformation area in the 2D simuation and the shape of this area may indicate how the energy is absorbed during the impact. For exampe, for the same energy absorption, it can be shaow and wide or it may be deep and narrow. Figure 7-5 shows a deformed cross-section of the honeycomb composite due to impact. It is reasonabe to assume a trapezoida shaped area to represent the deformed area. Figure 7-5 Estimation of deformed cross-section represented by a trapezoida shaped area (use 8L6P60 as an exampe) 193

194 The deformed area, S, in Figure 7-5 can be cacuated by the foowing equation: ( Lt Lb ) h' S [7-1] 2 where S is the area of the trapezoida shape, L t and L b are the engths of the top and bottom edge for the trapezoida shape, h represents the height of the trapezoid. The cacuated trapezoida area eads to a numerica expression of the deformation area. Deformation area ratio (%), denoted by R, can be used to express the percentage of the deformation area against the whoe composite cross-section area of the mode, which is defined as foows. S R 100 (%) [7-2] w h where w is the origina width and h is the origina height of the honeycomb structure in 2D with the unit of mm, and their vaue have been isted in Tabe 7-1. It has to be noticed that the deformation area generated above is based on the 2D modes and therefore, the cacuated deformation area ratio is ony a way to approximatey evauate the deformation dissipation capabiity of the mode in genera and more accurate deformation area are expected to be generated in 3D modes in the future. The depth of deformation Foowed by the above description of deformation area (S ), the depth of deformation for the modes can be generated as h from the Figure 7-5. It can aso be used as an performance index to evauate the impact performance of the honeycomb composites. In the current study, the depth of deformation ratio (%), denoted by D, is expressed as a percentage and the equation is shown beow: h' D 100 (%) [7-3] h 194

195 where h is the height of the composite. Comparing to the deformation area mentioned above, which is a numerica vaue to represent the deformation of the mode in two-dimension, the depth of deformation of the mode is aso a numerica vaue to express the deformation of the mode, however, it is in one-dimension. Strain energy density Strain energy density, defined as the strain energy absorbed per unit voume of the textie honeycomb composite, is used to describe the energy absorption performance. The strain energy density is determined by dividing the tota strain energy E by the voume V of the mode. Thus the strain energy density, denoted by the symbo e, can be expressed in the foowing form: e = V E [7-4] or, E= e V [7-5] where E is the tota strain energy absorbed by the mode with the unit of J, and V the tota origina voume of the mode with the unit of mm 3 and V=w h t, in which w, h and t are the origina width (mm), origina height (mm) and origina thickness (mm) of the honeycomb structure, respectivey. In the current FEA, the software (Mac.Mentat, 2005a) can generate the tota strain energy of the whoe mode as an output and this enabes the cacuation of strain energy density e (J/mm 3 ) easiy. The origina width (w), origina height (h) has been isted in Tabe 7-1, and the origina thickness (t) of the mode is assumed to be 120mm for a the modes, which is the same as for the rea composites. 195

196 Cassifications of the FE composite modes The textie honeycomb composite modes, with 12 varieties, are cassified into 4 groups in the current works as foows: 1). different ce size (8L3P, 8L4P, 8L5P, 8L6P) 2). different opening ange (8L6P30, 8L6P45, 8L6P60, 8L6P75, 8L6P90) b 3). different ce wa ength ratio of 1: (8L(6+3)P, 8L(4+3)P, 8L3P) b 4). different ce wa ength ratio of 1: (8L6P, 8L(4+6)P, 8L(3+6)P) f f Simuated resuts The simuated resuts from 2D FEA are isted and categorised into 4 groups according to their geometric parameters. Group I. Modes with different ce size In this group, textie honeycomb composites with different ce size (8L3P60, 8L4P60, 8L5P60 and 8L6P60) were modeed in 2D. The schematic of their deformation pattern under impact energy of 6J, 8.3J and 10J are isted in Tabe 7-5. Their depth of deformation ratio (D), deformation area ratio (R), and strain energy density (e) were recorded in Tabe 7-5 too. Tabe 7-5 Effect of ce size on modes under cyinder impact Impactor Type: Cyinder Mode Origina Shape 6J 8.3J 10J 8L3P60 196

197 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm L4P60 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm L5P60 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm L6P60 Deformed Pattern 197

198 D (%) R (%) e ( 10-6 ), J/mm Group II. Modes with different opening ange Modes with same ce wa ength but different opening anges from 30 to 90 are categorised into this group. They are 8L6P30, 8L6P45, 8L6P60, 8L6P75 and 8L6P90 respectivey. Tabe 7-6 iustrates their deformation detais after impact. Tabe 7-6 Effect of ce opening ange in the modes under cyinder impact Impactor Type: Cyinder Mode Origina Shape 6J 8J 10J 8L6P30 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm L6P45 Deformed Pattern 198

199 D (%) R (%) e ( 10-6 ), J/mm L6P60 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm L6P75 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm L6P90 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm b Group III. Modes with 1 f 199

200 b In this group, textie honeycomb composites with 1 f (8L3P60, 8L(4+3)P60, 8L(6+3)P60 were modeed in 2D and their FEA resuts are shown in Tabe 7-7. b Tabe 7-7 Effect of ce wa ratio ( 1) on the modes under cyinder impact f Impactor Type: Cyinder Mode Origina 8L3P60 Shape Deformed Pattern 6J 8J 10J D (%) R (%) e ( 10-6 ), J/mm L(4+3)P60 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm L(6+3)P60 Deformed Pattern 200

201 D (%) R (%) e ( 10-6 ), J/mm b Group IV. Modes with 1 f b In this group, textie honeycomb composites with 1 f (8L6P60, 8L(4+6)P60, 8L(3+6)P60 were modeed in 2D and their FEA resuts are shown in Tabe 7-8. Tabe 7-8 Effect of ce wa ratio ( b f ) on the modes under impact Impactor Type: Cyinder Mode Origina Shape 6J 8J 10J 8L6P60 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm L(4+6)P60 Deformed Pattern 201

202 D (%) R (%) e ( 10-6 ), J/mm L(3+6)P60 Deformed Pattern D (%) R (%) e ( 10-6 ), J/mm Deformation area under cyinder impact The deformation areas of the modes with different geometric parameters are isted schematicay in Tabe 7-5 to 7-8. Their deformation area ratio (%) are cacuated by using trapezoida area with their origina area from Equation 7-2. Obviousy, different amount of impact energy eads to different deformation area and generay, modes impacted under higher impact energy (10J) created more deformation area than that under ower impact energy (6J). It is aso noted from the figures in Tabe 7-5 that when the modes were impacted with the same amount of energy, the deformation area coud vary a ot if the geometric parameters of the mode change, and the foowing section wi discuss this more in detai. Deformation area of modes with different ce sizes 202

203 Depth of deformation ratio, D (%) Deformation Area Ratio (%) J 8J 10J L3P60 8L4P60 8L5P60 8L6P60 (a) Deformation area ratio (R) J 8J 10J L3P60 8L4P60 8L5P60 8L6P60 (b) The depth of deformation ratio (D) 203

204 Depth of deformation Ratio, D (%) FE Resut(E=8J) Experiment Resut(E=8J) 8L3P60 8L4P60 8L5P60 8L6P60 (c) Comparison betwwen experimenta and 2D FE resuts Figure 7-6 Deformation of modes with different ce sizes under impact energy of 6J, 8.3J and 10J The comparison of deformation area ratio (R) and the depth of deformation ratio (D) for the modes from sma to big ce size are shown in Figure 7-6(a) and (b). The impact energy are various at 6J, 8.3J and 10J. At first instance, it can be seen that the deformation area ratio increases when the impact energy is getting higher, and so does the depth of deformation ratio are the same. This is understandabe as more impact energy causes more structure deformation accordingy. Secondy, it is noticed from Figure 7-7(a) that the deformation area ratio for sampes with big ce size (8L6P) are generay arger comparing to the rest sampes under various impact energies and this indicates that big ce sized honeycomb composite modesare more easy to be damaged during the impact. The FE resuts for the depth of deformation ratio have been compared with experiment resuts in Figure 7-7(c) too, and generay, for the sampe of 8L4P60 and 8L5P60, the FE resuts are ony sighty ower than experiment resuts. Whie for 8L3P60 and 8L6P60, the simuated the depth of deformation ratio is smaer than the experiment 204

205 resuts. However, for both resuts from FE and experiment, it states that 8L6P60 encounters the deepest vertica dispacement and this caused arge deformation area which has been found in Figure 7-6(a) Combining with deformation area ratio and the depth of deformation ratio for 8L4P60 and 8L5P60 in Figure 7-7 (a) and (b), it seems that modes with medium ce size (8L5P60) has had a reativey smaer deformation area ratio whie their depth of deformation ratio is simiar. In another words, it means that if there is an object which impacts the mode and strikes up to a simiar distance, it wi cause ess damage if the ce size of the mode is medium sized. The resuts from previous experiments in Chapter 5 (Section ) aready states that sampes with a medium ce size ike 8L5P60 is recommended to be used in the appication as it shows up a considerabe force attenuation and energy absorption capabiity with a reasonabe materia handing property. Here, the FE resuts further noticed that 8L5P60 encounters ess damage under impact situation. Deformation area of modes with different opening ange The photographs of deformation area for the modes with different opening ange are isted in Tabe 7-6. When the ce opening ange is ess than 60, the deformation area forms an obvious trapezoida shape. However, it is ceary shown that when the ce opening ange exceeds 75 the deformation area is more or ess cose to a rectanguar shape. This indicates that when the opening ange is arge enough, not much of the impact energy is absorbed by the bucking rigidity of the ce was. Instead, much of the impact energy is passed through the ce was to do work on the other side of the modes. For the sampe of 8L6P90, the deformation area is tiny and this means when the ce opening ange enarges up to 90, the mode is very stiff and hard to deform. Simiar findings have been mentioned in the experiment resuts discussion too in Section too. 205

206 Deformation Area Ratio (%) J 8J 6J L6P30 8L6P45 8L6P60 8L6P75 8L6P90 Figure 7-7 Comparison of deformation area in modes with different opening anges From Figure 7-7, it is noted that 8L6P75 and 8L6P90 provide a smaer deformation area ratio than that in the rest of modes whatever the impact energy is 6J, 8.3J or 10J. This is due to the fact that the shape of the ce in modes of 8L6P75 and 8L6P90 is cose to rectanguar and bucking becomes the main form of deformation. Obviousy, the bucking rigidity of the ce was is much arger than the bending rigidity. Under a impact eves, the performance of 8L6P60, 8L6P75 and 8L6P90 is reativey simiar whereas for 8L6P30 and 8L6P45, the performance is more different, with arger impact energy causing more deformation. It indicates that for arger opening anges the impact energy eve woud not cause too much difference in the structure damage. It must be noted that the deformation area is ony the cross-section of the concaved deformation. A more accurate concusion shoud be drawn from the 3D modes. In contrast, if the opening ange is sma, the deformation area is more sensitive to the impact energy eve. This phenomenon is worth of further exporation. 206

207 Deformation Area Ratio (%) Deformation area of modes with different wa ratio ( b f ) b (i) 1: 8L3P60, 8L(4+3)P60, 8L(6+3)P60 f J 8J 10J L3P 8L(4+3)P 8L(6+3)P Figure 7-8 Comparison of deformation area ratio of modes with different ce wa ratio b ( 1). f Figure 7-8 shows a direct comparison of cyinder impact at three different impact energy eves and the modes are designed with the bonded and free wa ength ratio b more than one ( 1). It seems the modes with the ength ratio of 4:3 has encountered f the east damage whatever the impact energy various. 207

208 Deformation Area Ratio (%) Referring to the experiment resuts in Section , it has stated that with the increase of b f, the impact moduus of the sampes reduces which eads to a better force attenuation performance. However, the energy absorption between these three sampes is simiar. Here, from the FE resuts, it further reveas that the modes with a considerabe b f which is more than one and ess than two wi show a better damage toerance than the rest modes. This information again indicates that by modifying the bonded and free wa ength, it can optimize the mechanica performances of the textie honeycomb composites sufficienty. b (ii) 1: 8L6P60, 8L(4+6)P60, 8L(3+6)P60 f 30 6J J 10J L6P 8L(4+6)P 8L(3+6)P Figure 7-9 Comparison of deformation area in modes with different ce wa ratio b ( 1) f 208

209 b The numerica comparison of the deformation area ratio among the modes with 1 is iustrated in Figure 7-9. It can be seen that the mode with a medium bonded and free wa ength ratio of 4:6 demonstrates a east deformation area ratio under three different impact eves. The reasons for this performance are not cear at the moment and further investigations are needed in the future research work. And according to the experiment resuts in Section , there is not a significant reationship between the structure deformation and their transmitted force or energy absorption performance for the b subgroup with 1 either. f f History of dynamic contact force The present section investigates the contact force response of honeycomb modes under cyinder impact in order to obtain information how the different geometric design of the mode resists to the impact oads. Figure 7-10(a) to (d) show the history of dynamic contact force against time for the modes with different ce size, opening ange and ce wa ength ratio subject to the impact energy at 8J. The reference points are taken from the impact centre at the top surface of the mode and the average vaue has been cacuated. It can be seen that in genera, the contact response of the cyinder impact demonstrates an initia peak contact force and the curve fuctuate towards the end of the impact. This is because the rigidity of the materia provides resistances to the impactor and there are different yied stresses which are required to deform the structure and vaue of the stress are different depends on the structure response of the mode. In 2D panner FEA, the ce was are bucked and pastic-eastic deformations are occurred during the impact, which caused the ce was touched and cued to each other (Tan and Chen, 2005; Yu and Chen, 2006). 209

210 Contact Force(KN) Focusing on the cyinder impact mode in Figure 7-10(b), it can be seen that a the curves foow a simiar trend at eary stages of impact and the trends become more different towards the end of the impact whist 8L6P75 and 8L6P90 are exceptions and their peak contact force appears much ater than the rest modes. The opening ange of 75 and 90 provides a much stronger structure resistance to the impact due to the rectanguar shaped ces are mainy deformed by bucking mechanism and correspondingy, the mode must response in a different way to the impact oad comparing to others. And at the beginning of the impact, the mode responses to the impact whie the structure may not have experienced a arge amount of deformation, which deays the appearance of peak contact force accordingy. FE Contact Force (Different Ce Size/8J) Time(ms) 8L3P60 8L4P60 8L5P60 8L6P60 (a) Modes with different ce size 210

211 Contact Force(KN) Contact Force(KN) FE Contact Force(Different Opening Ange/8J) TimeI(ms) 8L6P30 8L6P45 8L6P60 8L6P75 8L6P90 (b) Modes with different opening ange FE Contact Force(Wa Ratio>=1) L(4+3)P60 8L(6+3)P60 8L3P Time(ms) b (c) Modes with 1 f 211

212 Contact Force(KN) FE Contact Force(Wa Ratio<=1) L(3+6)P60 8L(4+6)P60 8L6P Time(ms) b (d) Modes with 1 f Figure 7-10 Dynamic contact force of modes under the impact energy of 8J Figure 7-11 rearranges the modes according to their peak contact force under the impact energy of 8J. It can be seen that generay, modes with smaer ce size such as 8L3P and 8L4P provide higher peak contact force than the modes with arger ce size and this indicates that specimen with smaer ce size are more difficut to be deformed. The experiment resuts in Section aso reveaed the simiar materia properties as above because it is found that sampes with smaer ce size such as 8L3P and 8L4P had a higher impact moduus and their materia handing property is very rigid, which cause the sampes more difficut to be deformed. 212

213 Peak Contact Force(KN) Peak Contact Force (Cyinder Impact/8J) Figure 7-11 Peak contact force from cyinder impact b Figure 7-11 aso reveas that generay, modes with 1 f requires a higher force to be b deformed than the modes with 1 f and this indicates that onger bonded wa coud generate higher resistance to the impact Energy absorption performance 213

214 Energy Absorption Ratio (%) FE Resuts Experiment Resuts 0 Figure 7-12 Vaidation of energy absorption between FEA and experiment resuts According to the strain energy density (e) from Tabe 7-5, the absorption energy (E) from FEA are cacuated according to Equation 7-5. The energy absorption ratio (%) from FEA is worked out by diving the absorpted energy (E) with the initica kinetic energy (K), which is 8.31J in the current FEA work. The energy absorption ration (%) from experiment is isted in Tabe 5-1. These two energy absorption ratio from FEA and experiments are compared with each other in Figure Generay, compared with experiment resuts from impact tests in Figure 7-12, the energy absorption vaue from FEA has a good agreement on tendency expect for 8L6P90. In FEA, the mode is ideaized and the opening ange of the mode is exacty 90, in fact, in the rea experiment, the opening ange of the sampe is approximatey around 90 which is shown in Figure 4-4. This factor wi ead to the difference of energy absorption obtained from experimenta and FEA Comparison between ba and cyinder shaped impact 214

215 The foowing section wi investigate the difference between ba impact and cyinder impact. The impact energy for ba and cyinder impact is both set as 8.3J with initia impact veocity at 5.5m/s and the impactor mass is 0.55kg. Discussions wi be focus on it mechanica performance incuding contact force response and energy absorption performance. Schematic iustration of ba impact deformation Take group of sampes with different ce size (8L3P, 8L4P, 8L5P, 8L6P) as an exampe, generay, the deformation of ba impact is shaow than the cyinder impact, which is shown in Tabe 7-5 and Tabe 7-9, whist the contact area of ba impact is arger than the cyinder impact and this is mainy because the curvature of the ba edge can easiy touches the ce wa during the impact and there are mainy ine to ine deformation occurs. In the cyinder impact, the two corners on the bottom of the impactor touches the ce wa firsty which force the ce wa bending unti the whoe bottom ine of the impactor touches the origina or bended/bucked ce to continue the deformation. The schematic iustration of the ba and cyinder impact deformation process is isted in Figure

216 Tabe 7-9 Effect of ce size on its maximum dispacement and energy absorption for textie honeycomb composite modes under ba impact Impactor Type: Ba Mode Origina Shape 6J 8J 10J 8L3P60 Deformed Pattern Deformation depth ratio (%) e ( 10-6 ), J/mm L4P60 Deformed Pattern Deformation depth ratio (%) e ( 10-6 ), J/mm L5P60 Deformed Pattern Deformation depth ratio (%) e ( 10-6 ), J/mm L6P60 Deformed Pattern Deformation depth ratio (%) e ( 10-6 ), J/mm

217 Deformation process for ba and cyinder impact It is vita to understand the deformation process when the textie honeycomb composites are impacted by different shaped objects. The contact force response for cyinder and ba impact is shown in Figure 7-13 and they are taken at 8.3J impact for the mode of 8L3P60. In order to investigate the impact oad transfer and the deformation of cyinder and ba impact, Figure 7-14 speciay give a virtua views of the ce deformation process. The deformation views are taken at 8J impact. The different time steps are seected concerning characteristics deformation steps which corresponding to the points on the contact response diagram with [ ], are shown in Figure From Figure 7-13, it is seen that the individua contact curves are simiar in their trends at the beginning of the impact up to 4ms, but foows different rates. The peak contact force is higher for cyinder impact than in the case of ba impact. In other words, a cyinder shaped impactor penetrated more oading force at the beginning of the impact and this coud cause a faster rate of deformation, which may ead to a faster energy input to the mode at the initia stage of the impact and a different effect on the protection eve. By putting cyinder and ba impact contact response together in Figure 7-13, it is ceary that both of them can reach their peak contact force at around 2ms and subsequenty the contact force of ba impact starts to decrease then suddeny it increases from between 4ms and 5ms. Referring to the configuration in Figure 7-14(b) at 6ms, the ba impactor came in touch with the bonded wa of the ce in the centre of the ayer2 and this coud increase the contact force accordingy. The contact force curve went smoothy down unti it meets the configuration [5] and [6], it sighty went up and down again and virtuay this coud be the crushed ce wa touched the corner of the ce on the next ayer. 217

218 Figure 7-13 Comparison of contact force-time response of 8L3P60 under 8j by cyinder and ba impact [0] 0ms [1] 2ms 218

219 [2] 4ms [3] 6ms [4] 8ms [5] 10ms 219

220 [6] 12ms [7] 13ms (a) By cyinder impact (b) By ba impact Figure 7-14 Comparison of structure deformation under dynamic impact for mode 8L3P at 8J impact (a) by cyinder impact (b) by ba impact Contact response of ba impact Regarding the contact response of ba impact in Figure 7-15, it is noticed that the response of honeycomb structure to the ba impact doesn t perform identica as that of cyinder impact, which is shown in Figure The difference between this two types of impact is that the change of oading force is more frequenty during the ba impact and this causes more fuctuation in the contact response in Figure The curvature of the ba is the reason to cause this as it touches the ce wa differenty comparing to cyinder impactor. The curvature eads to the fact that more bending deformation occurs and it provides more opportunity for the deformed ce wa to contact each other which various the oading force step by step. 220

221 Contact Force(KN) Contact Force(KN) Contact Force Contact Force(KN) The contact force are characterised by a higher fuctuation in the ba impact, which indicates that more ce materia is invoved for stopping the impactor and this coud ead to a wider energy dissipation and reduced the eve of contact forces and it finay wi decrease the forces transmitted underneath and ower acceeration too. This information from FE resuts is usefu because it can hep the researchers to predict the performance of textie honeycomb composite under ba impact in their future work. FE Contact Force(Different Ce Size/8J Ba) FE Contact Force(Opening Ange/8J Ba) Time(ms) 8L3P60 8L4P60 8L5P60 8L6P Time(ms) 8L6P30 8L6P45 8L6P60 8L6P75 8L6P90 FE Contact Force(Wa Ratio>=1/8J Ba) FE Contact Force(Wa Ratio<=1/8J Ba) L(4+3)P60 8L(6+3)P60 8L3P L(3+6)P60 8L(4+6)P60 8L6P Time(ms) Time(ms) Figure 7-15 Dynamic contact force of modes under ba impact at 8J 221

222 Strain Energy Density (J/mm3) Energy absorption performance of ba impact Ba Cyinder 5 0 8L3P 8L4P 8L5P 8L6P Figure 7-16 Comparison between ba and cyinder energy absorption capabiity As anaysed in the above section, the impactor shape infuences the deformation pattern of the modes, therefore, their energy absorption performance is supposed to be different correspondingy. Figure 7-16 compared the strain energy density (e) between the ba and cyinder impact among the modes: 8L3P, 8L4P, 8L5P and 8L6P. Their impact energy is around 8J. However, from the figure, it is hard to find a trend to te whether the mode under ba impact wi absorb more or ess energy than cyinder impact and this need further investigation in the future. Nevertheess, from Figure 7-13 and 7-14, it can be seen that the cyinder impactor provides the faster strain energy input than the ba impactor and it is ikey that the rate of strain energy induction is reated to the contact area of the impactor, of which the cyinder impactor touches the surface by point to ine touch whie the ba impact is ine to ine touch. And aso the arger the contact area the wider distributed is the ce deformation, which eads to more pastic strains generated within a short period of time when comparing with an impactor with sma contact area. The deformation anaysis aso showed that the cyinder impactor deforms the structure faster than the ba impact 222

223 (Figure 7-13) and this proves that the arger is the contact area, the faster is the strain energy induction. Athough it is difficut to make any reiabe concusions at the current stage as it seems that an increase in contact area normay eads to an increase in impact energy oss caused by friction, therma and other forms of energy conversion, generay, a faster strain energy induction shoud ead to higher acceeration of the item underneath, because more impact energy is exposed underneath within a shorter period of time. From this point of view, a cyinder impactor woud be more threatening to the human being due to higher acceeration caused Vaidation of the simuation resuts with experiment resuts In the current section, the simuated resuts are put together with correspondent experiment resuts to seek out the simiarity of them. Figure 7-17 demonstrates the contact force response of 8L3P60 from 2D FEA and compares it with the resuts from experiment, the impact energy are both around 8J with impact veocity at 5.5m/s. It seems the contact force-time curve shares simiarities and their peak contact forces both cimes up around 0.65KN. The striking time for red curve (simuated resut) is onger than that of bue curve (experiment resut). And the simuated contact force has generated a second ower peak contact force whie there is ony one significant peak contact force in the experiment curve. The reason to expain this kind of difference coud be that FEA in 2D is a much simpe way to evauate the mechanica performance of the structure deformation comparing to the rea situation, therefore, a perfect match between experiment and simuation resuts wi not be justifiabe due to high compexity in the experiment testing. 223

224 Contact Force(KN) Contact Force (8L3P60) Experiment Simuated Time(ms) Figure 7-17 Comparison of contact force between experiment and simuated (2D) resuts for 8L3P FEA of 3D Textie Honeycomb Composites Creation of the geometric modes In this section, three sampes with different ce sizes are modeed in 3D to further vaidate their mechanica performance with experiment resuts. The actua sampes of textie honeycomb composites used for experimenta anaysis have been described in Section The geometric modes for FEA are created based on the fact that the impact is oaded in the centre of the top surface of the honeycomb mode. Figure 7-18 shows a quarter of the created mode. 224

225 Figure 7-18 Created honeycomb mode Due to the tremendous demand on computing resources and timing consuming in running the modes, i.e. one 3D honeycomb mode can take up to 3 days to finish one cacuation, it was decided that FEA wi ony be conducted for mode 8L3P60, 8L4P60 and 8L6P60. The dimensions of the modes and their ce parameters, and the dimension of the cyinder impactor are isted in Tabe 7-10 and Tabe Tabe 7-10 Dimension of cyinder impactor Impact Type: Cyinder Dimension (mm) Radium 15 Height