Comparison Study and Mechanical Characterisation of a Several Composite Sandwich Structures

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1 International Journal o Composite Materials 015, 5(1): 1-8 DOI: /j.cmaterials Comparison Study and Mechanical Characterisation o a Several Composite Sandwich Structures Jamal Arbaoui 1,,*, Yves Schmitt,3, Jean-Luc Pierrot, François-Xavier Royer 1 ENSTA Bretagne, MSN/LBMS/DFMS, rue François Verny, Brest, France LPMD/Université de Metz, 1 bld Arago, Metz, France 3 P.A. TECHNOLOGIES, 9 rue des Balanciers, Thionville, France Abstract Composite sandwich structures have excellent properties and they are widely used in the ields o high technology such as aeronautics and astronautics, etc. Investigations o the mechanical properties o composite sandwich structures play a vital role in determining their applicability in various engineering ields. In this study, we have developed a new core material, which is an original cellular composite based on polystyren cells named YmaCell. Bending and crash properties o YmaCell are determined and compared with a polypropylen honeycomb and a thermoplastic oam panel. The results show that the rigidity o the cellular composite YmaCell is better than that o the polypropylen honeycomb structures and the thermoplastic oam. The chemical bonds between the skins are likely to be a major actor or the higher perormances o the cellular composite. Keywords Composite, Short ibres, Sandwich structures, Crash, Bending 1. Introduction Sandwich panels typically consist o two thin ace sheets or skins and a lightweight thicker core. Commonly used materials or ace sheets are composite laminates and metals, while cores are made o metallic and non-metallic honeycombs, cellular oams, balsa wood or trusses. The ace sheets are typically bonded to the core with an adhesive, and carry most o the bending and in-plane loads. The core provides the lexural stiness and out-o-plane shear and compressive strength. The structural perormance o sandwich panels depends not only on the properties o the skins, but also on those o the core, the adhesive bonding the core to the skins, and the geometrical dimensions o the components. Sandwich composites with cellular core are widely employed in modern mechanical design, not only in the ield o aeronautical constructions, where they have been developed initially, but also in the ields o on-land transportation and marine constructions. Because o their characteristic eatures, such as the high lexural resistance and stiness [1], the high impact strength [, 3], the high corrosion resistance and the low thermal and acoustics conductivity [5, 6, 7], sandwich structures are in act preerred over conventional materials in various industrial * Corresponding author: jamal57010@yahoo.r (Jamal Arbaoui) Published online at Copyright 015 Scientiic & Academic Publishing. All Rights Reserved applications. Although large numbers o research projects have been perormed by various authors, the design o structural elements made rom sandwich composites is oten a diicult task. This is mainly because a reliable strength prediction needs the preliminary knowledge o the mechanical behavior o skins and core, as well as o the peculiar damage mechanisms [8, 9, 10], and ailure criteria that can be used under a complex loading. In the last years, the research and development on a large range o cellular composites has been explored. In nature cellular structures can be ound in plants and bird bones. Such structures have been reproduced so closely as possible to the natural ones. The basic principal or the production o such a materials is the association o a sophisticated system o sti-walled, tree-dimensional cells with a short-ibred composite material. In this paper a sandwich element with a novel cellular core named YmaCell are developed. Bending and crash properties o YmaCell are determined and compared with a polypropylene honeycomb and a thermoplastic oam panel. The panels are covered with dierent composite walls.. Sandwich Material A typical simply supported sandwich panel consists o two thin aces with a thickness o t, separated by a light and a weaker core o thickness h c, as illustrated in Fig.1. The overall depth o the panel is h and the width b. The aces are typically bonded to the core to provide a load transer

Jamal Arbaoui et al.: Comparison Study and Mechanical Characterisation o a Several Composite Sandwich Structures mechanism between the main components o the sandwich panel. Figure.

A structure o a sandwich composite [11] The lexural rigidity or a sandwich beam, denoted as D, is the sum o the lexural rigidities o the aces and the core measured with respect to the centroidal axis

2 Jamal Arbaoui et al.: Comparison Study and Mechanical Characterisation o a Several Composite Sandwich Structures mechanism between the main components o the sandwich panel. Figure. The our-point bending test [13] 3. Experimental Procedure Figure 1. A structure o a sandwich composite [11] The lexural rigidity or a sandwich beam, denoted as D, is the sum o the lexural rigidities o the aces and the core measured with respect to the centroidal axis o the entire section and can be expressed as: 3 3 E bt E btd Ecbhc D = + + = D + D0 + D 6 1 Where E and E c are the Young s modulus o the ace sheet and core, and d = t + h c. D is the bending stiness o a ace sheet about its own neutral axis, D 0 is the stiness o the ace sheets associated with bending about the neutral axis o the entire sandwich, and D c is the stiness o the core [1]. Since the core is sti in shear but sot generally, its Young s modulus is much smaller than that o the ace sheet. By assuming E c <<E and the ace sheets are thin, then 3 3 ( hc ) b h D E () 1 The shear stiness Q is given by equation (3): Q G c bh ( t) h c c (1) = (3) The ace stress is given by equation (4): P( L L σ 1) = (4) tbd In the core the shear stress is given by equation (5): Pmax τ c max = (5) bd The elastic delection w t or a sandwich beam at ( L loading points L1) is the sum o the lexural and shear delections, or a our-point bending (Fig..). P( L L1 ) ( L + L1) P( L L1 ) w t = w1 + w = + (6) 4D S 3.1. Material and Specimens The structural composite sandwich beams used in our experiment are made up o three core materials provided by PA Technologie, Fig. 3: - AIREX Foam; - Honeycomb polypropylen; - YmaCell is an original cellular composite based on polystyrene beads coated by epoxy resin which can be loaded with dierent short ibers or nanoparticles. Figure 3. Schematic o the cellular cores, (a) YmaCell, (b) poplypropylen honeycomb and (c) AIREX thermoplastic oam Characteristic Table 1. Characteristics o the cellular cores Polypropylen honeycomb AIREX oam YmaCell Polystyren cells Shear modulus (MPa) Shear stress (MPa) Density (kg/m 3 ) These cellular cores are covered with dierent composite skins: - Roving 3 Folds: Laminates based on epoxy resin and glass roving iber consists o 3-olds and 300 gm -. The overall iber volume raction is 8% or the composite, - T800/M300 ±45 and 0 /90 : Laminates with glass iber MAT (300 gm - ) and complex woven roving (800 gm - ) reinorced with epoxy resin. The overall iber volume raction is 8% or the composite. The ibers directions 0 /90 and ±45 correspond to the arrangement o the roving ibers relative to the loading direction, - Carbon Folds: Laminates based on epoxy resin and carbon iber consists o -olds and 500 gm -. The overall iber volume raction is 8% or the composite, - Twintex: The abric made o E-glass iber and polypropylene resin.

International Journal o Composite Materials 015, 5(1): 1-8 3 The sandwich panels used in these experiments were abricated using the VARTM process, Fig. 4.

Characteristics o various composites used in this work In order to characterize and compare the mechanical properties o the dierent cellular cores, experimental test series were conducted on a our

3 International Journal o Composite Materials 015, 5(1): The sandwich panels used in these experiments were abricated using the VARTM process, Fig. 4. Table summarises the thickness and mechanical properties o various composites used in this work. 3.. Mechanical Testing Bending Test Table. Characteristics o various composites used in this work In order to characterize and compare the mechanical properties o the dierent cellular cores, experimental test series were conducted on a our point bending machine. The bending test is done with respect to the NF norm using an INSTRON BE09 machine, Fig. 5. To check the reproducibility o the results, ive beams by composite type were tested. The crosshead displacement rate was 3mm/min. The sample dimensions are grouped in Table 3. Characteristic Roving 3 olds T300/M (±45 ) Carbone olds TWINTEX Young modulus (MPa) Shear modulus (GPa) Poisson's ratio Tensile strength (MPa) Laminate thickness (mm) Figure 4. Schematic o the VARTM Process Table 3. Specimen dimensions Length, L (mm) Span, b (mm) Skin Thickness (mm) Core Thickness (mm) distance between inner supports L 1 (mm) Distance between outer supports L (mm) Figure 5. INSTRON BE 09 machine used or the bending test

4 4 Jamal Arbaoui et al.: Comparison Study and Mechanical Characterisation o a Several Composite Sandwich Structures 3... Crash Test For the crash test a speciic apparatus has been developed in our laboratory, Fig. 6. The impactor is a system with interchangeable masses (rom 440 g to 4 kg) with a steel sphere o 10 mm de diameter as bloc proile, the drop height varying rom 10 to 85 cm. Impact energies up to 30 J can be developed with this system. Samples have been cut out in dierent areas o the panels to veriy the mechanical homogeneity and the eiciency o the production conditions. The dimensions o the specimens were chosen as 60 x 60 mm. The tests were perormed on a non-instrumented machine. Comparator measuring apparatus was used to record the indentation length o the sandwich material. Figure 6. Crash test 4. Results and Discussion 4.1. Bending Results Fig.7 shows the results rom the our-point bending tests or the three core materials: YmaCell alone, YmaCell reinorced with a short carbon ibers and YmaCell reinorced with a short glass ibers. The skins are made o the Twintex (thermoplastic composite composed the glass iber and polypropylene resin). These panels have been produced to study the eect o short ibers additions in the YmaCell core (0% o glass ibers and 4% o carbon ibers). From this igure, the bending behavior is similar and can be described in three principal phases: the irst phase is initial linear elastic behavior ollowed by a phase o nonlinear one in which the maximum loading is reached. In the last phase, a reduction in the load applied is observed till the total rupture o the samples. The linear behavior corresponds to the work o the skins in traction and compression, whereas the nonlinear behavior mainly depends on the core properties under the eect o the shear stress. This igure clearly shows the eect o the adding reinorcements in the core material. The lexural stiness is improved at the act that the reinorcements increase the shear modulus o the YmaCell. Figs. 8 and 9 represent the three retained cores with two dierent walls, roving 3 olds and carbon olds. I the general behavior appears to be the same, the ratio loading/displacement and the bending strength are dierent. The sandwich with carbon olds skin has higher bending properties than the roving 3 olds. For two lower the curves the walls are stuck on the cores. For the other one, the walls are impregnated with the same resin as used to prepare the composite mixture. These results show also that the YmaCell presents the high lexural rigidity relative to that o the polypropylen honeycomb and thermoplastic oam AIREX. Comparison the maximum load o the sandwich with carbon iber olds skin, we ound 400 N or YmaCell core, 50 N or Polypropylen honeycomb and 150 N or AIREX oam. This is can be explained by the presence o chemical bonds between the skins and the core contrary to the other structures where the bonding is simple. Fig. 10 show the results obtained with an YmaCell core covered with two dierent walls. These walls consist o the same thickness but the Young modulus dierent. It is observed that the increase o the Young modulus osters the increase o the bending stiness which given by: D = EI (7) Where D is the bending stiness, E is the Young modulus and I is the Moment o inertia. In Fig. 11, we keep the same thickness, the same Young modulus but we have introduces an orientation actor o the ibers in the skin. The bending stiness obtained in the case o perpendicular iber is higher that the iber with ±45 orientation. This is can be explained by the Changes o orientation actor which is ½ or T800/M300 0 /90 ibers and ¼ or T800/M300 ± Crash Test Results The Figs. 1 and 13 illustrate the same behavior during a crash test o three cores: YmaCell, AIREX oam and polypropylen honeycomb which is covered with two types o composite skins, roving 3 olds and carbon olds. It is observed that the structures which have the high shear stiness are the most indented. This allows us to conclude that the increase o the core shear modulus or skin lexural modulus promotes the increase o the contact energy and thus the indentation. The impact response o our structures is suitable in comparison with other structures [14]. From Fig. 14, it is ound that the same behavior or two cores which are YmaCell and polypropylen honeycomb covered with dierent skins (T800/M300, roving 3 olds, carbon olds and twintex). It is noted that the structures which have the high lexural stiness are the most indented. It absorbs low energy during bending which leads to conclude that the kinetic energy o the projectile is transormed mainly into indentation energy. However, the indentation values vary between.5 and 3 mm or the structures with YmaCell core, while or the structures with polypropylen honeycomb, the indentation values vary between 0.7 and 1.5 mm. Then, the indentation length is more important or YmaCell than or polypropylen honeycomb. This allows us to justiy the results obtained and outlined above in Figs. 1 and 13.

5 International Journal o Composite Materials 015, 5(1): Figure 7. Bending 4 points o the YmaCell alone and YmaCell reinorced with short carbon and glass iber Figure 8. Bending comparison o the cores with a roving 3 olds wall Figure 9. Bending comparison o the cores with a carbon olds wall

6 6 Jamal Arbaoui et al.: Comparison Study and Mechanical Characterisation o a Several Composite Sandwich Structures Figure 10. Bending comparison o the YmaCell covered with two dierent skins Figure 11. Bending comparison o the YmaCell covered with two same skins having dierent orientations Figure 1. Crash test comparison o the cores with a roving 3 olds wall

7 International Journal o Composite Materials 015, 5(1): Figure 13. Crash test comparison o the cores with a carbon olds wall Figure 14. Crash test- indentation comparison o the various structures 5. Conclusions In this paper, the core material, which is an original cellular composite, based on polystyren cells called YmaCell is developed and compared with a polypropylen honeycomb and a thermoplastic oam panel which already exists in the composite industry. Bending and crush properties o these materials are determined and compared. Some speciic conclusions drawn rom the bending and crash test are the ollowing: - Adding glass or carbon ibers allows to increase strongly the maximum loading charge. - The intended bending perormances (YmaCell better than the other composites) are attained. The bending properties are higher or the dierent walls. On the other hand, the crash energies are lower than those measured or the thermoplastic oam. - YmaCell has perormances close to or better than AIREX oam with T800/M300 0 /90 walls. - The rigidity o the cellular composite is better than that o the honeycomb structures. - The YmaCell also enables to obtain very powerul chemical bonds in comparison with other core materials. These experimental results will be used to develop numerical models using inite element method (FEM) calculations. ACKNOWLEDGEMENTS This work was supported by the scientiic department o REGION LORRAINE.

8 8 Jamal Arbaoui et al.: Comparison Study and Mechanical Characterisation o a Several Composite Sandwich Structures REFERENCES [1] Vinson J.R., 1999, The behaviour o sandwich structures o isotropic and composite materials, Technomic, Westport, [] Mines R.A.W., Worrall C.M., 1998, Gibson A.G. Low velocity peroration behaviour o polymer composite sandwich panels, Int. J. o Imp. Eng., 1, ,. [3] Torre L., Kenny J.M., 000, Impact testing and simulation o composite sandwich structures or civil transportation, Compos. Struct., 50, [4] Kootsookos A., Burchill P.J., 004, The eect o the degree o cure on the corrosion resistance o vinyl ester/glass ibre composites, Compos. A, 50, [5] Allard J.F., 1993, Propagation o sound in porous media: modelling sound absorbing materials, Elsevier Applied Science. [6] Sahraoui S., Mariez E., Etchessahar M., 001, Mechanical testing o polymeric oams at low requency, polym. test., 0, [7] Kalaprasad G., Pradeep P., George M., Pavithran C., Sabu Thomas., 000, Thermal conductivity and thermal diusivity analyses o low-density polyethylene composites reinorced with sisal, glass and intimately mixed sisal/glass ibres, Compos Sci. and Techn., 60, [8] Anderson M.S., 1959, Optimum proportions o truss core and web- core sandwich plates loaded in compression, NASA TN D-98. [9] Steeves C.A., Fleck N.A., 003, Collapse mechanisms o sandwich beams with composite aces and a oam core, loaded in three-point bending. Part I: analytical models and minimum weight design, Mech. Sci., [10] Steeves C.A., Fleck N.A., 004, Material selection in sandwich beam construction, Scri. Mat., 50, [11] Arbaoui J., 009, Etude comparative et caractérisation mécanique des structures sandwiches multicouches, PhD, Université de Lorraine, Metz. [1] Allen H.G., 1961, Analysis and design o structural sandwich panels, London: Pergamon Press. [13] Arbaoui J., Schmitt Y., Pierrout J.L., Royer F.R., 014, Numerical simulation and experimental bending behaviour o multi-layer sandwich structures, J. o Theo. and Appl. Mech., 5, [14] Anderson T., Madenci E., 000, Experimental investigation o low-velocity impact characteristics o sandwich composites., Compos. Struct., 50,

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