EXPERIMENTAL AND NUMERICAL ANALYSIS OF INSERTS IN SANDWICH STRUCTURES.

Transcription

1 EXPERIMENTAL AND NUMERICAL ANALYSIS OF INSERTS IN SANDWICH STRUCTURES. P. Bunyawanichakul*, B.Castanié**, J-J. Barrau** * IGMT, LMS Sup Aéro, BP Toulouse Cedex 4 ** IGMT, LGMT, Université Paul Sabatier, Bat 3PN, 3162 Toulouse Cedex (Corresponding author : bruno.castanie@supaero.fr) ABSTRACT In aeronautics, sandwich structures are widely used for secondary structures like flaps or landing gear doors. In the case of landing gear doors, the junction is made by a local reinforcement called an insert. This insert is made by a resin molded in the Nomex TM sandwich core. Such structures are still designed mainly using test results and the lack of an efficient numerical model remains a problem. The purpose of this study is on the one hand to perform experiments in order to be able to identify the failure modes and on the other hand to propose an efficient numerical model. Pull-out tests with cycling were conducted and 3D displacement measured by optical methods. The potential failure modes are numerous (delamination, local fiber breaking, skin/ core debonding, core crushing, core shear buckling, potting failure, etc.). Experiments demonstrated that, for the lower loads, the non-linearity and the hysteresis are mainly due to core shear buckling. From this observation, the nonlinear behavior of the core is identified by a 3 point-bending test. The shear-modulus damage law is then implemented on a non-linear finite element model and an acceptable correlation of the tests is achieved. As a consequence, some improvements of the technology will be proposed, manufactured and tested. Keywords : Insert; Potting, Junction; Sandwich structure 1. INTRODUCTION Sandwich structures consist of two thin, stiff, and strong faces which are separated by a thick, light and weaker core. This assembly provides a very high stiffness-to-weight ratio and also a high bending strength-to-weight ratio. The flexural rigidity of sandwich structures is enhanced without any additional weight. Their facesheets do not buckle even under quite high compressive stress [1]. Aircraft structures use a number of sandwich piece owing to the major requirement of weight-saving and a smooth surface. However, the core material is normally weak. Its low strength causes problem at the junction with the main structure of the aircraft. The failure can arise due to the great variation of stresses along discontinuous materials. It is then necessary to locally reinforce this weak point with materials stronger than the core alone. This reinforcement is called an insert. Insert technology is various and each type of insert has its own behavior [1-12]. However, inserts can be divided into two main types: partials inserts and through-thickness inserts. Both type have their own advantages. The first leaves one surface free but its load-carrying ability is low. The selection of insert type depends on the application and how the inserts are to be placed. One can find low load carrying inserts for attachment of cabin safety equipment and the structural attachment of walls, shelves, etc. High-load inserts can be found for example in flap and landing gear door junctions. The loads transferred by inserts are normally of 3 types: transverse, shear and torque. There are only few published works on inserts and most focus on transverse load. Besides, the available literature shows a lack of calculation methods and most inserts are firstly designed by simple formulas [1] and then validated by tests.this paper will study the use of inserts in a landing gear door under transverse load. The insert used in landing gear doors is a potted insert based on epoxy resin mixed with a modified amine in the same weight ratio. This resin mixture is injected into honeycomb cells in the vicinity of the circular load hole without cutting any cell walls of the core. In this study, sandwich and specific specimens were manufactured and tested under pull-out in order to identify the nonlinear

2 phenomena and the different types of damage. A damage scenario will then be proposed and a related finite element model made. The numerical model is validated by comparison with experimental results. 2. EXPERIMENTAL STUDY 2.1 Specimen material and preparation Two identical face sheets were manufactured from carbon fibre/epoxy tissue (G83/914) with quasi-isotropic layups [(/45// 45) 4 ] s. The thickness of the composite laminates was 2.6 mm. The equivalent elastic modulus of these plates, E x, was equal to 42 GPa. The core was made of NOMEX TM honeycomb 4.8 mm cell size, 48 kg/m 3 density and 2 mm thickness. The potted inserts was made with 3M Scotch-Weld TM 35-2B/A cured under 175 C for 1 hour (Figure 1). Then, the two skins were glued, with REDUX 322 film with the honeycomb. The specimen geometry was 14 x 14 x 25.2 mm (Figure 1(c)). The center of the specimen was drilled to put a countersunk titanium fastener (diameter 6.35 mm) for the pull-out test. A A A - A Potted insert (a) (b) (c) Figure 1 : (a) Potted insert injection, (b) 2 samples of cured potting and (c) Pull out test specimen. 2.2 Prestress and testing procedure A specific support tool was designed to fit into the machine (see Figure 2). The tool was composed of two steel plates linked by four rods. The upper plate had a 6 mm diameter hole that was drilled to support the specimen and to fit to the fork (see Figure 3). The lower plate was fixed to the testing machine by 6 bolts. The installation of the fastener required some prestressing obtained from applied torque. The preload generates residual stress within the specimen to modify the apparent stiffness of the structure. Fasteners should be tightened with a torque wrench to provide prestress before loading the insert. However, it remains difficult to know the effective pretension that is really generated during this operation. Therefore, experiments were also conducted to study the influence of different pretension procedure. For the tests presented, the specimen was installed as Figure 2 and was firstly loaded by the end of the fastener (see Figure 3). At the preload value, the metal part is just put in contact with the specimen and the nut is hand turned. Then, decreasing the loading, the specimen becomes prestressed. A set of experimental investigations concerning the preload at 4 N, 6 N and 1 N was carried out. The specimens were statically loaded by the fork using an Instron universal testing machine at a constant displacement rate of.5 mm/min until ultimate failure load. All experiments were performed at room temperature (~25 C) and ambient relative humidity (~65%).

3 Support displacement Specimen surface with adapter support Tooling potting strain gauge c b a D optical camera View from bottom Figure 2 : Tooling for the test and positioning of the strain gauge on the specimen surface. Prestress loading Pull-out loading Nut Washer Pull-out loading Fastener Fork Upper Plate Specimen with potted insert Figure 3 : Prestress installation and loading method. To be able to measure the displacements of a specimen s surface, a 3D optical camera is installed at the bottom of the tool. The camera is able to capture a 14 X 1 mm picture (more than half the specimen area) by 2 CCD cameras at 55 mm apart. For each stage of loading, the 3D coordinates of the specimen surface are calculated on the basis of digital image processing delivering the 3D displacement. It is possible that the tool can deform under high load so a dial comparator was also installed to measure displacement at the upper tool plate (see Figure 2).

4 A rosette was glued on the surface (its location is shown in Figure 2) to measure how strain is distributed on this zone. Strain gauges a, b c are relative to the orientations of carbon fibers at, 45 and 9 respectively. All the measurement data except the deflection of the specimen were continuously collected using a Yokogawa TM instrument. 2.3 Experimental results and discussion Specimens were quasi-statically tested until ultimate failure load. Failure was considered to have occurred when the specimen could not withstand any additional increase in load. The load-displacement curve of the first test specimen is shown in Figure 4 left. The displacement was plotted including a correction of the support displacement measured by a dial comparator installed as referred above. The curve was nonlinear considering the changes of slope while the load increases. It is also possible to plot the head of the screw penetration in the skin thanks to the optical measurement method (Figure 4 right). This is the result of the difference of displacement between a point on the fastener head and a point on the specimen surface in the immediate proximity of the head. According to two sets of results, the curve is globally nonlinear and may be divided into 4 parts (see Figure 4). The first area could be due to the setting up of contact between the fastener head and the specimen. The second part is nearly linear with no damages. Then, the load-displacement curve shows a global non-linear behavior due to damages. During the experiment, many noises into the specimen were heard from 55 N. The ultimate failure load was defined as the peak load 14 N with a z-displacement of 2 mm. The behavior looks like that of a perfectly plastic material and seems due to the penetration of the fastener head before the catastrophic failure. Figure 4 : Quasi-static loading and head penetration under pull-out test. After the experiment, the tested specimen was cut across its center at to analyze failure modes as shown in Figure 5. By visual observation, there are many failure modes existing in the specimen. For the skin below fastener head, the carbon fibre was broken by compression and shear. The potted insert was damaged in the upper zone by the stress higher than the allowable compressive. For lower zones of the insert, cracks appeared at 45. The honeycomb cell walls located near the insert were also damaged at 45. Thus, the failure modes identified are: skin delamination, local fiber breaking, skin/core debonding, core crushing, core shear buckling, potting failure in compression and shear. However, the results do not show how the damage occurs or the failure mechanism. skin delamination local fiber breaking skin/core debonding core shear buckling core crushing potting failure in shear potting failure in compression

5 Figure 5 : Final failure mode. 2.4 Investigation on the failure mechanism by C-Scan and optical microscopy To define the failure mode mechanism, a specimen tested only until 1 N was analyzed by ultrasonic C-Scan. This noncontact method can detect only defects in carbon-fibre composite faces. Figure 6 shows a result of a 39 x 41 mm image of the face sheet for the sandwich structure at the fastener head side. The large dark circular region at the centre of the image is the fastener head zone and the quasi-circular black trace presents the insert periphery. The carbon-fiber skin has no defects. Figure 6 : C-Scan image of a face sheet at the fastener head side. No detected skin delamination Core shear buckling Crack Figure 7 : -cuts showing failure of shearing cell wall near potting and image with scanning optical microscope. Another 1 N load specimen was cut to verify if skin delamination occurs or not. Only cell walls near inserts exhibit 45 fracture lines following the shear buckling of the core (Figure 7 left). The local area of the skin near the head of the screw was also observed by microscope camera with scanning. No delamination was detected in the skin. Nevertheless, this image shows that compressive insert failure occurred under the head of the screw area along the circular periphery of the insert. As both investigations show, the fastener transfers the load to the sandwich structure with no damage in the contact region even if the area is highly stressed. 2.5 Cycling test results after pretension and discussion The last investigation shows that damage to the honeycomb core and the potted insert occur early. Cycling tests were conducted to see how the rigidity behavior of the specimen varies and also the influence of the prestress load level. The load curves for prestress at 4N and 6N are shown in Figure 8 respectively curves (a) and curves (b) respectively. The load curves during pretensions are not presented here. Considering the load curves after prestress at 4 N (see Figure 8(a)), the first cycle at 2 N (blue) was carried out, then two cycles at 6 N (pink then brown), the third cycle at 8 N (blue) and finally the loading until the ultimate force was exerted (green). For the first cycle at 6 N, some noises were heard at approx-

6 imately 55 N which may be attributed to honeycomb cell wall buckling. Nonlinear behavior also occurs at this moment and hysteresis appears although no hysteresis was found for the first cycle at 2 N. The slope of the load curve also decreases which demonstrates loss of rigidity of the specimen and irreversible damage. The nonlinear contact behavior due to prestress does not seem to be involved here because of the hysteresis. The same kind of observation can be made for curves Figure 8 (b) with a specimen preloaded to 6 N. The specimen was tested in cycle at 2 N, twice at 4 N, twice at 6 N then, 8 N, 1 N and the last one was loaded until rupture. These tests and others for prestress at 8 N and 1 N demonstrate that there is a history effect with the prestress in the core Displacement of fastener head in Z-direction (mm) (a) Displacement of fastener head in Z-direction (mm) (b) Figure 8 : Load-displacement curves of cycle experiment after prestress at (a) 4 N and (b) 6 N. On Figure 9 left, the experimental load cycle curve also shows the behavior during prestress. During prestress (blue), a change of slope appears clearly at the same load around 5 N. This seems to correspond to the first failure mode of the sandwich. Then, the load curve increased continuously and quasi-linearly until the applied preload, here 95 N. At this stage, it can be seen that the displacement plotted comes from bending loading plus penetration of the fastener head. Then, the metallic part was put in contact with the specimen. The difference of slope during the decrease of loading is obviously due to the pretension. The fastener head remained in the hole about.35 mm depth. When the preload was finished, the specimen seemed to contain minor damage induced by this pretension method although it cannot be detected by visual observation. This specimen was continuously tested under pull-out cycling load at : 4 N, 6 N, 15 N and until ultimate failure. No hysteresis was found for loads under the preload value. The hysteresis curve appeared only when the force level was higher than the preload value (95 N). The specimen broke at a little less than 14 N The metallic part was put to contact with specimen here displacement of fastener head in Z-direction (mm) 1 Figure 9 : (left) Load-displacement curve of prestress at 95 N and cycling test, (right) analysis of the failure mechanism. Most experimental results displayed approximately the same ultimate load. Damage seemed to occurs under prestress loading at about 5 N. The first two failure mode are identified :

7 1] Honeycomb cell wall located near potted insert (zone 1 in Figure 9 right) may buckle under shear. 2] Potted insert under fastener head located near face sheet (point 2 in Figure 9 right) may break. Both failure modes seem to occur at the same load level. The following failure mode seems to be skin shearing which can lead to skin delamination and potting/skin debonding (zone 3 in Figure 9 right). Finally, the fastener head penetrates into the sandwich structure. At this step, a failure mechanism is proposed. However, no distinction can be made between the first two failure mode and further experiment should be conducted. 2.7 Special potting test A specific specimen was developed to study the resistance of potted inserts and skin and to investigate the failure modes without honeycomb. This small specimen is designed to fit into the same test tool and is made with the same potted insert materials except the lower skin which is replaced by aluminium (see Figure 1). The global load-displacement curve and protruding-head curve (penetration of the head) under the pull-out test were shown in Figure 1. Compressive potting failure is identified in the protruding-head curve by a change of slope at approximately 6 N. As previous analysis had shown that there was no damage at this load level in the skin, damage must occur in the potted insert. These experimental results confirm that cracks in the potted insert as in Figure 7 occurs early at about 6 N before skin delamination Fastener head displacement (mm) Figure 1 : Load-displacement curve and protruding-head curve of specific specimen Protruding-head displacment (mm) 2.8 Identification of the damage law of the honeycomb core Nonlinear behavior laws of the honeycomb and the potted insert are identified by two tests. The first law can be found by a 3- point bending experiment of sandwich beam specimens with aluminium face sheet. The second can be found by compressive testing of a potting resin 2 mm cube. Figure 11 right shows the load-deflection curve of the sandwich beam. The experiment was carried out in both L and W directions of the honeycomb (see Figure 11 upper left) and globally, the same behavior was found. Behavior was linear until the cell walls of the honeycomb started to fold at 45. The progressive appearance of shear buckling is observable. Then half of the 3- point bending test specimen was modeled to find the variation in shear stiffness of the core (see Figure 11). The mesh was made with volumic elements only and the material law is linear and isotropic. There were 3 elements along the skin thickness and 7 elements through the core thickness. The initial shear moduli obtained are 32 MPa (L-direction) and 24 MPa (W-direction). Then numerically, the initial shear modulus decreased and the intersection of the 3-point bending experimental curve and the new linear curve obtained by this variation of the shear core modulus gives the load level corresponding to shear strain and shear stresses. Thus, the shear stress versus shear strain of the honeycomb for both directions can be plotted as shown in Figure 12 left and the damage law of the honeycomb is obtained. The values were taken from the volumic element situated a quarter of the beam away to avoid boundary effects. To determine the potting stiffness, a compressive test was carried out and the stress-strain curve is reported in Figure 12 right. The ultimate compressive strength was defined at 5 MPa and the nearly perfect plastic behavior of the potting material can be noted.

8 . 3-point bending experimental curve F 7 F 8 G variation 1% 9% 8% 7% F 9 symmetry constraint F 1 Support Deflection (mm) Figure 11 : 3-point bending sandwich beam test and its numerical half model. τ (MPa) L-direction.8.6 W-direction γ Figure 12 : Shearing damage law of honeycomb in both directions (left) and compressive material property of potting (right). σ (MPa) ε 3. NUMERICAL STUDY 3.1 Description of the finite element model and identification of the damage laws According to the experimental investigations, a numerical model using the FE-code SAMCEF for the pull-out test of the potted insert is proposed. Firstly, it is possible to model only a quarter of the specimen and apply symmetry constraints. The model uses only volumic type elements. The screw, the adaptor and the lower part of the fork are also modeled. The mesh and boundary conditions are shown in Figure 13. Contact conditions are imposed between the fastener head and adaptor, adaptor and skin, skin and fork, fork and washer, washer and nut. In the model, the load is applied at the end of the fastener. The screw and the adaptor are made of steel. The sandwich skins are modelled by an equivalent orthotropic material. The moduli were found by classical tensile tests. E 1 (MPa) E 2 (MPa) E 3 (MPa) ν 12 ν 23 ν 31 G 12 (MPa) G 23 (MPa) G 31 (MPa) Steel 2.3 Composite laminate face sheet Potting Honeycomb core Table N 1: Material properties used in linear FE-analysis.

9 Table N 1 shows the linear material properties which are entered into the global finite element model. The damage behavior laws found for the honeycomb shear stresses were implemented directly (Figure 12 left). The potting material is assumed to be perfectly plastic with a plasticity threshold of 5 MPa. Symmetry constraints Z Support at radius 3 mm Y X Figure 13 : Meshing and boundary conditions of quarter specimen. 3.2 Comparison of numerical model and experimental results numerical model curve experimental curve Displacement (mm) Figure 14: Experiment and numerical modelling comparison and stress field analysis. In Figure 14, the red curve is from a special pull-out test where only the screw is loaded and no prestress applied to avoid any history effects. The black curve presents the load versus displacement in the z-direction of the fastener head curve computed by the model. In the experimental curve, the initial nonlinear behavior is attributed to contact phenomena and are very difficult to model. So, the numerical results are fitted to the experimental curve and a good agreement is found. 4. CONCLUSION This paper presents experimental investigations and a related numerical model of quasi-static pull-out tests of potted inserts used in landing gear doors. The importance of prestress is demonstrated because damage in the honeycomb appears thus generating an history effect. The different types of damage are identified and a failure mechanism is proposed. At first, the cell walls of the core buckle under shear. Secondly, damage appears in the potted insert under the head of the screw. Finally the head of the screw penetrates the skin because of the lack of resistance of the potted insert and the increase in shear stress in

10 the skin. Following these experimental conclusions, a nonlinear finite element model was made. It includes the nonlinear behavior law of the honeycomb which was identified by a simple 3-point bending test. Other material characteristics were identified by classic tests and the perfect plastic behavior of the potted insert was taken into account. Contact elements were also included in the model. A good agreement between experiment and the numerical model was found. The study is being pursued by testing a different way of introducing prestress and by analyzing and modeling the ultimate failure mode which is the penetration of the head in the skin. Finally the developments made in this study will be applied to other insert designs and loadings. REFERENCES [1] Zenkert D. : The handbook of sandwich construction. EMAS Publishing. [2] Noirot F., Ferrero J.-F., Barrau J.-J., Castanié B., Sudre M. : Analyse d inserts pour les structures sandwich composites. Mec. Ind. (2) 1, [3] Insert Design Handbook. ESA PSS 3-122, [4] Thomsen O. T. : Sandwich plates with through-the-thickness and fully-potted insert : evolution of differences in structural performance. Comp. Struc.(1998), Vol.4, No.2, pp [5] Thomsen O. T., Ritz W. : Analysis and design of sandwich plates with inserts-a high-order sandwich plate theory approach. Composites Part B 29B,1998, [6] US Military Handbook 23A (1974) : Design of flat circular sandwich panels loaded at an insert. [7] Youngquist W. G., Kuenzi E. W. : Stresses included in a sandwich panel by load applied at an insert. Forest. Forest Products Laboratory, Report No.1845, Madison 5, Wisconsin, [8] SHUR-LOK Corp. : Sandwich panel fasteners, Design manual, [9] SHUR-LOK Corp. : Fasteners for sandwich structure, Ed. 1th, [1] WITTEN Co.,Inc. : Product guide for high performance inserts for composites, [11] Andre S. : Synthese du programme d évaluation d inserts fortes épaisseurs pour application RTM. La recherche aérospatiale, [12] Astrom B. T., Mcgarva L. D. : Insert integration in thermoplastic-based foam core sandwich components. Journal of sandwich structures and materials, Vol.1-July 1999.