Behavior of Thin Sandwich Panel under Transverse Quasi-static and Impact Loading

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1 Behavior of Thin Sandwich Panel under Transverse Quasi-static and Impact Loading Ankush Sharma PhD Scholar, Solid Mechanics and Design Department of Mechanical Engineering, IIT Kanpur March 24, 2012 Abstract Several thin sandwich panels (thickness up to 4 mm) of different configurations were studied with the hope of developing alternative materials to the conventional mild steel (MS) sheet used for many practical applications like automobile body, cover of production machines such as lathe, drill press, etc. Thin sheets of MS having thickness close to 0.8 mm are widely used to make the body of automobiles. Three types of sandwich panels with the outer face sheets made of glass fabric/epoxy were fabricated by hand layup process for the present study: i) J-Core panel was made of layers of Jute fabric/epoxy as core material, ii) Xi-Core panel was made of polyester foam Coremat Xi/epoxy as core material, and iii) XM-Core panel was made of polyester foam Coremat XM/epoxy as core material. A very thin transition layer of glass chopped strand mat/epoxy was provided between the face sheets and the core to improve their bonding strength. The sandwich panels were tested under a transverse quasi-static loading and low velocity normal impact loading. Damage area, permanent deflection over damage area and failure modes were observed. With respect to yield point of 0.8 mm thick MS sheets, it was observed that the sandwich panels were having less damage area under static and impact loading. In addition, the area density in grams per square meter (gsm) of sandwich panels was much lower than that of 0.8 mm thick MS sheet. Keywords: Sandwich panel, Coremat, Jute, GFRP, damage area, failure mode. 1 Introduction Thin sheets of MS having thickness close to 0.8 mm are widely used to make the body of automobiles and outer cover (skin) of most of the machines such as lathe machine, drill press, etc. They are heavy, they require high maintenance cost and high tooling cost, and their design flexibility is poor. Thus, there is need to develop light weight, strong and attractive materials suitable for preparing cover of many machines. The sandwich structures are relatively lighter in weight and less expensive. In sandwich-structures, the moment of inertia of the cross-section of the structure is increased by separating the face-sheets with a low density material, known as core. On being subjected to loading, the face sheets are under maximum tension/compression and they largely contribute to the strength of sandwich panel. Face-sheets of sandwich panel are made of any high strength material like FRP laminates, metal sheets, etc. The most common types of core materials include Honeycomb structures such as aluminum honeycomb, nomex honeycomb, foam materials such as PU foam, PVC foam etc. A honeycomb core can produce stiff and very light laminates, but it finds its limitations because of its high cost and expensive shaping. PU foam work well for thick sandwich structure but are not suitable to make thin sandwich panels where the FRP face sheets are separated by a small distance usually between 2 to 4 mm. Polyester foam Coremat [1] are available as thin sheets of 1-3 mm thickness, are very light weight and are easy to use as core material. However, the resin is dispersed through the micro holes of Coremat making 1

2 core significantly heavier than foam. But Coremat/resin core is much lighter than the corresponding portion of a monocoque laminate. The Coremat sheets conform easily on most curved surfaces. For a double curvature surface with high curvatures, appropriate cuts can be easily made in the Coremat in such a way that it wraps the curved surface. Thin laminates employing a core made of jute fabric reinforced resin and glass fabric reinforced polymer (GFRP) face sheets also have high potential as far as cost and strength are concerned, compared to the conventional materials [2, 3]. Analysis of polymer composite plates subjected to transverse loading has received considerable attention of researchers and many articles are available [4-12]. Naik [13] observed that the transverse static central load for damage initiation is higher for woven fabric composites compared to those of cross-ply laminates made up of unidirectional layers. Abrate [14, 15] reported that the critical failure modes under impact were core buckling, delamination in the impacted face sheet, core cracking and fiber breakage in the facings. Both the skin configuration and the core structure, control the impact behavior. Ziileyha et al. [16] observed that the impact forces, centre of deflection and delamination damage are proportional to projectile velocity and impacted mass. Duration of contact increases as the mass increases, but while the projectile velocity increases, duration of contact is same. Resin impregnated carbon fiber components can offer a great deal of weight saving compared to high grade steel and aluminum, in high performance sport cars [17]. GFRP sandwich composites with Coremat, cotton fabric as core material have better static and dynamic strength [18, 19]. The local crash of the core of sandwich panel under the point of impact is expected to be reduced by separating the core in two layers by inserting an internal sheet inside the core [20]. Polymer matrix composite materials are finding increasing use in high performance applications because of their high inplane specific stiffness and specific strength. The skin material may be subjected to static loads or impacts due to tool drops, hail, etc. For effective use of composite structures, they should be able to withstand both transverse static and impact loading. The objective of the present work is to study the damage in composite plates while subjecting to both transverse static and impact loading conditions. 2 Development of Sandwich panel 2.1 Constituent materials For reinforcing face sheets, woven glass fabric (GF) was used, which was supplied by Owens Corning India, Ltd. It was balanced plain weave and area density was 360 gsm (g/m). A very thin transition layer of glass chopped strand mat (GCSM) was placed between GF and the core to improve the bonding strength between layers. GCSM was made of random short glass fibers and its area density was 30 gsm. Three types of sandwich panels were developed by using different core materials such as Jute fabric, polyester foam Coremat XM and polyester foam Coremat Xi. The area density of jute fabric was 368 gsm. Two layers of jute fabric were used to form the core. The polyester foam Coremat XM was provided with hexagonal cell pattern and the interspaced holes as shown in Figure 1(a). The porosity of foam at the walls between the cells was much higher, thus helping the foam to bend easily to complex shapes. Also, the resin filled into the highly porous walls, helped in joining the two face sheets. Its area density was 96 gsm and thickness was 2 mm [14]. Coremat Xi, also made of polyester foam, is supplied with the interspaced holes. Its area density was 62 gsm and thickness 2 mm [14]. The diameter of its holes was larger than that of the holes in Coremat XM. However no hexagonal cell pattern was provided to Coremat Xi as shown in Figure 1(b). The distance between the holes was 8 mm and the holes were arrayed in the diamond pattern. During fabrication of the sandwich structure, the resin filled these holes, thus generating the columnar structure between the two face sheets. The epoxy was used as the matrix material for impregnating the face sheets and the core material of the sandwich panel. Thermo-setting epoxy resin (commercially known as L12) and the room temperature hardener (commercially known as K6) were supplied by Atul Ltd., Balsad, Gujarat, India. 2

3 (a) (b) Figure 1: (a) Polyester foam Coremat XM and (b) Polyester foam Coremat Xi Fabrication of Sandwich panel Sandwich panels were fabricated using the hand layup process. The core material, GF and GCSM were cut into pieces of mm size. The epoxy was mixed with the 10% hardener thoroughly. One layer of GF was placed on thick glass base plate with a thin myler sheet in between as a releasing sheet. Then the resin layer was applied to wet the fibers completely, followed by a layer of GCSM. Then a layer of a core material was placed on the GCSM layer and rolled by using a roller to have uniform spread of resin. The resin that had been applied before Coremat was placed came out through the holes of the Coremat and resin columns were formed within the Coremat avoiding entrapment of air bubbles. Over the Coremat top, GCSM and GF were placed and wetted with the resin. Pressure was applied on sandwich panel by putting another thick glass plate with myler sheet in between and placing some dead weights. It was allowed to cure for 48 hours under room temperature. Three types of following sandwich panels were fabricated by hand layup process for the present study: 1. J-Core panel: Two layers of Jute fabric/epoxy as the core material, one layer of GF/epoxy in each face sheet and one layer of GCSM/epoxy in between the core and each face sheet. 2. Xi-Core panel: Coremat Xi/epoxy as the core material, one layer of GF/epoxy in each face sheet and one layer of GCSM/epoxy in between the core and each face sheet. 3. XM-Core panel: Coremat XM/epoxy as the core material, one layer of GF/epoxy in each face sheet and one layer of GCSM/epoxy in between the core and each face sheet. 2.2 Area density One of the main objectives of this study was to develop light weight sheet materials. The average area density with standard deviation of sandwich panels and the area density of 0.8 mm thick MS sheet are as shown in Figure 2. Average area density of J-Core panel, Xi-Core panel and XM-Core panel were 3621 gsm, 3028 gsm and 3004 gsm respectively which were much smaller than 6165 gsm of the 0.8 mm thick MS sheet. In fact, the area density of Coremat based sandwich panels, Xi-Core and XM-Core, were nearly 50% less than that of 0.8 mm thick MS sheet. The area density of jute based sandwich panel was nearly 40% less than that of 0.8 mm thick MS sheet. 3 Experimental studies The plates of dimensions mm made of all three types of sandwich panels and 0.8 mm thick MS sheet were used for the experimental studies under transverse quasi-static loading and impact loading. The loading was performed on the specimen clamped at all edges by the fixture shown in Figure 3. The unsupported area of the plate was mm. The cylindrical punch is used for the transverse central loading on small patch of 10 mm diameter rather than point load. The transverse quasi-static central loading was done at the rate of 0.1 mm/min on universal testing machine (UTM). 3

4 Figure 2: Area density of sandwich panels (J-Core, Xi-Core and XM-Core) and 0.8 mm thick MS sheet Figure 3: Fixtures for mechanical testing The impact drop test set up has been developed so as to perform the test under controlled conditions. It consists of a frame provided with the clamps for the seamless tube of inner diameter 34.5 mm. A short cylindrical piece with spherical head, made of mild steel weighing kg was dropped through the seamless tube to impact the specimen of size mm clamped at all four edges by the fixture shown in Figure 3. The length and diameter, in mm, of the cylindrical piece used were 200 and 30, respectively. The drop height of the cylindrical piece was determined by knowing the mass of cylindrical piece and impact energy. Three different seamless tubes of length, in mm, 1300, 1000 and 500 were used to carry out the drop test at 18, 12 and 6 Joules, respectively. The leveling screws were provided to the frame as well as to the fixture, so that the pipe could be adjusted perpendicular to the surface of the specimen to have normal impact during the test. The impacted specimens had clearly visible damage area on front and back side. The damage area and the deflection over the damage area of all three sandwich panels under impact loading were observed. Three specimens of each kind of sandwich panel were tested under transverse quasi-static central loading. The load versus deflection relations of all three kinds of sandwich specimens and 0.8 mm thick MS sheet were monitored. Figure 4 presents the load-deflection relations of one specimen of each kind of panel, J-Core panel, Xi-Core panel and XM-Core panel and 0.8 mm thick MS sheet respectively. The results of other specimens were similar. As expected, the mild steel yielded at a very early stage, it then plastically deformed with a very small transverse load and since the edges of the MS panel were clamped, in-plane tension developed in the specimen resulting into high slope of load versus deflection curve. However, due to extensive plastic deformation a fairly large size depression was observed after removal of the lateral load. On the other hand, all the three sandwich panels show linear elastic behavior up to almost 400 N lateral load. Beyond this load, the load deflection curve is non linear because of the tensile stresses generated in the face sheets of the panel due to rigid clamping of the test panels at all four edges. The load deflection behavior is thus non linear elastic almost up to the critical load. At the critical load, fibers on the face sheets are broken on the rear surface where the stresses are highly tensile. The breakage as seen on 0 and 90 directions of face sheet which was reinforced with balance plane weaved single glass fabric. On the 4

5 Figure 4: Load-deflection response of clamped composite plates and MS sheet under transverse quasi-static central loading. front face, fibers tend to delaminate under compressive stresses. Figures 5(a) and (b) show the damage on the front and rear faces, respectively, of the sandwich panel. (a) (b) Figure 5: Damage area of (a) front and (b) rear side. In 0.8 mm thick MS sheet, the plastic deformation was extensive under combined loading of bending and in-plane loads. When the lateral load was removed, the large area permanent depression was observed. On the other hand, the permanent deflection was negligible in case of sandwich panels. The sandwich panels regain their flatness all over except very small depression over damage area near to loading contact. It is thus felt that the damage is localized under the transverse load. This behavior is advantageous in repairing the cover of the machine (e.g. Automobile bodies) as only small area is required to be repaired. In case of steel cover, the repair is more difficult as the depression on large area is required to be repaired and matched with rest of the cover. In sandwich panels, the failure is initiated on the compressive side of the plate specimen under transverse quasi-static central loading. 4 Conclusions Three kinds of thin polymer sandwich panels having thickness up to 4 mm are investigated in this study. The sandwich panels are developed to explore whether they could replace steel body of an automobile and covers of different machines such as washing machine, lathe machine, drill press etc. The area density of panels is nearly about 40% to 50% less than that of MS sheet. The drop weight test set-up is used for normal impact test. The energy levels of 6, 9, 12 and 18 Joules are chosen to investigate the effect of impact loading on the sandwich panels. There was negligible permanent depression in case of sandwich panels as the panel restores its original shape when the load is released. 5

6 The sandwich panels clamped at all edges are tested under quasi-static plate test on UTM. The load versus deflection relation of sandwich panels made of GF face sheets and GCSM face sheets is nonlinear, while it is linear for Jute fabric face sheets. A very small spot damage of diameter less than 1 mm was found at the centre of impact on front face, because of small bending of front face. On the front face, fibres were also broken in the neighbourhood of centre of impact but the fibre breakage is much less in comparison to that of rear face. References [1] Kumar, A., Chandel, P. S. and Kumar, P., Experimental Study of Thin Sandwich Panel. Journal of Institution of Engineers, India ,2006. [2] Ahmed, K. S. and Vijayarangan, S., Tensile, Flexural and Interlaminar Shear Properties of Woven Jute and Jute Glass Fabric Reinforced Polyester Composites. Journal of Materials Processing Technology 207(1-3) ,2008. [3] Nangia, S. and Biswas, S., Jute composite: Technology and Business Opportunities. Proceedings of International Conference on Advances in Composites at IISc and HAL, Bangolore, India ,2000. [4] J. H. Park & S. K. Ha, Impact damage resistance of sandwich structure subjected to low velocity impact. journal of material processing technology ,2008. [5] Imieliska, L. Guillaumat, R. Wojtyra and M. Castaings, Effects of manufacturing and face/core bonding on impact damage in glass/polyester-pvc foam core sandwich panels. Composites Part B: Engineering 39, 6, ,2008. [6] H. N. Dhakal, Z. Y. Zhang, M. O. W. Richardson and O. A. Z. Errajhi, The low velocity impact response of non-woven hemp fiber reinforced unsaturated polyester composites. Composite Structures 81, 4, ,2007. [7] G. Caprino & V. Lopresto, Influence of material thickness on the response carbon fabric/epoxy panels to low velocity impact. composite science and technology 59, ,1999. [8] Aymerich F., Pani C. and Priolo P., Damage response of stitched cross-ply laminates under impact loadings. Engineering Fracture Mechanics 74, 4, ,2007. [9] Cantwell W.J. and Morton J., Comparison of the low and high velocity impact response of CFRP. Composites 20, ,1989. [10] Dorey G., Sidey G.R., Hutchings J., Impact properties of Carbon fibre/kevlar 49 fibre hybrid composites. Composites 1, 25-32,1978. [11] Kumar P., Rai B., Delaminations of barely visible impact damage in CFRP laminates. Composite Structures 23, 4, ,1993. [12] Scarponi C., Briotti G., Barboni R., Marcone A., Iannone M., Impact testing on composites and sandwich panels. Journal of Composite Materials 30, 17, ,1996. [13] Naik, N. K. and Nemani, B., Initiation of Damage in Composite Plates under Transverse Central Static Loading. Journal of Composite Structures 52, 2, ,2001. [14] Abrate, S., Localised Impact on sandwich structures with laminated facings. Journal of Appl Mech Rev 50, 69-82,1997. [15] Abrate, S., Impact on Composite Structures. Cambridge: Cambridge University Press,

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