Fatigue properties and failure mechanism of polyurethane foam core E- glass reinforced vinyl ester sandwich composites
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https://www.eduzhai.net International Journal of M aterials Engineering 2013, 3(4): 66-81 DOI: 10.5923/j.ijme.20130304.02 Fatigue Behavior and Failure Mechanism of PU Foam Core E-glass Reinforced Vinyl Ester Sandwich Composites Manujesh B. J1, Vijayalakshmi Rao2,* 1Department of M echanical Engineering, K.V.G. College of .Engineering, Sullia, D.K, 574327, Karnataka India 2Department of M aterials Science, M angalore University, M angalagangothri, M angalore, 574199, Karnataka India Abstract The present work is concerned with the study of flexural and fatigue behavior of E-glass fibre/ Vinyl es ter/Polyurethane foam sandwich composites. Four types of s andwich composites are prepared with polyurethane foam and E-glass fabric/ vinyl ester facesheet having glass: vinyl ester we ight percent ratio 65:35 by hand lay-up method followed by compression technique at room temperature. The specimens are then tested for flexural/fatigue behavior. The objective of the present work is to investigate the integrity of facesheet with core under fatigue on varying the density of PU foam and by changing the fibre architecture. The fatigue behavior is observed for frequencies 1Hz, 3 Hz, 5 Hz, 7 Hz and 9 Hz. The experimental study reveals that the cyclic load and test frequency play a crit ical role in determining fatigue strength. The fatigue strength increased with increase in density of PU foam. St iffness degradation increased with increase in the fat igue frequency. The failure modes observed are facesheet/core debonding, delamination and core shear failure. Keywords Fatigue Behavior, Sandwich Co mposites, Stiffness Degradation, Debonding, Delamination 1. Introduction A sandwich structure consists of two thin, stiff facesheets bonded to thick lightweight and weaker core material. The face sheets carry most of the bending and inplane loads while the core provides structural stiffness, and out of plane shear and compressive strength. Properties like high stiffness and specific strength has been exploited in applicat ion of s and wich co mp os ites as st ructu ral mat erials . The in creas in g us e o f s and wich co mp o s it es in a lo t o f eng in eerin g app licat ions h as mo t ivated res earchers to invest igate their p ropert ies. Du ring th e regu lar serv ice, sandwich composites are subjected to dynamic loads that may cause some induced stresses in the structures. Many applications like aircraft structures, marine vessels, car body parts and train & truck structures involve cyclic load ing, wh ich can d eg rad e t he mech an ical p rop ert ies o f t he co mposites and generate fat igue failure. Therefore their fatigue behavior is significant in reliability and ensuring safety of these sandwich structures. Hence an understanding of fatigue behavio r is important prior to its use in these applicat ions. Fat igue damage can be evaluated fro m the stiffness, residual strength or other mechanical properties. * Corresponding author: firstname.lastname@example.org (Vijayalakshmi Rao) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved Stiffness degradation occurs during the fatigue life. The flexu ral fatigue strength of sandwich structure depends on the strengths of outer facesheet, foam and interfacial bond strength between the facesheet and the light weight core. The failure of any one of these would cause failure of the sandwich structures[4, 5]. Fatigue behavior of foam materials like PVC, PU, PMI, polycarbonate foam materials used as core in sandwich structures have been reported by many researchers. Studies carried out by Zenkert and Backlund includes static tests to investigate the rigidity and stiffness of PVC foams. These studies showed that linear elastic fracture mechanics can be used to estimate the failure of PVC foams. Lilley et al. investigated fatigue crack growth in polyurethane foam using fracture mechanics parameters and have shown that using fracture mechanics approach it is possible to rationalize the crack growth and assess the effect of R-rat io. Similar work has also been executed by Yau and Mayer on polycarbonate foam under fatigue loading. Zenkert has investigated the fatigue properties and the physical behavior of fat igue damage in PVC foa m materia ls. Grenestedt has described crack propagation in PVC foams . Shipsha et al. have studied mode-I and mode-II modes of failure by crack p ropagation in PVC foams. Several reports are availab le on the fatigue behavior of foam core sandwich structures[13-18]. A thorough investigation on the fatigue crack format ion and gro wth has been made by Burmen & Zenkert on two different International Journal of M aterials Engineering 2013, 3(4): 66-81 67 sandwich configurations; with PVC & PMI foam cores and epoxy/glass facesheets. Damage in itiated in the zone of high shear stresses over the entire length of the zone and in the middle of the specimen. These micro damages grew together and formed a horizontal macro crack whose length depended on the size of the shear zone coupled to the length between the load supports. Once the macro crack had developed, the two crac k fronts kinked a way and gre w towards the face/core interfaces in directions corresponding to maximu m tangential stress. The effects of interfacial crack size and impact damage size on the shear properties and failu re mechanis ms of marine sandwich composite made of glass reinforced poly mer (GRP) skins and a polyvinyl chloride (PVC) foam core was reported by P S Tho mas et al.. An abrupt decrease in the static shear strength and fatigue resistance occurred when the interfacial crack length between the skin and core exceeded ~20-30 mm due to the failure mechanis m changing fro m wrin kling of the GRP skin to shear cracking of the foam core. The fatigue performance was reduced with increasing interfacial crack size, although the load capacity of the composite remained unchanged until complete failure of the core. The fatigue failure of the core is characterized by a crack initiat ion stage and a steady-state crack g rowth stage, with each stage having a different crack growth rate. The fatigue characteristics of polyurethane foam cored (PUF) co mposite sandwich structures were investigated using three-point bending tests. Three types of specimens (epo xy/glass-PUF-epo xy/glass, polyester/glas s-PUF-polyester/glass, and epoxy/glass-PUF-polyester/glass) were studied. Experimental results indicate that degradation of stiffness occurs due to debonding and sliding between the skin and the foam during fat igue cycles. Better performance of the epo xy/glass-PUF-epo xy/glass sandwich panels is most likely due to the superior properties of the outer thin skins. Most of the specimens failed within the foam region and not at the skin level due to debonding between the foam and the s kin [1 8] . O. Konur and F.L Mathews have shown that there is a close relationship between the properties of matrix, fibre and interface and thus affect the fatigue performance of the composite. St iffness degradation during fatigue has been reported by many researchers[20-24, 13, 14]. Review of literature reveals that there is an ample scope for the study of fatigue behavior of sandwich structures by varying the core density and fabric architecture in facesheet. Thus the present work discusses the fatigue characteristics and the fatigue damage initiat ion and growth in different sandwich s tru ctu res . polyurethane foam (PUF) core supplied by Po lynate foams Pvt. Ltd. Bangalore. The sandwich specimen face sheet is synthesized by using 2% Cobalt Octate accelerator, Methyl Ether Ketone Pero xide (M EKP), Di Methyl Acetamide (DMA) and Vinyl ester. The fiber to resin volume ratio is maintained as 65:35. The samples are cured at room temperature for 24 hours followed by 70° C in oven for post curing. The sandwich specimen’s specifications and various configurations used in the experiment are presented in Table 1. 2.2. Methods 2.2.1. Fle xura l Test/Static Three Point bending Test Flexu ral test was carried out according to ASTM C393. The dimension of the test specimen was 200 mm x 30 mm and overhang of 20 mm. The core th ickness was 24mm and the facesheet thickness was 3mm. Tests were initially performed to get relevant load levels for the fatigue tests. Four identical specimens within each configuration were tested at room temperature, in a co mputer controlled universal testing mach ine at a constant displacement rate of 2 mm/ min [Fig.1]. Figure 1. Three point bending setup 2. Experimental 2.1. Materials The sandwich specimens used in the present study comprise of four different grades of E-glass fabrics (supplied by Vet rotex /Saint Gobian, India) in vinyl ester resin supplied by Ecmas, Hyderabad and three varied densities of Figure 2. Specimen during fatigue testing 2.2.2. Fatigue Lad Testing Fatigue test was carried out according to ASTM C393. The fatigue tests were performed under a load controlled 68 M anujesh B. J et al.: Fatigue Behavior and Failure M echanism of PU Foam Core E-glass Reinforced Vinyl Ester Sandwich Composites sinusoidal cycle using a servo hydraulic testing machine. The experiments were performed using 3 po int bending as shown in Figure 2. The dimension of the test specimen was 200 mm x 30 mm and overhang of 20 mm. The core thickness was 24mm and the facesheet thickness was 3mm. The fatigue test frequency of 1Hz, 3 Hz, 5Hz, 7Hz and 9 Hz were chosen to observe any degradation of sandwich composites. The test was load controlled allo wing the displacement to vary. The variation in displacement was closely monitored. 60% of u ltimate bending load (Table 2) was chosen to predict the fatigue life of sandwich specimens. The tests were performed to failure or a maximu m of 105 cycles. The machine was set to stop automatically if the displacement during the test exceeded 20 mm either due to specimen failu re or stiffness degradation to avoid damage caused to the setup. Sandwich Type WR Resin Table 1. Sandwich composit es – specificat ions Fabric Type (E-Glass) Core Mat erial Woven Roving – 360 gsm Core Density(Kg/m3) CSM SBM Vinyl ester 3 mm Chopped Strand Mat-360 gsm Stitch Bond Mat -610 gsm PU Foam (24 mm thickness 100 - 300 CSM (S) Chopped Strand Stitch Mat- 420 gsm 3. Results and Discussion 3.1. Static Flexural Test The maximu m static bending load values of sandwich specimens with varied fibre arch itecture and density are presented in Table 2. The static fle xura l test results show that the fle xural stress increases with increase in core density. Table 2. Ultimate bending load (N) at failure for various sandwich specimens Sl. No. 1 2 3 Density (kg/m3) 100 200 300 CSM 149 195.6 219.18 CSM-S 145.19 193.97 213.23 WR 156 222.5 261.10 SBM 146.40 214.44 226.05 Bending strength at the ultimate bending load of sandwich specimens with varied fibre architecture and change in density under flexu ral test is presented in Table 3. The static flexural test results show that the flexural strength of the sandwich composites is dictated by density parameter. Table 3. Facing bending strength (FBS) (MPa) for various sandwich specimens Sl. No. Density (kg/m3) CSM CSM-S WR SBM 1 100 2.44 2.39 2.57 2.41 2 200 3.22 3.19 3.66 3.53 3 300 3.61 3.51 4.30 3.72 3.2. Fatigue Test The fatigue life is characterized as the nu mber of cycles to ult imate failure. There were no visual signs of damage in the specimens prior to specimen failure, but nearing to 105 cycles the damage occurred abruptly. The stiffness of the specimens under lower testing frequencies exhib ited no measurable change until the final load cycles prior to complete rupture. Figure 2 illustrates the still photograph of the fatigue testing, the bending of the sandwich can be clearly seen. The Wohler’s (S-N) curve for the four types of the sandwich specimens at five different test frequencies are given in Figures 3 -7. The fatigue behavior is found to be similar in all the four types of sandwich composites. A linear trend in fatigue curve is observed up to certain working cycles, later the trend shifts causing partial or co mplete co llapse of the specimens. International Journal of M aterials Engineering 2013, 3(4): 66-81 69 Fatigue Bending Strength (MPa) Fatigue Bending Strength (MPa) 1.3 1.2 (a) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 102 1.5 (c) 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 102 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 102 100 kg/m3 200 kg/m3 S 300 kg/m3 SD D S D 103 104 105 Fatigue Cycle (N) Fatigue Bending Strength (MPa) 1.3 1.2 (b) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 102 100 kg/m3 200 kg/m3 300 kg/m3 SD 103 104 105 Fatigue Cycle (N) Fatigue Bending strength (MPa) 100 Kg/m3 200 Kg/m3 300 Kg/m3 1.6 (d) 1.4 1.2 100 kg/m3 200 kg/m3 300 kg/m3 1.0 0.8 0.6 S D 0.4 S D 0.2 0.0 103 104 105 Fatigue Cycle (N) 102 103 104 105 Fatigue Cycle (N) Fi gure 3. Fat igue plot s of sandwich composit es at 1 Hz frequency. a)- CSM, b) CSM-S, c)-WR d) SBM (a) 100 Kg/m3 200 Kg/m3 300 Kg/m3 S D 103 104 105 Fatigue Cycle (N) Fatigue Bending Strength (MPa) 1.35 (b) 1.20 1.05 0.90 0.75 0.60 0.45 0.30 0.15 102 S D 103 104 Fatigue Cycle (N) 100 Kg/m3 200 Kg/m3 300 Kg/m3 105 Fatigue Bending Strength (MPa) 70 M anujesh B. J et al.: Fatigue Behavior and Failure M echanism of PU Foam Core E-glass Reinforced Vinyl Ester Sandwich Composites Fatigue Bending Strength (MPa) Fatigue Strength (MPa) 1.35 (c) 1.20 1.05 0.90 0.75 0.60 0.45 0.30 102 Fatigue Bending Strength (MPa) 100 kg/m3 200 kg/m3 300 kg/m3 1.4 (d) 1.2 1.0 0.8 0.6 S D 0.4 0.2 0.0 103 104 105 102 Fatigue Cycle (N) 103 104 Fatigue Cycle (N) Fi gure 4. Fat igue plot s of CSM-S sandwich composit es at frequencies a) 3Hz b) 5Hz c) 7Hz d) 9Hz 100 Kg/m3 200 Kg/m3 300 Kg/m3 105 1.5 (a) 1.4 1.3 1.2 1.5 100 Kg/m3 1.4 (b) 200 Kg/m3 1.3 300 Kg/m3 1.2 1.1 100 Kg/m3 200 Kg/m3 300 Kg/m3 Fatigue Strength (MPa) 1.1 1.0 S 0.9 0.8 D 102 103 104 105 Fatigue Cycles (N) 1 0.9 0.8 0.7 102 S D 103 104 105 Fatigue Cycle (N) 1.50 (c) 1.35 1.20 1.05 0.90 0.75 0.60 0.45 0.30 0.15 0.00 102 100 Kg/m3 1.4 (d) 200 Kg/m3 300 Kg/m3 1.2 S 1.0 0.8 D S 0.6 D 0.4 S 0.2 D 0.0 103 104 105 102 103 104 Fatigue Cycle (N) Fatigue Cycle (N) Fi gure 5. Fat igue plot s of WR sandwich composit es at frequencies a) 3Hz b) 5Hz c) 7Hz d) 9Hz 100 Kg/m3 200 Kg/m3 300 Kg/m3 105 Fatigue Strength (MPa) International Journal of M aterials Engineering 2013, 3(4): 66-81 71 Fatigue Strength (MPa) 1.4 (a) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 102 100 Kg/m3 200 Kg/m3 300 Kg/m3 S D 103 104 105 Fatigue Cycle (N) Fatigue Strength (MPa) 1.4 (b) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 102 100Kg/m3 200Kg/m3 300Kg/m3 S D 103 104 105 Fatigue Cycle (N) 1.4 (c) 1.2 1.0 100Kg/m3 200Kg/m3 300Kg/m3 1.4 (d) 1.2 1.0 100Kg/m3 200Kg/m3 300Kg/m3 Fatigue Strength (MPa) Fatigue Strength (MPa) 0.8 0.8 0.6 S 0.4 0.6 S 0.4 0.2 D 0.2 D 0.0 102 103 104 105 Fatigue Cycle (N) 0.0 102 103 104 105 Fatigue Cycle (N) Fi gure 6. Fat igue plot s of SBM sandwich composit es at frequencies a) 3Hz b) 5Hz c) 7Hz d) 9Hz At frequencies 1Hz and 3 Hz the specimens did not fail co mpletely when the machine was stopped. It was observed that all the specimens offered a high resistance against the fatigue load and least damage at 1 Hz. The test was stopped at a deflection of 20 mm, the maximu m deflect ion that can be attained. Though the specimens at 1 Hz were ab le to co mp lete 105 working cycles, there was marginal degradation of the core, as evidenced fro m the decrease in the fatigue strength. At 3 Hz and 5 Hz working cycles, samp les were found to be damaged, but all the samp les comp leted the set load cycles. At lower frequencies (up to 5Hz), no flaws were observed in the specimens, but a gradual decrement in the strength and enhancement of degradation rate, nearing to 105 cycles. At 7 Hz and 9 Hz frequencies the specimens fa iled and could not reach the e xpected 105 cycles. However CSM-S-300 and WR-300 sandwich co mposites reached almost 105 cycles. Although samples exh ibit core failure, it is found that samples take some load till its comp lete failure. All the samples have undergone damage at 9 Hz frequency. The stints of powdery foam started to evolve from the mid span of the core indicat ing the existence of core shear under cyclic loading. At higher test frequencies some specimens exh ibited cracking and tearing no ise before proceeding into typical failure modes. Fatigue strength increased with increase in the core density. The failure of the sandwich specimens 72 M anujesh B. J et al.: Fatigue Behavior and Failure M echanism of PU Foam Core E-glass Reinforced Vinyl Ester Sandwich Composites Fatigue Strength (MPa) with h igher density foams occurred at higher cycles. It can be seen fro m the plots Fig. 4 (a & b) that degradation init iated at much earlier cycles at high frequencies with increase in the density of the foams. 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 102 (a) SD 103 104 Fatigue Cycle (N) 100 Kg/m3 200 Kg/m3 300 Kg/m3 105 Fatigue strength (MPa) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 102 (b) S D 103 104 Fatigue Cycle (N) 100 Kg/m3 200 Kg/m3 300 Kg/m3 105 Fatigue Strength (MPa) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 102 (c) 100 Kg/m3 200 Kg/m3 300 Kg/m3 S D 103 104 105 Fatigue Cycle (N) Fatigue Strength (MPa) 1.4 (d) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 102 S D 103 104 Fatigue Cycle (N) 100 Kg.m3 200 Kg.m3 300 Kg.m3 105 Fi gure 7. Fat igue plot s of CSM sandwich composit es at frequencies a) 3Hz b) 5Hz c) 7Hz d) 9Hz Sandwich T ype Woven Roving CSM CSM-S SBM Table 4. The transition point and failure modes observed under fatigue testing Fatigue Behavior observed at 60% of Ultimate loading Density of PU Cycle to failure (MPa) Stiffness Degradation (%) Foam (Kg/m3) Lower Cycle (3 Hz) Higher Cycle Lower Cycle Higher Cycle (9 Hz) (3 Hz) (9 Hz) 100 16350 8186 27 51 200 21360 8900 20 42 300 27980 14050 17 34 100 14648 1923 35 59 200 16650 3219 23 45 300 17507 4885 22 39 100 12428 9860 33 53 200 21354 22150 20 39 300 30050 26064 16 37 100 21354 22150 31 54 200 30050 26064 22 40 300 16156 3455 19 43 Failure mode observed Debonding, Core Crack, Delaminat ion International Journal of M aterials Engineering 2013, 3(4): 66-81 73 Sl. No. 1 2 3 Table 5. FBS and CSS (MPa) for various sandwich specimens Density (kg/m3) 100 200 300 CSM FBS CSS 1.46 0.09 1.93 0.11 2.17 0.13 CSM-S FBS CSS 1.43 0.09 1.94 0.12 2.10 0.13 WR FBS CSS 1.54 0.09 2.20 0.13 2.58 0.16 SBM FBS CSS 1.44 0.09 2.12 0.13 2.23 0.14 The fatigue behavior o f all the sandwich co mposites exhibited a transition point, i.e., t ransition fro m a steady stable state region (S) to a deteriorating region (D) at which there is a sudden change in the slope. In the region S, there is a ma rgina l decrease in the fatigue strength. The debonding between faces heet and foam did not significantly affect the fatigue life behavior o f sandwich specimen keeping the rate of stress almost constant. The cycles at wh ich S to D transition observed is given in Tab le 4. At higher frequencies, the transition point is observed at lower cycles. Also, as the density of the foam increased the transition point is observed at higher cycles. After the transition point, the specimens exhib ited crack between the foam and the facesheet. Rate of decrease in fatigue strength is found to be more in the case of CSM sandwich composites and minimu m for CSM-S and WR sandwich co mposites. WR sandwich composites possess highest fatigue strength. The higher fat igue strength when compared to the other sandwich co mposites may be due to its highest bending strength and hence can resist high compressive and shear strength occurring during cyclic loading. A co mb ination of high shear and compressive properties at the interface may be the reason for the higher fatigue strength in WR sandwich composites. The bending strength of the sandwich composites depends mainly on the strength of the facesheets, foam core and the interfacial bonding between the facesheet and the foam. Core shear strength (CSS) and facing bending strength (FBS) at 60% of the ultimate bending load are given in Table 5. M. Kharwar Farooq et al., have analyzed the flexu ral behavior of sandwich composites with PVC core and glass/epoxy face sheets under fatigue. They observed initiat ion of damage in the facesheet and compression in the core at few hundreds of cycle. With the increase in nu mber of cycles, these damages propagated and interfacial debonding initiated between the facesheet and the core. This damage mechanis m continued with increase in nu mber o f cycles resulting in co mp lete debonding between the top facesheet and the core. Frequency in the range of 0.1-10Hz had no significant effect on stiffness degradation. Stiffness degradation increased with increase in core thickness. The effect of density of PVC core on the fatigue behavior of S2 glass fibre reinforced vinyl ester sandwich composites has been reported by K. Kanny & H. Mahfuz. The stiffness and failure load of sandwich beams increased with increased core density and fatigue strength decreased with increased stress and increased with core density. Fat igue failure post test analysis of PU foam cored carbon fibre/ epoxy sandwich composites by Basir Shafique and Amilear Qu ispitupa indicates core failure as the predominant damage mechanism followed by the interfacial failure. S.C Sharma et al.,[27, 28] have studied the fatigue behavior of polyester /glass/ PU/ glass/epoxy, Epo xy/glass/ PU/glass/epoxy and Polyester/glass/PU/glass/polyester sandwich composites. Epo xy/glass/PU/glass/epoxy sandwich structures exh ibited highest fatigue strength along with h igher stiffness degradation compared to other t wo types of sandwich panels. Lowest fat igue properties were obtained for polyester/glass/PU/glass polyester sandwich specimens with lo wer stiffness degradation. Most of the specimens failed within the foam region and not at the facesheet level due to debonding between facesheet and core. At 1Hz none of the specimens failed. Above the transition points , numerous cracks were seen between the foam and the facesheet. In the deteriorat ing region, debonding between the facesheet and the foam core played a major ro le. St iffness degradation increased with the increase in the fatigue frequency. The three failure modes observed were facesheet failure due to delamination, shear failure in the core and facesheet /core interface and core failure due to crushing stresses. Stiffness degradation was around 40% for epo xy and 30 % for polyester sandwich co mposites. The fatigue strength and stiffness degradation were mainly dependent on interfacial bond strength. In the initial cycles of fatigue loading, interfac ial bond strength was stronger, but gradually weakened at h igher cycles because of rubbing at the interface between the facesheet and the foam. Fatigue crack growth and life predict ion of PVC foam cored S2 glass epoxy sandwich co mposites under flexu ral loading have been studied by N. Ku lkarni et al.[5, 16] They observed that the fracture failure of the sandwich co mposite was controlled by the failure of the core. Crack propagation occurred in three stages i.e., core- facesheet debond, core shear follo wed by another core-facesheet debond. The first stage constituted around 85% of the fatigue life. Degradation in stiffness was only about 10%. The fatigue behavior of PUF 3-D woven glass fabric epoxy sandwich composites has been investigated by H. Judiwistra et al.. The 3-D woven glass fabric epoxy panels with PU foam showed excellent fatigue behavior and low stiffness degradation. Fatigue life was higher than 106 cycles and stiffness degradation lower than 6% at 80% bending ultimate load. 3.3. Damage Formati on During Fatigue Testing It is observed that the fatigue life of sandwich structures 74 M anujesh B. J et al.: Fatigue Behavior and Failure M echanism of PU Foam Core E-glass Reinforced Vinyl Ester Sandwich Composites solely depend on shear strength in the structure. In threepoint bending, besides shear stresses, tension and compression stresses are also generated in the core. Hence tension and compression stresses play a significant ro le resulting in lo wer shear strength. During fatigue testing the sandwich specimens are repeatedly undergoing tensile and compressive stresses at bottom and top facesheets respectively. This stress builds up shear stresses in the sandwich assembly. The ability of the core to balance theses stresses dictates the fatigue life of any sandwich composite. Maximu m shear stress is observed at the centre of the core, where the co mpressive stress balances the tensile stress. Fracture init iates at the centre of the core and there is a rise in temperature in the core. During compression and tension cycles, there will be an increase in the temperature at the interface due to the friction between the facesheet and core. The high shear stress along the centre of the core causes micro cracks or irregularit ies to initiate which only could be observed visually at a very late stage of the fatigue life o f the specimen. These micro cracks eventually grow in number and start to interact with each other forming a horizontal crack wh ich eventually separates core fro m the faces h eets . The fracture mechanics involved in sandwich composites under fatigue loading is complex and is associated with mo re than one failure modes due to anisotropic behavior of facesheets. Generally sandwich composites experience progressive fatigue degradation due to failure of the fibres, fibre stacking sequence and type of fatigue loading. The damage development under fatigue and static loading is similar but with the exception that the fatigue loading at a given stress level will cause additional da mage and this will be dependent upon the cycle frequency. The failu re mechanis ms observed under fatigue testing of sandwich composites are; i) Fibre breakage-interface de-bonding, ii) Matrix Cracking, iii) Interface shear failure with fib re pull-out iv) Delamination. Any co mb ination of the above four is possible and may cause fatigue damage which may result in reduced fatigue strength and stiffness. The degree of fatigue da mage is highly dependent upon materia l properties, facesheet, stacking sequence, applied load and nu mber of cy cles [30 ]. At high fatigue stress, cracks can in itiate on the first loading cycle and will then accumu late with increasing number of cycles. However cracks can develop even when the maximu m cycle stress is well below the static cracking threshold of cycles but these cracks will not take p lace until after many cycles, the actual nu mber depends upon the peak stress. Early init iation of matrix cracking in fatigue relative to static loading will lead to a decrease in the threshold for the onset of other types of damage. Delamination can propagate over many thousands of cycles thus resulting in separation of the laminate into discrete laminae wh ich will continue to support the fatigue loading. Figures 8 and 10 illustrate the various modes of sandwich failure during fatigue loading as a function of frequency and number o f cycles for lo w and h igh density sandwich composites observed in the present study. In lo w density sandwich co mposites, the crack path in the core at low frequencies is different fro m that at higher frequencies. However, the crack path in high density sandwich co mposites is similar at lower and higher frequencies (Fig.10). K. Kanny et al. have also observed that in lo w density sandwich composites, crack path in the core at lower frequencies is different fro m that at higher frequencies and the crack path in high density sandwich composites is similar at lower and higher frequencies. Fatigue failure is as a result of the combination of tension, compression and shear stresses. In low density sandwich composites (up to 200 kg/ m3), at lower frequencies i.e., up to 5 Hz, init iation of the damage in the facesheet and compression in the core occur at few hundreds of cycles. With increase in the number of cycles, interfacia l debonding in the upper facesheet/core results. This phase represents 75% of the fatigue life. During this period, only the core and the bottom facesheet could bear the applied load. With further increase in the nu mber of cycles, debonding starts at the lower facesheets also. At higher cycles, i.e., in the ‘D’ region cracks in the core is init iated at one side and propagated diagonally at 55° to the other side (Fig.8 a, b). Final stage involves delamination. M Kharwar Farooq et al have observed similar failure modes in the case of PVC core/glass/epoxy sandwich composites under fatigue. But at higher frequencies and lower cycles first stage involves crack initiat ion and propagation on the compression side just below the facesheet/core interface and the second stage is core shear and core crack propagation diagonally towards the tensile side and finally crack formation at the tensile side o f the sandwich just above the core/facesheet interface. At higher frequencies and higher cycles, crack propagates on the other side of the roller (loading point arrangement on the sandwich specimen) reaching the lower facesheet (Fig.8 c &d), resulting in delamination and debonding of the core/ facesheet interface. In PVC cored S2 glass fabric/epoxy sandwich co mposites S. Kulkarni et al[5, 16] have observed initial stage as crack initiat ion and propagation on the compression side just below the top facesheet/core interface. De la mination c rack was about 1-1.5 mm below the interface. This was followed by core shear and third stage was delamination at bottom facesheet/core interface causing the separation of core fro m the facesheet. The photographs of the failed low density sandwich composites under fatigue are given in Fig.9 (a & b). International Journal of M aterials Engineering 2013, 3(4): 66-81 (a) 75 (b) Low density/low frequency at lower cycles (c) Low density /low frequency at higher cycles (d) Low density/higher frequency at lower cycles Low density/higher frequency at higher cycles Figure 8. Failure modes of low density sandwich composites under fatigue loading (a) (b) Figure 9. Failure of low density sandwich composites With higher density composites (300 Kg/ m3) at lo w frequencies and lo wer cycles, crack is in itiated in the region of high shear stress i.e., in the middle of the core. The cracks nucleate at much faster rate due to repeated impact contact of the roller with the sandwich specimen. M icro cracks are formed in the beginning and finally grew to a macro crack as shown in the Figure 10(a). The macro crack formed is propagated at 55° towards one side of the facesheet/core interface. As the number of cycles increased, the macro crack p ropagated to the other facesheet/core interface, resulting in delamination and debonding (Fig. 10 b & Fig 11 e). Similar type of failure mechanis m has been observed in PVC cored glass reinforced v inyl ester by Burman & Zenkert. But at higher frequency and lo wer cycles, crack is formed in the core and propagated towards upper and lower facesheets. The direction of crack propagation is 55° to the original crack (Fig. 10 c). The specimens showed visible cracks in the mid height of the core. Grenesdelt et al., have observed the crack propagation angle ranging fro m 65° to75° in expanded PVC foam materials of different densities. In PVC cored g lass epoxy sandwich composites, Burman & Zenkert have reported that the angle at which the cracks kinked at final fracture was different for each crack ranging between 55°and 85°. As the number of cycles increased, micro cracks are init iated adjacent to the major crack formed in the middle of the core (Fig. 10 d). Mu ltiple cracks are formed due to the large difference in the elastic properties of the constituents of sandwich composite (Efacesheet » Einterface > E core). At higher cycles, dela mination and debonding of foam/facesheet occur very
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