Impact Damage Resistance of Carbon Fibre/Epoxy Composite Laminates Containing Short Kevlar Fibres

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1 Impact Damage Resistance of Carbon Fibre/Epoxy Composite Laminates Containing Short Kevlar Fibres Impact Damage Resistance of Carbon Fibre/Epoxy Composite Laminates Containing Short Kevlar Fibres Min-Seok Sohn ** and Xiao-Zhi Hu Department of Mechanical, Materials and Mechatronics Engineering The University of Western Australia, Nedlands, 9, WA, Australia Jang-Kyo Kim Department of Mechanical Engineering The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong Received: th September ; Accepted: st February SUMMARY The impact damage resistance of carbon fibre/epoxy matrix composites containing short Kevlar fibres as the interlayer material was studied. Instrumented low-velocity impact tests were conducted at several temperatures to study the impact fracture and damage mechanisms. The experimental results indicate that short Kevlar fibres improved the impact damage performance. Distinct damage patterns were observed on the front and back faces of the specimens tested at two extreme temperatures; - C and C. Crosssectional microscopy was performed to evaluate sub-surface damage; optical and scanning electron microscopy were used to study the fracture mechanisms of the laminates impacted at different temperatures. The main contribution of the interlayer material to the improvement in impact resistance was the enhanced fibre-bridging with the short Kevlar fibres at the interlaminar regions.. INTRODUCTION The mechanical properties and fracture performance of many thermoset epoxy resins used as the matrix in fibre reinforced plastics (FRP) composite laminates are temperature dependent because of their viscoelastic nature -. Therefore, extensive research has been done to investigate the temperature effect on impact-induced fracture. Although the glass transition temperature, T g, of most epoxy resins in current use is higher than their service temperature, it is highly probable that the service temperature can go up to T g or even higher for some load-bearing applications in supersonic transport aircraft. It is also important to evaluate composite properties in the low temperature range, i.e., - ~ - C. At such low temperatures, low energy impact loads (such as those involving birds, or debris thrown up from the runway) can have a detrimental effect because the material becomes extremely brittle at low temperatures. ** To whom correspondence should be addressed. Present address: Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Yusong Gu, Kusong Dong, -, Taejon -, Korea. Several studies have focused on the effect of temperature on the properties of polymer matrix composites under both static and impact loading conditions -. Quasi-static Mode-I interlaminar fracture energy for pure epoxy resin, G Ic increased with rising temperature. However, it is not entirely clear whether the fracture energy of FRP will increase or decrease with increasing temperature when the epoxy is incorporated into the FRP composite structure as a matrix material. An increasing trend for G Ic was found with increasing temperature for fibre composites with various matrix systems,. Russell and Street reported that G Ic of composites decreased with increasing temperature because residual stresses and constraint effects played a dominant role. Furthermore, if the composite laminates contain short fibres (e.g. short Kevlar fibres) or interleaves (e.g. tough polymer films) between the continuous fibre layers as toughness promoters, additional failure mechanisms need to be considered to explain the improved toughness results. Studies have also been carried out to investigate the effects of temperature and moisture on the extent of damage -9. A decrease in the delaminated area was found with increasing temperature when high-energy Polymers & Polymer Composites, Vol. 9, No.,

2 Min-Seok Sohn, Xiao-Zhi Hu and Jang-Kyo Kim impact loads were applied to carbon fibre/epoxy composite laminates in the temperature range from C to C. In contrast, Karasek et al reported a contradictory result on the impact resistance of graphite/epoxy composites without, and with, rubber modifications to the resin, depending on the moisture and temperature. The composites without rubber modification required increased energy to initiate damage due to the increase in resin ductility above T g and the increased extent of the damage, probably due to a decrease in resin strength. Meanwhile, composites with a rubber-modified epoxy matrix needed less energy to initiate damage at elevated temperatures due to the combination of reduced resin strength above T g and the unique stress distribution associated with the dispersed rubber phase. The extent of the damage was reduced due to the additional energy absorption mechanisms associated with the dispersed phase. A similar result was obtained for a carbon fibre/epoxy composite. Impact-induced fractures in FRP involve mainly delamination, intra-ply cracking, interfacial failures between the fibres and the resin matrix, translaminar failures including surface damage, and fibre fracture. Among these, delaminations critically reduce the composite stiffness and strength, although they are not visible on the composite surface. Therefore, many techniques have been developed to improve the delamination resistance of composites, e.g., fibre surface coating,, interleaving -, modification with toughened thermoset resin,9, and using hybrid fibres -, woven fabrics, short Kevlar fibres, and stitched fibres,. These techniques led to significant improvements in interlaminar fracture toughness, often more than % when through-thethickness (TTT) stitch fibres were used. The improvements in interlaminar fracture toughness, however, were at the expense of large reductions in composite strength in the fibre direction. In our previous studies,, short Kevlar fibres were successfully used as delamination suppressors which promoted crack-interface bridging and hence the R-curves of interlaminar fracture toughness under Mode-I, -II and mixed-mode loading conditions. In this paper, the influence of short Kevlar fibres on the resistance to impact-induced fracture and damage is investigated. Special emphasis is placed on temperature effects on impact response in the temperature range from C to C. The fracture mechanisms caused by the drop-weight impact are studied using optical, and scanning electron microscopes (SEM).. MATERIALS AND EXPERIMENTS NCT carbon fibre/epoxy prepregs (supplied by Newport Adhesives and Composites) were used to prepare the composite laminate specimens. The prepreg was of g/m in unit weight and consisted of k yarns. The nominal fibre volume percentage of the prepreg before curing was %. Ten-ply composite laminates were prepared by hand-lay up with a stacking sequence of [/9]. As-received DuPont Kevlar 9 fibres ( mm in length) were used as an interlayer material. During the lay-up process, Kevlar fibres of two different weight fractions,. wt % (equivalent to the weight-to-area ratio,. g/m ) and. wt % (. g/m ) over the total composite weight, were introduced on one side of each prepreg. The compression moulding technique was employed to cure the laminate according to the manufacturer s recommendation. The temperature of the mould was increased to C with a dwell time of min, followed by a further increase to C with a dwell time of min. The mould was then allowed to cool down to room temperature. A mould pressure of KPa was applied during curing. The composite laminates were then cut into the final size of x mm. Low-energy impact tests were conducted on an instrumented Dynatup drop-weight impact machine. An impact load was applied to the centre of the specimens with a test window mm in diameter using an impactor having a hemispherical nose. mm in diameter. The specimens were fixed in a hydraulic clamp equipped in the impact machine. The impact tests were performed with two different impact energies, J and J (impact distances were about and mm from the specimens, respectively), and at four different temperatures; C, room temperature (RT) ( C), 9 C and C. The temperature of C was chosen as the highest test temperature because it is equivalent to the curing temperature recommended by the manufacturer.. IMPACT RESPONSES The load-time (P-t) diagrams for the composites without and with. wt % short fibres for an impact energy of J at the four different temperatures are shown in Figures (a) and (b). The P-t diagrams for the composites containing. wt % short fibres were very similar to those for composites with. wt % short fibres. The duration of impact for both composites were also quite similar in the range from. ms to. ms in all temperature ranges. This implies that the impact duration depends on the material and structure of the composites rather than Polymers & Polymer Composites, Vol. 9, No.,

3 Impact Damage Resistance of Carbon Fibre/Epoxy Composite Laminates Containing Short Kevlar Fibres Figure Load-time (P-t) diagrams at various temperatures for: (a) the plain composite and (b) composite with. wt % Kevlar fibres of mm in length (a) J impact - C - (b) - C - room temperature room temperature - - Time (s) 9 C 9 C - - C C - - on the temperature, even though the resin matrix became more ductile at high temperatures. As the temperature increased, the incipient load, P i, as shown in the P-t curves became less distinct. So did the peak load, P m, at low temperatures, causing any measurement of energies before and after the peak load (based on the area under the P-t curves) to be unreliable because of the large scatter. Polymers & Polymer Composites, Vol. 9, No., 9

4 Min-Seok Sohn, Xiao-Zhi Hu and Jang-Kyo Kim The incipient load, Pi, and the peak load, Pm, taken from the P-t diagrams are plotted as a function of temperature in Figures and. The Pi and Pm values obtained at J were higher in general than at J. The Pi values were approximately constant between C and 9 C but dropped significantly at C. This confirms the result of a previous study for glass woven fabric composites. This temperature is equivalent to the prepreg cure temperature recommended by the manufacturer. The above observations shown in Figures and are a reflection of the viscoelastic behaviour of the epoxy resin matrix at elevated temperatures. Figure Incipient load, Pi, vs. temperature Incipient load, Pi (kn)... - Plain-J Plain-J Kevlar mm,.% - J Kevlar mm,.% - J Kevlar mm,.% - J Kevlar mm,.% - J - Temperature ( C).. Surface damage evaluation Figure Peak load, Pm, vs. temperature. Peak load, Pm (kn). Plain-J Plain-J Kevlar mm,.% - J Kevlar mm,.% - J Kevlar mm,.% - J Kevlar mm,.% - J Temperature ( C). EVALUATION OF IMPACT DAMAGE The front and back surfaces of impacted specimens containing. wt % short fibres were examined. Figure displays typical fracture patterns of the front and back surfaces tested at C and C. No distinct difference between the specimens with and without short fibres was found on the impact surfaces. The extent of damage produced at the front surface varied with temperature. The composite impacted at C showed an indent with some fractured fibres within the indent crater (Figure (a)), while that impacted at C exhibited major cracks running in the transverse direction, together with splitting along the fibre axis of the top layer (Figure (b)). The extent and shape of the damage produced at RT and 9 C were intermediate between the above two extremes at C and C. Figure Front and back surfaces of composites with. wt % Kevlar with an impact energy of J: (a) and (c) at C, and (b) and (d) at C (a) (b) (c) (d) Polymers & Polymer Composites, Vol. 9, No.,

5 Impact Damage Resistance of Carbon Fibre/Epoxy Composite Laminates Containing Short Kevlar Fibres Figure Delaminated area at every interface obtained by cross-sectional fractography; (a) the plain composite, (b) composite with. wt % Kevlar fibres mm in length and (c) composite with. wt % Kevlar fibres of the same length Delaminated area (mm ) (a) plain (RT) plain (9 C) plain ( C) The push-out type fracture was exhibited at the back surface 9. As shown in Figures (c) and (d), the damage became less extensive with increasing temperature. The damage produced at C contained extensive delaminations and splitting along the fibre axis of the bottom layer. The damage created at C was much smaller in size than that at C. The large damage area at a low temperature is mainly attributed to the brittleness of the epoxy resin at that low temperature. The increased ductility of the matrix at C led to more fracture on the front surface of impact than on the back surface. The upward movement of the fibres on the top ply, as shown in Figure (b), is a typical damage pattern, explaining the increased ductility of the matrix material. Delaminated area (mm ) (b) Interfaces Kevlar mm,. wt% (- C) Kevlar mm,. wt% (RT) Kevlar mm,. wt% (9 C) Kevlar mm,. wt% ( C).. Cross-sectional fractography Cross-sectional fractography was performed to study the effects of Kevlar fibre and temperature on the delaminated area at the laminar interfaces between the adjacent continuous carbon fibre layers. The impacted composite panels were cut along the impact centre in a cruciform shape into four pieces. Each piece was then sectioned by. ~ mm in width producing about strips from each specimen. The edges of the specimens were polished down to µm using diamond paste and observed on an optical microscope. The lengths of the long cracks (delaminations) observed at the interfaces were measured and used to predict the shape of the -D delamination area, using a computer software. Delaminated area (mm ) (c) Interfaces Kevlar mm,. wt% (- C) Kevlar mm,. wt% (RT) Kevlar mm,. wt% (9 C) Kevlar mm,. wt% ( C) Interfaces Figures (a), (b) and (c) display the delamination areas obtained at each interface for composites without and with. wt % and. wt % short fibres, which were damaged with an impact energy of J. As shown in Figure (a), the largest delamination areas at most interfaces were obtained at room temperature for the plain composite. There were significant variations in delamination area through the interface, and the delamination areas were higher than those for composites containing short fibres (Figures (b) and (c)). The delamination areas produced at C were largest for the composites with. wt % and. wt % Kevlar fibres, while those produced at C were smallest, as shown in Figures (b) and (c). It can be predicted from the trend shown in Figure that the plain composite will have the largest delamination area at C. For all laminates, the largest delamination areas were obtained at the 9 th interface that experienced the most bending during impact. Polymers & Polymer Composites, Vol. 9, No.,

6 Min-Seok Sohn, Xiao-Zhi Hu and Jang-Kyo Kim From the cross-sectional fractography, the delamination size of each composite specimen strip was measured and the delamination shape was evaluated as shown in Figures (a) and (b) for the composites containing. wt % Kevlar fibres tested at C and C, respectively. There were extended delaminations in the 9 fibre direction (which is the direction of the bottom ply) at the rd and 9 th laminar interfaces for both composites, while the damage in other interfaces had irregular shapes. The extent of damage produced at the st interface for the composite impacted at C was larger than that at C. This seems to contradict the result exhibited in Figure. However, the composite impacted at C had major cracks on the front surface that cannot be shown at the cross section in Figure. In summary, the total delamination area produced at C (Figure (a)) appears to be much larger than that at C (Figure (b))... Optical microscopy The composite specimens impacted at C and C with an energy of J were cut through the centre of impact using a diamond wheel cutting machine. The edges of the specimens were polished down to µm and examined using an optical microscope. The cross-sectional photographs are presented in Figures (a) and (b). First of all, the overall damage patterns appear to be very different in terms of the structural integrity maintained after impact. The specimen impacted at C (Figure (a)) had extensive fracture through delamination, intra-ply cracking, fibre breakage and transverse matrix cracks around the centre of impact. Delaminations are clearly seen between the individual carbon fibre layers. Two different patterns of intraply cracks were observed, which are long cracks running along the fibre/epoxy matrix interfaces and Figure Delamination patterns of the composites with. wt % Kevlar fibres impacted at (a) C and (b) C (a) fibre direction st nd rd th th th th th 9th (impact centre) Interfaces mm (b) fibre direction st nd rd th th th th th 9th (impact centre) Interfaces mm Polymers & Polymer Composites, Vol. 9, No.,

7 Impact Damage Resistance of Carbon Fibre/Epoxy Composite Laminates Containing Short Kevlar Fibres Figure Polymers & Polymer Composites, Vol. 9, No., Figure Figure 9 Figure Figure Cross-section views for the composites with. wt % Kevlar impacted with J at (a) C and (b) C small transverse matrix cracks connected the delaminations. The translaminar fracture contains extensive fibre fractures from the th carbon fibre layer. The fibre fractures became more extensive in the lower plies, further from the impact. The specimen experienced bending most seriously in the th layer. In fact, the th layer had been almost detached from the rest of the specimen and is not seen in the micrograph. The specimen impacted at C contained mainly long delaminations in the rd, th, th and 9th interlaminar regions, as shown in Figure (b). Some intra-ply cracks are evident in the lower half section of the specimen. It is interesting to note that the delaminations did not propagate directly beneath the impact region, but rather proceeded from the boundary region of the centre. No fibre fracture occurred. The vertical deflection, due to the impact are. mm and. mm for the specimens impacted at C and C, respectively. The difference is mainly due to the increased ductility of the resin matrix with rising temperature. The detailed micrographs at higher magnifications are shown in Figures to. Figure displays fracture and damage created at C in the nd to th carbon fibre layer. The interlaminar region containing Kevlar fibres between nd and rd carbon fibre layers remained undamaged after impact. In the nd carbon fibre layer, a long intra-ply crack was introduced in the vicinity of the interlaminar region between the nd and rd layers. In the rd layer, extensive intra-ply cracks were found. The impact load was high enough to fracture the fibres in the th layer in the translaminar direction. The fibre fractures became more severe in the layers near the back face due to the tensile stress applied upon impact. Figure 9 displays the adjacent area to the region shown in Figure with similar features. It is evident that the interlaminar regions containing Kevlar fibres are not affected by the impact, and intra-ply cracking is the main fracture phenomenon. As mentioned above, in the composite specimen impacted at C, delaminations are a main fracture mechanism. Figure shows the fractured region, including the th

8 Min-Seok Sohn, Xiao-Zhi Hu and Jang-Kyo Kim Figure Micrograph displaying a magnified crosssection from nd to th layer for the composite with. wt % Kevlar fibres of mm in length at C to th fibre layers below the centre of impact. Only intra-ply cracks are produced in the 9th fibre layer and no other fracture was found. This is in contrast to the fracture of specimens impacted at C. Figure is the micrograph showing the progress of delaminations associated with some intra-ply cracks... Scanning electron microscopy µm Figure 9 Adjacent area to the section shown in Figure µm Figure Micrograph displaying cross-section from th to th layer at C SEM was employed to study the fracture mechanisms at the 9th interface between the 9th and th carbon fibre layers. Figure shows the fracture surface of the specimen containing. wt % Kevlar fibres impacted by J at C. The th carbon fibre layer has been completely separated during the impact. It should be noted that the fracture surface in the right hand side of the micrograph represents the th carbon fibre plane, which shows matrix particles. In contrast, the 9th plane shown in the left-handed side of the micrograph had uniform matrix fracture patterns of hackles. Carbon fibres were pulled out by the th plane. In this micrograph, part of the 9th layer was removed showing extensive fibre fracture in the transverse direction. A typical Kevlar fibre fracture produced by an impact energy of J at C is exhibited in Figure. The major fracture mechanisms in Kevlar fibre include extensive splitting in the longitudinal direction. This type of Kevlar fracture has been produced under quasi-static loading conditions for carbon fibre/epoxy composite laminate. Figure displays the side view of the laminate including delamination between the 9th and th carbon fibre planes [ /9 ] for the specimen impacted by J at C. Some damaged Kevlar fibres are shown in the delaminated region. It seems however, that the fibres were detached from the matrix during the impact at C. Meanwhile, Figure shows an µm Figure Delamination and intra-ply crackings in composite impacted at C Figure th (right-handed side) and 9th (left-handed side) fracture surface of composite with Kevlar fibres impacted at C µm Polymers & Polymer Composites, Vol. 9, No.,

9 Impact Damage Resistance of Carbon Fibre/Epoxy Composite Laminates Containing Short Kevlar Fibres Figure Splitting in Kevlar fibres for the composite impacted at C Figure Delaminated area between 9th and th layers showing Kevlar fibres (- C) Figure Fibre bridging between 9th and th layers for the composite impacted at C extensive crack-tip bridge created by the epoxy resin matrix with Kevlar fibres. Initial stages of interfacial failures between the fibres and the matrix were found in the boundary region between the bridge and the 9th layer in the upper part of the micrograph. It is worth to mention that the Kevlar fibres show their positions upright between the two carbon fibre layers. This has not been observed for the other composite laminates, and is attributed mainly to the ductile deformation of the epoxy matrix at C, above Tg. In Figure, a similar bridging structure can be observed. The Kevlar fibres with the epoxy resin adhered well to the th carbon fibre layer, as shown in the bottom of the micrograph. This is in contrast to the delaminated area in the composite impacted at C, as shown in Figure. Figure depicts the delaminated area between the 9th and th carbon layers with the Kevlar fibres. A Kevlar fibre on the top of the other Kevlar appears to be highly stressed. As a result, the Kevlar on the top was bent and the surface was split because of the transverse tension applied during bending. This is another Kevlar fibre fracture mechanism that contributes to the improvement of delamination resistance for composites impacted at C. A higher magnification of the bent Kevlar fibre is shown in Figure. A peculiar Kevlar fracture was introduced. On the top of the bent fibre surface, a gap was induced with small cracks created in both the longitudinal and transverse directions. This may be caused by the intensively applied stress caused by the other Kevlar at the crossover location.. DISCUSSION There are three aspects of this study that are important for the impact responses of carbon fibre/epoxy composite laminates in the temperature range from - C to C. The first aspect is the response of the Figure Fibre bridging bonded to the th layer ( C) Polymers & Polymer Composites, Vol. 9, No., Figure Interactions of Kevlar fibres inhibiting delamination ( C)

10 Min-Seok Sohn, Xiao-Zhi Hu and Jang-Kyo Kim Figure Higher magnification of Figure resin matrix, which varies depending on temperature, i.e., viscoelastic behaviour. The strain energy release rate, G Ic of pure epoxy resin and the epoxy resin matrix in the composite laminates are known to show very different behaviour with an increase in temperature. In fact, the epoxy resin matrix is dominant in maintaining the structural integrity of the composite. The second aspect is the role of Kevlar fibres contained in the interlaminar regions. From the cross-sectional fractography (refer to Figure ), the plain composite laminate without Kevlar fibres did not show consistency in the change of delamination area at each interface, as compared with the results for the composites with Kevlar fibres. The interlaminar regions of the plain composites are so small that the composite cross-sectional views do not show distinct interlaminar regions. However, the composites containing Kevlar fibres form about to µm thick interlaminar regions with excessive resin and Kevlar fibres. The formation of thick interlaminar resin-rich regions decreases the fibre volume ratio over the whole composite body, which may reduce the composite bulk strength. However, because of their strong bond with the epoxy resin matrix, and their function as delamination inhibitor, the Kevlar short fibres efficiently promote crackinterface bridging. Overall, they improve composite delamination resistance and the flexural properties, maintaining the structural integrity of the whole composite. next to or 9 layers. The reason may be that the bending stiffness mismatch between the two adjacent plies is higher in the cross-ply layers, allowing many intra-ply cracks to occur. As shown in the optical micrographs in the previous section, extensive short and long intra-ply cracks were produced within the individual plies. However, the thicker interlaminar regions due to Kevlar fibres induce delamination fractures between the continuous carbon fibre layers at elevated temperature rather than intra-ply cracks. This does not necessarily mean that the delamination sizes are larger in the laminates containing Kevlar fibres than those without. In this case, the composite after impact maintains its structure integrity more efficiently, as seen in the cross-sectional fractographs (Figures (a) and (b)). This is one of the major reasons for the improved impact damage tolerance of the composite laminates at C. The number of Kevlar fibre ends is an important parameter for the fibre-bridging effect, as mentioned in the previous study. Therefore, the length of Kevlar fibres used in the current study needs to be modified to shorter lengths, i.e., - mm, to increase the number of fibre ends for the same weight of Kevlar fibres. In fact, Kevlar fibres of - mm in length were more effective in improving the quasistatic delamination resistance of the carbon fibre/ epoxy composite laminates than those of mm. They will further improve the crack-face bridging effect. It may also be worthwhile to note that poly(pphenylene-,-benzovisoxazole (PBO) short fibres, i.e., Zylon, have been successfully used to toughen composite laminates,. Zylon fibres have higher modulus and elongation ( GPa and. %) than Kevlar fibres (9 GPa and. %, respectively). The preliminary results have shown the effectiveness of Zylon fibres in improving the quasi-static interlaminar fracture toughness and impact resistance under a free-fall impact loading condition. Therefore, Zylon fibre was recommended as a promising interlayer material for fibre reinforced plastic composite laminates. The third aspect is the response of Kevlar fibres associated with fracture mechanisms due to temperature change. The thicker interlaminar region produced by adding Kevlar causes a beneficial effect of reducing intra-ply cracks at C. The formation of intra-ply cracks in the [ /9 ] laminate structure is more critical to the deterioration of impact resistance than it is in those laminates containing layers. CONCLUDING REMARKS The impact responses of carbon fibre/epoxy composite laminates without and with short Kevlar fibres were investigated under drop-weight loading conditions at various temperatures. Fractography of the composite specimens was performed using optical and scanning electron microscopy. Polymers & Polymer Composites, Vol. 9, No.,

11 Impact Damage Resistance of Carbon Fibre/Epoxy Composite Laminates Containing Short Kevlar Fibres The following concluding remarks can be made from this study. The delamination areas of the composites containing short Kevlar fibres were consistently smaller than those of plain composites, confirming the effectiveness of short fibres. Optical microscopy revealed that the composite laminates containing Kevlar fibres maintained their structural integrity more efficiently at high temperatures than at low temperatures. The highly efficient fibre-bridging due to the strong interface bond between the short Kevlar fibres and the epoxy resin matrix was observed in the specimen impacted at C. The short Kevlar fibres presented additional peculiar failure mechanisms, including splitting, fibrillation, and bending. REFERENCES. Russell A.J. and Street K.N., Moisture and temperature effects on the mixed-mode delamination fracture of unidirectional graphite/epoxy, ASTM STP, W.S. Johnson, Ed., American Society for Testing and Materials, Philadelphia, (9), 9-. Bibo G., Leicy P.J. and Kemp M., Hightemperature damage tolerance of carbon fibrereinforced plastics Part : Impact characteristics, Composites, (99), -. Bibo G.A. and Hogg P.J., High-temperature damage tolerance of carbon fibre-reinforced plastics Part : Post-impact compression characteristics, Composites,, (99), 9-. Davies P. and Benzeggagh M.L., Interlaminar mode I fracture testing. In Applications of Fracture Mechanics to Composite Materials, ed. K. Friedrich. Elsevier Science, Amsterdam, (99) -. Hashemi, S., Kinloch, A.J., and Williams, J.G., The effects of geometry, rate and temperature on the mode I, mode II and mixed I/II interlaminar fracture of carbon fibre/peek composites, J. Compos. Sci. Technol., (99) -. Karasek, M.L., Strait, L.H., Amateau, M.F. and Runt, J.P., Effect of temperature and moisture on the impact behaviour of graphite/epoxy composites: Part II Impact Damage J. Comp. Tech. & Res. () (99) -. Bouadi, H. & Sun, C.T., Hygrothermal effects on structural stiffness and structural damping of laminated composites, J. Mater. Sci., (99) 99-. Kellas, S., Morton, J. & Curtis, P.T., The effect of hygrothernal environments upon the tensile and compressive strengths of notched CFRP laminates. Part : Static loading, Composites, (99) - 9. Birger, S., Moshonov, A. and Kenig, S., The effects of thermal and hygrothermal ageing on the failure mechanisms of graphite-fabric epoxy composites subjected to flexural loading, Composites, (99) -. Levin, K., Effect of low-velocity impact on compression strength of quasi-isotropic laminate. In Proceedings of American Society for Composites: st Technical Conference. Technomic, Lancaster, PA (9) -. Lagace P.A., Delamination in composites: is toughness the key? st International SAMPE Symposium, April - (9) -9. Kessler A. and Bledzki A.K., Low velocity impact behaviour of glass/epoxy cross-ply laminates with different fibre treatments, Polym. Com. () (999) 9-. Hirai Y., Hamada H. and Kim J.K., Impact response of woven glass-fabric composites - I. Effect of fibre surface treatment, Comp. Sci. & Tech. (99) 9-. Masters J.E., Improved impact and delamination resistance through interleafing, Key Eng. Mater. (99) -. Lu W.H., Liao F.S., Su A.C., Kao P.W., Hsu T.J., Effect of interleaving on the impact response of a unidirectional carbon/epoxy composite, Composites () (99) -. Rechak S. and Sun C.T., Optimal use of adhesive layers in reducing impact damage in composite laminates, J. Rein. Plast.& Comp. 9 (99) 9- Polymers & Polymer Composites, Vol. 9, No.,

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