K.L. Choy et al./ Materials Science and Engineering 4278 (2000)187-194 the values appear to be particularly high if compared evidence of a reaction layer could be observed, though with similar glass-ceramic systems. For our matrix a more in depth investigation possibly including TEM rich, sample C, we expect a lower failure stress. This is would be beneficial in helping to explain the observed due to it being a composite with an equivalent volume fracture behaviour fraction of fibres and where these carbon theoretically have the same strength as the silicon carbide as shown 3.3. Interlaminar shear properties in Table 1. In the case of sample A, we would expect a value of aeu comparable to C. This would indicate a The results obtained at a cross-head rate of 1.3 mm strength well over 200 MPa Discrepancies between the min are summarised in Table 7. A typical load-dis- predicted results and the final outcome may well be due placement graph obtained during testing is shown in to interface degradation during processing, oxidation of Fig 3 carbon fibres at elevated temperatures and the brittle Interlaminar shear strength(ILS)tests conducted at a reaction layer formed between fibres and borosilicate span to depth ratio of 7 produced pure shear deforma matrix may well have produced what appears to be tion in all samples, making test results valid for calcula reduced mechanical properties. tion Damage tended to concentrate in the matrix Young's modulus values are in good agreement with regions only, and for cross-ply samples this meant similar glass-ceramic composites with a cross-ply lay shearing between lamina. The formation of a crack up[17] though sample C would be expected to have a corresponded to the rounded peak on the load maxim lower modulus value due to is lower volume fraction After shear crack formation the load drop is not The most interesting value is that obtained for sample catastrophic and a diminished load is maintained dur A(69 GPa)which is extremely low, especially when ing further deformation, corresponding to a substantial ROM predictions using data obtained in Table I where energy absorption Carbon HM fibres have a modulus almost 50% A similar study on a Las glass-ceramic reinforced greater than for silicon carbide, are considered. These with silicon carbide [18] determined that this load bear bility of the composite after shearing can be interfacial reactions must have taken place during pro- attributed to the combined effect of sliding resistance of cessing, resulting in fibre degradation and embrittle ment of the composite. Some initial SEM beams produced after failure characterisation of the interface was attempted but, no A study of the micrographs of these samples pro- vided an insight into the variation in results between Table 7 samples C and D, i.e. absence of matrix between the The interlaminar shear strength results plies which would result in reduced ILS strength. Mi crographs of C also indicated significant porosity be- ample TaPP(MPa) tween the plies in these matrix-rich regions. If these A 13.3+0.2 defects, that can reduce strengths of composites, could B 24.6+0.5 be reduced by altering the processing conditions, it may 33.9±43 be possible to achieve even higher ILs values that D 15.1+0.7 would results in improved off-axis properties for these composites The values obtained and the small scatter in results 400 were very encouraging and contrary to the data repro- 35 ducibility problems encountered, as stated in the intro- duction, by previous workers. ILS strengths for a 250 es cross-ply silicon carbide (Vr=35%) reinforced CAS found to lie in the range 35-45 Sample C having a similar volume fraction of fibres was 6 9 found to have a comparable iLs value of 33.9 +4.3 MPa indicating good agreement in our results with 100 published data. Sample C exhibited the highest value for all four samples. The other two cross-ply laminates (A and D)exhibited similar shear strength values, though the reasons for their diminished results (cf with C)can be attributed to different reasons. Failure, as stated, occurred in the matrix rich regions of the Fig. 3. Load-displacement curve for short beam shear test(Sample matrix meant shear failure was observed on application192 K.-L. Choy et al. / Materials Science and Engineering A278 (2000) 187–194 the values appear to be particularly high if compared with similar glass–ceramic systems. For our matrix rich, sample C, we expect a lower failure stress. This is due to it being a composite with an equivalent volume fraction of fibres and where these carbon theoretically have the same strength as the silicon carbide as shown in Table 1. In the case of sample A, we would expect a value of scu comparable to C. This would indicate a strength well over 200 MPa. Discrepancies between the predicted results and the final outcome may well be due to interface degradation during processing, oxidation of carbon fibres at elevated temperatures and the brittle reaction layer formed between fibres and borosilicate matrix may well have produced what appears to be reduced mechanical properties. Young’s modulus values are in good agreement with similar glass–ceramic composites with a cross-ply lay￾up [17], though sample C would be expected to have a lower modulus value due to is lower volume fraction. The most interesting value is that obtained for sample A (69 GPa) which is extremely low, especially when ROM predictions using data obtained in Table 1 where ‘Carbon HM’ fibres have a modulus almost 50% greater than for silicon carbide, are considered. These results enforce the idea that significant fibre/matrix interfacial reactions must have taken place during pro￾cessing, resulting in fibre degradation and embrittle￾ment of the composite. Some initial SEM characterisation of the interface was attempted but, no evidence of a reaction layer could be observed, though a more in depth investigation possibly including TEM would be beneficial in helping to explain the observed fracture behaviour. 3.3. Interlaminar shear properties The results obtained at a cross-head rate of 1.3 mm min−1 are summarised in Table 7. A typical load–dis￾placement graph obtained during testing is shown in Fig. 3. Interlaminar shear strength (ILS) tests conducted at a span to depth ratio of 7 produced pure shear deforma￾tion in all samples, making test results valid for calcula￾tion. Damage tended to concentrate in the matrix regions only, and for cross-ply samples this meant shearing between lamina. The formation of a crack corresponded to the rounded peak on the load maxim. After shear crack formation the load drop is not catastrophic and a diminished load is maintained dur￾ing further deformation, corresponding to a substantial energy absorption. A similar study on a LAS glass–ceramic reinforced with silicon carbide [18] determined that this load bear￾ing capability of the composite after shearing can be attributed to the combined effect of sliding resistance of the shear cracks and load bearing from the two half beams produced after failure. A study of the micrographs of these samples pro￾vided an insight into the variation in results between samples C and D, i.e. absence of matrix between the plies which would result in reduced ILS strength. Mi￾crographs of C also indicated significant porosity be￾tween the plies in these matrix-rich regions. If these defects, that can reduce strengths of composites, could be reduced by altering the processing conditions, it may be possible to achieve even higher ILS values that would results in improved off-axis properties for these composites. The values obtained and the small scatter in results were very encouraging and contrary to the data repro￾ducibility problems encountered, as stated in the intro￾duction, by previous workers. ILS strengths for a cross-ply silicon carbide (Vf=35%) reinforced CAS system were found to lie in the range 35–45 MPa [19]. Sample C having a similar volume fraction of fibres was found to have a comparable ILS value of 33.994.3 MPa indicating good agreement in our results with published data. Sample C exhibited the highest value for all four samples. The other two cross-ply laminates (A and D) exhibited similar shear strength values, though the reasons for their diminished results (cf. with C) can be attributed to different reasons. Failure, as stated, occurred in the matrix rich regions of the com￾posite. Samples A with a lower strength borosilicate matrix meant shear failure was observed on application Table 7 The interlaminar shear strength results Sample tc app (MPa) A 13.390.2 B 24.690.5 C 33.994.3 D 15.190.7 Fig. 3. Load–displacement curve for short beam shear test (Sample A)