K.L. Choy et al./ Materials Science and Engineering 4278 (2000 )187-194 4. Conclusions 450 00 Micrographs and image analysis work showed the four samples(A, B, C and D) exhibited a certain degree of inhomogeneity within the plies, where fibre bundling could be contrasted with less tightly packed areas 4009 Cross-ply laminates, and in particular Sample A and C exhibited matrix rich regions in between plies and sig- nificant porosity due to poor consolidation of the ma- 150 trix. Image analysis work determined fibre volume fractions and indicated porosity levels Sample A exhibited low transverse ou and matrix microcracking om due to its borosilicate matrix. Evi- 0 dence of low composite failure stress and Youngs modulus for this sample indicated the possibility of DEFLECTION (mm fibre/matrix reactions during processing resulting in embrittleness. Both glass-ceramic samples exhibited Fig. 4. Load-deflection curve for flexural testing(Sample C) and it as suggested tha the suppression of microcracking in the matrix was due Table 8 to mismatches in the coefficients of thermal expansions Flexural properties of some glass-ceramic composites between fibre/matrix, resulting in the matrix being un- Reference Matrix Fibre Lay-up vr%) acP EcP der compression. Final composite strengths were only obtained for 90)3s34±5530115 sample D, which was found to be in excess of 300 MPa, CAS Nicalon (0/90)s 19 320 and considerably higher than reported literature values on similar glass-ceramic systems. Only ILs could be performed on sample B(due to its unsuitable thick- ness). The short beam shear test was found to be successful in terms of data reproducibility, for the of a lower load. Sample B should necessarily exhibit a determination of the interlaminar shear properties of higher strength than A, even though they have the same these composites. Failure in matrix rich regions was matrix material due to its unidirectional lay-up. The observed and it was found that a layer of matrix results obtained of 24.6 MPa +0.5 must be dependent between plies, present in lower-fibre volume fraction on the significant porosity observed on both micro- composites improved this off-axis property of these graphs and during image analysis work materials. Flexural testing was only found to be suc cessful, when a correct span to depth ratio was chosen. 3. 4. Flexural properties Thus illustrating the poor off-axis composite properties; indicating the promising potential for this silicon car Samples A, C, and D were tested, only sampleC bide reinforced BMAS system, in terms of mechanical failed in a flexural manner required for calculations of properties apparent strength(oupp) and modulus(ecpP). For Sam This study appears to be the first to investigate the ple C, aapp was found to be 678+ 22 MPa and Eapp axial and off-axis properties of glass-ceramic com was 142+9 GPa. A load-displacement curve produced posites with varying volume fractions of fibres. While during testing is shown in Fig. 4 tensile strengths are dictated by the failure strength of Mixed mode failure, with multiple shearing for sam- the fibres, and increasing volume fraction of fibres ples A and d meant that apparent strengths and mod- results in an increase in strength, the opposite appears uli could not be obtained. This highlights the extremely to be the case for interlaminar shear strengths. This off poor off-axis properties for these composites, resulting axis property appears to be dependent on matrix rich in their anisotropic behaviour. Sample C failed in the regions between plies and increased ILS strengths are manner required for calculations. Strengths and moduli favoured by lower-fibre-volume fraction composite for similar glass-ceramic composites are presented in These phenomena must be thoroughly considered dur- Table 8. The value of acpp of 678+ 22 MPa is ex- ing composite design and criteria for the degree of remely high in comparison with those obtained from acceptable anisotropy should be taken into account the literature and indicates that the possibilities fo A homogeneous sample is of vital importance. Inhe these silicon carbide reinforced BMAS composites are mogeneous microstructures lead to large standard devi very promising in terms of mechanical properties. ation in results making design selection very difficult forK.-L. Choy et al. / Materials Science and Engineering A278 (2000) 187–194 193 Fig. 4. Load–deflection curve for flexural testing (Sample C). 4. Conclusions Micrographs and image analysis work showed the four samples (A, B, C and D) exhibited a certain degree of inhomogeneity within the plies, where fibre bundling could be contrasted with less tightly packed areas. Cross-ply laminates, and in particular Sample A and C exhibited matrix rich regions in between plies and sig￾nificant porosity due to poor consolidation of the ma￾trix. Image analysis work determined fibre volume fractions and indicated porosity levels. Sample A exhibited low transverse stu and matrix microcracking smu due to its borosilicate matrix. Evi￾dence of low composite failure stress and Young’s modulus for this sample indicated the possibility of fibre/matrix reactions during processing resulting in embrittleness. Both glass–ceramic samples exhibited high values for stu and smu, and it was suggested that the suppression of microcracking in the matrix was due to mismatches in the coefficients of thermal expansions between fibre/matrix, resulting in the matrix being un￾der compression. Final composite strengths were only obtained for sample D, which was found to be in excess of 300 MPa, and considerably higher than reported literature values on similar glass–ceramic systems. Only ILS could be performed on sample B (due to its unsuitable thick￾ness). The short beam shear test was found to be successful in terms of data reproducibility, for the determination of the interlaminar shear properties of these composites. Failure in matrix rich regions was observed and it was found that a layer of matrix between plies, present in lower-fibre volume fraction composites improved this off-axis property of these materials. Flexural testing was only found to be suc￾cessful, when a correct span to depth ratio was chosen. Thus illustrating the poor off-axis composite properties; indicating the promising potential for this silicon car￾bide reinforced BMAS system, in terms of mechanical properties. This study appears to be the first to investigate the axial and off-axis properties of glass–ceramic com￾posites with varying volume fractions of fibres. While tensile strengths are dictated by the failure strength of the fibres, and increasing volume fraction of fibres results in an increase in strength, the opposite appears to be the case for interlaminar shear strengths. This off axis property appears to be dependent on matrix rich regions between plies and increased ILS strengths are favoured by lower-fibre-volume fraction composite. These phenomena must be thoroughly considered dur￾ing composite design and criteria for the degree of acceptable anisotropy should be taken into account. A homogeneous sample is of vital importance. Inho￾mogeneous microstructures lead to large standard devi￾ation in results making design selection very difficult for Table 8 Flexural properties of some glass–ceramic composites Reference Fibre Lay-up Matrix Vf (%) scu app Ec app 20 (0 CAS Nicalon /90)3s 3495 115 530 (0/90) 17 320 – CAS Nicalon 4s 19 of a lower load. Sample B should necessarily exhibit a higher strength than A, even though they have the same matrix material due to its unidirectional lay-up. The results obtained of 24.6 MPa90.5 must be dependent on the significant porosity observed on both micro￾graphs and during image analysis work. 3.4. Flexural properties Samples A, C, and D were tested, only sample C failed in a flexural manner required for calculations of apparent strength (su app) and modulus (Ec app). For Sam￾ple C, su app was found to be 678922 MPa and Ec app was 14299 GPa. A load-displacement curve produced during testing is shown in Fig. 4. Mixed mode failure, with multiple shearing for sam￾ples A and D meant that apparent strengths and mod￾uli could not be obtained. This highlights the extremely poor off-axis properties for these composites, resulting in their anisotropic behaviour. Sample C failed in the manner required for calculations. Strengths and moduli for similar glass–ceramic composites are presented in Table 8. The value of scu app of 678922 MPa is ex￾tremely high in comparison with those obtained from the literature and indicates that the possibilities for these silicon carbide reinforced BMAS composites are very promising in terms of mechanical properties