K.L. Choy et al./ Materials Science and Engineering 4278 (2000)187-194 Table 5 Fibre volume fraction and pore Sample Fibre volume fraction, V(%) Porosity 300 ( magnification×100 250 35+1 5.55+0.92 200 The number of fields for fibre volume fraction and porosity were 115+0.3 45+4 0.9+0.85 where a is tensile strength, I is gauge length of exten- someter, P is load on sample and d is sample thickness The tensile results obtained are tabulated in table 6 0.2 0.6 and a typical stress-strain curve is shown in Fig. 2. STRAIN The stress-strain curves for tensile testing exhibit wo proportional limits, corresponding to initiation of Otu - transverse cracking in 90 plies transverse cracking in the 90 plies and matrix microc Omu=matrix microcracking in 0 plies stresses in the 0 plies. A lower transverse stress is expected for sample A, Fig. 2. Stress-strain curve observed of sample DProportional limits which has a matrix as opposed to samples C and for otu, and omu are shown by arrows. D whose BMAs matrix is a glass-ceramic having higher strengths. Sample C exhibits small variation in results whereas d has a large variation that makes the composite failure stress, cu. The microcracking comparison between the two glass-ceramic sample behaviour of the (0/90)3s, CAS composite with 45% difficult. A similar study conducted on a CAS matrix silicon carbide fibres [16], was found to be the highest posite reinforced with Nicalon fibres in a(0/90) literature value for cross-ply composites, with matrix figuration (V=45%)observed non-linearity for microcracking in the 0o plies and final composite failure stress-strain curves at 55 mPa which is much lower at 175 and 284 MPa respectively, indicating a much broader stress range between the two deformation than the observed value for our glass-ceramic mechanisms than for our composites. Narrow stress composites. ranges betw een microcracking The next deviation on the stress-strain curve, mark- hence, high stresses for the second proportional limit, ing the onset of matrix microcracking in the 0 plies is can be expected if the coefficients of thermal expansion again low. This was expected for the borosilicate sam-(CTE) of the matrix is lower than that of the fibre ple. In light of image analysis work conducted, the resulting in the compression of the matrix and subse- lower value of 210MPa for sample C(cf. with D)is quent suppression of matrix microcracking expected. We would expect composite containing only example, the CTE of silicon carbide(Tyranno)is 4.8x 33+ 2% of fibres (cf. with sample D which has Vr 10-6 K-I and the borosilicate glass matrix is 3.5 x 45+ 6%)to have a lower microcracking stress as dis- 10-6 K[3]. However, little is known about the cussed earlier where the Averston Cooper and Kelley residual stress distribution in this particular composite, (ACK) model predicted tresses for crack but this explanation could well explain the observed ing with increasing fibre volume fraction. The value of phenomena 261+8 MPa achieved for sample D is remarkably high The failure of the end tabs meant tensile strength for matrix microcracking stress and was very close to results for sample d only were obtained. As indicated Table 6 Measured tensile properties of samples A to D Gt (mPa Lu(%0) σmu(MPa) (%) (MPa) E(GPa) 0.06 his sample was unsuitably thick for tensile testing 97+1 0.08+0.04 0.04 l01+13 88±27 0.08+003 26l+8 0.38+0.3 290.320 100+1K.-L. Choy et al. / Materials Science and Engineering A278 (2000) 187–194 191 Table 5 Fibre volume fraction and porosity for compositea Fibre volume fraction, Vf Sample (%) Porosity (%) (magnification ×100) (magnification ×500) A 0.85 3591 90.03 B 5.55 4692 90.92 C 3392 1.1590.31 D 0.9 4594 90.85 a The number of fields for fibre volume fraction and porosity were 10 and 20, respectively. Fig. 2. Stress–strain curve observed of sample D. Proportional limits for stu, and smu are shown by arrows. where s is tensile strength, l is gauge length of exten￾someter, P is load on sample and d is sample thickness. The tensile results obtained are tabulated in Table 6 and a typical stress–strain curve is shown in Fig. 2. The stress–strain curves for tensile testing exhibit two proportional limits, corresponding to initiation of transverse cracking in the 90° plies and matrix microc￾racking at higher stresses in the 0o plies. A lower transverse cracking stress is expected for sample A, which has a glass/matrix as opposed to samples C and D whose BMAS matrix is a glass–ceramic having higher strengths. Sample C exhibits small variation in results whereas D has a large variation that makes comparison between the two glass–ceramic samples difficult. A similar study conducted on a CAS matrix composite reinforced with Nicalon fibres in a (0/90)3S configuration (Vf=45%) observed non-linearity for stress–strain curves at 55 MPa which is much lower than the observed value for our glass–ceramic composites. The next deviation on the stress–strain curve, mark￾ing the onset of matrix microcracking in the 0° plies is again low. This was expected for the borosilicate sam￾ple. In light of image analysis work conducted, the lower value of 210MPa for sample C (cf. with D) is expected. We would expect composite containing only 3392% of fibres (cf. with sample D which has Vf= 4596%) to have a lower microcracking stress as dis￾cussed earlier where the Averston Cooper and Kelley (ACK) model predicted increases in stresses for crack￾ing with increasing fibre volume fraction. The value of 26198 MPa achieved for sample D is remarkably high for matrix microcracking stress and was very close to the composite failure stress, scu. The microcracking behaviour of the (0/90)3s, CAS composite with 45% silicon carbide fibres [16], was found to be the highest literature value for cross-ply composites, with matrix microcracking in the 0° plies and final composite failure at 175 and 284 MPa respectively, indicating a much broader stress range between the two deformation mechanisms than for our composites. Narrow stress ranges between microcracking and final failure, and hence, high stresses for the second proportional limit, can be expected if the coefficients of thermal expansion (CTE) of the matrix is lower than that of the fibre, resulting in the compression of the matrix and subse￾quent suppression of matrix microcracking [3]. For example, the CTE of silicon carbide (Tyranno) is 4.8× 10−6 K−1 and the borosilicate glass matrix is 3.5× 10−6 K−1 [3]. However, little is known about the residual stress distribution in this particular composite, but this explanation could well explain the observed phenomena. The failure of the end tabs meant tensile strength results for sample D only were obtained. As indicated, Table 6 Measured tensile properties of samples A to D stu (MPa) otu (%) smu (MPa) omu (%) scu (MPa) Ec (GPa) A 50.6 0.06 152 0.2 – 69 B This sample was unsuitably thick for tensile testing C 9791 0.0890.04 210 0.044 – 101913 D 0.08 88927 261 90.03 98 0.3890.3 290, 320 10091