Y. Xu et al./ Ceramics International 27(2001)565-570 3. 2. Failure behavior under shear loading The shear failure behavior of the stress-deflection curve was similar to that of bending failure behavior and The shear strength of the 3D Hi-Nicalon SiC/Sic shown in Fig. 5. The obtained value of shear strength composites was measured by the short bending beam was 67.5 MPa which is much higher than that of 3D C method, and the span was 15 mm. Shear strength was SiC textile composites. Different from two-dimensional calculated by the following equation CFCCs and the other laminated composites, interlayer debonding was not observed in the present composites () The results indicated that the present 3D Hi-Nicalon Sic/SiC composites exhibited good shear resistant and where p is the maximum fracture load(N), b and h are isotropic properties the width and height of the sample respectively 3.3. Failure behavior of notched specimen under flexural loading In order to determine the fracture toughness of the composites, samples were notched and tested in a three-point-bending sample with a span of 40 mm and a loading rate of 0.05 mm mm-. The width of the notch was 0.01 mm. The value of fracture toughness(Kls) was calculated by using of the following expression 0.204060.8101 KIc 3-3.072+145()2-25.07 Displacement, mm +25.80 ig. 5. Stress-deflection curve under shear loading Fig. 6. Microstructure differences between(a) Hi-Nicalon SiC/SiC and (b)C/SiC composites Fig. 7. Failure behavior of the 3D Hi-Nicalon SiC/SiC composites with a notch3.2. Failure behavior under shear loading The shear strength of the 3D Hi-Nicalon SiC/SiC composites was measured by the short bending beam method, and the span was 15 mm. Shear strength was calculated by the following equation:
¼ 3p 4bh ð1Þ where p is the maximum fracture load (N), b and h are the width and height of the sample respectively. The shear failure behavior of the stress–deflection curve was similar to that of bending failure behavior and shown in Fig. 5. The obtained value of shear strength was 67.5 MPa which is much higher than that of 3D C/ SiC textile composites. Different from two-dimensional CFCCs and the other laminated composites, inter1ayer debonding was not observed in the present composites. The results indicated that the present 3D Hi-Nicalon SiC/SiC composites exhibited good shear resistant and isotropic properties. 3.3. Failure behavior of notched specimen under flexural loading In order to determine the fracture toughness of the composites, samples were notched and tested in a three-point-bending sample with a span of 40 mm and a loading rate of 0.05 mm mm
1 . The width of the notch was 0.01 mm. The value of fracture toughness (K1c) was calculated by using of the following expression: K1c ¼ 3pl 2bw2
1:93
3:07 a w þ 1:45 a w
2
25:07 a w
3 þ 25:80 a w
4 ð2Þ Fig. 5. Stress–deflection curve under shear loading. Fig. 6. Microstructure differences between (a) Hi-Nicalon SiC/SiC and (b) C/SiC composites. Fig. 7. Failure behavior of the 3D Hi-Nicalon SiC/SiC composites with a notch. 568 Y. Xu et al. / Ceramics International 27 (2001) 565–570