T. Ogasawara et al. Composites Science and Technology 65(2005)2541-2549 ●b=6 o b=9 mm 冒06 0 0.4 0100150200250300350 0.1mm Fig. 11. Effect of on-axis tensile stress on torsional rigidity G. The Loading direction torsional rigidity was normalized by the initial values of GJo Fig. 13. Optical micrograph of the replica films, illustrating matrix cracking within the transverse (90 )fiber bundles at 320 MPa The torsional rigidity GJ is normalized by the initial for a 6 mm width specimen, and 428 MPa for a 9 mm 320 MPa, is shown in Fig. 13. Beyerle et al. [4] reported width specimen, respectively. The torsional rigidity that the shear stiffness decreased under pure shear stres values were measured up to on-axis stress levels of (losipescu shear test). The experimental results suggest 320 MPa. Because the torsion moment versus twist that the shear stiffness degradation is caused by on-axis angle relation was linear, it is suggested that any crack tensile stress as well as shear stress, and this is due to propagation did not occur during the torsional tests transverse cracking in 90 fiber bundles and matrix The torsional rigidity finally decreased by 60%of the cracking in the 0o fiber bundles initial value at 320 MPa as shown in Fig. 11 The relationship between the transverse crack density Degradation of the torsional rigidity indicates that and maximum tensile stress is represented in Fig. 14 shear moduli Gx and Gex vary due to on-axis tensile The onset of transverse crack propagation is approxl- mately 40 MPa. The transverse crack initiation stress I-axis tensile stress is shown in Fig. 12, which repre- corresponds to the onset of decrease in GJ( see Fi sents the decrease of both Gry and G=r. As reported in I1), and the crack density reaches 11-12 mmat the previous study [91, SiC/SiC woven composites suffer 320 MPa. From Fig. 12, it is seen that the tensile stress from two major damage modes under tensile stress: (1) at the onset of decrease of Gxy coincides with the trans transverse cracking in the transverse(90%)fiber bundles, verse crack initiation stress (40 MPa), whereas almost and(2)matrix cracking in the longitudinal(0%) fiber no degradation of Ger is recognized until the tensile bundles.An optical micrograph of the replica films, stress of 100 MPa. In laminated composites, it is well illustrating transverse cracking within the 90o fiber known that in-plane shear modulus degrades due to bundles and matrix cracking in the 0 fiber bundles at 10 100150200250300350 Maximum tensile stress(MPa) Fig. 12. The relationship between shear moduli(Gxv, Gex) and on-axi Fig 14. Transverse crack density in the 90 fiber bundles as a function of the on-axis maximum tensile stressThe torsional rigidity GJ is normalized by the initial value GJ0. The ultimate tensile strength was 414 MPa for a 6 mm width specimen, and 428 MPa for a 9 mm width specimen, respectively. The torsional rigidity values were measured up to on-axis stress levels of 320 MPa. Because the torsion moment versus twist angle relation was linear, it is suggested that any crack propagation did not occur during the torsional tests. The torsional rigidity finally decreased by 60% of the initial value at 320 MPa as shown in Fig. 11. Degradation of the torsional rigidity indicates that shear moduli Gxy and Gzx vary due to on-axis tensile stress. The relationship between shear moduli and on-axis tensile stress is shown in Fig. 12, which repre￾sents the decrease of both Gxy and Gzx. As reported in the previous study [9], SiC/SiC woven composites suffer from two major damage modes under tensile stress: (1) transverse cracking in the transverse (90
) fiber bundles, and (2) matrix cracking in the longitudinal (0
) fiber bundles. An optical micrograph of the replica films, illustrating transverse cracking within the 90
fiber bundles and matrix cracking in the 0
fiber bundles at 320 MPa, is shown in Fig. 13. Beyerle et al. [4] reported that the shear stiffness decreased under pure shear stress (Iosipescu shear test). The experimental results suggest that the shear stiffness degradation is caused by on-axis tensile stress as well as shear stress, and this is due to transverse cracking in 90
fiber bundles and matrix cracking in the 0
fiber bundles. The relationship between the transverse crack density and maximum tensile stress is represented in Fig. 14. The onset of transverse crack propagation is approxi￾mately 40 MPa. The transverse crack initiation stress corresponds to the onset of decrease in GJ (see Fig. 11), and the crack density reaches 11–12 mm
1 at 320 MPa. From Fig. 12, it is seen that the tensile stress at the onset of decrease of Gxy coincides with the trans￾verse crack initiation stress (40 MPa), whereas almost no degradation of Gzx is recognized until the tensile stress of 100 MPa. In laminated composites, it is well known that in-plane shear modulus degrades due to 0 50 100 150 200 250 300 350 0 10 20 30 40 50 Shear modulus, Gxy, Gzx (GPa) Gxy Gzx Fig. 12. The relationship between shear moduli (Gxy,Gzx) and on-axis tensile stress. Fig. 13. Optical micrograph of the replica films, illustrating matrix cracking within the transverse (90
) fiber bundles at 320 MPa. 0 100 200 300 0 2 4 6 8 10 12 14 Maximum tensile stress (MPa) Transverse crack density (mm-1) Fig. 14. Transverse crack density in the 90
fiber bundles as a function of the on-axis maximum tensile stress. 0 50 100 150 200 250 300 350 0.2 0.4 0.6 0.8 1 Normalized torsional rigidity, GJ /G J 0 b =6 mm b =9 mm Fig. 11. Effect of on-axis tensile stress on torsional rigidity GJ. The torsional rigidity was normalized by the initial values of GJ0. T. Ogasawara et al. / Composites Science and Technology 65 (2005) 2541–2549 2547