Journal of the American Ceramic Society-Zhu et al. Vol. 82. N g ● Enhancec Hi- 郐- 60 20 10110210310410510107108 101102103104 Cycles to F Cycles to Failure Fig. 12. Maximum stress versus cycles to failure for fatigue in the Fig. 15. Maximum stress versus cycles to failure for fatigue in the Hi-NicalonTMSiC, enhanced SiC/SiC, and standard SiC/SiC compos- standard SiC/SiC composite in air and argon at 1300C ites in air at I300°C crep,1300°c … Argon 10-10 Stress, MPa mum creep strain rate as a function of stress in the standard SiC/SiC composite at 1300.C in air and argon 30um …Argo Fig. 16. Crack initiated at a large pore in the Hi-NicalonTM/SiC specimen crept in air at 1300C and 150 MPa for 480 s. The crack propagates to a fiber ized by the value obtained from the linear portion of the loop during the first loading)versus the number of cycles is shown in Fig. 24(a). At stresses 2120 MPa, the modulus decreases 102103 rapidly within ten cycles, then gradually decreases, and finally Time to Rupture, s decreases rapidly up to fracture. The shape of the curves is similar to the creep strain curves(Fig. 3). At stresses <105 Fig. 14. Time to rupture versus stress in the standard Sic/Sic com- MPa, the modulus initially remained constant up to 10 cycles posite at 1300C in air and argon and then monotonously decreased. At 75 MPa, the modulus remained constant up to 107 cycles, at which point the test was stopped. When the modulus decreased to 20%40% of the To understand the damage evolution and degradation mecha- original value, the specimens fractured nism during fatigue and creep, elastic moduli were measured. The change of the modulus during creep with time in ai Figure 23 shows the evolution of the stress-strain hysteresis ( Fig. 24(b)is similar to that during fatigue(Figs. 24(a). How- loops. The slope decreases and the width of the loops increases ever, the limiting modulus for fracture is 50%-60%, which is as the number of cycles increases. The former indicates th higher than that under fatigue. This result is the same as the decrease of the modulus, and the latter represents the decrease results for the standard SiC/SiC composite, 0 in which it was of the interfacial sliding resistance. The hysteresis loops move explained by the longer debonding of the interfaces under fa to the right along the strain axis(which is known as ratchetting) tigue. When cree?, 30 MPa, at this stress, the specimen did not because of time-dependent deformation. The modulus(normal with time, even atTo understand the damage evolution and degradation mecha￾nism during fatigue and creep, elastic moduli were measured. Figure 23 shows the evolution of the stress–strain hysteresis loops. The slope decreases and the width of the loops increases as the number of cycles increases. The former indicates the decrease of the modulus, and the latter represents the decrease of the interfacial sliding resistance. The hysteresis loops move to the right along the strain axis (which is known as ratchetting) because of time-dependent deformation. The modulus (normal￾ized by the value obtained from the linear portion of the loop during the first loading) versus the number of cycles is shown in Fig. 24(a). At stresses $120 MPa, the modulus decreases rapidly within ten cycles, then gradually decreases, and finally decreases rapidly up to fracture. The shape of the curves is similar to the creep strain curves (Fig. 3). At stresses #105 MPa, the modulus initially remained constant up to 104 cycles and then monotonously decreased. At 75 MPa, the modulus remained constant up to 107 cycles, at which point the test was stopped. When the modulus decreased to 20%–40% of the original value, the specimens fractured. The change of the modulus during creep with time in air (Fig. 24(b)) is similar to that during fatigue (Figs. 24(a)). How￾ever, the limiting modulus for fracture is 50%–60%, which is higher than that under fatigue. This result is the same as the results for the standard SiC/SiC composite,10 in which it was explained by the longer debonding of the interfaces under fa￾tigue. When creep tests are in argon, the modulus can decrease with time, even at 30 MPa; at this stress, the specimen did not Fig. 12. Maximum stress versus cycles to failure for fatigue in the Hi-Nicalon™/SiC, enhanced SiC/SiC, and standard SiC/SiC compos￾ites in air at 1300°C. Fig. 13. Minimum creep strain rate as a function of stress in the standard SiC/SiC composite at 1300°C in air and argon. Fig. 14. Time to rupture versus stress in the standard SiC/SiC com￾posite at 1300°C in air and argon. Fig. 15. Maximum stress versus cycles to failure for fatigue in the standard SiC/SiC composite in air and argon at 1300°C. Fig. 16. Crack initiated at a large pore in the Hi-Nicalon™/SiC specimen crept in air at 1300°C and 150 MPa for 480 s. The crack propagates to a fiber being oxidized. 122 Journal of the American Ceramic Society—Zhu et al. Vol. 82, No. 1