Y. Miyashita et al. /International Journal of Fatigue 24(2002)241-248 Loading direction Loading direction notch#+- 20 um Loading direction notch s To0um Observations of fatigue crack growth path in standard SiC/SiC with frequency of 1 Hz at room temperature: (a) at maximum stress of 60 MPa after 5000 cycles;(b) at maximum stress of 80 MPa after 1 cycle; (c)final fracture at maximum stress of 100 MPa after I cycle room temperature was higher than that at high tempera- 4.2. Crack initiation and propagation processes ture. Comparable test results from the literature which were obtained at room temperature and at 1000C for Fig 3 shows examples of the crack growth path in unnotched specimens [4 are also shown in Fig. 2. Not standard SiC/SiC at room temperature. In this instance rprisingly, the fatigue resistance of the notched speci- the notch was situated in a longitudinal fiber bundle mens was less than that of the smooth specimens. The which was oriented in the same direction as the loading shorter fatigue lives in notched specimens resulted not axis. A crack initiated at the interface between fiber and only from the stress concentration at the notch tip but matrix near the notch tip (indicated by an arrow in Fig also from differences in loading history. The applied 3(a)). As this crack was growing along the interface stress was increased stepwise during fatigue test in the fiber-cracking occurred( Fig. 3(b). The crack then con- present study, and fatigue damage may have developed tinued to grow by a combination of interfacial cracking at each applied stress level. The fatigue strength at and fiber-cracking. However, the cracks initiated not 800C is seen to be greater than at room temperature. It only at the notch, some cracks initiated at pores and is also noted that although the difference in fatigue life where fiber bundles of different orientation met between standard and enhanced SiC/SiC was in general (indicated by(A)in Fig. 3(c). Nevertheless, the final not significant, the standard material did have a slightly fracture process always involved considerable crack longer fatigue life than the enhanced material at room branching and a linking up with a crack initiated at the temperature and at 800oC notch( Fig. 3(c)Y. Miyashita et al. / International Journal of Fatigue 24 (2002) 241–248 243 Fig. 3. Observations of fatigue crack growth path in standard SiC/SiC with frequency of 1 Hz at room temperature: (a) at maximum stress of 60 MPa after 5000 cycles; (b) at maximum stress of 80 MPa after 1 cycle; (c) final fracture at maximum stress of 100 MPa after 1 cycle. room temperature was higher than that at high tempera￾ture. Comparable test results from the literature which were obtained at room temperature and at 1000°C for unnotched specimens [4] are also shown in Fig. 2. Not surprisingly, the fatigue resistance of the notched speci￾mens was less than that of the smooth specimens. The shorter fatigue lives in notched specimens resulted not only from the stress concentration at the notch tip but also from differences in loading history. The applied stress was increased stepwise during fatigue test in the present study, and fatigue damage may have developed at each applied stress level. The fatigue strength at 800°C is seen to be greater than at room temperature. It is also noted that although the difference in fatigue life between standard and enhanced SiC/SiC was in general not significant, the standard material did have a slightly longer fatigue life than the enhanced material at room temperature and at 800°C. 4.2. Crack initiation and propagation processes Fig. 3 shows examples of the crack growth path in standard SiC/SiC at room temperature. In this instance the notch was situated in a longitudinal fiber bundle which was oriented in the same direction as the loading axis. A crack initiated at the interface between fiber and matrix near the notch tip (indicated by an arrow in Fig. 3(a)). As this crack was growing along the interface fiber-cracking occurred (Fig. 3(b)). The crack then con￾tinued to grow by a combination of interfacial cracking and fiber-cracking. However, the cracks initiated not only at the notch, some cracks initiated at pores and where fiber bundles of different orientation met (indicated by (A) in Fig. 3(c)). Nevertheless, the final fracture process always involved considerable crack branching and a linking up with a crack initiated at the notch (Fig. 3(c))