244 Y. Miyashita et al. /International Journal of Fatigue 24(2002)241-248 Fig. 4 provides examples of the crack paths in the notch root, but this crack also did not propagate until enhanced SiC/SiC tested at room temperature.At a maximum stress of 130 MPa was reached. The second maximum stress of 60 MPa a crack initiated at one side crack then propagated and merged with a third crack of the notch root in the first loading cycle(Fig. 4(a)). initiated at a pore away from the notch(Fig. 4(c)),and the applied stress to 90 MPa, a new crack was observed third crack had been initiated at a maximum stress of fter 5000 loading cycles at a pore on the other side of 45 MPa early in the fatigue test but had not propagated until the 130 MPa maximum stress level was reached Fatigue crack growth paths in standard SiC/SIC at Loading direction 800C are shown in Fig. 5. A crack initiated from the notch and propagated along the interface between the fibers and the matrix, as indicated by arrows in Fig. 5(a) A second crack initiated from a pore far from the notch was observed, as indicated by arrows in Fig. 5(b). In this specimen, the crack which initiated from the pore played a dominant role in the final fracture process. As with the enhanced SiC/SiC. no significant difference in crack initiation and propagation behavior was found between room temperature and 800oC When the fiber orientation in front of the notch was notch normal to the loading direction as shown in Fig. 6,a crack initiated at the notch tip(Fig. 6(a)). This crack hen propagated rapidly along the interface between the fibers and the matrix as shown in Fig. 6(b) oading airection notch pore notch 20um Loading direction Loading direction Fig. 4. Observations of fatigue crack growth path in enhanced SiC/SiC with frequency of 10 Hz at room temperature: (a)at maximum Fig. 5. Observations of fatigue crack growth path in standard SiC/SiC ress of 60 MPa after 1 cycle;( b)at maximum stress of 90 MPa after with frequency of 10 Hz at 800C:(a)at maximum stress of 70 MPa 5000 cycles;(c)at maximum stress of 45 MPa after I cycle after 1000 cycles; ( b)at maximum stress of 71 MPa after I cycle244 Y. Miyashita et al. / International Journal of Fatigue 24 (2002) 241–248 Fig. 4 provides examples of the crack paths in enhanced SiC/SiC tested at room temperature. At a maximum stress of 60 MPa a crack initiated at one side of the notch root in the first loading cycle (Fig. 4(a)). However, the crack did not propagate. After increasing the applied stress to 90 MPa, a new crack was observed after 5000 loading cycles at a pore on the other side of Fig. 4. Observations of fatigue crack growth path in enhanced SiC/SiC with frequency of 10 Hz at room temperature: (a) at maximum stress of 60 MPa after 1 cycle; (b) at maximum stress of 90 MPa after 5000 cycles; (c) at maximum stress of 45 MPa after 1 cycle. the notch root, but this crack also did not propagate until a maximum stress of 130 MPa was reached. The second crack then propagated and merged with a third crack initiated at a pore away from the notch (Fig. 4(c)), and this linking up resulted in failure of the specimen. This third crack had been initiated at a maximum stress of 45 MPa early in the fatigue test but had not propagated until the 130 MPa maximum stress level was reached. Fatigue crack growth paths in standard SiC/SiC at 800°C are shown in Fig. 5. A crack initiated from the notch and propagated along the interface between the fibers and the matrix, as indicated by arrows in Fig. 5(a). A second crack initiated from a pore far from the notch was observed, as indicated by arrows in Fig. 5(b). In this specimen, the crack which initiated from the pore played a dominant role in the final fracture process. As with the enhanced SiC/SiC, no significant difference in crack initiation and propagation behavior was found between room temperature and 800°C. When the fiber orientation in front of the notch was normal to the loading direction as shown in Fig. 6, a crack initiated at the notch tip (Fig. 6(a)). This crack then propagated rapidly along the interface between the fibers and the matrix as shown in Fig. 6(b). Fig. 5. Observations of fatigue crack growth path in standard SiC/SiC with frequency of 10 Hz at 800°C: (a) at maximum stress of 70 MPa after 1000 cycles; (b) at maximum stress of 71 MPa after 1 cycle