Y. Miyashita et al. /International Journal of Fatigue 24(2002)241-248 245 Loading direction Fig. 6. Observations of fatigue crack growth path in standard SiC/SiC with frequency of 10 Hz at 800C: (a)at maximum stress of 90 MPa after 5000 cycles;(b)at maximum stress of 130 MPa after 50000 cycles The crack initiation and propagation processes in stan 9(a). On comparing Figs. 8(b)and 9(b), it is apparent dard SiC/SiC were similar to those in enhanced SiC/Sic that fiber pull-out is much more extensive at the higher both at room temperature and at 800oC temperature. Pull-out length of fiber at high temperature Based on the foregoing observations, the crack propa- is about 100 um, while that at room temperature is negli- gation processes were classified into two types as sche ble in average. This is the result of a decrease in bond- matically shown in Fig. 7 ing strength between the fibers and the matrix at elevated In Type la, the fibers ahead of the notch are parallel temperatures. From these fractographic observations to the loading direction, and final failure is caused by there is no significant difference of fracture morphology growth of a crack initiated from a notch with the crack between near-surface region and inside of the specimen propagating by a combination of fiber-breaking and Therefore, it may be speculated that the fracture mech- branching anisms observed on the surface coincide with those In Type Ib, the fibers ahead of the notch are perpen- within the bulk. However, further experiments for get- licular to the loading direction and final failure is caused ting direct evidence will be needed to make the fracture by the growth of a crack initiated from a notch with the mechanisms inside the specimen clear crack propagating along the interface between the fiber and the matrix 4. 4. Modulus of rigidity(MOR) In Type 2, a crack is initiated at the notch but does not grow until a second crack initiated at a pore grows In metals it is customary to treat the growth of a domi and joins with the first crack nant fatigue crack in terms of linear elastic fracture mech- anics with the rate of fatigue crack growth being a func 4.3. Fracture surface appearance tion of the stress intensity factor. However, in the SiC/SiC system such an approach is not feasible because ig. 8 shows fracture surface of the standard SiC/Sic of the complexity of the cracking process as described specimen tested at room temperature. Fig. 9 shows frac- above. Therefore, we seek for a parameter other than the ture surface of the standard SiC/Sic tested at stress intensity factor for estimating fatigue damage in 800C. There are three or four bundle thickness composite materials of this type. In order to characterize direction of the specimen as shown in Figs. 8(a)and the extent of fatigue damage in SiC/SiC compositeY. Miyashita et al. / International Journal of Fatigue 24 (2002) 241–248 245 Fig. 6. Observations of fatigue crack growth path in standard SiC/SiC with frequency of 10 Hz at 800°C: (a) at maximum stress of 90 MPa after 5000 cycles; (b) at maximum stress of 130 MPa after 50000 cycles. The crack initiation and propagation processes in stan￾dard SiC/SiC were similar to those in enhanced SiC/SiC both at room temperature and at 800°C. Based on the foregoing observations, the crack propa￾gation processes were classified into two types as sche￾matically shown in Fig. 7. In Type 1a, the fibers ahead of the notch are parallel to the loading direction, and final failure is caused by growth of a crack initiated from a notch with the crack propagating by a combination of fiber-breaking and branching. In Type 1b, the fibers ahead of the notch are perpen￾dicular to the loading direction and final failure is caused by the growth of a crack initiated from a notch with the crack propagating along the interface between the fibers and the matrix. In Type 2, a crack is initiated at the notch but does not grow until a second crack initiated at a pore grows and joins with the first crack. 4.3. Fracture surface appearance Fig. 8 shows fracture surface of the standard SiC/SiC specimen tested at room temperature. Fig. 9 shows frac￾ture surface of the standard SiC/SiC specimen tested at 800°C. There are three or four bundles to the thickness direction of the specimen as shown in Figs. 8(a) and 9(a). On comparing Figs. 8(b) and 9(b), it is apparent that fiber pull-out is much more extensive at the higher temperature. Pull-out length of fiber at high temperature is about 100 µm, while that at room temperature is negli￾gible in average. This is the result of a decrease in bond￾ing strength between the fibers and the matrix at elevated temperatures. From these fractographic observations, there is no significant difference of fracture morphology between near-surface region and inside of the specimen. Therefore, it may be speculated that the fracture mech￾anisms observed on the surface coincide with those within the bulk. However, further experiments for get￾ting direct evidence will be needed to make the fracture mechanisms inside the specimen clear. 4.4. Modulus of rigidity (MOR) In metals it is customary to treat the growth of a domi￾nant fatigue crack in terms of linear elastic fracture mech￾anics with the rate of fatigue crack growth being a func￾tion of the stress intensity factor. However, in the SiC/SiC system such an approach is not feasible because of the complexity of the cracking process as described above. Therefore, we seek for a parameter other than the stress intensity factor for estimating fatigue damage in composite materials of this type. In order to characterize the extent of fatigue damage in SiC/SiC composite