January 1999 Creep and Fatigue Behavior in Hi-Nicalon MSiC Composites at High Temperatures Fig. 17. Micrographs showing (a) cracks bridged by 0o fibers and(b)interfaces between fibers and the matrix being debonded in the Hi- NicalonTM/SiC specimen fatigued at 1300 C under the maximum stress of 150 MPa in air for 2.5 x 104 cycles fracture for up to lll h. At 60 MPa, fracture occurs at a critical and propagation of the matrix cracks in the specimens under modulus equal to 80% of the original value If the fibers have a lower creep resistance than the matrix, a trix cracks at stresses <105 MPa, according to the constant- adual decrease in the modulus during creep occurs because modulus stage( Fig. 24 ). Creep occurs during this stage, incu- creep of the bridge fibers transfers stress to the matrix and 2120 MPa, extensive matrix cracks are bridged by hbeaG bating damage for propagation of the matrix cracks. At stress causes matrix cracking and crack growth. 42-47 However, it is not known whether Hi-Nicalon TM fibers have a higher or lower Therefore, creep of the fibers promotes propagation of the creep resistance than the SiC matrix with the additives. Be- cracks, which leads to the decrease of the modulus. In argon, cause reduction of the elastic modulus reflects a multiplication the creep resistance of the fibers is lower. At a stress of 30 ohM (b) Fig. 18. (a) Severe oxidation 18(a).e in the Hi-NicalonTMSiC specimen crept in air at 1300oC and 150 MPa for 480 S; (b) high-magnification image of center region in fisfracture for up to 111 h. At 60 MPa, fracture occurs at a critical modulus equal to 80% of the original value. If the fibers have a lower creep resistance than the matrix, a gradual decrease in the modulus during creep occurs because creep of the bridge fibers transfers stress to the matrix and causes matrix cracking and crack growth.42–47 However, it is not known whether Hi-Nicalon™ fibers have a higher or lower creep resistance than the SiC matrix with the additives. Be￾cause reduction of the elastic modulus reflects a multiplication and propagation of the matrix cracks in the specimens under fatigue tests,38 the first loading did not produce extensive ma￾trix cracks at stresses #105 MPa, according to the constant￾modulus stage (Fig. 24). Creep occurs during this stage, incu￾bating damage for propagation of the matrix cracks. At stresses $120 MPa, extensive matrix cracks are bridged by fibers. Therefore, creep of the fibers promotes propagation of the cracks, which leads to the decrease of the modulus. In argon, the creep resistance of the fibers is lower. At a stress of 30 Fig. 18. (a) Severe oxidation damage in the Hi-Nicalon™/SiC specimen crept in air at 1300°C and 150 MPa for 480 s; (b) high-magnification image of center region in Fig. 18(a). Fig. 17. Micrographs showing (a) cracks bridged by 0° fibers and (b) interfaces between fibers and the matrix being debonded in the Hi￾Nicalon™/SiC specimen fatigued at 1300°C under the maximum stress of 150 MPa in air for 2.5 × 104 cycles. January 1999 Creep and Fatigue Behavior in Hi-Nicalon™/SiC Composites at High Temperatures 123