120 Journal of the American Ceramic Society-Zhu et al. Vol. 82. Ne 12 o Fatigue 。 Fatigue 0.8 0.4 ●目 0 500 1000 4000 TIme, s Cree ●cre o Fatigue 04 口 Fatigue 三0.5 02 輯H 15 Time,×104s Time,×1059 Fig. 3. Tensile creep strain versus time in the Hi-Nicalon TM/SiC composite under constant load (creep) and cyclic loading(fatigue)at 1300C in ir at the same maximum stresses(a)150,(b)120, (c)90, and(d)75 MPa) than that in air(Fig. 14). The fatigue life at a gi Iven debonding of the interfaces between the fibers and the matrix maximum stress in argon also is longer than that in air(F occurs. The 0 fibers bridge crack faces and, therefore, de- 15). The effects of the environment on the creep resistance in crease the driving force at the crack tip as a general bridging the standard SiC/Sic co te are opposite to those in the mechanism( Fig. 17). At a temperature of 1300C, the glas Hi-NicalonTM/SiC(Figs. 6 and 7)and the enhanced SiC/SiC ases become liquid, which flow into cracks. At room tem- composites. This result will be discussed in Section(2)of the ature, they become solid again and are situated in the cracks (Fig. 17(b). When the testing was performed in air, oxidation occurred at the interfaces and at the fibers and the matrix(Figs (4 Microscopic Damage and fracture 16 and 17). Fibers can be severely damaged by oxidation at Creep and fatigue cracks are always observed at large pores places near the edge of the specimen or close to the large pores among fiber bundles(Fig. 16). When cracks meet 0 fibers, (Figs. 18 and 16). However, such a severe oxidation is not 300 1300°c.Ai 三107 1000 Ma stres Time to Ri Minimum creep strain rate as a function of the maximum in the hi- the Hi-Nicalon TM/S posite under constant load (creep) cyclic load- lic loading( fatigue)at 1300 C in airlonger than that in air (Fig. 14). The fatigue life at a given maximum stress in argon also is longer than that in air (Fig. 15). The effects of the environment on the creep resistance in the standard SiC/SiC composite are opposite to those in the Hi-Nicalon™/SiC (Figs. 6 and 7) and the enhanced SiC/SiC composites.11 This result will be discussed in Section (2) of the Discussion. (4) Microscopic Damage and Fracture Creep and fatigue cracks are always observed at large pores among fiber bundles (Fig. 16). When cracks meet 0° fibers, debonding of the interfaces between the fibers and the matrix occurs. The 0° fibers bridge crack faces and, therefore, de￾crease the driving force at the crack tip as a general bridging mechanism (Fig. 17). At a temperature of 1300°C, the glassy phases become liquid, which flow into cracks. At room tem￾perature, they become solid again and are situated in the cracks (Fig. 17(b)). When the testing was performed in air, oxidation occurred at the interfaces and at the fibers and the matrix (Figs. 16 and 17). Fibers can be severely damaged by oxidation at places near the edge of the specimen or close to the large pores (Figs. 18 and 16). However, such a severe oxidation is not Fig. 4. Minimum creep strain rate as a function of the maximum stress in the Hi-Nicalon™/SiC composite under constant load (creep) and cyclic loading (fatigue) at 1300°C in air. Fig. 5. Time to rupture versus the maximum stress in the Hi￾Nicalon™/SiC composite under constant load (creep) and cyclic load￾ing (fatigue) at 1300°C in air. Fig. 3. Tensile creep strain versus time in the Hi-Nicalon™/SiC composite under constant load (creep) and cyclic loading (fatigue) at 1300°C in air at the same maximum stresses ((a) 150, (b) 120, (c) 90, and (d) 75 MPa). 120 Journal of the American Ceramic Society—Zhu et al. Vol. 82, No. 1