S. Zhu et al. /Composites Science and Technology 59(1999)833-851 750um 750pm 200pm Fig. 6. Fracture surfaces of cyclic fatigue specimens of Sic/SiC composites(4.3x10- cycles) at room temperat 10 Hz and (2.7x105 cycles)at 1000 C at 93.7 MPa in argon at 20 Hz. The load ratio is 0.1 for both temperatures. (a)RT, (b)1000oC, (c)RT, ( 1000°C. 3. The fiber pull-out engin or cyclic fatigue is higher length of the broken fiber. The fiber pull-out length is than that of monotonic tension at the same tempera- assumed equal to Lc/2. The sliding resistance of the ture. The sliding resistance of interface(Ti), can be cal- interface decreases with cyclic fatigue or increasing culated by the equation [55] temperature, when the fiber strength is assumed to be ti=ord/2L (1) constant (1. 1 GPa for Nicalon fiber). This is consistent with the experimental results on effects of temperature on where ar is the fiber strength, d is the diameter of the cyclic fatigue of Sic/SiC [63]. However, Eq (1)can only fiber(14um in the present SiC/SiC)and Lc is the shortest give a qualitative indication of the interfacial slidingThe ®ber pull-out length of cyclic fatigue is higher than that of monotonic tension at the same tempera￾ture. The sliding resistance of interface …i†, can be cal￾culated by the equation [55] i ˆ fd=2Lc …1† where f is the ®ber strength, d is the diameter of the ®ber (14mm in the present SiC/SiC) and Lc is the shortest length of the broken ®ber. The ®ber pull-out length is assumed equal to Lc=2. The sliding resistance of the interface decreases with cyclic fatigue or increasing temperature, when the ®ber strength is assumed to be constant (1.1 GPa for NicalonTM ®ber). This is consistent with the experimental results on e€ects of temperature on cyclic fatigue of SiC/SiC [63]. However, Eq. (1) can only give a qualitative indication of the interfacial sliding Fig. 6. Fracture surfaces of cyclic fatigue specimens of SiC/SiC composites (4.3102 cycles) at room temperature at 180 MPa in air at a frequency of 10 Hz and (2.7105 cycles) at 1000C at 93.7 MPa in argon at 20 Hz. The load ratio is 0.1 for both temperatures. (a) RT, (b) 1000C, (c) RT, (d) 1000C. 838 S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851