S. Zhu et al. /Composites Science and Te 9(1999)833-851 (C) Fig. 7. Schematic diag f fracture modes of oo bundles resistance, because the strength of the fibers depends on the gage length and the pull-out length is not exactly jual to Lc/2 Cyclic loading and unloading result in cyclic opening and closing of the matrix cracks, which leads to repe- ated shear stress and slip of interfaces between fiber and matrix in 0 bundles. This process promotes debonding nd increases the length of the debonded interface. The long debonded interface leads to long fiber pull-out Moreover, the repeated slipping of interface may cause the interphase (carbon coating layer)to fail and even the surface of fibers to wear Crushed fibers and debris were found on cyclic fatigue fracture surfaces at both room and high temperatures [55]. Two kinds of debris consist of micrometer-sized Sic debris particles and submicrometer-sized graphite dusts [20]. Similar phe- nomena were found in the present experiments. This is evidence of interface damage during cyclic fatigue 3. 2. Crack initiation and distribution Both observation on interrupted test specimens tested at stresses slightly higher than the proportional limit and in situ observation show that most of the cracks initiate at he sharp corners of large pores at the crossover points of the fiber bundle weave as shown in Fig. 8. The cracks are of three kinds cracks in o bundles cracks in 90 bun dles and cracks at crossover points of the weave. The percentage of cracks is greatest in 90 bundles followed by crossover points of 00/90 bundles for monotonic tension and cyclic fatigue at high stresses at both room and high temperatures. However, most cracks form in 0 bundles and then at crossover points of 0/90 bun dles for cyclic fatigue at low stresses at 1000C. There are few matrix cracks in the o fiber bundles in the rt tensile tested specimen The dominant damage mode changes from cracks in loading. (a)at 1000@ C and 118.8 MPa (6.9x10 cycles); (b) at room 90 bundles for cyclic fatigue at high stresses to cracking temperature and 170 MPa(3.3x 106 cycles)resistance, because the strength of the ®bers depends on the gage length and the pull-out length is not exactly equal to Lc=2. Cyclic loading and unloading result in cyclic opening and closing of the matrix cracks, which leads to repe￾ated shear stress and slip of interfaces between ®ber and matrix in 0 bundles. This process promotes debonding and increases the length of the debonded interface. The long debonded interface leads to long ®ber pull-out. Moreover, the repeated slipping of interface may cause the interphase (carbon coating layer) to fail and even the surface of ®bers to wear. Crushed ®bers and debris were found on cyclic fatigue fracture surfaces at both room and high temperatures [55]. Two kinds of debris consist of micrometer-sized SiC debris particles and submicrometer-sized graphite dusts [20]. Similar phe￾nomena were found in the present experiments. This is evidence of interface damage during cyclic fatigue. 3.2. Crack initiation and distribution Both observation on interrupted test specimens tested at stresses slightly higher than the proportional limit and in situ observation show that most of the cracks initiate at the sharp corners of large pores at the crossover points of the ®ber bundle weave, as shown in Fig. 8. The cracks are of three kinds: cracks in 0 bundles, cracks in 90 bun￾dles and cracks at crossover points of the weave. The percentage of cracks is greatest in 90 bundles followed by crossover points of 0/90 bundles for monotonic tension and cyclic fatigue at high stresses at both room and high temperatures. However, most cracks form in 0 bundles and then at crossover points of 0/90 bun￾dles for cyclic fatigue at low stresses at 1000C. There are few matrix cracks in the 0 ®ber bundles in the RT tensile tested specimen. The dominant damage mode changes from cracks in 90 bundles for cyclic fatigue at high stresses to cracking Fig. 7. Schematic diagram of fracture modes of 0 bundles. Fig. 8. Crack initiation sites and crack growth paths under cyclic loading. (a) at 1000C and 118.8 MPa (6.9104 cycles); (b) at room temperature and 170 MPa (3.3106 cycles). S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851 839