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 sliding
The ®ber pull-out length of cyclic fatigue is higher than that of monotonic tension at the same temperature. The sliding resistance of interface i, can be calculated by the equation [55] i 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 eects 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
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 repeated 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 phenomena 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 bundles 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 bundles 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