September 1998 Creep and Fatigue Behavior in an Enhanced SiC/Sic Composite at High Temperature 2275 Matrix Fig. 12. Fractures surfaces in a specimen of the enhanced SiC/SiC composite fatigued in air at 1300 C and a load of 120 MPa for 4. x 10 cycles ((a) general view and(b) former indicates a decrease of the modulus, and the latter further studied. Creep of the bridged fibers transfers stress to means a decrease of the interfacial sliding resistance. The he matrix and causes matrix cracking 30-33 As a result, the teresis loops move to the right along the strain axis, which is modulus decreases as the number of cycles increases known as ratchetting, because of time-dependent deformation Creep tests show similar results of the decrease of the modu- lus with creep time in the enhanced SiC/SiC composite. At 60 plot of the Young's modulus, normalized by the value MPa, the modulus remains constant during creep in air but from the linear portion during the first loading, versus the decreases as the time increases in argon( Fig. 15). Although the number of cycles is shown in Fig. 14. When the modulus applied stress for the creep test is 60 MPa below the matrix decreases to -80% of the original value. the frac- cracking stress(70 MPa), the damage led to degradation of the also matrix cra (E e2, where Er is the modulus of the fibers and V is the total mosphere. Observation of the specimen that was crept at 60 volume fraction of fibers in the composite. This observation MPa in argon shows that matrix cracks are initiated from the means that the matrix still contributes to the modulus of the large pores(Fig. 16); this condition may be due to the lower c At 120 and 90 MPa, the modulus remains constant, up to 10 of the fibers transfers the stress onto the matrix and causes and 10 cycles, respectively, and then decreases as the number matrix cracking. Moreover, the sealing of the cracks by the of cycles increases Matrix cracks formed by the first loading glass may be more effective in air than in argon, because there and during the early stage of fatigue are not sufficient to affect are more oxygen atoms available in air to react with the glas the modulus, which implies that there may exist a critical level forming particulates than in argon of damage for the decrease of modulus. which needs to be The limit modulus for fracture under creep tests in air and c 2x10e 0.8 0.6 40 150 MPa 04 --120 MPg 02 -A-90 MPa 0 3103 110102103104105108107 Strain Cycle Fig. 13. Evolution of the hysteresis l normalized by the value of the SiC/SiC composite fatigued in air at C and a load of 90 MPa ing(EEo), versus the number of cycles of the enhanced SiC/SiC composite under cyclic loading in air at 1300oCformer indicates a decrease of the modulus, and the latter means a decrease of the interfacial sliding resistance. The hys￾teresis loops move to the right along the strain axis, which is known as ratchetting, because of time-dependent deformation (creep). A plot of the Young’s modulus, normalized by the value from the linear portion during the first loading, versus the number of cycles is shown in Fig. 14. When the modulus decreases to ∼80% of the original value, the specimens frac￾ture. Eighty percent of the original modulus is still higher than (Ef Vf )/2, where Ef is the modulus of the fibers and Vf is the total volume fraction of fibers in the composite. This observation means that the matrix still contributes to the modulus of the composites. At 120 and 90 MPa, the modulus remains constant, up to 10 and 104 cycles, respectively, and then decreases as the number of cycles increases. Matrix cracks formed by the first loading and during the early stage of fatigue are not sufficient to affect the modulus, which implies that there may exist a critical level of damage for the decrease of modulus, which needs to be further studied. Creep of the bridged fibers transfers stress to the matrix and causes matrix cracking.30–33 As a result, the modulus decreases as the number of cycles increases. Creep tests show similar results of the decrease of the modu￾lus with creep time in the enhanced SiC/SiC composite. At 60 MPa, the modulus remains constant during creep in air but decreases as the time increases in argon (Fig. 15). Although the applied stress for the creep test is 60 MPa below the matrix cracking stress (70 MPa), the damage led to degradation of the Young’s modulus, and also matrix cracking, in the argon at￾mosphere. Observation of the specimen that was crept at 60 MPa in argon shows that matrix cracks are initiated from the large pores (Fig. 16); this condition may be due to the lower creep resistance of the fibers in argon than in air.27 The creep of the fibers transfers the stress onto the matrix and causes matrix cracking. Moreover, the sealing of the cracks by the glass may be more effective in air than in argon, because there are more oxygen atoms available in air to react with the glass￾forming particulates than in argon. The limit modulus for fracture under creep tests in air and Fig. 12. Fractures surfaces in a specimen of the enhanced SiC/SiC composite fatigued in air at 1300°C and a load of 120 MPa for 4.8 × 104 cycles ((a) general view and (b) cross-sectional view). Fig. 13. Evolution of the hysteresis loops in fatigue of the enhanced SiC/SiC composite fatigued in air at 1300°C and a load of 90 MPa in air. Fig. 14. Young’s modulus, normalized by the value of the first load￾ing (E/E0), versus the number of cycles of the enhanced SiC/SiC composite under cyclic loading in air at 1300°C. September 1998 Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature 2275