w. Xiaojun et al Composites Science and Technology 66(2006)993-1000 -RB-1560-5 83628Kv氵5" b下362Kv氵iu酯 IG52D-C/SIC-RB-1591-5 [m52T-CZSIC-RB-15IR-5 00715Ky xi50 200 d 83628Kv Fig 9. SEM micrographs of 2D-C/SiC tensile crept specimen at the near notch area at 1500C and 95 MPa in vacuum:(a)near the notch of the specimen after 0.5 h creeping, 150x;(b) near the notch of the specimen after 0.5 h creeping, 150x;(c) near the notch on lateral side of the specimen after 0.5 h creeping. 150x;(c) near the notch on lateral side of the specimen after 25 h creeping, 150x;(d) near the notch of the specimen after 25 h creeping, 1000x 3.4. The damage evaluated by elastic modulus resistance and residual strength, etc. In the present work, the creep damage has been evaluated by elastic modulus There are many methods to evaluate the damage of cera- The resonance frequency of the sample is measured by mic matrix composites, such as elastic modulus, electrical stopping the creeping test at different creep times. As elastic modulus is proportional to the square of the resonance fre- quency, variation of elastic modulus could be obtained indirectly by varying the resonance frequency. The damage parameter, D, Is given 一一b D=1-(E/E0)=(-f2)后2, where f and E are the resonance frequency and elastic modulus of the damaged material at time I, respectively ,, fo and Eo are the resonance frequency and elastic modu- lus of un-damaged material, respectively. Fig. 12 show the variation of damage with creeping time at different temperatures. It appears that the trend of damage curves Fig 10. The amount of micro-cracks as a function of time for 2 D-C/Sic under 1300 and 1500oC are similar. At the transient tensile creep specimen at the near notch area at 1300 and 1500.C and creep stage, damage curves increase rapidly with time 95MPa in vacuum: (a)was measured on the top surface of the specimen at after experience a relatively slow and slightly decreasing )was mea (c)was measured on the top surface of the specimen at 1300C, (d)was development stage, the damage curves start to increase measured on the lateral side of the specimen at 1300C agaln.3.4. The damage evaluated by elastic modulus There are many methods to evaluate the damage of cera￾mic matrix composites, such as elastic modulus, electrical resistance and residual strength, etc. In the present work, the creep damage has been evaluated by elastic modulus. The resonance frequency of the sample is measured by stopping the creeping test at different creep times. As elastic modulus is proportional to the square of the resonance fre￾quency, variation of elastic modulus could be obtained indirectly by varying the resonance frequency. The damage parameter, D, is given by D ¼ 1 ðE=E0Þ¼ðf 2 0 f 2 Þ=f 2 0 ; ð2Þ where f and E are the resonance frequency and elastic modulus of the damaged material at time t, respectively; f0 and E0 are the resonance frequency and elastic modu￾lus of un-damaged material, respectively. Fig. 12 shows the variation of damage with creeping time at different temperatures. It appears that the trend of damage curves under 1300 and 1500 C are similar. At the transient creep stage, damage curves increase rapidly with time; after experience a relatively slow and slightly decreasing development stage, the damage curves start to increase again. Fig. 9. SEM micrographs of 2D-C/SiC tensile crept specimen at the near notch area at 1500 C and 95 MPa in vacuum: (a) near the notch of the specimen after 0.5 h creeping, 150·; (b) near the notch of the specimen after 0.5 h creeping, 150·; (c) near the notch on lateral side of the specimen after 0.5 h creeping, 150·; (c) near the notch on lateral side of the specimen after 25 h creeping, 150·; (d) near the notch of the specimen after 25 h creeping, 1000·. 0 10 20 30 40 50 60 0 10 20 30 40 50 60 t (h) Quantity of microcracks a b c d Fig. 10. The amount of micro-cracks as a function of time for 2D-C/SiC tensile creep specimen at the near notch area at 1300 and 1500 C and 95 MPa in vacuum: (a) was measured on the top surface of the specimen at 1500 C, (b) was measured on the lateral side of the specimen at 1500 C, (c) was measured on the top surface of the specimen at 1300 C, (d) was measured on the lateral side of the specimen at 1300 C. 998 W. Xiaojun et al. / Composites Science and Technology 66 (2006) 993–1000