S. Zh et al./Composites Science and Technology 59(1999)833-851 843 The apparent activation energies for creep calculated r from Eq. 3)are shown in Fig. 14. The apparent acti vation energy for creep decreases with increasing stress where t, is time to rupture, B is a constant and N is the and tends to be a constant value at high stresses stress exponent for stress rupture. According to Eq (4) Nis58atl000°C,4.latl100°C,8.lat1200°Cand4.2 4.2. Creep rupture life at 1300%C. Most of these results are similar to the stress exponents for creep at high stresses. The value at The creep rupture time versus stress is shown in 1200C is unusually higher than others, since the data at Fig. 15. The curve can be fitted by the empirical relation 75 MPa are offset from others The steady-state creep strain rates versus time to SiC/SiC, 1000 C rupture is shown in Fig. 16. The data fall into a straight 0.004 line, i.e. fit the Monkman-Grant relationship [86] 0.003 where m is the strain rate exponent and CM-G is a con- stant. Fig. 16 shows that m is 0.72. The Monkman-Grant relation provides a method for creep life prediction of SiC/SiC, in Argon Time, s sic/sic,1300“c (b)0.005 n=5-6 0.004 0.003 90 MPa 100 0.001 Fig. 13. Tensile minimum creep strain rate versus stress in argon at 1000,1100,1200andl300°C 0 100200300400500600 SiC/SiC. 1300 C (c)0.0015 ⊥120MPa Q=165 kJ/mol ▲-60MPa 00012 106 苏0.000 75 MPa Q=235 kJ/mol 0.0006 0.0003 Q= 1040 kJ/mol Q=570 kJ/mol O 15104210425104 Time, s 17(10000K Fig 12. Tensile creep strain versus time at different stresses in argon Fig. 14. Tensile minimum creep strain rate as a function of absolute atl000and1300°C.(a)1000°C;(b)and(c)1300°C.The apparent activation energies for creep calculated from Eq. (3) are shown in Fig. 14. The apparent acti￾vation energy for creep decreases with increasing stress and tends to be a constant value at high stresses. 4.2. Creep rupture life The creep rupture time versus stress is shown in Fig. 15. The curve can be ®tted by the empirical relation tr ˆ B:ÿN …4† where tr is time to rupture, B is a constant and N is the stress exponent for stress rupture. According to Eq. (4), N is 5.8 at 1000C, 4.1 at 1100C, 8.1 at 1200C and 4.2 at 1300C. Most of these results are similar to the stress exponents for creep at high stresses. The value at 1200C is unusually higher than others, since the data at 75 MPa are o€set from others. The steady-state creep strain rates versus time to rupture is shown in Fig. 16. The data fall into a straight line, i.e. ®t the Monkman±Grant relationship [86] tr:"_ m ˆ CMÿG …5† where m is the strain rate exponent and CMÿG is a con￾stant. Fig. 16 shows that m is 0.72. The Monkman±Grant relation provides a method for creep life prediction of Fig. 13. Tensile minimum creep strain rate versus stress in argon at 1000, 1100, 1200 and 1300C. Fig. 14. Tensile minimum creep strain rate as a function of absolute temperature. Fig. 12. Tensile creep strain versus time at di€erent stresses in argon at 1000 and 1300C. (a) 1000C; (b) and (c) 1300C. S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851 843