844 S. Zhu et al. / Composites Science and Technology 59(1999)833-851 400 100 围 Time to P=T10-3.(7+logt) Fig. 15. Tensile stress versus time to rupture in argon at 1000, 1100, Fig. 17. Tensile stress versus Larson-Miller parameter at different 200and1300°C stresses in argon at 1000, 1100, 1200 and 1300C The constant,(C), of the Larson-Miller parameter IS of the order of 20 for metals and alloys. For monolithic silicon nitride, it is between 30 and 40, depending on the grade of silicon nitride [88, 89]. For oxides, it ranges from 10 to 22[90]. The values of the constant for Sic/ SiC composite are 5-10, determined by the data at four temperatures. The present data show that the Larson- ler parameter can be used for creep life prediction of 4.3. Creep damage an Creep rupture surfaces exhibit that the fracture mod Rupture, s mainly consists of 0o bundles'fracture(Fig. 18). This Fig. 16. Tensile minimum creep strain different from the fracture mode of monotonic tension different stresses in argon at 1000. 1 100. 1200 and 130000 at elevated temperatures, in which both 0 and 90 bundles fracture [55]. At low stresses fracture surfaces consist of two regions: one is rough, with evident fiber SiC/SiC composite. If the steady state creep rate is pull-out; another is plane, flush with the matrix known, the creep life can be easily calculated by Eq (5). [F 9) and(d) However, the creep rate is often difficult to measure for The longitudinal section of creep ruptured specimens a component In structures this case, some para shows that matrix cracking occurs in the rough region, meters may be useful for creep life prediction similar to that at high stress [Fig. 19(a) and(b)]. In the The Larson-Miller parameter(P)[87] is one of the plane region, fibers fracture flush with the matrix and useful icting creep life there are few cracks in the matrix [Fig. 19(c) metallic materials. The basic assumptions of it are that At high stresses, cracks initiated at large pores m=I and o is a function of stress. The present result between fiber bundles are bridged by intact approximately fulfill these assumptions. It can be used Crack propagation in 90 bundles is along fiber/matrix for correlating stress-temperature-life relationship in interfaces or connected by pores in the bundles. Crack SiC/SiC composite in the following expression propagation in 0 bundles at high stresses is similar to P=T(C +log tr ), (6) that which has been widely observed in unidirectional fiber reinforced CMcs. a difference is that there is bending of the bundles in plain-weave CMCs, which where T is the absolute temperature (in K)and tr is the produces stress concentration at the crossover points of time to rupture(in h). It was found that data at different fiber weave structures. This explains why the fiber pull- temperatures fall on the same line with the best fit when out length in plain-woven materials is smaller than that the constant C is between 5 and 10. Fig. 17 shows the in unidirectional materials. A crack propagation in the relation of stress with Larson-Miller parameter with C specimen crept at low stress. The crack cuts through the of 7 0%fibers. This means that creep fracture first took placeSiC/SiC composite. If the steady state creep rate is known, the creep life can be easily calculated by Eq. (5). However, the creep rate is often dicult to measure for a component in structures. In this case, some para￾meters may be useful for creep life prediction. The Larson±Miller parameter (P) [87] is one of the useful parameters used for predicting creep life in metallic materials. The basic assumptions of it are that m=1 and Q is a function of stress. The present results approximately ful®ll these assumptions. It can be used for correlating stress±temperature±life relationship in SiC/SiC composite in the following expression: P ˆ T…C ‡ log tr†; …6† where T is the absolute temperature (in K) and tr is the time to rupture (in h). It was found that data at di€erent temperatures fall on the same line with the best ®t when the constant C is between 5 and 10. Fig. 17 shows the relation of stress with Larson±Miller parameter with C of 7. The constant, (C), of the Larson±Miller parameter is of the order of 20 for metals and alloys. For monolithic silicon nitride, it is between 30 and 40, depending on the grade of silicon nitride [88,89]. For oxides, it ranges from 10 to 22 [90]. The values of the constant for SiC/ SiC composite are 5±10, determined by the data at four temperatures. The present data show that the Larson± Miller parameter can be used for creep life prediction of SiC/SiC. 4.3. Creep damage and rupture surface Creep rupture surfaces exhibit that the fracture mode mainly consists of 0 bundles' fracture (Fig. 18). This is di€erent from the fracture mode of monotonic tension at elevated temperatures, in which both 0 and 90 bundles fracture [55]. At low stresses fracture surfaces consist of two regions: one is rough, with evident ®ber pull-out; another is plane, ¯ush with the matrix [Fig. 18(c) and (d)]. The longitudinal section of creep ruptured specimens shows that matrix cracking occurs in the rough region, similar to that at high stress [Fig. 19(a) and (b)]. In the plane region, ®bers fracture ¯ush with the matrix and there are few cracks in the matrix [Fig. 19(c)]. At high stresses, cracks initiated at large pores between ®ber bundles are bridged by intact 0 ®bers. Crack propagation in 90 bundles is along ®ber/matrix interfaces or connected by pores in the bundles. Crack propagation in 0 bundles at high stresses is similar to that which has been widely observed in unidirectional ®ber reinforced CMCs. A di€erence is that there is bending of the bundles in plain-weave CMCs, which produces stress concentration at the crossover points of ®ber weave structures. This explains why the ®ber pull￾out length in plain-woven materials is smaller than that in unidirectional materials. A crack propagation in the specimen crept at low stress. The crack cuts through the 0 ®bers. This means that creep fracture ®rst took place Fig. 15. Tensile stress versus time to rupture in argon at 1000, 1100, 1200 and 1300C. Fig. 16. Tensile minimum creep strain rate versus time to rupture at di€erent stresses in argon at 1000, 1100, 1200 and 1300C. Fig. 17. Tensile stress versus Larson±Miller parameter at di€erent stresses in argon at 1000, 1100, 1200 and 1300C. 844 S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851