278 J. L Ryba/ Composites Science and Technology 68 (2008)274-282 口 Stean 需0.6 80.5 950c Fig. 6. Normalized residual strength. Finally, the test specimens that survived 100 h of stress rupture were tested under the monotonic tension test in the laboratory air test environment to measure their resid ual strength. This provided the estimate of damage devel- oped during the tensile stress rupture test over 100 h. ig. 6 shows these residual strengths which are normalized by the ultimate tensile strength at room temperature. This comparison clearly shows that the residual strength is less in the steam test environment than that in the laborator air environment at each elevated temperature. In addition, it is lower at 750 oC in either air or steam environment than its counterparts at 400C and 950C. further, the residual strength at 750oC under the steam test environment is the minimum among all the six test conditions of this study This suggests that intermediate temperature range test con- dition has relatively more detrimental effect on the stress rupture performance of the tested CMC system under both Fig. 7. Fracture Surface from monotonic tensile test at 950 C,(a) at laboratory air and steam conditions lower magnification(100x)and (b) at higher magnification(8000x 3.3. Damage mechanisms (marked by arrows as few such examples) along with deb- All tested specimens of this study were examined with onding between fiber and interphase or between interphase the scanning electron microscope(SEM) to observe failure and Sic matrix and damage mechanisms. In the case of monotonic tension The comparison of the typical fractured surfaces from tests under all four test conditions (room temperature, stress rupture tests under both laboratory air and steam 400C, 750C and 950C), fracture surfaces were rough, test conditions are shown in Figs. 8-10. These show images but still relatively fat and perpendicular to the loading for tests at 400C, 750C, and 950C(from left to right) direction. There were no distinctive features of the fracture under steam and laboratory air conditions. Fig. 8 shows which can be attributed to the change in the test tempera- these at lower magnification(100x)of one repr tre. Fracture surface was a typical of a woven brittle sample from each test environment. It can be seen that CMC system where the longitudinal tows showed the fiber the laboratory air tests show a relatively rougher fracture pull-out that did not change with test temperature from plane compared to the steam tests. Also, the tests con- room temperature to 950C. Therefore for the sake of ducted at 750C, in the intermediate range, have relatively brevity, only one case of fracture surface is shown at both flatter fracture surface with very little fiber pullout(Fig. 8b lower and higher magnifications in Fig. 7, where fiber pull- and e) whereas the 400C and 950C tests show pullout of out and fracture in matrix at different planes are clearly evi- longitudinal tows and planar fracture within the tows dent as well as the presence of fractured BN interphase (Fig. 8a, d, c and f. This indicates that the crackFinally, the test specimens that survived 100 h of stress rupture were tested under the monotonic tension test in the laboratory air test environment to measure their resid￾ual strength. This provided the estimate of damage devel￾oped during the tensile stress rupture test over 100 h. Fig. 6 shows these residual strengths which are normalized by the ultimate tensile strength at room temperature. This comparison clearly shows that the residual strength is less in the steam test environment than that in the laboratory air environment at each elevated temperature. In addition, it is lower at 750 C in either air or steam environment than its counterparts at 400 C and 950 C. Further, the residual strength at 750 C under the steam test environment is the minimum among all the six test conditions of this study. This suggests that intermediate temperature range test con￾dition has relatively more detrimental effect on the stress rupture performance of the tested CMC system under both laboratory air and steam conditions. 3.3. Damage mechanisms All tested specimens of this study were examined with the scanning electron microscope (SEM) to observe failure and damage mechanisms. In the case of monotonic tension tests under all four test conditions (room temperature, 400 C, 750 C and 950 C), fracture surfaces were rough, but still relatively flat and perpendicular to the loading direction. There were no distinctive features of the fracture which can be attributed to the change in the test tempera￾ture. Fracture surface was a typical of a woven brittle CMC system where the longitudinal tows showed the fiber pull-out that did not change with test temperature from room temperature to 950 C. Therefore for the sake of brevity, only one case of fracture surface is shown at both lower and higher magnifications in Fig. 7, where fiber pull￾out and fracture in matrix at different planes are clearly evi￾dent as well as the presence of fractured BN interphase (marked by arrows as few such examples) along with deb￾onding between fiber and interphase or between interphase and SiC matrix. The comparison of the typical fractured surfaces from stress rupture tests under both laboratory air and steam test conditions are shown in Figs. 8–10. These show images for tests at 400 C, 750 C, and 950 C (from left to right) under steam and laboratory air conditions. Fig. 8 shows these at lower magnification (100·) of one representative sample from each test environment. It can be seen that the laboratory air tests show a relatively rougher fracture plane compared to the steam tests. Also, the tests con￾ducted at 750 C, in the intermediate range, have relatively flatter fracture surface with very little fiber pullout (Fig. 8b and e) whereas the 400 C and 950 C tests show pullout of longitudinal tows and planar fracture within the tows (Fig. 8a, d, c and f). This indicates that the cracks 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Temperature Normalized Residual Strength Air Steam 4000C 7500C 9500C Fig. 6. Normalized residual strength. Fig. 7. Fracture Surface from monotonic tensile test at 950 C, (a) at lower magnification (100·) and (b) at higher magnification (8000·). 278 S. Mall, J.L. Ryba / Composites Science and Technology 68 (2008) 274–282