J M. Ehrman et al Composites Science and Technology 67(2007)1425-1438 Table 2 Retained properties of the N720/A specimens subjected to creep-fatigue interaction tests with the maximum stress of 125 MPa in laboratory air at 1200C Prior test duration (h) etained strength(MPa) Retained modulus(GPa) Failure strain (% 574 Fatigue, 10 s hold 38 216 Fatigue, 100s hold l12 .18 Cyclic-static l.04 marized in Table 2. All specimens tested r retained Stress-rupture behavior of the pre-fatigued and the as- 00% of their tensile strength. However, stiffness loss of processed specimens tested in air is summarized in Fig. 6 26-35% was observed, which appears to be independent In air prior fatigue has a dramatic effect on the subsequent of the maximum stress or the hold time. Full retention of creep life. For a given applied stress, creep lives of the pre- tensile strength indicates that no damage occurred to the fatigued specimens are one to two orders of magnitude fibers. The residual modulus of 60 GPa is consistent with longer than those of the as-processed specimens. At loading direction, indicating saturation of microcracks in out of 100 h, while the d specimen achieved a creep run- the matrix and no load transfer within the 90 tows. In only 4.25 h At 154 MPa, rupture time for the pre-fatigued team, run-out was not achieved in fatigue tests with hold specimen was 1.68h, an order of magnitude higher than the rupture time of 0. 27 h for the as-processed specimen The pre-fatigued specimen that achieved creep run-out at 3.3. Cyclic-static block loadings 125 MPa was subjected to a tensile test to failure at Previous study [27] revealed a significant difference in time to failure under sustained and cyclic loadings. Fatigue run-out of 10 cycles(x28 h at 1 Hz) was achieved at max- imum stress levels that caused creep failure in less than I h T=1200C. Air Cyclic-static block loading tests were conducted to evalu ate the effects of prior fatigue on creep performance. The number of cycles in the cyclic block was 10, the same as As-Processed. 154 MPa the fatigue run-out condition. The maximum stress levels o were 125 and 154 MPa in air. and 100 MPa in steam Results are included in Table 1. Because all specimens sub jected to cyclic-static tests survived the 10 cycles of the fatigue block, time to failure in Table I is in fact the time to failure obtained in the creep block. The total times to I Hz)to the creep rupture times given in Table/ ycles at failure can be calculated by adding 27.7 h(105 Time(h) T=1200C. Air ooC. stear 200IUTS 1000 Time(h) Time(h) Fig. 6. Creep stress vs time to rupture for Nextel 720/Alumina ceramic Fig. 7. Creep strain vs time for Nextel 720/Alumina ceramic composite composite at 1200C in laboratory air data from Ruggles- Wrenn at 1200C:(a)in laboratory air, (b) in steam environment. Creep data et al. [27) from Ruggles-Wrenn et al. [27]are also shown.marized in Table 2. All specimens tested in air retained 100% of their tensile strength. However, stiffness loss of 26–35% was observed, which appears to be independent of the maximum stress or the hold time. Full retention of tensile strength indicates that no damage occurred to the fibers. The residual modulus of 60 GPa is consistent with the composite modulus being due entirely to fibers in the loading direction, indicating saturation of microcracks in the matrix and no load transfer within the 90 tows. In steam, run-out was not achieved in fatigue tests with hold time. 3.3. Cyclic–static block loadings Previous study [27] revealed a significant difference in time to failure under sustained and cyclic loadings. Fatigue run-out of 105 cycles (28 h at 1 Hz) was achieved at max￾imum stress levels that caused creep failure in less than 1 h. Cyclic–static block loading tests were conducted to evalu￾ate the effects of prior fatigue on creep performance. The number of cycles in the cyclic block was 105 , the same as the fatigue run-out condition. The maximum stress levels were 125 and 154 MPa in air, and 100 MPa in steam. Results are included in Table 1. Because all specimens sub￾jected to cyclic–static tests survived the 105 cycles of the fatigue block, time to failure in Table 1 is in fact the time to failure obtained in the creep block. The total times to failure can be calculated by adding 27.7 h (105 cycles at 1 Hz) to the creep rupture times given in Table 1. Stress-rupture behavior of the pre-fatigued and the as￾processed specimens tested in air is summarized in Fig. 6. In air prior fatigue has a dramatic effect on the subsequent creep life. For a given applied stress, creep lives of the pre￾fatigued specimens are one to two orders of magnitude longer than those of the as-processed specimens. At 125 MPa, the pre-fatigued specimen achieved a creep run￾out of 100 h, while the as-processed specimen failed after only 4.25 h. At 154 MPa, rupture time for the pre-fatigued specimen was 1.68 h, an order of magnitude higher than the rupture time of 0.27 h for the as-processed specimen. The pre-fatigued specimen that achieved creep run-out at 125 MPa was subjected to a tensile test to failure at Table 2 Retained properties of the N720/A specimens subjected to creep–fatigue interaction tests with the maximum stress of 125 MPa in laboratory air at 1200 C Prior test type Prior test duration (h) Retained strength (MPa) Retained modulus (GPa) Failure strain (%) Fatigue, 10 s hold 117 219 57.4 0.24 Fatigue, 10 s hold 138 216 55.9 0.29 Fatigue, 100 s hold 128 212 50.7 1.14 Fatigue, 100 s hold 112 219 56.0 1.18 Cyclic–static 128 217 59.4 1.04 0 50 100 150 200 250 0.01 0.1 1 10 100 1000 Time (h) Stress (MPa) As-Processed, Ruggles-Wrenn 2006 Pre-Fatigued UTS T = 1200˚C, Air Fig. 6. Creep stress vs time to rupture for NextelTM720/Alumina ceramic composite at 1200 C in laboratory air. Creep data from Ruggles-Wrenn et al. [27]. 0.0 0.2 0.4 0.6 0.8 1.0 012345 Time (h) Strain (%) T = 1200 o C, Air As-Processed, 125 MPa Ruggles-Wrenn, 2006 As-Processed, 154 MPa Ruggles-Wrenn, 2006 Pre-Fatigued 125 MPa Pre-Fatigued 154 MPa 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Time (h) Strain (%) T = 1200o C, Steam Creep Stress = 100 MPa As-Processed Ruggles-Wrenn, 2006 Pre-Fatigued Fig. 7. Creep strain vs time for NextelTM720/Alumina ceramic composite at 1200 C: (a) in laboratory air, (b) in steam environment. Creep data from Ruggles-Wrenn et al. [27] are also shown. 1430 J.M. Mehrman et al. / Composites Science and Technology 67 (2007) 1425–1438