MB. Ruggles-Wrenn, P D. Laffey Composites Science and Technology 68(2008)2260-2266 2263 1E-03 rupture tests in interlaminar shear for the N720/ A ceramic composite ratory air and in steam environment stress(MPa) Time to rupture (s i 1.E-04IASteam Laboratory air 0.14 360.000 正105 3600004 3.21 2.1 1E07 T=1200° ' s time curves obtained at 1200 C in air and in steam are shown in Fig 5 10 The creep curves obtained at 6.5 MPa in air exhibit primary and Creep Stress(MPa) secondary creep regimes. Transition from primary to secondary Fig. 6. Minimum creep rate as a function of applied stress for N720JA ceramic creep occurs fairly early in creep life. Secondary creep is likely to composite at 1200C in air and in steam. persist for the duration of the creep lifetime. In reep run-out of 100 h is achieved at the shear stress of 6.5 MPa (78.5% ILSS The strains accumulated during 100 h at 6.5 MPa are compara to those obtained in the monotonic test. Creep curve produced in T=1200°c steam at the shear stress of 4 MPa also exhibits only primary and ILSS. Air ondary creep regimes. In contrast, creep curves obtained in at 5 and 6.5 MPa show primary, secondary and tertiary immediately. Secondary creep persists for x70% of the creep life 8 before transitioning to tertiary creep. Creep strain accumulation first increases as the applied shear stress increases from 4 to 5 MPa, then decreases as the applied stress increases to 6.5 MPa. It is noteworthy that in steam, all accumulated creep strains are at least an order of magnitude higher than the failure strain ob- tained in the monotonic test In steam creep run-out was achieved nly at 4 MPa(50% ILSS). 1.E+03 1.E+04 1.E+05 1.E+06 Minimum creep rate was measured in all tests. Creep strain rate as a function of applied stress is shown in Fig. 6. In steam, the min- mum creep rate increases by a factor of 10 when applied stress in- fg 7. Interlaminar shear stress vs time to rupture for N720/A composite at creases from 4 to 5 MPa. At 6.5 MPa, creep rate in steam is at least wo orders of magnitude higher than that in air. Stress-rupture Retained interlaminar shear strength values of the specimens behavior is summarized in Fig. 7, where applied shear stress is that achieved a run -out at 6.5 MPa in air and at 4 MPa in steam plotted vs time to rupture at 1200C in air and in steam. In are given in Table 2. The stress-strain curves obtained for the creep life(up to 100 h)appears to be relatively independent of ap- N720/A specimens subjected to pri in interlaminar shear plied stress up to 78% ILSS All creep tests conducted at 6.5 MPa inin air and in steam are presented in Fig. &a and b, respectively air achieved a run-out. For applied shear stress >5 MPa the pres- The IlSS of the specimen pre-crept at 6. 5 MPa in air has increased ence of steam dramatically reduced creep lifetimes. At 6.5 MPa, by nearly 37% compared to the ILSS of the as-processed specimen the reduction in creep life due to steam was 94% as compared to Conversely prior creep in steam has degraded the interlaminar the run-out of 100 h shear strength of N720/A by about 27%. The specimen pre-crept at 4 MPa in steam retained less than 75% of its ILSS. Prior creep in either environment had little qualitative effect on stress-strain 5 MPa T=1200°c 3.3. Composite microstructure 6.5 MPa 方20 pression to failure at 1200C in air is shown in Fig 9. Delamination Table 2 Retained interlaminar shear properties of the n720/A specimens subjected to prior 4 MPa creep in interlaminar shear at 1200C Creep stress etained interlaminar shear strength ailure strain 400006000080000 100000 Laboratory air 6.5 0.22 Fig. 5. Creep strain vs time curves for N720JA laminar shear stresses in the 4-6.5 MPa range at 1200" C in air and in steam.vs time curves obtained at 1200 C in air and in steam are shown in Fig. 5. The creep curves obtained at 6.5 MPa in air exhibit primary and secondary creep regimes. Transition from primary to secondary creep occurs fairly early in creep life. Secondary creep is likely to persist for the duration of the creep lifetime. In air, creep run-out of 100 h is achieved at the shear stress of 6.5 MPa (78.5% ILSS). The strains accumulated during 100 h at 6.5 MPa are comparable to those obtained in the monotonic test. Creep curve produced in steam at the shear stress of 4 MPa also exhibits only primary and secondary creep regimes. In contrast, creep curves obtained in steam at 5 and 6.5 MPa show primary, secondary and tertiary creep. Transition from primary to secondary creep occurs almost immediately. Secondary creep persists for 70% of the creep life before transitioning to tertiary creep. Creep strain accumulation first increases as the applied shear stress increases from 4 to 5 MPa, then decreases as the applied stress increases to 6.5 MPa. It is noteworthy that in steam, all accumulated creep strains are at least an order of magnitude higher than the failure strain ob￾tained in the monotonic test. In steam creep run-out was achieved only at 4 MPa (50% ILSS). Minimum creep rate was measured in all tests. Creep strain rate as a function of applied stress is shown in Fig. 6. In steam, the min￾imum creep rate increases by a factor of 10 when applied stress in￾creases from 4 to 5 MPa. At 6.5 MPa, creep rate in steam is at least two orders of magnitude higher than that in air. Stress-rupture behavior is summarized in Fig. 7, where applied shear stress is plotted vs time to rupture at 1200 C in air and in steam. In air, creep life (up to 100 h) appears to be relatively independent of ap￾plied stress up to 78% ILSS. All creep tests conducted at 6.5 MPa in air achieved a run-out. For applied shear stress P5 MPa the pres￾ence of steam dramatically reduced creep lifetimes. At 6.5 MPa, the reduction in creep life due to steam was 94% as compared to the run-out of 100 h. Retained interlaminar shear strength values of the specimens that achieved a run-out at 6.5 MPa in air and at 4 MPa in steam are given in Table 2. The stress–strain curves obtained for the N720/A specimens subjected to prior creep in interlaminar shear in air and in steam are presented in Fig. 8a and b, respectively. The ILSS of the specimen pre-crept at 6.5 MPa in air has increased by nearly 37% compared to the ILSS of the as-processed specimen. Conversely, prior creep in steam has degraded the interlaminar shear strength of N720/A by about 27%. The specimen pre-crept at 4 MPa in steam retained less than 75% of its ILSS. Prior creep in either environment had little qualitative effect on stress–strain behavior. 3.3. Composite microstructure A typical fracture surface of the DNS specimen tested in com￾pression to failure at 1200 C in air is shown in Fig. 9. Delamination Table 1 Results of creep-rupture tests in interlaminar shear for the N720/A ceramic composite at 1200 C in laboratory air and in steam environment Creep stress (MPa) Creep strain (%) Time to rupture (s) Laboratory air 6.5 0.14 360,000a 6.5 0.24 360,000a Steam 4.0 1.65 360,000a 5.0 3.21 29,040 6.5 2.13 20,400 a Run-out. 0.0 1.0 2.0 3.0 4.0 0 20000 40000 60000 80000 100000 Creep Strain (%) Time (s) T = 1200 ºC 6.5 MPa 5 MPa 4 MPa 6.5 MPa Air Steam Fig. 5. Creep strain vs time curves for N720/A composite obtained at applied int￾erlaminar shear stresses in the 4–6.5 MPa range at 1200 C in air and in steam. 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1 10 Creep Strain Rate (s-1) Creep Stress (MPa) Air Steam T = 1200 ºC Fig. 6. Minimum creep rate as a function of applied stress for N720/A ceramic composite at 1200 C in air and in steam. 0 2 4 6 8 10 1.E+03 1.E+04 1.E+05 1.E+06 Shear Stress (MPa) Time (s) Air Steam T = 1200 ºC ILSS, Air Fig. 7. Interlaminar shear stress vs time to rupture for N720/A composite at 1200 C. Table 2 Retained interlaminar shear properties of the N720/A specimens subjected to prior creep in interlaminar shear at 1200 C Creep stress (MPa) Retained interlaminar shear strength (MPa) Failure strain (%) Laboratory air 6.5 11.2 0.22 Steam 4.0 6.13 0.19 M.B. Ruggles-Wrenn, P.D. Laffey / Composites Science and Technology 68 (2008) 2260–2266 2263