J.M. Ehrman et al. Composites Science and Technology 67(2007)1425-1438 1429 T=1200℃c,Air T= 1200.C. Steam Max stress 125 MPa Max Stress= 100 MPa uggles-Wrenn, 2006 100s Hold Fatigue with 100 s Hold 0.50 0.00 Time(h) T=1200℃c,Ai T=1200℃c, Stean Max Stress= 125 MPa 0.75 Creep, Ruggles-Wrenn 2006 1.5 old 0.50 Fatigue with 10 s Hold 100 s Hold uggles-Wrenn 2006 Fatigue, Ruggles-Wrenn 2006 0.00 0.0 Time(h) 0.00 0.75 Time(h) Fig 3. Maximum strain as a function of time at 1200C in laboratory ai for the maximum stress of:(a)125 MPa and(b)154 MPa. Creep and ig. 4. Maximum strain as a function of time at 1200oC in steam Fi fatigue data [27] are also shown environment for the maximum stress of: (a)100 MPa and(b)125 MPa. Creep and fatigue data [27] are also shown. tors of how damaging the particular type of loading is Note that the steady-state strain rate was reached in all 1.0E-02 tests shown in Figs. 3 and 4. The steady-state strain rates 口 Steam, Fatigue as functions of maximum stress are presented in Fig. 5 1.0E-03 △ Steam, e 10s Hold In air. the rate of strain accumulation increases with unin ◆ Stean, e 100s Hold terrupted time spent at maximum stress. Introducing a 10-s hold into a fatigue cycle causes an increase in strain rate Increasing the hold time from 10 to 100 s results in a fur ther(nearly an order of magnitude) increase in strain rate 10E-06 In creep tests(a further increase in hold time) strain rate 10E-07 increases by yet another order of magnitude. These obser ations are consistent with results reported by Zawada 1.0E-08 et al. for N610/AS composite T=1200 1.0E-09 Presence of steam generally accelerates strain accumula ion under both cyclic and sustained loadings. Strain rates Max Stress(MPa tude higher than those obtained in air for a given fatigue g. s sd an rate as a intron en mumum appied stress at times of increasing duration are introduced into a fatigue cycle. In steam, strain rates produced in fatigue with hold ate, resulting in higher rates of strain accumulation. The time of any duration are close to those obtained in creep presence of steam accelerates damage initiation and and significantly higher than those obtained in fatigue. growth The damaging nature of sustained loading is evident. H Retained strength and stiffness of the specimens, which periods allow time-dependent damage mechanisms to initi- achieved run-out in fatigue tests with hold time, are sum-tors of how damaging the particular type of loading is. Note that the steady-state strain rate was reached in all tests shown in Figs. 3 and 4. The steady-state strain rates as functions of maximum stress are presented in Fig. 5. In air, the rate of strain accumulation increases with unin￾terrupted time spent at maximum stress. Introducing a 10-s hold into a fatigue cycle causes an increase in strain rate. Increasing the hold time from 10 to 100 s results in a fur￾ther (nearly an order of magnitude) increase in strain rate. In creep tests (a further increase in hold time) strain rate increases by yet another order of magnitude. These obser￾vations are consistent with results reported by Zawada et al. for N610/AS composite [29]. Presence of steam generally accelerates strain accumula￾tion under both cyclic and sustained loadings. Strain rates obtained in fatigue in steam are at least an order of magni￾tude higher than those obtained in air for a given fatigue stress. In air, the strain rates increase gradually when hold times of increasing duration are introduced into a fatigue cycle. In steam, strain rates produced in fatigue with hold time of any duration are close to those obtained in creep and significantly higher than those obtained in fatigue. The damaging nature of sustained loading is evident. Hold periods allow time-dependent damage mechanisms to initi￾ate, resulting in higher rates of strain accumulation. The presence of steam accelerates damage initiation and growth. Retained strength and stiffness of the specimens, which achieved run-out in fatigue tests with hold time, are sum- 0.00 0.25 0.50 0.75 1.00 1.25 1.50 0 10 20 30 40 50 60 Time (h) Strain (%) T = 1200˚C, Air Max Stress = 125 MPa Creep Ruggles-Wrenn, 2006 Fatigue with 100 s Hold Fatigue with 10 s Hold Fatigue, Ruggles-Wrenn 2006 0.00 0.25 0.50 0.75 1.00 01234 Time (h) Strain (%) Creep, Ruggles-Wrenn 2006 Fatigue with 100 s Hold Fatigue with 10 s Hold T = 1200˚C, Air Max Stress = 154 MPa Fatigue, Ruggles-Wrenn 2006 Fig. 3. Maximum strain as a function of time at 1200 C in laboratory air for the maximum stress of: (a) 125 MPa and (b) 154 MPa. Creep and fatigue data [27] are also shown. 0.0 0.5 1.0 1.5 2.0 2.5 012345 Time (h) Strain (%) T = 1200˚C, Steam Max Stress = 100 MPa Creep Ruggles-Wrenn 2006 Fatigue Ruggles-Wrenn 2006 Fatigue 100 s Hold Fatigue 10 s Hold 0.0 0.5 1.0 1.5 2.0 0.00 0.25 0.50 0.75 1.00 Time (h) Strain (%) T = 1200˚C, Steam Max Stress = 125 MPa Creep Ruggles-Wrenn 2006 Fatigue Ruggles-Wrenn 2006 Fatigue 10 s Hold Fatigue 100 s Hold Fig. 4. Maximum strain as a function of time at 1200 C in steam environment for the maximum stress of: (a) 100 MPa and (b) 125 MPa. Creep and fatigue data [27] are also shown. 1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 10 100 1000 Max Stress (MPa) Strain Rate (s-1) Air, Fatigue Steam, Fatigue Air, Fatigue 10 s Hold Steam, Fatigue 10 s Hold Air, Fatigue 100 s Hold Steam, Fatigue 100 s Hold Air, Creep Steam, Creep T = 1200˚C Fig. 5. Strain rate as a function of maximum applied stress at 1200 C. Creep and fatigue data [27] are also shown. J.M. Mehrman et al. / Composites Science and Technology 67 (2007) 1425–1438 1429