J M. Ehrman et al Composites Science and Technology 67(2007)1425-1438 from prior study [27] are included for comparison. Data in Table I and Fig. 2a demonstrate that in air static(creep) T=1200C. Air loading is significantly more damaging than cyclic loading For a given stress level, specimens tested in creep exhibit a much shorter lives than those tested in fatigue or fatigue a with hold time. Introduction of a hold period into the fati- gue cycle had no noticeable effect on time to failure for the g stress level of 125 MPa. All 125 MPa fatigue tests, regard less of the hold time, achieved a 100-h run-out. Apparently at this stress level, a 100-s hold time is not long enough to vith 100s Hold allow appreciable creep damage to develop and signifi uggles-Wrenn et al, 2006 cantly affect cyclic life. However, for the stress level of the specimen tested in fatigue achieved a run-out of 10sa07 154 MPa, superposition of even a shorter 10-s hold period 1000 onto the fatigue cycle noticeably reduced cyclic life. while cycles(28 h at 1 Hz), specimen subjected to fatigue with T= 1200 C Steam 10-s hold failed after 17.6 h. At 154 MPa, a 10-s hold per- showed that at 1200C creep strain rate of N720/A was 4@ 200 lod is sufficient to initiate creep damage. Prior study [27] UTS strongly dependent on the applied stress; increase in creep stress from 125 to 154 MPa caused an order of magnitude increase in creep rate Higher creep rate is likely responsible 100 A口 for acceleration of damage growth seen in the 154 fatigue 9 口 Fatigue, Ruggles-Wrenn et al,2001△ test with hold. Increasing the hold time at 154 MPa by an order of magnitude resulted in an order of magnitude ■ Fatigue with100 s Hold reduction in life. The specimen subjected to fatigue with 100-s hold survived only 2.73 h. It appears that creep dam 1000 age generated during the 100-s hold period of the first cycle b Time(h) was significant and continued to accumulate more rapidly as the cycling progressed. Finally in the case of the creep at 120 Fig. 2. Maximum stress vs time to failure for N720/A ceramic composite test,which represents the most damaging loading type, life shoa -C: (a)in air and (b)in steam Creep and fatigue data ([27] are also is reduced by another order of magnitude to 0. 27h. The presence of steam noticeably degrades fati well as creep performance of the CMC. Results also reveal loading would govern damage development and that in steam static loading remains considerably more creep would have little effect damaging than cyclic loading. At 154 MPa, the failure time Prior investigation [27] reported substantial strain accu- in fatigue test was 3.27 h, two orders of magnitude higher mulation in creep as well as ratcheting under cyclic loading than the failure time of 0.027 h obtained in creep [27]. at 1200C. Present results permit further assessment of the Superposition of a hold time onto a fatigue cycle dramati- effect of loading type on the rate of strain accumulation. cally degrades cyclic life in steam. At 100 MPa, fatigue test Maximum strain(i.e. strain at maximum stress)as a func achieved a run-out, surviving at least 30 h, while two fati- tion of time for cyclic tests conducted in air is presented in gue tests with 10-s hold failed after 2.36 and 4.63 h Intro- Fig 3, where creep and fatigue test results from [27]are ducing a 10-s hold reduces the cyclic life by an order of also included. Regardless of the maximum stress, strain magnitude bringing it down to the lifetimes obtained in accumulations are lowest in fatigue and highest in creep, creep. Note that fatigue tests with 100-s hold produced sim- while strains accumulated in fatigue with hold time fall in ilar lifetimes(1. 12 and 1.35 h). Similar observations can be the intermediate range. As expected increasing the hold made for the 125 MPa tests. While a 125 MPa fatigue test time results in larger strain accumulations. Maximum achieved a run-out surviving at least 46.2 h, fatigue tests strain vs time for tests conducted in steam is shown in with 10-s and 100-s hold failed after only 0.23 and 0.33 h, Fig. 4. In steam, evolution of maximum strain in cyclic respectively. These failure times are close to that produced tests with hold time is akin to that observed in creep at in creep. Apparently in steam, for stress levels >100 MPa, the same applied stress. This trend becomes more pro a 10-s hold is sufficiently long to activate the damage pro- nounced at higher stress levels. Conversely, fatigue tests cess associated with creep, which proceeds to dominate the produced little strain accumulation. These observations damage development and govern the cyclic life. Once the indicate that in air as well as in steam, the prevailing dam- said damage process sets in, increasing the hold time has age mechanisms operate under sustained loading. Of no appreciable effect on cyclic life. It is recognized that importance are the rates of strain accumulation, which some lower value of the hold time may exist below which reflect damage development under load and serve as indicafrom prior study [27] are included for comparison. Data in Table 1 and Fig. 2a demonstrate that in air static (creep) loading is significantly more damaging than cyclic loading. For a given stress level, specimens tested in creep exhibit much shorter lives than those tested in fatigue or fatigue with hold time. Introduction of a hold period into the fati￾gue cycle had no noticeable effect on time to failure for the stress level of 125 MPa. All 125 MPa fatigue tests, regard￾less of the hold time, achieved a 100-h run-out. Apparently at this stress level, a 100-s hold time is not long enough to allow appreciable creep damage to develop and signifi- cantly affect cyclic life. However, for the stress level of 154 MPa, superposition of even a shorter 10-s hold period onto the fatigue cycle noticeably reduced cyclic life. While the specimen tested in fatigue achieved a run-out of 105 cycles (28 h at 1 Hz), specimen subjected to fatigue with 10-s hold failed after 17.6 h. At 154 MPa, a 10-s hold per￾iod is sufficient to initiate creep damage. Prior study [27] showed that at 1200 C creep strain rate of N720/A was strongly dependent on the applied stress; increase in creep stress from 125 to 154 MPa caused an order of magnitude increase in creep rate. Higher creep rate is likely responsible for acceleration of damage growth seen in the 154 fatigue test with hold. Increasing the hold time at 154 MPa by an order of magnitude resulted in an order of magnitude reduction in life. The specimen subjected to fatigue with 100-s hold survived only 2.73 h. It appears that creep dam￾age generated during the 100-s hold period of the first cycle was significant and continued to accumulate more rapidly as the cycling progressed. Finally in the case of the creep test, which represents the most damaging loading type, life is reduced by another order of magnitude to 0.27 h. The presence of steam noticeably degrades fatigue as well as creep performance of the CMC. Results also reveal that in steam static loading remains considerably more damaging than cyclic loading. At 154 MPa, the failure time in fatigue test was 3.27 h, two orders of magnitude higher than the failure time of 0.027 h obtained in creep [27]. Superposition of a hold time onto a fatigue cycle dramati￾cally degrades cyclic life in steam. At 100 MPa, fatigue test achieved a run-out, surviving at least 30 h, while two fati￾gue tests with 10-s hold failed after 2.36 and 4.63 h. Intro￾ducing a 10-s hold reduces the cyclic life by an order of magnitude bringing it down to the lifetimes obtained in creep. Note that fatigue tests with 100-s hold produced sim￾ilar lifetimes (1.12 and 1.35 h). Similar observations can be made for the 125 MPa tests. While a 125 MPa fatigue test achieved a run-out surviving at least 46.2 h, fatigue tests with 10-s and 100-s hold failed after only 0.23 and 0.33 h, respectively. These failure times are close to that produced in creep. Apparently in steam, for stress levels P100 MPa, a 10-s hold is sufficiently long to activate the damage pro￾cess associated with creep, which proceeds to dominate the damage development and govern the cyclic life. Once the said damage process sets in, increasing the hold time has no appreciable effect on cyclic life. It is recognized that some lower value of the hold time may exist below which cyclic loading would govern damage development and creep would have little effect. Prior investigation [27] reported substantial strain accu￾mulation in creep as well as ratcheting under cyclic loading at 1200 C. Present results permit further assessment of the effect of loading type on the rate of strain accumulation. Maximum strain (i. e. strain at maximum stress) as a func￾tion of time for cyclic tests conducted in air is presented in Fig. 3, where creep and fatigue test results from [27] are also included. Regardless of the maximum stress, strain accumulations are lowest in fatigue and highest in creep, while strains accumulated in fatigue with hold time fall in the intermediate range. As expected increasing the hold time results in larger strain accumulations. Maximum strain vs time for tests conducted in steam is shown in Fig. 4. In steam, evolution of maximum strain in cyclic tests with hold time is akin to that observed in creep at the same applied stress. This trend becomes more pro￾nounced at higher stress levels. Conversely, fatigue tests produced little strain accumulation. These observations indicate that in air as well as in steam, the prevailing dam￾age mechanisms operate under sustained loading. Of importance are the rates of strain accumulation, which reflect damage development under load and serve as indica- 0 50 100 150 200 250 0.1 1 10 100 1000 Time (h) Max Stress (MPa) Fatigue, Ruggles-Wrenn et al, 2006 Fatigue with 10 s Hold Fatigue with 100 s Hold Creep, Ruggles-Wrenn et al, 2006 UTS T = 1200˚C, Air 0 50 100 150 200 250 0.01 0.1 1 10 100 1000 Time (h) Max Stress (MPa) Fatigue, Ruggles-Wrenn et al, 2006 Fatigue with 10 s Hold Fatigue with 100 s Hold Creep, Ruggles-Wrenn et al, 2006 UTS T = 1200˚C, Steam Fig. 2. Maximum stress vs time to failure for N720/A ceramic composite at 1200 C: (a) in air and (b) in steam. Creep and fatigue data [27] are also shown. 1428 J.M. Mehrman et al. / Composites Science and Technology 67 (2007) 1425–1438