M.B. Ruggles-Wrenn et al. International Journal of fatigue 30(2008)502-516 Summary of fatigue results for the N720/A composite at 1200C, in laboratory air and steam environments Test environment Max stress(MPa) Cycles to failure Time to failure(h) Fatigue at 0.1 HE Laboratory air 193 Laboratory air 136,1212 2.132 Steam 56,093 35 Steam 48.6 115 150 75 21 12 0.03 Fatigue at 1.0 Hz 33. Laboratory air 40.7 1.144 Laboratory air ,47 52 Laboratory 09.436 25 Steam 100,780 0 0.714 Steam 08 Steam 150 l1,782 Steam 02 0.06 0.81 fatigue at 10 Hz 1,0000102 0.77 Run-out, failure of specimen did not occur when the test was terminated Data from Ruggles-Wrenn et al. [44] that introduction of a short hold period(at the maximum Furthermore, in steam the influence of the loading fre- stress)into the fatigue cycle significantly degraded the fati- quency on fatigue life becomes dramatic. In laboratory gue performance of N720/A composite at 1200C in air air, the high 170 MPa fatigue limit was obtained at both and in steam. These results suggest that the loading rate 0.1 and 1.0 Hz In steam, the fatigue performance was best plays a significant role in damage development. Therefore at the frequency of 10 Hz. Yet, even at 10 Hz the in-steam investigation of the effects of frequency on fatigue behav- fatigue limit is only 150 MPa(78% UTS at 1200 C), ior, especially when conducted in steam environment is noticeably lower than what could be expected in air. At critical to assessing the durability of a given porous matrix the loading frequency of 1.0 Hz, the fatigue limit drops oxide-oxide CMC to 125 MPa(69% UTS at 1200C). As the frequency Tension-tension fatigue tests were conducted at the fre- decreases by another order of magnitude, the fatigue per quencies of 0. I and 10 Hz at 1200C in air and in steam. formance deteriorates drastically. At 0.1 Hz, run-out was Results are summarized in Table l. Results are also pre- not achieved even at the low stress level of 75 MPa(39% sented in Fig. 3 as the stress vs. time to failure curves. UTS at 1200C) Results of fatigue tests at 1.0 Hz from the prior study [44]are included in Table I and in Fig 3 for comparison Data in Table I show that the loading frequency has little T=1200'C, Steam ffect on fatigue performance in air. For the frequencies of 0 I and 1.0 Hz the fatigue limit in air was 170 MPa(88% UTS at 1200 C UTS at 1200C). This fatigue limit is based on the run- out condition of 10 cycles, approximate number of load ing cycles expected in aerospace applications at 1200C. could have resulted in a lower fatigue limit. Because the o 0 It is recognized that a more rigorous run-out condition fatigue performance was expected to improve with increas- ing loading frequency, no tests were conducted in air at the 01.0 Hz, Ruggles-Wrenn et al, 2006 口10Hz frequency of 10 Hz. In view of the excellent fatigue resis- tance and high fatigue limit obtained in air at the frequen cies 0. 1 and 1.0 Hz, an equally high in-air fatigue limit ould be anticipated at 10 Hz. Presence of steam causes noticeable degradation in fatigue performance. At all load- Fig 3. Fatigue S-N curves for NextelT720/alumina ceramic composite at ing frequencies investigated, the fatigue limits obtained 1200C in steam environment. Fatigue data at 1.0 Hz from ruggles Wrenn et al. [44]. Arrow indicates that failure of specimen did not occur steam are significantly lower than those obtained in air. when the test was terminatedthat introduction of a short hold period (at the maximum stress) into the fatigue cycle significantly degraded the fati￾gue performance of N720/A composite at 1200 C in air and in steam. These results suggest that the loading rate plays a significant role in damage development. Therefore investigation of the effects of frequency on fatigue behav￾ior, especially when conducted in steam environment is critical to assessing the durability of a given porous matrix oxide–oxide CMC. Tension–tension fatigue tests were conducted at the fre￾quencies of 0.1 and 10 Hz at 1200 C in air and in steam. Results are summarized in Table 1. Results are also pre￾sented in Fig. 3 as the stress vs. time to failure curves. Results of fatigue tests at 1.0 Hz from the prior study [44] are included in Table 1 and in Fig. 3 for comparison. Data in Table 1 show that the loading frequency has little effect on fatigue performance in air. For the frequencies of 0.1 and 1.0 Hz the fatigue limit in air was 170 MPa (88% UTS at 1200 C). This fatigue limit is based on the run￾out condition of 105 cycles, approximate number of load￾ing cycles expected in aerospace applications at 1200 C. It is recognized that a more rigorous run-out condition could have resulted in a lower fatigue limit. Because the fatigue performance was expected to improve with increas￾ing loading frequency, no tests were conducted in air at the frequency of 10 Hz. In view of the excellent fatigue resis￾tance and high fatigue limit obtained in air at the frequen￾cies 0.1 and 1.0 Hz, an equally high in-air fatigue limit could be anticipated at 10 Hz. Presence of steam causes noticeable degradation in fatigue performance. At all load￾ing frequencies investigated, the fatigue limits obtained in steam are significantly lower than those obtained in air. Furthermore, in steam the influence of the loading fre￾quency on fatigue life becomes dramatic. In laboratory air, the high 170 MPa fatigue limit was obtained at both 0.1 and 1.0 Hz. In steam, the fatigue performance was best at the frequency of 10 Hz. Yet, even at 10 Hz the in-steam fatigue limit is only 150 MPa (78% UTS at 1200 C), noticeably lower than what could be expected in air. At the loading frequency of 1.0 Hz, the fatigue limit drops to 125 MPa (69% UTS at 1200 C). As the frequency decreases by another order of magnitude, the fatigue per￾formance deteriorates drastically. At 0.1 Hz, run-out was not achieved even at the low stress level of 75 MPa (39% UTS at 1200 C). Table 1 Summary of fatigue results for the N720/A composite at 1200 C, in laboratory air and steam environments Test environment Max. stress (MPa) Cycles to failure Time to failure (h) Failure strain (%) Fatigue at 0.1 Hz Laboratory air 170 100,017a 278a 1.93a Laboratory air 170 136,121a 378a 2.13a Steam 75 56,093 156 3.35 Steam 100 17,498 48.6 1.80 Steam 125 1850 5.14 1.15 Steam 150 75 0.21 0.67 Steam 170 12 0.03 0.53 Fatigue at 1.0 Hzb Laboratory air 100 120,199a 33.4a 0.63a Laboratory air 125 146,392a 40.7a 1.14a Laboratory air 150 167,473a 46.5a 1.66a Laboratory air 170 109,436a 30.4a 2.25a Steam 100 100,780a 28.0a 0.71a Steam 125 166,326a 46.2a 1.08a Steam 150 11,782 3.27 1.12 Steam 170 202 0.06 0.81 Fatigue at 10 Hz Steam 150 1,000,010a 27.8a 0.77a Steam 170 11,387 0.32 1.03 a Run-out, failure of specimen did not occur when the test was terminated. b Data from Ruggles-Wrenn et al. [44]. 0 50 100 150 200 250 0.01 0.1 1 10 100 1000 Time to failure (h) ) aP M( ssert S 0.1 Hz 1.0 Hz, Ruggles-Wrenn et al, 2006 10 Hz UTS at 1200 ˚C T = 1200 ˚C, Steam Fig. 3. Fatigue S–N curves for Nextel720/alumina ceramic composite at 1200 C in steam environment. Fatigue data at 1.0 Hz from Ruggles￾Wrenn et al. [44]. Arrow indicates that failure of specimen did not occur when the test was terminated. M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516 505