508 10-mm 10 mm 10m Fig 8. Fracture surfaces(optical micrographs) of the N720/A specimens tested in cyclic fatigue at 1200C in steam at 0.1 Hz:(a)omax= 150 MPa, fr=0.21 h, Ef=0.67%;(b)omax= 125 MPa, Ir=5. 14 h, Er=1. 15%; and (c)omax= 100 MPa, tr= 486h, Ef= 1.80%. Size of the damage zone, the amount of fiber pullout and cyclic fatigue lifetime increase with decreasing fatigue stress level. 170 MPa, the evolution of strain with cycles observed at fiber pullout(see Fig 8), which accounts for larger accu- 0.1 Hz is similar to that at 1.0 Hz. Note that all tests con- mulated strains. In steam the evolution of maximum strain ducted in air achieved fatigue run-out. As seen in Figs. with cycles is strongly influenced by the loading frequency. 7a and b, strains accumulated in steam are considerably For a given fatigue stress level, the rate of strain accumula lower than those accumulated in air at the same fatigue tion increases with decreasing frequency. In the case of the stress and loading frequency. Generally, lower strain accu- 150 MPa tests, specimen cycled at 0. 1 Hz accumulated mulation with cycling indicates that less damage has 0.59% strain during the first 50 cycles, while those tested occurred, and that it is mostly limited to some additional at 1.0 and 10 Hz accumulated 0. 32% and 0. 25% strain matrix cracking. However, lower accumulated strains respectively. However, higher rate of strain accumulation observed in steam invariably correspond to shorter fatigue does not necessarily translate into higher failure strain. lives. In this case lower accumulated strains are more likely The decrease in loading frequency causes a dramatic Results in Table I reveal that in steam at a given loading strain accumulation. Once again considering the time for due to early bundle failures leading to specimen failure. decrease in fatigue life, hence allowing much less time for frequency, both the fatigue life and the accumulated strain tests, it is seen that the fatigue life of 11, 782 cycles increase with decreasing fatigue stress. This trend is partic-(3.27 h) produced at 1.0 Hz allowed for accumulated fail- ularly pronounced at the loading frequency of 0. 1 Hz. The ure strain of 1. 12%0, while at 0. 1 Hz a much shorter life 170 MPa test failed after 12 cycles accumulating only of 202 cycles (0.56 h)allowed for accumulated strain of 0.53% strain, while the 75 MPa test survived 56, 093 cycles only 0.67% and accumulated a much larger strain of 3.35%. Specimens Retained strength and stiffness of the specimens, which with longer cyclic lives also exhibited larger amounts of achieved a run-out, are summarized in Table 2, where Table 2 Retained properties of the N720/A specimens subjected to prior fatigue in laboratory air and in steam environment at 1200oC Fatigue Retained strength Strength retention Retained modulus Modulus retention Strain at failur MPa) environment (MPa) (%0) (GPa) %) Prior fatigue at 0. 1 Hz ≥100 0.3 ≥100 Prior fatigue at 1.0Hz ≥100 ≥100 ≥100 0.51 100 l74 Prior fatigue at 10 Hz 0.48 Retained properties measured at 1200C in laboratory air Data from Ruggles-Wrenn et al. [44]170 MPa, the evolution of strain with cycles observed at 0.1 Hz is similar to that at 1.0 Hz. Note that all tests con￾ducted in air achieved fatigue run-out. As seen in Figs. 7a and b, strains accumulated in steam are considerably lower than those accumulated in air at the same fatigue stress and loading frequency. Generally, lower strain accu￾mulation with cycling indicates that less damage has occurred, and that it is mostly limited to some additional matrix cracking. However, lower accumulated strains observed in steam invariably correspond to shorter fatigue lives. In this case lower accumulated strains are more likely due to early bundle failures leading to specimen failure. Results in Table 1 reveal that in steam at a given loading frequency, both the fatigue life and the accumulated strain increase with decreasing fatigue stress. This trend is partic￾ularly pronounced at the loading frequency of 0.1 Hz. The 170 MPa test failed after 12 cycles accumulating only 0.53% strain, while the 75 MPa test survived 56,093 cycles and accumulated a much larger strain of 3.35%. Specimens with longer cyclic lives also exhibited larger amounts of fiber pullout (see Fig. 8), which accounts for larger accu￾mulated strains. In steam the evolution of maximum strain with cycles is strongly influenced by the loading frequency. For a given fatigue stress level, the rate of strain accumula￾tion increases with decreasing frequency. In the case of the 150 MPa tests, specimen cycled at 0.1 Hz accumulated 0.59% strain during the first 50 cycles, while those tested at 1.0 and 10 Hz accumulated 0.32% and 0.25% strain, respectively. However, higher rate of strain accumulation does not necessarily translate into higher failure strain. The decrease in loading frequency causes a dramatic decrease in fatigue life, hence allowing much less time for strain accumulation. Once again considering the 150 MPa tests, it is seen that the fatigue life of 11,782 cycles (3.27 h) produced at 1.0 Hz allowed for accumulated fail￾ure strain of 1.12%, while at 0.1 Hz a much shorter life of 202 cycles (0.56 h) allowed for accumulated strain of only 0.67%. Retained strength and stiffness of the specimens, which achieved a run-out, are summarized in Table 2, where Fig. 8. Fracture surfaces (optical micrographs) of the N720/A specimens tested in cyclic fatigue at 1200 C in steam at 0.1 Hz: (a) rmax = 150 MPa, tf = 0.21 h, ef = 0.67%; (b) rmax = 125 MPa, tf = 5.14 h, ef = 1.15%; and (c) rmax = 100 MPa, tf = 48.6 h, ef = 1.80%. Size of the damage zone, the amount of fiber pullout and cyclic fatigue lifetime increase with decreasing fatigue stress level. Table 2 Retained properties of the N720/A specimens subjected to prior fatigue in laboratory air and in steam environment at 1200 C Fatigue stress (MPa) Fatigue environment Retained strength (MPa) Strength retention (%) Retained modulus (GPa) Modulus retention (%) Strain at failure (%) Prior fatigue at 0.1 Hz 170 Air 194 P100 51.7 82 0.38 170 Air 196 P100 55.6 80 0.39 Prior fatigue at 1.0 Hza 100 Air 194 P100 56.6 74 0.44 125 Air 199 P100 54.9 73 0.44 150 Air 199 P100 43.4 72 0.53 170 Air 192 P100 40.7 67 0.51 100 Steam 174 90 47.6 84 0.40 125 Steam 168 88 52.0 80 0.43 Prior fatigue at 10 Hz 150 Steam 184 96 55.4 93 0.48 Retained properties measured at 1200 C in laboratory air. a Data from Ruggles-Wrenn et al. [44]. 508 M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516