M.B. Ruggles-Wrenn et al. International Journal of fatigue 30(2008)502-516 modulus loss is likely due to the early failure of fiber bun 100MPa,1.0H125MPa,1.0Hz dles. while the modulus loss observed in other tests con 150MPa,1.0÷170MPa,1.0Hz ducted at 0.1 and 1.0 Hz is likely due to the progressive cracking of the matrix. Increase in frequency by another order of magnitude to 10 Hz has an even more appreciable 日1.04 normalized modulus was only 5% in the 150 MPa test, which achieved a run-out. and 8% in the 170 MPa test Continuous decrease in modulus observed both in air and in steam suggests progressive damage with continued T=1200°c,Air cycling. Because the fatigue damage is still evolving at 10 cycles (10 cycles at 10 Hz), the 10(10 at 10 Hz) fati 1.E+001E+011.E+021.E+031.E+041.E+051E+061.E+07 gue limit does not meet the criteria of a true endurance fati Cycles(N) gue limit proposed by Sorensen et al. [48 ]and may not be a true endurance fatigue limit 91目901h+12Ma10h 75 MPa 0.1 Hz Maximum cyclic strains as functions of cycle number for fatigue tests conducted at 1200C in air and in steam are 1.4EI-0150 MPa, 0.1 Hz 150 MPa, 1.0 Hz 0150 MPa, 10 Hz presented in Figs. 7a and b, respectively. It is seen that rat- 170MPa01h·170MPa1.0H170MPa,10H cheting takes place in all tests conducted at 1200C. In lab oratory air the rate of strain accumulation increases with increasing fatigue stress level. On the other hand, the loading frequency does not appear to have a strong effect on strain accumulation rate. For the fatigue stress of T= 1200C. Steam E+001.E+011.E+021.E+031.E+04 1E+061.E+07 a T=1200c.A ●100MPa,1.0H Fig. 6. Normalized modulus vs fatigue cycles at 1200C(a) in laborator 馨170MPa,1.0Hz ir and(b) in steam Data at 1.0 Hz from Ruggles-Wrenn et al. [44] A170 MPa, 0.1 Hz normalized modulus dropped by 5% in the 100 MPa test, 7% in the 125 MPa test. 8% in the 150 MPa test and 17% in the 170 MPa test. In air, the loading frequency appears to have little effect on the modulus change with cycles. Modulus loss of 18% observed in the 170 MPa test conducted at 0. 1 Hz is not significantly different from that at 1.0 Hz. Changes in normalized modulus as well as the 1E+001.E+011.E+021.E+031.E+041.E+051.E+06 influence of loading rate on modulus evolution become Cycle(N) more pronounced in steam(Fig. 6b). While in air the b reduction in normalized modulus was limited to 18% T= 1200 C Steam (170 MPa test at 0.1 Hz), in steam the normalized modulus 125MPa,0.1 loss reached 30%(170 MPa tests at 0. 1 and at 1.0 Hz. As 30日|→150MPa,01h 18%in the 75 MPa test, 20% in the 100 MPa test, s ge 2.5 F -170 MPa,0.1 Hz in air, in steam modulus loss increases with increasing fati- gue stress level. At 0.1 Hz, normalized modulus loss wa 2.0 F1-o-125 MPa,1.0H the 125 and 150 MPa tests, and 30% in the 170 MPa test. i 15F1-0-170 MPa 1.o t/ -o-150 Mpa, 10 Hz Increase in frequency by an order of magnitude had a 10E-170Mpa,10z noticeable effect on the modulus evolution with cycles. At 1.0 HZ. normalized modulus loss in run-out fatigue tests was 10% in the 100 MPa test and 16%o in the 125 MPa test In the 150 MPa test normalized modulus dropped by 17%o, 1E+001.E+011E+021E+031E+041.E+051.E+06 and in the 170 MPa test, by 30%0. Note that the normalized modulus loss of 30% was observed in the 170 MPa tests Fig. 7. Maximum strain vs fatigue cycles at 1200C: (a)in laboratory air conducted at both 0.1 and 1.0 Hz. in the case of the and(b)in steam environment Data at 1.0 Hz from Ruggles-Wrenn et al 170 MPa tests, which produced very short fatigue lifetimes, [44].normalized modulus dropped by 5% in the 100 MPa test, 7% in the 125 MPa test, 8% in the 150 MPa test, and 17% in the 170 MPa test. In air, the loading frequency appears to have little effect on the modulus change with cycles. Modulus loss of 18% observed in the 170 MPa test conducted at 0.1 Hz is not significantly different from that at 1.0 Hz. Changes in normalized modulus as well as the influence of loading rate on modulus evolution become more pronounced in steam (Fig. 6b). While in air the reduction in normalized modulus was limited to 18% (170 MPa test at 0.1 Hz), in steam the normalized modulus loss reached 30% (170 MPa tests at 0.1 and at 1.0 Hz). As in air, in steam modulus loss increases with increasing fati￾gue stress level. At 0.1 Hz, normalized modulus loss was 18% in the 75 MPa test, 20% in the 100 MPa test, 21% in the 125 and 150 MPa tests, and 30% in the 170 MPa test. Increase in frequency by an order of magnitude had a noticeable effect on the modulus evolution with cycles. At 1.0 Hz, normalized modulus loss in run-out fatigue tests was 10% in the 100 MPa test and 16% in the 125 MPa test. In the 150 MPa test normalized modulus dropped by 17%, and in the 170 MPa test, by 30%. Note that the normalized modulus loss of 30% was observed in the 170 MPa tests conducted at both 0.1 and 1.0 Hz. In the case of the 170 MPa tests, which produced very short fatigue lifetimes, modulus loss is likely due to the early failure of fiber bun￾dles, while the modulus loss observed in other tests con￾ducted at 0.1 and 1.0 Hz is likely due to the progressive cracking of the matrix. Increase in frequency by another order of magnitude to 10 Hz has an even more appreciable effect on the modulus change. At 10 Hz, the reduction in normalized modulus was only 5% in the 150 MPa test, which achieved a run-out, and 8% in the 170 MPa test. Continuous decrease in modulus observed both in air and in steam suggests progressive damage with continued cycling. Because the fatigue damage is still evolving at 105 cycles (106 cycles at 10 Hz), the 105 (106 at 10 Hz) fati￾gue limit does not meet the criteria of a true endurance fati￾gue limit proposed by Sorensen et al. [48] and may not be a true endurance fatigue limit. Maximum cyclic strains as functions of cycle number for fatigue tests conducted at 1200 C in air and in steam are presented in Figs. 7a and b, respectively. It is seen that rat￾cheting takes place in all tests conducted at 1200 C. In lab￾oratory air the rate of strain accumulation increases with increasing fatigue stress level. On the other hand, the loading frequency does not appear to have a strong effect on strain accumulation rate. For the fatigue stress of 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Cycles (N) sul udo Mdezil a mr oN 100 MPa, 1.0 Hz 125 MPa, 1.0 Hz 150 MPa, 1.0 Hz 170 MPa, 1.0 Hz 170 MPa, 0.1 Hz T = 1200 ˚C, Air 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Cycles (N) sul udo Mdezil a mr oN 75 MPa, 0.1 Hz 100 MPa, 0.1 Hz 100 MPa, 1.0 Hz 125 MPa, 0.1 Hz 125 MPa, 1.0 Hz 150 MPa, 0.1 Hz 150 MPa, 1.0 Hz 150 MPa, 10 Hz 170 MPa, 0.1 Hz 170 MPa, 1.0 Hz 170 MPa, 10 Hz T = 1200 ˚C, Steam a b Fig. 6. Normalized modulus vs. fatigue cycles at 1200 C (a) in laboratory air and (b) in steam. Data at 1.0 Hz from Ruggles-Wrenn et al. [44]. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Cycle (N) 100 MPa, 1.0 Hz 150 MPa, 1.0 Hz 170 MPa, 1.0 Hz 170 MPa, 0.1 Hz T = 1200 ˚C, Air 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Cycle (N) Strain % Strain % 75 MPa, 0.1 Hz 100 MPa, 0.1 Hz 125 MPa, 0.1 Hz 150 MPa, 0.1 Hz 170 MPa, 0.1 Hz 100 MPa, 1.0 Hz 125 MPa, 1.0 Hz 150 MPa, 1.0 Hz 170 MPa, 1.0 Hz 150 Mpa, 10 Hz 170 Mpa, 10 Hz T = 1200 ˚C, Steam a b Fig. 7. Maximum strain vs. fatigue cycles at 1200 C: (a) in laboratory air and (b) in steam environment. Data at 1.0 Hz from Ruggles-Wrenn et al. [44]. M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516 507