M B. Ruggles.Wrenn et al Composites: Part A 37(2006)2029-2040 run-out, a decrease in normalized modulus with cycling was still observed. Modulus loss increased with increasing Cycle 125 T=1200°c the 150 MPa test, and 17% in the 170 MPa test. Decrease in F Cycle 1 fatigue stress level In air, normalized modulus was reduced by 5%o in the 100 MPa test, 7% in the 125 MPa test, 8% in Cycle 103225 normalized modulus becomes more pronounced in steam 3 Cycle 2 environment. while modulus was limited to 17%, normalized modulus loss reached 30% in steam. In steam environment. normalized modulus loss in run-out fatigue tests was 10% in the 100 MPa test and 16% in the 125 MPa test. normalized frequency 1 Hz modulus in the 150 MPa test dropped by 17%, and in the 170 MPa test, by a significant 30% prior to failure at 202 00.1020.3040.50.60.70.80.9 cycles. Continuous decrease in modulus observed both in Strain(%) air and steam environments suggests progressive damage Fig. 6. Typical evolution of a stress-strain hysteresis loop with fatigue with continued cycling Because the fatigue damage is still cycles. the criteria of a true endurance fatigue limit proposed by Sorensen et al. [51] and may not be a true endurance presented in Fig. 7(a) and(b)for laboratory air and steam fatigue limit environments, respectively. It is seen that ratcheting takes Normalized modulus evolution with cycling at 1330.C is place in all fatigue tests conducted in air at 1200C. Onset shown in Fig. 5. Modulus loss in the 100 MPa in-air test was of ratchets g depends on the ma aximum tigue stress. Ear- % noticeably greater than the 5% modulus loss in the lier onset of ratcheting is observed in tests with higher max corresponding 1200.C test. This suggests accelerated dam- imum stress levels. In the 100 MPa test, there is little age growth, but may also be indicative of accelerated fiber change in accumulated strain for up to 10,000 cycles, only degradation at hig gher temperature. Contrary to the expec. tations, presence of steam caused little additional modulus degradation Modulus loss in steam was limited to 17% T=1200°c Hysteresis loops for a 100 MPa test conducted in air at Fatigue in Air 1200C are presented in Fig. 6. Results in Fig. 6 are repre- sentative of the hysteresis loop evolution with cycling observed in all fatigue tests reported herein. It is seen that most extensive damage occurs on the first cycle. Afterwards hysteresis loops stabilize quickly. Results in Fig. 6 reveal that ratcheting, defined as progressive increase in accumu lated strain with increasing number of cycles, continues throughout the test. Maximum and minimum cyclic strains as functions of cycle number for fatigue tests conducted at 1200C are E+001.E+011.E+021.E+031.E+041.E+051E+06 (a) Cycles(N T=1330°c 非翻 T=1200°c Fatigue in Steam R=0.05 1.0 1.E+021.E+0 E+001E+011E+021.E+031.E+041.E+051E+06 Cycles(N) Fig. 5. Normalized modulus vs fatigue cycles at 1330C in laboratory Fig. 7. Maximum and minimum strains as functions of cycle number at and in steam environment 1200C:(a)in laboratory air and(b)in steam environmentrun-out, a decrease in normalized modulus with cycling was still observed. Modulus loss increased with increasing fatigue stress level. In air, normalized modulus was reduced 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. Decrease in normalized modulus becomes more pronounced in steam environment. While in air the reduction in normalized modulus was limited to 17%, normalized modulus loss reached 30% in steam. In steam environment, normalized modulus loss in run-out fatigue tests was 10% in the 100 MPa test and 16% in the 125 MPa test. Normalized modulus in the 150 MPa test dropped by 17%, and in the 170 MPa test, by a significant 30% prior to failure at 202 cycles. Continuous decrease in modulus observed both in air and steam environments suggests progressive damage with continued cycling. Because the fatigue damage is still evolving at 105 cycles, the 105 fatigue limit does not meet the criteria of a true endurance fatigue limit proposed by Sorensen et al. [51] and may not be a true endurance fatigue limit. Normalized modulus evolution with cycling at 1330 C is shown in Fig. 5. Modulus loss in the 100 MPa in-air test was 15%, noticeably greater than the 5% modulus loss in the corresponding 1200 C test. This suggests accelerated dam￾age growth, but may also be indicative of accelerated fiber degradation at higher temperature. Contrary to the expec￾tations, presence of steam caused little additional modulus degradation. Modulus loss in steam was limited to 17%. Hysteresis loops for a 100 MPa test conducted in air at 1200 C are presented in Fig. 6. Results in Fig. 6 are repre￾sentative of the hysteresis loop evolution with cycling observed in all fatigue tests reported herein. It is seen that most extensive damage occurs on the first cycle. Afterwards hysteresis loops stabilize quickly. Results in Fig. 6 reveal that ratcheting, defined as progressive increase in accumu￾lated strain with increasing number of cycles, continues throughout the test. Maximum and minimum cyclic strains as functions of cycle number for fatigue tests conducted at 1200 C are presented in Fig. 7(a) and (b) for laboratory air and steam environments, respectively. It is seen that ratcheting takes place in all fatigue tests conducted in air at 1200 C. Onset of ratcheting depends on the maximum fatigue stress. Ear￾lier onset of ratcheting is observed in tests with higher max￾imum stress levels. In the 100 MPa test, there is little change in accumulated strain for up to 10,000 cycles, only 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 Cycles (N) Normalized Modulus (E/Eo) 100 MPa, Air 100 MPa, Steam 50 MPa, Steam T = 1330°C f = 1 Hz R = 0.05 Fig. 5. Normalized modulus vs fatigue cycles at 1330 C in laboratory air and in steam environment. 0 20 40 60 80 100 120 140 160 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Strain (%) Stress (MPa) T = 1200°C Fatigue in Air Max Stress = 100 Mpa frequency = 1 Hz R = 0.05 Cycle 10000 Cycle 103225 Cycle 1025 Cycle 125 Cycle 1 Cycle 2 Fig. 6. Typical evolution of a stress–strain hysteresis loop with fatigue cycles. 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 Cycles (N) Strain (%) T = 1200°C Fatigue in Steam T = 1200°C Fatigue in Air 100 Mpa 125 MPa 150 MPa 170 MPa Strain (%) Cycles (N) 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+06 100 MPa 150 MPa 170 MPa (a) (b) Fig. 7. Maximum and minimum strains as functions of cycle number at 1200 C: (a) in laboratory air and (b) in steam environment. M.B. Ruggles-Wrenn et al. / Composites: Part A 37 (2006) 2029–2040 2033