B. Ruggles-Wrenn et al. Composites: Part A 37(2006)2029-2040 then does ratcheting begin. Conversely, in the 150 MPa test itcheting begins after 1000 cycles, and in the 170 MP -e-100 MPa Air T=1200°c test, after only 250 cycles. Earlier ratcheting is accompa- 40 150 MPa. Air f=1H 2-170 MPa. Air R=0.05 nied with higher strain accumulation. Maximum strains ≥ accumulated in the 100. 150 and 170 MPa tests were +100 MPa. Steam t-125 MPa Steam 0.6%, 1.7%/, and 2.4%, respectively -150 MPa. steam As seen in Fig. 7(b), presence of steam accelerates rat cheting at 1200oC. For a given maximum stress, specimens 2o tested in steam exhibited a much earlier onset of ratcheting than those tested in air. In the 100 MPa test conducted in steam, ratcheting begins after 100 cycles, in the 150 MPa test, after 50 cycles, and in the 170 MPa test, after mere 10 cycles. Strain accumulations in the 100, 125, 150 and 1.E+001.E+011.E+021.E+031.E+041.E+051.E+06 170 MPa tests conducted in steam were 0.7%. 1.0% 0.7%, and 0.8%, respectively. Strains accumulated in the Fig 9. H 60 and 170 MPa tests in steam are considerably lower laboratory air and sis energy density(HED)vS fatigue cycles at 1200.Cin than those produced for the same fatigue stress levels in air. Generally, lower strain accumulation with cycling indi- cates that less damage has occurred, and that it is mostly limited to some additional matrix cracking. However, in 母100MPa,Air T the case of 150 and 170 MPa tests conducted in steam ◆100MPa. Stean low accumulated strains are more likely due to early bundle 70 ilures leading to specimen failure in accelerated ratcheting as well as in larger strain accumu- 9 lations with cycling(see Fig. 8). In the 100 MPa test at 1330C, ratcheting begins immediately, strain is accumu- lated rapidly, reaching a significant 3.9% at failure. At 1330C, presence of steam results in earlier failure. Fur- 10 m thermore, in steam environment, shorter cyclic lives are 2+001.E+011.E+021.E+031.E+041.E+051E+06 accompanied with lower strain accumulations Cycles(N) The hysteresis energy density(HED) behavior is shown in Figs. 9 and 10 for 1200 and 1330oC, respectively. The Fig 10. Hysteresis energy density (HED) vs fatigue cycles at 1330"C in HED values at 1200C are fairly small, with the average of xl0 kJ/m. Most traditional composites with interfaces and classical fiber debonding typically produce HED val- a slight decrease in HED with continued cycling. However, ues>80kJ/m'when fatigued above the proportional limit. upon closer examination the 1200C data reveals that at It is seen that for each stress level tested at 1200C, the 10 cycles the hed becomes stable for all tests except HED exhibits a significant decrease within the first 10 the 150 MPa test conducted in steam. Among tests repre- cycles. From this cycle number on there appears to be only sented in Fig 9, only the 150 MPa test in steam did not nieve run-out. In conventional composites, a decrease in HED with fatigue cycling is generally attributed to deg- T=1330°c radation of interfacial shear resistance at the fiber matrix 35·100MPa, Steam interface. For brittle matrix composites, it was also -A50 MPa. Steam observed [32] that continuous damage development, such as matrix cracking and fiber/matrix debonding, in a cycli cally loaded specimen may have a significant effect on the HED behavior. The hed behavior at 1330C is qualita- tively similar to that observed at 1200C. However,aver age HED values obtained in 100 MPa tests were higher at 1330C than at 1200C. The presence of steam appears to have little effect on the hed behavior at both tempe 李导是 tures investigated E+001.E+011E+021.E+031.E+0 E+051.E+06 Retained strength and stiffness of the fatigue specimens, Cycles(N) which achieved fatigue run-out, are summarized in Table 2. Fig 8. Maximum and minimum strains as functions of cycle number at Evaluation of retained properties is useful in assessing the 330C in laboratory air and in steam environment damage state of the composite subjected to prior loadingthen does ratcheting begin. Conversely, in the 150 MPa test ratcheting begins after 1000 cycles, and in the 170 MPa test, after only 250 cycles. Earlier ratcheting is accompa￾nied with higher strain accumulation. Maximum strains accumulated in the 100, 150 and 170 MPa tests were 0.6%, 1.7%, and 2.4%, respectively. As seen in Fig. 7(b), presence of steam accelerates rat￾cheting at 1200 C. For a given maximum stress, specimens tested in steam exhibited a much earlier onset of ratcheting than those tested in air. In the 100 MPa test conducted in steam, ratcheting begins after 100 cycles, in the 150 MPa test, after 50 cycles, and in the 170 MPa test, after mere 10 cycles. Strain accumulations in the 100, 125, 150 and 170 MPa tests conducted in steam were 0.7%, 1.0%, 0.7%, and 0.8%, respectively. Strains accumulated in the 150 and 170 MPa tests in steam are considerably lower than those produced for the same fatigue stress levels in air. Generally, lower strain accumulation with cycling indi￾cates that less damage has occurred, and that it is mostly limited to some additional matrix cracking. However, in the case of 150 and 170 MPa tests conducted in steam, low accumulated strains are more likely due to early bundle failures leading to specimen failure. In air environment, increase in test temperature results in accelerated ratcheting as well as in larger strain accumu￾lations with cycling (see Fig. 8). In the 100 MPa test at 1330 C, ratcheting begins immediately, strain is accumu￾lated rapidly, reaching a significant 3.9% at failure. At 1330 C, presence of steam results in earlier failure. Fur￾thermore, in steam environment, shorter cyclic lives are accompanied with lower strain accumulations. The hysteresis energy density (HED) behavior is shown in Figs. 9 and 10 for 1200 and 1330 C, respectively. The HED values at 1200 C are fairly small, with the average of 10 kJ/m3 . Most traditional composites with interfaces and classical fiber debonding typically produce HED val￾ues P80 kJ/m3 when fatigued above the proportional limit. It is seen that for each stress level tested at 1200 C, the HED exhibits a significant decrease within the first 10 cycles. From this cycle number on there appears to be only a slight decrease in HED with continued cycling. However, upon closer examination the 1200 C data reveals that at 104 cycles the HED becomes stable for all tests except the 150 MPa test conducted in steam. Among tests repre￾sented in Fig. 9, only the 150 MPa test in steam did not achieve run-out. In conventional composites, a decrease in HED with fatigue cycling is generally attributed to deg￾radation of interfacial shear resistance at the fiber matrix interface. For brittle matrix composites, it was also observed [32] that continuous damage development, such as matrix cracking and fiber/matrix debonding, in a cycli￾cally loaded specimen may have a significant effect on the HED behavior. The HED behavior at 1330 C is qualita￾tively similar to that observed at 1200 C. However, aver￾age HED values obtained in 100 MPa tests were higher at 1330 C than at 1200 C. The presence of steam appears to have little effect on the HED behavior at both tempera￾tures investigated. Retained strength and stiffness of the fatigue specimens, which achieved fatigue run-out, are summarized in Table 2. Evaluation of retained properties is useful in assessing the damage state of the composite subjected to prior loading. 100 MPa, Air T = 1330°C 100 MPa, Steam 50 MPa, Steam 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 (%) Fig. 8. Maximum and minimum strains as functions of cycle number at 1330 C in laboratory air and in steam environment. 100 MPa, Air 150 MPa, Air 170 MPa, Air 100 MPa, Steam 125 MPa, Steam 150 MPa, Steam T = 1200°C f = 1 Hz R = 0.05 0 5 10 15 20 25 30 35 40 45 50 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1. E+05 1.E+06 Cycles (N) Hysteretic Energy Density (kJ/m3) Fig. 9. Hysteresis energy density (HED) vs fatigue cycles at 1200 C in laboratory air and in steam environment. T = 1330°C f = 1 Hz R = 0.05 0 10 20 30 40 50 60 70 80 90 100 1.E+00 1. E+01 1.E+02 1. E+03 1.E+04 1. E+05 1.E+06 Cycles (N) Hysteretic Energy Density (kJ/m3) 100 MPa, Air 50 MPa, Steam 100 MPa, Steam Fig. 10. Hysteresis energy density (HED) vs fatigue cycles at 1330 C in laboratory air and in steam environment. 2034 M.B. Ruggles-Wrenn et al. / Composites: Part A 37 (2006) 2029–2040