M.B. Ruggles-Wrenn et aL. Composites: Part A 37(2006)2029-2040 and that fiber-matrix debonding is insignificant. Material exhibits typical fiber-dominated composite behavior. Fiber 口1200°c,Ar■1330°c,Air fracture appears to be the dominant damage mode. At △1200c,Seam▲1330c, Steam 23C, the ultimate tensile strength (UTS) was 169 MPa UTS at 1200C lastic modulus, 60 GPa, and failure strain, 0.35%. At 2 150 1200C, the UTS, elastic modulus and failure strain were 5 192 MPa, 75 GPa and 0.38%, respectively. These results UTS at1330°c agree well with those reported by COI Ceramics [50] The stress-strain behavior changes dramatically at 1330C. The stress-strain curve at 1330C is linear up to the proportional limit( 74 MPa), where non-linear behav- r sets in. At 1330C, the UTS, elastic modulus and fail- ure strain were 120 MPa, 42 GPa and 1.7/, respectively 1.E+001.E4011.E+021.E+081.E+041.E+05 While the UTS and the elastic modulus are significantly lower than those at 1200C, failure strain increases almost Fig 3. Fatigue S-N curves for NextelTM720/alumina ceramic composite tenfold at 1200 and 1330C, in laboratory air and in steam environment It is important to note that in all tension tests, as well as in all other tests reported herein, the failure occurred within the gage section of the extensometer. run-out condition of 10 cycles, approximate number o loading cycles expected in aerospace applications 3.2. Tension-tension fatigue 1200oC. It is believed that a more rigorous run-out condi tion would have resulted in a lower fatigue limit. Presence Degradation of fatigue performance in high-tempera of steam(a highly oxidizing environment) causes noticeable ture oxidizing environments remains among the key con- degradation in fatigue performance. At 1200C, the in- erns that must ddressed before using CMCs in steam fatigue limit is only 125 MPa (65% UTS 1200C). As seen in Fig. 3, increase in temperature from advanced aerospace applications. Therefore high-tempera- 1200 to 1330C results in significant degradation of the ture fatigue tests, especially when conducted in steam envi- in-air fatigue performance. Even at the low fatigue stress ronment are critical to assessing the durability of a given level of 50 MPa(42% UTS at 1330C) the run-out was CMC Tension-tension fatigue tests with a ratio, R of 0.05 not achieved As expected, steam environment even further vere performed at 1200 and 1330%C in air and in steam degraded an already poor fatigue resistance nvironments. Results are summarized in Table 1. where Of importance in cyclic fatigue is the reduction in stiff the maximum stress level and number of cycles to failure. and minima.sIs modulus determined from the maximum um stress-strain data points during a load Results are also presented in Fig. 3 as stress vs cycles to cycle), reflecting the damage development during cycling failure(S-N) curves for both temperatures and environ- normalized modulus(i. e. modulus normalized by the mod- ments.At 1200C the in-air fatigue limit was 170 MPa ulus obtained in the first cycle) is plotted vs fatigue cycles (88% UTS at 1200C). This fatigue limit is based on a It is noteworthy that although all in-air tests achieved Table 1 gue results for the N720/A composite at 1200 and °C.in ory air and steam environments Cycles to failure - A125 MPa, Air +125 MPa, Steam1-1H2oc Eh-100MPa, Air -100 MPa, Steam T= 120 Max stress(MPa) -e-150 MPa, Air --150 MPa, Steam Laboratory air 120,199 Laboratory air - 170 MPa, Air -+170 MPa, Stea Laboratory air 67,4732 Laboratory air l09.436 100.7804 Steam 1663262 Steam at1330°C 1E001E011E21E:01E:041E:051E:06 ry 25,852 Fig 4. Normalized modulus vs fatigue cycles at 1200C in laboratoryand that fiber-matrix debonding is insignificant. Material exhibits typical fiber-dominated composite behavior. Fiber fracture appears to be the dominant damage mode. At 23 C, the ultimate tensile strength (UTS) was 169 MPa, elastic modulus, 60 GPa, and failure strain, 0.35%. At 1200 C, the UTS, elastic modulus and failure strain were 192 MPa, 75 GPa and 0.38%, respectively. These results agree well with those reported by COI Ceramics [50]. The stress–strain behavior changes dramatically at 1330 C. The stress–strain curve at 1330 C is linear up to the proportional limit (74 MPa), where non-linear behav￾ior sets in. At 1330 C, the UTS, elastic modulus and fail￾ure strain were 120 MPa, 42 GPa and 1.7%, respectively. While the UTS and the elastic modulus are significantly lower than those at 1200 C, failure strain increases almost tenfold. It is important to note that in all tension tests, as well as in all other tests reported herein, the failure occurred within the gage section of the extensometer. 3.2. Tension–tension fatigue Degradation of fatigue performance in high-tempera￾ture oxidizing environments remains among the key con￾cerns that must be addressed before using CMCs in advanced aerospace applications. Therefore high-tempera￾ture fatigue tests, especially when conducted in steam envi￾ronment are critical to assessing the durability of a given CMC. Tension–tension fatigue tests with a ratio, R of 0.05, were performed at 1200 and 1330 C in air and in steam environments. Results are summarized in Table 1, where test temperature and environment are shown together with the maximum stress level and number of cycles to failure. Results are also presented in Fig. 3 as stress vs cycles to failure (S–N) curves for both temperatures and environ￾ments. At 1200 C the in-air fatigue limit was 170 MPa (88% UTS at 1200 C). This fatigue limit is based on a run-out condition of 105 cycles, approximate number of loading cycles expected in aerospace applications at 1200 C. It is believed that a more rigorous run-out condi￾tion would have resulted in a lower fatigue limit. Presence of steam (a highly oxidizing environment) causes noticeable degradation in fatigue performance. At 1200 C, the in￾steam fatigue limit is only 125 MPa (65% UTS at 1200 C). As seen in Fig. 3, increase in temperature from 1200 to 1330 C results in significant degradation of the in-air fatigue performance. Even at the low fatigue stress level of 50 MPa (42% UTS at 1330 C) the run-out was not achieved. As expected, steam environment even further degraded an already poor fatigue resistance. Of importance in cyclic fatigue is the reduction in stiff- ness (hysteresis modulus determined from the maximum and minimum stress–strain data points during a load cycle), reflecting the damage development during cycling. Change in modulus at 1200 C is shown in Fig. 4, where normalized modulus (i.e. modulus normalized by the mod￾ulus obtained in the first cycle) is plotted vs fatigue cycles. It is noteworthy that although all in-air tests achieved Table 1 Summary of fatigue results for the N720/A composite at 1200 and 1330 C, in laboratory air and steam environments Test environment Max stress (MPa) Cycles to failure Fatigue at 1200 C Laboratory air 100 120,199a Laboratory air 125 146,392a Laboratory air 150 167,473a Laboratory air 170 109,436a Steam 100 100,780a Steam 125 166,326a Steam 150 11,782 Steam 170 202 Fatigue at 1330 C Laboratory air 50 97,282 Laboratory air 100 1,519 Steam 50 25,852 Steam 100 347 a Run-out. 0 50 100 150 200 250 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Cycles (N) Stress (MPa) 1200°C, Air 1330°C, Air 1200°C, Steam 1330°C, Steam UTS at 1200°C UTS at 1330°C Fig. 3. Fatigue S–N curves for NextelTM720/alumina ceramic composite at 1200 and 1330 C, in laboratory air and in steam environment. 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 125 MPa, Air 125 MPa, Steam 150 MPa, Air 150 MPa, Steam 170 MPa, Air 170 MPa, Steam T = 1200°C f = 1 Hz R = 0.05 Fig. 4. Normalized modulus vs fatigue cycles at 1200 C in laboratory air and in steam environment. 2032 M.B. Ruggles-Wrenn et al. / Composites: Part A 37 (2006) 2029–2040