M B. Ruggles-Wrenn et al / Composites Science and Technology 68(2008)1588-1595 the furnace for testing in argon, the specimen instrumented with two S-type thermocouples was placed in argon envi- 0°90° ronment. In the 18 mm specimen gage section, the e max1- UTS =192 MPa mum deviation from the nominal test temperature was ±3° C in steam and±2 In argon. The temperature con- troller set points determined for testing in steam were pproximately 100C higher than those determined for ±45° testing in air. The set points determined for testing in argon UTS=55 MPa were approximately 115c above those determined fo Fracture surfaces of failed specimens were examined T=12 using SEM(FEI Quanta 200 HV)as well as an optical microscope(Zeiss Discovery V12). The SEM specimens Strain (%) were carbon coated All tests were performed at 1200C. In all tests, a spec- 10C sile stress-strain curves for N720/A ceramic composite at imen was heated to test temperature in 25 min, and held at temperature for additional 15 min prior to testing. All test specimens used in this study were cut from a single plate to ifications shown in Fig. 2. Tensile tests were performed typical fiber-dominated composite behavior. The average in stroke control with a constant displacement rate of ultimate tensile strength (UTS)was 190 MPa, elastic modu 0.05 mm/s in laboratory air. Creep-rupture tests were con- lus, 76 GPa, and failure strain, 0.38%. These results agree ducted in load control in accordance with the procedure in well with the data reported earlier [20, 23]. In the case of ASTM standard C 1337 in laboratory air, steam and the #45 orientation, the nonlinear stress-strain behavior argon. In all creep tests the specimens were loaded to the sets in at fairly low stresses(15 MPa). As the stress creep stress level at the stress rate of 15 MPa/s. Creep approaches 50 MPa, appreciable inelastic strains develop run-out was defined as 100 h at a given creep stress. In each rapidly. The specimen achieves a strain of 0. 27% at the max- test, stress-strain data were recorded during the loading to imum load. After the uts is reached, appreciable inelastic he creep stress level and the actual creep period. Thus both strains develop at near constant stress. These observations total strain and creep strain could be calculated and exam- are consistent with the results reported for the porous- ined. To determine the retained tensile strength and modu- matrix ceramic composites in the 45orientation [24, 25]. lus, specimens that achieved run-out were subjected to The elastic modulus(46 GPa)and UTS (55 MPa) obtained tensile test to failure at 1200C. In some cases one speci- for the +45 orientation are considerably lower than the men was tested per test condition. The authors recognize corresponding values for the 0/90 specimens. It is worthy that this is a limited set of data. However, extreme care of note that in all tension tests, as well as in all other tests was taken in generating the data. Selective duplicate tests reported herein, the failure occurred within the gage section have demonstrated the data to be very repeatable. This of the extensometer exploratory effort serves to identify the behavioral trends ind to determine whether a more rigorous investigation 3. 2. Creep-rupture should be undertaken Results of the creep-rupture tests for N720/A composite 3. Results and discussion with +45 fiber orientation are summarized in Table 1 where creep strain accumulation and rupture time are 3. Monotonic tension shown for each creep stress level and test environment Tensile stress-strain behavior at 1200C is typified inin Fig. 4. Creep curves produced in all tests at 15 and Fig 3. The stress-strain curves obtained for the 0/900 fiber 35 MPa exhibit primary and secondary creep regimes, but orientation are nearly linear to failure. Material exhibits no tertiary creep Transition from primary to secondary creep occurs late in creep life, primary creep persists during 叫+90 the first 40-50 h of the creep test. Note that creep run-out 8.0 of 100 h was achieved in all tests at 15 and 35 MPa, regard R=50 less of test environment While the test environment appears to have little influence on the appearance of the creep curves obtained at 15 and 35 MPa, it has a noticeable effect on the strain accumulated during 100 h of creep. For a given creep stress, the largest creep strains were accumu- lated in argon, followed by those accumulated in steam and Fig. 2. Test specimen, dimensions in air. In contrast, all creep curves obtained at 45 MPathe furnace for testing in argon, the specimen instrumented with two S-type thermocouples was placed in argon envi￾ronment. In the 18 mm specimen gage section, the maxi￾mum deviation from the nominal test temperature was ±3 C in steam and ±2 C in argon. The temperature con￾troller set points determined for testing in steam were approximately 100 C higher than those determined for testing in air. The set points determined for testing in argon were approximately 115 C above those determined for testing in air. Fracture surfaces of failed specimens were examined using SEM (FEI Quanta 200 HV) as well as an optical microscope (Zeiss Discovery V12). The SEM specimens were carbon coated. All tests were performed at 1200 C. In all tests, a spec￾imen was heated to test temperature in 25 min, and held at temperature for additional 15 min prior to testing. All test specimens used in this study were cut from a single plate to specifications shown in Fig. 2. Tensile tests were performed in stroke control with a constant displacement rate of 0.05 mm/s in laboratory air. Creep-rupture tests were con￾ducted in load control in accordance with the procedure in ASTM standard C 1337 in laboratory air, steam and argon. In all creep tests the specimens were loaded to the creep stress level at the stress rate of 15 MPa/s. Creep run-out was defined as 100 h at a given creep stress. In each test, stress–strain data were recorded during the loading to the creep stress level and the actual creep period. Thus both total strain and creep strain could be calculated and exam￾ined. To determine the retained tensile strength and modu￾lus, specimens that achieved run-out were subjected to tensile test to failure at 1200 C. In some cases one speci￾men was tested per test condition. The authors recognize that this is a limited set of data. However, extreme care was taken in generating the data. Selective duplicate tests have demonstrated the data to be very repeatable. This exploratory effort serves to identify the behavioral trends and to determine whether a more rigorous investigation should be undertaken. 3. Results and discussion 3.1. Monotonic tension Tensile stress–strain behavior at 1200 C is typified in Fig. 3. The stress–strain curves obtained for the 0/90 fiber orientation are nearly linear to failure. Material exhibits typical fiber-dominated composite behavior. The average ultimate tensile strength (UTS) was 190 MPa, elastic modu￾lus, 76 GPa, and failure strain, 0.38%. These results agree well with the data reported earlier [20,23]. In the case of the ±45 orientation, the nonlinear stress–strain behavior sets in at fairly low stresses (15 MPa). As the stress approaches 50 MPa, appreciable inelastic strains develop rapidly. The specimen achieves a strain of 0.27% at the max￾imum load. After the UTS is reached, appreciable inelastic strains develop at near constant stress. These observations are consistent with the results reported for the porous￾matrix ceramic composites in the ±45 orientation [24,25]. The elastic modulus (46 GPa) and UTS (55 MPa) obtained for the ±45 orientation are considerably lower than the corresponding values for the 0/90 specimens. It is worthy of 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. Creep-rupture Results of the creep-rupture tests for N720/A composite with ±45 fiber orientation are summarized in Table 1, where creep strain accumulation and rupture time are shown for each creep stress level and test environment. Creep curves obtained in air, steam and argon are shown in Fig. 4. Creep curves produced in all tests at 15 and 35 MPa exhibit primary and secondary creep regimes, but no tertiary creep. Transition from primary to secondary creep occurs late in creep life, primary creep persists during the first 40–50 h of the creep test. Note that creep run-out of 100 h was achieved in all tests at 15 and 35 MPa, regard￾less of test environment. While the test environment appears to have little influence on the appearance of the creep curves obtained at 15 and 35 MPa, it has a noticeable effect on the strain accumulated during 100 h of creep. For a given creep stress, the largest creep strains were accumu￾lated in argon, followed by those accumulated in steam and in air. In contrast, all creep curves obtained at 45 MPa R=50 50.0 76.0 8.0 9.0 5.0 Fig. 2. Test specimen, dimensions in mm. 0 50 100 150 200 0.0 0.1 0.2 0.3 0.4 Strain (%) Stress (MPa) T = 1200 ºC 0º/90º UTS = 192 MPa ±45º UTS = 55 MPa 0.5 Fig. 3. Tensile stress–strain curves for N720/A ceramic composite at 1200 C. 1590 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595