M B. Ruggles-Wrenn et al. Composites: Part A 37(2006)2029-2040 125 MPa, creep strain accumulation increases from 1%to T=1200°c 3.4%. However, for the creep stress of 154 MPa, creep strain accumulation reaches only about 0.6%/. It is note- worthy that in all creep tests, the accumulated creep strain significantly exceeded failure strain obtained in tension test It is important to recognize that the total strain incurred in 100 MPa. Steam a creep-rupture test represents a sum of two contributions: (1)that due to the initial loading up to the specific creep stress level, and (2) that accumulated during the actual 80 MPa, A creep od. for to 90% of total strai during the creep period. However, for specimen tested at 50000 00000 150000 200000 creep stress of 154 MPa, creep strain accounted only for Time(s) 33% of the total strain The creep curves produced in air environment at T=1200°c 1330C are qualitatively similar to those obtained at 1200C. Creep strain accumulation decreases from 5%to approximately 4% as the creep stress increases from 50 to 100 MPa. In both creep tests, close to 100% of the total 125 MPa, Steam strain was incurred during the creep period. Creep strain accounts for 98% of the total strain in the 50 MPa test 154 MPa, Steam and for 94% of the total strain in the 100 MPa test Results in Figs. Il and 12 demonstrate that specimens tested in steam environment produced creep curves that are qualitatively similar to those produced in air. As 154 MPa. Air expected, strains incurred during the initial loading to a given creep stress were not affected by the steam. Conversely, creep strains accumulated in steam Time(s) environment were significantly different from those accu Fig.11.Creep strain vs time for Nextel TM720 /alumina ceramic composite mulated in air. For both test temperatures and creep stress at 1200C in laboratory air and in steam environment: (a)at 80 and levels <100 MPa, specimens tested in steam accumulated 100 MPa, (b)at 125 and 154 MPa more creep strain than specimens tested in air. At 1200C, creep strain produced in the 80 MPa test con- ducted in steam was 67% higher than that produced at T=1330°c the same creep stress in air. At 1330C and creep stress of 50 MPa, creep strain accumulated in steam was 20% higher than that accumulated in air For creep stress levels >100 MPa, presence of steam resulted in lower creep strains and much lower creep lifetimes at both test ter atures. At 1200C, creep strains produced in steam ronment for creep stress levels of 100, 125 and 154 100 MPa. Steam 100 MPa. Air were, respectively, 53%, 73% and 92% lower than those produced at the same stress levels in air. At 1330C and creep stress of 100 MPa, creep strain accumulated in steam 50 MPa,Air was 60% lower than that accumulated in air Minimum creep rate was reached in all tests. Creep rate 1000 2000 Time(s) as a function of applied stress is presented in Fig. 13, where results of the present investigation are plotted together with Fig. 12. Creep strain vs time for NextelM720/alumina ceramic composite the data from Wilson and Visser [57] for Nextel 720 fibers at 1330C in laboratory air and in steam environment To further facilitate comparison between the creep proper ties of the fibers and the composite, the Nextel 720 fiber produced at higher stress levels. It is seen that all creep data adjusted for Vr=0. 22(volume fraction of the on-axis curves generated at 1200C in air environment exhibit pri- fibers in the N720/A composite)is also shown For both mary and secondary creep regimes. Transition from pri- temperatures, the minimum creep rates increase with ary to secondary creep occurs during the first 10% of increasing applied stress. At 1200C, as creep stress creep life. Secondary creep appears to be nearly linear to increases from 80 to 154 MPa creep rate increases by two failure. As the creep stress level increases from 80 MPa to orders of magnitude. Creep rates of the composite obtainedproduced at higher stress levels. It is seen that all creep curves generated at 1200 C in air environment exhibit pri￾mary and secondary creep regimes. Transition from pri￾mary to secondary creep occurs during the first 10% of creep life. Secondary creep appears to be nearly linear to failure. As the creep stress level increases from 80 MPa to 125 MPa, creep strain accumulation increases from 1% to 3.4%. However, for the creep stress of 154 MPa, creep strain accumulation reaches only about 0.6%. It is note￾worthy that in all creep tests, the accumulated creep strain significantly exceeded failure strain obtained in tension test. It is important to recognize that the total strain incurred in a creep-rupture test represents a sum of two contributions: (1) that due to the initial loading up to the specific creep stress level, and (2) that accumulated during the actual creep period. For specimens tested at creep stresses 6125 MPa, close to 90% of total strain was accumulated during the creep period. However, for specimen tested at creep stress of 154 MPa, creep strain accounted only for 33% of the total strain. The creep curves produced in air environment at 1330 C are qualitatively similar to those obtained at 1200 C. Creep strain accumulation decreases from 5% to approximately 4% as the creep stress increases from 50 to 100 MPa. In both creep tests, close to 100% of the total strain was incurred during the creep period. Creep strain accounts for 98% of the total strain in the 50 MPa test, and for 94% of the total strain in the 100 MPa test. Results in Figs. 11 and 12 demonstrate that specimens tested in steam environment produced creep curves that are qualitatively similar to those produced in air. As expected, strains incurred during the initial loading to a given creep stress were not affected by the presence of steam. Conversely, creep strains accumulated in steam environment were significantly different from those accu￾mulated in air. For both test temperatures and creep stress levels <100 MPa, specimens tested in steam accumulated more creep strain than specimens tested in air. At 1200 C, creep strain produced in the 80 MPa test con￾ducted in steam was 67% higher than that produced at the same creep stress in air. At 1330 C and creep stress of 50 MPa, creep strain accumulated in steam was 20% higher than that accumulated in air. For creep stress levels P100 MPa, presence of steam resulted in lower creep strains and much lower creep lifetimes at both test temper￾atures. At 1200 C, creep strains produced in steam envi￾ronment for creep stress levels of 100, 125 and 154 MPa were, respectively, 53%, 73% and 92% lower than those produced at the same stress levels in air. At 1330 C and creep stress of 100 MPa, creep strain accumulated in steam was 60% lower than that accumulated in air. Minimum creep rate was reached in all tests. Creep rate as a function of applied stress is presented in Fig. 13, where results of the present investigation are plotted together with the data from Wilson and Visser [57] for Nextel 720 fibers. To further facilitate comparison between the creep proper￾ties of the fibers and the composite, the Nextel 720 fiber data adjusted for Vf = 0.22 (volume fraction of the on-axis fibers in the N720/A composite) is also shown. For both temperatures, the minimum creep rates increase with increasing applied stress. At 1200 C, as creep stress increases from 80 to 154 MPa creep rate increases by two orders of magnitude. Creep rates of the composite obtained Fig. 11. Creep strain vs time for NextelTM720/alumina ceramic composite at 1200 C in laboratory air and in steam environment: (a) at 80 and 100 MPa, (b) at 125 and 154 MPa. 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 0 500 1000 1500 2000 Time (s) Strain (%) T = 1330°C 100 MPa, Air 50 MPa, Air 100 MPa, Steam 50 MPa, Steam Fig. 12. Creep strain vs time for NextelTM720/alumina ceramic composite at 1330 C in laboratory air and in steam environment. 2036 M.B. Ruggles-Wrenn et al. / Composites: Part A 37 (2006) 2029–2040