18 M.B. Ruggles-Wrenn, N.R. Szymczak/ Composites: Part A 39(2008)1829-1837 2.5 the high content of mullite, which has a much better creep resis- T=1200°c, tance than alumina [26 Conversely, in compression the creep- 2.0 100 MPa-Comilresswere rupture of the composite is largely dominated by an exceptionally eep weak porous alumina matrix. Both tensile and compressive creep 125 MPa are accelerated in the presence of steam. However, the degrading effect of steam environment is more pronounced in compression than in tension. While the tensile creep rate in steam is approxi mately an order of magnitude higher than that in air, the compres- 6 sive creep rate in steam can be as high as 10- times that obtained in for Stress-rupture behavior is summarized in Fig. 5, where creep stress magnitude is plotted vs time to rupture at 1200C in air and in steam. The tensile creep results from prior work [21, 2 100000 200000 300000 400000 are included for comparison. As expected, tensile creep life de Time(s) creases with increasing applied stress. However, in air compressive creep life(up to 100 h)appears to be relatively independent of ap- plied stress. All compressive creep tests conducted in air achieved a Tensile cree T=1200°c, Steam run-out. The presence of steam dramatically reduced creep life- Compressive creep times in both tension and compression In tension, the reduction in creep life due to steam was at least 90% for applied stress levels 60 MPa 40 MPa over 100 MPa, and -54% for the applied stress of 80 MPa. In com 125 MPa pression, creep lifetimes can be reduced by as much as 99.9% in the resence Retained compressive strength and modulus of the specimens that achieved a run- out in the -60 and -100 MPa creep tests con- ducted at 1200C in air are given in Table 3. Compressive stress- 80 MPa strain curves obtained for the N720/A specimens subjected to prior compressive creep are presented in Fig. 6 together with the com- pressive stress-strain curve for the as-processed material. Both 5000 10000 15000 specimens retained 100% of their compressive strength. However, prior compressive creep appears to have decreased compressive modulus. To evaluate the effects of compressive creep on tensile ir and(b)in steam. strength and stiffness, a specimen that achieved a run-out in Tensile creep data from Ruggles-Wrenn et al. [21, 25 are also shown. 80 MPa creep test was subjected to a tensile test to failure at 1200C Retained tensile strength and modulus are included in Ta- ble 3. Prior compressive creep caused a 30% decrease in tensile The compressive(tensile)creep curves produced in steam are strength and a 17% decrease in modulus. Tensile stress-strain qualitatively similar to the compressive(tensile)creep curves ob- behavior of the specimen subjected to prior compressive creep re- effect on creep strain and creep lifetime in both tension and com- Fig. 7). Note that the n720/ A composite subjected to 100 h of prior pression Tensile creep strain produced at 80 MPa in steam was x5 tensile creep at 80 MPa in air retained over 90% of its tensile times that obtained at 80 MPa in air. Yet at 100 and 125 MPa, the strength and over 86% of its tensile modulus [46]. Prior tensile tensile creep strains produced in steam were, respectively, 7% creep had no qualitative effect on tensile stress-strain behavior. and 30% lower than those produced in air. For compressive creep stresses of-40 and -60 MPa, creep strain magnitudes produced in steam were an order of magnitude higher than those in air. At 100 MPa, creep strain magnitude accumulated in steam was low- er than that in air. Note that in steam, creep strain magnitudes as 10E-01 well as creep lifetimes decrease with increasing creep stress mag- nitudes for both tension and compression. while the n720/A com- 10E-02 posite survived 100 h of tensile creep at 80 MPa in air, tensile creep 10E-03 run-out was not achieved in steam. Although all compressive creep tests conducted at 1200 C in air achieved a run-out in steam com 10E-04 pressive creep run-out was not achieved Minimum creep rate was reached in all tests. Creep strain rate 1.0E-05 magnitude as a function of applied stress magnitude is shown in Fig. 4, where results of previous work [21, 25 are also included. Re- 量1 sults in Fig. 4 show that at 1200C in air the compressive creep 1.0E-07 rate magnitudes are nearly an order of magnitude higher than 1.0E-08 the tensile creep rates produced at the same applied stress magni- T=1200°c tude. This result is hardly surprising, considering that in tension 10E-09 the creep-rupture of the N720/A CMc is likely dominated by 1000 creep-rupture of the Nextel 720 fibers. It is recognized that Nex tel M720 fiber has the best creep performance of any commercially available polycrystalline oxide fiber. The superior high-tempera- g. 4. Minimum creep rate magnitude as a function of applied at 1200"C in air and in steam Tensile creep data fro ture creep performance of the Nextel 720 fibers results from Ruggles-Wrenn et al [21, 25] are also showrThe compressive (tensile) creep curves produced in steam are qualitatively similar to the compressive (tensile) creep curves ob￾tained in air. Nevertheless, the presence of steam has a noticeable effect on creep strain and creep lifetime in both tension and com￾pression. Tensile creep strain produced at 80 MPa in steam was 5 times that obtained at 80 MPa in air. Yet at 100 and 125 MPa, the tensile creep strains produced in steam were, respectively, 7% and 30% lower than those produced in air. For compressive creep stresses of 40 and 60 MPa, creep strain magnitudes produced in steam were an order of magnitude higher than those in air. At 100 MPa, creep strain magnitude accumulated in steam was low￾er than that in air. Note that in steam, creep strain magnitudes as well as creep lifetimes decrease with increasing creep stress mag￾nitudes for both tension and compression. While the N720/A com￾posite survived 100 h of tensile creep at 80 MPa in air, tensile creep run-out was not achieved in steam. Although all compressive creep tests conducted at 1200 C in air achieved a run-out, in steam com￾pressive creep run-out was not achieved. Minimum creep rate was reached in all tests. Creep strain rate magnitude as a function of applied stress magnitude is shown in Fig. 4, where results of previous work [21,25] are also included. Re￾sults in Fig. 4 show that at 1200 C in air the compressive creep rate magnitudes are nearly an order of magnitude higher than the tensile creep rates produced at the same applied stress magni￾tude. This result is hardly surprising, considering that in tension the creep-rupture of the N720/A CMC is likely dominated by creep-rupture of the Nextel 720 fibers. It is recognized that Nex￾telTM720 fiber has the best creep performance of any commercially available polycrystalline oxide fiber. The superior high-tempera￾ture creep performance of the NextelTM 720 fibers results from the high content of mullite, which has a much better creep resis￾tance than alumina [26]. Conversely, in compression the creep￾rupture of the composite is largely dominated by an exceptionally weak porous alumina matrix. Both tensile and compressive creep are accelerated in the presence of steam. However, the degrading effect of steam environment is more pronounced in compression than in tension. While the tensile creep rate in steam is approxi￾mately an order of magnitude higher than that in air, the compres￾sive creep rate in steam can be as high as 105 times that obtained in air for a given stress. Stress-rupture behavior is summarized in Fig. 5, where creep stress magnitude is plotted vs time to rupture at 1200 C in air and in steam. The tensile creep results from prior work [21,25] are included for comparison. As expected, tensile creep life de￾creases with increasing applied stress. However, in air compressive creep life (up to 100 h) appears to be relatively independent of ap￾plied stress. All compressive creep tests conducted in air achieved a run-out. The presence of steam dramatically reduced creep life￾times in both tension and compression. In tension, the reduction in creep life due to steam was at least 90% for applied stress levels over 100 MPa, and 54% for the applied stress of 80 MPa. In com￾pression, creep lifetimes can be reduced by as much as 99.9% in the presence of steam. Retained compressive strength and modulus of the specimens that achieved a run-out in the 60 and 100 MPa creep tests con￾ducted at 1200 C in air are given in Table 3. Compressive stress– strain curves obtained for the N720/A specimens subjected to prior compressive creep are presented in Fig. 6 together with the com￾pressive stress–strain curve for the as-processed material. Both specimens retained 100% of their compressive strength. However, prior compressive creep appears to have decreased compressive modulus. To evaluate the effects of compressive creep on tensile strength and stiffness, a specimen that achieved a run-out in a 80 MPa creep test was subjected to a tensile test to failure at 1200 C. Retained tensile strength and modulus are included in Ta￾ble 3. Prior compressive creep caused a 30% decrease in tensile strength and a 17% decrease in modulus. Tensile stress–strain behavior of the specimen subjected to prior compressive creep re￾mained qualitative similar to that of the as-processed material (see Fig. 7). Note that the N720/A composite subjected to 100 h of prior tensile creep at 80 MPa in air retained over 90% of its tensile strength and over 86% of its tensile modulus [46]. Prior tensile creep had no qualitative effect on tensile stress–strain behavior. Fig. 3. Creep curves for N720/A composite at 1200 C: (a) in air and (b) in steam. Tensile creep data from Ruggles-Wrenn et al. [21,25] are also shown. 1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 10 100 1000 ABS Creep Stress (MPa) ABS Creep Strain Rate (s-1) Tension, air Tension , steam Compression, air Compression, steam T = 1200°C Fig. 4. Minimum creep rate magnitude as a function of applied stress magnitude for N720/A ceramic composite at 1200 C in air and in steam. Tensile creep data from Ruggles-Wrenn et al. [21,25] are also shown. 1832 M.B. Ruggles-Wrenn, N.R. Szymczak / Composites: Part A 39 (2008) 1829–1837