J.M. Ehrman et al. Composites Science and Technology 67(2007)1425-1438 10E02 likely due to the porous matrix. Fatigue cycling promotes BAs-Processed, Ruggles-Wrenn 2006 additional matrix cracking as evidenced by a decrease in 1.0E-0 stifness observed in fatigue run-out specimens. In addition, fatigue cycling likely causes the weakening of the fiber matrix interfaces as manifested by a strongly uncorrelated fiber fracture and brushy fracture surfaces produced in fati 1.0E-0 gue in air. It appears that prior fatigue in air serves to improve the damage tolerance and long-term durability 1.0E07 of the N720/A composite as demonstrated by improved creep resistance of this CMC. Note that prior fatigue in T= 1200C. Air steam does not result in similar improvements. It is possible that in steam the beneficial effects of cyclic loading weighed by negative effects of the additional sintering of the matrix. Considerably less brushy fracture surfaces pro- Fig8. Minimum creep rate vs applied stress for N720/A CMC at 1200 c duced in fatigue in steam would support this conjecture. in air. Creep data from Ruggles- Wrenn et al. [27] are also shown. 3. 4. Static-cyclic block loadings 1200C. It retained over 100% of its tensile strength and 76% of its modulus(see Table 2). Effects of prior creep on fatigue durability were investi Creep curves obtained in cyclic-static tests conducted in gated in static-cyclic block loading tests. The maximum ir and in steam are presented in Figs. 7a and b, respec- stress levels were 125 MPa in air and 100 MPa in steam tively. In air, prior fatigue causes a noticeable reduction The creep period was 2 h in air and 0.75 h in steam. In in accumulated creep strains. At 154 MPa, the as-processed air, the pre-crept specimen achieved a fatigue run-out dem pecimen accumulated creep strain of 0.61%o, nearly 3 times onstrating that 2 h of prior creep accompanied with the the 0. 22% creep strain produced by the pre-fatigued speci- creep strain of M0. 28% had no effect on fatigue life up to men. At 125 MPa, the pre-fatigued specimen achieved a run-out, accumulating creep strain of 0.60% in 100 h, 1.00 while the as-processed specimen accumulated 0.55% strain T=1200C. Air in only 0.27 h. In addition, it is seen in Fig. 8 that for a Fatigue Stress 125 MPa given creep stress, creep strain rates for the pre-fatigued specimens are at least an order of magnitude lower than those for the as-processed specimens, reflecting the benefi- cial effect of prior fatigue on the in-air creep performance. 5 0.50 -As-Processed, Ruggles. Wrenn 2006 However, in steam prior fatigue degraded the creep resis- tance. The pre-fatigued specimen failed after only 0.61 h having accumulated creep strain of 0.7%o, while the as-pro- cessed specimen survived 2.5 h producing creep strain of 1.41% 门720/A derives its damage tolerance 0.00 from a porous matrix. Therefore, the stability of the matrix 101520 porosity against densification is critical to the composite's a Time(h) long-term durability. Recent studies [32-34] investigated effects of thermal aging on the physical and mechanical T=1200C. Steam properties of composites consisting of NextelM720 fibers Fatigue Stress= 100 MPa and a porous matrix of mullite and alumina. For a com 0.75 posite with a pure alumina matrix, a porosity reduction of 6% was observed after a 10-min exposure at 1200C strengthening of the matrix and the fiber-matrix interfaces 850 [33, 34]. For a composite with a mullite/alumina matrix. was observed following aging at 1200C [32]. Additional 0.25 As-Processed sintering of the matrix during the aging treatments was Ruggles-Wrenn, 2006 considered to be associated predominantly with Al2O3. among the main manifestations was the increase in the 0.00 spatial correlation in the fiber failure locations within an 101520 individual tow and the increased amount of matrix mate- rial bonded to the fibers. The excellent fatigue resistance Fig 9. Maximum strain vs time for fatigue tests at 1200.C: (a)in air and of the N720/A composite at 1200C in air [27] is most (b)in steam Fatigue data from Ruggles-Wrenn et al. [27] are also shown1200 C. It retained over 100% of its tensile strength and 76% of its modulus (see Table 2). Creep curves obtained in cyclic–static tests conducted in air and in steam are presented in Figs. 7a and b, respec￾tively. In air, prior fatigue causes a noticeable reduction in accumulated creep strains. At 154 MPa, the as-processed specimen accumulated creep strain of 0.61%, nearly 3 times the 0.22% creep strain produced by the pre-fatigued speci￾men. At 125 MPa, the pre-fatigued specimen achieved a run-out, accumulating creep strain of 0.60% in 100 h, while the as-processed specimen accumulated 0.55% strain in only 0.27 h. In addition, it is seen in Fig. 8 that for a given creep stress, creep strain rates for the pre-fatigued specimens are at least an order of magnitude lower than those for the as-processed specimens, reflecting the benefi- cial effect of prior fatigue on the in-air creep performance. However, in steam prior fatigue degraded the creep resis￾tance. The pre-fatigued specimen failed after only 0.61 h having accumulated creep strain of 0.7%, while the as-pro￾cessed specimen survived 2.5 h producing creep strain of 1.41%. The N720/A composite derives its damage tolerance from a porous matrix. Therefore, the stability of the matrix porosity against densification is critical to the composite’s long-term durability. Recent studies [32–34] investigated effects of thermal aging on the physical and mechanical properties of composites consisting of NextelTM720 fibers and a porous matrix of mullite and alumina. For a com￾posite with a pure alumina matrix, a porosity reduction of 6% was observed after a 10-min exposure at 1200 C [33,34]. For a composite with a mullite/alumina matrix, strengthening of the matrix and the fiber–matrix interfaces was observed following aging at 1200 C [32]. Additional sintering of the matrix during the aging treatments was considered to be associated predominantly with Al2O3. Among the main manifestations was the increase in the spatial correlation in the fiber failure locations within an individual tow and the increased amount of matrix mate￾rial bonded to the fibers. The excellent fatigue resistance of the N720/A composite at 1200 C in air [27] is most likely due to the porous matrix. Fatigue cycling promotes additional matrix cracking as evidenced by a decrease in stiffness observed in fatigue run-out specimens. In addition, fatigue cycling likely causes the weakening of the fiber– matrix interfaces as manifested by a strongly uncorrelated fiber fracture and brushy fracture surfaces produced in fati￾gue in air. It appears that prior fatigue in air serves to improve the damage tolerance and long-term durability of the N720/A composite as demonstrated by improved creep resistance of this CMC. Note that prior fatigue in steam does not result in similar improvements. It is possible that in steam the beneficial effects of cyclic loading are out￾weighed by negative effects of the additional sintering of the matrix. Considerably less brushy fracture surfaces pro￾duced in fatigue in steam would support this conjecture. 3.4. Static–cyclic block loadings Effects of prior creep on fatigue durability were investi￾gated in static–cyclic block loading tests. The maximum stress levels were 125 MPa in air and 100 MPa in steam. The creep period was 2 h in air and 0.75 h in steam. In air, the pre-crept specimen achieved a fatigue run-out dem￾onstrating that 2 h of prior creep accompanied with the creep strain of 0.28% had no effect on fatigue life up to 1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 10 100 1000 Creep Stress (MPa) Creep Strain Rate (s-1) As-Processed, Ruggles-Wrenn 2006 Pre-Fatigued T = 1200˚C, Air Fig. 8. Minimum creep rate vs applied stress for N720/A CMC at 1200 C in air. Creep data from Ruggles-Wrenn et al. [27] are also shown. 0.00 0.25 0.50 0.75 1.00 0 5 10 15 20 25 30 Time (h) Strain (%) As-Processed, Ruggles-Wrenn 2006 2 h at 125 MPa T = 1200 ˚C, Air Fatigue Stress = 125 MPa 0.00 0.25 0.50 0.75 1.00 0 5 10 15 20 25 30 Time (h) Strain (%) T = 1200˚C, Steam Fatigue Stress = 100 MPa As-Processed Ruggles-Wrenn, 2006 0.75 h at 100 MPa Fig. 9. Maximum strain vs time for fatigue tests at 1200 C: (a) in air and (b) in steam. Fatigue data from Ruggles-Wrenn et al. [27] are also shown. J.M. Mehrman et al. / Composites Science and Technology 67 (2007) 1425–1438 1431