J M. Ehrman et al Composites Science and Technology 67(2007)1425-1438 the high temperature oxidation resistance [9]. The need for San Diego, CA)consisting of a porous alumina matrix environmentally stable composites motivated the develop- reinforced with NextelM720 fibers. There is no fiber coat ment of CMCs based on environmentally stable oxide con- ing. The damage tolerance of N720/A is enabled by the stituents [10-16]. porous matrix. The composite was supplied in a form of o The main advantages of CMCs over monolithic ceram- 2.8-mm thick plates comprised of 12 0/90 woven layers, are their superior toughness, tolerance to the presence with a density of 2.77 g/cm,, a fiber volume of 45% of defects, and non-catastrophic mode of failure. It is and matrix porosity of w24%. The fiber fabric was infil- widely accepted that in order to avoid brittle fracture trated with the matrix in a sol-gel process. The laminate behavior in CMCs and improve the damage tolerance, a was dried with a"vacuum bag"technique under low pres- weak fiber/matrix interface is needed, which serves to sure and low temperature then pressureless sintered [28]. deflect matrix cracks and to allow subsequent fiber pull- Representative micrographs of the untested material are out [17-20]. It has recently been demonstrated that a shown in Fig. 1. Fig. la shows 00 and 90 fiber tows as well matrix with finely distributed porosity can provide a simi- as numerous matrix cracks In the case of the as-processed lar crack-deflecting behavior [21]. This microstructural material, most are shrinkage cracks formed during process design philosophy implicitly accepts the strong fiber /matrix ing rather than matrix cracks generated during loading interface. The concept has been successfully demonstrated Porous nature of the matrix is seen in Fig. 1b for oxide-oxide composites [10, 13, 16, 22-24]. Resulting A servocontrolled MTS mechanical testing machine oxide/oxide CMCs exhibit damage tolerance combined equipped with hydraulic water-cooled collet grips, a com- ith inherent oxidation resistance. An extensive review of pact two-zone resistance-heated furnace, and two tempera- the mechanisms and mechanical properties of porous- ture controllers were used in all tests. An MTS TestStar II natrix CMCs is given in [25] digital controller was employed for input signal generation Porous-matrix oxide/oxide CMCs exhibit several behav- and data acquisition. Strain measurement was accom- ior trends that are distinctly different from those exhibited plished with an MTS high-temperature air-cooled uniaxial by traditional CMCs with a fiber-matrix interface. For extensometer. For elevated temperature testing, thermo these materials, fatigue is significantly more damaging than couples were bonded to test specimens to calibrate the fur- creep Zawada et al. [26]examined the mechanical behavior nace on a periodic basis. The furnace controller(using a of a porous-matrix Nextel610/Alumino-silicate composite non-contacting thermocouple exposed to the ambient envi- at 1000C and reported that the CMc exhibited excellent ronment near the test specimen)was adjusted to determine fatigue performance, but low creep resistance. Ruggles- the power setting needed to achieve the desired tempera- Wrenn et al. [27] investigated fatigue and creep behaviors ture of the test specimen. The determined power setting of N720/A composite at 1200C. The results revealed was then used in actual tests. The power setting for testing excellent fatigue resistance in laboratory air, the material in steam environment was determined by placing the spec exhibited high fatigue limit, long fatigue life and near imen instrumented with thermocouples in steam and 100% strength retention. However, N720/A produced short repeating the furnace calibration procedure. Thermocou- creep lives, revealing low creep resistance at 1200oC. Pres- ples were not bonded to the test specimens after the furnace ence of steam significantly degraded the fatigue as well as was calibrated. Tests in steam employed an alumina sus- the creep performance of the CMC ceptor(tube with end caps), which fits inside the furnace. Because creep was shown to be considerably more dam- The specimen gage section is located inside the susceptor aging than cyclic loading to porous-matrix oxide-oxide with the ends of the specimen passing through slots in CMCs [26, 27), mechanical performance under combined the susceptor. Steam is introduced into the susceptor static-cyclic loading in high-temperature oxidizing environ-(through a feeding tube) in a continuous stream with a ments remains among the key issues that must be addressed slightly positive pressure, expelling the dry air and creating before using these materials in aerospace applications. The a 100% steam environment inside the susceptor objective of this study is to investigate the influence of hold All tests were performed at 1200C. Each test specimen imes at maximum stress on fatigue performance of Nex- was heated to 1200C in 25 min, and held at 1200C for [T720/Alumina, a porous-matrix oxide-oxide CFCC, in additional 15 min prior to testing. Dog bone shaped speci laboratory air and in steam environments. In addition, this mens of 152-mm total length with a 10-mm-wide gage sec- effort aims to assess the effects of prior loading history on tion were used in all tests. Tensile tests were performed in mechanical behavior and durability of N720/. The com- displacement control with a constant rate of 0.05 mm/s in posite microstructure, as well as damage and failure mech- laboratory air. All creep-fatigue interaction tests were per anisms are also discussed formed in load control and employed the ratio R(mini- mum to maximum stress of 0.05, and the loading/ unloading frequency of 1 Hz. Cyclic test with hold time 2. Experimental procedure consists of a standard fatigue cycle with a hold time of either 10 or 100 s superimposed at the maximum stress The material studied was NextelTM720/Alumina(N720/ The run-out is set at 100 h, which is consistent with the ser A), an oxide-oxide CMC (manufactured by COI Ceramics, vice life expected in aerospace applications at that temperthe high temperature oxidation resistance [9]. The need for environmentally stable composites motivated the develop￾ment of CMCs based on environmentally stable oxide con￾stituents [10–16]. The main advantages of CMCs over monolithic ceram￾ics are their superior toughness, tolerance to the presence of defects, and non-catastrophic mode of failure. It is widely accepted that in order to avoid brittle fracture behavior in CMCs and improve the damage tolerance, a weak fiber/matrix interface is needed, which serves to deflect matrix cracks and to allow subsequent fiber pull￾out [17–20]. It has recently been demonstrated that a matrix with finely distributed porosity can provide a simi￾lar crack-deflecting behavior [21]. This microstructural design philosophy implicitly accepts the strong fiber/matrix interface. The concept has been successfully demonstrated for oxide–oxide composites [10,13,16,22–24]. Resulting oxide/oxide CMCs exhibit damage tolerance combined with inherent oxidation resistance. An extensive review of the mechanisms and mechanical properties of porous￾matrix CMCs is given in [25]. Porous-matrix oxide/oxide CMCs exhibit several behav￾ior trends that are distinctly different from those exhibited by traditional CMCs with a fiber–matrix interface. For these materials, fatigue is significantly more damaging than creep. Zawada et al. [26] examined the mechanical behavior of a porous-matrix Nextel610/Alumino-silicate composite at 1000 C and reported that the CMC exhibited excellent fatigue performance, but low creep resistance. Ruggles￾Wrenn et al. [27] investigated fatigue and creep behaviors of N720/A composite at 1200 C. The results revealed excellent fatigue resistance in laboratory air, the material exhibited high fatigue limit, long fatigue life and near 100% strength retention. However, N720/A produced short creep lives, revealing low creep resistance at 1200 C. Pres￾ence of steam significantly degraded the fatigue as well as the creep performance of the CMC. Because creep was shown to be considerably more dam￾aging than cyclic loading to porous-matrix oxide–oxide CMCs [26,27], mechanical performance under combined static-cyclic loading in high-temperature oxidizing environ￾ments remains among the key issues that must be addressed before using these materials in aerospace applications. The objective of this study is to investigate the influence of hold times at maximum stress on fatigue performance of Nex￾telTM720/Alumina, a porous-matrix oxide–oxide CFCC, in laboratory air and in steam environments. In addition, this effort aims to assess the effects of prior loading history on mechanical behavior and durability of N720/A. The com￾posite microstructure, as well as damage and failure mech￾anisms are also discussed. 2. Experimental procedure The material studied was NextelTM720/Alumina (N720/ A), an oxide–oxide CMC (manufactured by COI Ceramics, San Diego, CA) consisting of a porous alumina matrix reinforced with NextelTM720 fibers. There is no fiber coat￾ing. The damage tolerance of N720/A is enabled by the porous matrix. The composite was supplied in a form of 2.8-mm thick plates comprised of 12 0/90 woven layers, with a density of 2.77 g/cm3 , a fiber volume of 45%, and matrix porosity of 24%. The fiber fabric was infil￾trated with the matrix in a sol-gel process. The laminate was dried with a ‘‘vacuum bag’’ technique under low pres￾sure and low temperature, then pressureless sintered [28]. Representative micrographs of the untested material are shown in Fig. 1. Fig. 1a shows 0 and 90 fiber tows as well as numerous matrix cracks. In the case of the as-processed material, most are shrinkage cracks formed during process￾ing rather than matrix cracks generated during loading. Porous nature of the matrix is seen in Fig. 1b. A servocontrolled MTS mechanical testing machine equipped with hydraulic water-cooled collet grips, a com￾pact two-zone resistance-heated furnace, and two tempera￾ture controllers were used in all tests. An MTS TestStar II digital controller was employed for input signal generation and data acquisition. Strain measurement was accom￾plished with an MTS high-temperature air-cooled uniaxial extensometer. For elevated temperature testing, thermo￾couples were bonded to test specimens to calibrate the fur￾nace on a periodic basis. The furnace controller (using a non-contacting thermocouple exposed to the ambient envi￾ronment near the test specimen) was adjusted to determine the power setting needed to achieve the desired tempera￾ture of the test specimen. The determined power setting was then used in actual tests. The power setting for testing in steam environment was determined by placing the spec￾imen instrumented with thermocouples in steam and repeating the furnace calibration procedure. Thermocou￾ples were not bonded to the test specimens after the furnace was calibrated. Tests in steam employed an alumina sus￾ceptor (tube with end caps), which fits inside the furnace. The specimen gage section is located inside the susceptor, with the ends of the specimen passing through slots in the susceptor. Steam is introduced into the susceptor (through a feeding tube) in a continuous stream with a slightly positive pressure, expelling the dry air and creating a 100% steam environment inside the susceptor. All tests were performed at 1200 C. Each test specimen was heated to 1200 C in 25 min, and held at 1200 C for additional 15 min prior to testing. Dog bone shaped speci￾mens of 152-mm total length with a 10-mm-wide gage sec￾tion were used in all tests. Tensile tests were performed in displacement control with a constant rate of 0.05 mm/s in laboratory air. All creep–fatigue interaction tests were per￾formed in load control and employed the ratio R (mini￾mum to maximum stress) of 0.05, and the loading/ unloading frequency of 1 Hz. Cyclic test with hold time consists of a standard fatigue cycle with a hold time of either 10 or 100 s superimposed at the maximum stress. The run-out is set at 100 h, which is consistent with the ser￾vice life expected in aerospace applications at that temper- 1426 J.M. Mehrman et al. / Composites Science and Technology 67 (2007) 1425–1438