M. B. Ruggles-Wrenn et al. International Journal of Fatigue 30(2008)502-516 Fig. 1. As-received material: (a)overview, optical microscope and (b) porous nature of the matrix is evident (SEM to achieve the desired temperature of the test specimen. Fatigue run-out was defined as 10 cycles at 0.1 and The determined power setting was then used in actual tests. 1.0 Hz, and as 10 cycles at 10 Hz. The 10 cycle count The power setting for testing in steam was determined by value represents the number of loading cycles expected in placing the specimen instrumented with thermocouples in aerospace applications at that temperature. Fatigue run- steam environment and repeating the furnace calibration out limits were defined as the highest stress level, for which procedure. Thermocouples were not bonded to the test run-out was achieved. Note that in the case of all run-out specimens after the furnace was calibrated. Tests in steam tests, the failure of specimen did not occur when the test environment employed an alumina susceptor (tube with was terminated. Cyclic stress-strain data were recorded end caps), which fits inside the furnace. The specimen gage throughout each test. Thus stiffness degradation as well section is located inside the susceptor, with the ends of the as strain accumulation with fatigue cycles and/or time specimen passing through slots in the susceptor. Steam is could be examined. All specimens that achieved run-out introduced into the susceptor(through a feeding tube) in were subjected to tensile test to failure at 1200C in labo- a continuous stream with a slightly positive pressure, expel- ratory air to determine the retained strength and stifness ling the dry air and creating 100% steam environment inside the susceptor(see Fig. 2). 23. Microstructural characterization All tests were performed at 1200C. In all tests, a spec imen was heated to test temperature in 25 min, and held at Fracture surfaces of failed specimens were examined temperature for additional 15 min prior to testing. Dog using SEM(FEI Quanta 200 HV) as well as an optical bone shaped specimens of 152 mm total length with a 10- microscope(Zeiss Discovery V12). The SEM specimen mm-wide gage section shown in Fig. 2 were used in all tests. were carbon coated. In addition, energy-dispersive X-ray Tensile tests were performed in stroke control with a con- spectroscopy (EDS) analysis was performe using an stant displacement rate of 0.05 mm/s at 1200C in labora- EDAX Genesis 4000 EDS system tory air. The effects of frequency on the fatigue behavior were evaluated in tension-tension fatigue tests conducted 3 Results and discussion at the frequencies of 0. 1 and 10 Hz at 1200oC, in labora tory air and in steam environments. Fatigue data at 3.1. Monotonic tension 1.0 Hz from prior work [44] is included for comparison All fatigue experiments were carried out in load control Tensile results obtained at 1200C were consistent with with the ratio R(minimum to maximum stress) of 0.05. those reported earlier [44, 47]. The ultimate tensile strength (UTS)was 190 MPa, elastic modulus, 76 GPa, and failure strain, 0.38%. It is worthy of note that in all tests reported herein, the failure occurred within the gage section of the extensometer Creep was shown to be considerably more damaging than cyclic loading to oxide-oxide CMCs with porous Fig. 2. Test specimen, dimensions matrix [43, 44]. Recently Ehrman et al. [45] demonstratedto achieve the desired temperature of the test specimen. The determined power setting was then used in actual tests. The power setting for testing in steam was determined by placing the specimen instrumented with thermocouples in steam environment and repeating the furnace calibration procedure. Thermocouples were not bonded to the test specimens after the furnace was calibrated. Tests in steam environment employed an alumina susceptor (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, expel￾ling the dry air and creating 100% steam environment inside the susceptor (see Fig. 2). 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. Dog bone shaped specimens of 152 mm total length with a 10- mm-wide gage section shown in Fig. 2 were used in all tests. Tensile tests were performed in stroke control with a con￾stant displacement rate of 0.05 mm/s at 1200 C in labora￾tory air. The effects of frequency on the fatigue behavior were evaluated in tension–tension fatigue tests conducted at the frequencies of 0.1 and 10 Hz at 1200 C, in labora￾tory air and in steam environments. Fatigue data at 1.0 Hz from prior work [44] is included for comparison. All fatigue experiments were carried out in load control with the ratio R (minimum to maximum stress) of 0.05. Fatigue run-out was defined as 105 cycles at 0.1 and 1.0 Hz, and as 106 cycles at 10 Hz. The 105 cycle count value represents the number of loading cycles expected in aerospace applications at that temperature. Fatigue run￾out limits were defined as the highest stress level, for which run-out was achieved. Note that in the case of all run-out tests, the failure of specimen did not occur when the test was terminated. Cyclic stress–strain data were recorded throughout each test. Thus stiffness degradation as well as strain accumulation with fatigue cycles and/or time could be examined. All specimens that achieved run-out were subjected to tensile test to failure at 1200 C in labo￾ratory air to determine the retained strength and stiffness. 2.3. Microstructural characterization 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. In addition, energy-dispersive X-ray spectroscopy (EDS) analysis was performed using an EDAX Genesis 4000 EDS system. 3. Results and discussion 3.1. Monotonic tension Tensile results obtained at 1200 C were consistent with those reported earlier [44,47]. The ultimate tensile strength (UTS) was 190 MPa, elastic modulus, 76 GPa, and failure strain, 0.38%. It is worthy of note that in all tests reported herein, the failure occurred within the gage section of the extensometer. 3.2. Tension–tension fatigue Creep was shown to be considerably more damaging than cyclic loading to oxide–oxide CMCs with porous matrix [43,44]. Recently Mehrman et al. [45] demonstrated Fig. 1. As-received material: (a) overview, optical microscope and (b) porous nature of the matrix is evident (SEM). R=50 50.0 76.0 8.0 9.0 5.0 Fig. 2. Test specimen, dimensions in mm. 504 M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516