J.M. Ehrman et aL/Ce tes Science and Technology 67(2007)1425-1438 r Fig. 1. As-received material:(a)overview, optical microscope and (b) porous nature of the matrix is evident(SEM ature [29]. Cyclic-static block loading test block of 10 fatigue cycles followed by static le loadin ses a and 100 s were selected to simulate realistic conditions for with turbine ts[29,31 the same maximum stress. The run-out is defined as Results are summarized in Table l where test environ- 100 h at creep stress Static-cyclic block loading test con- ment, test type and maximum stress are shown together sists of static loading(2 h in air and 0.75 h in steam) fol- with time to failure. Results are also presented in Figs. 2a lowed by cyclic loading with the same maximum stress. and b as maximum stress vs time to failure curves for air In this case the run-out is defined as survival of 105 fatigue and steam, respectively. Fatigue and creep-rupture results cycles. In each test, stress-strain data were recorded during cyclic loading as well as during creep periods. Thus stifness degradation and strain accumulation with fatigue cycles Table 1 achieved a run-out were subjected to tensile test to failure composite at /on its of creep-fatigue interaction tests for the N720/A and/or time could be examined. All specimens that Summary of res and steam environments at 1200C to determine the retained strength and stiffness. Test type Max stress(MPa Fracture surfaces of failed specimens were examined Laboratory air ng SEM(FEI Quanta 200 HV) as well as an optical Fatigue, 10 s hold The SeM 1464 128a Energy-dispersive X-ray spectroscopy (EDS) analysis was Fatigue. 100s hold 112 performed using an EDAX Genesis 4000 EDS system 54 17.6 Fatigue. 100s hold 154 2.75 3. Results and discussion Fatigue 125 40.7° Fat 154 C 125 4.25 3.. Monotonic tension 154 Fatigue-creep 125 Tensile results obtained at 1200C were consistent with Fatigue-creep those reported earlier [27, 30]. The ultimate tensile strength Steam environment (UTS)was 190 MPa, elastic modulus, 76 GPa, and failure Fatigue, 10s hold 4.63 strain, 0.38%. It is worthy of note that in all tests reported Fatigue, 10s hold herein,the failure occurred within the gage section of the Fatigue, 100 s hold Fatigue. 100s hold Fatigue. 10s hold 0005 1.12 Fatigue. 100s hold 3. 2. Fatigue with hold times F 30.0 F Fatigue with hold times at maximum stress were CI 2.49 conducted at 1200°C and in steam. For each envi- creep b 1.25 0.24 ronment, fatigue stress levels were selected according to Fatigue-creep 100 0.61 the results of the previous study [27] to be at or below the fatigue limit, yet above the creep run-out stress. Hence, comparison Results of fatigue and creep tests from prior work [27] are included fo stress levels of 125 and 154 MPa were used in air and stress Run-out levels of 100 and 125 MPa. in steam the hold times of 10 From Ruggles-Wrenn et al. [27]ature [29]. Cyclic–static block loading test comprises a block of 105 fatigue cycles followed by static loading with the same maximum stress. The run-out is defined as 100 h at creep stress. Static-cyclic block loading test con￾sists of static loading (2 h in air and 0.75 h in steam) fol￾lowed by cyclic loading with the same maximum stress. In this case the run-out is defined as survival of 105 fatigue cycles. In each test, stress-strain data were recorded during cyclic loading as well as during creep periods. Thus stiffness degradation and strain accumulation with fatigue cycles and/or time could be examined. All specimens that achieved a run-out were subjected to tensile test to failure at 1200 C to determine the retained strength and stiffness. Fracture surfaces of failed specimens were examined using SEM (FEI Quanta 200 HV) as well as an optical microscope. The SEM specimens were carbon coated. 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 [27,30]. 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. Fatigue with hold times Fatigue tests with hold times at maximum stress were conducted at 1200 C in air and in steam. For each envi￾ronment, fatigue stress levels were selected according to the results of the previous study [27] to be at or below the fatigue limit, yet above the creep run-out stress. Hence, stress levels of 125 and 154 MPa were used in air, and stress levels of 100 and 125 MPa, in steam. The hold times of 10 and 100 s were selected to simulate realistic conditions for turbine engine exhaust components [29,31]. Results are summarized in Table 1, where test environ￾ment, test type and maximum stress are shown together with time to failure. Results are also presented in Figs. 2a and b as maximum stress vs time to failure curves for air and steam, respectively. Fatigue and creep-rupture results Fig. 1. As-received material: (a) overview, optical microscope and (b) porous nature of the matrix is evident (SEM). Table 1 Summary of results of creep–fatigue interaction tests for the N720/A composite at 1200 C in laboratory air and steam environments Test type Max stress (MPa) Time to failure (h) Laboratory air Fatigue, 10 s hold 125 103a Fatigue, 10 s hold 125 146a Fatigue, 100 s hold 125 128a Fatigue, 100 s hold 125 112a Fatigue, 10 s hold 154 17.6 Fatigue, 100 s hold 154 2.75 Fatigue b 125 40.7a Fatigue b 154 46.5a Creep b 125 4.25 Creep b 154 0.27 Fatigue–creep 125 101a Fatigue–creep 154 1.68 Steam environment Fatigue, 10 s hold 100 4.63 Fatigue, 10 s hold 100 2.36 Fatigue, 100 s hold 100 1.35 Fatigue, 100 s hold 100 1.12 Fatigue, 10 s hold 125 0.22 Fatigue, 100 s hold 125 0.31 Fatigue b 100 30.0a Fatigue b 125 46.2a Creep b 100 2.49 Creep 100 1.25 Creep b 125 0.24 Fatigue–creep 100 0.61 Results of fatigue and creep tests from prior work [27] are included for comparison. a Run-out. b From Ruggles-Wrenn et al. [27]. J.M. 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