ASTM E2760-19e1
- standard by ASTM International, 11/01/2019
- Standard Test Method for Creep-Fatigue Crack Growth Testing
- Category: ASTM
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Full Description
1.1 This test method covers the determination of creep-fatigue crack growth properties of nominally homogeneous materials by use of pre-cracked compact type, C(T), test specimens subjected to uniaxial cyclic forces. It concerns fatigue cycling with sufficiently long loading/unloading rates or hold-times, or both, to cause creep deformation at the crack tip and the creep deformation be responsible for enhanced crack growth per loading cycle. It is intended as a guide for creep-fatigue testing performed in support of such activities as materials research and development, mechanical design, process and quality control, product performance, and failure analysis. Therefore, this method requires testing of at least two specimens that yield overlapping crack growth rate data. The cyclic conditions responsible for creep-fatigue deformation and enhanced crack growth vary with material and with temperature for a given material. The effects of environment such as time-dependent oxidation in enhancing the crack growth rates are assumed to be included in the test results; it is thus essential to conduct testing in an environment that is representative of the intended application.
1.2 Two types of crack growth mechanisms are observed during creep/fatigue tests: (1) time-dependent intergranular creep and (2) cycle dependent transgranular fatigue. The interaction between the two cracking mechanisms is complex and depends on the material, frequency of applied force cycles and the shape of the force cycle. When tests are planned, the loading frequency and waveform that simulate or replicate service loading must be selected.
1.3 Two types of creep behavior are generally observed in materials during creep-fatigue crack growth tests: creep-ductile and creep-brittle (1)2. For highly creep-ductile materials that have rupture ductility of 10 % or higher, creep strains dominate and creep-fatigue crack growth is accompanied by substantial time-dependent creep strains near the crack tip. In creep-brittle materials, creep-fatigue crack growth occurs at low creep ductility. Consequently, the time-dependent creep strains are comparable to or less than the accompanying elastic strains near the crack tip.
1.3.1 In creep-brittle materials, creep-fatigue crack growth rates per cycle or da/dN, are expressed in terms of the magnitude of the cyclic stress intensity parameter, ΔK. These crack growth rates depend on the loading/unloading rates and hold-time at maximum load, the force ratio, R, and the test temperature (see Annex A1 for additional details).
1.3.2 In creep-ductile materials, the average time rates of crack growth during a loading cycle, (da/dt)avg, are expressed as a function of the average magnitude of the Ct parameter, (Ct)avg (2).
Note 1: The correlations between (da/dt)avg and (Ct)avg have been shown to be independent of hold-times (2, 3) for highly creep-ductile materials that have rupture ductility of 10 percent or higher.
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ASTM E2760-19e1
ASTM E2760-19
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