Abstract
Finite element simulations are performed to predict plasticity-induced crack closure and creep-fatigue crack growth rates in two heat-resistant steels, the austenitic 20Cr-25Ni (Alloy 709) and martensitic 9Cr-1Mo (modified 1Cr-9Mo) steels. In a creep-fatigue cycle, total crack growth rate is usually computed by the addition of the fatigue crack growth rate during cyclic loading and creep crack growth rate during hold time. Two-dimensional finite element analyses of compact tension specimens are performed to simulate crack growth under cyclic and time-dependent loading conditions, with the consideration of elastic-plastic and creep deformations of the material at the crack tip. The simulations quantify the effect of hold time on crack opening load induced by the combined action of plasticity-induced crack closure and creep-induced stress relaxation at the crack tip. It is shown that increasing hold time during a creep-fatigue cycle results in a decrease of crack-tip opening load, thus increasing crack growth rate during the next cyclic loading. Experimental results of creep-fatigue crack growth rates in Alloy 709 are presented. The finite element simulations predicted crack growth rates under creep-fatigue loading in agreement with experimental values for both Alloy 709 and modified 9Cr-1Mo steels.
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