Crack-Tip Stress Fields and Crack Growth Under Creep-Fatigue Conditions

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Abstract
Fatigue crack growth at elevated temperatures is a complicated problem since widespread creep deformation, in addition to elastic deformation, can accompany crack extension, and since the micromechanisms of crack growth comprise grain boundary cavitation, the striation mechanism, and corrosive effects. In the present paper, the time-dependent crack-tip stress fields in an elastic-nonlinear viscous body are analyzed for variable load. From the results, it is concluded that for slow cycling the C*-integral is the load parameter that determines the crack-tip fields and therefore the crack growth rate. If rapid load variations occur within otherwise slow cycles, the stress intensity factor, ΔKI, also affects the crack-tip fields; ΔKI determines the severity of the stress peaks that follow rapid load changes. For rapid cycling, the time-dependent stressing and straining near the crack tip is governed by ΔKI. If rapid cycling is unbalanced, a time-dependent component is superimposed on the rapidly varying ΔKI-controlled field. This time-independent component can be described by the C*-integral only for R-ratios near unity (that is, for high mean stress and small amplitude). The general case of unbalanced rapid cycling remains unresolved. Characteristic frequencies ω1 are developed to distinguish slow and rapid cycling, and a characteristic time t1 is given to decide whether a given load increase within a cycle is rapid or slow.The stress fields are combined with simple models for the striation and cavitation mechanisms of crack growth. The resulting crack growth rates, da/dN, are compared with a small number of experimental results from the literature.