Fatigue crack initiation and stage-I propagation in polycrystalline materials. II: Modelling

Abstract

Based on the discussion of the micromechanisms involved with fatigue crack initiation presented in part I, an analytic model of stage-I fatigue crack initiation and the subsequent stage-II crack propagation based on a plastic strain intensity factor, ΔK p, is proposed. In this model, the stage-I crack initiation process is simplified as a straight line on a log-log plot of d a/d N against ΔK p. An inflection point that divides the crack growth into the initiation and propagation categories is introduced as the plastic crack propagation threshold, ΔK pth, whose value is determined by [(ΔJ)E] pth 1 2 , where ΔJ is the range of the J-integral and E is the Young's modulus. Following this, the integration of the crack growth rate as a function of the plastic strain intensity factor yields a fatigue crack initiation life estimate whose error is less than 5%. For comparison, the experimental data presented in this paper are also discussed on the baskis of the parameters ΔK and ΔK eff that have been proposed by other researchers.