Cyclic deformation of ferritic steel—II. Stage II crack propagation

Abstract

The crack propagation process is dominated by the formation of ductile fatigue striations and the classical “one load cycle per striation” concept is only valid in a narrow crack propagation rate interval. However, over 4 orders of magnitudes in crack propagation rates the striation rates is independent of the ΔK, which implies that the number of cycles necessary to form one striation probably is greater than one. The similarities between the fatigue damage processes in the cyclic plastic zone and in plastic strain controlled specimens are documented in detail. The micromechanisms of crack propagation is related to a local plastic collapse of the dislocation substructure at the crack tip. A model for fatigue crack propagation rate predictions has been developed. The model is a refinement of the existing accumulated damage, LCF-models and the most important parameters taken into account are: the constant striation spacing, a realistic dynamic cyclic yield stress, the Coffin-Manson constants, a threshold plastic strain and a constant which coincide with the average grain size.