The effect of overload cycles on fatigue crack propagation in two structural steels

Abstract

A study has been made of the effect of a single peak overload cycle on the rate and mechanisms of fatigue crack propagation in two structural steels. Controlled-rolled, cerium treated C-Mn and C-Mn-Nb-V steels were used and all overload cycles were 100% larger than the range of constant amplitude loading. The imposition of an overload cycle caused a retardation of subsequent crack growth, and the durations of retardation was affected by the values of ΔK b , R and the yield strength of steel. The increments of crack growth that were affected by the overload cycle were compared with the radius of the corresponding overload plastic zone and predictions by the Wheeler-Willenborg model. Surface displacement measurements indicated that the amount of through-thickness contraction in the crack plane before and after the overload cycle, had a significant effect on the increment of crack growth, even when the predominant state of stress at the crack tip during the overload cycle was plane strain. The results of stress relieving experiments suggest that the main mechanism causing the retardation of crack stress growth in this study was the influence of residual compressive stresses which produced crack closure forces.