Surface Fatigue Carck Propagation Behavior in Residual Stress Fields Distributed along the Thickness of Plates : On Fatigue Carck Propagation Rate toward the Depth of Surface Precracks

Abstract

Effects of residual stresses on the fatigue crack propagation behavior from surface precracks were studied using either water-cooled or oil-quenched plate specimens where the residual stress changed along the thickness but the hardness was nearly constant. The surface crack shape during fatigue was approximated by a half-ellipse, the depth of which was influenced by the residual stress distribution. The weight function method was used to evaluate the stress intensity factors, K_r, at the deepest point of the surface crack in the residual stress field. The fatigue crack propagation toward the depth was accelerated or decelerated according to the positive or the negative values of K_r by the tension or the compression residual stress generating inside, respectively. Their propagation rate was evaluated quantitatively by including the value of K_r in the maximum stress intensity factor, K_max, for the parameter (ΔK)^P (K_max)^q controlling the fatigue crack propagation rate, where ΔK is the stress intensity factor range and p, q the empirical coefficients satisfying the equation of p + q = 1.