Fatigue propagation of surface flaws in round bars: A three-parameter theoretical model

A three-parameter fracture mechanics model is proposed to theoretically analyse the propagation of an elliptical-are part-through flaw in a round bar subjected to constant cyclic amplitude axial or bending loads. The edge flaw presents an aspect ratio alpha = a(el)/b(el) (a(el), b(el) = ellipse semi-axes) and a relative crack depth xi = a/D, where a and D are the depth of the deepest point on the crack front and the bar diameter, respectively. Additionally a parameter s = a(el)/a (ellipse shifting) defines the distance of the ellipse centre from the bar circumference. The surface flaw growth occurs according to preferred patterns which tend to converge to an inclined asymptotic plane in the diagram of alpha against s and xi.