FATIGUE PROPAGATION OF SURFACE FLAWS IN ROUND BARS - A 3-PARAMETER THEORETICAL-MODEL

A three-parameter fracture mechanics model is proposed to theoreticall y analyse the propagation of an elliptical-are part-through flaw in a round bar subjected to constant cyclic amplitude axial or bending load s. 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 (elli pse shifting) defines the distance of the ellipse centre from the bar circumference. The surface flaw growth occurs according to preferred p atterns which tend to converge to an inclined asymptotic plane in the diagram of alpha against s and xi.