A three-parameter model for fatigue behaviour of circumferential surface flaws in pipes

A three-parameter model, recently proposed by the authors for part-through-cracked round bars, is employed herein to examine the shape evolution of a circumferential external surface defect in a metallic round pipe under fatigue loading. The elliptical-arc flaw presents an aspect ratio α=ael/bel (ael,bel=ellipse semi-axes) and a relative crack depth ξ=a/t, where a and t are the depth of the deepest point on the crack front and the pipe wall thickness, respectively. The third parameter of the model is the ellipse shifting s=ael/a, which defines the distance of the ellipse centre from the external boundary of the pipe cross-section. Thick- and thin-walled pipes are considered. Fatigue crack propagation paths in the diagram of α against s and ξ are numerically obtained for different initial flaw configurations under cyclic bending loading.