Circumferential surface flaws in pipes under cyclic axial loading

The behaviour of a circumferential external surface ¯aw in a metallic round pipe under cyclic axial loading is examined. The defect is assumed to present an elliptical-arc shape with aspect ratio a =a/b (a, b are the ellipse semi-axes), whereas the relative depth x of the deepest point on the front is equal to the ratio between the maximum crack depth, a, and the pipe wall thickness, t. Thick- and thin- walled pipes are considered. The propagation path of the surface ¯aw is obtained in the diagram of a against x through a two-parameter theoretical model. A simpli®ed model (iso-K criterion) is also applied to predict the stable crack growth. Finally the unstable crack growth process is analysed.