Strain Concentrations in Pressurized Dented Pipes

Abstract

Knowledge of the strain distribution in a dent is crucial in assessing the severity of damage to pipes. Accordingly this paper presents the results of a series of tests to measure strains and displacements in previously dented aluminium pipes subject to increasing internal pressure. Indentation of unpressurized and pressurized pipes was caused by a rigid sphere and resulted in short smooth dents of depths up to 16 per cent of pipe diameter. Strain changes on the external pipe surface were monitored by strain gauges and a photoelastic coating. The results identify two stationary crescent-shaped regions of high external strain at the axial extremities of the initial dent, where strain concentration factors of 10 were recorded in the early (elastic) stages of pressurization, increasing to a maximum of 28 in one particular dent. The results have immediate application to the assessment of a gouge within a dent, which has long been recognized as the most severe form of mechanical damage to pipelines and can result in failures at very low internal pressures. A parallel series of burst tests by the authors revealed that gouges located near the axial extremities of the initial dent may cause premature failure. The strain concentration results presented here help to explain this behaviour, and indicate the size, shape and location of the regions of high external strain.