STRAIN CONCENTRATIONS IN PRESSURIZED DENTED PIPES

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 rests to measure strains and displacements in p reviously dented aluminium pipes subject to increasing internal pressu re. Indentation of unpressurized and pressurized pipes was caused by a rigid sphere and resulted in short smooth dents of depths up to 16 pe r cent of pipe diameter. Strain changes on the external pipe surface w ere monitored by strain gauges and a photoelastic coating. The results identify two stationary crescent-shaped regions of high external stra in at the axial extremities of the initial dent, where strain concentr ation factors of 10 were recorded in the early (elastic) stages of pre ssurization, increasing to a maximum of 28 in one particular dent. The results have immediate application to the assessment of a gouge withi n a dent, which has long been recognized as the most severe form of me chanical damage to pipelines and can result in failures at very low in ternal pressures. A parallel series of burst tests by the authors reve aled that gouges located near the axial extremities of the initial den t may cause premature failure. The strain concentration results presen ted here help to explain this behaviour, and indicate the size, shape and location of the regions of high external strain.