The influence of accidentally appeared stress raisers, on the components lifetime duration

Abstract

The pipelines for fluids transport have usually to support variable loadings, because of fluid pressure and temperature variation, but also as a result of some environmental effects. On the other hand, some possible errors in the use of equipments and machinery, around the pipes, may lead to accidentally introducing of stress raisers, having different types and sizes, on the pipe surfaces. The presence of stress raisers may lead to significant decreases of pipe lifetime duration. The paper is focused on some fatigue tests, made on specimens that were longitudinally cut from a steel pipe, used for oil transport. Three types of stress raisers were artificially introduced on the specimen surfaces, with spherical, conical, and respectively pyramidal shape, and with three different values of depth (obtained for different levels of down force on the corresponding indenter). The fatigue tests were conducted, using pulsating loading cycles, with the same maximum stress level, situated below the yield point of the steel. The number of loading cycles to failure was established, for each tested specimen, and on this basis it was possible to distinguish the influence of both the type and size of stress raiser, on the remaining lifetime duration, for the studied pipeline.