Interaction between surface breaking and embedded circumferential flaws in tension loaded (pressurized) steel pipes

Abstract

This paper examines the interaction of adjacent surface breaking and embedded circumferential flaws in terms of crack driving force. A novel finite element modelling methodology for adjacent planar flaws is adopted, based on element deletion in a densely meshed grid. Element deletion provides a unique capability for modelling multiple surface breaking or embedded flaws, since the mesh design is independent of the geometry of the flaw(s). A newly developed postprocessing procedure allows for the calculation of crack driving force (both linear-elastic and elastic-plastic), based on displacements along the flaw flanks.The methodology is employed to scrutinize established flaw interaction rules for surface breaking and embedded co-planar flaw pairs in a tension loaded pipe. The evaluation comprises two aspects: the increase in crack driving force due to flaw interaction, and the conservatism introduced by re-characterizing the flaw pair into a single bounding flaw. A full-factorial parametric study is conducted, containing 36 flaw combinations and two load cases, i.e., axial tensile loading in absence and presence of internal pressure. The results highlight (potentially strong) limitations of current analytical flaw interaction rules with respect to conservatism, the influence of load on flaw interaction, and underestimation of primary geometrical influence factors. The presence of internal pipe pressure, additional to axial stress, is found to have little effect on the results.