Strain-Based Failure Criteria for Sharp Part-Wall Defects in Pipelines

Abstract

Failure criteria in current engineering critical assessment procedures for defects in pipelines and welds are stress-based. For example, failure is presumed to occur when the net section average stress reaches some arbitrary flow stress. These approaches are unrealistic for defects of limited length where loading of the net section (ligament) is essentially strain controlled. In order to improve upon this, the authors developed a strain-based failure criterion for part wall pipe defects in terms of the maximum ligament plastic extension. While this criterion[l] provided a basis for assessing the criticality of blunt defects, with respect to plastic collapse, it did not address sharp or planar defects which promote fracture. As a defect becomes sharper, failure is determined more by local strain at the defect tip which is typically characterized by the crack tip opening displacement (CTOD). This paper describes the development of a sharp/planar defect strain-based failure criterion which relates the maximum ligament extension to the critical CTOD of the material. Two and three dimensional non-linear finite element analyses are used to determine local root extensions of circumferential defects which can be related to the loading, defect and pipe dimensions. The root extensions are calibrated to standard CTOD measurements through non-linear finite element analysis. The failure criterion development process considers various defect lengths, material work hardening rates and material models. The failure criterion is compared with analytical and experimental data to demonstrate its predictive capability. The end result of this work is the development of an alternative acceptance criterion for sharp weld defects permitting more effective repair decisions to be made based on a more uniform level of reliability.

Strain-based design criteria of pipelines

“…The approach to tensile strain capacity of pipelines has been the hot spot (Dinovitzer, Brian, & Graville, 1996;Graville & Dinovitzer, 1993;Liu, Liu, & Zhang, 2008b;Wang et al, 2002;Wang et al, 2004). The basic approach is based on the inter-relationship between the longitudinal pipe stress-strain properties, the toughness and stress-strain properties of the weld and HAZ, flaw size and location, and the applied loads.…”

Fracture control assessment in the base metal and welded joint of a pipeline steel

“…Fracture control studies were initiated at Battelle Memorial Institute ~40 years ago, where initiation, propagation and arresting behaviour of defects were analysed in full‐scale pressurized pipes and correlated to small scale tests 6 . Since then, a very large amount of research has been developed on steels, 3 , 4 , 7 –10 welding 11 –16 and fracture control 17 –23 to enhance the reliability of pipelines.…”