Evaluation of the Fracture Behavior of Cold-Worked Elbows With Prescribed Cracks

Abstract

Cold-working of elbows and other fittings results in higher strength of the material and has traditionally been considered to be beneficial for piping applications. Although it is known that cold-working leads to spatiotemporal variation in the grain size and the concomitant material property and fracture toughness variations they are not routinely investigated. Hence, this paper reports the findings from elbow fracture experiments conducted at 550°F and internal pressurized conditions (2,250 psi) on TP304 cold-worked elbows. The objectives for conducting the tests were to evaluate the effects of cold-working on the applicability of existing techniques such as an “Original” Net-Section-Collapse (NSC) and recently developed “Apparent NSC” equations for pipes with different inner diameter (ID) surface crack (SC) depths and lengths. This is to determine the failure moments and the plastic reduction factor (PRF) obtained to translate these to piping system evaluations. Preliminary comparisons of the experimental findings with the maximum stress predictions existing for straight pipes and elbow fracture prediction methods developed in the International Piping Integrity Research Group (IPIRG)-2 program were revisited, along with the verification of the applicability of the American Society of Mechanical Engineers (ASME) B2 stress indices and flaw acceptance criteria.