Fracture response of X65 pipes containing circumferential flaws in the presence of Lüders plateau

Abstract

A yield discontinuity or Lüders plateau can be observed in tensile tests conducted on seamless pipe manufactured to API 5L X65 strength grade steel. Such material behaviour is associated with strain localisation which can significantly affect the fracture behaviour of X65 steel pipe subjected to plastic strain. This study considers the Lüders plateau, using the so-called “up-down-up” (UDU) constitutive model, in finite element (FE) analyses of seamless X65 pipes containing circumferential surface-breaking cracks and subjected to axial plastic straining. The softening modulus of UDU model was found to significantly affect the simulated evolution of plasticity, crack driving force and crack-tip fields of the cracked pipe. The FE analysis results were validated against the full-scale pipe test data. It was found that by correctly selecting the softening modulus, a suitable level of accuracy and conservatism was obtained by using an UDU model in FE analyses for assessing fracture response of flawed pipes which show Lüders plateau behaviour. In contrast, the existing stress- and strain-based fracture assessment solutions generally underestimate the crack driving force in the Lüders plateau phase.