Consideration of Yield Discontinuity in the Elastic-Plastic Fracture Analysis of Circumferentially Flawed Pipes

Abstract

This study examined the fracture behaviour of pipes containing surface flaws oriented circumferentially and made from a material that exhibits yield discontinuity (known as Lüders plateau) with the view of making recommendations for the assessment of pipes subject to high level of plasticity. Starting with the fundamental and first principles, uniaxial tensile tests were carried out with the use of digital image correlation (DIC) to observe the formation and propagation of Lüders bands quantitatively. Finite element (FE) analyses were then carried out to simulate the Lüders banding phenomenon in uniaxial tensile specimens and consequently cracked pipes. Different material models were adopted in FE analyses, including the stress-strain curve with a flat stress plateau neglecting upper yield stress, and the so-called ‘up-down-up’ (UPU) stress-strain curve for refining crack driving force predictions. The numerical analysis of tensile tests demonstrated that UPU stress-strain model satisfactorily simulated the main macroscopic features of Lüders band observed in the experiment. FE analysis of flawed pipes using both flat and UPU stress-strain curves produced a similar trend in the crack tip opening displacement (CTOD)-strain trajectory as that obtained from large-scale testing. It was seen that the shape of the UPU stress-strain curve, particularly the magnitude of softening, considerably affects the magnitude of crack driving force in the flawed pipe. However, the strain localisation associated with Lüders banding was not observed in the circumferentially flawed pipe in the case of using the flat stress-strain curve. The CTOD crack driving force obtained from simulations was lower than the CTOD obtained from experiments in the Lüders plateau regime, even with the consideration of ductile tearing. Finally, as a result of this study, recommendations on the optimum choice of material parameters were made for more accurate predictions of crack driving force in the presence of yield discontinuity.