Numerical Simulation of Flat-to-slant Ductile Fracture Transition in Notched Plate

Abstract

The effort of this study is to develop a damage model for simulating large ductile crack extension accompanied by a flat and a slant surface observed in the hydrostatic full-scale tests up to rupture for API X80 line pipes with an axial through-wall notch. A small and thin specimen with a shallow through-thickness notch having a large ligament (sub-sized Charpy V-notch (Sub-CVN) specimen subjected to 3-point bending) is used for a fundamental verification study. The Sub-CVN specimen provided the similar macroscopic process as well as micro-mechanism for ductile cracking, where a flat-to-slant fracture transition accompanied by a transition from an equi-axed dimple mode to a shear-slip mode fracture is presented. On the basis of the observed mechanism of ductile crack growth, a numerical ductile damage model for predicting the large ductile crack growth resistance is proposed taking into account an effect of the Lode parameter on ductile fracture into the present damage model proposed by authors. The crucial consideration is that the damage parameters to be identified can be correlated to the material properties. In this model, the shear-dominated stress state is assumed to advance the acceleration of damage evolution due to a shear localization mechanism at lower damage level even at the same stress triaxiality condition, so that the critical damage fraction dependent on the Lode parameter is phenomenologically employed in this model. The modified damage model reproduces the same ductile crack profile together with the same micro-mechanisms for forming the chevron-shaped flat surface as those observed in the experiment. The possibility to reproduce the flat-to-slant fracture transition is also demonstrated by the simulation.