Influence of stress states on cleavage fracture in X70 pipeline steels

Abstract

Stress state is a primary factor controlling the ductile fracture behavior of steels, which is typically represented as the combination of the stress triaxiality and Lode angle parameter. The cleavage fracture properties of pipeline steels at low temperatures are usually assessed under plane strain conditions, such as using the fracture mechanics experiments. In this study, the cleavage fracture properties of a X70 steel at liquid nitrogen temperature (−196 °C) are characterized over a broad range of stress states. A comprehensive experimental program is carried out by performing tensile tests using various flat specimens of different geometries immersed in liquid nitrogen, including shear, central hole, notched dog bone, and side grooved plane strain. Pronounced plasticity occurs prior to the final fracture within the tested range of stress states at the very low temperature. Anisotropy effects are considered by conducting tensile tests of fracture specimens along the rolling, diagonal and transverse directions. Finite element simulations of corresponding experiments are performed using an evolving quadratic plasticity model to extract the local stress state variables to establish the fracture criteria, which are formulated based on the critical values of plastic strain and maximum principal stress. The fracture strain of the investigated material at liquid nitrogen temperature is affected by the stress triaxiality, Lode angle parameter and loading direction.