Application of an Instability-Based Fracture Model for Prediction of Crack Initiation in Modified 9Cr–1Mo Steel

Ductile fracture of modified 9Cr–1Mo steel was investigated using smooth and notched bar tension tests. Round and flat notched specimens with varying notch acuity ratio were used to investigate the influence of stress state on the fracture strain. Simulations of the experiments were performed using the extended finite element method, together with an instability-based crack initiation criterion based on the micromechanics of void coalescence. The parameters in the plasticity model were determined using smooth bar tension tests, whereas the void nucleation parameters were calibrated from experiments on a shallow-notched specimen. Subsequently, the load–deflection responses and strain to crack initiation have been predicted for round and flat notched specimens with various notch acuities, and central notched specimens with notches oriented at various angles to the loading axis. Using a single set of parameters, the model was able to quantitatively predict fracture in different specimen geometries encompassing a range of stress states.