Observations of cleavage initiation in ferritic steel

Local Approach (LA) to cleavage fracture models have been under development since the 1980s with the aim of predicting cleavage fracture, including accounting for the effects of temperature, crack-tip geometry and irradiation. This work presents observations of particle behaviour and micro-crack nucleation under stress conditions similar to those ahead of a crack tip, achieved by testing bespoke tensile specimens with a highly constrained volume at temperatures in the material’s ductile to brittle transition region. This region was examined post-test using a scanning electron microscope to identify and characterise micro cracks and micro-crack initiation defects, such as particles. Supporting finite element analyses were performed to predict the mechanical fields acting on the defects. The results suggested that:
• micro-cracks nucleated in the form of damaged, spherical regions around the initiation particle with a cloud of crack tips, rather than as single penny shaped cracks as assumed in LA models;
• these damaged regions can be much larger than the initiating particle;
• nucleation was more often caused by failure of the particle-matrix interface than particle cracking.