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.