Micromechanical modeling of ductile fracture initiation to predict fracture toughness of reactor pressure vessel steels

Two micromechanical model of ductile fracture are investigated and have been applied to two reactor pressure vessel steels, 18MND5 and A508 Cl.3: the Beremin model, based on the Rice et Tracey void growth model, and the damange work model that combines the plastic strain work to the work spent in void growth. Due to the local nature of these models, finite element analysis needs to be performed to derive stress and strain history in order to obtain the damage kinetics of the material and gecometry uvder consideration. Tensile tests were performed on various gecomctrise (notched and precracked tensile) and sizes. All specimens are in large scale yielding condition. It is found that while the critical void growth ratio decreases with the triaxiality ratio, the critical damage work is not affected. Geometry, size and orientation effects are also investigated. These effects aer well described by these micromechanical models. Similar concepts are applied to sharp notch and crack tip situations. An addition parameter, the so-celled characteristic distance, characterizing the process zone size, is introduced. The fracture toughness, derived from the notched bars,is within experimantal uncertainties in reasonable agreement with the results obtained for cracked geometries.