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

Two micromechanical models of ductile fracture are investigated and have been applied to two reactor pressure vessel steels, 18MND5 and A508 CI.3: the Beremin model, based on the Rice et Tracey void growth model, and the damage 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 geometry under consideration. Tensile tests were performed on various geometries (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 are well described by these micromechanical models. Similar concepts are applied to sharp notch and crack tip situations. An additional parameter, the so-called characteristic distance, characterizing the process zone size, is introduced. The fracture toughness, derived from the notched bars, is within experimental uncertainties in reasonable agreement with the results obtained for cracked geometries.