BRITTLE-FRACTURE OF NUCLEAR PRESSURE-VESSEL STEELS .1. LOCAL CRITERION FOR CLEAVAGE FRACTURE

Brittle fracture of nuclear pressure vessel steels has been investigat ed on the basis of physical and mechanical modelling for processes of nucleation, start and propagation of cleavage microcracks in the mater ial substructure, as changed by plastic deformation. It is shown that depending on stress triaxiality, temperature and material properties t he brittle fracture in macrovolume may be controlled either by cleavag e microcrack nucleation or by the propagation condition. Cleavage micr ocrack nucleation according to conditions formulated by the authors de pends on the effective and maximum principal stress and temperature. S uch microcracks, as experimental research has shown, may be nucleated at plastic strains from about 1 to 40% depending on test temperature a nd stress triaxiality. The brittle fracture critical stress S-c is det ermined as stress for cleavage microcrack start and propagation throug h the deformation substructure of a material. The dependence describin g the variation of S-c with plastic strain under various loading histo ries is theoretically obtained and experimentally confirmed. It is est ablished that S-c is a monotonously increasing function of accumulated plastic strain-Odqvist's parameter-and does not depend on loading his tory. On the basis of experimental and theoretical investigations perf ormed, a new formulation for local criterion of cleavage fracture is p roposed. (C) 1997 Elsevier Science Ltd.