Brittle fracture of nuclear pressure vessel steels—I. Local criterion for cleavage fracture

Abstract

Brittle fracture of nuclear pressure vessel steels has been investigated on the basis of physical and mechanical modelling for processes of nucleation, start and propagation of cleavage microcracks in the material substructure, as changed by plastic deformation. It is shown that depending on stress triaxiality, temperature and material properties the brittle fracture in macrovolume may be controlled either by cleavage microcrack nucleation or by the propagation condition. Cleavage microcrack nucleation according to conditions formulated by the authors depends on the effective and maximum principal stress and temperature. Such microcracks, as experimental research has shown, may be nucleated at plastic strains from about 1 to 40% depending on test temperature and stress triaxiality. The brittle fracture critical stress S c is determined as stress for cleavage microcrack start and propagation through the deformation substructure of a material. The dependence describing the variation of S c with plastic strain under various loading histories is theoretically obtained and experimentally confirmed. It is established that S c is a monotonously increasing function of accumulated plastic strain—Odqvist's parameter—and does not depend on loading history. On the basis of experimental and theoretical investigations performed, a new formulation for local criterion of cleavage fracture is proposed.