Competing fracture mechanisms in the brittle-to-ductile transition region of ferritic steels

Theoretical models are used to investigate the propagation of fracture in the transition region of ferritic steels. Four mechanisms of fracture are allowed: transgranular brittle and ductile and grain boundary brittle and ductile. As fracture propagates decisions are made at each stage about which of these mechanisms will be operative in the next grain or grain boundary. These decisions are based on the relative energies of the different mechanisms, which are functions of temperature, and the orientation of the stress axis. The simulations, which are two-dimensional, enable the proportions of the four mechanisms to be deduced and hence the overall energy of the fracture surface to be determined. The most striking feature of the results is that there is a much greater scatter of mechanisms and of energies than is found in corresponding low temperature and high temperature simulations. This is consistent with experimental results obtained on ferritic steels.