Strain hardening and rate-sensitivity effects on the ductile–brittle transition

A thorough study of the flow and fracture parameters for pure Fe and its binary alloys of Ni and Si at their respective ductile–brittle transition temperatures is presented. The problem of how the region at the root of blunt notch in such low yield strength materials may reach its cleavage fracture stress is assessed. For example, the cleavage fracture stress of Fe-1%Ni, which is 1260 MPa, requires a factor of 7.6 increase in the static yield stress which is 166 MPa at the transition temperature. Recent analyses by Rice et al. have shown that large stress elevations may be achieved by strain hardening in the plastically-constrained region. Nevertheless, the present study demonstrates that these elevations by themselves are not sufficient. Strain hardening exponents n = δ ln σ/δ ln ε ranging from 0.19 to 0.25, and. strain rate sensitivity exponents m* = δ ln ε/δ ln σ* ranging from 5 to 11, were found at the ductile-brittle transition. By incorporating both of these effects it is shown that the necessary stress elevations may be achieved.