CONSTRAINT EFFECTS ON THE DUCTILE-BRITTLE TRANSITION IN SMALL-SCALE YIELDING

The effect of constraint on the ductile-brittle transition is investig ated numerically under small scale yielding conditions in plane strain . An elastic-viscoplastic constitutive relation for a porous plastic s olid is used to model ductile fracture by the nucleation and subsequen t growth of voids to coalescence. Two populations of second phase part icles are represented, large inclusions with low strength, which resul t in large voids near the crack tip at an early stage, and small secon d phase particles, which require large strains before cavities nucleat e. Adiabatic heating due to plastic dissipation and the resulting ther mal softening are accounted for in the analyses. Cleavage is modeled i n terms of attaining a temperature and strain rate-independent critica l value of the maximum principal stress over a specified material regi on. A characteristic length is associated with each failure mechanism; the large inclusion spacing for ductile failure and size of the cleav age region for brittle failure. The crack growth predictions are conse quences of the constitutive characterization of the material, so that the present study is free from ad hoc assumptions regarding appropriat e crack growth criteria. The numerical results show that the extent of cleavage fracture relative to ductile fracture is strongly controlled by temperature and constraint. Fracture resistance curves are obtaine d for a range of conditions. Copyright (C) 1996 Elsevier Science Ltd