Dissipation in porous metal plasticity and ductile fracture

The framework of thermodynamics of irreversible processes (TIP) enables the coherent formulation of constitutive equations for porous metal plasticity and viscoplasticity, expressions for the dissipated and stored energies, and a new form of the heat balance equation including void growth damage. These expressions are in good agreement with existing thermographic measurements, which provides additional confidence in the model. The linear stability analysis of a perturbation is applied to the whole set of equations. It gives some features of ductile fracture. Depending on the material parameters and on strain rate, shear localisation is induced by thermal softening, void growth damage, or a combination of both. Localisation is classically inhibited by inertia terms at very high strain rates, and postponed by viscosity. The combination of these phenomena creates an "adiabatic nose" in a strain-strain rate diagram. In the case of an axisymmetric stress tenser, that prevails for example in the centre of the necking zone of a round tensile specimen, a pure opening localisation can be obtained. The limitations of the analyses are mentioned. (C) 2001 Elsevier Science Ltd. All rights reserved.