Mechanism-Based Susceptibility to Adiabatic Shear

Summary Shear localization in ductile metals at high strain rates is potentially affected by many physical phenomena and material properties. Mechanisms tending to correlate with localization include thermal softening from dislocation mobility, geometric softening from lattice reorientation or twinning, ductile fracture from void linkage or related microstructure degradation, dynamic recrystallization, solid–solid or solid–liquid (that is, melting) transitions to a softer phase, and initial defects and inhomogeneities. Work hardening, strain-rate hardening and heat conduction tend to impede localization, as does gradient regularization in the context of damage and plasticity models. Here, a model framework encompasses physical mechanisms with internal state variables governed by basic kinetic laws. Effects are included in the constitutive equations and energy balance for a simple shear problem with superposed pressure. The response is parameterized using temperature as the independent variable. Asymptotic approximations for small rate sensitivity suggest the critical localization strain past peak load is inversely proportional to the square root of the magnitude of curvature of the homogeneous, adiabatic stress–strain curve. Solutions relate constitutive properties to applied strain at which localization is anticipated. Results can provide insight into the temporal sequence of events leading to terminal failure. Depending on properties and initial conditions, structural changes such as dynamic recrystallization and material damage are shown to induce localization in some cases, whereas localization is triggered by thermal softening alone in other cases.