Phenomenological Theories of Elastoplasticity and Strain Localization at High Strain Rates

In this paper certain fundamental concepts underlying the phenomenological theories of elastic-plastic deformations at finite strains and rotations are presented, and some of the commonly discussed theories are summarized, emphasizing the constitutive parameters which influence strain localization and material instability often observed in finite deformation of ductile materials. Particular attention is paid to the thermodynamic basis of inelastic deformation. Conditions for the existence of inelastic potentials are discussed. The results are presented in terms of a general material strain and its conjugate stress, and then specialized for particular applications, emphasizing quantities and theories which are reference- and strain measure-independent. Rate-independent and rate-dependent elastoplasticity relations are developed, starting from a finite deformation version of the J₂-plasticity with isotropic and kinematic hardening, and leading to theories which include dilatancy, pressure sensitivity, frictional effects, and the noncoaxiality of the plastic strain and the stress deviator. A class of commonly used deformation plasticity theories is then examined and its relation to nonlinear elasticity is discussed. The question of plastic spin, and its relation to the decomposition of the deformation gradient into elastic and plastic constituents, is reviewed in some detail, and it is shown that this decomposition yields explicit relations which uniquely define all spins in terms of the velocity gradient and the elastic and plastic deformation rates, hence requiring no additional constitutive relations for the plastic spin. The phenomenon of strain localization at high strain rates is illustrated and discussed, and a series of numerical results are given. Finally, a recent breakthrough in elastoplastic explicit computational algorithms for large-strain, large-strain-rate problems is briefly reviewed.