Modeling of Anisotropic Inelastic Behavior

Abstract

An experimental capability, developed at Lawrence Livermore National Laboratory (LLNL), is being used to study the yield behavinr of elastic-plastic materials. The objective of our research is tQ develop better constitutive equations for polycfystalline metals. We are experimentally determining the multidimensional yield surface of the material, both in its initial sfate and as it evolves during large inelastic deformations. These experiments provide a mom complete picture nf material behavtor than can be obiained from traditional uniaxial tests. Experimental results show that actual material response can differ significantly from that predicted by simple idealized models. These restdfs are being used to develop impmved constitutive models of anisotmpic plasticity for use in continuum computer codes. IntroductionAt a micmstmctural level, polycrystdllne metals am composed of aggregates of individual crystals, each of which has KS own orientation and properties. When subjected to loading, metals initially exhibit rwverslble deformation, due fo the stmtchlng of the lattice. When the loads become suMclentlY large, permanent deformations can occur through a number of mechanisms. such as dislocation motion. twinning, or grain boundary slldlng. As a consequence of having randomly distributed grain orientstions, annealed polycrystalllne metals typically exhibit isotmplc behavior with respect to a reference conflguratlon; that is, at a given point in the mnterIal, the material response nf a specimen in any direction is the same. This includes the elastic behavior and the initial yield behavior. Hnwever, significant processing nf materlala, or even moderate plastlc deformations, can cause grains which wem inltlaiiy randomly oriented to become aligned, resulting In behavior which is anisotmpic, where material response in different directions Is quite different.The ability of numerical simulations to predict the behavior of systems involvlng materials ucldergolng large deformations IS contingent upon hwing a reallstic model of the material behavior. Such models must be accurate in the fnli range of possible loading conditions tm which the materials may be subjected. Use of overly simplified models In regimes where they are not well suited can serlOUSIYcompmmlse the valldity of a simulation. Many problems of engineering Interest involve metals undergoing large deformation under multlaxial staks of stress and the need for rellabie models for these applications can hardly be overemphasized. Experimental data demonstrate that simple models for plasticity commonly used In numerical codes do not accurately predict material behavior under these conditions. Engineering models of polycrystalllne metals generally omit mlcmstructural details and describe the effective macroscopic behavior in terms of a phenomenological continuum model. Viewed fmm the macroscopic perspective, the initial material response is path-independent and thew is a one-tin one correspondence between stress and strain. However, If the deformation or loads become ...