Development of a Theoretically Sound Cap Model for Fitting ISST-Type Material Behavior.

Incremental elastic-plastic constitutive models which include both an ultimate failure envelope and a work-hardening yield surface, or cap, are commonly used in ground shock calculations to simulate geologic material behavior. The basic premise of elastic-plastic constitutive models is the assumption that certain materials are capable of undergoing small plastic permanent as well as elastic recoverable strains at each loading increment. This report describes the development of a three-dimensional elastic-plastic work-hardening constitutive relationship in which the shear modulus is a function of plastic volumetric strain and the second invariant of the strain deviation tensor. It contains 1 the general description of the new model including the proof of its theoretical soundness, 2 selected mathematical orms of its various response functions, and 3 an example model fit to the material properties specified for ISST Layer 2.