Sandia's GeoModel is a generalized plasticity model that was developed primarily for geological materials, but is also applicable to a much broader class of materials such as concretes, ceramics, and even some metals. Nonlinear elasticity has been incorporated through empirically fitted functions found to be well suited for a wide variety of materials. The yield surface has been generalized to include any form of inelastic material response including pore collapse and growth. Deformation-induced anisotropy is supported in a limited sense through kinematic hardening. Applications involving high strain rates are supported through an overstress model. Inelastic deformation can be associated or non-associated, and the GeoModel can employ up to 40 material input parameters in the rare case when all features are needed, but simpler idealizations (such as linear elasticity, or Von Mises yield, or Mohr-Coulomb failure) can be replicated by simply using fewer parameters.
1. INTRODUCTION
Simulating deformation and failure of natural geological materials (such as limestone, granite, and frozen soil) as well as rock-like engineered materials (such as concrete [1] and ceramics [2]) is at the core of a broad range of applications, including exploration and production activities for the petroleum industry, structural integrity assessment for civil engineering problems, and penetration resistance and debris field predictions for the defense community. For these materials, the common feature is the presence of microscale flaws such as porosity (which permits inelasticity even in purely hydrostatic loading) and networks of microcracks (leading to low strength in the absence of confining pressure and to noticeable nonlinear elasticity, rate-sensitivity, and differences in material behavior under triaxial extension compared with triaxial compression).
For computational tractability and to allow relatively straightforward model parameterization using standard laboratory tests, the Sandia GeoModel strikes a balance between first-principals micro-mechanics and phenomenological, homo-genized, and semi-empirical modeling strategies. The over-arching goal is to provide a unified general-purpose constitutive model that can be used for any geological or rock-like material that is predictive over a wide range of porosities and strain rates. Being a unified theory, the GeoModel can simultaneously model multiple failure mechanisms, or (by using only a small subset of the available parameters) it can duplicate simpler idealized yield models such as classic Von Mises plasticity and Mohr-Coulomb failure. Thus, running this model can require as many as 40 parameters for extremely complicated materials to only two or three parameters for idealized simplistic materials.
2. GEOMODEL OVERVIEW
The GeoModel shares some features with earlier work by Schwer and Murry [3] in that a Pelessone function [4] permits dilatation and compaction strains to occur simultaneously. For stress paths that result in brittle deformation, failure is associated ultimately with the attainment of a peak stress and subsequently work-softening deformation. Tensile or extensile microcrack growth dominates the micromechanical processes that result in macroscopically dilatant (volume increasing) strains even when all principal stresses are compressive. At higher pressures, these processes can undergo strain-hardening deformation associated with macroscopically compactive volumetric strain (i.e., pore collapse).
