Geomaterials like soft rocks and overconsolidated clays exhibit strain hardening and strain softening behaviors in a certain range of confining pressure. The structures of such materials as soft rocks and concrete may experience structural instability or shear bands under extreme loads In engineering. This paper reports a computational constitutive modeling method based on adaptive constitutive finite element method(FEM) and the simplified yet unified disturbed state constitutive(DSC) model for highly nonlinear deformations in the structures of geomaterials with softening and joints. The method is characterized by incorporating the computational and constitutive modeling into one to solve engineering failure problems such as collapse or large dams, The computational constitutive results agree with experimental observations in the footing benchmark problem.
As a modern challenge, it is hard to model material and structural instabilities computationally and accurately with the help of traditional FEA and plasticity model. Adaptive finite element method(FEM) is a method to adapt or guide itself to better subsequent computation by use of previous computational information so as, to achieve prescribed accuracy. As amassed material instability may finally trigger structural instability, de Borst et al.[1993] pointed out that the root cause of the structural instability is owing to loss of ellipticity and continuity of conventional constitutive equations of softening materials. Zienkiewicz et al[1995] found that the ellipticity condition is not sufficient or reliable in numerical analysis and believed that the adaptive FEA and traditional plasticity model can solve some localization problems. Therefore, for analyzing highly nonlinear deformation of localization problems such as interfaces and joints and shear bands, and complex materials with both hardening and softening, the newly unified computational constitutive model is considered to be nearly optimal and a priority in this report. Based on the traditional elasto-plasticity model, the disturbed state constitutive(DSC) model provides a unified basis for constitutive modeling including elastic, plastic and creep deformations, mircrocracking, damage and softening, stiffening, and cyclic fatigue underthermomechanical loading [Desai and Zhang 1997). It includes intrinsically regularization, localization, characteristic dimension and avoidance of spurious mesh dependence. Simplification of the constitutive model and its incorporation into adaptive finite element computation are one of major concerns in this paper. By simplifying the disturbed state constitutive model and furthering the work done[Desai and Zhang 1997], The computational constitutive model(CCM) is proposed as a concept and implemented in the developed highly nonlinear finite element analysis program(DSC-FEAP) based on the simplified disturbed state constitutive model and new adaptive finite element method. The adaptive finite element-meshing indicator is based on a combined uniform-mesh-stress-accuracy-disturbance function. In the range of elasto-plastic deformations (with hardening), the uniform element stress meshing indicator using assumed stress finite element method[Zhang 1993, Zhang and Desai 1994, Zhang and Chen 1997] is adopted, and after the peak point or in the range of strain softening, the disturbance adaptive remeshing indicator_representing deviatoric plastic strains or localization is applied. The features of the developed model and software are hierarchical for metals.
