A parametric study on a granular micromechanics continuum-based hemivariational approach: unraveling the emergence of critical states in granular materials

A numerical model was established in earlier work to investigate the macroscale critical state, which determines the mechanical behavior of sheared granular materials. This paper explores the behavior of this model by conducting a parametric study that varies the constitutive parameters over a wide range. This study is essential to define the combination of material parameters that will lead to the emergence of critical state along the classical response. According to the typical critical state behavior, while the material volume and stress remain unchanged under large shear deformation, the material continues to deform. The critical state concept is examined using a granular micromechanics approach within a numerical framework. In this model, elastic and dissipation energies for a generic grain-pair interaction are adapted using a hemivariational principle. Karush–Kuhn–Tucker-type conditions are derived through a hemivariational principle, providing evolution equations for damage and plastic irreversible phenomena. The coupled damage and plasticity, which are crucial for material strength properties, are associated with grain-pair contact loss and irreversible deformation. Notably, damage-elastoplastic spring elements are described in order to link the micro and macro mechanisms, using orientationally based grain-pair interactions, decomposed into normal and tangential directions. The material properties of specimens with different initial density states are adapted according to dilatancy/compaction characteristics to achieve the idealized critical state behavior. The present model is then applied to simulate the stress and volumetric strain behaviors under varying characteristic compression constitutive parameters.