A damage evolution law enriched by microscopic mechanisms for structured sand in mechanical loading

Abstract

Damage evolution law is widely used to describe the bond degradation process of natural soils in their constitutive models. In mechanical loading, the macroscopic damage of natural soils originates from the breakage of inter-particle bonds. Respecting the granular nature of structured sand (a typical natural soil), a static damage variable and a kinematic damage variable that quantitatively characterize the extent of bond breakage have already been theoretically derived previously in the framework of granular mechanics. In this study, the two damage variables are evaluated from discrete element method (DEM) simulations of structured sand. After reviewing features of damage evolution laws in previous constitutive models, a new damage evolution law is proposed based on the results of DEM simulations which cover a wide range of stress levels and loading paths. The proposed law is formulated in the mean stress–plastic deviatoric strain (p–$$\varepsilon_{\text{s}}^{p}$$) space to mathematically describe the evolutions of the two theoretically derived damage variables. In this proposed law, the macroscopic damage is thought to be driven by a deviatoric damage mechanism and an isotropic damage mechanism. An incremental segment of loading path in the p–$$\varepsilon_{\text{s}}^{p}$$ space (dp, $$d\varepsilon_{\text{s}}^{p}$$) therefore is decomposed into two parts: a pure deviatoric segment (dp = 0) and a pure isotropic segment ($$d\varepsilon_{\text{s}}^{p} = 0$$), respectively. The damage rates along the two decomposed segments are derived from the simulated constant-p shear tests and isotropic compression tests. The observed coupling effect of the two mechanisms and the effect of the intermediate principal stress are considered properly. The proposed law is finally validated against DEM simulation results along various stress paths and satisfactory consistency is obtained.