SHPB experiments and FDM-DEM coupled simulations on calcareous sand under uniaxial to triaxial deformation

Abstract

The impact behavior of sand is paramount to geotechnical and protective engineering. Although the one-dimensional impact responses of sand are extensively studied, the behavior of shear under impact loading remains poorly understood as the shear stresses in uniaxially compressed sand cannot attain the failure state. This paper reports the macroscopic responses of crushable calcareous sand under uniaxial to triaxial deformation by split Hopkinson pressure bar (SHPB) experiments. In addition, the mesoscopic mechanism is analyzed by coupled finite difference method (FDM) and discrete element method (DEM) numerical simulations. With an increasing lateral confined stiffness, the p-q curves exhibit a clockwise rotation, signifying a deviation of the stress path from the critical state line (CSL) during the transition from triaxial to uniaxial deformation, meanwhile the Mohr circle experiences a shift from intersecting to tangential contact with the failure line, indicating that shear failure occurs under triaxial deformation. The stress-strain relationships for both compression and shear exhibit four distinct regimes, the initial elastic stage mainly overcomes intergranular friction through particle rotation; the yield stage mainly compresses the skeleton by contribution of displacement; the hardening stage experiences critical particle breakage and then promotes particle displacement and rotation; and the unloading stage with strain softening caused by the asynchrony between impact loading and deformation. Both compressed and shear yield stress increases with an increasing confined stiffness, but the shear friction angle decreases. This indicates that the governing factor in triaxial compression under impact loading is the stress state rather than kinetic friction. Particle breakage in triaxial compression state tends to approach the splitting mode, while in uniaxial squeezing state it tends to approach the Mixed mode.