The Role of Grain Morphology and Individual Grain Fracture on Granular Material Behavior

Granular materials, such as sand, are multi-scale materials combining an immense number of crushable particles to create an assembly. Numerical approaches to modeling sand’s mechanical behavior often use simplifications, such as treating the volume as a continuum or representing the particles with rigid, idealized spheres using the discrete element method. These approaches overlook crucial nonlinear phenomena, such as individual grain morphology and fracture, which have been shown to influence sand’s strength and plasticity behavior. This research investigated methods to incorporate such phenomena , focusing on simulating single grain crushing tests and 1D confined compression of Ottawa sand using finite element analysis (FEA) codes, GEODYN-L and ABAQUS, to resolve internal stresses and strains.

Grain morphology was incorporated through finite element meshes derived from x-ray computed tomography images. Two approaches were employed to incorporate grain fracture: a damage mechanics model (GEODYN-L) and a brittle cracking model (ABAQUS). The specified ultimate tensile strength was shown have the greatest influence on the peak force associated with catastrophic grain splitting. The brittle cracking model was determined to be too computationally demanding due to mesh refinement requirements.

Investigating grain properties like orientation and coordination number revealed their impact on catastrophic splitting force, underscoring the necessity of incorporating accurate grain morphology due to its effect on grain arrangement within the granular assembly.

Simulating single particle compression of elastic quartz spheres, this research also examined the impact of mesh discretization approaches and finite element types on capturing high contact stresses that lead to fracture, determining that quadratic tetrahedral elements rather than linear tetrahedral elements could balance accuracy with computational efficiency.

Finally, this study utilized FEA of 1D confined compression to determine that incorporating grain fracture softened the overall stress-strain response, with the softening directly correlating to a decrease in the ultimate tensile strength. A representative volume element size based on the maximum grain diameter was also recommended for capturing macroscale stress behavior in granular assemblies under 1D confined compression, considering grain morphology and fracture, which is especially important when computational resource constraints prevent explicit modeling of individual grains in large assemblies.