Multiscale modeling and characterization of granular matter: From grain kinematics to continuum mechanics

Granular sands are characterized and modeled here by explicitly exploiting the discrete-continuum duality of granular matter. Grain-scale kinematics, obtained by shearing a sample under triaxial compression, are coupled with a recently proposed multiscale computational framework to model the behavior of the material without resorting to phenomenological evolution (hardening) laws. By doing this, complex material behavior is captured by extracting the evolution of key properties directly from the grain-scale mechanics and injecting it into a continuum description (e.g., elastoplasticity). The effectiveness of the method is showcased by two examples: one linking discrete element computations with finite elements and another example linking a triaxial compression experiment using computed tomography and digital image correlation with finite element computation. In both cases, dilatancy and friction are used as the fundamental plastic variables and are obtained directly from the grain kinematics. In the case of the result linked to the experiment, the onset and evolution of a persistent shear band is modeled, showing—for the first time—three-dimensional multiscale results in the post-bifurcation regime with real materials and good quantitative agreement with experiments.