Mechanical characterisation of highly interlocked granular metamaterials

Granular materials exhibit fascinating and complex states across scales. Although the mechanical properties of materials with convex and regular particles have been extensively studied in the literature, the properties of materials with highly concave particles are still widely unexplored. Particle shape plays a key role in the packing, mechanical and rheological properties of granular systems; yet a straightforward link between particle shape and shear strength or flowability at the bulk scale has not been established. A new class of granular mechanical metamaterials exhibit extraordinary properties compared to conventional materials, such as high capacity to interlock, due to their complex particle shapes. In this work, we use the Discrete Element Method to gain micromechanical insights into the behaviour of systems made of these complex particles. Assemblies of granular metamaterials are sheared under triaxial stress conditions to establish micro-to-macro links between particle-scale features and bulk-scale behaviour. It is found that intense interlocking of these concave particles at the micromechanical level serves as the origin of apparent cohesion at the bulk scale.