Tailored Mechanical Response of 3D Microgranular Crystals with Hierarchical Architecture

Abstract

Granular media exhibit extraordinary impact-mitigating properties due to their nonlinear grain-to-grain interactions, enabling efficient energy dissipation and wave perturbation under dynamic loading—behaviors unattainable in conventional monolithic materials. Recent efforts have sought to engineer granular systems with tunable mechanical responses, though few have begun to realize them as functional architected materials. Here, we introduce a two-level architected granular framework that programs spherical microgranular media across both grain-level (ellipsoidal microvoids) and bulk granular packing-level architectures, offering surprising control over static and dynamic properties. Using nanoindentation experiments, we reveal tunable quasi-static stiffness behavior, where hollow architected granular packings can exhibit superior mass-normalized energy dissipation compared to their fully dense counterparts. Finite element simulations uncover a structurally engineered Poisson effect, enabling nonlocal contact mechanisms that enhance load-bearing capacity across different packing structures. Future custom direct impact experiments demonstrate a potential route the effectiveness of our multi-scale design in dynamically programming energy dissipation. Our findings demonstrate that a hierarchical granular crystal exhibits enhanced specific energy absorption at a fraction of the weight of their fully dense counterparts and unique nonlocal stress redistribution, surpassing classical granular mechanics through architectural design. This work establishes a path toward lightweight, tunable, and impact-resistant metamaterials, with broad applications in nonlinear waveguiding, energy dissipation, and protective systems.