Session: 12-16-03: Drucker Medal Symposium

Paper Number: 99249

99249 - High-Strength Engineered Granular Crystals 

Granular materials can be defined as a large collection of discrete, macroscopic particles which are in general non-cohesive. These materials, ubiquitous in everyday life and in industry (food, pharmaceutics, powder metallurgy, geology, mining…), are seemingly simple systems, but they actually display a wide range of complex mechanical responses. For example depending on confinement, sand can flow like a liquid or be as stiff and strong as a solid, with a rich set of mechanisms that include the formation of clusters, localized regions of stress transfer, jamming, shear bands, shear-induced dilatation. Randomness in the packing of grains makes it however difficult to design, optimize and exploit these deformation mechanisms for broader engineering applications. Granular materials are also usually based on spherical or ellipsoidal grains, which in terms of mechanical performance spherical shapes is sub-optimal: The random packing of spheres lead to a packing factor (solid volume fraction) of only 0.55 to 0.64 which is significantly smaller than the upper bound volume fraction in closed packed spheres (0.74), which itself is much lower than 1 because spheres are not space filling shapes. As a result, mechanical loads applied to typical granular materials are channeled along thin “force lines” that occupy only a small fraction in the material while most grains remain free of stress.

Here we follow a new approach where we consider granular materials as engineering materials by design. We fabricated and tested strong granular crystals at the intersection between traditional granular materials, topologically interlocked materials and mechanical metamaterials. We considered grains in the shape of rhombic dodecahedra (RD) which is an attractive geometry because it tessellates space with a face-centered, fully dense cubic (FCC) crystalline structure. We also considered truncated octahedra (TO), which tessellate space with a body-centered cubic (BCC) crystalline structure. For reference we also fabricated and tested granular materials based on spheres, since they are common in granular mechanics. Fabrication of individual grains was carried out with a high-resolution 3D printer based on digital light processing, which produced individual grains which are fully dense, isotropic and with a smooth surface roughness comparable to injection molding. We used an ABS-like photocurable material, which is relatively stiff and strong. Importantly, the dimensions of the individual grains were chosen so their volume was held constant (32 mm3) across all granular geometries. The granular systems we considered contained N=800 grains each, which occupied an approximate volume of 50×40×80 mm3 (with some variations depending on the packing factor). Finally, for each grain geometry we considered a random packing, and a crystallized packing. Granular crystals were created by placing the grains in a rectangular container, which was then subjected to high amplitude vibrations to form fully dense large granular crystals. Triaxial compression tests revealed strengths up to ~10 times higher than traditional granular materials. These granular crystals display a rich set of mechanisms: Nonlinear deformations, crystal plasticity reminiscent of atomistic mechanisms, cross-slip, shear-induced dilatancy, micro-buckling. Once fully understood and harnessed, we envision that these mechanisms will lead to granular engineering materials with unusual and attractive combinations of mechanical performances. 

Presenting Author: Francois Barthelat University of Colorado

Presenting Author Biography: Francois Barthelat is Professor of Mechanical Engineering at the University of Colorado Boulder. He obtained his PhD from Northwestern University in 2006, and was a Professor in Mechanical Engineering at McGill University (Montreal, Canada) from 2006 to 2019. Francois Barthelat founded the Laboratory for Advanced Materials and Bioinspiration to explore key structures and mechanisms in natural materials, and to develop new bioinspired, high-performance materials. Dr. Barthelat and his students have discovered new deformation and fracture mechanisms in bone, in mollusk shells and in fish scales. They have also pioneered new bioinspired materials and systems which they are now implementing in engineering applications. Dr. Barthelat is the recipient of the Hetényi Award for best research paper in Experimental Mechanics, of a Discovery Accelerator Supplement from NSERC, and of a Department of National Defence /NSERC Discovery Grant Supplement. Barthelat serves on the editorial board of Scientific Reports, Bioinspiration and Biomimetics, Mechanics of Materials and the Journal of the Mechanical Behavior of Biomedical Materials.

Authors:

Francois Barthelat University of Colorado
Navdeep Karuriya University of Colorado