Engineered granular crystals as platform for new materials, new mechanics and new functionalities
Abstract
Traditional granular materials are defined as a large collection of discrete, non-cohesive macroscopic particles which interact through frictional contact. This class of materials is seemingly simple, but their mechanics is remarkably rich and complex. The field of granular mechanics has to this day been dominated by the study of systems that already exist in nature or in industrial processes: Soils, cereal grains in hoppers and silos, industrial scale powder processing. Models and experiments on granular materials are usually conducted with grains of already given grain sizes, shapes, arrangement (often random), loading or flow conditions. Typical granular materials are far from optimal in terms of mechanical performance: Load transfer between individual grains is highly localized to small contact areas and thin force lines in granular materials. Moreover the randomness of typical granular materials makes it hard to design and achieve high stiffness, high strength, or any other advanced structural feature or functionalities. A newer approach considers granular materials as engineering materials by design: the geometry of the individual grains, their arrangement and the composition of their interfaces can be tailored and optimized to achieve specific properties and functionalities. In this project we propose to systematically explore and exploit the assembly, deformation and failure of ÒengineeredÓ granular materials using a materials-science / mechanics-based approach that combines modeling, fabrication and testing. We will exploit the assembly of space filling solids such as the rhombic dodecahedron into fully dense and strong granular crystals more than 10 times stronger than regular granular materials. We will explore how the geometry of the grains, their arrangements (amorphous vs. crystalline), and their surface properties govern stiffness, deformation and strengthening mechanisms, granular slip systems and the initiation and propagation of macroscale dislocations. By surface treatment and functionalization of the individual grains, and by infiltration of the granular crystal with second phases we will create features such as high surface hardness, low friction, reversible cohesion, strong adhesion between the grains or shear thickening effects. Finally, larger scale structural features such as structural and/or interface gradients will be considered to create extreme gradients of stiffness to mitigate the effects of localized loads or stress concentrations, or for Òcase hardeningÓ of granular components. Multilayered architectures will also be studied in this project for the management of cracks, elastic waves or shock waves. The methods we propose combine discrete element modeling with enhanced grain-grain interaction models, 3D printing and injection molding for the fabrication of individual grains with custom geometries, surface functionalization, vibration-directed assembly for the creation of large granular crystals, grain-on-grain mechanical experiments using micromanipulators, and large scale mechanical tests with in-situ 3D imaging. This new generation of granular materials will offer new characteristics and features which can be used for versatile and reconfigurable construction materials, or as a platform for protective materials with resistance to extreme mechanical loads. Finally, these new granular crystals will be manipulated and tailored to model atomistic mechanisms of deformations in complex crystals that involve directional (covalent) bonding, which is not currently possible with traditional sphere-based assemblies and colloidal physical models.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jun 25, 2021
- Source ID
- W911NF2110075
Entities
People
- Francois Barthelat
Organizations
- Army Contracting Command
- United States Army
- University of Colorado Boulder