An experimental platform for granular crystals under impact loading
Abstract
Traditional granular materials are defined as a large collection of discrete, non-cohesive macroscopic particles which interact through frictional contact. Typical granular materials are however far from optimal in terms of mechanical performance. 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. Through the ARO Grant ?Engineered granular crystals as platform for new materials, new mechanics and new functionalities? we are systematically exploring and exploiting the assembly, deformation and failure of ?engineered? granular materials using an approach that combines modeling, fabrication and testing. 3D printed grains with specific polyhedral shapes are assembled into fully dense FCC, BCC, HCP or diamond cubic crystals. In quasi-static loading rates these crystals are about 10 times stronger than regular granular materials, and they display a rich set of deformation and failure mechanisms: Nonlinear deformations, crystal plasticity reminiscent of atomistic mechanisms, cross-slip, shear-induced dilatancy, microbuckling. Granular materials where grains interact by normal contact are already known to attenuate elastic wave propagation by trapping energy and even disintegrating shock waves. However, there is currently no data on dynamic loading of fully dense granular crystals where frictional sliding dominates inelastic deformations. Our recent discrete element models of fully dense FCC granular crystals suggest that at high loading rates, the granular crystals develop a plane shock wave behind which massive grain sliding occurs in a homogenous fashion. In addition, the models show a significant attenuation of impulse in these materials, and a large amount of energy dissipation by friction. Experiments are now needed to validate these models, measure properties at high rates and to develop specific granular architectures for impact mitigation. The velocity of the longitudinal elastic waves and shock waves in our granular crystals is relatively low (below 300 m/s), and therefore elastic waves and shock waves can be captured using impacts at speeds below 10 m/s. To this effect, we propose to acquire a drop weight impact tower to capture the mechanics and properties of fully dense granular crystals at high rates of deformation. The standard impact tower configuration will be modified using acrylic input and output bars to achieve wave separation and to produce data free of spurious reflections and vibrations. High-speed imaging will be used to capture the mechanics of deformation and to validate our models. We also propose a variant with a photoelastic setup to capture the dynamic formation of force lines in 2D granular crystals. This new experimental capability will greatly accelerate our understanding of granular crystals under impact loading, within the current project and for future projects. In addition, students at all levels will use this equipment and get trained in scientific areas relevant to ARO and DoD: mechanics of materials, dynamics, material design for impact mitigation, experimental mechanics. Finally, the proposed equipment will accelerate the development of advanced granular systems that can find applications for rapid and versatile construction of static structures, or as light weight protective materials in a multitude of applications (e.g. buildings, body armor, vehicles, etc.).
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 09, 2023
- Source ID
- W911NF2310346
Entities
People
- Francois Barthelat
Organizations
- Army Contracting Command
- United States Army
- University of Colorado Boulder