Development of adaptive granular metamaterials for impact mitigation

Abstract

Granular crystals exhibit highly nonlinear impulse responses and are therefore excellent candidate materials for applications such as vibration absorption, impact mitigation, and shock protection. This work presents a new class of adaptive metamaterials in the form of granular crystals immersed in MR fluids which offer the unique ability to reversibly generate rheological defects at any spatial location using external magnetic fields. To demonstrate the utility and efficacy of these tunable metamaterials, a novel experimental methodology was developed to visualize the wave propagation through granular crystals immersed in opaque fluids subjected to low-speed impact loading. This experimental approach relied on a drop-tower-based setup to subject the granular crystals immersed in MR fluids to low-speed impact loading. The kinematic and strain fields in each grain at any instant during impact loading were calculated using a combination of high-speed imaging and digital image correlation. The experimental methodology was employed to illustrate the influence of “point” defects generated by using an external magnetic field on the wave propagation in granular immersed granular crystals. In this letter, a single rheological “point” defect was introduced in the center of the granular crystals using external magnetic fields of varying magnitudes, and the influence of the strength of the magnetic field on the wave dynamics was quantified. The experimental measurements demonstrated that the strength of the magnetic field has a significant influence on the wave propagation process, and it is possible to control the spatial kinetic and strain energy distribution by varying the magnetic field.