Development of Adaptive Granular Crystals Immersed in Active Fluids for Impact Mitigation Public
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Numerous numerical and experimental efforts in literature have extensively investigated the wave propagation through dry granular media with lack of cohesion at inter-particle contacts. In contrast to dry granular materials, the mechanics and wave dynamics of wet and immersed granular media have not garnered adequate attention due to various complications posed by the presence of cohesive interactions at inter-particle contacts along with various dissipative effects in these materials. The goal of the current work is to derive fundamental knowledge about dissipative phenomena and defect interactions in immersed granular crystals in order to aid the development of a novel class of adaptive granular metamaterials. The unique ability to introduce defect patterns at any spatial location in an immersed granular crystal can allow for highly effective control of the magnitude and direction of wave propagation. The current work explores a novel strategy to introduce point defects at any spatial location in granular crystals immersed in magnetorheological fluids using external fields. In order to realize this vision, novel experimental visualization techniques and a mathematical framework for the investigation of the dynamic response of dry and immersed granular crystals were developed. A drop-tower based experimental setup was developed to investigate wave propagation through the 2D assembly of cylindrical particles 1ʺ in length and 1/2ʺ in diameter completely submerged in a secondary fluid medium under impact loading at a projectile velocity around 6 m/s. The deformation of the individual grains in granular crystals was recorded by a high-speed camera at a 40000 fps frame rate, and the kinematics and the strain fields in each individual particle were computed using Digital Image Correlation. These experimental measurements, in conjunction with the Granular Element Method (GEM) based mathematical framework was employed to calculate the inter-particle forces in the granular crystals. These experimental and computational tools facilitated a quantitative assessment of the influence of lateral constraints imposed by the sidewalls on the wave propagation in finite granular crystals. A detailed analysis of these experiments established that the directional nature of the wave propagation in finite granular crystals resulted in strong wave reflection from the sidewalls. It was also noteworthy that the two reflected waves from the two opposite sidewalls result in destructive interference, and the depth where most of the kinetic energy is concentrated can be adjusted by varying the lateral size of the granular crystal. In order to aid the development of adaptive granular metamaterials, a comprehensive experimental investigation was performed to quantify the influence of point defects on the wave propagation in granular crystals and provide a deeper understanding of interactions between multiple point defects. It was found that there is a significant scattering and reflection from defects and the exact nature of such effect depends on the location of the defects, and by varying the location of the defects, it is possible to guide the waves to certain parts of the crystal. Since the presence of a secondary fluid in immersed granular crystals completely alters the physics of wave propagation, the wave dynamics of immersed granular crystals was also investigated over a wide range of viscosity for the secondary fluids. The experimental measurements for the wave decay characteristics as a function of viscosity were quantitatively assessed using the wave attenuation constant that describes the spatial decay of the waves in granular crystals. It was noteworthy that the wave decay for the granular crystals immersed in low viscosity fluids increased with increasing fluid viscosity. While this trend for low viscosity secondary fluid was consistent with other literature reports, the wave decay characteristics for highly viscous secondary fluids exhibited a rather unexpected trend with an increase in viscosity resulting in wave attenuation. Thus, the dissipative effects due to secondary fluid are quite complex and cannot be described using the existing numerical models for immersed granular media that rely only on the viscous drag effects. Finally, the utility of adaptive granular crystals immersed in active fluids for impact mitigation was illustrated through a series of low-speed impact experiments with granular crystal immersed in an MR fluid subjected to an external magnetic field. The magnetic field of varying strength was applied using a strong DC electromagnet at a specific spatial location in the granular crystal immersed in a typical MR fluid. These experimental measurements for the kinematics of the individual grains in the adaptive granular crystal established that the presence of a magnetic field resulted in the formation of localized rheological defect that has a significant influence on the wave propagation in granular media. The magnitude of the magnetic field had a significant influence on the wave scattering and attenuation characteristics in granular crystals immersed in MR fluids. Thus, this clearly illustrates that the application of external magnetic fields in adaptive granular metamaterials represents an effective method to control the direction and magnitude of wave propagation during impact loading.
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