Browsing by Subject "Impact mitigation"
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Item 2.5D and conformal negative stiffness honeycombs under static and dynamic loading(2019-05-13) Debeau, David Alexander Robbins; Seepersad, Carolyn; Haberman, Michael R; Kovar, Desiderio; Roach, AllenNegative stiffness honeycombs have been shown to provide nearly ideal impact mitigation with elastically recoverable configuration and mechanical behavior. This capability allows for reliable mitigation of multiple impacts, which conventional honeycombs cannot accommodate because of plastic deformation and collapse. A more in-depth characterization of the mechanical behavior of these negative stiffness honeycombs is presented. The starting point is a 2.5D configuration in which the negative stiffness honeycomb configuration is varied in-plane and extruded out-of-plane. Impact mitigation is investigated by subjecting the 2.5D honeycombs to various drop heights on a purpose-built, drop-test rig. Several embodiments of the 2.5D honeycomb are designed and tested, including nylon versus aluminum, constrained versus unconstrained, and altered configurations with different numbers of rows and columns of negative stiffness elements. While the 2.5D configuration performs well in response to in-plane loading, it is not designed to accommodate out-of-plane loading. A conformal negative stiffness honeycomb design is introduced that conforms to curved surfaces and accommodates out-of-plane loading that is not orthogonal to the load concentrator on top of the honeycomb. Quasi-static mechanical and dynamic mechanical impulse testing of the conformal honeycomb are conducted to characterize the mechanical performance of the conformal design. The final chapter includes a multi-element study that demonstrates how multiple elements perform in an assembly in a more realistic setting. A FEA framework is built to automate the simulation of the 2.5D and conformal negative stiffness honeycomb designs. The framework is built within the commercial Abaqus® FEA package using its Python scripting interface. Automating the design, meshing, loading, and boundary conditions allows for rapid design iteration. Simulations using the FEA framework are compared to experimental quasi-static, impact, and impulse tests. The conformal design was developed to be manufactured additively. The additive manufacturing process introduces sources of potentially significant geometric and material property variability that affect the performance of the honeycombs. The FEA framework is used to conduct a predictability and reliability study that incorporates several sources of variability into the analysis and returns estimates of the expected force threshold and its distribution.Item On the dynamic response of polymeric foams(2019-12-06) Fulton, Andrew Harvey; Ravi-Chandar, K.This study examined the mechanical response of four polymeric foams under quasi-static and dynamic loading conditions to explore the protective impact mitigation applications of polymeric cellular materials. Polyethylene, polyurethane, and polystyrene, all with relative densities between 1.5-3%, and a bilayer polymeric foam with a trademarked name Zorbium were the four foams analyzed. With the extensive use of image analysis techniques (digital image correlation), the quasi-static local strain behavior of these cellular materials was determined. The dynamic impacts, using a gas gun apparatus and high-speed camera imaging in direct impact experiments, were evaluated to study the dynamic shock response. The experiments generally show a much greater dynamic stress response for the foams compared to their quasi-static responses and suggest a significant difference from the slow rate compressive response. Higher impact velocities resulted in shock formation and propagation. Using conservation of momentum and Rankine-Hugoniot jump conditions, the stresses in the foams were determined. The Shock-Hugoniot of impact velocity and shock wave speed was generated from the experimental results to characterize the high-speed dynamic response of the four foams. The two-layer composite foam, with differing stiffnesses and similar pore sizes, in slow and fast impact experiments, is discussed and analyzed using similar methods used for polyethylene, polyurethane, and polystyrene foams. Finally, the impact problem is examined numerically using the method of characteristics; the results of these simulations are used to determine the force transmission characteristics of protective foam layers. This method is shown to be an effective tool for design