2.5D and conformal negative stiffness honeycombs under static and dynamic loading

Date
2019-05-13
Authors
Debeau, David Alexander Robbins
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Abstract

Negative 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.

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