Abstract
We present a basic analysis that establishes the metrics affecting the energy absorbed by multilayer cellular media during irreversible compaction on either a mass or volume basis. The behaviors at low and high impulse levels are distinguished through the energy dissipated in the shock. The overall mass of an energy absorbing system (comprising a cellular medium and a buffer) is minimized by maximizing the non-dimensional dissipation per unit mass parameter for the cellular medium, Λ ≡ U m ρ s / σ Y , where U m is the dissipation per unit mass of the cellular medium, ascertained from the area under the quasi-static compressive stress/strain curve, σ Y the yield strength of the constituent material and ρ s the density of the material used in the medium. Plots of Λ against the non-dimensional stress transmitted through the medium, σ tr / σ Y demonstrate the relative energy absorbing characteristics of foams and prismatic media, such as honeycombs. Comparisons with these benchmark systems are used to demonstrate the superior performance of micro-lattices, especially those with hollow truss members. Numerical calculations demonstrate the relative densities and geometric configurations wherein the lattices offer benefit. Experimental results obtained for a Ni micro-lattice with hollow members not only affirm the benefits, but also demonstrate energy absorption levels substantially exceeding those predicted by analysis. This assessment highlights the new opportunities that tailored micro-lattices provide for unprecedented levels of energy absorption for protection from impulsive loads.
Highlights
Concepts for protecting structures from impulsive loads are of current interest
The buffer acquires a momentum per area, M, equal to the transmitted impulse per unit area, Itrans, with associated kinetic energy/area, KE M 2 / 2mb [1]
We examine the potential of all known classes of cellular materials for attaining this objective and reveal a new regime wherein energy absorption per unit mass is maximized by using hollow micro-lattices, a consequence of their favorable topological characteristics
Summary
Concepts for protecting structures from impulsive loads are of current interest. The prototypical threat comprises a shock in air characterized by an overpressure p0 with associated impulse/area, I. We examine the potential of all known classes of cellular materials for attaining this objective and reveal a new regime wherein energy absorption per unit mass is maximized by using hollow micro-lattices, a consequence of their favorable topological characteristics. While cellular materials such as stochastic foams and honeycomb are readily available, fabrication of appropriate micro-lattice structures has been challenging. Regime I: When 2 , shock effects in the medium are minimal, and the maximum dissipation/area is quasi-static, with U pl Dh. Equating the dissipation to the kinetic energy gives the minimum thickness to arrest the buffer as: hmin. In regime I, a minimum exists in the total (buffer and cellular medium) mass per area to mitigate the overpressure (Appendix) given by:. By equating the loads required for elastic buckling and yielding, it has been demonstrated that truss-based cellular media are yield (rather than buckling) dominated at relative densities that approximately satisfy [3], 36 Y2
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