Impact behavior of negative stiffness honeycomb materials
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Negative stiffness honeycombs are architected metamaterials that utilize elastic buckling to absorb mechanical energy. Relative to conventional honeycomb materials, they offer several advantages, including the ability to recover their initial configuration and offer consistently repeatable mechanical energy absorption. In this paper, fully recoverable negative stiffness honeycombs are fabricated from thermoplastic and metallic parent materials. The honeycombs are subjected to quasistatic and impact loading to demonstrate the predictability and repeatability of their energy absorption characteristics across a variety of loading conditions. Results indicate that these honeycombs offer nearly ideal shock isolation by thresholding the acceleration of an isolated mass at a predetermined level and that this thresholding behavior is highly repeatable as long as the magnitude of the mechanical energy imparted to the system does not exceed the energy absorption capacity of the honeycomb. I. INTRODUCTION
Negative stiffness honeycombs are periodic arrangements of unit cells that buckle elastically when subjected to mechanical loading that exceeds a designed threshold. In the configuration considered in this paper and illustrated in Fig. 1(a), the honeycomb comprises multiple rows and columns of curved beams that are extruded out of the plane of the honeycomb. As the honeycomb is subjected to in-plane compressive loading [Fig. 1(b)], the honeycomb exhibits positive in-plane mechanical stiffness followed by a region of serrated plateau stress in which each row of the curved beams successively snaps from one first-mode-buckled shape to another. Under displacement-controlled loading, this snap-through behavior manifests itself as regions of negative stiffness in the force–displacement plot. The number of snapthrough events is equivalent to the number of rows of curved beams in the honeycomb and the order in which the snap-through events occur depends on minor imperfections in the as-built honeycomb. The magnitude of the force threshold is directly proportional to the number of columns of curved beams and depends on the shape of each beam, specifically its length, in-plane thickness, outof-plane depth, and apex height (see Refs. 1–3). If the geometry is properly designed, the honeycomb will return to its initial configuration upon release of the load. As described in previous publications,1,2 these negative stiffness honeycombs provide several advantages for shock isolation applications. Like conventional positive
Contributing Editor: Lorenzo Valdevit a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.7
stiffness honeycombs,4–6 negative stiffness honeycombs exhibit a region of positive stiffness followed by a region of plateau stress as the rows of curved beams buckle sequentially. This region of plateau stress provides nearly ideal shock isolation, which is defined as energy absorption at a constant force level or threshold.7 Force thresholding allows the honeycomb to absorb energ
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