Selective Laser Sintering of Negative Stiffness Mesostructures for Recoverable, Nearly-Ideal Shock Isolation

Honeycomb materials are well known for providing lightweight stiffness, strength, and energy absorption capabilities. For most honeycomb materials, energy absorption occurs when individual cells collapse progressively. Although it is possible for honeycombs with very low relative density to collapse via elastic buckling, honeycombs with typical relative densities collapse due to plastic yielding and buckling of the cell walls, such that the energy absorption is nonrecoverable. In this paper, mono-stable negative stiffness unit cells are investigated for constructing honeycomb mesostructures with high levels of recoverable energy absorption. Negative stiffness is achieved by incorporating curved beams into each unit cell. When subject to transverse loading, the curved beams exhibit negative stiffness behavior as they transition from one curved geometry to another in a snap-through type of motion that absorbs energy elastically at a relatively constant plateau stress. The plateau stress at which this energy absorption occurs can be tailored via the geometry of the unit cell. Preliminary experiments indicate that the structures can absorb significant amounts of energy by requiring nearly-constant-force to increase deformation as the structure transitions between snap-through configurations. Unlike traditional honeycombs, the negative stiffness mesostructures are self-resettable and therefore reusable. Using SLS as a means of fabrication, they can also be customized for specific shock events and even functionally graded to offer shock isolation for transient loads of various amplitudes.