Dynamic Homogenization of Acoustic Metamaterials with Coupled Field Response

AbstractAcoustic metamaterials (AMM) are heterogeneous materials with dynamic subwavelength structures that can generate useful effective responses of interest to ultrasonic imaging applications such as negative refraction and zero index. Traditional effective medium models fail to capture details of frequency dependent AMM response and can give non-causal properties. This work derives non-local expressions for effective properties for an infinite periodic lattice of heterogeneities in an isotropic fluid using conservation of mass and momentum and the equation of state. The resulting model correctly predicts a causal effective material response by considering coupling between the ensemble-averaged volume strain and momentum fields

Determining the Complex Young’s Modulus of Polymer Materials Fabricated with Microstereolithography

Microstereolithography is capable of producing millimeter-scale polymer parts having micron-scale features. Material properties of the cured polymers can vary depending on build parameters such as exposure time and laser power. Current techniques for determining the material properties of these polymers are limited to static measurements via micro/nanoindentation, leaving the dynamic response undetermined. Frequency-dependent material parameters, such as the complex Young’s modulus, have been determined for other relaxing materials by measuring the wave speed and attenuation of an ultrasonic pulse traveling through the materials. This method is now applied to determine the frequency-dependent material parameters of polymers manufactured using microstereolithography. Because the ultrasonic wavelength is comparable to the part size, a model that accounts for both geometric and viscoelastic effects is used to determine the material properties using experimental data.Mechanical Engineerin

Negative Stiffness Honeycombs for Recoverable Shock Isolation

Negative stiffness honeycomb materials are comprised of unit cells that exhibit negative stiffness or snap-through-like behavior. Under an external load of small magnitude, a negative stiffness honeycomb exhibits large effective elastic modulus, equivalent to those of other standard honeycomb topologies. When the external load reaches a predetermined threshold, the negative stiffness cells begin to transition from one buckled shape to another, thereby absorbing mechanical energy and mechanically isolating the underlying structure. When the external load is released, the honeycomb returns to its original topology in a fully recoverable way. In this paper, theoretical and experimental behavior of negative stiffness honeycombs is explored, based on FEA modeling and experimental evaluation of laser sintered specimens. Additive manufacturing enables fabrication of these complex honeycombs in regular or conformal patterns. Example applications are also discussed.Mechanical Engineerin