Measurement of additively manufactured anisotropic three-dimensional underwater pentamode materials

Abstract

Pentamode (PM) materials are three-dimensional (3D) elastic lattice materials that can be designed to match the acoustic impedance of water while also minimizing shear modulus of the sample over wide frequency ranges. Further, the lattice structure provides the degrees of freedom necessary to produce strongly anisotropic stiffness that is useful for transformation acoustics [Su et al., J. Acoust. Soc. Am. 141(6) (2017)]. In the current work, anisotropic sub-wavelength 3D metallic PM samples were fabricated using additive manufacturing for measurement of their effective material properties. The samples were distributed uniformly in a one-dimensional, water-filled resonator tube. An electrodynamic shaker excited the system and the system response was recorded using a hydrophone positioned near the top of the tube. The resulting resonance frequencies were then used to infer the phase speeds for each mode in the fluid-filled elastic waveguide. Two effective medium models were used to infer the PM-water mixture properties: (i) Wood-Mallock mixture law and (ii) a self-consistent micromechanical model that explicitly considers material anisotropy. Experimental results are compared with simulations and observations are drawn on material property extraction using the resonance tube technique when elastic anisotropy is present. [Work supported by ONR.]