Non-contact vibration transmissibility measurements of an additively manufactured pentamode material

Pentamodal (PM) metamaterials are elastic structures that are designed to support purely longitudinal acoustic wave propagation over a wide band of frequencies. Further, these metamaterials can be designed to have anisotropic stiffness and be impedance matched to water [doi.org/10.1121/10.0009161]. Due to the complexity of three-dimensional (3D) PM microstructures, metal additive manufacturing techniques must be employed to physically realize these materials to achieve the specifically tailored impedance and anisotropic sound speeds over the desired frequency range. Previous work designed and built an additively manufactured titanium PM material measuring 98.9x 98.7x87.2 mm3 which was successfully characterized in a fresh-water tank. The current work presents vibration transmissibility measurements on the same PM sample. The structure was mounted on an electro-dynamic shaker which provided a broadband base excitation. Base acceleration and out-of-plane surface accelerations of each exposed face were then measured using point accelerometers and a scanning laser Doppler vibrometer, respectively, and the process was repeated for multiple sample orientations. The observed vibrational modes demonstrate the influence of geometric asymmetry introduced by the truncated domain of the PM lattice. Measurements were compared to a fully-featured finite element model of the sample using COMSOL Multiphysics showing good agreement in mode shapes. [Work supported by ONR]