Investigation of an additively manufactured pentamode material using non-contact vibration transmissibility measurements

Pentamodal (PM) metamaterials are one potential means to enable concepts from transformation acoustics in underwater applications because they can be designed to support anisotropic longitudinal wave motion over wide frequency bands while simultaneously being impedance matched to water [Su et al., J. Acoust. Soc. Am. 141(6), (2017)]. In three dimensions, PM materials can be fabricated using additive manufacturing (AM) to create elastic lattice structures whose microstructure can be tailored to produce the desired anisotropic sound speed and impedance in a target frequency band. This work presents measurements of the vibrational response of a PM block designed to display anisotropic stiffness. The sample measures approximately 100x100x85 mm and is constructed from titanium using direct metal laser sintering. Measurements were made on a sample mounted on an electro-dynamic shaker providing base excitation between 0.05 and 20i kHz. A scanning laser Doppler vibrometer measured out-of-plane surface velocity with normals both parallel and orthogonal to the excitation axis. The finite extent of the sample introduces geometric asymmetry associated with domain truncation, which is observable in the vibrational modes. Measurements were made for various sample orientations and compared to full-featuredfinite element models. Simulation-experiment comparisons and other findings will be presented and discussed. [Work supported by ONR.]