Design and testing of three-dimensional additively manufactured anisotropic underwater pentamode materials

Three-dimensional (3-D) pentamode (PM) materials are used in underwater applications because they can yield impedance matching with water and eliminate shear modes over a wide frequency range while resisting shear deformation in the static limit. Further, a PM lattice can also yield significant anisotropy in sound speed, which is useful for devices that rely on transformation acoustics [Su et al., J. Acoust. Soc. Am. 141(6) (2017)]. The present work attempts to verify the predicted anisotropic sound speeds of additively manufactured anisotropic 3-D PM samples. A titanium sample approximately 80 × 80 × 60 mm was suspended in front of a plane-wave source in a water tank to measure the time of flight for wavefronts with and without PM samples present. The measurements were conducted using broadband chirp signals generated by the source, while a scanning hydrophone recorded the response in front of the sample at constant depth. Deconvolution methods were used to extract the impulse response at each scan point and changes in the arrival times of the wavefronts were observed. Frequency domain observations were also made to determine impedance matching characteristics and desired quasi-fluid behavior. Experimental and simulation results will be presented and discussed. [Work Supported by ONR.]Three-dimensional (3-D) pentamode (PM) materials are used in underwater applications because they can yield impedance matching with water and eliminate shear modes over a wide frequency range while resisting shear deformation in the static limit. Further, a PM lattice can also yield significant anisotropy in sound speed, which is useful for devices that rely on transformation acoustics [Su et al., J. Acoust. Soc. Am. 141(6) (2017)]. The present work attempts to verify the predicted anisotropic sound speeds of additively manufactured anisotropic 3-D PM samples. A titanium sample approximately 80 × 80 × 60 mm was suspended in front of a plane-wave source in a water tank to measure the time of flight for wavefronts with and without PM samples present. The measurements were conducted using broadband chirp signals generated by the source, while a scanning hydrophone recorded the response in front of the sample at constant depth. Deconvolution methods were used to extract the impulse response at each scan point...