Improved Nonlinear Ultrasound Sensing based on 3D Printing Phonon Crystals

Nonlinear ultrasound is widely popularized by its high sensitivity to the micro-cracks and material damage at the early stages by sensing the second harmonic. However, the inherent nonlinear characteristics generated by equipment is the main bottleneck for making full use of nonlinear ultrasound. Here we adopt the phonon crystals plate as mechanical filter to eliminate the influence of the inherent nonlinear high harmonic. The metamaterial unit cell contains the PLA mass block and connective layer that can be fabricated by 3D printing. A strong resonance effect occurs between the mass block and Lamb waves and a stop band is opened due to the local resonance mechanism. On the contrary, the connective layer is used to make the array into single unit. Dispersion and transmission curves are calculated to analyze the bandgap theoretically. The experiments on the aluminum plate are carried out to verify the bandgap transmission and the elimination of inherent nonlinear high harmonic influence. The results show that phonon crystals can effectively suppress the influence of the inherent nonlinear harmonics and have great potential in simple, automatable, and accurate nonlinear ultrasound structural health monitoring and nondestructive testing applications.