Nonlinear Ultrasonic Detection Enhanced by 3-D Printed Phononic Crystals

Nonlinear ultrasonic methods are a novel nondestructive testing (NDT) and evaluation technology due to their high sensitivity to microdefects. However, the inherent nonlinearity of ultrasonic equipment hinders the feature extraction with microdefects information and limits the full-scale use of nonlinear ultrasonic NDT techniques. In this work, we propose a material-enabled filter using phononic crystals (PCs) as functional units to eliminate the unapplicable inherent nonlinearity component. The PCs composed of mass blocks and connective layers symmetrical about the host plate are simultaneously processed by melting deposition of polylactic acid (PLA). The finite element method with Bloch-Floquet boundary condition is implemented to obtain the bandgap at 100 kHz. Based on the mass-spring model, the mechanism of bandgap formation is proved to be attributed to the negative effective mass density band induced by the local resonance of mass block. The PCs successfully open a propagation wave transmission gap and eliminate the inherent nonlinearity to improve the nonlinear detection of microdefects in the aluminum plate. This work provides a useful methodology for the wide application of PC-based nonlinear detection methods in the NDT field.