3D-printed phononic crystal filters for second harmonics separation in nonlinear ultrasonic non-destructive testing

Higher harmonic method is widely employed to detect and evaluate early material degradation in nonlinear ultrasonic non-destructive testing. However, the separation of extremely weak nonlinear harmonics (especially second harmonics) from received signals represents a significant challenge. Therefore, it is feasible to use the bandgap property of phononic crystals to achieve second harmonics separation. In this paper, numerical simulations are performed to verify the filtering capability of the filters. The influence of the unit-cell's spatial arrangement in the filter is also numerically investigated. Meanwhile, 3D-printed cubic-structured phononic crystal filters are employed to experimentally demonstrate the efficient filtering capability for separating. It is found that the accuracy of 3D-printing can significantly affect the filtering capability. The 3D-printing accuracy would be increased with the filtering frequency of the filter. This study provides numerical and experimental basis for the application of the 3D-printed cubic-structured phononic crystal filter in NDT.