Metal additively manufactured phononic materials as ultrasonic filters in nonlinear ultrasound measurements

Nonlinear ultrasound (NLU) is a nondestructive evaluation method that is sensitive to damage at length scales well below those detected by conventional ultrasonic methods. Micro- and nano-scale damage correlates to the second harmonic generated by a sinusoidal wave as it propagates through a material. However, NLU measurements are plagued by experimentally-induced nonlinearities and require careful calibrations that have limited them to laboratory measurements. Here, we propose the use of additive manufacturing (AM) phononic materials with ultrasonic filtering properties to reduce extraneous nonlinearities. To do this, finite element simulations were first used to design and analyze phononic materials to transmit an ultrasonic wave but forbid the propagation of its second harmonic. Phononic filters were then fabricated with AM and experimentally characterized in the ultrasonic regime. Results show that the phononic materials behave as low-pass filters, where the cut-off frequency is controlled by the unit cell geometry and also influenced by defects and microstructure from the AM process. Finally, the phononic filters were incorporated into NLU measurements, demonstrating the removal of extraneous nonlinearities and thus better isolating second harmonic generation in a test sample. This work suggests that AM phononic materials could improve NLU and other nondestructive evaluation measurements.