Lamb waves excited by an air-coupled ultrasonic phased array for non-contact, non-destructive detection of discontinuities in sheet materials

Lamb wave excitation in sheet materials is possible using non-contact, non-destructive evaluation methods such as air-coupled transducers, pulsed Laser excitation and electro magnetic acoustic transducers (EMATs). However, these methods are limited to specific groups of materials or lack flexibility when testing specimens with different geometries or compositions. In previous work we demonstrated air-coupled Lamb wave excitation using a custom ultrasonic phased array (UPA). In this work we utilized the UPA for localizing two different discontinuities in a steel sheet specimen. The steering capability of the UPA is used in order to find the optimum coupling angle. The time of flight (TOF) of the Lamb wave, reflected at the discontinuities, is measured using a laser Doppler vibrometer (LDV). The defect location is calculated using the group velocity derived from the coupling angle and the theoretical value from the dispersion diagram. The setup is characterized and our results show mean localization errors of 1.39 mm and 1.42 mm at distances of up to 800 mm using the optimum coupling angle. This can be further optimized using a finer angular step size. This way, the system is suitable for a wide range of non destructive evaluation (NDE) applications.