Simulation of Lamb Waves Excited by an Air-Coupled Ultrasonic Phased Array for Non-Destructive Testing

Lamb waves are widely used in non destructive evaluation (NDE) applications for inspection of sheet materials or pipes. However, simulation models of wave excitation focus on frequency thickness products above 1 MHz.mm and higher order Lamb wave modes. We use our custom ultrasonic phased array (UPA), in order to excite Lamb waves without contact and at much lower frequencies thickness products of 40 kHz.mm. We modeled our setup and simulated the steering capabilities of the array and the resulting airborne excitation of the zero order Lamb wave modes in a 1 mm-thick specimen. The simulation results of the coupling angle and wave propagation at defects are in good agreement with our measurements. We simulated an optimum steering angle of 37.5 degrees the measured angle was at 34.3 degrees. Both, simulation and measurement show the beamforming characteristic of the UPA with several local maxima. The time delay at the point of measurement between the Lamb wave and its reflection at the defect are within the uncertainty of the measurement. The simulation enables us to predict measurements at different positions, and, thus, allows us to improve sensor placement in the setup.