Non-contact ultrasound with optimum electronic steering angle to excite Lamb waves in thin metal sheets for mechanical stress measurements

A non-contact method for measuring mechanical stress in thin steel sheets using air-coupled ultrasound with electronic beam steering is presented. This allows the automatic adjustment of the optimum steering angle to excite the target Lamb wave mode for a wide range of thicknesses of the specimens, which is advantageous compared to conventional setups with fixed angles. By inducing mechanical stress into the metal sheet, a change of group velocity is measurable, as expected. The experimental setup consists of our 2D ultrasonic phasedarray, positioned above a steel sheet, emitting an ultrasound beam at an arbitrary angle. At the opposing end of the steel sheet, a microphone receives the leaky Lamb wave. We electronically sweep the angle of the ultrasound beam, matching the optimal coupling angle for different thicknesses of the steel sheets. Further, we demonstrate a sample NDT application, where we induce mechanical stress into the steel sheet, changing the group velocity in the material. Based on the arrival times measured with an oscilloscope, we calculate the group velocities for variable mechanical stresses. The signals received by the microphone show that the acoustic signal caused by the Lamb wave arrives before the air-coupled component of the ultrasound. The group velocities measured show good agreement to a linear relation for mechanical stresses up to 100MPa. Moreover, the measured group velocities match the expected values based on the calculation of the proper dispersion curves for asymmetric and symmetric Lamb waves for phase and group velocity at 40kHz and the given thickness of the steel sheet.