Plate thickness and transducer distance dual inversion with dry contact ultrasonic Lamb wave transducers

Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. Recent development in dry contact Lamb wave transducers enables one to efficiently excite Lamb waves without the aid of liquid couplant. However, in order to perform thickness measurement in a plate-like structures, the transducer to transducer distance has to be fixed and known to a good accuracy, which places strict mechanical requirements during actual implementation. In this paper, we propose a novel technique that utilizes spectrum analysis on the receiver signal and invert for both the plate thickness and transducer to transducer distance. A pair of pin transducers are used to excite and detect the A ₀ mode Lamb wave in a plate and the receiver waveform is captured for a windowed FFT. The signal phase changes with both the plate thickness and the transmitter to receiver distance, but with different sensitivity coefficients. Therefore, with proper calibration, we are able to use an optimization algorithm to perform simultaneous thickness and distance inversion. With the relaxed requirement for known transducer to transducer distance, this technique extends the advantages of the dry contact transducers with added implementation flexibility