The generation, propagation, and detection of Lamb waves in plates using air-coupled ultrasonic transducers

Air-coupled nondestructive testing has become feasible following recent improvements in air-coupled transducer design. However, the large acoustic impedance mismatch between air and solid materials does not allow normal incidence pulse-echo inspection. Nevertheless, air-coupled transducers can be used for the generation and detection of Lamb waves, the receiver being outside the field of the specular reflection. A finite element study of the generation of Lamb waves in plates from a finite air-coupled transducer, the interaction of these waves with defects, and their detection using an air-coupled receiver is described. These predictions are compared with experimental results obtained on a variety of specimens using a pair of 1-3 composite, air-coupled transducers. The use of an ideal collimated beam in the model, instead of using the real pressure field generated by the transducers, is demonstrated to have a negligible effect on the predicted Lamb waves. It is shown both theoretically and experimentally that the measured amplitude of the Lamb waves is very sensitive to the alignment of the transducers with respect to the test structure, misalignment of 0.6° reducing the amplitude by around 50%. The detection of notches of various depths in steel plates is investigated, and the sensitivity of the technique with different choices of incident mode and different positions of the receiver with respect to the defect is discussed. It is shown that in metal structures it is most satisfactory to use the a0 mode since it has a large out-of-plane surface displacement and so is easy to excite.