THE GENERATION, PROPAGATION, AND DETECTION OF LAMB WAVES IN PLATES USING AIR-COUPLED ULTRASONIC TRANSDUCERS

Air-coupled nondestructive testing has become feasible following recen t improvements in air-coupled transducer design. However, the large ac oustic impedance mismatch between air and solid materials does not all ow normal incidence pulse-echo inspection. Nevertheless, air-coupled t ransducers can be used for the generation and detection of Lamb waves, the receiver being outside the field of the specular reflection. A fi nite element study of the generation of Lamb waves in plates from a fi nite air-coupled transducer, the interaction of these waves with defec ts, and their detection using an air-coupled receiver is described. Th ese predictions are compared with experimental results obtained on a v ariety of specimens using a pair of 1-3 composite, air-coupled transdu cers. The use of an ideal collimated beam in the model, instead of usi ng the real pressure field generated by the transducers, is demonstrat ed to have a negligible effect on the predicted Lamb waves, it is show n 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 degrees reduci ng the amplitude by around 50%. The detection of notches of various de pths in steel plates is investigated, and the sensitivity of the techn ique with different choices of incident mode and different positions o f the receiver with respect to the defect is discussed. It is shown th at in metal structures it is most satisfactory to use the a(0) mode si nce it has a large out-of-plane surface displacement and so is easy to excite. (C) 1996 Acoustical Society of America.