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

Abstract

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°r educing 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 a 0 mode since it has a large out-of-plane surface displacement and so is easy to excite.

Air-coupled ultrasonic through-transmission thickness measurements of steel plates

“…Previous work on steel consists of imaging of spot-welds [32] and excitation of the lowest order antisymmetric mode for imaging of defects [33]. Further applications of air-coupled ultrasound can be found in the review by Chimenti [12].…”

Characterisation of hidden defects using the near-field ultrasonic enhancement of Lamb waves

“…The use of ultrasonic waves with displacements throughout the thickness of a material, such as Lamb waves in sheets and guided waves in pipes [14,15] , enables inspection of areas of the system that cannot be accessed directly, such as the internal surface of a pipe [8,16] . Several long-distance ultrasonic inspection methods exist that monitor in the defect far-field (defined as the region starting at a distance of several wavelengths away from the defect [17] ) through changes in the reflection or transmission of guided waves as they interact with surfacebreaking defects, and these methods are capable of estimating the position and depth of defects over a distance of several metres [8,9] .…”

Crack growth monitoring at CFRP bond lines

“…2a)). This slanted transmission mode projects a periodic pressure distribution on the surface of the plate, which leads to a lamb wave [6]. This lamb wave emits air-coupled ultrasound on both sides of the plate in the same angle as the transducer (Fig.…”