Ultrasonic Lamb wave tomographic scanning

Lamb waves are guided ultrasonic waves capable of propagating relatively long distances in plate-like structures such as airframe skins. Their propagation depends on frequency-thickness and material properties, and because structural flaws present changes in effective thickness and/or material properties Lamb waves can be employed to assess the integrity of these structures. For aging aircraft structures a full integrity evaluation can be a time-consuming operation, but with Lamb wave techniques this evaluation can be performed with waves propagating along one dimension of the inspection area as the probing transducer pair is moved in the perpendicular dimension. Such an approach yields information about the presence of flaws within the scanned area. Then, in order to quantitatively characterize the flaws, Lamb wave measurements can be made for a number of projections and an image of the flawed region can be reconstructed tomographically In this paper, contact scanning Lamb wave tomography for metallic aircraft structures with flaws is discussed as a practical technique for quantitative nondestructive evaluation.