PARALLEL PROJECTION AND CROSSHOLE LAMB WAVE CONTACT SCANNING TOMOGRAPHY

Lamb waves are guided ultrasonicwaves capable of propagating relatively long distances in thin plates and thin laminated structures, such as airframe skins, storage tanks, and pressure vessels. Their propagation properties in these media depend on the vibrational frequency as well as on the thickness and material properties of the structure. Structural flaws such as disbonds, corrosion, fatigue cracks, and voids represent changes in effective thickness and material properties, and therefore measurement of variations in Lamb wave propagation can be employed to assess the integrity of these structures. Lamb wave measurements can be made for a number of relative transducer positions (projections) and an image of the flawed region can be reconstructed tomographically. This paper presents a new technique in which two contact piezoelectric transducers are independently scanned along parallel lines in a fashion analogous to that commonly used in seismic crosshole tomography. These results are compared to those for parallel projection Lamb wavetomography data collected with an automated contact scanning apparatus. The advantages and drawbacks of these two methods in the development of automated tomographic Lamb wave scanners for quantitative mapping of thickness variation in platelike materials are discussed.

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