Computerized tomography (CT) algorithms have been used mainly in the medical field but their powerful capabilities are being exploited more and more in industrial applications. This paper demonstrates that the technology is capable of detecting material loss on real aircraft components using embedded piezoelectric sensors on hidden surfaces. The work is novel in more than one respect. Firstly, it demonstrates that Lamb wave ultrasonic tomography can be used to accurately map material loss on an exposed aircraft surface with sensors embedded on the structure's hidden surface. Hidden, in this case, refers to the surface that is not exposed to the atmosphere-the underneath of an aircraft wing, for example. Secondly, it compares tomographic images generated by fan-beam back projection and the signal difference coefficient methods, showing clearly that the latter are more sensitive to material loss.

Variations of Lamb wave propagation reflect some changes in effective thickness and material properties caused by such structural flaws as corrosion, fatigue cracks, disbonds and voids that can then be mapped via a reconstructed tomographic image. Ultrasonic Lamb wave tomography can be used to evaluate structural integrity based on the variations in features extracted from measurements made by a transducer array from a reference point in time. In this paper, several tomographic imaging techniques, such as the filtered backprojection algorithm, the algebraic reconstruction technique and the reconstruction algorithm for probabilistic inspection of damage are compared, and the advantages and drawbacks of these methods, as well as practical considerations such as reconstruction fidelity, quality, efficiency and the minimum number of sensors required for each array geometry, are discussed, and some application examples are given.

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Ultrasonic guided waves, due to theirs capability of interrogating a large structure from a single sensor position, has been proven as a promising tool for structural health monitoring (SHM). In this paper, we present two imaging approaches of utilizing guided wave leave-in-place sensors for early detection of defects in plate-like structures as well as for monitoring the defect growth. The first approach is based on a guided wave tomographic technique, in which the region surrounded by a sparse sensor array is monitored. The second one is a phased array approach, in which sensors are attached to a structure in a compact format to form an array. The region subjected to inspection and monitoring is the region outside the array. Both techniques have shown excellent capability of determining damage size, location, and severity.