A comb transducer model for guided wave NDE

A countless number of guided wave modes at particular frequencies could be selected for a particular NDE problem, each point producing special sensitivities by way of wave structure across the thickness of the component being studied and also specific penetration powers as a result of interface and surface displacement values and subsequent energy leakage into neighboring media. The mode and frequency choice has a strong influence on NDE and flaw detection, classification and sizing potential as well as an ability to propagate guided waves over long distances, despite the presence of coatings and other surrounding media. The approach to mode and frequency selection is therefore crucial, which can ultimately be based on theoretical and/or experimental means. One aspect of a theoretical approach beyond dispersion curve analysis includes theory of elasticity computations of displacement distributions across a structure. Focus can be on achieving in-plane or out-of-plane optimal values on a surface or at a specific location inside a structure in an attempt at flaw analysis or improved penetration power. From an experimental point of view, an angle beam transducer at a specific angle can be used to achieve a particular phase velocity value. Unfortunately, the presence of a phase velocity spectrum due to a transducer source influence, size and velocity pattern, as well as the frequency spectrum itself, often limits the ability to specifically achieve the particular mode and frequency of choice. Multiple modes can be obtained. An alternate transducer choice to the angle beam transducer can be a multiple element array or comb based on various design choices of element size, spacing and pulsing schedules to produce specific modes and frequencies. The purpose of this paper is to present a model and subsequent solution to a boundary value problem that can evaluate the source influence as a function of the comb transducer design parameters. Advantages of the comb transducer, the mathematical model and analysis, and sample experimental results are all presented in the paper along with an insight into future directions.