Mutual Impedance of Eddy-Current Coils Above a Second-Layer Crack of Finite Length
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A model is presented to determine the change in mutual impedance between a driver and pickup coil due to eddy-current interaction with a crack in a conductive two-layer plate system where the plates are separated by a non-conducting layer of constant thickness. Here, the impedance change of interest arises from a through-thickness crack of finite length located in the hidden lower plate. The model assumes that both conducting plates are electromagnetically thin and is therefore valid for large electromagnetic skin-depth. The model predictions are compared with experimental measurements using a pair of cylindrical air-cored coils arranged to simulate an eddy-current sliding probe. Agreement between the calculated and measured impedance change is obtained over the frequency range 10 Hz–3 kHz for which the plates can be treated as electromagnetically thin, and reasonable qualitative agreement was obtained at higher frequencies. A method for computing the induced current streamlines in the presence of the crack is also described and visualisation of the computed streamlines is used to assist in the interpretation of the impedance change for different probe orientations. These results provide a valuable initial insight into the interaction between a buried crack in a multilayer structure and the electromagnetic fields induced and detected by eddy-current sliding probes.
KeywordsEddy-current testing Driver-pickup coils Mutual impedance Crack modelling Nondestructive testing
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Conflict of interest
The authors declare that they have no conflict of interest.
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