The influence of a bobbin probe's radial offset on eddy current testing of tubes

Investigation results of the influence of a bobbin probe's radial offset on the eddy current testing of metallic tubes are presented in this research. First, an analytical model of a driver pick-up bobbin probe with a radial offset is formulated by employing the second-order vector potential. Then, the effect of the radial offset on the eddy current density distribution and sensor signal is simulated and quantitatively evaluated. It is found that the radial offset leads to a non-uniform distribution of eddy current density, and this offset accumulates the eddy current density towards the direction of movement of the bobbin probe. As a result, the sensor signal changes and the measurement accuracy is degraded. The simulation results also reveal that this influence becomes more obvious for larger radial offset distances and higher excitation frequencies. This research demonstrates that the influence of the radial offset of a bobbin probe cannot be neglected, and measures are required to be undertaken for a precise measurement of the electromagnetic properties of the tube and an accurate evaluation of tube wall thinning.