A Reflective Eddy Current Probe With New Structure Applied To Crack Detection

As to nondestructive testing of the metal parts with insulation wrapped layer such as main poles of electric locomotive, the eddy current testing is the first choice. So in this paper a new structure of eddy current probe was designed where the reflective detection principle was used. The excitation coil and the detection coil winded separately, at the same time, the detection coil was perpendicular to the exciting coil. So the new structure of eddy current probe can greatly reduce the impact of the detection coil output caused by drive coil magnetic field. The optimal structural parameters of probe were obtained by theoretical derivation and simulation. Experimental system was constructed. The influence of excitation signal frequency, lift-off, crack depth and crack width on the output of detection coil were discussed by experiments.Based on the proposed probe structure in this paper, the simulation indicates that as the coil become thicker, the sensitivity of the probe is decrease, as the coil diameter become larger, the linear range of the probe is increase, but the sensitivity will be reduced. Inside diameter has little or no effect on sensitivity and linearity.Through experiments, there are some valuable conclusions as follows,(1) The peak and phase difference of output signal are rise with the increase of driving frequency, which is coincident with the theoretical results. When the driving frequency is 3KHz, there is a higher sensitivity for the selected probe parameters and detection objects.(2) Regardless of cracks, the peak of output signal is rise with the increase of the lift-off, while the phase difference of detection and incentive signal have little effect with the increase of the lift-off.(3) The peak and phase difference with cracks are greater than that without cracks, it can be judged that whether there is a crack according to the sudden change of the peak and phase difference under the conditions that the fluctuation of lift-off is small.(4) The peak and phase difference of output signal are rise with the increase depth of crack.(5) The peak and phase difference of output signal increase with the increase width of crack, but the value of change is small.At present, the maximum lift-off of the designed probe is only up to about 3.5mm. It deserves how to further optimize the parameters of probe, improve the shape and arrangement of detection coil for enhancing the detection sensitivity and the maximum height of lift-off.