Eddy current thermography detection method for internal thickness reduction in ferromagnetic components based on magnetic permeability perturbation

Abstract

Eddy current thermography (ECT), as an emerging nondestructive testing (NDT) technique, has been used for defect detection in many critical components. However, the skinning effect of eddy currents limits the ability of ECT to detect internal defects in thick-walled pipes. An ECT detection method for thickness reduction of ferromagnetic components based on magnetic permeability perturbation (MPP-ECT) under DC magnetization is proposed. The thickness reduction cause MPP phenomenon on the surface of ferromagnetic components. Then under high-frequency AC excitation, the thinning area affected by MPP will produce a different thermal response from the normal area, which is recognized and captured by an infrared camera. The mechanism of MPP-based thinning defect detection is analyzed through a theoretical model, and the relationship between thinning thickness, relative permeability and thermal response is established. The feasibility of the MPP-ECT detection method is verified through a series of simulations and experiments. The experimental results show that the method can effectively detect the thinning defect of 4.2 % wall thickness on the back of 12 mm thick specimens. The thermal response of both the thinning and normal areas decreases with increasing magnetization intensity, and the thermal response of the thinning area decreases with increasing thinning thickness. However, the thermal contrast (peak-to-peak value of thermal response) between the two regions increases with the increase of magnetization intensity and thinning thickness. This method can be used for detection under high lift off and weakens the skin effect of ECT for the internal thickness reduction, which has great practical value.