A Novel Eddy Current Method for Defect Detection Immune to Lift-Off

Abstract

Eddy current (EC) sensors have been applied to detect defects in many years. Since both defects and the lift-off variation will lead to measurement signal fluctuation, it is necessary to distinguish the defect response and lift-off variation response. In this work, a novel eddy current method is proposed for defect detection immune to lift-off via a triple-coil structure. Firstly, a simplified theoretical model is built to investigate the relationship between the lift-off and the inductance variation. It is found the lift-off is proportional to the logarithm of inductance variation in real part under high frequency. In terms of sensor design, the triple-coil sensor consists of two pairs of transmitting-receiving coils, from which one of the receiving coils is larger than the other one. With such sensor structure, the small size coil aims to detect the defect, while the larger size coil aims to calibrate lift-off. The proposed method is verified by experiments with the sensor scanning over a Titanium specimen with defects of different sizes. The results indicate the proposed method can achieve efficient and precise defect detection immune to lift-off.