Reduction of lift-off effect in pulsed eddy current testing for surface hardness classification of ferromagnetic steel

In the field of pulsed eddy current (PEC) testing, the lift-off effect can mix up the signal features used for surface hardness classification. In order to improve the accuracy of dynamic PEC testing for the hardness classification of steel, we propose a method to reduce the lift-off effect. By acquiring PEC signals under different lift-offs and extracting features, the relationships between each feature and lift-off are studied. According to the relationships, the lift-off effect is reduced by a linear combination of features, and a combined feature for hardness classifi-cation is obtained. Both the simulation and experimental results have shown that the method can effectively overcome the influence of the lift-off fluctuation within 1.2 mm. Based on this method, a new type of PEC probe array with different lift-offs combinations has been designed to simplify the testing process. The proposed method and PEC probe array have been successfully applied in the surface hardness classification of the hub bearing groove, which increases the accuracy by more than 30 % compared with the traditional method.