Abstract
Alternating current field measurement (ACFM) has been widely used to detect and evaluate metal surface cracks. However, the lift-off caused by various attachments significantly affects the accuracy of crack size assessment. In this article, a simple method is proposed to evaluate surface cracks without the limitation of lift-off. The theoretical models of ACFM and the lift-off effect are developed. The influence of lift-off on magnetic field response signals is studied with the aid of the finite-element method (FEM). The ACFM test system is set up, and the experiments are carried out to evaluate surface cracks of different dimensions at different lift-off distances. The test results show that the length and depth of surface cracks can be accurately evaluated at an unknown lift-off distance by the proposed method.
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