Optimal Frequency of AC Magnetic Flux Leakage Testing for Detecting Defect Size and Orientation in Thick Steel Plates

AC magnetic flux leakage (ACMFL) testing is a novel method for the detection of defects of metallic materials. Conventional ACMFL probes excited by a single-frequency (SF) sine signal have limitations in accurately detecting the defect size and identifying the defect orientation. In this article, a multifrequency ACMFL (MF-ACMFL) method is proposed to address these issues. First, we give a thorough analysis of the effects of excitation frequencies on magnetic flux density under different defect sizes and orientations using the finite-element method. The numerical results indicate that the optimal frequencies are 400 and 500 Hz to detect the defect size, and that to identify the defect orientation are approximately 400-500 Hz. Then, we compare the receiver signal under SF and MF excitations and estimate the size of the defect including its length, width, and depth under oblique angles of 0 degrees and 30 degrees, whose results validate the effectiveness of the proposed method. Finally, measuring the depths and orientations of the defect on the thick steel plate verifies the correctness of the numerical results and the advantage of the proposed method in the orientation identification of the defect. Thus, the proposed scheme is an effective complementary measure for the existing ACMFL tools.