Investigation of DC Electromagnetic-Based Motion Induced Eddy Current on NDT for Crack Detection

Cracks caused by long-term cyclic loading and harsh working environment may seriously lead to the catastrophic industrial safety accidents. Therefore, a quantitative and accurate characterization for the cracks is necessary to ensure safety of the key component (e.g. rolling contact fatigue of high speed rail track in particular). This article investigates the motion induced eddy current (MIEC) of a direct current (DC) electromagnetic nondestructive testing (NDT) and proposes an approach to quantitative characterization the cracks in moving ferromagnetic material. The velocity effect as well as the distribution of MIEC of the ferromagnetic material under the DC electromagnetic probe are investigated firstly. Then, the relationships between the crack depth/width and detection signals are discovered. Finally, a quantitative characterization method of crack depth/width is proposed by experiment. The investigations indicate that for DC electromagnetic NDT, the dragging effect is related to the rate of the MIEC diffusion time, which is inversely proportional to the conductivity and permeability of metal, therefore, the higher speed, the more obvious the dragging effect is. The experiments show that the crack depth/width can be characterized quantitatively by the peak separation and differential value. The investigation in this article show that the DC electromagnetic based on MIEC can be used not only for the crack detection in moving ferromagnetic metals such as bearings, gears, etc., but also for the quantitative characterization of cracks when there is rapid relative motion between the detection devices and the ferromagnetic metals (e.g., rail, pipe, etc.).