Design and study of novel square electromagnetic sensing based on stepped eddy current thermography

Abstract

Nondestructive testing of internal corrosion defects is a key task to evaluate whether the material is repaired and replaced. In this paper, a novel square ferrite magnetic open sensing structure is designed for detecting volumetric corrosion defects on the inner surface of oil and gas storage tanks. A theoretical model of the structure is developed and the characteristics of the guided distribution of the electromagnetic field are resolved. The proposed model provides a region of interest with a uniform magnetic field. This greatly improves the detectability and thermal comparison of corrosion defects on the inner surface. In addition, the inspection is performed under weak excitation conditions and in the full open field of view of the infrared camera. This configuration greatly improves the portability and efficiency of workpiece inspection. Several open architecture sensors are evaluated and compared regarding temperature rise uniformity, model passability, and lightweight level. Meanwhile, the geometrical parameters of the square sensor, the operating parameters, and the material properties of the detected samples are investigated, including the coil height, the cross-arm length, the lifting distance, the operating frequency, the relative magnetic permeability of the sample, and the electrical conductivity of the sample. Finally, the detection capability of the sensors was verified through experiments with different steel plate thicknesses and defect morphologies.