Quantitative Stress Evaluation and Defect Identification in Ferromagnetic Steels Based on Residual Magnetic Field Measurements

Abstract

In this paper, the correlation between the residual magnetic field (RMF) and the applied load is investigated. Tensile tests were carried out to measure RMF signals on the surface of 30CrNiMo8 steel specimens with three types of machined defect shapes. Results show that the RMF curves of the three different defective specimens demonstrate similar overall evolution patterns during the loading process, while the magnetic signals exhibit noticeable differences in the defect area. It suggests that the profiles of the stress-induced RMF curves are strongly dependent on the defect’s shape, notch width, and load level. An improved method is proposed to extract some quantitative characteristic parameters from the magnetic signals. The characteristic parameters that reflect the fluctuation degree are in quadratic polynomial relation with the applied load, which can be potentially used to evaluate the applied load acting on a ferromagnetic material with a macro defect. The characteristic parameters that reflect the acting range seem to be independent of the applied load, and the normal ones are capable of capturing the defect’s location and shape. This paper presents a supplement for quantitative defect identification for discontinuities in ferromagnetic steels by RMF measurements.