Evaluation of stress concentration degree of ferromagnetic steels based on residual magnetic field measurements

The aim of this research is to correlate the residual magnetic field (RMF) with the stress concentration degree of ferromagnetic steel subjected to tensile stress. Specimens of X80 pipeline steel were machined into smooth plates with circular holes and rectangular holes for various stress concentration factors. Variations of the tangential component and the normal component of the RMF signals with the applied tensile load were studied. Relationships of the maximum gradient Kmax of the RMF components with the applied tensile load and the stress concentration factor were investigated, respectively. Magnetic parameter β based on the maximum gradient Kmax was introduced to characterize the stress concentration degree. The magnetic parameter β shows significant advantages compared with the maximum gradient Kmax when characterizing the stress concentration degree. Correlations of its average value βm with the stress concentration factor and the defect area were both studied. This research will greatly promote the development of quantitative evaluation method of the stress concentration degree using the metal magnetic memory (MMM) testing.