Metal magnetic memory technique used to predict the fatigue crack propagation behavior of 0.45%C steel

• A novel and passive NDT method, i.e. MMMT method, is proposed.
• Both tangential and normal components of magnetic signals were investigated.
• The prediction of crack propagation by abnormal magnetic signals was discussed.
• A linear relationship was found between the parameters and fatigue crack length 2a.
• The parameters can be potentially used to evaluate the crack propagation state.

Monitoring fatigue crack propagation behavior of ferromagnetic components is very important. In this paper, the tension–tension fatigue tests of center cracked tension (CCT) specimens were carried out; the variation regularity of both tangential and normal components of magnetic signals during fatigue process were investigated. The results showed that the initial abnormal signals which appeared at the notch were reversed after cyclic loading. The abnormal magnetic signals became more significant with the increase of fatigue cycles and reversed again after failure. The characteristic parameters, i.e., the peak value of tangential component, Btp, and maximum gradient value of normal component, Km, showed similar variation trends during the fatigue process, which can be divided into three different stages. An approximate linear relationship was found between the characteristic parameters and fatigue crack length 2a. The feasibility of predicting the fatigue crack propagation using the abnormal magnetic signals was discussed. What's more, the variation and distribution of the magnetic signals were also analyzed based on the theory of magnetic charge.