Investigation of Magnetic Memory Signals Induced by Dynamic Bending Load in Fatigue Crack Propagation Process of Structural Steel

Metal magnetic memory effect, induced by applied stress under the excitation of the geomagnetic field, has attracted a lot of attentions due to its unique advantages of stress concentration identification and early damage detection for ferromagnetic materials. To further investigate the regularity of magnetic memory signals in the fatigue crack propagation process under the dynamic bending load, the surface magnetic field intensity \(H_{p}(y)\) of ferromagnetic structural steel was measured throughout the dynamic three-point bending fatigue tests; variation of \(H_{p}(y)\) and its maximum gradient \(K_{max}\) were studied; meanwhile the possibility of using \(K_{max}\) to predict the fatigue crack propagation was discussed. The results showed that \(H_{p}(y)\) was relatively stable at different loading cycles and its maximum value appeared at the fatigue crack area before the specimen fractured; instead the \(K_{max}\) increased exponentially with the increase of loading cycles, and an approximate linear relationship was found between \(K_{max}\) and crack length 2a. The cause for this phenomenon was also discussed.