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 Hp(y)\documentclass[12pt]{minimal}
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\begin{document}$$H_{p}(y)$$\end{document} of ferromagnetic structural steel was measured throughout the dynamic three-point bending fatigue tests; variation of Hp(y)\documentclass[12pt]{minimal}
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\begin{document}$$H_{p}(y)$$\end{document} and its maximum gradient Kmax\documentclass[12pt]{minimal}
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\begin{document}$$K_{max}$$\end{document} were studied; meanwhile the possibility of using Kmax\documentclass[12pt]{minimal}
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\begin{document}$$K_{max}$$\end{document} to predict the fatigue crack propagation was discussed. The results showed that Hp(y)\documentclass[12pt]{minimal}
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\begin{document}$$H_{p}(y)$$\end{document} was relatively stable at different loading cycles and its maximum value appeared at the fatigue crack area before the specimen fractured; instead the Kmax\documentclass[12pt]{minimal}
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\begin{document}$$K_{max}$$\end{document} increased exponentially with the increase of loading cycles, and an approximate linear relationship was found between Kmax\documentclass[12pt]{minimal}
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\begin{document}$$K_{max}$$\end{document} and crack length 2a. The cause for this phenomenon was also discussed.
