This paper focuses on engineering application of metal magnetic memory (MMM) method. High-cycle fatigue test were applied to a large number of defect-free samples, and MMM signals were captured before fatigue fracturing. A concept of critical magnetic intensity (Hp) derivative (dHp/dx) of cracking was proposed. To study the characteristics of Hp and its derivative (dHp/dx) during cracking, fatigue test were applied to a total number of 50 pieces of Q235, 16MnR and base metal and welded specimens. In different fatigue periods, MMM signals were tested and microscopic metallographic were observed, so as to quantitatively analyze Hp and dHp/dx from both macro and micro angles. The results show that various materials differ in their critical dHp/dx limit, which could be the fatigue crack initiation time. It thus proves that MMM has special efficiency in the diagnosis of premature defects and stress concentration of metal structures.

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