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The detection of crack initiation in steel Q235, 16MnR and 20 g was carried out by metal magnetic memory (MMM) technique. Fifty specimens of welded and base metal were tested. Both MMM testing and metallographic examination were done to them when unloaded at the different phase of high‐cycle fatigue testing, and hence MMM signals before and after crack initiation could be recorded. The values of magnetic intensity gradient were calculated, and whose critical value was determined and proposed for early defects detection. The results show that the magnetic intensity (Hp) curve became concave responding to the occurrence of stress concentration, and its gradient (dHp/dx) increased greatly; at the critical dHp/dx, no change occurred to the microstructure, but beyond the critical value, dHp/dx increased suddenly and a large number of intragranular slips were found in the microstructure. Three different kinds of materials have different critical values of magnetic intensity gradient.

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