Article Article
Effect of Defect Depth on Stress Evaluation of Carbon Steel Using the Metal Magnetic Memory Technique

Based on magneto-mechanical effect and equivalent theory, the influence of the depth of precut reference defects on stress was evaluated using the metal magnetic memory (MMM) technique. To evaluate a precut defect in the surface of experimental samples, an equivalent technique is used; a three-dimensional electrically controlled displacement system is employed to keep constant the detection parameters for collecting the normal component Hp(y) signal of the experimental sample; the magnetic intensity gradient, corresponding to the location of the precut defect, is extracted based on the least squares technique; and finally, the change on the magnetic intensity gradient as the depths of the precut defects increase is discussed. The results show that the liftoff of the sensor probe is important for the Hp(y) signal, and 1.0 mm is its optimal value. As tensile load increases, the Hp(y) signal turns anticlockwise around a zero-crossing point, and the magnetic intensity gradient changes linearly; when the tensile load reaches the yield load, the change of the magnetic intensity gradient is less obvious as the tensile load increases further. When the tensile load is the same, the magnetic intensity gradient, corresponding to the location of the precut defect, increases as the depth of the defect increases; thus, the influence of defect depth on stress evaluated with the MMM technique can be corrected. Finally, the experimental results are discussed based on the piezomagnetic effect and the model of the finite depth slit, and the change in orientation of the magnetic domains as tensile load increases is seen as the main reason.


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