Magnetic method is one of prominent candidates for NDE method to detect the degradation before the initiation of cracking. Because of the magneto-elastic coupling between domain wall and lattice defects, the hysteresis loop is affected by the strain field produced by lattice defects such as dislocations, grain boundaries, and impurity atoms. Extensive works revealed simple relationships between dislocation density and magnetic properties of the major hysteresis loop that the coercive force H{sub}c is in proportion to the square root of dislocation density ρ. Nevertheless, the relation of minor loops to lattice defects was less studied in detail because of strong dependence of minor loops on magnetic field amplitude.
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