Measurement of magnetic properties of low alloy steel used as a pressure vessel in nuclear reactors, which were degraded due to tensile deformation and cyclic load, was carried out. Magnetic hysteresis curves change depending on the load condition. In case of the tensile loading, there were three staged magnetic property changes. In the elastic region, magnetic properties have no change. Over the 0.2% yield stress, the residual magnetic flux density and the incremental permeability decrease while there is no change of the coercive force. When plastic deformation proceeds, the coercive force increases with the increasing residual strain while the residual magnetic flux density and the incremental permeability decrease. Those finally reach the saturation with the material work hardening. In case of fatigue degradation accumulated by the cyclic load, the same tendency observed. Based on the above results, we performed the sensitivity analysis of magnetic property changes for the mechanical load. Finally we concluded that the dB/dH at H=0 is the most sensitive among the promising magnetic parameters for early degradation due to the mechanical loads. The feasibility of application of this magnetic technique to the evaluation of degradation due to neutron mechanical and thermal loading is now under consideration.
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