Abstract
Magnetic properties of an iron-based steel derived from its major hysteresis loop are very sensitive to its microstructures and have been, therefore, used for nondestructive evaluation (NDE) of degradation in infrastructure made by ferromagnetic materials. The permeability obtained from small ac field excitation may be a more useful parameter for NDE of ferromagnetic materials than one obtained from a major hysteresis loop from the viewpoints of electricity consumption and real-time measurements. In this paper a relation between ac permeability, calculated from impedance of coil elements, and dislocation density or grain size of pure iron were clarified. A potential of NDE using ac permeability is also discussed.