New development of sensitive magnetic sensors for nondestructive evaluation of structural materials

The diagnosis of iron-based materials was performed by using many magnetic sensors such as a small Hall element for leakage flux observations after polarizations, a locally sensitive fluxgate sensor available down to 100pT, by a pick-up coil for magnetic noise observation and by a triple-fold coil for eddy current detector. The spatially high resolution of the magnetic sensors was attained as narrow as 100 mu m or less with the same order lift-off distance depending on the situation. By using these detectors, we detected the positions with the relatively high stress in comparison with the other positions. Residual stress detection less than 0.1 % in ferromagnetic material was attained in this study by deriving the expectations of field strength for magnetic noise occurrence. Positions with small holes inside the material were precisely detected by an eddy current detector, composed of a triple-fold coil with thin amorphous ribbon and attached from the material surface. The small magnetic field emission from a new type flux gate sensor was also attained by composing a double coil set for flux gate with the opposite coil current coil each other.