NONDESTRUCTIVE MAGNETIC DETECTION OF PLASTICIZED AREA USING SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE

The relationship between the magnetic and structural changes induced by the application of a tensile load to a carbon steel specimen was investigated by correlating the change in the spatial second-order gradient component of the magnetic flux density d2Bz/dr2 with the appearance of the Luders bands on the specimen surface. The magnetic change was measured using a nondestructive evaluation (NDE) system with a concentric second-order superconducting quantum interference device (SQUID) gradiometer of low temperature superconductor. The area where peaks and folds appeared in the 3-dimensional map of d2Bz/dr2 corresponded to the area where Luders bands appeared on the specimen surface. The dimensions of the area where peaks and folds appeared had a strong correlation with the residual strain of the specimen. It was concluded that the magnetic NDE using SQUID has the ability to detect the area plasticized by fine deformation in ferromagnetic materials.