NONDESTRUCTIVE DETECTION OF DISLOCATIONS IN STEEL USING A SQUID GRADIOMETER

We have developed a magnetic flux imaging system. Magnetic flux image (MFI) shows the distribution of magnetic flux density over plate like specimen. The change of the pattern in the MFI by the tensile deformat ion has been investigated in a carbon steel without applying magnetic field using the system. A clear striped pattern appeared after deforma tion of the fine plastic strain between 0.2 and 1.0% before strain-har dening. Luders bands, which are groups of slip bands, appeared on the plate surface and were observed at the corresponding locations of the stripes in the MFIs. This shows that the fine deformation is nondestru ctively detected using the MFI.