Nondestructive detection of dislocations in steel using a SQUID gradiometer

We have developed a magnetic flux imaging system. The magnetic flux image (MFI) shows the distribution of magnetic flux density over a plate like specimen. The change of the pattern in the MFI by tensile deformation has been investigated in a carbon steel without applying magnetic field using the system. A clear striped pattern appeared after deformation by fine plastic strain between 0.2 and 1.0% before strain-hardening. 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 nondestructively detected using the MFI.