Non-Destructive Testing of Structural Materials Using a Scanning Hall-Sensor Microscope-Influence of Plastic Deformation on Surface Magnetic Fields of Low Carbon Steel SM400B-

Abstract

Using a scanning Hall-sensor microscope with an active area 25µm×25µm, we measured the magnetic field distributions due to spontaneous magnetization on flat surfaces of low carbon steel SM400B with a yield point of 326N/mm2 and a pulling strength of 469N/mm2 under a static condition of terrestrial magnetism. Referring to the stress-strain curve, we applied a uniaxial stress to the sample and increased the stress level following the pre-scheduled experimental run, while measuring the magnetic profiles both for the stress state and after its release. The magnetic profile in the elastic stress nearly agrees with that after the release, and also agrees with that for the virgin state. However, as the stress level increases beyond the yield point and gives the residual strain after the stress release, there appear marked changes only in the results after the release compared to the others, which are attributed to the occurrence of Lüders bands. The positions with such changes in the magnetic profile move along a length direction of the sample, with extension of the Lüders bands to the whole part. The study shows the possibility to obtain information on a degree of strain progress in steels using magnetic images and to open up a practical way for non-destructive testing.