Evaluation of residual stresses and plastic deformations for iron-based materials by leakage magnetic flux sensors

Leakage magnetic flux distribution in plate-type specimens of iron-based materials has been investigated using a magnetic sensor of 120-mum square GaAs Hall element for the three types of A533B, SUS304 and S25C with plastic strains up to 8.0%. As reference data, the residual stress distribution has also been obtained by X-ray diffraction method, where an irradiation spot was 1 mm in diameter. The data of the leakage magnetic flux normal to specimen surface (B-z) were converted into the first derivative dB(z)/dx to elucidate the spatial gradient of B-z with respect to the axial positions of the specimen (x-axis), and the data of the axial residual stress (sigma (x)) obtained by the X-ray diffraction method were also transformed into the first derivative as d sigma (x)/dx. It was found that there were weak correlations between the absolute values of the dB(z)/dx and d sigma (x)/dx for A533B (correlation coefficient r=0.282) and for S25C (r=0.387), respectively, while no significant correlation for SUS304 (r=-0.050). Standard deviations sigma for frequency distribution of the dB(z)/dx were calculated by statistical processing to estimate the amount of plastic strains accumulated in the specimens. The sigma derived for P533B and S25C increased remarkably at the beginning stage of the plastic deformation where Luder's band was generated, while the sigma for SUS304 increased with increasing plastic strains. It was concluded that the leakage magnetic Bur observation for iron-based materials by using GaAs Hall elements was a promising non-destructive evaluation (NDE) method to detect residual stresses and the amount of plastic deformations. (C) 2001 Elsevier Science B.V. All rights reserved.