Detection of Plastic Deformation and Fatigue Damage in Pressure Vessel Steel by Leakage Magnetic Flux Sensors

抄録

In order to detect material degradations in pressure vessel steel of A533B employed as a component material in light water reactors (LWRs), the leakage magnetic flux distribution on plate-type A533B specimens has been examined using a magnetic sensor of 120μm square GaAs Hall element. As reference data, the residual stress distribution has also been obtained by X-ray diffraction method where an irradiation spot was 1mm in diameter. The data of the leakage magnetic flux normal to the specimen surface (B_z) measured by the Hall sensor were converted into the form of the first derivative dB_z/dx by calculating the gradient of the B_z along the axial direction of the specimen (x-axis), and the data of the axial residual stress (σ_x) obtained by the X-ray diffraction method were also transformed into the differential form dσ_x/dx. It was found that there existed a correlation between the absolute value of the dB_z/dx and the dσ_x/dx for the tensile specimens (correlation coefficient r=0.282). Full width at half maximum (FWHM) for the frequency distribution profile of the dB_z/dx was calculated. The FWHM of the tensile specimens increased remarkably at the beginning stage of the plastic deformation where the Lüder's band was generated; The FWHM of the fatigued specimens increased with stress cycling. It was suggested that the leakage magnetic flux measurement by the GaAs Hall sensor was a promising non-destructive evaluation (NDE) method to detect the amount of plastic deformation and fatigue damage.