Yoshihiro Isobe

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

Abstract Leakage magnetic flux distribution in plate-type specimens of iron-based materials has been investigated using a magnetic sensor of 120-μm 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 d B z /d x 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 ( σ x ) obtained by the X-ray diffraction method were also transformed into the first derivative as d σ x /d x . It was found that there were weak correlations between the absolute values of the d B z /d x and d σ x /d x 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 σ for frequency distribution of the d B z /d x were calculated by statistical processing to estimate the amount of plastic strains accumulated in the specimens. The σ derived for A533B and S25C increased remarkably at the beginning stage of the plastic deformation where Luder’s band was generated, while the σ for SUS304 increased with increasing plastic strains. It was concluded that the leakage magnetic flux 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.

Detection of plastic deformation and estimation of maximum value of residual stress in low carbon steel by X-ray stress analysis using statistical techniques

Abstract Residual stress distribution on plate-type tensile specimens of low carbon steel with applied plastic strains up to 6.45% has been measured by X-ray stress analysis. Standard deviation for the frequency distribution profile of the residual stress was calculated by statistical processing to estimate the amount of macroscopic plastic strain applied on the specimen. Since adequate understanding of the maximum value of the residual stress (σmax) has been important for the maintenance of the nuclear power reactor components, the σmax was also estimated from the limited number of the experimental data by statistics of extreme. It was found from above estimation that the X-ray stress analysis supported by the statistical techniques was a promising non-destructive evaluation (NDE) method to detect the plastic deformation and to estimate the σmax in S25C low carbon steel.

The Observations of Martensitic Transformations in SUS316 by Using a Sensitive Flux-Gate Sensor

The martensitic transformations (MT) in austenite stainless steel (SUS316) were observed by using a sensitive Flux-Gate (FG) sensor just after the magnetizations by the permanent magnet in the terrestrial field. MT in the small deformation range less than 5% elongation, was visualized by the 2-dimensional profile at each position together with the profile of the thickness deformations with an accuracy of several microns. We found the local MT enhancements up to 400[nT] at the positions along the changes of the thickness. In this experiment, the observations of MT were able to avoid the effects of the external fields or of the ac fields which was generated by the gating function in the FG sensor.

New development of sensitive magnetic sensors for nondestructive evaluation of structural materials

The diagnosis of iron-based materials was performed by using many magnetic sensors such as a small Hall element for leakage flux observations after polarizations, a locally sensitive fluxgate sensor available down to 100pT, by a pick-up coil for magnetic noise observation and by a triple-fold coil for eddy current detector. The spatially high resolution of the magnetic sensors was attained as narrow as 100 μm or less with the same order lift-off distance depending on the situation. By using these detectors, we detected the positions with the relatively high stress in comparison with the other positions. Residual stress detection less than 0.1 % in ferromagnetic material was attained in this study by deriving the expectations of field strength for magnetic noise occurrence. Positions with small holes inside the material were precisely detected by an eddy current detector, composed of a triple-fold coil with thin amorphous ribbon and attached from the material surface. The small magnetic field emission from a new type flux gate sensor was also attained by composing a double coil set for flux gate with the opposite coil current coil each other.