Ｘ線材料強度 シンクロトロン放射光によるＸ線フラクトグラフィ 疲労破面直下の残留応力分布

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The residual stress distributions just beneath the fatigue fracture surface were measured using synchrotron radiation with three different wavelengths, i.e., three different penetration depths. The residual stress distributions were estimated from three kinds of diffraction data by the following process. First, a temporary residualstress distribution in the depth direction is assumed. Theoretical 2θ-sin2Ψ diagrams for each wavelength, where each has a different penetration depth, are calculated by the cosΨ method developed by one of the authors. The sum total of the differences between the theoretical and experimental values of the diffraction angle in 2θ-sin2Ψ diagrams is calculated. This total value is minimized by changing the assumed stress distribution by the quasi-Newton optimization method. Finally, optimized 2θ-sin2Ψ diagrams for each penetration depth and detailed stress distribution are determined. The true surface residual stress is obtained from this stress distribution.No effect of load ratio R(=Pmin/Pmax) on the residual stresses of the fatigue fracture surfaces in low-carbon steels was observed when the sin2Ψ method was used for stress measurement. However, the residual stresses became higher with increasing R when these were measured by the proposed method. On the basis of this, the stress intensity factor range, ΔK, can be estimated from the residual stress on the fatigue fracture surface.