X-Ray Fractography Using Synchrotron Radiation

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The residual stress distributions just beneath the fatigue fracture surface were measured using synchrotron radiation of three different wavelengths having three different penetration depths. The residual stress distributions were estimated from these three diffraction data by the following process. First, a temporary residual stress distribution in the depth direction is assumed. Theoretical 2θ-sin2ψ diagrams for each wavelength are calculated by the cosψ method developed by one of the authors. The 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 using the quasi-Newton optimization method. Finally, the optimized 2θ-sin2ψ diagrams for each diffraction and the detailed stress distribution are determined. The true surface residual stress is obtained from this stress distribution. No effect of the load ratio R (=Pmin/Pmax) on the residual stress of the fatigue fracture surface in low-carbon steels was observed when the sin2ψ method was used for stress measurement. However, it was found in this synchrotron study that the residual stress became higher with increasing R. On the basis of this, the stress intensity factor range, ΔK, can be estimated from the residual stress on the fatigue fracture surface.