X-Ray Stress Measurement of the Specimen with a Steep Stress Gradient in Its Near Surface Layer

Abstract

X-ray stress measurement is widely applied to the estimation of residual stress. In real machine parts, residual stress is often observed to have a steep gradient in their near surface layers. In the present study, the effect of the stress gradient on the stress data obtained by the X-ray stress measurement based on the sin²ψ method, was investigated experimentally and analytically.
Three kinds of steel specimens were prepared, namely, (A) a shot-peened spring steel plate with decarburized surfaces, (B) a finish ground 0.55%C steel round bar and (C) a shot-peened spring steel plate with non-decarburized surfaces. The residual stress distributions of (A) and (B) specimens showed, respectively, steep negative and positive stress gradients in the near surface layers. But the residual stress near the surface of (C) specimen remained almost constant. The characteristic X-ray used was CrK_α radiation.
Experimental data for (C) specimen, when plotted on the sin²ψ-cosecθ diagram, can almost be represented by a straight line. But for (A) and (B) specimens, the data on the same diagram nearly fit to the concave and convex curves, respectively.
The sin²ψ-cosecθ diagrams of the specimens with a steep stress gradient near the surface were also obtained by calculating the Bragg's angle as the weighted average value of the diffracted X-ray intensity. The analytical results mostly agree with the experimental ones.
According to the theoretical analysis of residual stress on the assumption that it varies linearly with depth and σ_x=σ_y, the stress obtained by using the sin²ψ method with CrK_α radiation was found to correspond to that of the layer 4.4μ deep from the surface.