Pitfalls of Layer Removal Techniques in X-Ray Residual Stress Measurements

The nondestructive aspect of X-ray residual stress analysis is often heralded as its most important attribute. Unfortunately, errors caused by the presence of steep subsurface stress gradients and difficulties in interpreting surface results often limit the usefulness of surface data. More often than not, subsurface data are required. This necessitates successive layer removal via electropolishing techniques. Correction procedures will be briefly reviewed and examples presented showing common residual stress distributions. It will be shown that, for most cases, the assumptions on which traditional correction procedures are based are usually abrogated by the method of layer removal. Fortunately, the corrections are small and can be neglected except when the stress gradient into the depth is sufficiently steep that the exponentially weighted average of the stress over the volume of irradiation is not representative of the true stress in that region. Other difficulties including coarsening of the grain size and changes in preferred orientation with layer removal will be illustrated.