Problems With Non-destructive Surface X-ray Diffraction Residual Stress Measurement

Because surface measurements are non-destructive, xray
diffraction is often considered as a method of
residual stress measurement for quality control testing.
Unfortunately, errors caused by the presence of a
subsurface stress gradient as well as difficulties in
interpreting surface results often limit the usefulness of
surface data. The magnitude of the potential errors,
both in measurement and in interpretation, depends
upon the nature of the subsurface residual stress
distribution which can only be determined
destructively. Although residual stress distributions
subject to these problems are commonly encountered in
practice, the question of the validity of non-destructive
surface results is seldom adequately considered.
Examples are presented showing common residual
stress distributions produced by grinding, nitriding and
shot peening which are subject to errors in
measurement and/or interpretation when measured only
at the surface. The methods for determining the
subsurface residual stress distributions and correction
for penetration of the x-ray beam are discussed along
with examples of their application. The need to
determine the subsurface stress distribution in order to
verify the accuracy of surface measurements is
emphasized.