This paper briefly describes the theoryand methods of x-ray residual stress
measurements.
Residual stresses can be defined as the stresses which remain in a material in the
absence of any external forces. There are many stress determination methods. Some of
those methods are destructive and some are nondestructive. X-ray residual stress
measurement is considered as a nondestructive method.
X-ray diffraction together with the other diffraction techniques of residual stress
measurement uses the distance between crystallographic planes as a strain gage. The
deformations cause changes in the spacing ofthe lattice planes from their stress free
value to a new value that corresponds to the magnitude of the residual stress. Because of
Poisson’s ratio effect, if a tensile stress isapplied, the lattice spacing will increase for
planes perpendicular to the stress direction, and decrease for planes parallel to the stress
direction. This new spacing will be the same in any similarly oriented planes, with
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respect to the applied stress. Therefore the method can only be applied to crystalline,
polycrystalline and semi-crystalline materials.