Deformations and diffraction

Measuring residual stress via diffraction relies on a tight correlation between macroscopic deformations (the concern of engineers) and the variation of diffraction angles (what is measured).

Deformation (compressive or tensile) changes distances between crystal planes, which translates into a modification of the corresponding diffraction angles. For a given family of atomic planes, the angle goes up for compression areas and down for tensile areas. In both cases, diffraction peaks tend to broaden because there is not a single interplane distance any longer but rather a distance distribution.

To prove this, it is enough to deform a metal sample placed in an X-ray or neutron beam.