Residual stresses are mechanical stresses that are present in materials and components but cannot be attributed to an external load. They are caused by plastic deformation and are generally present to varying degrees in every component. Component residual stresses occur in almost all manufacturing and processing procedures and can have a significant influence on the strength and service life of components. If residual stresses and operating stresses overlap unfavorably, material failure can occur without the external load reaching the strength limit! Conversely, the operational strength of components can be improved by the targeted introduction of residual stresses. Quantitative information about the size and orientation of residual stresses are therefore important quality characteristics and can be damage-relevant influencing factors. X-ray diffraction (XRD) is a proven method for determining the residual stresses in crystalline materials.

Non-destructive residual stress analysis on metal parts using X-ray diffraction (XRD)

Short-wave X-rays are diffracted by the crystallites of the component material (Bragg reflection). The 2-theta method can be used to derive information about the atomic distances in the crystal lattice (so-called lattice plane distances). In an elastically stressed material, changes in the lattice plane spacing occur compared to the stress-free state. This is used in X-ray residual stress analysis with the sin2ψ method. X-ray diffraction is performed on the sample at different tilt angles and thus enables precise detection of length changes in the atomic distances in the tilt direction. Using elasticity constants, these changes in length can be converted into compressive or tensile stress values (specified in MPa). The residual stress can thus be analyzed in a specific component direction. The stress tensor is calculated by tilting in three different component directions.
The information depth of the X-rays is relatively low and is only a few µm. The residual stress is therefore determined at the component surface. By repeated, localized material removal using electrolytic etching, it is possible to derive a depth profile of the residual stress without significantly changing the residual stresses in the component. This can be done with very high spatial resolution.

Apparatus setup: Diffractometer

Our compact diffractometer is mounted on a 6-axis robot. This setup makes it possible to examine components of different shapes and sizes. We carry out precise residual stress measurements on a wide variety of metals and alloys (e.g. steel, Ni, Cu, Ti, Al, etc.). Different X-ray tubes are used for specific materials in order to achieve optimum measurement results.
Artifact-free removal of the component surface by electrolytic etching enables measurements of the residual stresses present at different component depths. This allows stress curves to be displayed in a depth profile.