Every component, tool or any welded structure is under residual stresses. Even raw materials, which are used to manufacture and heat-treat components, are not free of residual stresses.
Residual stress is tension within a closed system which is shielded from external forces or moments. The residual stresses equalize each other over the cross-section of a component, for which reason residual tensile and compressive stresses always appear together. The most reliable and most accurate method for measuring residual stress is the X-ray diffractometer analysis (RDA) according to DIN EN 15305.