Residual stresses are those that remain in a component after external forces are removed; they are self-equilibrating in nature and are often caused by deformation or uneven heating during manufacture; particularly casting, forging, forming, or welding operations. Residual stresses are significant in the failure of components as they contribute to fracture, fatigue, stress corrosion cracking (SCC), hydrogen — assisted cold cracking, hydride formation, or lead to unacceptable deformation during manufacture. An understanding of residual stress is essential in developing new components, materials, and joining techniques for nuclear-energy systems.

As most metal-forming operations involve heating and or deformation, residual stresses are almost always present due to differential thermal-strains, phase trans­formations, or plastic mismatch. These mismatches cause elastic strains, which result in residual stresses. The structural issues that arise from residual stresses are of two types, those that are conventional structural-integrity issues (fracture, fatigue, stress corrosion cracking susceptibility, creep crack growth, hydride formation, and dis­tortion) and issues that come about due to the interaction of residual stress and radiation with service exposure (stress relaxation, creep, and swelling; all induced by radiation).

Structural integrity assessments of nuclear components rely on accurate values of the residual stresses; in the absence of better information these must be con­servatively assumed to be equal to the yield strength, leading to small critical-defect sizes and loads. The regular use of residual stress measurements by neutron dif­fraction has been able to safely reduce the conservatism of these estimates by providing accurate, validated measurements. Stresses cannot be measured directly, only the elastic strains locked into the material. Broadly speaking, there are two methods of measuring residual stresses; compliance methods and methods that measure lattice strains in crystalline materials (typically metals) [1—3]. Compliance methods assess deformations that occur during cutting or other methods of material removal. Lattice strains are typically measured by diffraction and comparing lattice spacings in the strained and unstrained condition, this can be done by X-ray (including synchrotron) and neutron diffraction. Neutron diffraction has many advantages over other methods of measuring residual stresses due to its good penetrating power and spatial resolution within the bulk of the component.

A difficulty with diffraction measurements is that a stress-free sample is often required for reference. Furthermore, the compositional strain-variation that may occur across welds leads to unavoidable sources of error due to so-called chemical strains. There may be significant variation in the weld position and composition between the stress-free sample and the measured component. Where one stress component is known to be near zero measurement at a range of angles normal to the surface using the sin20 technique [3] can obviate the need for a stress-free sample.