Neutron strain measurement

The use of neutrons has extended the technique of diffraction strain measurement from an essentially two-dimensional, near-surface tool using X-rays to a true three-dimensional method. The depth scale has gone from micrometres to millimetres. This initially gave rise to subsurface measurements of type I residual macrostresses in weldments and type II residual microstresses in anisotropic and multiphase metals, and composites. Soon the possibilities of in situ applied stress measurements became apparent and today they represent a significant portion of the work being carried out. This perspective focuses on the current state of the art, the prospects and developments necessary for further progress. The transition from two-to three-dimensions raised the issues of stress free reference values, methodology for general tensor measurements, beam optics and experimental design. Workers invested great effort in resolving these issues. In addition, the development of pulsed source instruments has enabled in situ measurements of slip, fatigue, load partitioning, twinning, high T stress-strain response, thermal cycling and the shape memory effect. Most recently, the SNS instrument, VULCAN, has introduced torsion and continuous recording of data which can later be binned in the shortest statistically significant time intervals.