Welding or other non uniform heat treatments, and plastic deformation during fabrication, can leave strong residual stresses locked-up within components. These stresses can affect the component life in service as they may add to applied loads causing fatigue and failure. Consequently engineers have to allow for their presence, and they have sought for many years an absolute, nondestructive means of measurement of the internal residual stress fields within weldments and fabricated components. The engineer dreams of obtaining a simple small ‘black-box’ with a probe which could be placed on the surface of the component. On dialling the depth and direction of required stress, its value would be read off on a display! Unfortunately this ideal is still some way off full realisation, although a wide range of techniques exist to give limited information on the stress field (1). The engineer also has recourse to computer calculations of stress fields using finite element methods, or analytical models.