Effect of spatial variation of stress-free lattice spacings on measured residual stresses

Abstract

When neutron diffraction is used to measure residual stresses, estimates of the unstressed lattice spacing ( d0) are often required. These are generally measured on a sample that is considered representative of the component. In the case of welded samples, the possibility of variation in weld shape between the d0 sample and the area where stresses are determined must be taken into account. To assess this, d0 values are measured on an area considered representative of the sample, and then are re-measured on a sample cut from the actual strain measurement plane. The residual stresses are re-calculated with these new d0 values and the results compared to those based on the initial d0 measurement. Methods of estimating these errors are discussed. The errors in residual stress that arise from the spatial variation of d0 values are greater than the commonly reported peak fitting errors. These errors can be estimated and should be included in the reported residual stress measurements. Cutting a d0 sample from the measurement plane makes this a destructive technique.