Stress measurement by X–ray diffraction using film techniques

Abstract

For some X–ray stress measurement applications the photographic film methods may be preferred to diffractometer methods. The principles applied to the former are reviewed and the accuracy and sources of error are examined. The main procedures include the standard ‘two exposure’ method (including the use of the sin2 method), and the use of a relationship depending on the asymmetry of rings at oblique incidence. A number of apparent errors in the literature are explained and resolved. The simplest formula for the second method takes the form s̀=k.δR.In the most favourable cases the accuracy of stress measurement is of the order of ±30 MN/m2 or ±2 ton f/in2. Larger errors arise for hardened surfaces with microstresses, and are likely to be larger for the second method. Both methods obviously give the most favourable results when sharp diffraction rings are obtained. However with proper care and appreciation of the factors considered, practically usefulresults are obtained under most circumstances. The treatment is illustrated by particular reference to surface residual stress measurement on steels.