The Use of Combs for Evaluation of Strain-free References for Residual Strain Measurements by Neutron and Synchrotron X-ray Diffraction

Abstract

The measurement of residual strain using diffraction techniques relies on the determination of a change in lattice parameter relative to a reference or “strain-free” lattice parameter. Therefore, obtaining a relevant reference lattice parameter is perhaps the most important part of any diffraction-based strain experiment. This is particularly important in samples where one may expect a changing reference, for example in a weld where microstructure may vary due to heating. One method of determining appropriate references is the use of “combs” machined from the sample. The teeth should essentially be macro-strain relieved and reveal information about microstructural and intergranular strain variations. It is shown that there is often significant variation in measured diffraction angles along the length of a comb and that the most reliable measurements for strain-free reference are at the ends of the teeth. A study is presented of the application of strain-free combs in friction-stir welded plates.