You really can’t say what is the “best” method without knowing more. Why do you want to know the residual stress and what will you use the results for? What are the dimensions of the pipe? What are the details of the weld, such as the type of weld process and filler metal used? Do you need to know surface or subsurface stresses? Which stress component(s)? In short, no method can measure everything, so the best method depends on what is most important.

In general, aluminum is good for residual stress measurements because the residual stresses tend to be high relative to the elastic stiffness, which gives good “signal,” i.e., elastic strain, for either diffraction or relaxation methods. X-ray diffraction or hole drilling can often give good results for surface and near-surface stresses. For bulk stresses, you can get good results with the contour method or the deep hole method, even for very large parts, and with neutron diffraction for medium sized parts and maybe synchrotron diffraction for thinner parts. Some aluminum alloys and especially welds can be hard to measure with diffraction methods because of microstructural issues like texture and intergranular strains that confound the results, so be careful. Of course, the other methods have error sources as well, so you should always be careful.

Thanks Doc.