X-ray measurement of near surface residual stress in textured cold-worked stress-relieved Zr–2.5%Nb pressure tube material

In the present study, the near surface residual stress in Zr–2.5%Nb pressure tubes was measured using X-ray diffraction. Both cold rolled (pilgered) and stress-relieved tubes were studied. Since the material is highly textured and the hexagonal close packed crystal lattice has anisotropic elastic constants, special care is required in collection and analysis of experimental data to minimise errors. The standard multi-exposure technique of X-ray residual-stress measurement results in inaccuracies for textured material. Diffraction intensities depend on factors like inclination of the sample, the rotation of the beam and the position of detectors. Low intensity due to texture can result in increased uncertainties in observed stress value. Anisotropy of elastic constants in textured materials can further increase the errors in measured stress. A general approach has been developed for accurate determination of the stress tensor in textured material. The measured stresses for Zr–2.5%Nb pressure tubes were highest and compressive along the rolling direction (RD) in the pilgered samples. The stress-relieved samples showed a small tensile component along the RD. Errors up to 20% are introduced if isotropic elastic constants are used in the analysis instead of the more realistic anisotropic elastic constants calculated for the observed texture.