Neutron Diffraction Assisted Residual Stress Analysis in Welded Structures

Abstract

Welding residual stresses in structural components can significantly compromise their performance and lifetime. Prediction of welding stresses based on numerical modeling has not yet proven to be reliable, while measurement of such stresses based on NDT remains a challenging task. It is shown in this paper that neutron diffraction is a reliable non-destructive method for residual stress analysis in structural weldments. The Large Component Neutron Diffraction Facility (LCNDF) at the High Flux Reactor (HFR), Petten has facilitated residual stress measurements in various weldments, including large steel piping welds. A key issue in applying neutron diffraction to welds is the reliable estimation of the stress-free lattice distance in the heat affected zone and weld pool and in all directions of interest. Results of numerous investigations at HFR show that this is achievable by testing small coupons, cut from a companion weld specimen, which are nearly free of macro-stresses and consequently it is reasonable to be used as reference specimens. In fact, the feasibility of this approach has been demonstrated in monolithic and bimetallic welds. In this paper residual strain/stress data in five welded specimens, based on neutron diffraction, are presented. The results presented in this paper consistently show that, by ignoring the spatial and directional variation of the reference lattice distance, which is exhibited throughout the weld pool and the heat affected zones, erroneous strain data can be derived leading to non self-equilibrating internal stress estimates.KeywordsResidual stressweldingneutron diffractionlarge componentscoupons