Determination of the Welding Residual Stress Field by Diffraction Methods and Studying Its Behavior Under Uniaxial and Multiaxial Mechanical Loading

Weld fatigue strength is currently the bottleneck to designing high performance and lightweight welded structures using advanced materials. In addition to loading conditions, environmental aspects, geometrical features and defects, it has been proven that studying the influence of residual stresses on fatigue performance is indispensable. The extent of the influence is however a matter of discussion. A deeper insight into the source of the welding residual stresses on the basis of sound physical principles would increase the awareness of the extent of their true threat to the structural integrity. In this article the influence of the uniaxial and multiaxial loading on the relaxation of welding residual stresses in small specimens and large components out of different steels namely S235JRG2, S355J2G3, S355J2H, P460NL, 690QL and S1100QL will be presented. X ray diffraction analysis has been used for the determination of residual stress profiles in surface layers. For residual stress analysis in deeper layers synchrotron and neutron diffraction techniques were applied as complementary methods. A clear recognition of the difference between initial welding residual stresses in small and large scale specimens was observed. Tensile residual stresses as high as the yield strength could appear in large scale welded specimens. That was not the case for small scale welds due to the low grade of restraint. Nevertheless in the flat and tubular butt welds the highest residual stresses were obtained in the weld centerline. At the weld toe which is critical regarding fatigue crack initiation, the magnitude of the residual stresses is lower. Beside that it was observed that in the case of relaxation the first load cycles especially in components out of low strength steels are decisive. The influence of loading conditions and local mechanical properties on the relaxation of welding residual stresses was investigated based on principles of solid mechanics. The von Mises failure criterion was able to describe the relaxation behavio