Influence of residual stress and heat affected zone on fatigue failure of welded piping joints

In recent decades, some unexpected fatigue failure occurred in welded joint of metal structures under cyclic loading. In many cases, the cause for the failure could not be detected. A study at NC State University revealed that welding procedure could be one of the factors that was not appropriately considered in current design methodologies. The welding procedure can influence the strain response near weld toe in two ways: one is by generating residual stress, and the other is changing material properties in the heat affected zone (HAZ). It was the primary objective in this investigation to determine the influence of these two factors on strain response by conducting experiments and performing numerical simulations of welded piping joints.;In the experimental study a series of residual stress data were measured, using x-ray and neutron diffraction techniques, for welded piping joints. The measured results revealed that the initial maximum compressive residual stress of stainless steel piping joints is higher than the yield stress of base metal. Moreover, the axial residual stresses of stainless steel piping joints are mostly relaxed after 5 cycles. The change of mechanical material properties due to high temperature exposure was studied by conducting experiments on tubular specimens. It was obtained that some mechanical material properties changed after subjecting to high temperature cycles, and the changed material was correlated to the peak temperatures. A modified thermo-mechanical material heterogeneity model was then developed to improve the initial residual stress simulation at the weld toe. The heterogeneous material properties coupled with the Chaboche model were used for subsequent fatigue response simulation. Quarter-point elements were applied at the stress concentration locations. The analysis results showed that fatigue response and residual stress relaxation can be simulated well. Final two simulations in this research showed that the presence of initial residual stress influences strain amplitude and strain mean, both of which could influence the fatigue life of welded joints.