Residual Stresses in Butt and Socket Welded Joints Xiangyang Lu and Tasnim Hassan Center for Nuclear Power Plant Structures, Equipment and Piping, North Carolina State University, Raleigh, NC ABSTRACT A finite element scheme for simulation of welding residual stresses is presented in this paper. Detailed thermal and residual stress analyses for butt-welded and socket-welded pipes are performed using ANSYS and ABAQUS. The calculated distribution of axial stress and hoop stress at welded joint are validated against experimental results. Furthermore, residual stress relaxation under high amplitude cyclic loading is studied. INTRODUCTION Fatigue failures in many nuclear power plants have resulted in unscheduled plant downtime and considerable financial loss ([ 1], [2]). Most of these fatigue failures occurred at welded-joints of small-bore piping or at fillet weld attachments under high- and low-cycle vibration conditions. Despite the general notion that the current fatigue design methodologies are conservative, fatigue failures are still occurring and sometimes much earlier than the estimated life. Hence, understanding the mechanisms that contribute to welded joint fatigue failures and incorporating that knowledge into design procedures in a scientific manner are critical. Fatigue behavior of welded joint is complicated due to many factors, such as stress concentration, environment and residual stress. This study will make an effort to estimate residual stresses in socket-welded joints initially and after fatigue loading. Residual stresses that arise in welded joints by heating and cooling cycles during the welding process is an important factor in fatigue failure of welded structures. It has been shown that a tensile residual stress in welded structure can be as high as the yield stress and can have detrimental effect on the fatigue life of welded structures ([3], [4], [5]). Magnitude and distribution of residual stress in a welded joint depend on the weld-bead sequences ([6], [7]). In the presence of high residual stress, materials at or near welded joints experience repeated excursions into the plastic range, even for small amplitude fatigue loading cycles. Moreover, residual stress acts as a mean stress to the fatigue loading. In the presence of mean stress, repeated excursion of "stress-controlled" loading cycles into the plastic range can lead to degradation and failure of structures due to the accumulation of deformation or ratcheting ([8], [9]). This fatigue failure mechanism is not well understood and has not been incorporated into design in a scientific manner. In this paper, detailed thermo-elastic-plastic finite element analyses are performed to estimate residual stresses during welding process. The calculated distribution of axial stress and hoop stress at welded joint are validated using experimental results. Furthermore, residual stress relaxation under high amplitude cyclic loading is studied. FINITE ELEMENT ANALYSIS In order to understand the effect of welding residual stress on fatigue failure, it is essential to numerically simulate its initial distribution and subsequent response of material (relaxation and ratcheting) during fatigue loading. This requires a robust, reliable and numerically efficient method of modeling residual stresses. Methods of residual stress calculation in butt welded joints have been developed by Ueda et al. [ 10], Rybicki et al. [ 11 ], Wilkening and Snow [ 12], Brickstad and Josefson [13]. Most of these studies were conducted based on either axisymmetric or two-dimensional plane assumption. Although some of the basic residual stress characteristics can be reasonably captured with axisymmetric model as shown by Karlsson [ 14] and Dong et al. [ 15], it is also demonstrated that the residual stresses in welded joints are not always symmetric. High tensile and compressive residual stress concentrations have been observed at welding start/end point (Shack et al. [16], Karlsson and Josefson [ 17] and Lindgren and Karlsson [ 18]). Fatigue failures in welded structures usually initiate at the stress concentration point. Hence, a two-dimensional simplified model can not always simulate the fatigue failure of welded joints accurately. Therefore, it is essential to conduct a detailed three-dimensional analysis in order to simulate the fatigue responses of welded joints. Along this line, some insightful results on welding residual stresses for butt-welded joints are published ([5], [15], [17], [19], [20]). Methods developed by Rybicki et al. [11], Wilkening and Snow [12], Li [5], Brickstad and Josefson [13] are used in this study. The numerical method for predicting residual stresses of welded joints consists of two parts: heat transfer and residual stress analyses. The prediction of welding residual stresses using the finite element method can be simplified by uncoupling the thermal and mechanical aspects of the problem. In this approach the heat transfer analysis can be solved independently of the stress analysis [21 ]. The link between the heat transfer and stress analyses is obtained through the temperature history, which is the input as thermal load for residual stress calculation. First a transient thermal analysis is performed during which
Residual Stresses in Butt and Socket Welded Joints
Jun 04, 2023
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