Effect of constitutive model on residual stress development in the pressure tube rolled joint

Abstract

Being one of the most important joints in a pressurized heavy water reactor (PHWR), pressure tube rolled joint is critical during the construction and operation. This joint is formed through the roll expansion process that leads to an interference fit between the Zr 2.5%Nb pressure tube and stainless-steel end fitting. For integrity of this joint, a sufficient pull-out strength and leak tightness must be maintained, while having lower undesirable residual stresses, especially in the transition zone. The roll expansion process parameters are controlled to achieve optimum contact pressure and limit the tensile residual stress. The parameters of this roll expansion process are designed either through experiments or through numerical simulations. One important aspect that is often ignored in the numerical simulations is the effect of material constitutive behavior of the Zr 2.5%Nb alloy, particularly the anisotropic yield behavior and the cyclic stress-strain response. In this paper, these two aspects of constitutive behavior are investigated, using three-dimensional finite element simulations, to understand their influence on the residual stress development in the roll expansion process. The cyclic behavior was modelled using Chaboche nonlinear kinematic hardening model. The model parameters were extracted from low cycle fatigue experiments with the strain amplitude equivalent to that seen in roll expansion process. The results show that constitutive behavior, especially the cyclic behavior, has a significant impact on the residual stress. Anisotropic yielding of the pressure tube was also seen to affect the tensile residual stress and the contact pressure, although to a lesser extent. Finally, after comparison between four combinations of constitutive behavior, a simple isotropic yield with isotropic hardening model is suggested to predict the tensile residual hoop stress in the rolled joint, conservatively.