PSI - Issue 60

R.P. Pandey et al. / Procedia Structural Integrity 60 (2024) 324–334 R. P. Pandey/ Structural Integrity Procedia 00 (2023) 000 – 000

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of the pressure tube is of the order of 8.5e-6/deg.C, whereas the corresponding value for the end-fitting is of the order of 9.5e-6/deg.C. Hence, the end-fitting expands more compared to the pressure tube and this in turn relaxes the residual stress by an amount corresponding to satisfaction of compatibility of geometry at temperature of interest. The distribution of residual stress is very complex in the rolled interface between the pressure tube and the end-fitting because ofpresence ofgrooves.The presence ifgrooves disrupts the residualstress distribution frompurely compressive (along the length of the tube) to a complex pattern involving compressive and tensile residual stresses as shown in Fig. 10 for a typical 220 MWe rolled joint of Indian PHWR. The residual stresses relax at high temperature as the coefficient of thermal expansion of the end-fitting is higher compared to that of the pressure tube. The variation of hoop residual stress with temperature along axis of the pressure tube in the rolled joint region is shown in Fig. 10. For evaluating pull-out strength of the rolled joint at different temperature, FE simulation has been carried out and a typical pull-out geometry of the rolled joint is shown in Fig. 11(a). For simulation of the pull out process, the end-fitting has been prevented from moving in axial direction as can be seen from the boundary condition. A uniform displacement has been provided to the end cross-section of the pressure tube as shown in Fig. 11(a). The typical distribution of von Mises residual stress in the longitudinal cut-section of the joint is shown in Fig. 11(b) for the pull-out simulation carried out at 800 K. It may be noted that the pressure tube has come out of the locked region in the grooves due to application of tensile pull-out load. The actual load-displacement curve during the pull out process at different temperatures (27 o C to 627 o C) is shown in Fig. 12. It may be noted that the locally plastically expanded region of pressure tube gets unlocked from the grooves in the end-fitting at maximum load and after that the load decreases gradually instead of falling drastically to zero. This behavior can be explained by the presence of frictional traction force between the pressure tube outersurface and inner surface of the end-fitting.

Fig. 11: (a) Schematic representation of boundary and loading conditions used during FE simulation of pull-out process of the pressure tube from the rolled joint; (b) Distribution of von Mises stress in a longitudinal cross-section as observed during simulation of the pull-out process of the pressure tube from the grooves of the end-fitting as obtained from FE simulation at 800 K (527 deg. C)

The contact stresses generate the required normal force and the sliding movement generates necessary frictional drag, which is reflected in gradual decrease of load with applied displacement in Fig. 12. The maximum load in Fig. 12