Hydraulic Expansion of the Tube-to-Tubesheet Joints: A Finite Element Analysis

Abstract

The integrity and quality of tube-to-tubesheet joints are of a prime concern in the heat exchangers industry. These joints become extremely critical when it comes to the nuclear-power plants. Three main techniques are in use to produce these joints, namely, mechanical rolling, explosive forming and hydraulic expansion. In this paper the hydraulic expansion of tubes into tubesheets is modeled by the use of the finite element method. The model consists of a single tube and a surrounding sleeve with two dimensional axisymmetric geometry. The uniform pressure loading is incrementally applied on the inner surface of the tube end within the sleeve. Von-Mises yield criterion along with isotropic work hardening rule is implemented to cope with the plastic flow 'of the material. The updated Lagrangian description of motion is implemented in order to account for the geometric nonlinearities. The frictional contact between the outer tube surface and the tubesheet hole is modeled without recourse to any gap elements. This modelling is achieved by the use of the contact algorithm in the general purpose finite element program, INDAP. Friction action is accounted for through the use of Coloumb's friction law. Several case studies are analyzed in order to investigate the main and interaction effects of the maximum applied hydraulic pressure and the initial radial clearance on the integrity and the quality of the expanded joint. The joint integrity is looked at in terms of the magnitude and distribution of the residual contact pressure. On the other hand the quality is assessed through the residual stress distributions along the tube wall. This study shows that the finite element analysis is capable of simulating the T/TS joint closely enough to be considered as a standard tool in a design code. It offers a new insight into the explanation of the break-off of the joint strength as the expanding pressure level is increased beyond a well defined optimum value. Exploring the interaction effect of both design parameters on both performance measures, it becomes possible to accommodate the differing conclusions reported in the published literature. As a result of this investigation, the designer would acquire a better understanding of the complex deformation process, assess the quality of the joint and predict the effects of changing the two design parameters on the integrity and quality of the joint.