Effects of Thickness Offset on the Tube-to-Tube Sheet Expansion Joint Strength: An Experimental Evaluation

Uniform wall thickness all around the circumference is significant to get the right tube–to-tube sheet expansion joint pull-out strength. Thickness offset at the swaged ends of tubes reduces the joint strength and leak-proof characteristics of joints. Apparent wall reduction (%WR) is used to evaluate the expansion joint strength. Hybrid expansion of hydraulic and roller expansion was considered for this study. Hydraulic expansion pressure of 190 MPa, and roller expansion pressure corresponding to 6, 8, 10, and 12%WR were used. This paper focuses on tube thickness offsets of 0.125, 0.25, and 0.5 mm, microstructure and hardness values of expanded tubes and pull-out force at various %WR. The result shows that the hardness value has a linear relationship with pull-out force up to 12%WR for uniform thickness tubes. Thickness offset tubes` reduction in pull-out force was observed compared to uniform thickness tubes at higher %WR. There are evident severe plastic strain and grain rotation at the thinner sections. Thickness offset of 0.125 mm tubes does not severely affect joint strength when expanded to 12%WR. At 0.25 mm and 0.5 mm thickness offset tubes can safely expand up to 9.5% and 7.8%WR, respectively. Thickness offset above 0.5 mm is not acceptable.