Analysis and Optimization of Autofrettaged and Shrink-Fitted Compound Cylinders under Thermo-Mechanical Loads

Cylindrical shells have large industrial applications ranging from pressure vessels, engine cylinders and hydraulic chambers to chemical and power plants and they are typically subjected to severe mechanical or thermo-mechanical environmental conditions. The fatigue life, pressure and thermal load bearing capacities of thick-walled cylinders can be considerably improved by inducing near the bore compressive residual hoop stresses. Shrink-fit and autofrettage processes have been effectively applied to generate favorable compressive residual stresses. The main goal of this research study is to fundamentally investigate the compound cylinders subjected to autofrettage and shrink-fit processes and develop new design processing technique and practical design optimization strategies to enhance their fatigue life under cyclic thermo-mechanical loads. First, the residual stresses of compound cylinders subjected to different combinations of shrink-fit and autofrettage processes have been evaluated using the developed finite element model in the ANSYS environment. The stresses due to different cyclic thermo-mechanical loads have also been calculated for the different combinations of compound cylinders considering the fully coupled thermo-elastic finite element model. To validate the finite element model, an experimental setup has been designed to measure the temperature history at three different locations through the wall thickness and also hoop strain at the outer surface of a two-layer compound cylinder under internal quasi-static and cyclic thermal loads. The experimental results have then been compared with those obtained from the finite element model. Moreover, to compare the performance of compound cylinders under different thermo-mechanical loads, the fatigue life due to cyclic pressure, cyclic thermal pulses and cyclic combined thermo-mechanical pulses has been calculated using ASME code for high pressure vessel. Next, to enhance the residual stress distribution along the wall thickness of the cylinder, a new double autofrettage process has been introduced. In the proposed double autofrettage process, an outer autofrettage cycle is performed prior to a standard inner autofrettage cycle. This can provide an increase in the beneficial compressive residual stresses at the near bore area of the cylinder while decreasing the detrimental tensile residual stress at the outer part of the cylinder. The proposed process has then been utilized to construct new combinations of autofrettage, shrink-fit and double autofrettage processes. The residual stress distribution through the thickness and fatigue life of these new combinations have been evaluated and compared with those based on conventional combinations of shrink-fit and autofrettage processes. Finally, a practical design optimization methodology has been developed to identify the optimal configuration of autofrettaged and shrink-fitted cylinders. Optimization problems based on the high-fidelity finite element model is computationally very expensive and may not render accurate optimum results. Considering this in the presented research, design of experiment (DOE) and response surface method (RSM) have been used in combination with the finite element model to create smooth response surface functions which can accurately describe the behavior of the residual hoop stresses with respect to the change of design variables. The developed response surface functions have been effectively utilized in the design optimization problems to simultaneously maximize the residual compressive hoop stress and minimize the residual tensile hoop stress through the thickness of the compound cylinder. Nonlinear mathematical programming technique based on the powerful sequential quadratic programming (SQP) algorithm has been used in combination with the genetic algorithm (GA) in order to accurately capture the global optimal solutions. At the end, the residual hoop stress distribution and fatigue life of the optimum configurations for each combination of autofrettage and shrink-fit processes have been evaluated and compared