Research And Simulation Of Thermal Autofrettage Technique For Internally Pressurized Thick-walled Cylinders

Autofrettage technique is usually adopted by internally pressurized thick-walled cylinders to increase bearing capacity and extend fatigue life when subjected to high or ultrahigh pressure. Hydraulic autofrettage and swage autofrettage are the most common methods, but there are certain difficulties in these autofrettage process. The thermal autofrettage is achieved by creating temperature gradient in the wall of the cylinder. It is easy to handle, safe and inexpensive. Therefore, it is of great importance to investigate the theory of thermal autofrettage and analyze the effects of temperature gradient on bearing capacity.Basic research on some important phases of the thermal autofrettage for internally pressurized thick-walled cylinder is carried out with the combination of theory method and numerical simulation, which could provide a theoretical guide for the application of thermal autofrettage.The primary investigation contents of this paper are as follows:First, the theory of thermo-elastic stresses was systematically studied.On the basis of the elastic-perfectly plastic material, The calculation formulas such as threshold temperature differences of initial yielding at inner wall, the total stresses due to internal working pressure and thermal load, the relationships between the maximum bearing capacity after autofrettaged and temperature gradient were obtained by using the yield criterion of Tresca. The distribution rules of thermo-elastic stresses and the influence of temperature gradient on bearing capacity were summarized. Second, mechanical models of elastic and plastic zone were built. On this basis, the formulas of thermal stresses, residual thermal stresses and total stresses under plane stress condition for the phases of elasto-plastic deformation were obtained. The numerical solutions ofelasto-plastic interface and threshold temperature difference of initial yielding at the outer wall were solved, the distribution curves of stresses were shown by MATLAB programming. Then the proper autofrettage technique was studied for internally pressurized thick-walled cylinders at this phase. Third, the simulation model of thermal autofrettage was built.Based on thermal-structural coupling analysis and plastic analysis, the autofrettage process was simulated, predicting the distribution of elasto-plastic stresses and the location of elasto-plastic interface. The calculation results from theory method and simulation were compared,which confirmed the accuracy of theory analysis and the validity of simulation.It is shown that at the phase of elastic deformation, the maximum bearing capacity increases linearly with the increase of temperature gradient and internally heating suits for autofrettage. At the plastic phase,thermal residual stresses due to outer heating are compressive stresses on the inner side of the cylinder. The maximum bearing capacity does not increase linearly with temperature gradient’s increase when employing these compressive residual stresses for the pre-stresses of the cylinder.These features could offer the reference for the further research on the bearing capacity after thermal autofrettaged. ANSYS simulation could be an important approach to simplify calculation process and improve research efficiency.