Influence of autofrettage on pressure limits and fatigue life of pressurized thick - walled cylinders

Abstract

In optimal design of thick-walled cylinder, there are two main objectives to be achieved, increasing its strength-to-weight ratio and extending its fatigue life. This can be achieved by generating a residual stress field in the cylinder wall prior to use, a process known as autofrettage. Two different cylindrical components are proposed in this study; a plain and a stepped thick-walled cylinders. They are modelled using two-dimensional axisymmetric elements, and analysed for optimisation of autofrettage pressure and fatigue life. A Finite Element (FE) Method using ABAQUS is carried out on the cylinders to develop a procedure in which the autofrettage process is determined numerically, resulting in a reduced maximum equivalent stress distribution. Cylindrical pressure vessels often have a fluctuating internal pressure load and can fail through fatigue. For this purpose a fatigue life evaluation of the cylinders is performed, using FE-SAFE, to evaluate the structural integrity of autofrettaged vessels. A technique for elastic-plastic analysis of thickwalled cylinder under internal operating pressure is proposed where the performance of the cylinders is evaluated for different levels of autofrettage. The results reveal three scenarios in the design of thick-walled cylinders. For maximum load carrying capacity, non-autofrettage is suitable when, in service, the whole wall thickness will be yielded. Full autofrettage is suitable when, during subsequent operation, yielding is limited at the inner surface. Optimum autofrettage of the cylinder is suitable if a minimum equivalent stress is to be achieved. FE simulation shows that the effect of external step on the optimum autofrettage is not significant. Experiments are carried out to validate the numerical results of residual stress. There is a good agreement between the FE simulation and the strain measurements. In fatigue analysis, the fatigue life initially increases with autofrettage level, reaching a maximum optimum level and then decreases. The optimum autofrettage leads to an optimum fatigue life which is found to be about 3.24 times greater than non-autofrettaged cylinders. The analytical solutions are compared to numerical results and a very good correlation in form and magnitude is obtained.