Optimum Autofrettage and Shrink-Fit Combination in Multi-Layer Cylinders

The interest in the use of layered cylinders that combine autofrettage and shrink fit in order to extend fatigue lifetimes is increasing. As the number of layers increases, the sequential order of assembly and the size of each layer become more important. To achieve the most benefical result, a design optimization method is required. In this investigation, the optimum design of a three-layered vessel for maximum fatigue life expectancy under the combined effects of autofrettage and shrink fit has been considered. To obtain optimum size of each layer and to optimize the initial stress distribution, the numerical optimization procedure known as the Simplex search method is employed here. The thickness of each layer, shrink-fit pressures, and autofrettage percentages are treated as design variables. Under stress constraints, the operational sequences for assembly of a layered vessel have been formulated in order to lead to optimum results, defined as maximum life expectancy. The fatigue life consideration is based on ASME code provisions and standards for high pressure vessel technology, which define the allowable final crack depth for multilayer vessels. The proposed procedure has been carried out on a number of examples. The results show that, with proper combination of operations significant life enhancement can be achieved using the optimization procedure.