The cylindrical vessels are used for storing fluids at high pressure. If the magnitude of the internal/external pressure is closer to the yield strength of the material used, then no thickness of the material will prevent the failure of the vessel. Hence shrink-fitted compound cylinders are used, which can store the fluids at higher pressure closer to the yield stress of the material. Optimally designed compound cylinder has equal maximum hoop stress in both - the inner and outer cylinders. The value of this hoop stress is closer to the value of yield stress of the material used. There are many parameters in the design of compound cylinder. Out of them only a few are important. Three important parameters are chosen for optimization – interface diameter, interference and outside diameter, keeping other parameters such as material, internal diameter, etc constant. The optimization is highly nonlinear and if the number of parameters is large, solution time will be more. Compound cylinders have historically been designed such that the maximum shear stress is equal in each cylinder. This is the optimum condition for yielding of the cylinder, since both cylinders yield at the same pressure. If compound cylinders are subjected to fatigue, this is not the case. A better design criterion is to equate the maximum tensile stress in each cylinder, since the maximum tensile stress controls fatigue crack propagation. I have used maximum tensile stress (Hoop stress) criterion to arrive at the optimum design of compound cylinder. This paper describes the method of determining the optimum dimensions of both the cylinders made of specified material and to withstand a specified internal pressure so that the volume (and weight) is minimum. The results obtained are verified by using ANSYS finite element analysis packages.
Key words: Optimum design, compound cylinder, maximum tensile stress, finite element analysis (FEA).
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