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Primary Stress Evaluations for Redundant Structures

“Primary Stress Evaluations for Redundant Structures” W. J. O’Donnell, J. S. Porowski, B. Kasraie, G. Bielawski, and M. L. Badlani, presented at the ASME Pressure Vessel and Piping Conference, Denver, Colorado, July 25-29, 1993, ASME PVP Vol. 265, Design Analysis, Robust Methods and Stress Classification.
The ASME Boiler and Pressure Vessel Code defines the permissable methods of analysis and provides conservative criteria for evaluation of stresses obtained by elastic and inelastic analyses. The Code leaves the choice of method to the user who decides whether to stay with elastic analyses, or to use a more effective method of analysis which considers nonlinear response of the material in order to improve the design. The current state of technology allows stresses and strains to be obtained even in a complex structure within a rational engineering effort. This paper discusses the use of simplified inelastic solutions to demonstrate structural integrity as required in the ASME code, with potentially significant reductions in the weight of the structure. Finite element analysis can be used to provide a more effective distribution of primary stresses in redundant structures. The analyses is performed for the benchmark problem of cylinder-to-plate shell under internal pressure. The model includes thin and thick plates. Primary stresses must remain in equilibrium with external and internal loads, and should nowhere violate the yield strength of the material. Only mechanical loads are considered in the primary stress category. Evaluation of primary stresses is the first important step in the analysis of Class I nuclear vessels, followed by analyses of secondary stresses and fatigue to justify the structural integrity of complex components. However, for the majority of pressure vessels fabricated for the chemical, power, and other industries, the analysis of primary stresses is the only analysis required by the code to justify the vessel design. Sizing the plates and other redundant components of such vessels affects the basic competitiveness of the design. Reducing the weight of such vessels by more effective determinations of primary stresses is therefore an important engineering concern.