Abstract: This paper conducts a study on the inconsistent practices and overly conservative assessment results associated with the "algebraic sum of three principal stresses" criterion which is commonly adopted in domestic engineering design and based on elastic stress analysis. Taking the structure of an inclined nozzle on a pressure vessel cylindrical shell as a case study, elastic and elasticplastic analysis were carried out respectively with a comparative analysis of the evaluation results. The results show that the evaluation based on the "algebraic sum of the three principal stresses" of the total stress from elastic analysis exceeds the limit value by 22.5%, rendering the structure unqualified; but the evaluation by elastic-plastic analysis obtain a qualified result with a safety margin of approximately 85% remaining. It is demonstrated that the evaluation of this structure using the "algebraic sum of the three principal stresses" of the total stress based on elastic stress analysis leads to an overconservative design. Therefore, it is recommended to prioritize the adoption of elastic-plastic analysis methods and criteria for localized strain failure assessment in special complex structures, thereby achieving more rational and optimized design.

Key words: localized excessive strain, principal stress, pressure vessel, elastic analysis, elastic-plastic analysis