Simplified Elastic-Plastic Analysis Method Using Ke Factors Obtained for the Real Structural Model

Abstract

This paper describes a new simplified elastic-plastic analysis method, which utilizes a plastic strain multiplication factor (Ke factor) obtained from elastic-plastic finite element analysis (FEA) results for the same structural model in the design stress calculation. ASME Code, Sec. III specifies a simplified elastic-plastic analysis method which can be used when PL+Q intensity exceeds the 3Sm limit, provided that the rules to prevent thermal stress ratchet are satisfied. The conventional method requires using Ke factors given by a closed-form equation having a value of PL+Q intensity as a variable. The elastically calculated peak stresses need be multiplied by the Ke factors, before performing the fatigue analysis. The Ke factors in the Code were derived from strain multiplication factors calculated for rather simple structural elements, which are assumed to cover a wide range of structural components. Consequently, although the rule can be applied safely to most of the cases, the results are usually too conservative. On the other hand, when PL+Q intensities are near 3Sm level, it has been pointed out based on experiments and analyses that the current Ke has a lack of margin. We propose to use the Ke factors obtained by FEA of the real structural model, in order to avoid the foregoing overconservatism and the lack of margin. The procedure also makes it unnecessary to extract PL+Q category, which is necessary in the conventional evaluation method. Elastic and elastic-plastic FEAs were performed for the axisymmetric nozzle in a vessel, and the effectiveness of the proposed procedure was shown in a practical case. Generalization of the procedure is also discussed.