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

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.