Comparison of New Proposal for Simplified Elastic-Plastic Analysis and Code Case N-779

Abstract

In the fatigue analysis in Section III design by analysis, the evaluation is performed in terms of alternating stress, but the evaluation curves are alternating strain – life curves that were converted to alternating pseudo-stress by formally multiplying elastic-plastic (total) strains by a modulus of elasticity that is representative of the elastic regime. The corresponding evaluation method would be an elastic-plastic analysis where the calculated strain amplitude is multiplied by the modulus of elasticity that is provided with the Code fatigue curve to obtain the alternating pseudo-stress. When the alternating stress is calculated by elastic analysis but is found to be in excess of the elastic regime, corrections need to be applied that account for the difference between stress from an elastic analysis and the pseudo-stress that would be obtained from an elastic-plastic analysis. Performing such a correction is the objective of simplified elastic-plastic fatigue analysis. An alternative to the simplified elastic-plastic fatigue analysis in Appendix XIII, XIII-3450, is given by Code Case N-779. A new proposal has been developed recently, with the objective to be less conservative in most cases than the very conservative method in Appendix XIII, XIII-3450, and more straightforward to apply than Code Case N-779. The new proposal has been validated with elastic-plastic analysis results. The present paper details a comparison to Code Case N-779 over a large range of parameters. The causes for discrepancies and implications are discussed in detail.