Verification of the EPP Code Case for Strain Limits Evaluations by Inelastic Analysis Method

An ASME Section III Division 5 code case, N-861, for the evaluation of strain limits based on the elastic-perfectly plastic (EPP) methodology has recently been published. A key feature of the EPP methodology is the application of the EPP finite element analysis method with a pseudo yield stress to bound component response under elevated temperature cyclic service. The simplified inelastic approach in Division 5, Appendix HBB-T that is based on the elastic analysis results is not applicable at the elevated temperature range where creep and plasticity cannot be distinguished and unified viscoplastic model is required to describe the deformation behavior. The EPP strain limits code case overcomes such limitations. It also has the distinct advantage that stress classification, which is required by the simplified inelastic approach, is not needed. Thus, it is ideally suited for modern-day finite element technology. The conservatism of the EPP strain limits code case was verified for some simple geometries. In this paper, a viscoplastic constitutive model calibrated to the experimental data for 316H stainless steel is used to conduct a full inelastic analysis. The calculated strain accumulation is compared with that obtained from using the EPP code case approach.