Abstract
Per ASME Code Section III Div. 5, design of a Class A component for elevated temperature service must meet the functional requirements specified in the Design Specifications (NCA-3211.19) to be acceptable. Also it needs to satisfy the requirements for a design by analysis, either in HBB-3200 or in the Design Rules for components, while under the loadings described in HBB-3111.2 and the Design Specifications; and satisfies the general design rules of HBB-3130 and the applicable design rules for components. The creep-fatigue assessment is part of the Code requirements to meet the strain and deformation limits for a design by analysis for a Class A metallic pressure boundary component as required in HBB-3200, and a Class SM metallic core support structure as required by HGB-3200. As suggested by the Code, the analysis can be either elastic analysis, elastic perfectly plastic analysis, or inelastic analysis. Although the inelastic analysis is more complicated than the elastic analysis method considering the required time-dependent material properties to account for plasticity and creep, the elastic analysis method has more complicated rules than the inelastic analysis since the elastic strain range must be adjusted to account for plasticity and creep. The adjustments are usually over-conservative that makes it difficult to pass the required limits under most design situations by elastic analysis method. This paper compares the creep-fatigue evaluation procedures and results using elastic analysis method and inelastic analysis method, emphasizing the safety margin each analysis method can achieve.
