A Method for Directly Assessing Elastic Follow Up in 3D Finite Element Calculations

Abstract

Elastic follow up significantly influences the creep and creep-fatigue life of structural components in elevated temperature service. As such, assessing the amount of follow up a component sees under a given loading history is critical in properly designing a component for high temperature use. Current methods for quantifying elastic follow up based on an elastic follow up factor use simple constitutive models and are available only for simple component geometries. This work describes a method for assessing the follow up factor, as a function of position and time, from a finite element simulation. The method recovers the simple, analytic solutions for simple geometries and material models but can also provide follow up factors for complicated components, loading histories, and material models. We use this new method to assess the range of follow up factors in a variety of prototypical high temperature structural components. This information can be used to improve design methods by better understanding the magnitude of follow up present in typical high temperature components. The method can also be used to support new design methods that require time-dependent point values of the follow up factor across a component.

Design and Scoping Tests on Alloy 617 Using Notched Specimen Geometry to Validate Methods for Multiaxial Stress Relaxation

“…In the multiaxial stress state, the elastic follow-up factor can be calculated using Eq. ( 10) (Messner et al 2019):…”

Report on FY 2023 Research and Development on Specially Designed Creep-fatigue Experiments on Alloy 617 in Support of Improving Creep-fatigue Evaluation Approaches

“…In the multiaxial stress state, the elastic follow-up factor can be calculated using Eq. 25 (Messner, M. C. et al 2019):…”

Draft Rules for Alloy 617 Creep-Fatigue Design Using an EPP+SMT Approach

“…In our previous work [14,15], we used a simple two bar model, one bar representing the test section in series with another bar representing the driver section, to evaluated the elastic follow up applied to the test section from an inelastic simulation. We used the tangent definition of elastic follow up provided in [33] to determine the elastic follow up from an inelastic simulation. We also performed EPP cyclic analysis to determine the steady cyclic strain range for the test section.…”

“…We performed two analyses for each SMT tests: 1) inelastic analysis using the Alloy 617 constitutive model provided in [34] to evaluate the elastic follow up and 2) cyclic EPP analysis using the method described in Section 3.1 to determine the EPP cyclic stable strain. We calculated the elastic follow up factor, using the tangent definition provided in [33]. Figure 4.17 shows example results from analyses.…”