Preliminary description of a new creep-fatigue design method that reduces over conservatism and simplifies the high temperature design process

Abstract

The report provides the initial description of a new creep-fatigue design method for structural components in high temperature nuclear service. The new method is based on an integrated elastic-perfectly plastic (EPP) analysis and Simplified Model Test (SMT) approach that reduces over conservatism, improves the treatment of elastic follow up, and simplifies the design procedure, when compared with the current creep-fatigue design methods in ASME Boiler and Pressure Vessel Code. Developing the design charts for the EPP-SMT design method requires extrapolating SMT test data as a function of hold time and follow up factor. The report develops the preliminary design charts for Alloy 617 at temperatures between 800°C and 950°C by combining two extrapolation approaches developed in a previous work. The report also presents a comparative analysis between the EPP-SMT design method and the current ASME creep-fatigue design methods by evaluating design life of two sample geometries under different loading conditions. Results from the comparative analysis verify the EPP-SMT design charts but suggest the requirement of additional test data in the low strain range regime for improving the extrapolation procedure that will further reduce the over conservatism in the creep-fatigue damage evaluation. The report also concludes that the EPP-SMT design procedure can account for effect of primary load on creep-fatigue life by using a fixed, bounding value of follow up in constructing the design charts. The conclusions to this report describe the future work required to complete this new design method so it can be codified through a nuclear Code Case Preliminary description of a new creep-fatigue design method that reduces over conservatism and simplifies the high temperature design process

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

“…This chapter provides a commentary describing the rationale behind the proposed design rules. For some elements of the design method, as discussed below, the commentary summarizes the development work while referring to previous ART sponsored work [5,[14][15][16] for details. The commentary provides detailed discussion on the recent works including the procedure for generating the design charts, the effect of elastic-follow up on creep-fatigue damage evaluation, and finally, validating the overall design method through comparing with failure data from p-SMT tests.…”

“…Note the change in Eq. 4.5 from previous work [14] where the value of was assumed to be 1, i.e. a linear correlation with fixed slope of -1 between cycles to failure and strain range in the log-log space for experimental test data in the high strain range regime.…”

“…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.…”

“…Our previous work [14] describes several approaches to include the primary load effect in the EPP+SMT design method. However, the recommendation is to use the increased strain range from EPP analysis due to primary load and combine it with a bounding elastic follow up factor applied to the design allowable cycles.…”

“…This chapter is also a commentary describing the rationale behind the design rules. For some elements of the design method the commentary summarizes the development work from past research [5,[14][15][16]. This chapter also discusses a procedure for generating the design charts in detail.…”