Abstract
Creep-fatigue (CF) interaction at elevated temperature is the most damaging structural failure mode for materials under cyclic loads. In the last 40 years, significant efforts have been devoted to elevated temperature code rule development in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Section III, Division 5, Subsection HB, Subpart B) to ascertain conservative structural designs against CF failure. However, the current CF evaluation procedure is complex and overly conservative. Recently, an alternative CF evaluation method, Simplified Model Test (SMT) based design methodology, is being developed with the purpose of simplifying and improving the CF evaluation process. The concept of this approach is to perform CF evaluation using a set of design curves developed directly from experimental CF data and avoid the separate evaluation of creep and fatigue damage.
Experimental CF failure data are the key in finalizing the SMT-based design curves. In assessing available data sources, major data gap was found at low strain ranges of below 0.3% and/or at hold times of longer than one hour. The lack of data under these conditions is due to many practical constraints such as the unrealistically long test duration and difficulties in controlling experimental parameters. In this study, an experimental method is proposed based on the concept of damage summation to generate CF failure life information at low strain ranges and long hold times. The CF test was designed for Alloy 617 at 950°C with elastic follow-up effect to slow down the stress relaxation process during the hold time. The test specimen was cycled at higher strain ranges to accumulate enough CF damage, followed by cycling at lower strain ranges and longer hold times, to allow for a reasonable test time to generate failure data. Results from the CF test are presented, and information generated through this method in verifying the SMT-based design curves is evaluated.
