With inelastic analyses requiring a substantially greater expenditure of effort over elastic analyses, there is strong incentive to establish an elastic design criterion for ratchetting that is not excessively conservative and consequently restrictive.
To establish design curves for ratchetting, the Ansys computer program was used to develop elastic-plastic-creep solutions for several cases of a long, sodium-filled pipe. The pipe was subjected to a fluid temperature history which consists of a rapid down-transient followed by an isothermal rise to operating temperature, and a 500 h holdtime at that temperature. The parameters of the analytical solutions covered the range of interest for sodium-cooled nuclear systems. The only material considered was Type 304 stainless steel.
Design curves were developed for ratchet strain as a function of elastic primary and secondary stress intensities and temperature. The diagrams presented here are of similar format to those given by Bree.
The results of this study indicate that the existing design rules of FRA-152 Revision 3 and of the Liquid Metals Engineering Center (LMEC) supplement to the ASME Code Case 1331-5 for loading cycles with long holdtimes are very restrictive, with one exception. That exception is Rule A.2.1.2(4) of the LMEC rules. When compared to the results of the study, the rule is consistently and reasonably conservative for all loadings and temperatures studied.
In this study it was also desired to correlate the ratchetting results of the straight pipe to the analytical ratchetting results for more complex geometries. However, the correlation was not completed in time for this report.