GENERALIZATION OF THE STRAINRANGE PARTITIONING METHOD FOR PREDICTING HIGH TEMPERATURE LOW CYCLE FATIGUE LIFE AT DIFFERENT EXPOSURE TIMES

High Temperature Low Cycle Fatigue (HTLCF) life depends upon both the waveform of loading (or strainrange) and the frequency (or exposure time). Conventional Strainrange Partitioning (CSRP) method takes into account the effect of waveform of loading on the cyclic life. Four different strainranges (or waveforms) are considered in the CSRP method. A separate life relationship is generated for each of the four different strainranges in the CSRP method. In this study, the CSRP method is generalized to incorporate the effect of exposure time on the life relationships of the strainranges involving creep strain. The Generalized Strainrange Partitioning Method (GSRP) consists of (i) Steady State Creep Rate (SSCR) Modified SRP life relationships and (ii) Exposure Time (ET) Modified SRP life relationships. The GSRP life relationships were established by conducting baseline experiments on 316 stainless steel at 1500F. At a given inelastic strainrange, a reduction in the generic cyclic life was observed with an increase in the exposure time for all the strainranges involving creep strain. Experiments were also conducted on the 316 stainless steel at 1500F to determine the Frequency Modified Life relationship (FML) and the Ductility Normalized SRP life relationships (DNSRP).; Three existing life prediction methods (CSRP, FML & DNSRP) are compared with the new method (GSRP). The results of this study indicate that the new method (GSRP) is better than the other life prediction methods. Numerous verification experiments involving complex loading waveforms were also conducted on the stainless steel used in the baseline experiments. Life predictions of the above verification experiments by the newly proposed GSRP method are closer to the experimentally observed lives than the life predictions by the other methods. Hence, the verification experiments established the superiority of the GSRP method over the other life prediction methods considered in this dissertation.; Post failure fractographic and metallographic analyses revealed that the reductions in the cyclic life at longer exposure times are attributable to (i) Sensitization of the steel due to the precipitation of carbides along the grain boundaries, (ii) Oxidation and (iii) Internal damage within the bulk of the material during prolonged periods of creep.