MEAN STRESS MODELS FOR LOW-CYCLE FATIGUE OF A NICKEL-BASE SUPERALLOY

The aim of this study was to relate observed low-cycle fatigue respons e under asymmetric loading to mean stress effects on the cyclic behavi our and fatigue life of nickel-base Incoloy 901 superalloy at room tem perature. Based on experimental data, a mean stress relaxation model w as proposed, which described the dependence of mean stress on the tota l strain range and on cyclic plasticity. It was found that cyclic soft ening and mean stress relaxation began when plastic strain started to develop, and increased in importance with increasing strain range. Bot h depended on the plastic strain range rather than on monotonic plasti c deformation. For any given strain ratio there was a value of strain range corresponding to peak mean stress sustained. Beyond this strain range, mean stress relaxation began. A mean stress strength constant r epresenting the apex on the sigma(m) axis of the Goodman diagram was i ntroduced in order to modify the Goodman criterion, resulting in a mod ified strain-life relation. Both the mean stress-relaxation model and the strain-life model were evaluated against LCF test data.