In-phase and out-of-phase low-cycle thermal-mechanical fatigue tests were carried out on 316L austenitic stainless steel specimens controlled by computer. A nonlinear kinematic hardening model with internal variables was developed to describe the cyclic stress strain behavior of thermal-mechanical fatigue and a very good approximation of the hysteresis loops was obtained by comparing with the experiments. In order to predict the lifetime, a typical stabilized hysteresis loop of thermal-mechanical cycling was considered to be the combination of a great deal of parts at different temperatures. Each part has a different contribution rate of the energy density to the damage by introducing a damage factor depending on the temperature. With isothermal fatigue data an equivalent total strain energy density was deduced as a damage parameter and with which the calculation for thermal-mechanical fatigue lifetime prediction gave satisfactory results corresponding to test data.

Effects of thermal exposure (TE) on high cycle fatigue (HCF) behaviours in Ni-based superalloy GH4169 have been investigated in this study. GH4169 specimens were thermally exposed at 650°C for 1, 2, 7, 14 and 21 days before fatigue test. These TEs produced eruptions, bulge-like features and pits at surface due to oxidation of carbides/nitrides. Fatigue life was not reduced evidently by TE as compared to the specimens in virgin conditions, indicating the insignificant influence of TE on HCF life. Fracture morphology analysis showed fatigue crack mainly initiated from oxides, crystallographic facets and subsurface inclusions. Although long TE is not necessarily related to crack initiation from oxides, crack initiation from oxides is generally associated with a shorter fatigue life.