Thermomechanical fatigue (TMF) is a unique type of fatigue process in which a component is simultaneously subjected to fluctuating loads and temperature. Isothermal life prediction techniques are often not applicable to TMF conditions since mechanical properties are temperature dependent with different damage mechanisms. There are two major cycles in TMF: the in-phase (IP) cycle where the maximum strain peak coincides with the maximum temperature and the out-of-phase (OP) cycle where the maximum strain and the lowest temperature coincide. Experimental and analytical methods are developed to address the effect of thermomechanical strain cycling on coated nickel base superalloy IN-738LC material which is a γ' (Ni3Al) strengthened material used primarily for land based gas turbine blades. The coating system was a NiCoCrAlY overlay type. Tubular specimens in the two conditions, coated and uncoated, were primarily tested in out-of-phase (OP) TMF loading with a temperature range of 482–871°C. Using a viscoplastic concept which accounts for strain/temperature cycling response of substrate and coatings in terms of hysteresis loops which characterize the evolution of stress/strain/cycle up to mid-life cycle, a life prediction model was developed incorporating the effect of creep (strain hold-period), environment, and temperature. Test results show the OP TMF type cycle is the most damaging cycle for the coated IN-738LC material when compared to both in-phase and isothermal cycles. All experiments were strain-controlled with a triangular waveform and a strain-ratio A = εamp/εmean = ∞.

A biaxial fatigue model for axial-torsion strain cycling is proposed which includes elastic and plastic strain life regimes at elevated temperatures. The biaxial fatigue model is based on the concept that the transition from the plastic strain region to the elastic strain region on the bilinear fatigue life curve occurs at a cycle at which the elastic and plastic strain components of the total applied strain are equal. This transition corresponds to the intersection of the elastic and plastic strain lifelines of either a uniaxial or a torsional strain cycling condition. The transition point can combine the fatigue strength and fracture ductility coefficients of the material. The von Mises yield criterion was modified by using the Davis and Connelly triaxiality factor (TF) to account for the effects of the stress state and strain rate on the fracture ductility at elevated temperatures, which decreases as TF increases. Experimental data by the authors as well other available data were analyzed on the basis of the proposed model.

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Three sections of piping removed from a styrene furnace were metallugically examined. The piping was fabricated from alloy 800H and was service-exposed at temperatures in the range of 621 to 774°C (1150 to 1425°F) for times in the range of 73,500 to 90,000 hours. These samples were investigated by metallurgical studies and mechanical testing to determine the effect of the prolonged high-temperature service on the integrity of the components. A few specimens from the samples were re-annealed to determine if the properties could be restored to their original values. A few more specimens were re-annealed and aged for 1000 hours to determine if significant changes would occur during short-time exposure to high temperature. With one exception, the service-exposed samples exhibited microstructures and properties that were comparable to mill annealed and short-time exposed material. Modest increases in strength and reduction in ductility accompanied the exposure. The exception was material exposed to the highest temperature for the longest time. Here, a significant decrease in the ultimate strength and ductility was observed in a test at 704°C (1300°F).