In-situ ESEM study of thermo-mechanical fatigue crack propagation

An experimental procedure to determine crack propagation rates under thermo-mechanical loading is presented. The experiments were conducted in-situ within an environmental scanning electron microscope, and images of the crack tip region were taken throughout the load cycles. The mechanical load was provided by a load controlled electrically driven load stage. Thermal heating of the crack region was provided by means of resistance heating, and cooling by water flowing through the grips holding the specimen. In-phase experiments with the mechanical load following the temperature, as well as out-of-phase experiments with maximum mechanical load coinciding with minimum temperature, were conducted. Temperature cycles with a range from 300 °C to 550 °C, and from 300 °C to 630 °C with cycle times of 55 s and 75 s, respectively, were performed. Crack propagation rates as well as fracture surface morphologies were compared with corresponding results obtained under iso-thermal conditions at 550 °C and 630 °C. It was found that the temperature at which maximum mechanical load was applied was decisive for both the crack propagation rate and the fracture surface morphology, regardless of whether the temperature was cycled or kept constant. The study indicates that iso-thermal fatigue propagation data can be used to predict crack propagation rates provided that iso-thermal data are taken from the temperature at which the mechanical load peaks are known.