Thermo-mechanical Fatigue Crack Propagation in Inconel 718

Abstract: Thermo-mechanical fatigue crack propagation in Inconel 718 has been studied with different methods. In-situ observations with a scanning electron microscope (SEM) were performed, the crack propagation process was followed cycle by cycle and the influences from load, temperature, and microstructure on the crack growth were analyzed. To measure the displacements an image analysis technique was developed and to measure the contact between the fracture surfaces the electrical potential drop signal was measured over the crack mouth. The phenomenon of crack closure has been investigated by measuring the displacements between the fracture surfaces close to the crack tip and by measuring the electrical contact between the crack surfaces. A study of cracks under steady state growth conditions and subjected to an overload was also conducted. Thermo-mechanical fatigue was performed in-situ within the SEM and a heating device was developed to generate thermal cycles. A heating wire was placed on the test specimen and fixed with aluminum oxide paste to heat the test specimen, and the grips on the load cell were cooled with water. Differences between thermo-mechanical cycles in-phase and out-of-phase were compared with iso-thermal cycles in the temperature intervals 300–550°C and 300–630°C. These tests showed significant differences in crack propagation mechanisms and crack propagation rates. Thermo-mechanical fatigue tests were also performed using cylindrical specimens with a diameter of 6.3 mm. To generate the thermal cycles, a system blowing alternately hot and cold air through a nozzle onto the test specimen was constructed. Thermo-mechanical load cycles were performed in-phase and out-of-phase with different load ratios R in the temperature range 200–550°C. Crack closure level and crack length were measured using the potential drop technique. A correlation between effective J-integral range and crack propagation rate was found after compensating for crack closure, making the ?Jeff a possible fracture mechanical parameter for prediction of crack propagation rates under given loading conditions.