Effects of loading waveform on fatigue crack growth of Udimet 720 Li superalloy

The influence of loading waveform, including constant load tests, on fatigue crack growth rate (FCGR) of the Ni-base superalloy Udimet 720 Li has been investigated in air at the temperature of 700 °C, using corner crack specimens. The fatigue crack growth is controlled by oxidation mechanisms and the mode of fracture is completely intergranular. The loading ramp has the same effect on crack growth rate as the unloading ramp. The time spent at maximum load produces higher crack growth rate than the same time consumed during loading or unloading ramp. High strain rate due to high loading ramp of about 300 kN/s followed by slow unloading leads to considerable increase in crack growth rate, which is attributed to interaction between crack tip plasticity, creep and oxidation.