INFLUENCE OF STRESS RATIO AND TIME AT MAXIMUM LOAD ON THE FATIGUE CRACK GROWTH BEHAVIOUR OF IN718 AT 600°C

摘要

The effects of dwell time at maximum load and stress ratio on the fatigue crack propagation rate of the nickel base superalloy IN718 at 600°C were analysed. The total applied cyclic frequencies ranged from 0.0015Hz to 15Hz and the stress ratios were 0.05, 0.5 and 0.8. Three regimes of FCG behaviour were observed: cycle dependent, time dependent and mixed. The increase in stress ratio produced a significant increase in fatigue crack propagation rate in the time dependent regime. To a first approximation, transition frequencies from intergranular to transgranular failures were obtained at high stress ratios. The different failure mechanisms were identified by SEM observations on the fracture surfaces.