Effects of AIr and Inert Environments on the Near Threshold Fatigue Crack Growth Behavior of Alloy 718

The near threshold fatigue crack growth behavior of alloy 718 was studied in air and helium environments at room temperature and at 538°C. Tests were performed at 100 Hz and at load ratios of 0.1 and 0.5. At room temperature and at 538°C, the Δ K th values in helium were lower than in air. The Δ K th values in air decreased with increasing load ratio. These results can be explained with a model that involves the accumulation of oxide in the crack which enhances crack closure. In the air tests, the oxide build‐up on the fatigue fracture surfaces at Δ K th was of the order of magnitude as the crack tip opening displacement. In the helium tests, no significant build‐up of oxide on the fracture surface at threshold was found.