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

Abstract

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 AK,, values in helium were lower than in air. The AK,, 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 AKlh 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. NOMENCLATURE da/dN = fatigue crack growth rate K,, = stress intensity factor to close the crack K,,, = maximum stress intensity factor K,,, = minimum stress intensity factor AK = alternating stress intensity factor equal to K,,, -K,,, AKerr= effective AK equal to K, , , -KcI AKth = threshold alternating stress intensity factor R = load ratio equal to K,,/K,,, uys = yield stress E = elastic modulus Kthmax=AKth/(l -R ) CTOD = crack tip opening displacement SIMS = secondary ion mass spectrometry.

On the Effect of Testing Frequency on High and Very High Cycle Fatigue Behavior of AA2024-T3, Ti-6Al-4V, and Inconel 718

“…In order to determine the threshold stress intensity factor range and the fatigue crack initiation behavior Eq 3 was fitted to an experimentally derived S-N curve with 20 Hz testing frequency in order to quantify the values of the fatigue strength constants a and c. The value of the threshold SIF range for 20 Hz of testing frequency was taken from the literature for all the materials (Ref 32,35,[44][45][46][47][48][49][50]. Once the fatigue strength constants were derived for 20 Hz they were used to determine the threshold stress intensity factor range for the other testing frequencies, 90 and 1000 Hz.…”