Effects of Stress Ratio and Hold-Time on Fatigue Crack Growth in Alloy 718

Crack growth rate da/dN in Alloy 718 at 650°C, compared on the conventional ΔK basis, increased with increase in R (K_min/K_max) and also generally with inclusion of hold time at K_max and K_min. An exception, however, was that a 1-min hold at a K_min just below the threshold K for creep crack growth caused retardation of crack growth. Correlation of da/dN for various R levels was better with K_max than with other parameters such as K_min, ΔK, and Δ J-integral. Fatigue crack growth in the alloy was found to be time dependent and K_max controlled under these test conditions and consequently the best overall correlation was given by da/dt with K_max. An unusual observation was that the addition of hold time to the loading cycle at near zero K_min (R = 0.05) caused an increase in da/dN, as the result of an environmental interaction.