Effect of Load History on Near-Threshold Fatigue Crack Growth at Elevated Temperature in a Nickel-Based Superalloy

Studies have shown that the ΔKth,i versus σ* relationship can be used to interpret load sequence effects in early fatigue crack growth under flight-spectrum loading. The objective of this study is to determine whether this applies to load history effects in early crack growth when elevated temperature with and without associated corrosive environment is involved. Tests were performed to establish the ΔKth,i versus σ* relationship for Waspaloy at 650°C in air and in rarified nitrogen. Experiments were performed to study load sequence effects on early fatigue crack growth in the same material using specially designed programmed loads designed to induce contrasting levels of near-tip residual stress through periodic overload-underload sequences. These are followed by quantitative fractography of the fatigue fracture surfaces in order to investigate whether observed load sequence effects on crack growth rate can be explained by the ΔKth,i versus σ* relationship.