Fatigue behavior of small cracks in an aircraft turbine disk alloy

The fatigue behavior of small cracks was investigated in powder metallurgy Gatorized IN100 at 649C. Cracks were initiated at laser defects or at intrinsic micropores. Fatigue crack growth rates were measured by the plastic replication technique over the crack length regime from 100 μm to 1 mm. Tests were performed in load control under high stress (σ_max ≤ 0.9σ_ys) elastic fatigue conditions with -1 ≤ R_σ ≤ 0.1 and = 10 cpm. Strain control tests were performed under elastic-plastic fatigue conditions with Δε_t = 0.6% to 1.2%, R_ε = 0, and frequency = 10 cpm. Under elastic fatigue conditions the LEFM stress intensity factor range correlated small crack (100 μm < a <1 mm) with long crack growth rates and consolidated small crack growth rates at different stress levels. Small crack growth rates at different R_σ ratios were consolidated using the Walker-modified stress intensity factor range. The results for the elastic tests are discussed in terms of the validity of employing LEFM at high stress levels and small crack sizes. Small crack growth rates under elastic-plastic fatigue conditions were consolidated using EPFM parameters including: the pseudostress intensity factor (ΔPK), the equivalent stress intensity factor (ΔK_eq) and the cyclic J-integral (ΔJ). Consolidation of small crack growth rates was achieved only when the total stress range or total strain range was used in calculating ΔPK, ΔK_eq and ΔJ. A linear relationship between Δσ_t and Δσ_eff was found using Newman's crack closure model. The role of the compressive stress-strain excursion in promoting higher crack growth rates is discussed. Anomalous crack growth behavior was not observed for small Mode I cracks under the conditions considered here.