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Near-Threshold Fatigue Crack Growth Behavior of Fine-Grain Nickel-Based Alloys

Abstract Constant-K max fatigue crack growth tests were performed on two fine-grain nickel-base alloys – Inconel 718 (DA) and Rene 95 – to determine if these alloys exhibit near-threshold time-dependent crack growth behavior observed for fine-grain aluminum alloys in room-temperature laboratory air. Test results showed that increases in K max values resulted in increased crack growth rates, but no evidence of time-dependent crack growth was observed for either nickel-base alloy at room temperature. Introduction In cases where fatigue lives primarily depend on the early stages of crack growth, it is critical to understand the fatigue crack growth (FCG) characteristics in the near-threshold regime, defined schematically in Figure 1. Considerable research has shown that an increase in FCG rates (d a /dN) occurs with increasing stress ratio (R = K min /K max ) due to a reduction in crack closure effects (ref. 1). This R effect, schematically shown in Figure 1, is especially pronounced near the crack growth threshold. However, research has suggested that crack closure does not account for all stress ratio effects when R ranges from 0.5 to 0.95, and for K