Fatigue Crack Propagation Threshold

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Abstract
Service-type fatigue loading conditions are critically analyzed with respect to their relevance for a fatigue crack growth threshold. It was found that a threshold should be experimentally determined as a function of the monotonic-plastic zone size (i.e., as a function of Kmax). For each Kmax the cyclic-plastic zone size depends on the applied ΔK and threshold testing should cover the range ΔK ≈ ΔKth and ΔK (R = 0) for tension fatigue. For a given plastic zone, Kmax and ΔK fixed, the threshold ΔKth could occur anywhere between Kmax and Kmin. This study presents experimental procedures and furnishes results for Inconel 617 and a Ti-6A1-4V alloy which allow one to analyze the influence of the three parameters cited above. It demonstrates the following: • The threshold decreases “linearly” with increasing monotonic-plastic zone size; this confirms existing data. • The cyclic-plastic zone has minimal influence on the fatigue threshold with Ti-6A1-4V, while this influence is substantial in the case of Inconel 617. • A shift of threshold between Kmax and Kmin does not alter the magnitude of the threshold.