Anomalies in Long-Crack Propagation at Low ΔK in Some Engineering Alloys.

In this article, we discuss an unusual pattern in long-crack behavior at low stress intensity factor ranges ΔK (below ΔK), characterized by an initial dip, followed by a plateau, and then an acceleration in fatigue crack growth (FCG) rate. This unanticipated FCG behavior was first observed experimentally in the IMI 834 alloy and reported by Marci in 1996. Such an anomaly is only reported from experimental observation but cannot be understood or explained using the plasticity, roughness, or oxide-induced crack closure assumptions.

Unusual Fatigue Crack Growth Behavior of Long Cracks at Low Stress Intensity Factor Ranges.

In this article, we characterize and review the unusual lack of threshold in fatigue crack growth (FCG) behavior for some alloys at low values of stress intensity factor ranges ΔK and its implications to damage-tolerant design approaches. This unusual behavior was first observed by Marci in 1996 in IMI 834 alloy. Conventional applications of linear elastic fracture mechanics to FCG analysis at constant R-ratio (or K) assumes that (da/dN) decreases monotonically with decreasing ΔK and approaches the threshold value of ΔK with (da/dN) ≤ 10 mm/cycle for a given R (or K).