Small Cracks and High-Cycle Fatigue

The problem of high-cycle fatigue (HCF) failures that has recently plagued the aircraft jet-engine industry is associated with the rapid growth of small defects under the action of high-frequency vibrational or resonant loading; such defects have been associated with regions of microstructural damage attributed to such processes as foreign object damage, surface fretting and low-cycle fatigue. Since the size of these defects is generally below characteristic inspection limits, traditional design and life-prediction procedures do not specifically address this problem. Moreover, from a fracture-mechanics perspective, small fatigue cracks, of dimensions less than ∼500 μm, often behave in an apparently anomalous fashion, compared to conventional results on through-thickness large cracks, of dimensions greater than ∼5 mm (e.g., in compact-tension specimens). Such “anomalous” small-crack behavior is not limited to metallic alloys but is also seen in ceramics and intermetallics. It is the intent of this paper to briefly review what a small crack is and how its behavior may differ from that of long cracks. Indeed, there are several definitions of small cracks which relate to their size in comparison to the dimensions of the microstructure, the local inelastic zones (e.g., plastic-zone size) ahead of the crack tip, and the zone of crack-tip shielding (e.g., from crack closure) behind the tip. The relevance of this to the HCF problem and more broadly to fatigue life prediction in general will also be discussed.