Fatigue behaviour of two nickel-base alloys I: Experimental results on low cycle fatigue, fatigue crack propagation and substructures

Abstract

The low cycle fatigue and fatigue crack propagation of Inconel 718 alloy and Waspaloy were investigated at 25, 550 and 650 °C. Microstructural observations by means of optical, transmission and scanning electron microscopy were extensively employed both in bulk low cycle fatigue specimens and within the plastic zone surrounding fatigue cracks in order to establish in which regime of crack growth rate the crack propagation process resembles the low cycle fatigue phenomenon. These qualitative observations indicate that there exists a close correspondence between the near-threshold regime and stage I crack initiation in low cycle fatigue. At higher crack growth rates the situation is more complex and is briefly discussed. Quantitative measurements of cyclic plastic strain amplitude inferred from the distance between intense deformations bands observed in either low cycle fatigue specimens or within the plastic zone around fatigue cracks are given. These measurements show a very large reduction in the equivalent plastic strain amplitude as the near-threshold crack propagation regime is reached.