Very high cycle fatigue crack initiation mechanisms in different engineering alloys

The fatigue crack initiation mechanisms prevalent in high strength martensitic steel grades, hot rolled plate duplex stainless steels, cold rolled strip duplex stainless steel and a super alloy grade were compared. The fatigue testing of all the grades was conducted in the VHCF regime using an ultrasonic fatigue testing equipment operating at 20 kHz. Scanning electron microscope (SEM) observations of the fracture surfaces revealed the presence of a microstructure controlled initial growth of short fatigue cracks in all the tested grades. Fracture surfaces of the failed specimens of a high strength martensitic steel grade revealed the typical fine granular area (FGA) within the fish-eye area around the internal inclusions. Fatigue crack initiation in the cold rolled strip duplex stainless steel grade occurred at surface defects left over by the cold rolling process of this grade. However, the presence of FGA around the surface crack initiating defect was observed similar to the internal crack initiations in the high strength martensitic steels. By mapping the FGA size development during VHCF loading, as obtained from fracture surfaces, FGA growth results were obtained. A similar study on hot rolled plate duplex stainless steel grades, 2304 SRG and LDX 2101, revealed the presence of an initial crystallographic growth region (CGR) in which crack growth direction is changed by microstructural barriers such as phase and grain boundaries. The early plastic fatigue damage accumulation occurred predominantly in one phase or at the austenite-ferrite phase boundaries. On the other hand, an initial transcrystalline fatigue crack growth was observed in the Ni-based super alloy grade Inconel 718. (C) 2016 The Authors. Published by Elsevier B.V.