Study of crack initiation or damage in very high cycle fatigue using ultrasonic fatigue test and microstructure analysis

• A new concept on local plasticity exhaustion for damage in metal matrix during very high cycle fatigue.
• Extensive experimental work for 4 alloys with different microstructures using TEM and SEM, EBSD.
• A new analysis method of strain mapping using EBSD to describe local plasticity exhaustion.
• Mechanism on subsurface non-defect fatigue crack origin (SNDFCO) was studied. Very few reports on this.
• VHCF behavior of a metal material is strongly dependent upon its microstructure.

Fatigue damage behaviors of four metal materials in the very high cycle fatigue (VHCF) regime have been studied using ultrasonic fatigue test and microstructure analysis. The results show that the fatigue crack initiation in VHCF regime could occur at subsurface non-defect fatigue crack origin (SNDFCO), where the accumulated cyclic strains or damage in the specimens were highly localized, especially in the materials with some softer phase, where the local maximum strain can be eight times higher than the average strain value in the specimen. This high strain localization can cause a local plasticity exhaustion that leads to a stress concentration and consequently fatigue crack initiation, and finally the formation of SNDFCO. For pure single phase austenitic material, strain localization can also occur due to dislocation accumulation at or near grain boundaries, which can become fatigue crack initiation origin in the VHCF regime. The results in this study show that fatigue damage and crack initiation mechanisms in the VHCF regime can be different in different metals due to the mechanisms for local plasticity exhaustion.