ON A MECHANISM FOR VHCF

摘要

In a 2008 paper [1] which relied heavily upon the experimental work of Oguma [2] with Ti-6AI-4V, it was proposed that an important step in the VHCF process was the wearing away of crack closure at the crack tip to enable crack propagation . The Ti-6-4 data were obtained under axial loading conditions, and no plateau between surface and sub-surface fatigue lifetimes was observed. Such results are consistent with the proposal of Murakami et al. [3] that the plateau is an artifact arising because of testing and method of data analysis of small, 3mm diameter, specimens of steel tested in bending. Murakami found no such plateau in the testing of 7 mm diameter specimens under axial load conditions. The present paper considers that the ODA (optically dark area) in steels represents a region of intense wear where much of the VHCF lifetime is consumed to enable crack propagation to take place. The available evidence for such a mechanism is discussed, particularly with respect to Ti-6AI-4V.