While fatigue is ubiquitous and is responsible for a large proportion of failure in engineering structures, we understand little regarding the mechanism of crack initiation and early-stage crack growth especially for very high cycle fatigue (VHCF) behavior. We identified in this report the microstructural evolution during interior crack initiation and early growth in a martensitic stainless steel in VHCF regime by constant and variable amplitude loading method. We observed in the post-mortem samples after VHCF a discontinuous gradient layer composed of ultrafine grains and coarse grains in the fractured surfaces using both electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The gradient ultrafine grains are resulted from excessive cyclic plastic deformation in the vicinity of the crack origin or crack tip, which promotes the formation of crack initiation. The equivalent crack growth rate in FGA (fine granular area) is also estimated based on the "tree ring" patterns under variable amplitude loading, which is in the magnitude of 10 −12-10 −11 m/cyc.

Specimen geometry and loading condition usually have a great influence on the fatigue strength of metallic materials, which is an important issue in evaluating the reliability of component parts. In this paper, a rotating bending fatigue test is performed at first on an hourglass specimen and a notch specimen of a high strength titanium alloy. Experimental results indicate that, in terms of local stress, the notch specimen endures higher fatigue strength in comparison with the hourglass specimen due to its relatively smaller control volume. Then, a probabilistic control volume method is proposed for correlating the effects of specimen geometry and loading condition on the fatigue strength based on Weibull distribution and the concept of control volume. A simple formula is obtained for the fatigue strength in relation to control volumes, in which the parameter is the shape parameter of Weibull distribution of fatigue strength. The predicted results are in good agreement with the present experimental data for high strength titanium alloy and the data for the high strength steel and the full scale EA4T axle in the literature.