Characteristic of interior crack initiation and early growth for high cycle and very high cycle fatigue of a martensitic stainless steel

In this paper, interior crack initiation and early growth are investigated to determine the high cycle and very high cycle fatigue of a martensitic stainless steel by constant and variable amplitude loading fatigue tests. It is shown that the tested material presents a very large fine granular area (FGA) size (72-176 mu m), and the value of the stress intensity factor range for FGA remains almost constant irrespective of the stress amplitude and the fatigue life. The equivalent crack growth rate in FGA is of the magnitude 10(-13)-10(-11) m/cyc as determined by the "tree ring" patterns on the fracture surface under variable amplitude loading. Moreover, transmission electron microscopy shows both refined and coarse grains in the extracted samples beneath the FGA, i.e., there is no inevitable relationship between FGA morphology and refined grains beneath the FGA. Additionally, multi-site crack origins are observed under both constant and variable amplitude loadings. This indicates that the crack tends to initiate from the larger inclusion in the highly stressed region for the inclusion-induced fatigue failure