The paths of small fatigue cracks in high-strength steels initiated from inclusions and small defects

In fatigue, the early growth mechanisms, paths, and rates of the microstructurally small cracks are not well known. Growth of subsurface - undetected - cracks cannot be monitored in real time, and postfracture fractography is complicated because of the statistical nature of local microstructure. Furthermore, hammering or sliding often damages the fracture surface before the test is stopped. We addressed this challenge with two approaches. This paper deals with fractographic details connected to local microstructure obtained by milling and imaging with a focused ion beam in a scanning electron microscope (SEM). The results provided input such as crack growth on adjacent planes and their coalescence, as well as formation of the "optically dark areas" that play a key role in the early growth and initiation of small cracks in fatigue. A subset of data consisting of eleven 100Cr6 bearing steel specimens loaded at the same stress amplitude, with fatigue lives (Nf) in the range of 10·106 < Nf < 650·106 loading cycles are studied and discussed. The second test series revealed that very early initiation and crack arrest are typical for small notches in the 34CrNiMo6 QT steel. The relation of crack path and microstructure, along with their connection to the optically dark area, is discussed.