Evaluation of crack growth behavior and probabilistic S–N characteristics of carburized Cr–Mn–Si steel with multiple failure modes

Interior induced fatigue of surface‐strengthened steel under constant and variable loading: Failure mechanism and damage modeling

“…It should be noted that under these two loading conditions, the location of inclusions, ie, the crack nucleation sites, all can fall into two categories. First, majority of crack nucleation sites are confined to the matrix region under these two loading conditions, as shown in Figures A and A, which is similar to the conventional failure behavior of surface‐hardened alloy steels in the long life regime . Secondly, a small amount of crack nucleation sites distributed in the carburized layer are also observed, as shown in Figures D and D.…”

“…For the same axial loading with uniform stress distribution on the cross‐section area of specimen, the CA or VA essentially has no effect on the inclusion size. The inclusion sizes sometimes also tend to decrease with the increase of fatigue life, and sometimes are regardless . The reason why such a difference appears between CA and VA conditions is still unsolved at the present stage.…”

“…It is known that the fatigue process is extremely sensitive to the surface state of component or part because most of cracks easily initiates from free surfaces of cyclically loaded materials. To avoid this, a number of common surface‐strengthening technologies such as carburizing, nitriding, and shot peening are still widely adopted to raise the fatigue resistance of component or part . However, the VHCF tests show that the surface‐strengthening sometimes cannot improve the fatigue properties of structural materials .…”

“…For alloy steels, the interior failure is mainly caused from the interior nonmetallic inclusions or inhomogeneous microstructures . Generally, a fisheye‐shaped crack occurs on the fracture surface, in which a peculiar area, named “fine granular area (FGA)” by coauthor Sakai, sometimes can be observed in the vicinity of crack nucleus, but sometimes it does not occur even in the life regime beyond 10 8 cycles . Some ideas or models such as “Polygonization and debonding,” “Depressive decohesion of spherical carbide,” “Continuous grain refinement,” “Numerous cyclic pressing,” “Hydrogen embrittlement‐assisted cracking,” “Repeating contact of crack surfaces,” and “Stage I‐like crystallographic crack propagation” have been proposed to explain the formation mechanism of the FGA from the various viewpoints.…”

“…6,7 However, the VHCF tests show that the surface-strengthening sometimes cannot improve the fatigue properties of structural materials. 7,8 This is mainly attributed to a transition of crack nucleation site from the surface to the interior or subsurface. Especially under the impact of surfacestrengthening, the probability of interior failure is much increased.…”