The fatigue crack origin region is the region where the fatigue crack nucleates and initiates. The fatigue crack first nucleates and propagates here. Modern research generally believes that the material crack usually originates from the material surface or near the surface, mainly because the material surface needs to bear special stress state and environmental conditions. Microscopically, The binding force on the grain on the surface of the material is smaller than that inside the material, and it is easy to slip. For gray cast iron, the casting holes on the surface and near the surface and the tip of flake graphite are easy to cause stress concentration, which provides conditions for the nucleation and initiation of fatigue cracks. The figure shows the low magnification morphology of fatigue fracture and fatigue crack origin zone of three grades of gray cast iron.
It can be seen from the figure that the fatigue fracture of the three grades of gray cast iron samples is black, the fracture surface is rough and uneven, and there are casting holes near the surface. These casting holes are easy to form stress concentration and become the source of cracks. It is generally believed that the fatigue crack of gray cast iron originates from the places where stress concentration is easy to form, such as casting holes and the tip of flake graphite, especially the sharp parts of casting holes and flake graphite. A large number of fatigue cracks and casting holes are found in the area indicated by the solid line arrow in the figure. These cracks continue to develop into the interior of the material, forming a crack propagation area. The initial growth rate of fatigue crack is small. When it expands to a certain area and the applied stress exceeds the current bearing limit of the material, the material will break instantaneously.