Firstly, the fatigue crack nucleates at the stress concentration. Under the action of stress, the fatigue crack begins to expand. When the stress exceeds the material bearing limit, it will break suddenly. The graphite in gray cast iron mainly exists in the form of flake, and the stress concentration is easy to form at the tip of flake graphite. At the same time, there are some casting defects in gray cast iron, so the fatigue performance of gray cast iron is poor. Wang duxiong et al. Studied the low cycle fatigue behavior of high carbon equivalent gray cast iron. Due to its high carbon content, high carbon equivalent gray cast iron has high brittleness, and the number of fatigue cycles is much lower than that of carbon steel, and the number of cycles decreases rapidly with the increase of stress amplitude. Under the action of different strain amplitude, it is found that the fracture surface is uneven and there is no obvious necking phenomenon, which belongs to typical brittle fracture. Wang duxiong and others also found that some defects in theare prone to stress concentration, which seriously reduces the mechanical properties of the matrix. Fatigue cracks preferentially nucleate at the defects, expand around the grain boundary, proliferate on multiple different cleavage surfaces, and form cleavage steps and cleavage tongues.
Yuan Yongli et al. Conducted tensile test and fatigue test on gray cast iron to analyze the low cycle fatigue behavior of gray cast iron. From the analysis of cyclic stress-strain curve, it is concluded that the presence of graphite sheet will reduce the strength of gray cast iron matrix, which is equivalent to the formation of multiple holes in the matrix, which will reduce the tensile strength and have little impact on the compressive strength, Therefore, the compressive strength of gray cast iron is greater than the tensile strength. When the gray cast iron material is subjected to cyclic load, the surface inclusions, internal casting defects and flake graphite tips in the gray cast iron are easy to form stress concentration. Fatigue cracks preferentially sprout and form nuclei in these parts, and expand around the grain boundary after the crack is formed. On the one hand, the second phase particles in gray cast iron hinder the propagation of fatigue crack, on the other hand, promote the generation of secondary crack and prolong the fatigue life of the material. Through the analysis of fracture morphology, it is found that there are obvious river patterns and a large number of tearing edges in the fatigue fracture of gray cast iron, indicating that the fatigue fracture of gray cast iron has the characteristics of quasi cleavage fracture. At high magnification, smooth cleavage steps and fatigue bands can be observed, and fatigue cracks along grain boundaries can be observed on the fracture of gray cast iron, indicating that the fatigue fracture of gray cast iron has the characteristics of intergranular fracture. Therefore, the fatigue fracture mechanism of gray cast iron belongs to the composite fracture mechanism of quasi cleavage fracture and intergranular fracture.