Effect of graphite on initiation and development of tension compression fatigue crack in gray cast iron

The deformation of any material under external load will be affected by its own microstructure and properties and external environmental factors. Various factors accelerate the process of material failure to varying degrees. In the high-frequency tension compression fatigue properties of three grades of gray cast iron studied in this paper at room temperature, the effects of surface roughness and loading frequency on the tension compression fatigue strength of gray cast iron are excluded through preliminary tests. The tension compression fatigue properties of gray cast iron mainly depend on the gray cast iron material itself. The tension compression fatigue failure mechanism of gray cast iron is studied, and the effects of its own microstructure and casting defects on the failure process are mainly analyzed.

(a) crack initiation (b) crack propagation

The figure shows the morphology of fatigue crack growth zone of fatigue fracture of low alloy HT250. It is generally considered that under the action of cyclic stress, 90% of the fatigue cracks of the material originate from the surface or near surface of the material. Microcracks are easy to form at the tip of flake graphite of gray cast iron material on the surface or near surface. As shown by the solid line arrow in Figure (a), microcracks are first formed at the tip of flake graphite, causing the fragmentation of matrix structure, Due to the great difference in mechanical properties between graphite and matrix, the crack begins to expand along the graphite sheet. When the crack extends to other matrix structures or encounters other graphite branches, the crack path may change and continue to expand along other graphite. As shown by the solid arrow in figure (b).