Gray cast iron has been used as a component in various mechanical parts, such as the blocks and heads of automobile and marine engines, cylinder liners for internal combustion engines, and machine tool bases. It is desirable because of its good castability and machinability, damping characteristics, and high ratio of performance to cost. On the other hand, the weak graphite flakes present in gray cast iron act as stress concentrators and negatively affect the strength of this material. It is therefore important to know the relationship between the distribution of graphite flakes and the strength or fracture of gray cast iron. In this study, a tensile test of gray cast iron was carried out using a plate specimen in a scanning electron microscope, and the microscopic deformation was observed on the surface of specimen. Particularly, the change in the size of graphite flakes during the tensile test was examined, and the observed trend was discussed. We found from the experimental results that the dimensional changes in the graphite flakes varied in the observed area, and that the final fracture occurred in an area where a relatively large dimensional change was observed. This suggests that the fracture location or the critical parts of gray cast iron, can be predictable from the dimensional changes of the graphite flakes at an early stage of deformation.

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