In order to elucidate the effect of non-uniform stress distribution on the notch strength of cast iron, some plate specimens with edge notches or a hole were examined at room temperature and the liquid nitrogen temperature. Non-elastic stress analysis was performed by FEM to discuss the fracture criterion.
The notched specimens with the elastic stress concentration factor of 1.6∼5.7 ruptured at the load 5∼40% smaller than the nominal stress criterion, but 8∼200% larger than the maximum stress criterion. The maximum stress at the notch root was estimated as 15∼80% higher than the tensile strength of the material.
These results were explained by introducing the“boundary layer depth”δ of 2∼4mm, which was equivalent to the size of 3∼5 of graphite eutectic cells. The effect of the stress distribution decreased at the liquid nitrogen temperature where the cast iron showed almost an elastic and brittle behaviour. At that temperature, the value of δ was about 0.7mm, which was nearly the size of an eutectic cell.