Damage effect in the fracture toughness of nodular cast iron

In order to understand the toughness of nodular cast iron, the flow stress have been studied numerically by using Gurson's model considering that nodular cast iron can be regarded as a porous material. The damaged zone ahead of the crack tip is studied by SEM observations on the pre-polished surface of a CT 25 specimen, before and after ductile tearing. It is shown that the damaged zone is very large in the GGG 40 nodular cast iron (through almost the whole ligament ahead of the crack), so the Linear Elastic Fracture Mechanics is not valid in nodular cast iron for small specimens. The force versus crack opening displacement curves calculated in 2D for plane strain and plane stress conditions give respectively an upper and a lower limit as compared with the experimental curve. Only 3D calculations lead to a good fit with the experimental curve. This result shows that the stress state ahead of the crack tip of a nodular cast iron is intermediate between plane stress and plane strain conditions, due to the high density of voids inside the damaged material.