DAMAGE EFFECT IN THE FRACTURE-TOUGHNESS OF NODULAR CAST-IRON

In order to understand the toughness of nodular cast iron, the flow st ress have been studied numerically by using Gurson's model considering that nodular cast iron can be regarded as a porous material. The dama ged 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 tea ring. It is shown that the damaged zone is very large in the GGG 40 no dular cast iron (through almost the whole ligament ahead of the crack) , so the Linear Elastic Fracture Mechanics is not valid in nodular cas t iron for small specimens. The force versus crack opening displacemen t curves calculated in 2D for plane strain and plane stress conditions give respectively an upper and a lower limit as compared with the exp erimental curve. Only 3D calculations lead to a good fit with the expe rimental curve. This result shows that the stress state ahead of the c rack tip of a nodular cast iron is intermediate between plane stress a nd plane strain conditions, due to the high density of voids inside th e damaged material.