Damage effect on the fracture toughness of nodular cast iron: Part-II. Damage zone characterization ahead of a crack tip

Abstract

In order to understand the fracture toughness of nodular cast iron, the damage zone was studied by Scanning Electron Microscope (SEM) observations of the polished surface of a CT 25 specimen before and after ductile tearing. Damage is defined as decohesion at the graphite/matrix interface. It is shown that the damage zone is very large in nodular cast iron (almost throughout the whole remaining ligament ahead of the crack tip), so linear elastic fracture mechanics (LEFM) are not valid for small specimens. The size of the damage zone was calculated analytically by introducing a damage initiation criterion which was based both on observations of the debonding of the interface between matrix and graphite nodules and on measurements of the pressure sensitivity of cast iron. To take into account the actual boundary conditions, the damage zone was also calculated by numerical modeling using the modified Gurson’s model and by considering the nodular cast iron as a porous material. The calculated results led to good agreement with the damage zone observations. Plane stress and plane strain calculations yielded nearly the same size plastic zone. This result is opposite to those obtained for fully dense materials.