Graphite nodule morphology as an indicator of the local complex strain state in ductile cast iron

Compression tests were carried out on ductile cast iron with different sample geometries in order to understand the influence of the stress triaxiality on the local strain. The tests were performed in dry friction conditions involving a complex local stress and strain state with a severe barrelling. The numerical simulation of the compression tests was achieved by using finite element analysis. Local strain was evaluated using microstructural quantifications. A relationship between the nodule strain, experimentally determined at different stress-strain stages, and the numerical simulation is proposed. The numerical predictions agree with the distribution of the experimental aspect ratio defined as the ratio between the major and minor axis of the graphite nodules. This study shows that the nodule strain is a good indicator of the total material strain at room temperature for different triaxiality states and complex strains. In addition, it was highlighted that graphite nodules cannot always be considered as ordinary voids during the plastic strain process.