Damage Effect on the Fracture Toughness of Nodular Cast Iron: Part I. Damage Characterization and Plastic Flow Stress Modeling

After chemical, morphological, and mechanical characterization of ductile cast iron, the damage mechanisms were studied by tensile tests inside the scanning electron microscope (SEM). The evolutions of Young’s modulus and of Poisson’s ratio were measured in uniaxial tensile tests. Compression tests were used to measure the pressure sensitivity coefficient of the flow stress. The damage is produced by early initiation of cavities at the pole cap of graphite nodules by debonding of the interface, followed by the growth of cavities. The mechanical behavior was modeled in the elastic region by calculating the Hashin-Shtrickman bounds. This provided the elastic constants for the graphite nodules. The plastic behavior was modeled by considering that the graphite nodules were replaced by voids. The critical interfacial stress for debonding was determined by analytical as well as by finite-element calculations. The growth rate of cavities was deduced from the evolution of the Poisson’s ratio and was compared with predictions from Gurson’s potential. The stress-strain behavior could be modeled either by extension of the Mori-Tanaka analysis in the plastic range or by finite-element computations. This allowed a fair prediction of the observed behavior.