Overload effects on fatigue cracks in ferritic-pearlitic ductile cast irons

Matrix microstructure (e.g., phases volume fraction, grains size and grain distribution) and graphite nodules morphology peculiarities (e.g., nodularity level, dimension, distribution etc.) strongly affect the mechanical behavior and damaging micromechanisms in Ductile Cast Irons (DCIs). Concerning the influence of the graphite nodules, it depends both on the matrix microstructure and the loading conditions (e.g., static, quasi-static or cyclic loadings). The influence of graphite nodules on the damaging micromechanisms is not univocally identified. Some authors proposed to consider the graphite nodules as voids embedded in a more or less ductile matrix; other authors recently proposed a more complex contribution of the graphite nodules, suggesting a mechanical properties gradient inside the graphite nodules, with the graphite elements – matrix debonding as only one of the possible damaging micromechanisms. In this work, three different ferritic-pearlitic DCIs were investigated, focusing the damaging micromechanisms due to overloads applied on fatigue cracked Compact Type specimens. Scanning Electron Microscope (SEM) and Digital Microscope (DM) observations were performed on the lateral surfaces of the overloaded specimens following a step by step procedure: SEM observations were mainly focused on the damaging mechanisms in graphite nodules; DM observations were mainly focused on the damaging mechanisms in the ferritic-pearlitic matrix.