Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 462-468; DOI: 10.3221/IGF-ESIS.30.56 466 Nodule 1 is characterized by the highest TF values (up to 1) and shows a 3D damage development due to the high triaxiality level. The damage initiation (Fig. 7, white arrows) seems to correspond to the increase of the triaxiality factor and it is observed both the debonding between the graphite nodule and the pearlitic matrix and the initiation and growth of internal cracks (so called “onion-like” mechanism). The video shows the damage increase with the equivalent Von Mises stress – crosshead displacement evolution. The DM lateral surface analysis shows a damage that initiates in the graphite nodules and, only for higher values of the equivalent Von Mises stress, shows slip bands. The increase of the slip bands density implies the initiation of microcracks, mainly corresponding to the nodule equator (Fig. 7d and e). Considering other nodules with analogous triaxiality values, it is possible to suppose that graphite – matrix debonding is more important with higher triaxiality values (but this is to be confirmed by further investigations). Figure 7 : EN GJS700-2 ductile cast iron (Nodule 1, Fig. 5 and 6). Digital Microscope in situ lateral surface analysis performed on notched specimen (a-e); black arrows show the loading direction. Diagram: Triaxiality Factor - Crosshead displacement evolution. Figure 8 : EN GJS700-2 ductile cast iron (Nodule 1, Fig. 5 and 6). Digital Microscope in situ lateral surface analysis performed on notched specimen (a-e); black arrows show the loading direction. Diagram: Triaxiality Factor - Crosshead displacement evolution. Nodule 2 is characterized by the lowest triaxiality factor values (comparable to the values obtained with the ferritic DCI, Fig. 1). For these conditions, it is possible to describe the damaging evolution according to the following stage: