Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 462-468; DOI: 10.3221/IGF-ESIS.30.56 463 matrix-graphite debonding is considered and all the other observed damaging micromechanisms are considered as negligible. Recent experimental results showed that the role played by the graphite nodules is more complex, depending both on the matrix microstructure [9-12] and on the presence of mechanical gradient inside the graphite nodules [13-14], probably due to physical phenomena during the solidification and cooling process. Focusing the triaxiality influence on the damaging micromechanisms and considering ferritic DCIs [15], it is possible to observe that the triaxiality values are not really high and the deformation is mainly developed along the loading directions (Fig. 1 and 2). 0.25 0.30 0.35 0.40 0.45 0.0 0.2 0.4 0.6 0.8 1.0 Triaxiality Crosshead displacement [mm] (b) (c) (d) (e) (f) (a) ‐ Not defined Figure 1 : EN GJS350-22 ductile cast iron. Left: Digital Microscope in situ lateral surface analysis performed on notched specimen (red point indicates the investigated nodule). Right: Evolution of the Triaxiality Factor with the crosshead displacement [15]. Figure 2 : EN GJS350-22 ductile cast iron. Scanning Electron Microscope in situ lateral surface analysis performed on notched specimen [15]. Focusing pearlitic DCIs, and considering smooth specimens [8], they are characterized by the absence of irreversible damage only for very low stress values. An irreversible damage is observed already in the elastic stage, with cracks that initiate and propagate at the graphite nodules pole cap. Cracks initiation and propagation is also observed in the pearlitic matrix. Stress increase implies both cracks propagation in graphite nodules, and matrix plastic deformation with cracks