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F. Iacoviello et alii, Frattura ed Integrità Strutturale, 25 (2013) 102-108 ; DOI: 10.3221/IGF-ESIS.25.15 105 propagation direction), respectively Fig. 3 and 4 (crack propagates from left to right). It is worth to note that both propagation micromechanisms seem to be discontinuous: according to the observed lateral surfaces, ferritic lamellae seems to be the first to fracture while cementite ones are the last to be broken, playing a ligament role. These fracture themselves when fatigue crack macroscopically propagates. A SEM failure analysis performed on the crack surface allows to observe: - a “pseudo-cleavage” propagation mechanism, corresponding to the pearlite delamination; - the presence of “pseudo-striations” that are not connected to the macroscopic crack growth rate, but, better, to the pearlite lamellae spacing, corresponding to the transgranular mechanism. Figure 3 : Fatigue crack propagation. Pearlite “delamination”. Figure 4 : Fatigue crack propagation. Transgranular propagation. Considering the interaction between fatigue crack and graphite nodules, two main damaging morphologies are observed with an analogous probability: - debonding between the pearlitic matrix and the graphite nodules, sometimes with the initiation and propagation of a secondary crack in the nodule center, probably corresponding to the nodule nucleation site (Fig. 5); - a sort of internal debonding between a “nodule core” and a “nodule shield” (Fig. 6), supposedly connected to the graphite nodules solidification and growth process (as observed more frequently in ferritic DCI [9]). Furthermore, graphite nodules can also imply the crack bifurcation, as in Fig. 7. This is due to the crack tip stress field that induces a damage also in the nodules around the crack tip with different damaging morphologies, as in Fig. 8 and 9. Figure 5 : Pearlitic matrix-graphite nodule debonding (with secondary crack in the nodule center). Figure 6 : Graphite nodule internal debonding. Figure 7 : Crack bifurcation. Figure 8 : Damaged graphite nodule near the crack. Figure 9 : Damaged graphite nodule near the crack.