13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Focusing on the pearlitic DCI, the “onion like” mechanism is observed often also for higher ΔK values (Figure 7; arrows inside the graphite nodule). If the crack meets the nodule “tangentially”, matrix – graphite debonding is also observed (Figure 8). It is worth to note that the interaction between the pearlite lamellae and the fatigue crack depends on their reciprocal orientation: in fact, in Figure 7 (arrows in the matrix) the pearlitic lamellae are almost orthogonal to the crack path, with a consequent “transgranular” crack: performing a traditional SEM observation of the crack surface, these cracked lamellae have a morphology that is analogous to fatigue striations (Figure 10). If pearlitic lamellae are almost parallel to the crack path, ferrite is the preferential propagation path (arrows in Figure 7), and a traditional SEM observation of the crack surface shows a sort of cleavage. The analysis of the microstructure influence on fatigue crack propagation in a ferritic-pearlitic DCI (with “bulls eye” microstructure morphology) is complicated by the phases distribution in the metal matrix. For all the investigated R values, considering lower ΔK values, rrpz values (always calculated considering the investigated DCI as a homogeneous material) are always comparable to dmax values. For higher ΔK values, the graphite nodules presence is less critical. Ferritic zones around the graphite nodules are comparable to the rrpz values also for the higher ΔK values. Considering the different tensile mechanical behavior of ferrite and pearlite, during fatigue loading, with K that ranges between Kmax and Kmin, deformation level in the involved microstructure components (ferrite and pearlite) is quite different: - For higher K values (nearby Kmax), plastic deformation level in ferritic shields is higher than in pearlitic matrix, due to the higher ferrite ductility; - For lower K values (nearby Kmin), pearlitic matrix induces a compression stress state on ferritic shields and, consequently, on graphite nodules, with an increase of crack closure effect importance. The superposition of this mechanism to the compression stress state due to the reversed plastic zone is more and more evident with the increase of ΔK and R values, with a consequent increase of the fatigue crack propagation resistance (as observed in Figure 4). Figure 10. DCI EN GJS500-7, SEM fracture surface analysis (R = 0.1, ΔK = 15 MPa√m). Figure 11. DCI EN GJS500-7, SEM fracture surface analysis (R = 0.75, ΔK = 8 MPa√m). Ferritic-pearlitic DCI fracture surfaces are characterized by the presence of cleavage in ferritic shields around the graphite nodules (Figures 10 and 11) and, analogously to the pearlitic, “striations” are manly due to a “transgranular” damaging mechanism of pearlitic lamellae. All the
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