Issue34

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 34 (2015) 406-414; DOI: 10.3221/IGF-ESIS.34.45 410 is characterized by the presence of some small metal particles embedded in the matrix. Considering that fatigue crack propagation results in DCIs is usually characterized by a good repeatability [10] and that, for lower R values, the matrix influence is almost negligible [9], Fig. 11, fatigue crack propagation resistance of the ferritized DCI seems to be influenced by the presence of the degenerated graphite nodules (Fig. 12). 10 10 -10 10 -9 10 -8 10 -7 10 -6 100% F GJS350-22 R = 0.1 R = 0.5 R = 0.75 50% F + 50% P GJS500-7 R = 0.1 R = 0.5 R = 0.75 100% P GJS700-2 R = 0.1 R = 0.5 R = 0.75 da/dN [m/cycle]  K [MPa m 1/2 ] 4 50 10 -10 10 -9 10 -8 10 -7 10 -6 10 50 4 da/dN Ferritic DCI da/dN Ferritized DCI da/dN [m/cycle]  K [MPa m 1/2 ] Figure 11 : Matrix and loading conditions influence on DCIs ferritic-pearlitic fatigue crack propagation resistance [6]. Figure 12 : Degenerated graphite elements influence on fatigue crack propagation in ferritic DCI. Considering that the long annealing treatment modified both the dimension and the shape of the graphite nodules, and considering the hypothesis that the graphite obtained directly from the melt during solidification (nodule core) is characterized by a different behaviour with respect to the graphite of the outer shield (obtained during the cooling process by means of a solid diffusion process), it is hard to define to contribution of all the geometrical and material parameters (graphite nodules shape and dimension and graphite mechanical properties gradient) on the da/dN-  K results. Analysing the fatigue crack path (Fig. 13), it is possible to observe that the path tortuosity is analogous to the one observed with the ferritic DCI [9, 10]. Secondary crack are frequent but short. The main difference concern the interaction between the graphite nodules and the crack: the debonding between the graphite nodules and the matrix or the “internal” debonding between the nodules core and the nodules shield are difficult to observe: more frequently, fatigue crack propagates inside the graphite nodule. Figure 13 : Ferritized DCI. Crack path. Fracture surface SEM observations allows to identify the interaction mechanism between the fatigue crack and the “degenerated” graphite nodules (Fig. 14, 15). Fatigue crack propagates inside the graphite shield obtained during the long annealing treatment, probably following the interfaces between graphite and metal particles. Secondary cracks nucleate and propagate corresponding to the interface between the shield obtained during the long annealing treatment and the “original” graphite nodule. The main crack propagates along this interface, with an apparent graphite nodule