13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- reaction violence and to promote graphite nodulizing, controlling the effect of impurities on nodules morphology and the matrix microstructure [6]. Different graphite elements nucleation theories are proposed in literature, mainly based on “heterogeneous mechanisms” (e.g. gas bubble theory, graphite theory, silicon carbide theory etc.) [7], and also different graphite nodules growth mechanisms are proposed [8]. The results obtained by other authors [9-11] by means of nanoindentation tests and by means of Micro Raman Spectroscopy and Electron Probe Microanalysis confirms the presence of a substructure in graphite nodules and of a gradient of mechanical properties, with cementite plates/particles that are trapped in and around the spherulite and multicomponent particles of Mg, Fe, S, C, etc. that are trapped in the spherulite or accumulate on the edges of the spherulite upon solidification. It is worth to note that, although the graphite tensile resistance is not negligible if compared to ferrite, it is its compression strength is absolutely not negligible, with compression resistance values that can be even 200 MPa [12, 13]. Fatigue crack propagation resistance in DCI is usually investigated according to ASTM E647 [14], analyzing the evolution of the crack growth rate (da/dN) with the increase of the stress intensity factor amplitude (ΔK), [15-19]. Considering that DCIs are characterized by a substantially composite microstructure, with graphite nodules that are a consistent volume fraction (usually about 10-15 %), the material homogeneity condition can be considered as critical in order to apply Linear Elastic Fracture Mechanics principles, and use the stress intensity factor range (ΔK) in order to characterize the stress conditions at the crack tip. In this work the fatigue crack propagation resistance of a three different ferritic-pearlitic DCI has been re-analyzed [19, 20]. 2. DCIs fatigue crack propagation: Materials and tests results analysis The investigated ferritic-pearlitic DCIs were obtained by means of chemical composition control: as a results, investigated matrix microstructures ranged from a completely ferritic DCI up to a completely pearlitic one (Figures 1-3). Chemical compositions and phases volume fractions are in Tab. 1-3. Figure 1. DCI EN GJS350-22 microstructure (100% ferrite). Figure 2. DCI EN GJS500-7 microstructure (50% ferrite – 50% pearlite). Figure 3. DCI EN GJS700-2 microstructure (100% pearlite). Table 1. DCI EN GJS350-22 chemical composition (100% ferrite). C Si Mn S P Cu Cr Mg Sn 3.66 2.72 0.18 0.013 0.021 0.022 0.028 0.043 0.010 Table 2. DCI EN GJS500-7 chemical composition (50% ferrite – 50% pearlite). C Si Mn S P Cu Cr Mg Sn 3.65 2.72 0.18 0.010 0.03 - 0.05 0.055 0.035
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