13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- possible interactions between fatigue crack and graphite nodules are observed, probably due to the crack propagation direction with respect to the nodule position and to the graphite element nodularity: - graphite nodule – matrix debonding (probably more frequent); - onion like mechanism; - fracture inside the graphite nodules. 5. Conclusions Fatigue crack propagation resistance in DCI is usually investigated according to ASTM E647, investigating the evolution of the crack growth rate (da/dN) with the increase of the stress intensity factor amplitude (ΔK). Considering that DCIs are characterized by a substantially composite microstructure, with graphite nodules that are a consistent volume fraction (usually about 10-15 %), and considering the different mechanical behavior of the microstructure components, the material homogeneity condition can be considered as critical in order to apply Linear Elastic Fracture Mechanics principles. In this work, the fatigue crack propagation resistance of a three different ferritic-pearlitic DCI has been re-analyzed, investigating the crack propagation micromechanisms and reconsidering the stress intensity factor range (ΔK) ability to characterize the stress conditions at the crack tip. According to the observed crack propagation micromechanisms and to LEFM considerations it is possible to summarize as follows: - Ferritic DCI: considering lower nominal ΔK and R values, max pzK r and rrpz values that are comparable to maximum values of the graphite elements diameter; as a consequence the material homogeneity condition is not respected. - Pearlitic DCI: max pzK r and rrpz values that are comparable to maximum values of the graphite elements diameter for almost all the investigated loading conditions; as a consequence the material homogeneity condition is not confirmed. - Ferritic-pearlitic DCI (with “bulls eye” microstructure morphology): for all the investigated R values, considering lower ΔK values, rrpz values are always comparable to the graphite elements diameter. Corresponding to higher ΔK values, the graphite nodules presence is less critical; however, ferritic zones around the graphite nodules comparable to the rrpz values also for the higher ΔK values. As a consequence of this analysis, it is possible to conclude that, in order to analyze the fatigue crack propagation resistance of ferritic-pearlitic DCIs, stress intensity factor range ΔK is not able to describe the effective stress state at the crack tip for all the investigated conditions and it should be considered only a first approximation of the stress state based on the wrong hypothesis of a homogenous material. 6. References [1] http://www.ductile.org/didata/Section2/2intro.htm [2] D. K. Millis, P. A. Gagnebin, B. N. Pilling, Cast Ferrous Alloy, US patent 2485760, issued 1949-10-25. [3] L.R. Jeckins, R.D. Forrest, Properties and selection: iron, steels and high performance alloys. ASM Handbook Ductile Iron, Metal Park (OH) ASM International, 1 (1993) 35-55. [4] R.G. Ward, An Introduction to the Physical Chemistry of Iron and Steel Making. Arnold, London, 1962.
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