13th International Conference on Fracture June 16-21, 2013, Beijing, China -8- Not every graphite flake that opens inside contributes to the final crack but once they have opened it affects the mechanical properties of the cast iron. It has been identified in this investigation that the first fracture observed in the material actually occurs inside the graphite flakes followed by delamination of the graphite-matrix interface. The non-linear elastic behaviour and the decrease in Young´s modulus observed in flake cast iron is proposed by the authors to a result of these two permanent deformation phenomenon´s in the material. 5. Conclusions The fracture behaviour of flake cast iron can be summarized in a few steps of main events. The first noticeable microstructural feature due to the axial loading is the opening of graphite. Graphite flakes opens inside the flakes and this feature can be found in graphite flakes independent of its orientation to the applied load. As a second important and noticeable event is the delamination of the graphite-matrix interface. Parallel to this event one can see that the graphite that had opened inside opens even more. The third crack feature is the local plasticity at graphite tips laying perpendicular to the loading. Fourth and last important feature is the bulge of the matrix at the delaminated interface, where the crack later will propagate. Some of the graphite with an opening inside had also generated a delamination of the graphite-matrix interface. Development of bulges start once the plastic deformation at graphite tips is evidence but stresses close to and above σys0.2% is needed to clarifying that a bulge is developed and not just local plastic deformation. At applied loadings just before rapture the fracture behaviour of flake cast iron results in multiple cracking, which makes it possible for the main crack to “jump” relatively large distances due to the network of flaky graphite. The weakest points in the sample will then link together which results in a rough topography of the fracture surface. As a final occurrence of fracture behaviour in flake cast iron subjected to axial loading the crack propagates in one of the following alternatives: • Through the opening inside the graphite. • Through the delaminated interface. • Via the closest distance between graphite tips where the local plastic deformation results in a ductile-like fracture appearance. • At the pearlitic grain boundaries leaving a ductile-like fracture. • Straightforward through the matrix resulting in a cleavage fracture. • Along the ferrite-cementite interface giving cleavage fracture. Grain orientation and changes of LAGB’s in flake cast iron due to axial loading is very small, if existing, the relative fine step-size used could not show any differences. Fractography give instant insight why the crack propagated to the right in Figure 1 and not on the left side as large graphite flakes where located just below the surface to the right and not to the left. Analyse of the fractured surface revealed both ductile and cleavage fracture and an increase in fracture topology where multiple cracks was easily detected. Acknowledgment The authors would like to thank Maqsood Ahmad from Volvo Powertrain for providing the material. Agora Materiae graduated school at Linköping University. References [1] A. Diószegi, V. Fourlakidis, I.L. Svensson, Fracture Mechanics of Gray Cast Iron, Material Science Forum, 649 (2010) 517-522. [2] L. Haenny, G. Zambelli, Strain Mechanisms in Grey Cast Iron, Engineering Fracture Mechanics, 18 (1983) 377-387. [3] H.T. Angus, Cast Iron, Butterworth, London, 1976.
RkJQdWJsaXNoZXIy MjM0NDE=