13th International Conference on Fracture June 16-21, 2013, Beijing, China -7- 4. Discussion Even though pearlitic flake cast iron is widely used and is an important engineering material the knowledge on its fracture behaviour and the deformation due to axial loading is still very limited. One of the main goals with this research was to gain deeper knowledge on the deformation and fracture behaviour of a pearlitic flake cast iron. The use of the special designed stage in the SEM together with EBSD analysis can give a unique insight in the deformation and fracture behaviour of the material studied. In this study the amount of pre-existing LAGB’s made the analysis of their changes due to loading very difficult and no clear effects where shown. The great amount of LAGB’s detected is an effect of the solidification process and the material used had not been stressrelived. The heat treatment should decrease the amount of LAGB’s once the internal thermal residual stresses from the solidification process minimizes. One aim with this study was to investigate the LAGB changes and the grain orientation due to axial loading and from these results no changes were detected. The deformation of a flake cast iron under axial loading can be divided into several different main events. First deformation detected in the material was the development of voids inside graphite flakes. This material deformation was detected at loads as low as 50 MPa and the strength of the weakest slip plane in the graphite is approximately 40 Mpa which then makes the fact that it separates at 50 Mpa very understandable. With compressive residual stresses presence at the surface this deformation would not be seen at this load-level and thanks to careful polishing this effect could be discovered. Interesting to see is that when increasing the load these voids can be found in graphite flakes independent of its orientation to the load axis. Delamination of the graphite-matrix interface was the second kind of deformation detected in the material. This deformation requires slightly more applied load and a suggested explanation for this is that the interface forces between the two phases is just greater than the weakest slip plane in the graphite. By looking at the stress strain curve seen in Figure 3 one can see that curve deviates from its linear path at stresses between 50 and 90 Mpa and it is proposed by the authors that this is an effect of both the delamination of the graphite-matrix interface and the presence of voids inside the graphite flakes. These effects are also responsible in the change of Young´s modulus as seen in for flake cast iron subjected to cyclic loading, less elastic deformation is possible for this multi-phased material. Plastic deformation at graphite tips was the third permanent deformation detected in the material. Loads generating stresses in the magnitude of 140 Mpa and more results in plastic deformation at graphite tips. With increasing load the size of the plastic zone is increased and also amount of plastic deformation around graphite tips increases with increasing load. This is expected to be seen but not that the sizes of voids inside graphite flakes to decrease for those voids lying a bit away from the real crack path. Obviously some kind of stress relaxation occurs in the material when the pearlite starts to hardens due to the plastic deformation at the graphite tips. The last type of deformation detected was the development of bulges just before rapture, as a consequence of stress concentrations at the graphite. When study the final fracture it was then evident that the crack path had been via these bulges. The plastic deformation at these bulges is severe and a phenomenon of local necking is present. Fracture behaviour becomes more and more complex as the applied load increases and several distinct features acting simultaneously resulting in final failure. The graphite tips acts as notches and clearly the plastic deformation at the tips indicates where the crack later will propagate. Due to multiple cracking not every graphite flake with local plastic deformation at the tip will be a part of the final cracking. Fractography reviled a ductile fracture where the bulges earlier had been detected leading to the conclusion that severe plastic deformation at the tips results in the ductile-like fracture found in the material.
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