13th International Conference on Fracture June 16-21, 2013, Beijing, China -5- see Figure 3. At this stage the changes in the matrix is still too small to be detected with the set-up used. When increasing the load, (P>PL) the interface between the flaky graphite and the matrix starts to separate at several places in the material, mostly graphite flakes oriented perpendicular to the load shows this phenomenon. Still the cracks/voids earlier founded inside the flaky graphite opens more independent of the graphite’s orientation, but the degree of opening is more evidence in graphite perpendicular to the load axis. The pearlitic matrix also starts to undergo microplasticity at several graphite tips, the matrix undergo local plastically deformation which can be seen in Figure 4 and Figure 5. Figure 4. Plastic deformation at graphite tips and a bulge is evidence in the left picture. To the right multiple cracking with crack path both inside the graphite and at the graphite-matrix interface can be seen. The delamination of the graphite-matrix interface is easy to identify and when increasing the load even more, above 200 MPa, small bulges in the separated graphite-matrix interface is evidence of the weakest pearlite, see Figure 4. Here at these bulges the matrix limited plasticity results in a crack path when the load is increased even more, local necking in the matrix. At loads above σys0.2% the crack also finds its way through the matrix in the ferrite-cementite interface as can be seen in Figure 5 below. Figure 5. Cracking in the ferrite-cementite interface between two graphite tips.
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