13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- (usually about 10-15%) and considering that fatigue crack propagation is characterized by the presence of a “reversed” or “cyclic” plastic zone, rrpz (four times lower than the monotonic value corresponding to Kmax) and that the tensile load reduction from the σmax, and the presence of the surrounding elastic body, implies a compression condition at the crack tip, it is evident that for lower applied ΔK and/or R values: - it is quite difficult to consider ferritic-pearlitic DCIs as homogeneous material; - compression stress conditions are completely developed in the graphite elements at the crack tip. Furthermore, it is necessary to remember that ferritic-pearlitic DCIs microstructure components are characterized by different mechanical properties: - σY of ferrite depends on the grain diameter, but can be assumed between 180 and 320 MPa [21]. - σY of pearlite depends on the lamellae spacing, but can be assumed between to 400 and 800 MPa [21]. - graphite tensile strength is quite low (25 – 30 MPa, max [22]), but it’s compression resistance is higher (even 200 MPa [23]). On the basis of these considerations, the influence of the graphite nodules on the fatigue crack propagation should be dependent on the microstructure: - ferritic matrix: for lower R and ΔK values the homogeneity condition is not respected; considering the rrpz, it is evident that for all the investigated R values, for lower nominal ΔK values, the compression stress state is almost completely developed in the graphite nodules. Corresponding to higher ΔK values the homogeneity condition can be considered as respected, but the crack tip plastic zone can’t be considered negligible (especially in plane stress conditions). - pearlitic matrix: for all the investigated loading conditions the homogeneity condition is not respected, with dmax values comparable with max pzK r values (with the exception of the higher nominal ΔK and R values); furthermore, for almost all the investigated loading conditions, the compression stress state in the reversed plastic zone is completely developed in the graphite nodules. - ferritic-pearlitic matrix: considering the bulls eye structure of ferrite around the nodules, and the different mechanical behaviour of ferrite and pearlite, corresponding to lower nominal ΔK values, the homogeneity condition is not respected. Focusing the reversed plastic zone, corresponding to the lower ΔK values, rrpz values are comparable to dmax values, with the compression state that is developed inside the graphite nodules. Considering the ferritic grains around the graphite nodules, the problem of the material homogeneity is evident also for ΔK values in the Paris stage. 4. Fatigue crack propagation micromechanisms In this section, on the basis of the considerations of Section 3, and considering results already published in [19, 20], fatigue crack propagation micromechanisms in ferritic-pearlitic DCIs are re-analyzed. Considering the ferritic DCI, the main interactions between the fatigue crack and the graphite nodule are shown in Figure 5 and 6. For lower nominal ΔK values ( max pzK r and rrpz values are comparable to dmax values) the compression state is completely developed in the graphite nodules and secondary cracks initiate and propagate inside the nodules, with a sort of “onion-like” morphology (Figure 5). The increase of the applied ΔK implies a modification of the interaction
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