Issue 13

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 13 (2010) 3-16; DOI: 10.3221/IGF-ESIS.13.01 14 Figure 64 : Austempered DCI (R = 0.1,  K = 15 MPa√m). Figure 65 : Austempered DCI (R = 0.1,  K = 9 MPa√m). Figure 66 : Austempered DCI (R = 0.1,  K = 20 MPa√m). C OMMENTS AND C ONCLUSIONS ccording to the experimental results shown in this work, it is evident that it is necessary both a good graphite nodules morphology control and a microstructure optimization, in order to increase the DCI fatigue crack propagation resistance. Both phases volume fraction and phases morphology are important parameters, and microstructure importance is not only connected to its intrinsic fatigue crack propagation resistance, but also to the possible interactions with nodules. Graphite elements do not only act as “crack arresters”, due to their peculiar shape: in fact, depending on matrix microstructure, they can also increase the DCI fatigue crack propagation resistance by means of an increase of the crack closure effect, with a consequent decrease of the  K value that is effective at the crack tip. For all the investigated DCI, graphite elements “ductile” or “fragile” debonding seems to be one of the main damaging micromechanism, with a respectively more or less evident plastic deformations of the matrix around the nodules. A model of the interaction between fatigue crack and graphite nodules during debonding, for different applied K values, is shown i n Fig. 67. Figure 67 : Microstructure influence on graphite nodules – matrix debonding. Considering pearlitic DCI, fragile debonding implies a negligible matrix plastic deformation: as a consequence, graphite nodule merely play as “mechanical obstruction”, mechanical reducing the crack closure corresponding to K min . Ferritic A