Issue 13

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 13 (2010) 3-16; DOI: 10.3221/IGF-ESIS.13.01 12 3D fracture surface analysis The aim of this experimental procedure was to analyze the microstructure influence on graphite nodules – matrix debonding and to quantify a ductile component in this damaging mechanism (Fig. 50, 51) . Almost all the investigated voids are characterized by “L > D”, for all the investigated microstructures. It implies that a ductile component in the debonding mechanism is always present. Microstructure strongly affects the experimental results distribution. Pearlitic DCI is characterized by the lowest differences “L - D” (completely fragile debonding corresponds to “L – D = 0”), and fully ferritic DCI is characterized by the higher “L-D” values (higher ductile deformation during debonding). Ferritic-pearlitic DCI shows intermediate “L-D” values. This is probably due to the different mechanical behaviour of ferritic shields and pearlitic matrix that induces a compression stress state in ferritic shields corresponding to K min , and a consequent reduced ductile debonding. ADI is characterized by “L-D” experimental results distribution that is similar to ferritic-pearlitic DCI and probably crack closure mechanisms are the same as in ferritic-pearlitic DCI, due to the presence of residual ferrite around graphite elements. Differences in the mechanical behaviour of pearlite and bainite are not so relevant. In fact, ferritic-pearlitic and austempered DCI crack growth rates are comparable for all the investigated experimental conditions ( Fig. 14) . Also the analysis of voids depths “K” as a function of the approximation sphere diameters “D” allows to obtain an analogous classification of the importance of the ductile deformation in the debonding mechanism, with the fully pearlitic microstructure that is characterized by “K ≤ D/2” (completely fragile spheroids debonding) and other investigated microstructures that are characterized by a higher importance of the ductile deformation in the debonding mechanism, with a consequent higher scatter of the experimental results. Figure 50 : Four investigated ductile irons. Approximation sphere diameter – void diameter. Figure 51 : Four investigated ductile irons. Approximation sphere diameter – void depth. LOM transversal crack paths analysis Considering ferritic DCI, LOM transversal crack path analysis confirms graphite nodules disintegration as an important damaging mechanism (Fig. 52 and 53) , and the presence of residual graphite inside cavities is also evident (Fig. 54) . Pearlitic DCI is characterized by an absolutely fragile debonding, without graphite element disintegration and without residual graphite inside cavities (Fig. 55, 56 and 57) . Considering both ferritic and pearlitic DCI, the observations concerning the ductile or fragile debonding do not depend on the loading conditions (R and applied  K). Ferritic-pearlitic GJS500-7 DCI crack profile is characterized by the presence of partially disintegrated and sound graphite nodules, whith residual graphite that could be present inside cavities (Fig. 58, 59 and 60) . The interface between pearlitic matrix and ferritic shields seems to act as a preferential crack propagation path (Fig. 59) , together with the graphite nodules – ferritic shields interfaces. Considering the ferritic-pearlitic DCI obtained annealing a pearlitic DCI, it is possible to observe a higher density of partially disintegrated graphite nodules, if compared to ferritic-pearlitic GJS500-7 DCI (Fig. 61) . However, many sound graphite nodules are always present and their debonding from matrix does not show an evident plastic deformation of the matrix. 0 20 40 60 80 100 0 20 40 60 80 100 100% F 50% F + 50% P 100% P ADI Void diameter "L" [  m] Approximation sphere diameter "D" [  m] 0 20 40 60 80 100 0 10 20 30 40 50 100% F 50% F + 50% P 100% P ADI Void depth "K" [  m] Approximation sphere diameter "D" [  m]