Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 462-468; DOI: 10.3221/IGF-ESIS.30.56 467 - Initiation and growth of matrix – graphite nodule debonding (Fig. 8b, white arrows). - Initiation and growth of the debonding between the “nodule core” and the “nodule shield” (“onion-like” mechanism, red arrows), presence of some slip bands and initiation of short cracks (few microns) in the matrix, Fig. 8c. - Increase of the slip bands density and growth of the short crack in the matrix, Fig. 8 d. - Strong increase of the slip bands density and evident increase of the microcracks, Fig. 8e. Although the matrix is not ductile, the deformation along the loading direction observed in the Nodule 2 is analogous to the one observed in the ferritic DCI (Fig. 1). SEM observations SEM observations allow to confirm, with a higher detail, the damaging micromechanisms already observed by means of the DM observations (Fig. 9). The highest deformations along the loading direction are observed for the lowest values of the triaxiality factor. Figure 9 : SEM lateral observation of the fracture surface (central notch radious, R = 12.5 mm) The higher resolution of the SEM analysis allows to identify some further details: focusing the onion like mechanism, it is possible to observe that the microcrack initiation in the “onion like mechanism” can be characterized by a sort of “exfoliation” of the graphite nodule in numerous layers (Fig. 10b). Figure 10 : SEM lateral observation of the fracture surface: onion-like mechanism and multiple cracks (black arrows show the loading direction). (a) before the tensile test; (b) during the tensile test. C ONCLUSIONS n this work, triaxiality influence on damaging micromechanisms in a pearlitic DCI have been investigate by means of tensile tests performed on notched specimens and observations of the lateral surface by means of a SEM and of a DM. According to the experimental results, the influence of triaxiality on the damaging micromechanisms in a ferritic DCI, it is possible to summarize the following conclusions: I