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A. De Santis et alii, Frattura ed Integrità Strutturale, 26 (2013) 12-21; DOI: 10.3221/IGF-ESIS.26.02 12 Graphite nodules features identifications and damaging micromechanims in ductile irons Alberto De Santis, Daniela Iacoviello Sapienza Università di Roma, Dipartimento di Ingegneria Informatica, Automatica e Gestionale Antonio Ruberti, Italy Vittorio Di Cocco, Francesco Iacoviello Università di Cassino e del Lazio Meridionale, DICeM, via G. Di Biasio 43, 03043 Cassino (FR), Italy iacoviello@unicas.it A BSTRACT . Ductile irons mechanical properties are strongly influenced by the metal matrix microstructure and on the graphite elements morphology. Depending on the chemical composition, the manufacturing process and the heat treatments, these graphite elements can be characterized by different shape, size and distribution. These geometrical features are usually evaluated by the experts visual inspection, and some commercial softwares are also available to assist this activity. In this work, an automatic procedure based on an image segmentation technique is applied: this procedure is validated not only considering spheroidal graphite elements, but also considering other morphologies (e.g. lamellae). K EYWORDS . Ductile irons; Damaging micromechanisms; Image segmentation; Level sets. I NTRODUCTION amaging micromechanisms evolution in ductile cast irons (DCIs) is strongly influenced by the matrix microstructure and by the graphite nodules morphology. Considering recent experimental results [1-6], the role played by the graphite nodules is not merely connected to a matrix-graphite debonding mechanism, followed by voids nucleation and growth as described in [7-10]. Graphite nodules damaging micromechanisms can be classified as follows: - Graphite – matrix debonding (Fig.1 – Video 1); - “Onion-like” mechanism (Fig. 2 – Video 2); - Crack initiation and propagation in the “nodule core” (Fig. 3 – Video 3). Fig. 1 – 3 (and Videos 1 – 3) are obtained according to the procedure described in [1-6] and refers to a fully pearlitic DCI. In order to obtain the videos, a “step by step” testing procedure allowed “in situ” scanning electron microscope (SEM) observations and a commercially available morphing software was used (at least, 10 step for each video). The pearlitic matrix – graphite nodule debonding (Fig. 1 - V1) is characterized by a debonding nucleation that is obtained for loading conditions that corresponds to the elastic stage: debonding nucleation is evident in the nodule “pole” and becomes more and more evident with the increase of the macroscopic deformation. The “onion –like” mechanism is probably connected to a mechanical properties gradient in the graphite nodule, probably due to the graphite nodule nucleation and growth mechanisms. This gradient has been observed in ferritic DCI by means of nano indentation tests [11]. The “onion-like” mechanism nucleates already corresponding to a stress value of about 600 MPa (elastic stage): some cracks nucleate on the higher-left side of the nodule (Fig. 2 - V2). The increase of the macroscopic deformation implies a propagation of the first cracks and the nucleation of new cracks, allowing to define a D