Issue 30

V. Di Cocco et alii, Frattura ed Integrità Strutturale, 30 (2014) 462-468; DOI: 10.3221/IGF-ESIS.30.56 464 propagation in pearlitic matrix. Matrix – graphite elements debonding is seldomly observed, with a frequency that seems higher than the one observed in ferritic containing DCIs. In this work, the influence of a high stress triaxiality level on the damaging micromechanisms in a pearlitic DCI is investigated by means of tensile tests performed on notched tensile microspecimens and observing the specimens lateral surface evolution during the test by means of a scanning electron microscope and of a digital microscope (“in situ” tests). I NVESTIGATED MATERIAL AND EXPERIMENTAL AND NUMERICAL PROCEDURES n this work, an almost fully pearlitic DCI (5% ferrite) with a high graphite elements nodularity, higher than 85%, with a graphite volume fraction of about 9-10%, was investigated (chemical composition in Tab. 1, %wt). C Si Mn S P Cu Cr Mg Sn 3.59 2.65 0.19 0.012 0.028 0.04 0.061 0.06 0.098 Table 1 : Investigated fully pearlitic DCI chemical composition (EN GJS 700-2). Experimental procedure Investigated DCI was cut into microtensile specimens with a length x width x thickness equal to 25 x 2 x 1 mm, respectively, with a central notch. Three different notch radii were considered, R = 2, 5, 12.5 mm (e.g., Fig. 3; notch radius R = 2 mm). Notched specimens were metallographically prepared (both with and without chemical etching, Nital 1). Tensile tests were performed using with a tensile holder (Fig. 4): specimens lateral surfaces were observed by means both of a scanning electron microscope (SEM), focusing the damaging micromechanisms in the graphite nodules, and of a Digital Microscope (DM), focusing the damage evolution in the ferritic matrix. The reason of this double observation process is connected to the possibility to use the DM as a portable device, allowing in situ observations. 1 Figure 3 : Notched specimen. Figure 4 : Tensile holder with microtensile specimen (a) ; fatigue testing machine (b) . Specimens deformation and applied load were measured by means of a Linear Variable Differential Transformer (LVDT) and two miniature load cells (10 kN each), respectively (tensile holder and the testing machine are shown in Figs 2a and 2b, respectively). Videos were obtained by means of a morphing commercial software. I