numero25

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 25 (2013) 102-108 ; DOI: 10.3221/IGF-ESIS.25.15 104 Reversed plastic zone radius, rpz r : - between 0.024 and 0.314 mm (plane stress conditions); - between 0.008 and 0.105 mm (plane strain conditions). Considering that the maximum values of the graphite nodules diameters (about 40-50  m), it is evident that, for lower  K and R values, a pearlitic DCI cannot be considered as an homogeneous material, with graphite nodules diameters that are comparable to the main fracture mechanics geometrical parameters (first of all, the reversed plastic zone radius). Does this inhomogeneity have consequences on the crack propagation paths? The aim of this work is focused on the analysis of the influence of graphite nodules on damaging micromechanisms and on crack path, considering both cyclic loading and overloads and considering the role played by the pearlitic matrix. I NVESTIGATED MATERIAL AND EXPERIMENTAL PROCEDURE n this work a pearlitic DCI (EN GJS700-2) was considered. Investigated DCI chemical composition is shown in Tab. 1: it is characterized by an almost fully pearlitic microstructure and by a high graphite elements nodularity (higher than 85%). C Si Mn S P Cu Mo Ni Cr Mg Sn 3.59 2.65 0.19 0.012 0.028 0.04 0.004 0.029 0.061 0.060 0.098 Table 1 : DCI EN GJS700-2 chemical composition (95% pearlite, 5% ferrite). In order to analyze the damage evolution during fatigue crack propagation, or after overloads, 10 mm thick CT (Compact Type) specimens lateral surfaces were previously metallographically prepared. Long fatigue cracks (18-19 mm) with negligible crack tip plastic zones, were obtained performing two or three times the force shedding procedure described in ASTM E647 [10], with the applied  K value that follows the relationship:   0 C a a 0 K K e        (3) The decrease of the applied  K value implies a decrease of the crack growth rate and of the crack tip plastic zone radious: when a very low crack growth rate value was obtained (about 10 -10 m/cycle), the  K 0 value was increased, allowing to the fatigue crack to propagate again up to the final crack length (18-19 mm). Tests were performed using a computer controlled servohydraulic machine in constant stress ratio conditions (R=P min /P max = 0.1), considering a 20 Hz loading frequency, a sinusoidal loading waveform and laboratory conditions. Crack length measurements were performed by means of a compliance method using a double cantilever mouth gage and controlled using an optical microscope (x40). At the end of this procedure, a long fatigue crack was obtained, with a negligible crack tip plastic zone. Subsequently, static overloads were applied in order to generate crack tip plastic zones: after each applied overload, Scanning Electron Microscope (SEM) and Digital Microscope (DM) specimens lateral surface observations were performed, considering both the crack path and the crack tip zone, investigating the damaging micromechanisms. SEM observations were mainly focused on the graphite nodules damaging analysis, while DM allowed a more complete analysis of the damage evolution in the pearlitic matrix (e.g., by means of the observation of slip lines evolution, less evident if observed by means of a SEM). E XPERIMENTAL RESULTS AND COMMENTS Fatigue crack propagation damaging micromechanisms ccording to the SEM observations performed on Nital 2 etched specimens, fatigue crack propagation in pearlitic DCI is strongly influenced by the microstructure. Focusing the pearlitic microstructure, fatigue crack can propagate both along the ferritic lamellae with a sort of delamination (usually, when pearlitic lamellae orientation is more or less parallel to crack propagation direction), and with a transgranular mechanism (preferentially, when pearlitic lamellae orientation is more or less orthogonal to crack I A