PSI - Issue 28

Evangelia Nektaria Palkanoglou et al. / Procedia Structural Integrity 28 (2020) 1286–1294 E. Palkanoglou et al. / Structural Integrity Procedia 00 (2019) 000–000

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(flake) iron, nodular graphite particles in ductile iron and a mix of the previous two in compacted graphite iron. Compacted graphite iron (CGI) is an industrial material used in a diversity of applications ranging from machinery to automotive parts. Its extensive use is thanks to its excellent casting qualities, fine mechanical and thermal properties, as well as its competitive price. Despite the increased use of CGI in industrial applications, its fracture at the microscale is still not fully understood. This is because of its complex microstructure comprising graphite particles of different shapes and sizes are embedded in a metallic matrix. Three main shapes were identified for graphite particles: nodular, vermicular and flake (Fig. 1). Flake graphite particles tend to be very long and thin, whereas vermicular ones have an intermediate shape between flake and nodular (Lim and Goo, 2011).

Fig. 1: Microstructure of CGI with different shapes of graphite particles

Graphite plays a significant role in the development of interfacial debonding in cast irons. Being very brittle, graphite particles tend to debond from the metallic matrix (Nicoletto et al., 2009). This decohesion, combined with stress concentration at their sharp edges, often leads to initiation of microcracks, which can then coalesce and propagate along the interface between graphite and metallic matrix, leading to total failure (Nicoletto et al., 2009). The identification of debonding as the main mechanism of fracture in CGI was experimentally confirmed. Under uniaxial tensile conditions, debonding was reported before yielding of ferritic matrix, between 150 and 200 MPa (Qiu et al., 2016a). Debonding was also identified in experiments, where the microstructure was subjected to thermal exposure and then tensile load was applied. In this case, thermal exposure softened the microstructure and, as a result, debonding occurred before the stress level in the matrix reached 50 MPa (Qiu et al., 2016b). Similar evidence was found in cyclic-loading studies of ductile iron, where graphite spheroids loose from the surface either partially or completely (Cocco et al., 2014). Modelling efforts for cast irons can be classified in phenomenological or micromechanical studies, with the main focus to date being either grey or ductile iron. Initial modelling attempts based on a phenomenological approach involved modification of yield surface and hardening parameters in order to consider the effect of microstructural features (Hjelm, 1994; Josefson et al., 1995; McLaughlin and Frishmuth, 1976). On the other hand, microstructure based models were reported with graphite or matrix constituents represented based on experimental observations (Andriollo et al., 2015; Andriollo et al., 2016). Recent modelling studies on CGI focus on fatigue performance and crack-path prediction; however, they do not discuss graphite debonding, especially under thermal loading. CGI is vulnerable to high temperatures associated with long-term loading. As a result of mismatch in coefficients of thermal expansion of the two constituents, purely thermal load can result in damage due to interfacial debonding . Although most applications of CGI are subjected to such loading conditions, debonding has not been taken into consideration in existing models for CGI, as parameters that affect it have not been identified yet. This lack of knowledge is partially attributed to the complex microstructure, which is difficult to characterise and model adequately. Therefore, in this paper debonding is investigated numerically, with the emphasis on the identification of parameters that affect it. The study is based on a micromechanical approach with specific inputs obtained from statistical analysis of SEM micrographs and mechanical testing at various temperatures.