PSI - Issue 42

Minghua Cao et al. / Procedia Structural Integrity 42 (2022) 777–784

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Minghua Cao et al. / Structural Integrity Procedia 00 (2019) 000–000

diameter of spherical graphite fully fixed boundary condition 2 length of square matrix in 2D 3 length of cubic matrix in 3D major length of vermicular graphite minor length of vermicular graphite periodic boundary condition area of square matrix (2D) volume of cubic matrix in 3D volume fraction of graphite volume of spherical graphite in 3D

1. Introduction Cast iron is a modern engineering alloy, widely used in industry because of its great mechanical, and thermal properties, excellent wear resistance and competitive prices. Increasing applications of compacted graphite iron (CGI) that include cylinder heads, cylinder blocks (Dawson, 2008) and disc brakes (Behera, 2012) attracted research interest in the automotive industry. A microstructure of CGI comprises graphite inclusions and a metallic matrix. Graphite particles are classified as spherical, vermicular, and flake graphite according to their circularity (Fig. 1).

Spherical graphite

Vermicular graphite

Flake graphite

20 μ m

Fig. 1. Microstructure of compacted graphite iron.

As a brittle and soft material, graphite typically experiences interfacial debonding, when it disconnects from the metallic matrix and behaves as a void (Dawson, 2008). Interfacial debonding is the main debonding mechanism in compacted graphite iron (Qiu et al., 2016) and can cause initiation of fatigue cracks (Endo and Yanase, 2014; Kohout, 2001). Due to CGI’s use in automotive engines, it is often exposed to high-temperature conditions. In this case, interfacial debonding is caused by different coefficients of thermal expansion of the two phases. However, there is not enough information in the literature about the thermal deformation mechanism of CGI at the microscale. Under severe thermal loads, after thermal deformation and debonding, the formed microcracks can connect and form a network of large cracks, further propagating and potentially leading to total failure (Qiu et al., 2016). Exposed to high-temperature environment, CGI softens. Debonding was observed in CGI at the stress level surpassing 495 MPa at room temperature (Yue et al., 2019), but its initiation in graphite can happen below 50 MPa at a temperature of 723 K (450 °C) (Qiu et al., 2016). The tensile strength of CGI decreases at temperatures higher than 300 °C (Selin, 2010). Hence, thermal loading influences the thermomechanical properties of graphite and interfacial debonding damage at the microscale; still this has not yet been fully investigated.