PSI - Issue 68

Minghua Cao et al. / Procedia Structural Integrity 68 (2025) 828–834 M. Cao et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction As a modern engineering material, cast iron has been extensively utilized in industry thanks to its outstanding wear resistance as well as thermal and mechanical properties. Compacted graphite iron is an important type of cast irons; its growing use includes disc brakes [1], cylinder heads and blocks, attracting interest in the automotive industry. The microstructure of CGI consists of graphite inclusions within a metallic matrix (Luo et al. (2024)). Based on their shape, graphite particles are categorized into spherical, vermicular, and flake inclusions (Fig. 1).

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Fig. 1. Microstructure of compacted graphite iron As a brittle and soft material, graphite tends to experience interfacial (graphite-matrix) debonding under loads, where it separates from the metallic matrix and effectively acts as a void (Dawson (2008)). This interfacial debonding is a primary fracture mechanism in CGI (Qiu et al. (2016)) and a key factor in the initiation of fatigue cracks (Kohout (2001); Endo and Yanase (2014)). As a result of the use of CGI in automotive engines, often exposed to high temperature conditions, interfacial debonding is caused by a mismatch in coefficients of thermal expansion (CTE) of the two phases. Microcracks, caused by thermal deformation and debonding can connect, coalesce a network of larger cracks that propagate and may ultimately lead to failure (Qiu et al. (2016)). In high-temperature environments, CGI becomes softer. While debonding was observed in CGI at stress levels exceeding 495 MPa at room temperature (Wu et al. (2019)), it can initiate in graphite at stress levels below 50 MPa at a temperature of 723 K (450 °C) (Qiu et al. (2016)). Hence, thermal loading influences the thermomechanical properties of graphite and affects the interfacial debonding damage at microscale; still this has not yet been fully investigated. Additionally, considerable data available on carbon and graphite properties for different types of CGI. The grade of CGI 350 to CGI 550 showed an increasing hardness trend and different microstructure (Sirtuli et al. (2024)). Ferritic CGI (CGI GJV-300) exhibits greater wear compared to CGI GJV-450 and grey iron thanks to its higher ductility, which results in increased local plastic deformation in the matrix during sliding wear under higher loads (Sirtuli et al. (2024)). Phenomenological and micromechanical modelling approaches are commonly used to numerically study the thermomechanical behaviour of cast irons. In phenomenological models, the yield surface and hardening parameters are modified to account for microstructural features, using empirical descriptions (Frishmuth and McLaughlin (1976)). In contrast, micromechanical models directly simulate the material’s microstructure, drawing from experimental analysis (Andriollo et al. (2019); Yang et al. (2021)). In some cases, the material constituents are treated as anisotropic (Andriollo et al. (2016)) or elastoplastic (Andriollo et al. (2015)). In this study, both the graphite and metallic matrix were regarded as homogeneous and isotropic at microscale. Thermal debonding, as a primary fracture mechanism, remains under-researched in terms of thermal deformation and damage under pure thermal loading conditions. Additionally, it is widely recognized that the morphology of graphite inclusions influences the macroscopic performance of CGI. This study focuses on examining the impact of morphology and CTE of graphite particles on thermal deformation and the damage behaviour of CGI under pure thermal loading, utilizing three-dimensional numerical models. 2. Introduction 2.1. Geometry Representative volume element (RVE) is regarded as a statistical representation of the material at the macroscopic scale (Seleš et al. (2021)). A set of 3D RVE models was developed with a free top surface to allow for thermal expansion of both graphite and matrix. The geometric parameters of all numerical models were determined based on