PSI - Issue 82

Abhijit Joshi et al. / Procedia Structural Integrity 82 (2026) 91–97 A. Joshi et al./ Structural Integrity Procedia 00 (2026) 000–000

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treatment, the morphology of these particles plays a critical role in the strength of cast irons. The traditional cast iron, known as grey iron or flake graphite iron (FGI), includes graphite particles as flakes with sharp edges that act as stress concentrators, resulting in a brittle material behaviour under tensile loading. Even though FGI is brittle, it is still widely used thanks to its low cost, high wear resistance, high castability and machinability, very good thermal conductivity and excellent damping characteristics (Grote and Antonsson, 2009). Compacted (or vermicular) graphite iron (CGI) is a modern cast iron often considered a replacement for FGI in various applications because it provides better static and fatigue strength, higher stiffness, while retaining high castability and good thermal conductivity (Pierce et al., 2019). The graphite particles in CGI have rounded edges and they appear in coral-like formations that improve adhesion with the matrix resulting in its superior stregth. Although CGI was discovered in 1948, its large-scale production did not start until late 1990s as it needs very stringent control of magnesium and sulphur. Scanning electron microscopy (SEM) images of material studied in this research are shown in Fig. 1, highlighting the complex morphology of graphite particles as well as pearlitic-ferritic structure of its matrix. This version of CGI is called EN-GJV-450; which is a pearlitic form of CGI as evident in the microstructural images.

Fig. 1. SEM images of CGI microstructure (a) showing vermicular graphite and matrix; (b) highlighting graphite, ferrite and pearlite particles.

CGI and FGI are widely used in automotive cylinder heads and engine blocks with operating temperatures in the range of 400 °C to 500 °C. These parts experience considerable thermal and mechanical stresses, and their long-term use at high temperatures makes them susceptible to creep and thermo-mechanical fatigue failures. Multiple studies have covered failure investigations of valve-bride regions of cylinder heads (Joshi et al., 2023; Zhang et al., 2020); with focus on high- and low-cycle fatigue with some attention to creep and plasticity. There are only few publications in the literature with emphasis on study of creep in cast irons (Jing et al., 2022; Joshi et al., 2023; Wu et al., 2018). Our recent paper (Joshi et al., 2025) presented experimental studies of tensile and compressive creep in CGI and calculated parameters necessary for creep simulations, which can be helpful to understand the effect of graphite morphology on the creep behaviour of CGI. In general, there is very limited understanding of the spatial micro-scale elasto-visco-plastic behaviour of CGI, and there are no published micromechanical models available in the literature about these effects for CGI. This paper presents the suggested methodology and approach for development of such models and includes results from some early steps in this process. The objective of micromechanical modelling of CGI is to understand the material behaviour at the microstructural level under the application of macroscopic loading. The microstructural models can show the local stress distributions, helping to quantify the effect of stress-concentration features such as inclusion of graphite particles in the matrix. With introduction of creep mechanism in the micromechanical models, evolution of creep with time and associated stress distribution can be analysed. 2. Simulation setup The micromechanical models in this paper are based on a 2D representative-volume-element (RVE) approach, widely reported in the literature for cast iron (Andriollo and Hattel, 2016; Bonora and Ruggiero, 2005; Zhang et al., 2018). The models are created in commercial finite-element software package Abaqus. The method used to account for graphite particles is an important consideration in the micromechanical models and two approaches - modelling the graphite particles as void and as an isotropic solid can be found in the literature (Andriollo and Hattel, 2016;