PSI - Issue 37

Evangelia Nektaria Palkanoglou et al. / Procedia Structural Integrity 37 (2022) 209–216 E. N. Palkanoglou et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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of graphite particles in the microstructure, there can be an infinite number of potential interactions. The interaction between two neighbouring particles and the parameters that influence this interaction are discussed in this work. To this aim, a 2D unit cell that encloses two graphite inclusions presented as ellipses is generated. The effect of distance between the particles on graphite decohesion and matrix plasticisation is studied numerically for different boundary conditions. The investigation is conducted for pure thermal loading since CGI is vulnerable at high temperature excursions due to the mismatch on the CTE of its constituents. However, the proposed methodology can be extended to consider either mechanical or thermomechanical loading. 2. Methodology 2.1. Microstructure characterisation A set of CGI micrographs was obtained with scanning electron microscopy (SEM) and post-processed using an image processing software (Image J). This program can recognise distinctive microstructural constituents based on their greyscale shades and assess their different geometrical features, e.g., the volume fraction of graphite particles, as well as their shape, size and distance between them (Palkanoglou et al., 2020). In order for the software to calculate these data, adjustment of the particles’ outline shape was required. In this work an ellipse was selected as the most appropriate outline shape for the graphite inclusions in CGI microstructure since it enables an adequate estimation of nodularity parameter. Statistical analysis of the obtained results was further used to generate a statistically equivalent unit cell (Kanit et al. 2003; Kanouté et al. 2009). Table 1: Minimum and maximum values of geometrical parameters of graphite particles in CGI (Palkanoglou et al., 2020). Graphite fraction (%) Perimeter (μm) Area (μ m 2 ) Major axis (μm) Minor axis (μm) Distance (μm) 2.2. Micromechanical model Normally, a unit cell includes numerous inclusions that interact with each other as the unit cell is loaded (Hill, 1963). Since the interaction between two graphite particles is investigated in this work, a unit cell enclosing only two graphite inclusions was created, assuming effective properties for the surrounding material that represents the metallic matrix. This assumption allows for the consideration of the remaining particles in microstructure and can be easily justified given that the volume fraction of graphite is much lower than that of ferrite (Table 1). Following the results of microstructure characterisation and the assumptions mentioned above, a 2D unit cell was generated, comprising two graphite inclusions, represented as ellipses, embedded in a square domain (metallic matrix). Although different shapes of graphite particles can be identified in CGI microstructure (Fig. 1), interaction between vermicular and nodular inclusions was investigated in this study, due to a complex shape of a flake particle. The geometrical configuration of the models studied herein is depicted in Fig. 2. All dimensions used were determined by the results of microstructure characterisation, assuming a volume fraction of 8.2% for graphite. The four interaction cases investigated here considered either two vermicular inclusions or one vermicular and one nodular particle. The focus of the study was on the effect of distance between the particles; hence, two configurations were considered for each case: one with minimum distance between inclusions and one where the particles had equal distances between them and with the domain boundaries. The maximum distance was selected so that both particles were not too close to the boundaries and remain unaffected by any edge effects. Further, to investigate thermal debonding, an additional layer was incorporated around each graphite particle, representing the interface and allowing for localisation of damage in it. The interfacial layer was created as an offset of the elliptical particle and its thickness was selected equal to 1 μm based on results of microstruct ural analysis and on our previous studies investigating the 5.20-11.37 3.54-315.88 0.99 6086.96 0.60-67.96 0.52-28.51 3.18-25.00