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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1. Introduction Compacted graphite iron (CGI) has been used as a substitute to grey iron for a number of engineering applications, such as pipes, machinery and automotive since its discovery in 1948 (Dawson, 2009). Its extensive industrial use was triggered by its enhanced thermal and mechanical properties, superior casting qualities, as well as competitive price. CGI consists mainly of iron, carbon (usually in form of graphite or carbide), and silicon. Graphite particles of varying shapes and sizes are embedded in a metallic matrix, creating CGI’s complex microstructure ( Fig. 1), which is difficult to characterise. Although the shape and size of graphite particles are random, this can be partially controlled using some tracing elements in the solidification process. On the other hand, the distance between neighbouring particles cannot be controlled by any means (for a given volume fraction) and remains arbitrary.

Vermicular graphite

Flake graphite

10 μ m

Nodular graphite

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

CGI was not appropriately studied at the microscale despite its extensive industrial use. Its failure mechanisms at microscale were not fully understood together with parameters that influence them. Graphite inclusions have a significant effect on the fracture mechanism of CGI. Such particles are unable to deform at the same level and rate as their surrounding matrix material due to their lower coefficient of thermal expansion (CTE) and their brittle, soft nature. The mismatch between the deformation abilities of the two constituents can lead to decohesion of inclusions from the matrix (Nicoletto et al., 2009). Apart from the reduced loading capacity due to debonding, microcracks can also initiate and coalesce into larger cracks. Such larger cracks propagate along the debonded particles, creating a network of cracks that eventually can lead to total failure (Qiu et al., 2016a; Qiu et al., 2016b). Generally, the approaches used for modelling the mechanical behaviour of CGI fall into two categories: phenomenological and micromechanical ones. A phenomenological approach involves the modification of yield surface and hardening parameters with a view to describe the macroscopic behaviour of CGI considering any microstructural features and fractures mechanisms (Josefson and Hjelm, 1992, 1995; Josefson et al., 1995; McLaughlin and Frishmuth, 1976). On the other hand, micromechanical directly account for each constituent and identify all the parameters that affect their behaviour (Andriollo et al., 2015a, 2015b). However, the influence of interaction between two neighbouring particles on the failure of microstructure is rarely incorporated in modelling schemes for CGI since (i) phenomenological models refer to the macroscale and, hence cannot be used to study this phenomenon and (ii) most micromechanical are based on single-inclusion formulations. A micromechanical approach that considers the interaction of graphite particles is the direct incorporation of microstructure into a finite element code. However, this approach cannot be used to study the influence of parameters such the distance, shape, and size of the inclusions. Interaction of graphite inclusions can cause the stress concentration in the matrix area between them, accelerating the coalescence and further propagation of microcracks, already initiated after debonding. Due to the random location