PSI - Issue 28

Evangelia Nektaria Palkanoglou et al. / Procedia Structural Integrity 28 (2020) 1286–1294 E. Palkanoglou et al./ Structural Integrity Procedia 00 (2019) 000–000

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2. Methodology 2.1. Microstructure analysis and statistical characterisation

A set of scans of CGI microstructures was produced using a scanning electron Microscope (SEM) and analysed with the image processing software ImageJ. This software can identify different microstructural constituents based on their shades in greyscale and evaluate various geometrical characteristics such as the fraction of graphite particles as well as angles and distances between neighbouring particles. Statistical representation of the obtained results may be then employed for the construction of a statistically equivalent unit cell (Kanit et al., 2003; Kanouté et al., 2009). A critical point for this analysis is the adjustment of the outline shape of particles. In this work, an ellipse was selected as it enables a better estimation of nodularity, which is a dimensionless parameter defined as the fraction of particle area to perimeter. The range of this feature is from zero to one, with values higher than 0.80 corresponding to a nodular shape, while lower indicate an elliptical one. Additionally, nodularity values lower than 0.20 correspond to very thin elliptical shapes that resemble needles (flake particles). These geometrical parameters are evaluated based on pixel recognition, with an uncertainty equal to 0.5 pixels/mm. The results of statistical analysis of ImageJ are presented in Table 1 and Fig. 2

Table 1: Minimum and maximum values of geometrical parameters of graphite particles in CGI

Area (μm 2 )

Major axis (μm)

Minor axis (μm)

Graphite fraction (%)

Perimeter (μm)

5.2-11.37

3.54-315.88

0.99-6086.96

0.602-67.96

0.52-28.51

10 15 20 25 30 35

0.0-0.19 0.2-0.39 0.4-0.59 0.6-0.79 0.8-1 0 5

volume fraction (%)

nodularity

Fig. 2: Average nodularity of graphite particles in volume fraction.

It is noted that the volume fraction of graphite ranged from 5.2 to 10.3 for CGI. Minimum and maximum values of the perimeter of a typical graphite particle was 3.54 μm and 315.88 μm respectively. The corresponding values for the area are 0.99 μm 2 , respectively. For those graphite particles that diverge from the spherical shape, their major and minor axis were found to range from 0.602 to 67.96 and 0.52 μm and 28.51 μm. The distribution of average nodularity of graphite particles in a micrograph of CGI. Fig. 2 demonstrates that most particles fall in the group of average nodularity (0.20-0.59), supporting the evidence that CGI has graphite particles of intermediate shape (Matsushita et al., 2015). In addition, the fraction of nodular graphite was slightly higher than the corresponding of flake graphite, indicating that CGI tends to have a microstructure closer to ductile iron than flake one.