Figure 3. Schematic representation of the roundness shape factor.
c) Roundness Shape Factor: Fraction of the area oc- cupied by the particle to the area of a circle with a diameter equal to the maximum feret of the particle.
Roundness Shape Factor = Aparticle / A for maximum feret
All three shape factors are valid methods of estimating the degree of “roundness” of a graphite particle. However, it is important to be aware that camera resolution and magnifica- tion used for the analyses are likely to have an influence on the values of particle perimeter obtained. Therefore, trying to compare two analyses results conducted on different mi- croscopes and at different magnifications are more likely to yield different results if either the sphericity or compactness shape factors are used and particularly with sphericity.
IMPACT OF SELECTING DIFFERENT IMAGE ANALYSIS PARAMETERS
Depending on the type of microstructure being analyzed, the image analysis parameters selected may or may not have an impact on the result (nodule count and nodularity) obtained. For example, when analyzing a sample with a very high nodu- larity, modifying the shape factor type and value will have a limited impact on the final result obtained, since the vast majority of graphite particles are very round. However, when analyzing a DI microstructure with a significant amount of degenerated graphite, or when comparing the image analysis results of two microstructures with nodules of very different size distribution, the selection of these initial parameters may have a significant impact on the result obtained.6
To visualize the effect of changing the type and the mag- nitude of the shape factor, or the trap size, analyses were conducted on two different microstructures (Figures 4 a and 4b), both with a high degree of nodularity, at three dif- ferent magnifications (50x, 100x, and 200x). The raw data associated with these analyses were then treated to simu- late the effect of changing the trap size or the shape factor on the results obtained. Figures 5-7 describe the impact of the selection of the analysis parameters on the nodule count (Nod/mm2
% Nodularity by Area). International Journal of Metalcasting/Volume 8, Issue 2, 2014
Figure 4. Microstructures of samples analyzed for the study of the influence of various IA parameters; a) TM2, and b) TCC2.
Figures 5-7 provide a quick visual impact of selecting one parameter as opposed to another. In Tables 1-3, the trap size has been fixed at 10µm, which allows a better comparison of the effect of varying the magnification and shape factor on the nodularity and nodule count. All the results presented in Figures 5-7 as well as in Tables 1-3 were obtained with a fixed number of fields analyzed, i.e. 25 fields. Changing the magni- fication while keeping the number of fields analyzed constant will result in significant differences in the number of particles analyzed. For example, if X particles are analyzed at 200x, that number will increase to 4X at 100x, and to 16X at 50x.
The trends observed for these two samples are:
• A higher magnification used in the analysis al- lows the shape factors to be more “discriminating” in their ability to determine if a graphite particle is round enough; the result, lower nodularity and nodule count values.
• The nodule count and nodularity values obtained using a sphericity shape factor are the one most in- fluenced by a varying magnification.
), and on nodularity (% Number of Nodules, and
• In general, for both nodularity and nodule count, the most significant variation is observed when changing the magnification from 100x to 200x. In- creasing the shape factor by 0,1 can decrease sig- nificantly the calculated nodularity value obtained, especially for samples with a nodularity <90%, and especially for the nodularity based on Number %.
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