# Effect of Unit-Cell Size on the Barely Visible Impact Damage in Woven Composites
## 4 Results and Discussion
### Impact Testing
#### 4.2.3 X-ray CT Failure Analysis
Again, most of the damage occurs at the backside, which is typical for low-velocity impact damage. Nevertheless, the unit-cell size effect is very significant. In the case of the ORT-PW (Figure 9a), the frequency of the z-binding yarns is double the frequency of the ORT-TW (Figure 9b) counterpart. In other  
Figure 8: Ortho-slices and segmented damage highlighted in red in locations where there are no binding yarns (the top is the front side of the impact and the bottom is the backside) for: (**a**) NCF, (**b**) 2D-PW, (**c**) ORT-PW and (**d**) ORT-TW. words, the unit-cell size of the ORT-PW is half of the ORT-TW. This enables the ORT-PW architecture to resist the induced impact damage more than the ORT-TW. In both cases, the z-binding yarns act as crack/delamination stoppers, hence delamination or matrix cracking grows as long as it is not arrested by any z-binding yarn. This results in longer delamination and larger damage size in the case of the ORT-TW as opposed to the ORT-PW architecture. So far the discussion and comparison have been made only based on 2D planar data. To confirm the findings in 3D space, volume reconstruction and segmentation are inevitable. Here, the 3D morphology and distribution of interyarn impact damage are of interest and simple automatic thresholding could not fit this purpose. Thus, a routine was developed to segment interyarn damage (mostly in-plane but may deviate when z-binding yarns exist) in a reliable and repeatable way across different structures. The full composite volume was firstly segmented out to define the region-of-interest for further image processing. This was followed by applying the top-hat segmentation tool to the composite volume. Line kernels sized 40 lying in-plane were used as seeds to extract black features (in this case, the impact image) and the masking range of 48-255 was selected. The selected damage volume was then denoised by removing all isolated islands smaller than 5 voxels in 3D to provide the final segmented interyarn damage volume for all types of architectures as shown in Figure 10.  
The damage volume fraction (see Table 2) was calculated by dividing the volume of the segmented impact damage by the analysed image volume (width \(\sim\)19 mm \(\times\) height \(\sim\)22 mm \(\times\) thickness) of the composite panel before impact. It is clear from the comparison that the same trend observed from the C-scan results is prevailing. The NCF architecture experiences the maximum damage due to the single impact followed by the 2D-PW. Both 3D woven architectures experience less damage with the ORT-PW having the least damage volume fraction. This demonstrates first the effect of the z-binding yarns in resisting the induced damage due to out-of-plane loading compared to laminated and 2D woven composites. Second, the frequency of the z-binding yarns through the thickness also has a noticeable effect on the damage resistance. The higher the z-binding yarns' density, as in the ORT-PW case, directly translates into less damage volume fraction compared to the ORT-TW. Figure 9: Ortho-slices and segmented damage highlighted in red in locations where there exist binding yarns in the 3D woven composites (the top is the front side of the impact and the bottom is the backside) for: (**a**) ORT-PW and (**b**) ORT-TW. Figure 10: 3D X-ray CT reconstructed volume with the segmentation of damage due to impact highlighted in red (**left**) and 2D top view (**right**). \begin{table}
\begin{tabular}{c c c c c} \hline  & **Volume of Segmented Damage (mm\({}^{3}\))** & **Composite Panel** (mm)** & **Scanned Area** (mm\({}^{3}\))** & **Damage Volume** (mm\({}^{3}\))** \\ \hline NCF & 42.02 & 2.63 & 18.97 \(\times\) 22.16 & 3.80 \\
2D-PW & 17.78 & 2.48 & 18.97 \(\times\) 22.16 & 1.71 \\ ORT-PW & 0.62 & 2.48 & 18.97 \(\times\) 22.16 & 0.06 \\ ORT-TW & 13.97 & 2.64 & 18.97 \(\times\) 22.16 & 1.26 \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of the X-ray segmentation data.