# Effect of Unit-Cell Size on the Barely Visible Impact Damage in Woven Composites
## 4 Results and Discussion
### Impact Testing
#### 4.2.2 Damage Characterisation
The z-binding yarns' path effect is further discussed in this study using the X-ray CT ortho-slices. When it comes to energy absorption, the results confirm the findings obtained from the C-scan images. The NCF architecture which experienced the largest damage absorbed approximately 1 and 2 more Joules compared to the 2D-PW counterpart and the two 3D woven architectures, respectively. In the case of the 3D woven architectures, despite the comparable energy absorption of both the ORT-PW and ORT-TW, the damage is less severe in the former. Moreover, two conventionally used parameters, in literature [15], to assess the level and severity of damage and the resistance of the composite laminates to impact are: (i) the damage degree parameter (DD) and the absorbed energy per delaminated area (E\({}_{\text{DA}}\)/A). These two parameters are widely used for the comparison of composites that possess similar areal density and number of layers, which is the case in this research study. The damage degree parameter is defined as the normalised absorbed energy per the initial impact energy. The higher the value of (DD), the less resistance of a specific type of composite to impact loading. On the contrary and based on its definition, the higher the (E\({}_{\text{DA}}\)/A) value, the more capacity a specific composite structure has to resist impact. This is achieved either by absorbing less energy or by experiencing less damage due to impact. Both parameters are compared in Table 1 for all the tested architectures. The conclusion confirms the previously highlighted differences in the sense that NCF is found to be the least impact resistant architecture while ORT-PW is found to be the most impact-resistant counterpart. The (DD) parameter for ORT-PW is approximately 15% less than NCF and the (E\({}_{\text{DA}}\)/A) parameter is almost 2.3 times of the NCF counterpart. The ORT-TW results suggest that it is not very different from the 2D-PW architecture, which again highlights the importance of the careful design of the unit-cell size in the 3D woven composites in resisting damage induced due to impact. However, both architectures, 2D-PW and ORT-TW, still demonstrate higher impact resistance as opposed to the NCF specimens. The C-scan image analysis is useful in providing planer information about the induced damage, but it does not provide any information regarding the thickness or the out-of-plane direction. Therefore, using the dent depth calculations by incorporating the 3D DIC analysis complements the understanding of the phenomena. The comparison of the dent depth, denoted by the z-coordinate, measured by the 3D DIC is detailed in Figure 7. It is \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **C-Scan Area** & **Absorbed Energy** & **DD Parameter** & **E\({}_{\text{DA}}\)/A** \\  & **(\%)** & **(J)** & **(J/J)** & **(J/cm\({}^{2}\))** \\ \hline NCF & 1.81 \(\pm\) 0.29 & 14.38 \(\pm\) 0.31 & 0.96 \(\pm\) 0.02 & 5.30 \(\pm\) 0.11 \\
2D-PW & 1.05 \(\pm\) 0.12 & 13.34 \(\pm\) 0.12 & 0.89 \(\pm\) 0.01 & 8.47 \(\pm\) 0.08 \\ ORT-PW & 0.66 \(\pm\) 0.08 & 12.23 \(\pm\) 0.59 & 0.82 \(\pm\) 0.04 & 12.35 \(\pm\) 0.60 \\ ORT-TW & 1.02 \(\pm\) 0.03 & 12.81 \(\pm\) 0.93 & 0.85 \(\pm\) 0.06 & 8.37 \(\pm\) 0.61 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the intensity of damage analysis. again clear that the affected area, because of the low-velocity impact, is larger compared to the other architectures. By plotting a line across the width of a representative specimen from each architecture (see Figure 7), the level of plastic deformation, in the form of the dent depth, can be analysed. Regardless of the specimen architecture, it seems like the dent depth due to the impact is very comparable, with a value of \(\sim\)0.2 mm. However, again it is noticed that the architecture with the shallowest dent is the ORT-PW, followed by the 2D-PW and then the NCF. The dent depth of the ORT-TW seems to be the deepest but this might be just due to the damage being very localised at maybe the z-binding yarn location.