# 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  
In an impact test, the majority of the energy is absorbed by the impacted specimen in the form of permanent induced damage while the remaining part of the energy is stored in the form of elastic energy due to the specimen deformation. As a function of time, the energy can be calculated by numerically integrating the area below the impact force-displacement curve. Figure 5a depicts a representative energy vs. time impact curve [29]. This curve clearly differentiates the elastic component of the energy from the absorbed energy component by the asymptotic dashed line shown (see Figure 5a). The higher the absorbed energy, the larger the damage induced in the specimen due to impact. When applying this principle on the comparison among the four tested architectures in this study (see Figure 5b), it is found that the NCF architecture absorbs the highest energy followed by the 2D-PW, then ORT-TW and finally the ORT-PW. This suggests that the induced damage size should be expected to follow the same pattern. This can be supported by the ultrasonic C-scanning results as well as the damage quantification using the X-ray CT data. Figure 4: Impact force–displacement curves for the impacted specimens: (**a**) NCF, (**b**) 2D-PW, (**c**) ORT-PW and (**d**) ORT-TW. After all the specimens were impacted, the ultrasonic C-scanning was used to have a better idea of the level of the internal induced damage. A representative C-scan of each architecture is shown in Figure 6. Just by visual inspection, it was difficult to spot the difference among all the different architectures as the impact was a BVID; however, thanks to the C-scans, a clear distinction can be made. Before even calculating the size of the damaged area, it is obvious that the shape of the induced damage is dependent on the architecture. For the NCF case, which has the minimum waviness among all the tested architectures, the shape of the damage induced is characterised by a cross (0\({}^{\circ}\)/90\({}^{\circ}\)) shape. This can be attributed to the fact that, due to impact loading, splitting occurs in the 0\({}^{\circ}\) plies, which have longer floats as opposed to the 90\({}^{\circ}\) counterparts [29]. Moving on to the 2D-PW case, this cross shape almost disappears with a relatively smaller damage region, but still, it is larger compared to the 3D woven architectures. For both ORT-TW and ORT-PW, the damage size is smaller but still, the ORT-PW demonstrates the smallest induced damage region. Figure 5: Energy versus time: (**a**) schematic explaining the elastic and absorbed component and (**b**) representative sample for each architecture. Figure 6: C-scanning and digital image correlation (DIC) results for the impacted specimens. Impact damage resistance is generally evaluated in light of the size of induced damage and the energy absorption. The comparison among all the architectures using these two parameters should help in understanding the effect of the fabric/composite architecture. Therefore, the size of the induced damage was calculated using MATLAB image segmenting tool for all the architectures using the C-scan images. Table 1 summarises the percentage of the damaged area normalised by the total area of the specimen. It is clear that the NCF architecture experienced the largest damage due to impact followed by the 2D-PW. When it comes to the 3D woven architectures, although both architectures experienced less damage as opposed to the NCF, a significant difference is observed between the ORT-PW and the ORT-TW specimens. The induced damage in the case of ORT-TW is almost 1.5 times the damage in the case of the ORT-PW, which clearly highlights the effect of the z-binding yarns frequency or in other words, the unit-cell size of the weave on the impact resistance of the woven composites. This is very much in line with observations, previously reported in [30; 31; 32; 33; 34; 35], explaining the role of the z-binding yarns in resisting delamination propagation in 3D woven composites for different loading conditions.