# 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  
Generally, low-velocity impact results in internal damage such as matrix cracking, fibre damage and fibre-matrix debonding. As discussed by [PERSON] et al. [36], The level of damage caused by low-velocity impact depends on two primary factors including the fabric architecture and resin toughness. The resin toughness factor is outside the scope of this study. Nevertheless, to further understand the role of fabric architecture and the z-binding yarns in delamination and impact resistance, postprocessing of the 3D reconstructed volume from the X-ray CT was essential. The X-ray CT data was analysed using Avizo 2019.1 software from Thermo Fisher Scientific. The 32-bit data were first converted to 8-bit with the grey level range of 0-255. As can be seen in the CT sections/ortho-slices in Figure 8, the carbon fibre tows appear white, which corresponds to a higher grey value, and the polymer matrix appears in grey and the impact damage in black. Different phases can therefore be extracted/segmented based on thresholding the grey value in the CT images. For a simple and fair comparison among the four architectures, the damage due to the impact is discussed on three levels. The first is comparing the ortho-slice images captured along the warp (0 direction) plane in locations where there are no z-binding yarns. The second is to compare only the damage of the 3D woven architectures (ORT-PW and ORT-TW) along a plane sectioning the z-binding yarn's path. Finally, the 3D reconstructed volume of all the architectures along with the 3D segmented damage volume are detailed  
Figure 7: Dent depth comparison for representative cases of all the tested specimens along the cutting plane shown. and the 2D ortho-slice damage discussion is complemented with the 3D data to provide a comprehensive comparison of the impact-induced damage for all architectures. In locations where no z-binding yarns exist (see Figure 8), The dominant damage mechanisms represent the typical damage mechanisms due to low-velocity impact [37]: matrix cracking (due to shear and bending stresses), delamination (because of the large out-of-plane deformation and interlaminar shear) and localised fibres/yarns breakage (directly beneath the impactor at the front side and tensile failure at the backside due to bending). The severity of each damage mechanism differs from one architecture to the other. In the case of the NCF specimen (see Figure 8a), interlaminar delamination between all the adjacent plies, extending from the impact location, is very obvious as well as surface plies splitting. This is also accompanied by extensive matrix cracking and fibre/yarn breakage especially at the backside of the specimen, away from the impact location in the thickness direction. The interlaminar delamination, however, in the case of the woven architectures (2D-PW, ORT-PW and ORT-TW), is guided by the waviness of each architecture. The severity of the damage beneath the impact location at the front side is very minimal compared to the damage experienced at the backside. Tensile matrix cracking and fibre/yarn breakage are dominant especially in the 2D-PW (Figure 8b) and ORT-TW (Figure 8d) specimens. Moreover, this type of damage is much localised in the ORT-TW case, which could explain the reason why the dent depth previously measured in Section 4.2.2 using the DIC was found to be the largest. By visual inspection of the damage size among all the architectures in these locations, it is clear that the ORT-PW (Figure 8c) experiences the least damage, but this should be also confirmed in other planes as well as the 3D reconstructed volume as discussed later. Moving on to locations where z-binding yarns exist (see Figure 9), a comparison can be made between the two 3D woven architectures, the ORT-PW and ORT-TW. Although the aforementioned dominant damage mechanisms (matrix cracking, delamination and fibre/yarn failure) are still the same, it is very obvious that the size of them is much smaller in the planes where the z-binding yarns exist.