# 3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing
## 3 Results and Discussion
### Low-Velocity Impact Tests
#### 3.2.4 Non-Destructive Evaluation via Phased Array Ultrasonics and IR Thermography  
The 3R laminates were repaired after the impact test. The repair process was evaluated with the phased array ultrasonics technique to investigate the efficiency of the resin responding. Figures 18 and 19 show that most of the defects that were created during the manufacturing process were eliminated after the repair. Figure 19 presents a comparison between the c-scan prior to and after repair. A few delaminations still remained between the middle laminae of the composite (Figure 19c). Figure 16: IR thermography of damaged initial 3R laminates at 9 J (**left**) and 15 J (**right**). Figure 15: IR thermography of damaged RTM6 laminates at 9 J (**left**) and 15 J (**right**). \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**9 J Impact**} \\ \hline
**Type of CFRP Specim** & **Energy Absorbed (J)** & **Repair Efficiency** \\ \hline
3R Initial & 5.79 \(\pm\) 0.05 & \\
3R Required & 5.76 \(\pm\) 0.06 & 99.3\% \\ \hline
**Type of CFRP Specim** & **Max Load (N)** & **Repair Efficiency** \\ \hline
3R Initial & 6.30 \(\pm\) 0.28 & \\
3R Required & 5.76 \(\pm\) 0.11 & 91.4\% \\ \hline  & \multicolumn{2}{c}{**15 J Impact**} \\ \hline
**Type of CFRP Specim** & **Energy Absorbed (J)** & **Repair Efficiency** \\ \hline
3R Initial & 9.41 \(\pm\) 0.17 & \\
3R Required & 9.95 \(\pm\) 0.45 & 105.8\% \\ \hline
**Type of CFRP Specim** & **Max Load (N)** & **Repair efficiency** \\ \hline
3R Initial & 8.11 \(\pm\) 0.31 & \\
3R Required & 7.08 \(\pm\) 0.76 & 87.3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Repair efficiency values of 9 and 15 J impacts. Figure 17: Representative load/time and load/displacement plots of impacts at 9 and 15 J of initial and repaired specimens. Figure 20 presents the IR thermographs of the 3R laminates after the repair process. Although the delaminations were fully repaired, a few defects from the manufacturing process remained and acted as heat traps near the surface of the laminate (red/dark spots on the surface of the laminate). The delaminations of the repaired 3R laminates caused by the second impact test are illustrated in Figure 21.  
The 9 J energy impact led to a delamination with a mean diameter of 36.1 mm \(\pm\) 1.63 mm. More extensive delamination occurred in the repaired 3R laminates after the 15 J impact. The mean diameter of the damage was 48.82 mm \(\pm\) 3.54 mm (Table 5). When compared with the initial specimens, the repaired specimens had an increased damaged area diameter by 36.6% and 37.4% for the 9 J and 15 J impact, respectively. Figure 22 depicts a bar chart with a comparison of the mean damage diameter for all the inspected laminates. Figure 19: 3R laminate (**a**) initial, (**b**) repaired, and (**c**) a-scan, b-scan, and c-scan of defects. Figure 18: Repaired 3R laminate: C-scan of a representative specimen. Figure 21: C-scan of repaired 3R laminate after 9 J (**left**) and 15 J (**right**) low-velocity impact (A is the area, C is the circle length and D is the diameter of the circle). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{4}{c}{**Debonding Diameter (mm)**} \\ \hline \multicolumn{4}{c}{**Before Repair**} \\ \hline Laminate & 9 J & 15 J & 9 J & 15 J \\
1 & 23.29 & & 34.27 & \\
2 & & 35.23 & & 46.31 \\
3 & & 34.52 & & 51.32 \\
4 & & 36.75 & & \\
5 & & 35.8 & & \\
6 & & 35.02 & & Used for CAI \\
7 & & 35.86 & & \\
8 & 27.78 & & 37.4 & \\
9 & 27.81 & & 36.63 & \\
10 & 28.75 & & & \\
11 & 25.24 & & & Used for CAI \\
12 & 25.69 & & & \\ Average & 26.43 & 35.53 & 36.1 & 48.82 \\ SD & 2.05 & 0.78 & 1.63 & 3.54 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Debonding diameters of the impacted 3R laminates. Figure 20: IR thermography pictures of the repaired 3R laminates after impact at 9 J (**left**) and 15 J (**right**). The second damage, which was caused by the impact test on the repaired 3R specimens, was also detected by IR thermography by applying pulse heating. The results were in agreement with the phased array ultrasonics.