# 3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing
## 3 Results and Discussion
### Low-Velocity Impact Tests
#### 3.2.2 Non-Destructive Evaluation via Phased Array Ultrasonics and IR Thermography  
The impact test caused extensive internal delaminations inside the laminates. Phased array and pulsed IR-thermography were employed to assess the size of the damaged area after the impact at both energy levels. More specifically, for the RTM6 laminates, measurements were performed after the manufacturing to evaluate the quality of the process. The measurements were repeated after the impact test for the identification of the damage (Figure 9). For the 3R laminates, the above NDE characterization stages were employed, followed by the evaluation of the repair process, the second damage by impact test, and the identification of the extension of the second damage (Figure 10). \begin{table}
\begin{tabular}{c c c} \hline \hline  & **9 J Impact** & \\ \hline
**Type of CFRP Specim** & **Energy Absorbed (J)** & **Knockdown Effect** \\ \hline Conventional Resin (RTM6) & 5.56 \(\pm\) 0.12 & \\
3R Resin & 5.79 \(\pm\) 0.05 & +4.1\% \\ \hline
**Type of CFRP Specim** & **Max Load (N)** & **Knockdown Effect** \\ \hline Conventional Resin (RTM6) & 6.42 \(\pm\) 0.15 & \\
3R Resin & 6.30 \(\pm\) 0.28 & \(-\)1.8\% \\ \hline \hline  & **15 J Impact** & \\ \hline
**Type of CFRP Specim** & **Energy Absorbed (J)** & **Knockdown Effect** \\ \hline Conventional Resin (RTM6) & 9.20 \(\pm\) 0.09 & \\
3R Resin & 9.41 \(\pm\) 0.17 & +2.2\% \\ \hline
**Type of CFRP Specim** & **Max Load (N)** & **Knockdown Effect** \\ \hline Conventional Resin (RTM6) & 8.42 \(\pm\) 0.28 & \\
3R Resin & 8.11 \(\pm\) 0.31 & \(-\)3.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Knockdown effect values of 9 and 15 J impacts. Figure 10: Phased array and IR-thermography test plan on 3R impact specimens. Figure 9: Phased array and IR-thermography test plan on RTM6 impact specimens. The complete volume of each laminate was evaluated by employing the built-in stitching function to detect any defects induced during the manufacturing process, such as delaminations, poor impregnation of the fibers, voids, etc. In Figure 11 (left), a representative phased array c-scan of an RTM6 laminate is depicted. The blue color corresponds to the bottom surface of the composite. No defects were detected during the manufacturing process. On the other hand, phased array scans of the 3R laminates after manufacturing (Figure 11 (right)) revealed numerous defects which occurred at various depths due to the poor impregnation of the CF by the 3R resin. Poor impregnation of the fibers led to voids manifested as trapped air at the matrix-fabric interphase or between/inside yarns. The void content varied both between different areas of the same laminate and among different laminates. Pulsed IR-thermography confirmed the c-scan results. In Figure 12 (left), the RTM6 laminate was of high quality, with a uniformly distributed thermal behavior. Contrariwise, the 3R laminate in Figure 12 (right) presented a few heat traps that emerged at almost 5 s to 10 s of the heating cycle. Imperfections that appear at this timeframe of the heating cycle correspond to defects located near the surface of the laminate. These heat traps were small voids that remained from the manufacturing process of the composite. Ultrason inspection after the impact test revealed delaminations inside the composite. This type of damage (barely visible impact damage: BVID) is introduced inside the composites and can expose them to danger as visual inspections on the surface cannot detect them. BVIDs can act as crack initiators, leading to further delamination extension and potentially disastrous results [24]. The laminates were tested under two different impact energies. Two representative ultrasonic scans of the RTM6 laminates are depicted in Figure 13 (left) and (right) for 9 J and 15 J energy levels, respectively.