# 3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing
## 4 Conclusions  
This study focused on the evaluation of the knockdown effect between a conventional typical aerospace-grade epoxy resin (RTM6) and a vitrimer epoxy resin. The vitrimer resin was re-processable, repairable, and recyclable (3R-resin). The repair efficiency of the 3R composites was also estimated. Various mechanical tests were performed simultaneously with NDE techniques such as impedance spectroscopy, acoustic emission, IR- thermography, phased array ultrasonics, and electrical resistance change method. ILSS and low-velocity impact results presented a negligible knockdown effect of about 3-4%, proving that the 3R resin had similar mechanical properties to the conventional (RTM6) resin. Repair efficiency values of 72% were also calculated for the ILSS geometry. At low-velocity impact specimens, the values recovered by 87% in terms of average maximum load and by 105% in terms of the absorbed energy at two energy levels (9 J and 15 J). The impedance spectroscopy was used to evaluate the impregnation of the carbon fibers and  
Figure 32: Representative lap strap test of the repaired 3R composite with ERCM monitoring. offered a complete view of the electrical behavior of the ILSS geometry. Conventional specimens exhibited higher impedance than 3R specimens. This showed that carbon fibers were better impregnated by the conventional resin than by the 3R resin. The impedance values of the 3R specimens were similar to the carbon fiber values, denoting the appearance of shortcuts between the fibers, which indicated poor wetting/impregnation by the 3R resin. An increase in the impedance occurred in both types of specimens due to delamination. The impedance was restored after the repair of the 3R specimens, while the damage in the repaired specimens caused a further reduction in the magnitude of the impedance, probably by exposing the carbon fibers. The phased array ultrasonics offered a picture of the remaining defects from the production and the repair processes and a quantitative damage detection on the impacted specimens, with high accuracy at impact tests. The IR thermography confirmed the phased array results with high sensitivity at the small defects on the surface of the impacted specimens, which acted as heat traps. The knockdown effect was evaluated approximately at \(\approx\)\(-3.0\%\) in terms of maximum load or \(\approx\)\(-1.9\%\) using the maximum strength values at CAI tests after a 9 J impact. At the 15 J impact, the knockdown effect of 3R specimens was \(-4.3\%\) in terms of maximum load and \(-1.1\%\) in compression strength. The lap sharp, a geometry that can simulate the stiffening of a composite panel, exhibited a knockdown effect of about 3% between the RTM6 and 3R composites. However, lap sharp specimens recovered about 110% of their initial values due to the welding phenomenon that occurred during the repair process. The acoustic emission technique provided crucial information about the evolution of the fracture process. The RTM6 specimens presented a sudden rupture as the AE activity accumulated only at the point of lap sharp failure, denoted by a higher population of low-amplitude hits (matrix cracks) and fewer hits at a high amplitude (shear damages and fiber breakages). On the other hand, the 3R specimens had a rich low-amplitude AE activity from the early stages of the test, declaring crack formation in the matrix before the failure. These cracks were due to manufacturing defects identified by ultrasonics and IR-thermography which acted as initiation points. Additionally, after the repair process, the AE technique revealed the existence of initial cracks as well as the effects of the loading history. The electrical resistance change method was applied to the lap sharp simultaneously with the mechanical testing. ERCM proved to be a valuable technique for monitoring deformations before the debonding initiation and the progress of the damage with consistency and high sensitivity in real time. Methodology, M.K., G.F., K.T., A.R.d.L., D.C.Z. and S.W.