# 3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing
###### Abstract  
In this study, the mechanical properties of purposefully synthesized vitrimer repairable epoxy composites were investigated and compared to conventional, commercial systems. The purpose was to assess the knockdown effect, or the relative property deterioration, from the use of the vitrimer in several testing configurations. Mechanical tests were performed using ILSS, low-velocity impact, and compression after impact configurations. At modeled structure level, the lap strap geometry that can simulate the stiffening of a composite panel was tested. Several non-destructive evaluation techniques were utilized simultaneously with the mechanical testing in order to evaluate (i) the production quality, (ii) the damage during or after mechanical testing, and (iii) the repair efficiency. Results indicated that the new repairable composites had the same mechanical properties as the conventional aerospace-grade RTM6 composites. The electrical resistance change method proved to be a valuable technique for monitoring deformations before the initiation of the debonding and the progress of the damage with consistency and high sensitivity in real time. In terms of repair efficiency, the values ranged from 70% to 100%. v 2022 2022018 July 202218 July 202  
evaluation techniques (NDE) such as the IR-thermography [25; 26] and ultrasonics [27] are used on the composite parts. These techniques can also be employed for the evaluation of any repair process performed on the damaged part or structure [28; 29]. The repair of thermoset epoxy polymers and composites is challenging. The chemical bonds formed after the polymerization are irreversible; thus, the repair is problematic or even impossible [23]. Except for the conventional and well-known self-healing methods (extrinsic or intrinsic) [30], a new category of thermoset epoxy resins, called vitrimers, is nowadays being developed and studied. These epoxies have all the advantages of thermoplastics as they are re-processable, repairable, and recyclable by introducing dynamic covalent bonds into their polymeric network [31]. In vitrimers, permanent crosslinks are replaced with exchangeable bonds that can rearrange thermally (or under another stimulus) via dissociative or associative mechanism [32]. At a specific temperature, above the glass transition temperature (Tg), or topology freezing temperature (Tv), these bonds can rearrange to repair resin/fiber delaminations and resin microcracks in the damaged part [33]. On the other hand, vitrimers retain the properties of the typical thermoset under service temperature, such as the resistance to environmental degradation, the good thermal properties, and, more crucially, their high mechanical properties [34; 35]. [PERSON] et al. [36; 37] developed a new epoxy vitrimer resin called 3R (re-processable, repairable, recyclable) resin. The vitrimer 3R resin was, for the first time, applied at the manufacturing of fiber-reinforced composites using standard and well-known processes. In this study, in order to assess and establish the vitrimer 3R technology, the fracture mechanisms of commercial (composites manufactured with a typical aerospace grade resin) and 3R epoxy composites were investigated and compared. Specifically, the relative property deterioration, henceforth called the \"knockdown effect\", was assessed in relation to the mechanical properties for several testing configurations. The knockdown effect is defined as the percentage change of the initial mechanical properties of a commercial composite material after the incorporation of a repair technology. Numerous well-established geometries were tested, such as the interlaminar shear strength (ILSS), the low-velocity impact, and the compression after impact configuration. At modeled structure level, the lap strap geometry that can simulate the stiffening of a composite panel was examined [36]. The previous tests were employed on both conventional and 3R carbon fiber-reinforced polymers (CFRPs).