# Mechanical Properties Assessment of Low-Content Capsule-Based Self-Healing Structural Composites
## 1 Introduction  
Composite materials have been a topic of intense research since their introduction in a variety of engineering fields. However, polymer composites are mostly susceptible to damage during their service. The damage accumulation degrades the mechanical performance of the material and leads often to catastrophic failure [1]. In light of these issues, scientists inspired from biological systems have designed structural materials with a recovery mechanism triggered by the damage itself. The self-healing functionality can be addressed to a composite material either by incorporating capsules within the polymeric matrix that contains the active healing agent [2], or alternatively by creating vaccines within its structure that deliver the healing agent to the damaged site [3, 4], or by employing an intrinsic self-healing polymer as the matrix material [5]. Simultaneously, different integration techniques have been developed according to the selected self-healing system [6]. Focusing more on capsule-based systems, when the capsules are ruptured by a damage event, the self-healing mechanism is triggered through the release of the healing agent. Subsequently, the local healing agent reacts upon contact with the embedded catalyst and restores the desired functionality. Therefore, it is crucial for both components to be present in the damaged area in high quantities, so that the chemical reaction can sufficiently take place [2]. Over the lastyears, extensive efforts have been made by researchers towards the assessment of the mechanical properties of capsule-based polymer systems. In the majority of the cases that have been studied, the microcapsules are developed by a poly-ureaformaldehydehyde (pUF) shell, encapsulatingdicyclopentadiene (DCPD) as the liquid healing agent [1; 7; 8] and/or epoxy resin [9; 10; 11]. The chemical mechanism in these systems relies on the ring opening metathesis polymerization (ROMP) of the healing agent and the catalyst. Grubbs' catalyst is well known for its direct catalytic action [12]. However, it is particularly air and moisture sensitive and quite costly. While recognizing the high rate of self-healing efficiency of DCPD and Grubbs' catalyst healing system [7; 13], significant drawbacks have been observed regarding the incorporation of these aforementioned system into fiber reinforced polymer (FRP) materials by using conventional manufacturing techniques [14; 15; 16; 17]. In view of this, it is of high importance to have efficient compatibility between the commonly used epoxy matrix and the embedded self-healing system. More specifically, [PERSON] et al. [18] developed microcapsules containing a diglycidyl ether bisphenol-A (DGEBA) epoxy monomer encapsulated in a pUF shell as the healing agent and Scandium (III) Triflate (Sc(OTf)3) as the catalyst. By implementing the self-healing system (20%wt. capsules and 0.5%wt. catalyst) into tapered double cantilever beams (TDCB) the self-healing polymer managed to recover its initial fracture strength at approximately 80%. Furthermore, one more successful attempt to incorporate the healing agent into an epoxy matrix was noted by [PERSON] et al. [19]. In their work, they encapsulated Bisphenol-A (DGEBA) in UF capsules and they integrated them within the matrix material along with an imidazone hardener. The self-healing performance was evaluated in terms of fracture toughness, showing a total recovery of 106% when 20%wt. capsules were added to the matrix. Towards the same direction, [PERSON] et al. [20] prepared UF microcapsules with a diameter of 200 \(\upmu\)m, containing a core solution of 5 wt%. epoxy-chlorobenzene [21]. In situ fracture testing with short groove TDCB specimens embedded with 5-20%wt. loadings of the epoxy-containing microcapsules, demonstrated healing efficiencies up to 100%.