# Mechanical Properties Assessment of Low-Content Capsule-Based Self-Healing Structural Composites
## 1 Introduction
The incorporation of microcapsules as a healing system within an FRP material is an attractive recent research area where limited effort has been devoted to the investigation of the self-healing efficiency and also the effects of the integration process regarding the mechanical properties of the final self-healing composite [21]. Capsule-based composite studies are generally focused not only on healing agents, but also on the rupture process, mixing process, micro-structure, and fabrication techniques. As is well known, fiber reinforced materials experience several failure modes during their active lifetime [22]. In more detail, failure mechanisms in a single lamina, include reinforcing fiber failure, matrix cracking, failure of the matrix/fiber interface as well as delamination between individual plies. In a representative study, [PERSON] et al. [23] demonstrated that the introduction of 15%wt. of UF capsules within the 'local' matrix of the central ply in a woven glass fiber-reinforced epoxy composite resulted in a decreased interlaminar fracture toughness in Mode I and Mode II at approximately 30% while the peak load decrease reached at 15% and 10%, respectively, as compared to plain composites. Although capsules containing ethyl phenylacetate (EPA) for self-healing purposes did not take place for composite samples due to the presence of bare fibers on the crack plane and to the reduction of EPA diffusion into the matrix. In other work, [PERSON] et al. [24] produced a two-component healing system consisting of UF microcapsules containing epoxy and CuBr2(2-MeIm)4 (the complex of CuBr2 and 2-methylimidazole) as hardener. In the case of 10%wt. of the microcapsules and 2%wt. of the latent hardener, the highest recovery of fracture toughness (GIC) was observed at 68%. [PERSON] and his co-workers [17] sealed (DCPD) into microcapsules made from UF resin. Then, the microencapsulated monomer (20%wt.) was added into epoxybased composites combined with powdered Grubbs' catalyst. The healing at 80 \({}^{\circ}\)C yielded at 80% recovery of the virgin interlaminar fracture toughness in double cantilever beam (DCB) specimens. A two-part healing chemistry was incorporated by [PERSON] et al. [25] in a woven glass/epoxy fiber-reinforced composite. The healing system was comprised of a single type of microcapsules containing silanol end-functionalized polydimethylsiloxane, and a crosslinking agent, polydiethoxysilane, and a second type containing dibutyltin dilaurate catalyst in the solvent hexylacetate. At a concentration of 12 vol.%, a 30% decrease in the short beam strength and at 15 vol.%, a 15% reduction in storage modulus was measured. Even fewer are the references that study the correlation between the Acoustic Emission (AE) method and the self-healing performance in capsule based FRP composite. On polymeric level,Szebenyi et al. [26] collected data by detecting the number of hits as a function of displacement and crack propagation and correlated them with the progress of failure during TDCB tests. The accumulation of these hits was significantly higher in the case of the encapsulated material, which can be attributed to capsule fracture phenomena. Summarizing, the self-healing concepts, especially the capsule-based ones, have been proved as a step-forward towards the development of advanced FRP composites. However, some drawbacks are currently under investigation in order to enable the development of such multifunctional materials. The aim of the current study is to provide new insights in the mechanical and acoustical performance of self-healing fiber reinforced composites integrated with a low content of UF capsules as the self-healing carrier. The healing agent of bisphenol-A/epichlorohydrin (EPON 828) was selected due to its compatibility with the epoxy matrix and the solid-state catalyst of Scandium (III) Triflate (Sc(OTf)3) due to its increased catalytic ability, low toxicity and high stability [27].