# Mechanical Properties Assessment of Low-Content Capsule-Based Self-Healing Structural Composites
## Materials and Methods
### Composites Manufacturing  
Composite plates intended for three points bending and Mode II experiments were manufactured according to ASTM D7264/D6264M-07 and AITM 1.0006, respectively. The selected manufacturing method was wet layup for all the 12 layers. The wet layup technique was selected as composites are developed with lower values of fiber volume fraction (vf). The matrix domination with regards to the fibers intensify the polymerization process once capsules are activated.  
UF capsules were implemented in polymers, as previous in house work, at two different contents 5 and 10%wt. following the literature review as stated in the introduction section. The combination of 10%wt. UF capsules with 2.5%wt. scandium triflate exhibited the most promising results in terms of knock down effect and healing efficiency. For that reason, those contents were selected for further implementation within FRPs.  
The self-healing efficiency of a capsule-based system is dependent strongly on the dispersion of the capsules into the epoxy material. In view of this, 10%wt. capsules were incorporated into the epoxy modified resin and manual mixing was performed until the mixture was homogeneous. Subsequently, the hardener of the epoxy was added. Due to the high activity of the scandium triflate, it was incorporated right before the fabric impregnation, at a content of 2.5%wt. The final mixture was placed in a vacuum chamber for degassing in order to eliminate air inclusion during mixing process.  
The fabric impregnation followed layer by layer. The matrix material containing the self-healing agents (Figure 1a) was impregnated at the lower (3 rd and 5 th) and upper (9 th and 11 th) layers to avoid the neutral mid layer at three-point bending loading conditions. In the case of Mode II tests the mid layer was the area of interest and therefore capsule-modified matrix material (Figure 1a) was used for the carbon fabric impregnation, while the neat matrix material (Figure 1b) impregnated the remaining layers. A 13 \(\upmu\)m thick PTFE film was also placed in the middle plane of Mode II composite plate to generate the starter crack, which is required according to AITM 1.0006. Figure 2 depicts the schematic of the reference and the modified mid plane.T  
Figure 1: (**a**) Mixing process of the capsule modified matrix material for the manually impregnation of the composite through wet layup technique. (**b**) Mixing process of the neat matrix material for the manually impregnation of the composite through wet layup technique.  
The consumables that have been also employed for the wet layup process were the peel ply at the top of the laminate to provide a smooth surface to the composite, the prforated release film which allows the removal of the excess resin, the breather to absoMrb the excess resin.  
Curing was undertaken according to the manufacturer's recommendations at room temperature for 24 h and 15 h at 60 \({}^{\circ}\)C in a program-controlled oven. The fiber volume fraction was calculated at 40%.  
The mechanical properties of the aforementioned modified composites were compared with those of the reference (no healing functionality).