# Mechanical Properties Assessment of Low-Content Capsule-Based Self-Healing Structural Composites
## 3 Results and Discussion
### Three-Point Bending Results  
The flexural strength and modulus were assessed regarding the knock down effect (Equation (3) due to the capsule's integration and also for their recovery after the healing process (Equation (4). A decrease of up to 5% and 6% was measured for flexural strength and modulus, respectively, as shown in Figure 9a,b. Furthermore, the representative curve for a reference and a modified specimen is presented in Figure 9c. Several parameters have been reported as being responsible for the knock down factor (i.e., capsules size and content, method of incorporation etc., [33]), one of the most crucial is the dispersion of the capsules within the matrix material. Capsules were deliberately integrated at certain layers, as detailed described in section \"2.3 Composite manufacturing\", in order to reduce the knock down effect. It should be noted that the measured decrease is not considered crucial as the developed material can provide an additional functionality, that of self-healing. The percentage of the damage index due to the unloading/reloading conditions was measured for the developed materials according to Equation (5). The induced damage index followed the same trend for both reference and modified material at each property. More precisely, for flexural strength (FS) 21% and 23%, and flexural modulus (FM) 23% and 25, respectively, to the reference and modified material as presented in Figure 10a. A representative diagram of load-extension for a modified specimen is presented in Figure 10b during loading/reloading conditions. Figure 8: Mode II experiments main results for (**a**) knock down effect and healing efficiency regarding the fracture toughness (**b**) typical behavior of representative sample of reference, modified before and after healing. Figure 10: (**a**) Percentage of damage index during unloading/reloading protocol for flexural strength and flexural modulus. (**b**) Load-extension for modified specimen during loading/reloading conditions. Figure 9: Knock down effect for (**a**) flexural strength, (**b**) flexural modulus and (**c**) representative curve of load-extension for reference and modified specimen up to fracture. Once damage was quantified the healing efficiency was evaluated. In Figure 11a,b the average values of self-healing efficiency are observed. An additional comparison is mentioned that of the reference material when reloaded up to fracture after the damage index. The aim is to compare the after and before healing properties, not only with the modified material, but also with the reference with no healing functionality. A representative diagram from a specimen of each category is shown in Figure 11c comparing the reloading behavior of modified before, after healing and the reference. A proportion of 16% for flexural strength and 30% for flexural modulus was recovered after the healing process when compared to the unhealed modified material. The adhesion between the capsules and the matrix proved to be sufficient and compatible to provide recovery. It is worth mentioning that the modified material after the healing process performed better during reloading up to fracture even to the reference material at the same conditions. More precisely, both flexural strength and modulus were increased up to 14%. A representative diagram of a specimen at each category is shown in Figure 11c, comparing exclusively the reloading behavior of the modified before, after healing (M\({}_{\text{BH}}\), M\({}_{\text{AH}}\)) and the reference (RR). Figure 12 summarizes all of the results for each concept and involves an explanatory schematic for each comparison. Table 1 provides the final outcomes for the behavior of the developed materials. Figure 11: (**a**) Flexural Strength Healing Efficiency. (**b**) Flexural Modulus Healing Efficiency. (**c**) load-extension of reloading conditions for a reference specimen, modified before and after healing. The supremacy of the material after the healing process confirms the successful activation and performance of capsules.