# Mechanical Properties Assessment of Low-Content Capsule-Based Self-Healing Structural Composites
## 3 Results and Discussion
### Acoustic Emission Results  
Recordings of AE during Mode II experiments are presented in Figure 13. Reference and modified specimens reported marginal differences in terms of the number of hits at maximum load. Capsules breakage recordings were most probably eliminated due to the relatively high selected threshold of 40 dB [27]. It is worth mentioning that the acoustic activity of the healed specimens was,as anticipated, slightly higher than those of the reference and unhealed ones. This could be attributed to the partially repaired damage and for this reason the acoustic emission activity started at an earlier stage, resulting in more overall AE hits.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Main Results**} & **Flexural Strength (FS)** & **Flexural Modulus (FM)** \\  & **(MPa)** & **(GPa)** \\ \hline KNE \%: R vs. M & \(-5\) & \(-6\) \\ \hline HE \%: M\({}_{\mathrm{AH}}\) vs. M\({}_{\mathrm{BH}}\) & \(+16\) & \(+30\) \\ \hline HE \%: M\({}_{\mathrm{AH}}\) vs. RR & \(+14\) & \(+14\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main results for the knock down effect and the self-healing efficiency regarding the flexural strength and modulus of the tested materials.  
Figure 12: Unloading/Reloading protocol for three-point bending tests. Values and schematic of each measurement. Abbreviations: AE, Acoustic Emission; AH, after healing; BH, before healing.  
Figure 13: Average AE hits of Mode II experiments for all specimens.  
In the case of 3 PB experiments, one may hypothesize that the total recovery measured may indicate no new damage within the composite until the loading stops and thus, there is no new damaged area to be healed. To investigate this phenomenon and assess whether damage was indeed healed or not, the protocol of unloading/reloading was utilized in order to determine the felicity effect (FE) which refers to the presence of detectable/significant acoustic emission activity at a load level below the previous maximum applied. According to the literature [34], the difference at the load onset of emission is a reliable indicator of new damage to the FRP. As load increases and the material suffers more damage, the AE hits increase, and the FE becomes more pronounced. Furthermore, the Felicity Ratio (FR) was calculated based on Equation (6) [35]. It is described as the fraction of load at which the acoustic emission is recorded to the previous maximum applied load. A decreasing FR corresponds to a growing level of damage in the structure being monitored.  
\[FR=\frac{\textit{Load (F2) at onset of significant AE during reloading}}{\textit{Load (F1) at onset of significant AE during loading}}<1 \tag{6}\]  
The onset of emission at an earlier load during reloading was confirmed and is shown in Figure 14. The presence of some level of damage, possibly at the level of matrix microcracks, which eventually triggers the embedded healing system is thus ensured with the aid of acoustic emission observations. Figure 14 depicts the felicity effect and a calculated FR of 0.95 for one representative modified specimen. In all tested samples the felicity ratio was less than one and this was used as a prerequisite for the validity of the above argumentation.