# Mechanical Properties Assessment of Low-Content Capsule-Based Self-Healing Structural Composites
## Materials and Methods
### Fracture and Bending Experiments
#### 2.6.2 Knock Down Effect and Healing Performance Assessment  
As mentioned, the current study is structured in two main investigations. The first covers the assessment of the performance of composites by comparing and evaluating the properties of the reference material to the modified capsule-based composite system. The change in performance is commonly referred to as the knock-down effect. The second investigation deals with the self-healing efficiency of the modified composite. It is anticipated the recovery of the measured property up to a percentage once the composite is integrated with a healing functionality.  
Mode II Experiments  
In the case of MODE II tests, specimens were loaded up to the load drop point which coincided with the first acoustic emission signs. The knock down effect (KNE) was calculated as a percentage according to Equation (1).  
Modified samples after load/reload followed the healing cycle of 48 h at 80 \({}^{\circ}\)C in a controlled oven. All samples were after left to cool down at room temperature. Experiments were conducted again at the same conditions after the healing process was completed. Healing efficiency (HE) regarding the modified material is presented as a ratio of the healed to the unhealed measured property and is based on Equation (3). At least five specimens were tested for each material.  
\[\textit{KNE \%}=\frac{\text{Reference Property - Modified Property}}{\text{Reference Property}}\star 100 \tag{1}\]  
\[\textit{HE \%}=\frac{\text{Property AH}}{\text{Property BH}}\star 100 \tag{2}\]  
where AH is the measured property After Healing and BH the initial property Before Healing.  
Regarding the knock down effect the same protocol was followed for three-point bending experiments and measurements calculated according to Equation (3).  
Concerning the self-healing assessment, it is taken into account that fiber breakage is a catastrophic type of failure for composites and capsules fail to be sufficient in such an extensive damage. Thus, it is recommended fiber breakage to be avoided and focus is given on matrix microcracking.  
To this view, the first category of specimens either reference (R) with no capsules or modified (M) with capsules loaded up to fracture in order to obtain the materials properties. Subsequently, the second category of specimens subjected to unloading/reloading three-point bending conditions in order to induce in a controlled manner new damage within the composite. Loading was interrupted by one unload/reload cycle. The loading was terminated at the first acoustic emission activity (AE) indicating the presence on new matrix microcracking damage. Reloading up to fracture separated either to specimens before (BH) or after the healing cycle (AH) The objective is to measure the residual strength and modulus of each material when damage is induced during loading. Figure 5 depicts the flowchart of the loading conditions. At least five specimens were tested for each concept while reference samples were tested at their initial state (un-healed).  
\[\textit{KNE \%}=\frac{\text{Reference Property at Fracture - Modified Property at fracture}}{\text{Reference Property at fracture}}\star 100 \tag{3}\]  
\[\textit{HE \%}=\frac{\text{Property AH \emph{reloading up to fracture}}-\text{Property BH reloading up to fracture}}{\text{Property BH reloading up to fracture}}\star 100 \tag{4}\]  
where AH is the measured property after healing and BH the initial property before healing.  
The new damage index was measured for both reference and modified materials according to Equation (5).  
\[\text{Damage Index }\%=\frac{\text{Property at fracture}}{\text{Property reloaded up to fracture}}\ast 100 \tag{5}\]