# Repair Performance of Self-Healing Microcapsule/Epoxy Resin Insulating Composite to Physical Damage
## 3 Results and Discussion
### Repair Performance of Mechanical Damage
#### 3.1.3 Change of Insulation Strength to Scratch Damage  
Breakdown strength is one of the most important macroscopic performances of insulating material, which can be affected by damage defects. Moreover, the scratch defect can equivalent to the structural damage. Therefore, alternating current (AC) breakdown strength of damaged samples is measured. From Section 3.1.1, the damaged samples were heated for 30 min. This experiment can reflect the influence of physical damage on the epoxy resin insulating material and the repair effect of composite on its electrical properties. Due to the breakdown data usually have great dispersion, the Weibull distribution (as shown in Formula (1)) is used to reduce the error [28]. \[F(U)=1-\exp\left[-\left(\frac{U}{\alpha}\right)^{\beta}\right], \tag{1}\]  
where, \(U\) is the breakdown voltage value; \(\alpha\) is the scale parameter, indicating the breakdown voltage value when the material failure probability is 63.2%; and \(\beta\) is the shape parameter, indicating the dispersion of data. The AC breakdown strength is shown in Figure 6. The breakdown strengths of epoxy resin before and after doping the microcapsule are maintained at about 45 kV/mm, which indicates that the 1 wt. % microcapsule has little effect on the insulation strength of epoxy resin. On the one hand, the impurities and interface defects introduced by microcapsule will destroy the original continuous structure of matrix material. It can make the internal electric field distribution uneven, thus reducing the electrical insulation performance of matrix [29]. On the other hand, the microcapsule distributes evenly in epoxy resin, which can reduce the influence of impurities on the internal structure of matrix. In addition, the local state can be formed by the interface between microcapsule and epoxy matrix, resulting in many charge traps. The migration of carrier can be affected by charge traps. In other words, the microcapsule can shorten the average free path of carrier, which improves the insulation performance of matrix material [1]. Therefore, the breakdown strength of composite is not decreased significantly, which can meet the application requirements of epoxy resin insulating material. After the scratch treatment, the breakdown strengths of the pure sample and its composite are all reduced by about 10 kV/mm. Moreover, the breakdown point occurs at the scratch site. It is proved that the physical defect (such as a scratch) can directly affect the electrical insulation performance of insulating material, threatening the safe operation of electrical equipment. And the similar decrease  
Figure 6: (a) Weibull probability. (b) Alternating current AC breakdown strength of epoxy resin. amplitude of breakdown strength proves that the 1 wt. % microcapsule has little effect on the original properties of epoxy resin. There are three main reasons for the influence of scratch on breakdown strength: (1) According to the theory of electrical-mechanical aging, the micro-defect in the material can distort the electric field and form a high field strength area at the damage site [11]. And the electric field at the scratch was more concentrated, so the breakdown strength was reduced. (2) Physical damage would directly reduce the thickness in the direction of breakdown. At the same external electric field, the field strength at the scratch was greater. Therefore, the breakdown would occur at the scratch. (3) The physical damage can be equivalent to the filling of another dielectric. In this experiment, the oil used in the test replaced the epoxy resin in the damaged parts. And the insulation strength of oil is lower than that of epoxy resin. Considering the interface charge effect, more space charge would be accumulated at the interface between oil and epoxy resin [29], which exacerbates the electric field distortion and caused the breakdown phenomenon. After heating, the breakdown strength of pure epoxy resin is fluctuated slightly, and there is no obvious recovery.