## 1 Introduction  
Due to their superior specific strength, stiffness and design flexibility, fiber-reinforced polymer (FRP) composite laminates are widely used in numerous aerospace and engineering applications. However, FRP laminates are susceptible to damage arising from low-velocity impact [1]. Often, an impact event creates internal damage with a minimal external indication, also called barely visible impact damage (BVID). The effect of this hidden damage on laminate properties can be significant, especially in compression where strength may be reduced by up to 50% [2]. Hence, it is necessary to regularly perform nondestructive inspections or implement a structural health monitoring (SHM) system to detect the presence of damage and prevent catastrophic failure of structure [3]. Consequently, large factors of safety are incorporated into the design to ensure their safety and reliability, which lead to composite structures with higher weight and thicker sections. Traditionally, once damage has been detected within a composite structure, the approach has been to design and undertake temporary or structural repair. The alternative solution to these problems is the application of self-healing FRP composites. Self-healing has the potential to mitigate the damage resulting from an impact event, thereby providing an opportunity to improve the design allowables for FRPs or offer other benefits such as reduced maintenance and inspection schedules [4]. The concept of healing materials was explored in polymers in the context of crack healing in the early 1980s by [PERSON] and [PERSON] [5]. This initial work was focused on understanding how cracks could be repaired, or healed, using a solvent welding approach. In this study, [PERSON] and [PERSON] one of the central concepts of self-healing, the notion of healing efficiency, which is given by Equation (1):  
\[\eta=\frac{\Gamma_{\textit{n}\textit{n}\textit{n}\textit{n}\textit{n}\textit{n} \textit{n}}}{\Gamma_{\textit{n}\textit{n}\textit{n}\textit{n}\textit{n}}}, \tag{1}\]  
where \(\Gamma\) is a material property of interest. This property is frequently fracture toughness, but other metrics, such as modulus or ultimate strength, have been adopted by researchers when attempting to quantify healing [6]. Most experimental methods in the evaluation of self-healing efficiency are destructive. Various nondestructive and SHM methods have been employed for damage quantification, but only a few of them have been considered to quantify healing, namely acoustic emission (AE), micro-X-ray computer tomography (\(\mu\)CT), digital image correlation (DIC), Raman spectroscopy and optical microscopy. In this overview, we focus only on studying potential methods for impact damage healing efficiency which can be determined by nondestructively. The acoustic emission technique is used widely for structural health monitoring purposes. Piezoelectric sensors are attached to the structure to record the response under the excitation of the cracking sources. The parameters of the recorded signals are related to the fracture processes and enable the monitoring of fracture from the start until final failure. Fracture mechanisms like matrix cracking and debonding show different AE signatures, allowing the evaluation of the fracture stage as the load changes. The results in [7] indicate that AE provides the possibility to characterize the restoration action of textile-reinforced cement due to the existence of a self-healing agent aiming at closing the matrix cracks. The attenuation of the acoustic waves is also sensitive enough to indicate the physical filling of the cracks by the self-healing agent. Acoustic emission is often utilized for studying self-healing of cracks in cementitious composites. [PERSON] et al. [8] experimentally investigated by both passive and active AE self-healing of cracks in the cement matrix due to microencapsulated healing agents.