## 1 Introduction  
Advanced composite materials nowadays possess a leading position in the aeronautics industry for primary aircraft components (e.g., Airbus A380, Boeing 787). Due to their exceptional specific mechanical properties, corrosion resistance and capability of tailored manufacturing, composite materials are employed for numerous aerospace components such as the wings, fuselage or the aircraft interiors. In fibre reinforced composites, the lightweight advantage is achieved by efficient design and involves structural anisotropy. This anisotropy is accompanied by more complex damage initiation and propagation mechanisms than those commonly encountered in conventional isotropic structural materials. A major disadvantage of composite materials is that they are sensitive to impact damage, e.g. hail damage [1]. The impact is classified into low, intermediate, high and hyper-velocity, according to the velocity of the projectile. Different impact types correspond to different energy transfer from the projectile to the target, which affects energy dissipation inside the material and consequently damage initiation and propagation [2, 3]. The ballistic impact is characterised as high-velocity when the velocity of the projectile ranges from 50 m/s to 1000 m/s [3]. For glass fibre reinforced polymers (GFRPs) under ballistic impact, two dominant damage mechanisms are responsible for the energy absorption of the projectile, fibre breakage and delamination [4]. Delaminations between plies usually cannot be detected by visual inspection and therefore, may pose a serious threat to the structural integrity of load-bearing components. A lot of research has been invested into the efficient detection and localization of the extent of damage, particularly in the development of various non-destructive testing (NDT) techniques. Among the most prevalent NDT techniques in the aeronautic field are ultrasonics, IR thermography, vibration methods,radiographic inspection, Eddy currents, electrical methods, etc. [5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. The majority of the techniques mentioned above require both the removal of the component to be tested and the utilisation of bulky and heavy sensors and equipment. The solution of permanently integrating or embedding a sensor in a structure is ideal for the in situ monitoring of the structural integrity or the structural health monitoring (SHM) of the composite during its service life [9]. SHM is vital in providing on-site or even on-line damage detection and topography. Among several SHM techniques, those related to the electrical response to damage can provide information about the structural degradation of the material with high sensitivity and precision. Moreover, the sensing elements may be an integral part of the composite structure, providing self-sensing functionalities [11; 15; 16; 17]. In recent years, numerous studies have been focused on developing self-monitoring systems based on impedance spectroscopy (IS). In various cases, the monitoring system is based on introducing two metallic sheets at the outer surfaces of the fibre reinforced laminate, thus creating a capacitor structure [18; 19; 20]. [PERSON] et al. manufactured a diagnostic film with printed sensors/actuators to monitor the curing process and detect the damage in composites. These sensors can be placed either on the surface or embedded within the composite [16; 17]. However, embedded sensors may also act as damage initiation sites, compromising the mechanical performance of the structure. Self-sensing is defined as the ability of the material to provide information about its structural integrity. The self-sensing functionality for structural components presents significant advantages compared to the inclusion of non-structural elements, such as a sensor inside the structure of the composite. Since the sensing element is an integral part of its structure, structural aggravation is minimized [21].