## 1 Introduction  
Applications of composite materials are nowadays spreading from the aircraft and marine industry to the manufacturing industry and sports equipment. In almost all applications, composites are exposed to cyclic mechanical loading. In addition, various types of impacts can occur during the lifetime of the product. Impacts on composite can be visible or barely visible (very difficult to visual inspection or not possible to detect by visual inspection) [1, 2]. Although visible impacts cause greater damage to the material (visible surface densts, fiber breakage, matrix cracking, and multiple delaminations between plies) and bigger loss of compression and tension strength than barely visible impact damage (BVID), they can be revealed by visual inspection and by other methods easier than BWID. Barely visible impacts cause delamination of the composite material, which leads to buckling and a significant decrease of compressive strength of the composite [3]. The internal damage of the composite material in form of delamination also grows under cyclic loading and can lead to fatal damage [2, 4]. Detection of BWID in composites is therefore of great interest. There are many Non-Destructive Testing (NDT) methods that can reveal the damage. However, NDT methods mostly require the structure to be taken out of service (ultrasonic C-scan, speckle techniques, X-ray micro-computed tomography), which is costly [3; 4; 5]. New sensors and new approaches are therefore being developed to reveal damage to composite materials. NDT methods are also being modified for in-situ monitoring [6; 7; 8; 9]. Monitoring in construction materials can also be carried out using the non-destructive self-acoustic method [10] with which it is possible to detect structure deformation. There are two different approaches to the inspection of impact damage to composite materials in-situ. The first approach is to monitor the response of the material itself (contact ultrasonic inspection, acoustic emission, eddy current, electromechanical impedance, ultrasonic guided waves, speckle interferometry, thermography, etc.) [11; 12]. For self-sensing purposes, the electrical conductivity of the fibers (carbon-fiber composites) [9; 13; 14; 15], or the electrical conductivity of the matrix can be used by adding conductive particles to the matrix, e.g., carbon nanotubes [16; 17]. The second approach is to perform in-situ monitoring with the use of sensors embedded during manufacturing or attached sensors (fiber Bragg grating, crack propagation gauges, comparative vacuum monitoring, crack wire, carbon fiber sensors, carbon nanotube thread sensor, etc.) [7; 18; 19; 20]. It is not possible to say which inspection method is the best, because each application has different requirements, and each inspection method has some advantages and some weaknesses. Ideal, the method should be of high resolution, good localization, quick, easy to operate, and cost-effective. Methods, which give precise information about the size and nature of the damage, are time demanding and often need laboratory conditions (e.g., immersion, taking part out of service, etc.). Between high precision methods, we can classify ultrasonic immersion C-scan, X-ray radiography, X-ray micro-computed tomography, and speckle techniques. Other methods are applicable for in-field impact damage detection [3] but are also time demanding such as visual inspection, mechanical impedance inspection, in-field ultrasonic inspection, shearography (heat/vacuum method), thermography. Detection Techniques based on guided waves are also very effective in the identification of impact damage [12; 21]. Monitoring methods based on the electric conductivity of the composite material were successfully so far on small scale coupons (100 mm \(\times\) 100 mm) [13; 16]. Impact damage detection using optical fibers with Bragg gratings for BVID was presented [22], the drawback of the optical fibers is their quite high cost and difficulties during manufacturing due to the handling of the optical fibers themselves.