## 3 Results and Discussion  
Tensile stress measurements using magnetic microwires were, for the first time, used outside of laboratory conditions directly in the composite construction. The goal of the measurements was to verify the proposed methodology in real practice. Therefore, a tensile stress sensor based on a magnetic microwire was, for the first time, used in an aircraft, with the primary structural components made from composite materials. The microwire samples were prepared using the Taylor-Ulitovsky method at the University of P. J. Safarik in Kosice (Figure 4a); the chemical composition of the microwire was Fe\({}_{77.5}\)Si\({}_{7.5}\)B\({}_{15}\). These samples, with a length of 2 cm, were placed on the top layer at the upper side of the main beam of the aircraft wing and covered by one layer of the glass fiber fabric, as can be seen in Figure 4b. The longitudinal axis of the microwire had the same direction as the main beam. For the attachment, epoxi resin L285 with the hardener H285 was used. Since as the tensile stress sensor was implemented in the aircraft structure additionally and was not a construction part of the aircraft, the sensing part was hardened only in the hangar environment. These experiments were performed in co-operation with the Incoff Aerospace company, which provided for the experiments its Shark UL aircraft, which is an ultralight aircraft owned by this company. The placement of the sensing element with the disproportionate width marked by the green line can be seen in Figure 5b. Thanks to this unique measurement technology, it is possible to connect the sensing device and to perform contactless defectoscopy during the standard aircraft ground inspection. The real-time measurements can be visualized through the built-in IPC (Industrial Personal Computer) at the oscilloscope screen. The experimental measurement workstation is shown in Figure 6.  
Figure 3: Measurement block diagram. Figure 4: Glass-coated magnetic microwire (**a**) fabrication process; (**b**) implementation into the glass fiber fabric. Figure 5: Shark UL (**a**) before the placement of the sensing element, (**b**) after the placement of the sensing element. Figure 6: Shark UL aircraft with the prototype of the measurement workstation during the tests. The series of measurements was performed on the ground. One of the experimental measurements can be seen in the Figure 7. In this case, the aircraft was swung manually around the longitudinal axis by a single worker. This movement induced the maximum tensile stress in the main wing beam. During the measurements, a sampling frequency of 500 Hz was used. The ambient temperature was 30 \({}^{\circ}\)C and, from the measurement output, the ambient magnetic field was subtracted, as shown in Equation (2). Therefore, the deviation from the Earth's magnetic field did not affect the measurement and only the deviation from the initial state representing no applied tensile stress is shown in Figure 7.  
From the results of the spectral analysis, it can be clearly seen that the dominant frequency is around 1 Hz. It corresponds to the expected natural frequency of the aircraft. For a better visualization, only the detail of the FFT (Fast Fourier Transform) analysis in the frequency range from 0 Hz to 5 Hz is shown in Figure 8.  
Figure 8: FFT (Fast Fourier Transform) analysis of the signal obtained during the aircraft swing movement. Figure 7: Record of the microwire switching field deviation during the aircraft swing movement. Due to the fact that the scope of the study was to perform dynamic tests in which the deviation from the steady state is determining, the recorded data were centered and filtered by applying a low-pass FIR (Finite Impulse Response) filter with a cut-off frequency of 15 Hz. We suppose that not only the natural frequency but also all of the performed mechanical movements are below this cut-off frequency. The dominant frequency of the swing movement is surrounded by the frequencies caused by the amplitude modulation of the movement deviation.