## 3 Experimental Measurement
### Realization of Experimental Measurement
#### 3.2.2 Operational Measurement--Antenna Mounting  
After performing tensometric measurements at the so-called unit load, the measurement was performed during the actual assembly of the antenna. As part of this measurement, displacement measurements were also performed by the Leica device, which required surface preparation (spray application) in the measurement area (Figure 11). The connection of the strain gauges and the setting of the sampling frequency were identical to the test measurement.  
The time record of the strains during antenna mounting on the unpainted roof B is shown in Figure 12 and on the painted roof C in Figure 13. Since the assembly tightening process can be considered a quasi-static loading phenomenon, the sampling frequency was set to 50 Hz. Significant peaks corresponding to sudden changes (e.g., at 43 s), such as object impact, etc., do not affect the value after tightening (in 195 s). The description of the individual stages of measurements is as follows:  
1--setting the antenna to the mounting position,  
2--tightening the nut,  
3--stabilization of measured values,  
4--loosen the nut,  
5--antenna removal,  
6--spray application for surface scanning (this phase was carried out only on the painted roof C).  
Figure 11: Scanning of the analyzed area when mounting the antenna on the roof C.  
The procedure for evaluating the measured strains was identical to that of the test measurement. The values of the normal stresses and reduced stresses for the state after antenna assembly (stage 3) calculated according to Equations (1) and (2) are given in Table 4.  
Figure 14 shows a graphic interpretation of direction of normal stresses in measuring places after antenna installation (Figure 13--stage 3), calculated according to Equation (3)  
\[\tan\varphi=\frac{2\varepsilon_{b}-\varepsilon_{a}-\varepsilon_{c}}{\varepsilon_ {a}-\varepsilon_{c}}. \tag{3}\]  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Measuring Place** & \(\sigma_{\text{max}}\) **(MPa)** & \(\sigma_{\text{min}}\) **(MPa)** & \(\sigma_{\text{red}}\) **(MPa)** \\ \hline BL1 & \(-1.94\) & \(-7.50\) & \(6.74\) \\ BL2 & \(3.93\) & \(-7.33\) & \(9.90\) \\ BP1 & \(1.88\) & \(-6.37\) & \(7.48\) \\ BP2 & \(0.68\) & \(-7.18\) & \(7.54\) \\ CL1 & \(-3.14\) & \(-10.22\) & \(9.07\) \\ CL2 & \(-0.39\) & \(-12.58\) & \(12.39\) \\ CP1 & \(-5.31\) & \(-15.97\) & \(14.09\) \\ CP2 & \(-2.69\) & \(-15.53\) & \(14.38\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Values of normal stresses and reduced stresses after antenna mounting.  
Figure 12: Time record of strains during antenna mounting—roof B.  
Figure 13: Time record of strains during antenna mounting—roof C.  
The advantage of strain gauge measurements under operational load is the possibility of validation of the results with numerical modelling. The difference is mainly in boundary conditions. Experimental measurements are performed on a real model with real bonds, while numerical modelling depends on the definition of bonds, contacts, etc. The disadvantage of the strain gauge measurement is the evaluation of deformations and stresses only in places of applied gauges, while numerical modelling allows examination in any place. At present, numerical modelling methods are most often used in the selection of experimental evaluation sites. When it is not possible to obtain necessary information by the above procedure, the experimenter's own experience plays an important role. There are certain dependences between deformations and stresses, which was also confirmed by the results of tensometric measurements. However, it should not be forgotten that stress levels in the material of real structures and equipment, in addition to the load from operation, also consider residual stresses from production. In the case of thin sheets, these can result in higher levels of deformation.