## 4 Results
### Tensile Test with Strain and Temperature Varying--Test Model 4  
Test Model 4 is a test with Conf.1 and the SP2 specimen. The sample is subjected to various mechanical deformations and temperature variations. Four models were tested, with temperature varying up to 140 \({}^{\circ}\)C and load up to 6 kN, generating a maximum mechanical stress around 25 MPa, corresponding to a deformation of 125 \(\upmu\)m/m, for the SP2 that has the cross-sectional area of 240 mm\({}^{2}\). Figure 12 presents the result of the values that maximize parameter S21 for test 4.  
Figure 11: Model 3 test results—values that maximize the parameter S21 (dB) by the mechanical deformation exerted (\(\upmu\)m/m). (**a**) Test Model 3-1, where the variation in parameter S21 from the undeformed state to the maximum strain state was approximately 0.7 dB. (**b**) Test Model 3-2, where the variation in parameter S21 from the undeformed state to the maximum strain state was approximately 0.7 dB.  
The results obtained in the four tests performed on Model 4 showed a variation \(\Delta\)S21 of approximately 0.15 dB. In all curves, it is observed that the interpretation of S21 with strain is minimal for this range of strains, from 0 to 125 \(\upmu\)m/m. The result is compatible with that observed for the specimens tested in test 3. The signal has a small level of hysteresis during the charging cycles, which does not harm the power transmission efficiency between the transducers. All four specimens tested with temperature variation show S21 peak amplitude curves with similar behavior, where 100 \({}^{\circ}\)C was the temperature at which the S21 signal had the highest peak values. In the temperature range from 30 to 100 \({}^{\circ}\)C, the peak value of the S21 signal changes. For tests 4-1 and 4-2, \(\Delta\)dB is approximately 0.6; for tests 4-3 and 4-4, this value is about 0.8. In the four cases, the signal decayed sharply after 100 \({}^{\circ}\)C and up to 140 \({}^{\circ}\)C, varying between 1.2 and 1.3 dB. This variation was a little higher than in the Model 1 test because of the influence of deformation.  
It is important to reiterate that in all tests, the peak value of S21 consistently remained between \(-4.0\) and \(-2.5\) dB. The configuration studied here efficiently fulfills the proposed function of interrogating sensors by ultrasonic telemetry through the metal wall. Even in the worst cases, the small variations in S21 of 1 dB do not disrupt the energy transfer and data flow between the transmitter-receiver pair.  
It is crucial to note that the adhesive undergoes significant changes in its properties, including color alteration and the appearance of a burning odor when exposed to tem  
Figure 12: Test Model 4 result—values that maximize the parameter S21 (dB) by the strain (\(\upmu\)m/m) and the temperature variation. Each color in the graph represents a different temperature, and each point a different strain. (**a**) Test 4-1: The \(\Delta\)S21 for the strain influence is +0.15 dB, and the maximum variation in parameter S21 is \(-1.3\) dB for 100 \({}^{\circ}\)C with strain and 140 \({}^{\circ}\)C without strain. (**b**) Test 4-2: The \(\Delta\)S21 for the strain influence is +0.1 dB, and the maximum \(\Delta\)S21 is \(-1.4\) dB for 100 \({}^{\circ}\)C with strain and 140 \({}^{\circ}\)C without strain. (**c**) Test 4-3: The \(\Delta\)S21 for the strain influence is +0.2 dB, and the maximum \(\Delta\)S21 is \(-1.3\) dB for 100 \({}^{\circ}\)C with strain and 140 \({}^{\circ}\)C without strain. (**d**) Test 4-4: The \(\Delta\)S21 for the strain influence is +0.15 dB, and the maximum \(\Delta\)S21 is \(-1.4\) dB for 100 \({}^{\circ}\)C with strain and 140 \({}^{\circ}\)C without strain.  
peratures above 120 \({}^{\circ}\)C. This underscores the importance of considering temperature as a significant factor that can influence the performance of the system, as shown in Figure 13.