## 3 Experimental Tests and Results
### Results from Experimental Campaigns  
To compare with the results from the numerical modeling, thermograms obtained at three instants (50 s, 100 s, and 150 s) during the experimental tests with the sample without a defect and that with a defect are shown in Figure 15. In accordance with the numerical modeling (thermograms in Figure 10), the thermograms show the controlled defect area, which was hotter than the healthy area, on the specimen with a defect (Figure 15b). These elevated temperatures can be attributed to the presence of air, which showed a lower thermal conductivity compared with that of the epoxy adhesive. Another observation referred to the impact of the non-uniformity of the microwave beam on the thermograms of the defect-free specimen, resulting in the generation of additional areas of increased temperature (Figure 15a). That was also the case in the numerical modeling (Figure 10a).  
The temporal evolutions of the temperature on the defect and non-defect regions for both samples are shown in Figure 16.  
In accordance with the results from the numerical modeling (Figure 12b), the temperature curves clearly show modulation due to the excitation (microwave signal). After the heating phase (150 s), the modulation disappeared from the temperature curves. In the defect area (red solid curve), the temperature increased from 15.2 \({}^{\circ}\)C to 16.2 \({}^{\circ}\)C, while that of the defect-free region (blue dotted curve) increased only from 14.9 \({}^{\circ}\)C to 15.3 \({}^{\circ}\)C (Figure 16b). On the other hand, there was only a small difference between both observed regions on the sample without a defect (both curves in Figure 16a), although there was an effect of the non-uniformity of the microwave beam.  
The temporal thermal contrast between both samples is also shown in Figure 17. Both samples may have had different initial surface temperatures before the test. This is why the initial temperatures were subtracted from the thermal contrast. Obviously, the maximum thermal contrast (0.7 \({}^{\circ}\)C) occurred at the end of the heating phase. It should be noted that it is the thermal contrast that allows us to see the defect area on the thermograms.  
Figure 15: Thermograms at different instants: (**a**) sample without the controlled defect and (**b**) sample with the controlled defect.  
It is important to notice that the comparison between the experimental approach and the numerical modeling, especially the results with the modulated excitation, showed a higher temperature at the defect area than the area without a defect (due to the presence of the air having smaller thermal conductivity than that of the adhesive). Another common point of the results from both approaches is the influence of the non-uniformity of the microwave beam on the thermograms. This non-uniformity was generated by the reflection  
Figure 16: Time–temperature characteristics obtained for both samples: (**a**) sample without a controlled defect and (**b**) sample with a controlled defect.  
Figure 17: Calculated temperature difference, which was obtained by subtracting the initial temperature distribution from subsequent thermograms, for both defective and non-defective areas.  
of the waves with the antenna and the sample. This phenomenon must be considered in the detection with the MIRT method.