## 6 Experimental Results and Discussion
### High-Temperature Experiments  
As was mentioned in the theoretical section, high-temperature EMATs have been designed so far only for thickness measurements and thus an EMAT that can withstand high temperatures and excite guided waves is still required. A first approach for that would be the selection of the suitable high Curie magnets and high-temperature coil or the design of a cooling system so that the EMAT would be as efficient as possible at high temperatures. However, all of the above may result in a more complicated design. Hence, a further study about room temperature, guided wave EMATs, and their performance at high temperatures should be conducted prior to the design of a new EMAT. Hence, the EMATs were tested from ambient temperature to 180 \({}^{\circ}\)C with a step of 10 \({}^{\circ}\)C; the maximum operating temperature of these EMATs is 250 \({}^{\circ}\)C and therefore they were tested up to 180 \({}^{\circ}\)C only, so that any serious and irreversible damage will be avoided. The EMATs were continuously exposed to the heat source with zero lift-off during the rise in temperature, while the overall time they were exposed to the heat was equal to 15 min. This set of experiments was conducted three times. Figure 11a-d show the signal received at room temperature, 60 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, and 180 \({}^{\circ}\)C, respectively. Firstly, in the signal obtained at room temperature, both the reflections from the plate edges and the first two reflections from the defect are clear. However, it is obvious that the amplitude of the second reflection from the plate diminishes greatly after 60 \({}^{\circ}\)C, while at 100 \({}^{\circ}\)C and 180 \({}^{\circ}\)C it  
Figure 10: Amplitude change of signal transmitted against **(a)** lift-off increase; **(b)** power increase. can hardly be noticed. Similarly, the amplitute of the first reflection from the defect and the forth reflection from the plate decreases with the increase in temperature. Figure 12 shows how the amplitude of the signal transmitted decreases with temperature rise; it is clear that the amplitute dwindles almost linearly with the rise in temperature. However, the amplitude of the signal transmitted in 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C was slightly larger than the amplitude at room temperature in the areas marked in red in Figure 12. Also, the amplitude error alters with temperature rise. A reason for that may be the ground connection between the EMAT and the specimen. The thermal conductivity of stainless steel is low and therefore the specimen was not heated up uniformly; as a result, the plate bended and the mechanical connection between the EMAT ground and the specimen altered with temperature rise due to the gradient of the bend. This mechanical/electrical connection significantly influences the amplitude of the signal transmitted and thus may be the reason for the amplitude increase at 30 \({}^{\circ}\)C and 40 \({}^{\circ}\)C.  
Time shifting is also observed, as was expected due to the change in the wave velocity at high temperatures, presented in the theoretical study. The third reflection from the plate shifts in time and starts coming closer to the second reflection from the crack, leading to an increase of the magnitude of the latter. Consequently, the experimental results match the theoretical. Temperature compensation should take place and both the ToF of the reflections as well as the drop of their amplitude may be two features that can be further processed and used for the identification of the temperature of the structure being tested. Room-temperature EMATs cannot be used for the monitoring of high-temperature structures (\(>\)100 \({}^{\circ}\)C); however, a high-temperature EMAT specifically designed for the monitoring of  
Figure 11: Signal received at **(a)** room temperature; **(b)** 60 \({}^{\circ}\)C; **(c)** 100 \({}^{\circ}\)C; **(d)** 180 \({}^{\circ}\)C.  
high-temperature structures (\(>\)200 \({}^{\circ}\)C) should be compared with the performance of this EMAT up to 100 \({}^{\circ}\)C.