## 4 Experimentation and Discussion  
The authors' practical experience suggested that these testing techniques can be applied to the inspection of in-field railway electric poles. If inspection is performed by the scheme of Figure 4, i.e., by using natural grounding, one can avoid digging out an underground pole section due to the temperature distribution at the ground level allowing for the evaluation of the electric current flowing down through rebars. The corresponding surface temperature pattern reflects the severity and localization of damage in the pole underground section. In its turn, the monitoring of a 1-2 m long above-ground pole section allows for the detection of such defects as thinning of rebar cross-section, the presence of corrosion products, concrete delaminations, etc. To simultaneously view the rear side of the poles during a single heating cycle, one can use, as mentioned above, IR mirrors or additional IR cameras. In TNDT, test results can often be improved by increasing heating power. In order to avoid damaging a pole protective layer by using additional electrodes, it is convenient to apply non-contact inductive heating by the scheme of Figure 5 in a narrow circular area on the pole surface. By moving both the heater and the IR camera, one can observe pole temperature response under constant heating conditions. Since the thermal conductivity of concrete is relatively low, an optimal inspection area is located several centimeters from the heated zone, thus being unaffected by powerful thermal radiation from the heating zone. A similar procedure was used by [PERSON] in the 1970s to test quality of nuclear fuel elements [14]. Theoretical aspects of line heating were discussed elsewhere [12], and its practical implementation was done in the inspection of boiler sections [15]. Experimental section of this study was performed on the Transsiberian railway (JSCo \"Russian Railways\") by using a pilot experimental setup. We have checked 14 operating poles with a pre-stressed steel reinforcement of the SK type (same as simulated) by using three NDT methods: IR thermographic, ultrasonic, and vibroacoustic. Thermal stimulation of poles was accomplished by means of an inductive heater powered from an autonomous gasoline generator that was mounted on a truck. The inductor coil was moving up along a test pole with velocity from 5 to 10 mm/s by using a special lifting device (see Figure 13a). Bottom sections of the poles under test were dug out by about 0.5 m to allow their direct IR surveying. Each pole area was inductively heated for about 20 s, the same as in the modeling. Figure 12: Temperature derivative (\(\Delta T/\Delta\tau\)) vs. time (\(\tau\)) (see caption for Figure 11). Thermal NDT was accomplished on a 1 m long pole area including a 0.5 m long underground section. The inspected area was outlined with aluminum markers that were visible in both IR and visual images. Pole surface temperature was monitored on a dry sunny day (ambient temperature 25-30 \({}^{\circ}\)C) by using a FLIR P60 IR camera mounted on a tripod (Figure 13a). A typical IR thermogram of a defect-free pole is presented in Figure 13b to clearly show the warmed-up subsurface reinforcement. Recorded images were processed to produce synthetic images containing extreme values of parameters chosen as significant--for example, images of maximal temperatures achieved during the whole thermal process (the so-called \"maxigrams\"). Figure 14a shows a defect-free pole section located above the ground level. It is clearly seen that the surface 'footprints' of the reinforcement are regularly located and characterized by the same excess temperature in the range from 7 to 8 \({}^{\circ}\)C. In the case of a damaged pole (Figure 14b), the corresponding maxigram shows a distinct area of decreased temperature that apparently corresponds to debonding between the concrete and the reinforcement. Figure 13: (**a**) Inspecting quality of electric railway poles: test site view; (**b**) typical IR thermogram.