## 2 Inspection Schemes  
The authors have suggested the stimulation of hidden metallic rebars by applying electric current or inductive heating. In both cases, the infrared (IR) thermographic monitoring of dynamic temperature distributions on the external bottom section of a pole allows for evaluating the quality of reinforcement. Since steel, concrete, air, and corrosion products differ by their thermal properties, significant test parameters, such as magnitude of temperature anomalies and their behavior over time, depend on the presence of corrosion products and the quality of contact between rebars and concrete. Both power and duration of electric pulses should be optimized to ensure detectable surface temperature signals.  
There are four test configurations enabling the internal stimulation of rebars. In the first scheme, a source of electric current 5 is directly connected to rebars 2 via contact clamps 6 (Figure 2). Figure 3 shows the use of electrodes 7 placed on the surface of concrete without direct access to rebars. The third test configuration uses the natural grounding of concrete poles that are deeply buried in soil. In this case, one conductor is connected to a natural or artificial earth electrode 8 (Figure 4). However, the authors' practical experience has shown that the most effective technique is inductive stimulation of reinforcement (Figure 5) using high-frequency inductors 10 allowing contactless heating of metallic rebars inside concrete poles.  
Figure 3: Using surface electrodes for heating rebars: (1—test object, 2—rebars, 3—IR imager, 4—cables, 5—current source, 7—surface electrodes).  
Figure 2: Direct connection of current source to reinforcement: (1—test object, 2—rebars, 3—IR imager, 4—cables, 5—current source, 6—contact clamps).  
Below we describe general principles and a step-by-step procedure of TNDT of railway electric poles by using inductive heating.  
The inspection is performed by using at least one IR camera 1 (Figure 5). The inspection area can be expanded by using additional IR cameras or IR mirrors 11 made of materials with high reflectivity in the IR spectral band (for example, polished aluminum, etc.). A scheme of electric energy supply intended for rebar stimulation should be chosen taking into account the configuration and mass of test objects. Stimulation time and power are to be optimized by modeling particular test cases. Note that, by delivering a maximal heating power, one must avoid material damage accompanied by the appearance of burns and slags.  
Figure 4: TNDT by using pole natural grounding and earth electrode: (1—test object, 2—rebars, 3—IR imager, 4—cables, 5—current source, 6—contact clamps, 8—natural/artificial earth electrode, 11—IR mirror).  
Figure 5: TNDT of electric railway poles by using inductive heating: (1—test object, 2—rebars, 3—IR imager, 4—cables, 5—current source, 9—high-frequency current transducer, 10—high-frequency inductor, 11—IR mirror).  
Synchronously with turning on a stimulation source, one starts recording a sequence of IR thermograms that reflects the evolution of pole surface temperature over time. Note that most of the thermal signals appropriate for further evaluation take place at a cooling stage when hidden defects can be detected by their specific temperature patterns on the inspected surface. By analyzing IR images, a thermographer is to outline areas with rebars and detect possible hidden defects, whose identification strongly depends on thermographer's experience. It has been found that an additional detection criterion, along with defect pattern amplitude and shape, is a rate of temperature variations in suspicious areas, i.e., the corresponding temperature derivatives.  
The efficiency of the above test procedure depends on how uniform the heating of rebars is. This can be achieved by: (1) using transverse reinforcement (if available); (2) binding reinforcement with additional wires; (3) using grounding elements or special surface electrodes.