# Investigation of Acoustic Emission of Cracks in Rails under Loading Close to Operational
## 2 Materials and Methods  
Railroad rail R43, manufactured from E76F high-carbon steel, was chosen as the object under study. The chemical composition and mechanical properties of E76F high-carbon steel are presented in Tables 1 and 2, respectively. The tested specimens were in the form of rail fragments with a thickness of 10 \(\pm\) 1 mm. After cutting, the surfaces of the fragments were milled and ground to obtain the appropriate surface quality. On the cut fragments, thin notches were made over the entire thickness of the rail at a defined depth, simulating a defect. The width of the finished notch is between 0.25 and 0.3 mm. The notch depth is between 2 and 4 mm, depending on the specimen. Specimens with three different notch locations were investigated: in the rail web, in fillet transitions from the rail web to the head, and from the web to the foot. A photograph of one of the rail fragments with a notch in the web is shown in Figure 1a. A fatigue crack of a defined length was grown on the notched rail fragments by means of the Instron 8801 testing machine (Figure 1b). Smooth cyclic loading was performed by compression with a sinusoidal cycle with a frequency of 10 Hz and a cycle asymmetry coefficient R = 0. The length of the cracks was between 0.5 and 2.5 mm. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Steel**} & \multicolumn{6}{c}{**The Content of Chemical Elements (\% wt.)**} \\ \cline{2-7}  & **C** & **Mn** & **Si** & **V** & **Al** & **S** & **P** \\ \hline E76F & 0.71–0.82 & 0.75–1.15 & 0.25–0.6 & 0.03–0.15 & \(\leq\)0.02 & \(\leq\)0.025 & \(\leq\)0.025 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: E76F steel chemical composition (reference data). Figure 1: Rail fragment (**a**); fatigue crack growing in Instron 8801 testing machine (**b**); test of the prepared fragment (**c**). Testing of rail specimens with a notch and fatigue crack was carried out on an Instron 5982 universal testing machine in compression loading mode. An in-situ photograph of the specimen during the experiment is shown in Figure 1c. Figure 2 shows the frequency response of the AE sensor GT200 used in the experiment. The sensor has high sensitivity in the frequency band of 50-200 kHz, with the highest transformation coefficient at a frequency of about 180 kHz. Even though the sensor is resonant, its sensitivity remains relatively high (about 40-50 dB) in the high-frequency region, which makes it possible to register AE signals in the frequency range of 300-400 kHz. The loading of the specimen was carried out by cyclic compression according to a programmed cycle (see Figure 3), simulating the passage of a train carriage at a low speed (between 3 and 4 km/h) with maximum cycle stress of 15, 30, and 45 kN. Each rail was loaded with 120 cycles, which approximately corresponds to the loading of the rail during train travel. AE signals were registered during the tests using the industrial system A-Line DS (produced by LLC \"INTERUNIS-IT\"). The measuring chain consisted of resonant AE sensor GT200 (with 180 kHz resonant) and preliminary digital amplifiers ALS with 26 dB gain coefficient (produced by LLC \"INTERUNIS-IT\"). The intrinsic noise of the equipment, preamplifier, and sensor was 20 dB. The acoustic peak background noise level after placing the specimen in the grips of the testing machine was 30 dB, and the amplitude discrimina  
Figure 3: Cyclorama for testing rail fragments with a maximum load of 45 kN. Figure 2: Frequency response of AE sensor GT200. tion threshold was set equal to 45 dB. The data acquisition used a 100-500 kHz digital filter to exclude the noise of the testing machine. Metallographic studies, which were carried out to identify the microstructure features of the rail material, were performed on microsections cut in the longitudinal and transverse directions. Microstructure studies were carried out using optical microscopy at magnifications of 100x-1000x on a Zeiss Observer Z1m microscope.