# Investigation of Acoustic Emission of Cracks in Rails under Loading Close to Operational
## 3 Results and Discussion  
When testing defect-free rail specimens, AE hits with amplitudes of 45-55 dB were registered, and the dependence of the amplitude values on time in accordance with the applied load is shown in Figure 4. AE hits were registered mainly in the phase of increasing load with the uniform activity of no more than 25-30 hits per cycle (or 2-5 hits per second). During the loading of defect-free specimens, the more reasonable sources of AE activity seem to be various external noises. However, the waveform and spectrum of the measured signals indicate that the AE source has a microscopic size and is located inside the specimen. All signals have a repeatable waveform with a low duration between 50 and 150 \(\upmu\)s (Figure 5a,c). At that, two types of frequency spectrums were registered (Figure 5b,d). AE waveforms are presented without gain coefficient compensation. In 80% of cases, the spectrum has a resonance character in the frequency band 100-200 kHz near the resonant frequency of the sensor (Figure 5b). In 20% of other cases, the spectrum  
Figure 4: Dependence of AE hits amplitudes on time (blue) and dependence of loading force on time (red) for defect-free specimen. contains a high-frequency part in the region 300-450 kHz (Figure 5d). The waveform and spectrum of AE signals shown in Figure 5 are similar to a signal represented in [20], which are generated by cyclic loading of specimens from rail steel on the stage before the crack onset. We can conclude that during elastic deformation of a defect-free specimen, observed AE corresponds to deformation of the inner structure of the material, i.e., a kind of \"structural noise\" of the testing object is formed. The short duration and fast rise time of the AE signals exclude internal noise as a probable cause of AE generation. It is known that the short scale of the signals corresponds to the short scale of the process, that typical for a source of AE [24]. The fast rise time could be evidence of the rapid release of elastic energy, which is typical for the destruction processes. Such signals are not typical for external noises like friction, vibration, and the noise of the testing machine. The effect of \"structural noise\" from porous collapse in granular Mg-Ho alloys has been investigated in the fundamental works [25; 26], and the AE hits corresponding to this process have been characterized and identified with specific values of AE hit amplitudes. A study [27; 28] noted that the appearance of AE hits with a short rise time can also be caused by the deformation of the cementie in the structure of high-carbon steel. AE signals similar in waveform to those shown in Figure 5 were obtained in [29] at the elastic deformation stage; the authors explain the appearance of such signals by welding inclusions in the material structure. A study of the microstructures of rail steel specimens was carried out to identify possible sources of structural noise. The microstructure of the rail consists of quasi-eutectoid pearlite grains along the boundaries of which the ferrite phase is located (see Figure 6a). At the same time, in the study of non-etched microsections, non-metallic inclusions elongated in the rolling direction were revealed (Figure 6b). The SEM study of the rail fracture made it possible to establish that most non-metallic inclusions are manganese sulfides MnS (Figure 6c,d) and complex manganese silicates 2 MnO\(\cdot\)SiO\({}_{2}\). It is known that inclusions of manganese sulfides in pearlite steels can significantly affect the activity of acoustic emission under loading even in the elastic region [30], so we associate the appearance of such signals with a debonding or cracking of these inclusions. Figure 5: Typical waveform (**a**,**c**) and spectrums (**b**,**d**) of AE signals obtained during the loading of defect-free specimens. Figure 7 shows typical distributions of amplitudes for different maximum cycle stress values: 15, 30, and 45 kN, respectively.