# Use of Surface Acoustic Waves for Crack Detection on Railway Track Components--Laboratory Tests
## 3 Results
### Measured Wave Signals Recorded at Specific Cirumferential Points of the Rail-Wave Transmission  
Figures 15 and 16 show plots of the signals measured with the gampt setup (described in Section 2.2) at the four sensor locations illustrated in Figure 3. Due to the narrow-band signal characteristic of the actuator, a band-pass filter with a center frequency of 1 MHz and a bandwidth of 0.2 MHz was used to clean the recorded signals of unwanted noise. The measurements in Figure 15 were taken before the rail was subjected to the loading cycles at the wheel-rail test rig; Figure 16 represents the measurements at the end of the rolling contact fatigue experiment after 310,000 loading cycles. These signals illustrate the propagation of a surface wave along the rail circumference in an undamaged and a damaged rail, respectively. Again, the actuator was located at the left rail head bottom. The distinction between the main surface wave packet, and other surface and bulk waves due to reflections, is based on the velocity of the main/direct surface wave and thus on its position on the time axis (that changed only slightly due to the manual positioning of  
Figure 14: Attenuation of transmitted R-wave with respect to the undamaged state depending on crack depth, number of cracks and receiver location; (**a**) for 1 crack, (**b**) 2 cracks, (**c**) 3 cracks.  
the transducers for each measurement). Correspondingly, the occurrence of the surface wave at the individual sensor locations is indicated by a yellow box. All signals were normalized with respect to the peak value of the first wave packet at sensor position HL occurring at \(\sim\)15 us, which represents the interrogation wave before it hit the gauge corner (and the eventual head checks there). This normalization implies that the surface wave that always overlapped with other waves (reflections, bulk waves) was the main wave component to interact with the surface cracks, whereas any corresponding interaction of reflected waves and bulk waves can be considered negligible. The latter was also confirmed by the simulation studies where the individual reflections can be traced.  
Figure 16: Signals measured along rail circumferential positions (HL, HR, HBR, WR) on the rail with severe head check damage after 310,000 load cycles, normalized with respect to the peak amplitude at position HL.  
Figure 15: Signals measured along rail circumferential positions (HL, HR, HBR, WR) on the undamaged rail, normalized with respect to the peak amplitude at position HL.  
All surface wave packets recorded at the follow-up locations experienced attenuation with increasing travelling distance, and possible presence of surface defects with increasing number of load cycles. In addition to the dominant peak of the surface wave, numerous rail-geometry dependent reflections can be noted. Due to the repeated manual procedure of sensor placement at the individual locations, sensor positions were not completely identical for measurements at different loading cycles. Therefore, surface wave arrival times varied slightly and were not used for further analyses. Instead, the shape and amplitude of the time signals were compared and related to head check formation.  
Comparing the signals recorded at the rail head positions HL and HR shown in Figures 15 and 16, respectively, it can be noted that the signal-to-noise ratio increased with the number of load cycles. An explanation for this phenomenon might be that the initially rough surface of the pristine rail was reduced by the repeated passage of the test wheel.  
Looking at the measurements for the damaged state in Figure 16, it can be recognized that the shape of the surface wave packet became more and more elongated with increasing travelling distance and hence harder to detect; this is especially true for sensing location WR, where the distortion is so pronounced that the peak of the R-wave signal disappeared completely.