# Use of Surface Acoustic Waves for Crack Detection on Railway Track Components--Laboratory Tests
## 3 Results
### EFIT Visualization of Surface Wave Propagation in a Rail
The differences arise from the fact that in one case a single crack dominated the reflection, whereas in the other cases multiple cracks contributed to the reflected wave. In general, the reflected signal amplitude ranges for all simulation variants were in the same order of magnitude. Figure 8: Simulated signal as received at position HL, without crack, and due to reflection at 1 crack, 2 cracks, and 3 cracks, respectively. The crack depth as shown in the sketch is 0.5 mm. The red circles mark the part of the signal coming from the interaction with the cracks. v denotes the velocity component at the receiver position, projected to the surface normal. Overall, the simulations indicate that reflection is a less promising measurement arrangement, due to the complexity of the reflected signals and the difficulty identifying a clear correlation between crack depth and reflected signal amplitude. EFIT Calculation of the Wave Signals Received at Specific Cirumferential Points of the Rail-Wave Transmission  
In terms of transmission, Figures 11-13 show the simulated signals at positions HR (right side of the rail head), HBR (bottom right side of the rail head), and WR (right side of the rail web; cf. Figure 3 for the positions), respectively. Each figure is split into three subplots a, b and c illustrating the changes in the transmitted R-wave packets due to the interaction with different numbers of cracks, i.e., one crack (a, top), two cracks (b, middle), three cracks (c, bottom). The individual plots depict the surface normal velocities for the undamaged and damaged states with cracks of 0.5 mm and 1 mm depth. In addition to the individual signals, the envelope of each signal is shown, to indicate the change in the amplitude of the transmitted wave with different crack depths. Figure 10: EFIT simulation surface normal velocity signal recorded at HL, crack depth 0.5 mm. Figure 9: EFIT simulation surface normal velocity signal recorded at HL, crack depth 1 mm. Figure 11: Change of transmitted R-wave received at position HR with increasing number of cracks and crack depths up to 1 mm; (**a**) 1 crack (**b**) 2 cracks (**c**) 3 cracks. Figure 12: Change of transmitted R-wave received at position HBR with increasing number of cracks and crack depths up to 1 mm; (**a**) 1 crack (**b**) 2 cracks (**c**) 3 cracks. For all receiver locations the following can be deduced: With respect to R-wave packet width, the presence of a single crack (Figures 11-13, plot a) did not hinder the identification of the wave packet for crack depths between 0.5 and 1 mm. The shape of the wave packet remains distinctive with a clear beginning and end. However, with increasing number of cracks (Figures 11-13, plots b and c), the signal-to-noise ratio was reduced drastically and the R-wave packet in time became stretched and smeared. Regarding mode conversion of the R-wave packet at the crack faces and the subsequent occurrence of faster travelling bulk waves in the computed time signals, it can be seen in all plots that the initially coherent signals began to split \(\sim\)18 \(\upmu\)s before the arrival of the transmitted R-wave packet. The deviation of the signal envelopes naturally correlates with the number of cracks and depth, since the crack tips provide the appropriate centers for mode conversion. To better compare the wave attenuation in transmission mode and possible effects due to receiver location, the results shown in Figures 11-13 were condensed by extracting the attenuation of the envelope peak intensities with respect to the undamaged state for all receiver positions and damage states, plotted in Figure 14. Figure 14a shows a plot of the attenuation of the R-wave at the respective receiver positions due to a single crack at the rail gauge corner versus the crack depth. Figure 14b,c show the same type of plot for two and three cracks, respectively.