# Use of Surface Acoustic Waves for Crack Detection on Railway Track Components--Laboratory Tests
## 5 Concluding Remarks  
In the present contribution it has been demonstrated by means of simulation and experiment that rail gauge corner cracks (head checks) can be detected by surface acoustic waves with a frequency of 1 MHz. In a pitch-catch setup, the attenuation correlates with crack depth if the excitation frequency is chosen appropriately according to the depth range of interest. This correlation was demonstrated by a series of measurements during a rolling contact fatigue test where head checks developed; attenuation due to crack depth measured at the end of the test agreed reasonably well with simulation results and theoretical results from the literature. In order to proceed from crack detection to crack-depth measurement, the method needs to be calibrated so that the depth of the head checks can be reliably deduced from the measured attenuation. To this purpose, extensive experimental testing will be necessary, with individual tests being terminated after different numbers of cycles and thin sections being taken (the most reliable, albeit destructive, method of crack-depth measurement). Additional challenges are the dependence of the attenuation on crack numbers and crack spacing, which has been already briefly addressed in the present work, and a possible dependence on the microstructure of the material (rail grade). As it was revealed that the repeated application of the transducers led to additional experimental scatter, this method is recommended mainly for a permanent sensor setup at selected track locations. Hence, the SAW excitation and measurement setup with transducers fixed on the rail will be the next step towards waysside condition monitoring using  
Figure 18: Thin section of rail gauge corner (perpendicular to the running direction of the wheel to match the simulations sketched in Figure 8) taken after 310,000 load cycles (wheel passes) at the measurement cross-section, (**a**) head checks with 0.62 mm depth, (**b**) detailed view. SAW. A fixed mounting of smaller SAW transducers will improve the reproducibility and hence the scatter of the data. Due to the reduction of influence from manually attaching the transducer for each measurement (see Section 2.2), the identification of the desired surface wave component on the time axis will be easier. In this way it is expected not only that head check formation can be detected at the earliest stage, but also that crack depth development can be monitored closely over time, thereby providing important information for maintenance scheduling in a condition-based maintenance concept. In the early stages of crack formation, with cracks less than approx. 0.1 mm in length, rust, surface roughness and changes in rail geometry due to abrasive wear could influence the SAW signals and prevent clear distinction between these changes and the onset of crack formation. However, the simulation and measurement results indicate that cracks exceeding a length of about 0.5 mm should be clearly detectable with SAW in the proposed frequency range. Our eventual aim is wayside condition monitoring with inexpensive, small and reliable transducers implemented in or attached to the rails at \"hotspots\" on the railway network. This is intrinsically different to the EMAT-based scanning approach, e.g., as proposed in ref. [15], because our application implies finding the sending and receiving positions on the rail where the wheel passage allows it, i.e., on the rail sections below the running band. Furthermore, our numerical and experimental results indicate that it is not sufficient to use specimens with single cracks or artificial slots to calibrate a condition monitoring device for depth measurements of head checks in track, because these gauge corner cracks usually evolve as crack networks rather than single cracks. Conceptualization, S.E. and C.G. ; methodology, C.G. ; software, R.H.; performance of the experiments, D.K., C.G. and S.E. ; validation, formal analysis, C.G. and R.H.; data curation, C.G. and H.-P.G. ; writing--original draft preparation, S.E.