# Use of Surface Acoustic Waves for Crack Detection on Railway Track Components--Laboratory Tests
## 2 Test Specim and Methods  
### Generation of Head Checks in a 1:1 Wheel-Rail Test Setup  
The evolution of gauge corner cracks (head checks) in rails is related to the wheel-rail contact that in turn depends on the geometries of the rail and the passing wheels as well as the local friction conditions. One aim of building a 1:1 wheel-rail laboratory setup was to identify the contact conditions that lead to this specific damage pattern observed in track. A more detailed description of the test rig, testing procedure and the evolution of head checks can be found in ref. [1; 8]. To monitor the generation and growth of head checks, a 1.5 m long standard rail (material R260, rail profile 60E1 (UIC60)) was subjected to cyclic testing in the voestalpine wheel-rail test rig with friction-load parameters that are known to produce the targeted damage pattern \"head checks\" [1; 8]. The corresponding RCF experiment took approximately 2 weeks; SAW measurements were performed during dedicated breaks, hence the following results exhibit unequal measurement intervals. After approximately 15,000 load cycles (i.e., wheel passes), cracks had initiated in the rail; after \(\sim\)25,000 cycles, a typical head check pattern started to evolve; and after 70,000 cycles the head check pattern clearly covered the full length of wheel-rail contact. Figure 1 shows an image of the head check pattern at the end of the experiment, i.e., after 310,000 wheel passes. Note that in contrast to the experiments performed by [PERSON] et al. [7], where \"_the specimens were initially polished in order to remove any unwanted rust which would affect the propagation of Rayleigh wave_\", the aim of the present study was to investigate rails with a typical RCF damage pattern and close to service conditions; therefore, the rail was deliberately left unpolished, retaining any existing rust or wear particles. Pitch-Catch Setup for the Excitation and Measurement of Quasi-Rayleigh Waves with Contacting Transducers  
In a recent publication, [PERSON] et al. gave an overview on the advantages and disadvantages of contact and non-contact methods to excite and measure Rayleigh waves [7]; however, the literature cited therein only covers non-contact setups. As the aim of the present study was to test the ability to detect head checks using the SAW pitch-catch setup with transducers attached to the rail, i.e., aiming at wayside condition monitoring, the authors chose piezoelectric comb transducers in a pitch-catch setup. These transducers are widely used in nondestructive guided wave examinations, and are characterized by easy operation, low cost, and high sensitivity [7; 8; 9]. For signal excitation and measurement, the readily available commercial system gampt GS200 [9] was used. The pitch-catch setup consisted of two piezo transducers combined with a comb adaptor to introduce and measure a narrowband surface acoustic wave with a center frequency of 1 MHz. In 2001 [PERSON] published a sketch of a similar comb transducer and detailed the wave generation theory behind it [10]. In the case of the gampt 1 MHz setup, the comb consisted of 10 contacting teeth with a spacing of 2.89 mm corresponding to a Rayleigh wave speed of 2889 m/s in aluminum. The first experiments revealed that manually attaching the transducers to the rail (see Figure 2a) resulted in strong fluctuations in the excited and measured surface acoustic wave peak intensities, due to the fluctuating quantities of coupling agent between transducer and comb as well as between comb and rail. To improve the reproducibility of the signals, a transducer holder was designed that fixed the transducer on the rail via magnets, and controlled the contact pressure of the transducer to the rail via a spring, as shown in Figure 2b. Figure 1: Head check pattern achieved at the end of the RCF experiment in the 1:1 wheel–rail test rig at voestalpine (scales are in cm).