## 3 Experimental Set-Up  
In this research, an ultrasonic sensor, Epoch(tm) 650, was integrated within the end effector of a KUKA KR 16 robotic system to compare its efficacy against a manual process. To assess the performance of the novel system, sacrificial holes (internal defects) and slots (surface cracks) were machined into a steel railway track specimen of grade R260 (Figure 1). The sacrificial features were machined via wire electrical discharge machining, and the flaws ran through the full height of the track (Figure 2). The details of these features are presented in Table 1.  
The Epoch(tm) 650 used in this study utilises a piezoelectric transducer with an integrated receiver to propagate ultrasonic energy into the specimen. The selected Angle Beam Wedge Transducers are single-element transducers used for flaw detection and sizing. The unit transmits a primary shear wave into a test piece and the design allows it to be easily scanned back and forth over the inspected part. This is unlike straight-beam testing, where the sound beam will travel at the generated angle down to the bottom of the test piece and then be reflected upward at the same angle. In selected-angle beam testing, by moving the probe back and forth, the sound beam will sweep across the full height of a detected flaw. This scanning motion enables the inspection of the entire flaw either through direct reflection from a second acoustic path (Figure 3a,b) or through a second acoustic path (Figure 3c,d). These transducers are typically used for the inspection of welds and of cracks on pipes, tubes, forgings, castings and machined components. They are Atlas European Standard Transducers, designed to meet inspection criteria referenced throughout Europe, and feature standard connectors and common frequencies. The two sensors use a 70- and 60-degree refracted shear wave (frequency range between 4 MHz and 5 MHz) and have a near-field distance in steel of 30 mm. Before inspection, both sensors were tested using NDT Calibration and Reference Test Blocks. The blocks used were a U8880016 2214M 5-STEP 1018 steel block and a U8880046 TB1065-1 ISO 7963 miniature steel block (ISO 7963 Ultrasonic testing). For both the automated robotic and manual experiments, the rail specimen was studied by running the probe along the centre of the head of the rail, specifically to investigate the surface cracks (Figure 4a). Subsequently, the web region (Figure 4b) was scanned for the internal defects created in the form of holes. Both scanning approaches were carried out at a low speed in the direction observed in Figure 5. Glycerin was applied as a couplant between the transducer and the test piece to facilitate more efficient sound transmission and a stronger signal strength. The probe was held stationary when the known flaws were located to allow the successive generation of data. The data collected from the Epoch(tm) 650 are presented in a graphical A-Scan waveform display. The pulse travel time is depicted in the horizontal direction, whilst the echo amplitude is depicted in the vertical direction [28]. The system adopted allowed the amplification of the signals of interest and manipulation of the time base to permit focusing on the area containing flaws. Furthermore, the waveform-averaging software allowed the calculation of an average of the successively acquired A-scans, which improved the sign-to-noise ratio on detection of the static flaws [28]. For the automated robotic experiment, the ultrasonic probe was located within a universal gripper housing that is compatible with the use of angled probes (Figure 6a).