## 5 Discussion  
This study presents additional insight into the potential of railway track flaw detection through the use of robotics for the application of enhanced rail safety. Initial results generated for the surface crack study demonstrated that the transducer was insensitive to the detection of surface cracks. This observation can be supported by the investigation by [PERSON] and [PERSON], who concluded that when using conventional probes, high frequencies in the range of 5 MHz result in a higher resolution. Shorter wavelengths increase the sensitivity of the system to any small inhomogeneity, such as the rough surface of the cracks or crystal grains in the material [30]. Therefore, as the tests within this investigation were performed using a frequency between 4 MHz and 5 MHz, this could have been a plausible explanation for increased wave scattering, thereby resulting in poor data-capture performance. However, previous studies have demonstrated that using a wedged probe compensates for increased scattering by providing a delay line and an angled incident wave [31]. Although wedged probes were used within this study, the results demonstrate that the both the automated robotic and manual systems are not appropriate for surface crack detection. The underlying cause of these adverse results is substantiated by studies indicating that conventional ultrasonic testing is very challenging when used as a tool to inspect cracks close to the surface. This is due to the existence of an area referred to as the 'dead zone'. Defect-deflected echoes located a few millimetres in front of the transducer were found to be overshadowed by disturbing signals caused by the piezoelectric element ringing inside the transducer [32]. Anandika mitigated this issue by adopting a wedged phased-array technique [31]. Unlike a conventional ultrasound's single fixed-angle element (determined by the wedge), the advantage associated with the array phase system is that it utilises beam steering to create beams at multiple angles, allowing multi-angular defect testing via a single probe [33]. In this way, the location of the noise signals with respect to the surface defect position shifts, thereby increasing the probability of detection of the surface anomalies. Furthermore, it offers the advantage of a far more effective spatial sampling frequency, thereby increasing the scanning efficiency [31]. Considering these findings, repeating the investigation with a 90\({}^{\circ}\) wedged probe might result in a different outcome. The angle of inclination of the wave would generate a surface wave, producing a stronger reflected echo from the surface defects, which suppresses the unwanted modes in the 'dead zone' [34]. The investigation of the web region of the railway track confirmed that both the automated robotic and manual systems could successfully detect internal defects in the form of holes ranging from 2.5 to 3 and 6 mm. Furthermore, low variance was observed between  
Figure 9: Comparison of the mean sound path DGS from all experiments (blue—Manual 60\({}^{\circ}\); green—Robotic 60\({}^{\circ}\); orange—Manual 70\({}^{\circ}\); purple—Robotic 70\({}^{\circ}\); dots—2.5 mm; vertical lines—3 mm; horizontal lines—6 mm). the sacrificial defect size and the mean DGS measured by both configurations. The findings related to the manual system suggest that the flaw detector with set parameters provided a reliable means for measuring the defect size. The proposed robotic configuration, specifically with the 70\({}^{\circ}\) wedged probe, was also found to be effective at accurately measuring the defect size. Furthermore, the mean DGS readings were also similar to those generated by the manual system. Extending the study to measure a range of defect sizes in different orientations, and scanning in different directions, would provide further insight into the effectiveness of the proposed robotic system for use in scanning actual rails.