## 4 Results
### Internal Defects  
The experiments on internal defects were conducted by running the scanning probe along the web region. For both the automated robotic and manual system configurations, a clear distinction could be witnessed between clear regions and defect regions in the resulting A-scan image amplitude, as evidenced in Figure 7. In Figure 7a, no signal is present (no green spiked peak), meaning that in that specific region, there are no cracks. Figure 7b,c both show a signal (a green distinct peak), and this is observed when using the probe manually and when using the robotic configuration, therefore proving that cracks can be detected both manually and automatically. Importantly, in both the manual and robotic configurations, all the defects were identified by a clear change in signal; specifically, higher amplitudes were observed in regions with defects. The Epoch(tm) 650 system compared the signal amplitude obtained via the calibration data and identified the corresponding depth of the defect. This confirmed the effectiveness of both systems in successfully detecting the hole defects by scanning along the web region. After running the full cycle of tests for both configurations, all DGS data were recorded. Results generated for holes of the same size were clustered to assess repeatability and statistical significance. Internal defects were clustered as holes with a diameter of 2.5 mm, 3 mm and 6 mm. A One-Sample T-test was conducted to compare the mean sample defect size data of each system configuration (manual 60\({}^{\circ}\) wedge, robotic 60\({}^{\circ}\) wedge, manual 70\({}^{\circ}\) wedge and robotic 70\({}^{\circ}\) wedge) against the actual defect size. Also, the standard deviation, mean and two-sided probability value (\(p\)-value) were also calculated. For the manual system measurements, correlations to the actual defect sizes were obtained for all hole sizes when using both angled wedged probes. For the robotic system measurements, correlation to the actual defect size was only witnessed on the 3 mm hole cluster when using the 60\({}^{\circ}\) wedged probe. However, when using the 70\({}^{\circ}\) wedged probe, all hole sizes were correctly identified. The subsequent \(p\)-value (set at 0.05) statistical analysis of the mean values obtained from the experiments concluded that all internal defect measurements corresponded well to the actual values. This confirmed that the size characterisation of internal defects was attainable by both the robotic and manual system, with the robotic system showing slightly less consistency and accuracy when adopting the 60\({}^{\circ}\) wedged probe. Figure 8 displays the T-test analysis comparing the mean DGS values from the manual and automated configurations when utilising the 60\({}^{\circ}\) and 70\({}^{\circ}\) probes independently. Marginally lower variance was witnessed between the manual and robotic configurations when analysing the 2.5 mm and 3 mm defects with the 70\({}^{\circ}\) probe. In particular, the resultant \(p\)-value exceeded the 0.05 level of significance, as displayed in Table 2. However, for the 6 mm defect, the difference between the two probes was indicated as being significant in both systems, as shown in Table 2. Overall, the accuracy for both the robotic and manual configurations was higher when using the 60\({}^{\circ}\) and 70\({}^{\circ}\) wedged probes to analyse the 2.5 mm and 3 mm defect sizes. Figure 7: **(a)** A-Scan image, 60\({}^{\circ}\) probe, no flaw region. **(b)** A-Scan image, 60\({}^{\circ}\) probe, defect region, manual configuration. **(c)** A-Scan image, 60\({}^{\circ}\) probe, defect region, robotic configuration. The second dependent variable that was compared was the recorded sound path, which indicates the depth and location of the internal defect.