## 3 Results and Discussion
### Infrared Thermography (IRT)
This smallest detectable AR is not guaranteed for each heating duration because there is an interaction between the \(\Delta\)T, D/t, and AR. In addition, the smallest detectable AR is controlled by the cooling duration in each cycle. If the cooling duration is long enough for that specific heating time, after completing all of the heating-cooling cycles, then not enough heat will be accumulated in the tank wall thickness to induce the required thermal gradient. On the contrary, if the cooling duration is too short, then the edges of the defects become saturated and there is insufficient contrast between the defect and the solid. Moreover, some other environmental parameters, such as relative humidity and wind speed, may affect the accuracy of the results. For instance, as experienced by the authors, on a windy day, the surface of the tank cools faster during the cooling phase due to heat loss from the surface exposed to the wind, so there is less heat penetration. Results shown in Figures 8-11 indicate that the heating-cooling combinations of 30-90, 30-120, 40-40, 40-80, and 40-120 are the best options when the wall thickness is 12.7 mm (0.5 in), but the heating-cooling combination of 3  
For the west side, where the defects were created in the wall thickness of 19.1 mm (0.75 in), heating times of 40 s and 60 s resulted in comparable heat penetration into the wall thickness. The minimum AR detected was 0.79 for a heating time of 20 s and 0.30 for all other heating times. Therefore, 40 s should be selected as the shortest heating time to achieve the highest possible D/t and detect the smallest AR. When comparing the findings from the evaluation performed in the morning and the afternoon, it appears that the afternoon evaluation may provide greater accuracy by detecting a smaller AR for the west side while maintaining the same level of accuracy in detecting defects for the east side. This suggests that completing the evaluation in the afternoon will provide a higher possibility of detecting smaller defects for both the east and west sides. In addition, the smallest detectable AR highly depends on the cooling duration between the two heating cycles. Regardless of the cooling time, a heating time of 40 s or 60 s yields the same smallest detectable AR. Therefore, a heating-cooling combination of 40-40 or 40-80 can be applied in the field for five cycles as the shortest and most effective combination. The recommended heating-cooling combinations of 40-40 and 40-80 allow the inspector to heat one or two more spots and then come back to the first spot to start the next cycle which helps to speed up the evaluation process. Comparing \(\Delta\)T values reported for the 40 s heating duration, the highest D/t, and the smallest AR can be detected when \(\Delta\)T > 4 \({}^{\circ}\)C (7.2 \({}^{\circ}\)F). If 0 \({}^{\circ}\)C (0 \({}^{\circ}\)F) < \(\Delta\)T < 4 \({}^{\circ}\)C (7.2 \({}^{\circ}\)F), then the authors recommend increasing the heating duration to 60 s and following one of the combinations of 60-10, 60-20, or 60-60, which would allow for a greater heat deposit on the surface and therefore compensate for the lower \(\Delta\)T. The combination of 60-60 allows the inspector to heat the next spot while performing IRT on the first spot which would be  
Figure 11: Characterization of the defects detected when the heating time was 60 s on the (**a**) east side and (**b**) west side. Figure 10: Characterization of the defects detected when the heating time was 40 s on the (**a**) east side and (**b**) west side. in the cooling phase, allowing for an optimized inspection time. If \(\Delta\)T \(<\) 0 \({}^{\circ}\)C (0 \({}^{\circ}\)F), then the accuracy of the defect detection decreases dramatically. Therefore, the authors do not recommend performing IRT when \(\Delta\)T is negative.