## 4 Conclusions  
In this work, the effectiveness of employing infrared thermography (IRT) and ultrasonic testing (UT) techniques for evaluating high-density polyethylene (HDPE) liquid storage tanks was investigated. IRT was conducted using heating-cooling cycles of varying durations. The initial thermal gradient (\(\Delta\)T) that existed on the tank wall was measured for every single experiment as one of the largest impacts on evaluation accuracy. Phased array ultrasonic testing (PAUT) was taken into consideration because it requires access to just one side of the tank and enables the inspection of liquid-filled tanks. The following general conclusions can be drawn from this study:  
* The interaction of sample geometry, initial thermal gradient, defect location, and defect aspect ratio will impact the accuracy of IRT in detecting subsurface defects in HDPE ASTs. * The distance of the camera to the tank, the angle of view, the heating source power, and the distance of the heating power to the tank will impact the accuracy of the IRT measurements. * The best time for conducting IRT is on sunny days and between 9 a.m. to 6 p.m. Since the evaluation of the tank is time-consuming, the evaluation can be started from the east side in the morning and completed on the west side in the afternoon. * Completing the IRT evaluation in the afternoon (after 2:30 p.m.), will provide a higher probability of detecting smaller defects for both the east and west sides. * If \(\Delta\)T < 0 \({}^{\circ}\)C (0 \({}^{\circ}\)F), then the accuracy of the defect detection diminishes dramatically. Therefore, it is not recommended to perform IRT if this condition exists. Figure 19: The waviness of the interior wall of an HDPE AST. When the thickness of the tank is 12.7 mm (0.5 in) and \(\Delta\)T > 6 \({}^{\circ}\)C (10.8 \({}^{\circ}\)F), 30 s of heating for five consequent intervals should be sufficient to allow the heat to penetrate the tank wall. The combination of 30-90 (30 s heating with 90 s cooling) is recommended when the thickness of the tank is 12.7 mm (0.5 in). * When the thickness of the tank is 19.1 mm (0.75 in) and \(\Delta\)T > 4 \({}^{\circ}\)C (7.2 \({}^{\circ}\)F), the heating-cooling combinations of 40-40, 40-80, or 40-120 are recommended. * The recommended distance of the IR camera to the tank is between 0.5 m to 1.5 m (1.6 ft to 4.9 ft) and the recommended angle of view is between 0\({}^{\circ}\) to 30\({}^{\circ}\) relative to the direction perpendicular to the tank wall. * In addition to IRT, PAUT can be utilized to examine HDPE ASTs in greater detail. PAUT was able to detect defects as small as 4.4 mm (0.17 in) in diameter in this study. * The massive liquid inside the tank, the attenuation of sound in HDPE, and the waviness of the tank wall interior will increase the background noise in PAUT investigations and reduce the capability of defect detection and measurement by UT. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13031396/sl](https://www.mdpi.com/article/10.3390/app13031396/sl)]([https://www.mdpi.com/article/10.3390/app13031396/sl](https://www.mdpi.com/article/10.3390/app13031396/sl)), Tables S1-S8. Conceptualization, A.B. and A.S.B. ; Methodology, A.B. and A.S.B. ; Formal Analysis, A.B. ; Investigation, A.B., T.Q.T., Y.L., M.D., M.S.S., M.M.M.D. and A.H.; Data Curation, A.B., T.Q.T., Y.L., M.D., M.S.S., M.M.D. and A.H.; Writing--Original Draft Preparation, A.B., T.Q.T., M.D., M.S.S. and M.M.D. ; Writing--Review and Editing, A.B., M.M.D. and A.S.B. ; Visualization, A.B., T.Q.T. and M.M.D. ; Supervision, A.S.B. ; Project Administration, A.S.B. ; Funding Acquisition, A.S.B. All authors have read and agreed to the published version of the manuscript. This research was funded by Virginia Transportation Research Council (VTRC), Project 117955. The data presented in this study are available on request from the corresponding authors. The authors thank [PERSON] at Virginia Transportation Research Council (VTRC) for his help during this study.