## 2 Experiment Methodology
### Infrared Thermography (IRT)
Previous studies utilized the step heating method with three minutes of continuous heating [8; 9]. Due to the semi-infinite size of the HDPE AST, compared to the laboratory-scale water tank used in previous studies, the IRT technique, with step heating--even with 10 min of continuous heating--was not efficient for the field investigation in this study. Short-step heating was not enough to induce an adequate thermal gradient and to therefore obtain an observable contrast between the defects and the intact areas; on the other hand, long-step heating reduced the contrast between the defect edge and the original solid area. Therefore, according to the preliminary investigations, step-heating and thermal wave imaging were combined for this study. To induce waveform heat, the heat was deposited on the surface by following alternative heating and cooling cycles. For this study, the grouping of one heating phase and one cooling phase is termed one cycle. Different heating and cooling duration combinations were considered for this study (Table 1) to investigate the optimum heating-cooling cycles. For each combination of heating-cooling durations (e.g., 20 s heating with 60 s cooling), six cycles were completed and the IRT was conducted during each cooling phase. The first two cooling phase durations per heating time group were kept constant (e.g., 10 s and 20 s), and the subsequent four cooling phase durations were changed as a function of the heating time duration. This arrangement in selecting the cooling phase duration was chosen for the sake of the practical implementation and a faster field investigation. The length of the heating phase is only a fraction of the cooling phase, enabling an inspector to perform multiple thermal cycles across the tank simultaneously. For instance, in the combination of 20 s heating and 140 s cooling, the inspector can heat seven areas (7 \(\times\) 20 s = 140 s) and then return to the first spot to start the second cycle of IRT measurement. This heating-cooling cycle allows the heat to penetrate the sample in the form of waves without reducing the contrast between the defects and the sounding area. Therefore, the authors believe that applying a waveform heat distribution on the HDPE AST causes the heat to penetrate deeper, increasing the contrast between the defect edges, and consequently revealing more subsurface defects. For this reason, the optimum heating-cooling durations need more investigations relative to what was established by the authors in the previous two studies. The IRT evaluation and measurement for each heating-cooling combination were completed twice: one was performed in the morning and one was performed in the afternoon; this reasoning is explained in Section 3.1. For this study, a Fluke TiX580 thermal imager (IR camera), with the features shown in Table 2, was used. The IR camera settings used in this study can be found in Table 3. The distance between the camera and the object varied from 0.8 m to 1.1 m (2.6 ft to 3.6 ft). Inspectors in the field are interested in evaluating as many tanks as possible in one day, without performing further analysis. Therefore, in order to expedite the field investigation, the temperature span on the camera was set to automatic. In the previous studies performed by the authors, the camera was set manually which narrows down the temperature range for better defect detection, but is also more time consuming. No further image processing was performed and the smallest detected defect size was obtained directly from the raw \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Heating Time (s)** & & & & & **Cooling Time (s)** & & & \\ \hline
20 & 10 & 20 & 40 & 60 & 80 & 100 & 120 \\
30 & 10 & 20 & 30 & 60 & 90 & 120 & - \\
40 & 10 & 20 & 40 & 80 & 120 & - & - \\
60 & 10 & 20 & 60 & 120 & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Heating and cooling duration combinations considered for each cycle during IRT measurement.