## 2 Experiment Methodology
### Ultrasonic Testing (UT)  
While the IRT reflection technique can help to identify the existence of superficial subsurface defects, IRT is not able to quantify the defect depth, defect size, or whether there are stacked defects. Therefore, UT, specifically phased array ultrasonic testing (PAUT), was used as a supplementary technique for defect quantification. PAUT probes are composed of multiple ultrasonic elements that act as a synthetic aperture to \"sweep\" its focus without moving the probe [24; 25], thereby providing significantly more versatility and range than conventional pulse-echo methods. Since the probe's focus is \"swept,\" even defects that are stacked can be detected, which would not be possible by conventional pulse-echo methods [26]. PAUT has shown great capabilities to detect embedded defects in HDPE pipes and joints [27; 28; 29; 30; 31; 32; 33]. By calibrating the measuring equipment to the P-wave velocity of the AST material with knowledge of the wall thickness and object temperature, PAUT can locate hidden abnormalities. One disadvantage of the application of UT to HDPE is the potential for significant acoustic attenuation and dispersion due to the viscoelasticity of HDPE, with energy losses reported from 7 dB/cm (18 dB/in) to as much as 15 dB/cm (38 dB/in) [26; 27; 34]. Therefore, based on the AST wall thickness and defect size, a range of ultrasonic frequencies needs to be considered; for instance, while higher frequencies will likely yield a better resolution, the signal will diminish due to increased scattering [35]. Another challenge with PAUT is that more peaks are generated in the response signals from deep defects as a result of waves propagating through the irregularly damaged area, intact area, and corner. The extra signals make it difficult to determine the defect tip and starting point, which are necessary for determining the geometry and location of the defect. This limitation in the technique is known as a corner trap. In this research, a Proceq Flaw Detector 100 ultrasonic instrument was used in combination with a phased array water wedge probe from SensorScan with 2.25 MHz. Water wedges have been used in other UT studies of HDPE [8; 30; 31; 32], primarily because the relatively slow acoustic velocity of HDPE would result in a negative refraction from other commonly-used PAUT wedge materials. A wave velocity calibration was performed with a pulse-echo probe before starting each measurement. The settings used for the calibration are summarized in Table 4.  
A previous study showed that the thickness of the HDPE--at least the thickness of typical HDPE AST walls--does not impact the sound wave velocity measurement [8]. The HDPE temperature does affect velocity such that, as the temperature decreases, the sound wave velocity increases [8; 36; 37]. In addition, it has been shown that the outdoor weathering of HDPE ASTs does not appear to considerably affect the sound wave velocity [8]. \begin{table}
\begin{tabular}{c c} \hline Detector resolution & 640 \(\times\) 480 (307,200 pixels) \\ Field of view & 34 \({}^{\circ}\)H \(\times\) 24 \({}^{\circ}\)V \\ Temperature measurement range & \(-\)20 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C (\(-\)4 \({}^{\circ}\)F to 1832 \({}^{\circ}\)F) \\ Accuracy & \(\pm\)2 \({}^{\circ}\)C or 2\% (whichever is greater) \\ Thermal sensitivity (NETD) & \(\leq\)0.05 \({}^{\circ}\)C at 30 \({}^{\circ}\)C target temp (50 mK) \\ Frame rate & 60 Hz \\ Infrared spectral band & 7.5 \(\upmu\)m to 14 \(\upmu\)m (long wave) \\ \hline \end{tabular}
\end{table} 
Table 2: IR camera properties reported by the manufacturer. \begin{table}
\begin{tabular}{c c} \hline Emissivity & 0.78 \\ Background & 23 \({}^{\circ}\)C (73 \({}^{\circ}\)F) \\ Transmission & 100\% \\ Range & \(-\)20 \({}^{\circ}\)C to 100 \({}^{\circ}\)C (\(-\)4 \({}^{\circ}\)F to 212 \({}^{\circ}\)F) \\ \hline \end{tabular}
\end{table} 
Table 3: Settings of the IR camera used for the study.