## 3 Results and Discussion
### IRT Challenges for the Field Evaluation of HDPE ASTs  
Although IRT is a great technique for defect detection in many industries, it has some challenges for the inspection of HDPE ASTs. Some of the challenges found during this study are listed below:  
* IRT can be used only as an indicator of the defects' existence and does not provide information about the location and depth of the defects. Supplementary evaluation with UT can further assess the location and depth of defects. This is also true when using IRT on other materials.
* Defect detection accuracy highly depends on ambient weather conditions.
* IRT is not able to detect small defects (AR < 0.3) or very deep defects (D/t > 0.83).
* IRT is not able to detect non-leaking cracks propagated from the internal side of the AST towards the surface. This has been discussed in more details in Ref [9].
* IRT is labor intensive and requires personnel to heat areas in different cycles.
* IRT is time-consuming for HDPE ASTs, due to the material's low thermal conductivity. The operator should be prepared with multiple charged batteries for the equipment.
* Using various IR cameras from various manufacturers or various models produced by the same manufacturer could affect the accuracy of defect detection.
* Water droplets due to rain, fog, or other activities close to the AST can interfere with defect detection.
* Because many sites will contain a number of ASTs that need to be inspected, this task can become tedious. Therefore, to expedite the evaluation process, the inspection can be limited to the lower sections of the AST (i.e., waist height and below) where failures are most frequently reported.
* IRT can be completed on the lower (i.e., waist-high and below) sections of the AST without the need for any additional equipment. If there is an accident or a concern close to the top of the AST, then the evaluation process may be challenging in the absence of hydraulic lifters or cranes.
* The IRT equipment needs to have access to electrical power. A portable generator is useful in the field.
* Appropriate electrical access for IRT is challenging to obtain. The heaters recommended for performing IRT have a large current draw and will trip most conventional circuits. It is recommended either to use two separate electrical sources or to use a sufficiently capable portable generator.
* The heaters used for IRT can reach extremely high temperatures. Careful consideration must be taken to ensure that the operator is properly insulated from the heat generated by the infrared heaters.
* To prevent fire or electrical hazards, it is recommended to turn off the heaters for 10 min every 20 min or for 15 min every 30 min. Aside from proper insulation, this is the best way to keep the heaters within a comfortable heat range for the operators.  
The authors recommend using stacked infrared heaters, such as those being used for paint curing, in order to reduce these hazards, keep the operator safe, and deposit a more uniform temperature on the area under study.
* Heating around the fittings is challenging and causes non-uniform heat deposits. Hoses connected to the outlets prevent the application of heat on the surface behind them. Moreover, heating these hoses may inadvertently cause damage to the hoses and gaskets.
* Since an operator is moving the heater, it is not guaranteed that exactly the same area heated during one cycle is being heated in the next cycle. There is likely to be a gap between the heated spots, as shown in Figure 18. Therefore, it is possible that some sections of the tank may be missed during the inspection.
* Discolorations, stains, superficial scratches, and information tags interfere with defect detection, which may reduce the speed of evaluation by numerous false detections.