## 2 Standard Nondestructive Testing and Evaluation Techniques
### Ultrasonic Testing  
Ultrasonic testing (UT) is an NDT technique that employs high-frequency (HF) sound waves to characterize and inspect internal defects or cracks in materials. Figure 2 shows two diagrams that depict the principle of ultrasound testing [12]. In Figure 2a, we can see that an ultrasonic source emits sound waves onto an obstacle, and a detector co-located with the source measures the reflected waves. Furthermore, the reflections are utilized to measure the defects mostly on the surface. Another UT technique is shown in Figure 2b, where acoustic waves are transmitted through the specimen. The source and detector are not co-located and are positioned at different points. The reflections are utilized to detect internal cracks. Various works exist within the literature that utilize this NDT technique. For example, [PERSON] et al. [20] described the integration of ultrasonic-based NDT with wireless sensor networks (WSNs) to monitor materials continuously. [PERSON]'s proposed system was successfully deployed to monitor the thickness of tungsten and steel compound metal sheets used in vibration screens. Thus, such an WSN-based NDT technique proved its effectiveness in achieving low-cost, real-time wireless material examination, paving the way for its practical use [20]. Figure 1: A block diagram demonstrating the steps of dye or liquid penetrant testing: (**a**) apply penetrant to specimen; (**b**) wash excess penetrant after some time; (**c**) apply developer to make the defect with penetrant more visible. Figure 2: Diagrams that demonstrate two techniques for performing ultrasound testing [12]: (**a**) a tecnique based on measuring the reflections from the specimen, and (**b**) a technique based measuring the signals propagating through the specimen. Similarly, [PERSON] et al. [21] reviewed studies on the use of UT for defect detection on metallic, nonmetallic, and plastic pipes, with the results presented as ultrasonic C-scans. Their review highlights UT's effectiveness in detecting defects smaller than 0.8 mm in size [21]. Furthermore, [PERSON] et al. [22] utilized UT to investigate the structure and dimensions of inclusions found within the defective metallic pipes in natural gas pipelines. Their system detected defects at various depths in the metallic pipes; their solution showed UT's effectiveness in detecting defects at multiple depths within a material [22]. UT methods were utilized to characterize and evaluate the performance of carbon-fiber-reinforced polymers (CFRPs) and glass-fiber-reinforced polymers (GFRPs) that are used in many applications [23; 24; 25; 26]. In [23], [PERSON] et al. utilized two different UT methods to determine various parameters about the CFRP. [PERSON] et al. [24] used UT testing to study CFRP-concrete specimens subjected to accelerated aging conditions. In [25], [PERSON] et al. used pulsed UT testing to evaluate the bonding between composite materials and substrates; here, different composite materials (CFRPs and GFRPs) were applied to various substrates (concrete, masonry, and polyurethane). Lastly, in [26], [PERSON] et al. utilized the UT method to monitor the damage and degradation of wet GFRP composites. A sub-field of UT, termed guided wave ultrasonic testing (GWUT), is used in buried metallic pipeline systems. The waves in GWUT can propagate along pipes in circumferential and longitudinal directions, providing an advantage over the purely longitudinal nature of UT waves [27]. Using this approach, [PERSON] et al. [5] presented a paper on detecting cracks in polyethylene pipes. A laboratory experiment was conducted using an ultrasonic testing system made of low-priced piezoceramic transducers arranged in a pitch-catch configuration. The authors of [5] found that detecting cracks with lengths smaller than the wavelength of the interacting acoustic wave mode requires greater crack depth for detection. On the other hand, cracks with lengths greater than or equal to the interacting mode wavelength are comparatively easier to detect [5].