## 6 Discussion  
This paper breaks down significant advancements and developments in MNDT techniques, highlighting the most significant turning points in detecting and evaluating cracks across various materials. Microwave NDT techniques have emerged as remarkable alternatives to traditional NDT methods. They offer enhanced efficiency and effectiveness, primarily due to their high sensitivity and resolution in detecting surface and subsurface defects [51]. In contrast to traditional NDT methods, MNDT techniques employ electromagnetic waves in the microwave spectrum, enabling deeper material penetration and better defect characterization without harmful radiation. A unique aspect of MNDT lies in the method of energy transmission into the testing environment. As a result, coupling can occur via air or a suitable dielectric, and there is usually no need for special treatment or coupling agents [42]. The first MNDT technique discussed in this paper is the far-field method, which employs microwave propagation characteristics in the far-field region to detect defects. On the other hand, near-field methods, with their innovative probe designs like coaxial probes and rectangular waveguides, have demonstrated remarkable capability in identifying minute crack inconsistencies, providing a detailed analysis of crack geometry and properties [51]. It is worth noting that both of these methods possess their limitations: the far-field method suffers from limited spatial resolution, whereas the near-field method suffers from requiring short-range distances between the transducer and specimen under test, making it essential to choose between the two methods depending on the requirements of the application of interest. The resonator method was also among the MNDT techniques discussed. This method further extends the capabilities of near-field and far-field NDT by leveraging the sensitivity of resonant structures to minute changes in material properties, offering a high degree of precision in crack detection. The distinction between the near-field, far-field, and resonant methods is based on the wavelength produced by the source and the dimensions of the radiating component, and choosing between these methods is dependent on the specific application area. Additional MNDT techniques discussed were microwave imaging and microwave reflectometry, which are typically utilized for detecting defects in materials. These two techniques are effective in detecting changes in material properties, including composition, density, or structural flaws. Thus, they are employed for various applications, such as industrial inspection, medical imaging, and material characterization. Both methods rely on the interaction of microwave signals with materials to extract useful information, often using different equipment and software for data analysis. The spatial resolution accuracy depends on the algorithm employed for microwave imaging; however, this approach can be computationally expensive [106], representing a drawback of the microwave imaging technique. On the other hand, a crucial limitation of microwave reflectometry is the sensitivity of the measurements to minor changes in the lift-off distance separating the probe from the object being tested [98]. Variations in lift-off as small as 0.05 mm can significantly affect the measurements, potentially obscuring the detection of defects [98]. Aside from these traditional MNDT approaches, there are various emerging techniques in MNDT, such as microwave microscopy, active microwave thermography (AMT), and chipless RFID. Microwave microscopy is excellent for its ability to characterize high-resolution materials and is essential for identifying minute defects across various materials, particularly metals, semiconductors, and dielectrics [111; 112]. Active microwave thermography, divided into passive and active types, enhances defect localization and depth quantification, with innovative techniques like pulsed phase thermography offering improved detection aptitudes [115; 116; 117; 118; 119; 120; 121].