## 3 Microwave NDT Techniques  
Microwave nondestructive testing (MNDT) and evaluation refers to techniques that utilize electromagnetic signals at frequencies in the range of 300 MHz to 300 GHz to inspect materials and identify potential defects noninvasively and without causing damage [38,51,52]. MNDT was introduced in the 1950s, garnering additional interest in the 1960s [38]. The 1990s marked a significant revival in research and development activities in this field, which was expanded to encompass nondestructive evaluation (NDE), where material properties \begin{table}
\begin{tabular}{c c c c c c} \hline
**NDT Technique** & **Working Principle** & **Advantages** & **Limitations** & **Applications** \\ \hline \multirow{3}{*}{Visual inspection} & \multirow{3}{*}{Direct observation} & \(\bullet\) & Simple & \(\bullet\) & Only detects surface defects & \(\bullet\) & Welding [44] \\  & & \(\bullet\) & Cost-effective & \(\bullet\) & Skill-dependent & \(\bullet\) & Structures [45] \\  & & \(\bullet\) & Immediate results & & & \\ \hline \multirow{3}{*}{Dye penetrant testing} & \multirow{3}{*}{Dye reveals surface defects} & \(\bullet\) & Sensitive to small defects & \(\bullet\) & Only detects surface defects & \(\bullet\) & Welding [17,18,46] \\  & & \(\bullet\) & Used on a variety of materials & \(\bullet\) & Requires clean surfaces & \(\bullet\) & Castings [18] \\ \hline \multirow{3}{*}{Ultrasonic testing} & \multirow{3}{*}{Sound waves detect flaws} & \multirow{3}{*}{High depth of penetration} & \multirow{3}{*}{\(\bullet\)} & \multirow{3}{*}{Requires expertise} & \(\bullet\) & Thickness measurement [20,22] \\  & & & Accurate positioning and sizing defects & & & Internal flaws detection in metal [27] \\  & & & & & & Welding [18,47] \\  & & & & & & Composites monitoring and characterization [23, 24, 25, 26] \\ \hline \multirow{3}{*}{Eddy current testing} & \multirow{3}{*}{Electric currents identify defects} & \(\bullet\) & High sensitivity to surface defects & \(\bullet\) & Only applicable to conductive materials & Coating thickness measurements [31] \\  & & \(\bullet\) & Portable & \(\bullet\) & Limited depth & \(\bullet\) & Crack detection [6,30,34,48] \\ \hline \multirow{3}{*}{Magnetic particle testing} & \multirow{3}{*}{Magnetic fields highlight flaws} & \multirow{3}{*}{High sensitivity} & \multirow{3}{*}{Immediate} & \multirow{3}{*}{\(\bullet\)} & Only applicable to ferromagnetic materials & \(\bullet\) & Automotive cracks [33, 34, 35] \\  & & & & & Surface assessment & \(\bullet\) & Aerospace [3] \\ \hline \multirow{3}{*}{Radiography testing} & \multirow{3}{*}{X-rays/gamma rays image defects} & \multirow{3}{*}{\(\bullet\)} & Deep and surface detection & \(\bullet\) & Use of ionizing radiation & \(\bullet\) & Casting inspection [49,50] \\  & & \(\bullet\) & Permanent record & \(\bullet\) & High cost & \(\bullet\) & Pipelines integrity assessment [36,37] \\ \hline \end{tabular}
\end{table} 
Table 1: Overview of standard NDT techniques. \begin{table}
\begin{tabular}{c c c c} \hline
**NDT Technique** & **Minimum Detectable** & **Penetration Depth** & **Relative Cost** \\ \hline Dye penetrant testing & \(\approx\)0.02–0.1 mm & Surface only & Low \\ \hline Ultrasonic testing & \(\approx\)0.1–1 mm & Up to several 100 mm (dependent on frequency) & Moderate–high \\ \hline \multirow{3}{*}{Eddy currents testing} & \(\approx\)0.01–0.5 mm & \(\approx\)1–10 mm (dependent on material conductivity and frequency) & Moderate \\ \hline Magnetic particle testing & \(\approx\)0.05–0.2 mm & \(\approx\)1–2 mm (surface and near-surface) & Low–moderate \\ \hline \multirow{3}{*}{Radiography testing} & \(\approx\)0.1–1 mm & Several centimeters to decimeters (dependent on radiation energy) & High \\ \hline \multirow{3}{*}{Microwave testing} & \multirow{3}{*}{\(\approx\)0.5–2 mm} & Several millimeters to centimeters (dependent on material’s electric properties and frequency) & Moderate \\ \hline \end{tabular}
\end{table} 
Table 2: Quantitative comparison of NDT techniques. were quantitatively analyzed [38; 53].