## 2 NDT Method Preliminaries  
There is a variety of inspection technology approaches. Their use depends on the structure or material to be inspected, cost and effectiveness. Method-wise, they offer pros and cons, and hence a requirement analysis can be a good starting point to be able to select an appropriate method for the relevant inspection application. Note that each technique utilises a different sensing technology and property. A useful categorisation of NDT methods relative to defect types has been seen in previous works [16; 17; 18]. Table 1 lists a snapshot of the standings of such methods, which relate to those presented in this review paper [17]. We also include an indicator of \"applicability\" as _suitable, weak_ or _limited_. This relates to the level of information outcome of the NDT method for the said defect type and defect detection performance (ability). For example, X-ray-based NDT is challenged by detecting delamination defects in general, with open delamination possible to detect using X-ray computed tomography (CT). Moreover, we give a brief description of these main inspection methods. X-ray radiography is a well-established technique which provides a permanent record (using the X-ray-sensitive film) and is used in NDT inspection. X-rays (or \(\gamma\)-rays, which have a better penetration ability) passing through the sample and the radiation intensity are different if any defects are encountered (see Figure 1c). This principle was successfully used to discriminate different materials for an object by using neutron and X-ray beams simultaneously [19]. However, it can provide a 2D line scan, and it might be difficult to identify the depth information in the case of defect characterisation or localisation. X-ray CT is a developed version that can give 3D images with depth information. The authors of [20] applied X-ray CT with terahertz testing to obtain 3D images for CFRP composite defect characterisation. Even though both can give 3D images using different wavelengths and frequencies of electromagnetic radiation, they were used together to obtain the optimum contrast and resolution. It should be noted that the inspection process is simple, while the equipment can be complex and have a high cost. Radiography also raises health concerns and is performed with care. In addition, both have limited capabilities because of the orientation of delamination or crack defects, as shown in Table 1[17]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Defects**} & \multirow{2}{*}{**X-ray**} & \multirow{2}{*}{**Ultrasonic**} & \multirow{2}{*}{**Penetrant**} & \multicolumn{2}{c}{**METHODDS**} & \multirow{2}{*}{**Eddy Current**} & \multirow{2}{*}{**Therography**} & \multirow{2}{*}{**Acoustic Emission**} \\  & & & & & & & & \\ \hline
**Porosity or Voids** & ✓ & ✓ & \(\odot\) & \(\odot\) & \(\odot\) & \(\odot\) & ✓ \\ \hline
**Delamination** & \(\odot\) & ✓ & \(\odot\) & \(X\) & \(X\) & \(\odot\) & ✓ \\ \hline
**Debonding** & ✓ & ✓ & \(\odot\) & \(X\) & \(X\) & \(\odot\) & ✓ \\ \hline
**Foreign Bodies** & ✓ & ✓ & \(\odot\) & \(\odot\) & \(\odot\) & \(\odot\) & ✓ \\ \hline
**Cracks** & \(\odot\) & ✓ & \(\odot\) & ✓ & ✓ & \(\odot\) & ✓ \\ \hline
**Surface** & \(X\) & \(\odot\) & ✓ & ✓ & ✓ & ✓ & ✓ \\ \hline
**Internal** & ✓ & ✓ & \(X\) & \(\odot\) & \(\odot\) & \(\odot\) & ✓ \\ \hline
**Limit** & Orientation-dependent & Dead zone effect & Only open to surface defects & Ferromagnetic materials only & Conductive materials only & Small thickness & Lack of size and shape \\ \hline
**Advantage** & Inspection process is simple & Portable and good depth resolution & Suitable for mass- manufactured products & Rapid for complex areas & Suitable for hard-to-reach response & Useful for quick defects \\ \hline \hline \end{tabular}  
\end{table} 
Table 1: NDT method standings relative to defect types. Another important method is ultrasonic inspection, which uses ultrasound wave propagation inside materials (Figure 1b).