## 2 Methods and Materials
### Type of Defects  
This section examines the two most widely used materials in aircraft manufacturing: metallic and composite materials. It explores various types of defects associated with these materials, as investigated by other researchers in the field. The focus is on how these defects impact aircraft maintenance and the methods used to inspect and characterise them. The Table 7 below summarises typical types of defects found during the manufacturing process or during in-service in aerospace materials, divided into metallic and composite categories. Metallic materials often suffer from issues like fatigue, corrosion, and wear, which weaken their structure. In contrast, composite materials are more sensitive to problems such as delamination, fibre cracking, and impact damage, which affect their layered and bonded nature. This breakdown shows the unique damage challenges faced by each material type. The table above corresponds to the image below, providing a detailed view of the shape, location, and unique attributes of each defect. This illustration showcases typical material defects found in both composite and metallic structures. Figure 10(a) represents defects in composite materials, such as impact damage, delamination, porosity, and fibre/matrix cracking. Figure 10(b) shows a jet engine turbine blade with thermal cracks and delamination between ceramic coatings and CFRP layers. Figure 10(c) displays honeycomb panel defects, \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Component** & **Applied Loads/Conditions** & **Material Requirements** & **Example Materials** \\ \hline Wing & Subjected to bending during & High tensile and compressive & 2024 Al-based alloy: Moderate \\  & flight; additional loads during & strength & yield strength (324 MPa), good \\  & taxiing, take-off, and landing; & & fracture toughness (37 MPa m\({}^{1/2}\)), \\  & compression on the upper & & high elongation (21\%); \\  & surface and tension on the lower & & Polymer Matrix Composites (PMC) \\  & surface during flight & & such as CFRP: High strength \\  & & & (3450-4830 MPa), elastic modulus \\  & & & (224–241 GPa), high-temperature \\  & & & resistance (290–345 °C) \\ \hline Fuselage & Exposed to high cabin pressure & High tensile and shear strength & 2024 Al-based alloy: Used for \\  & and shear loads & & moderate yield strength and \\  & & & durability \\ \hline Exhaust Nozzle & Exposed to high temperatures & High-temperature capability & Ceramic Matrix Composites \\  & & & (CMC) \\ \hline Aircraft Brakes & Subjected to extremely high & High-temperature resistance & Carbon fibre-reinforced silicon \\  & temperatures (up to 1200 °C & \\  & under emergency conditions) & & carbide \\ \hline Ailerons, Flaps, & High specific strength and & Lightweight with high & Polymer Matrix Composites \\  & & & strength-to-weight ratio & (PMC): Both thermoset and \\  & & & thermoplastic PMCs \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Material requirements and example materials for key aircraft components [99]. including core crushing, water ingress, and skin-to-core debonding. Finally, Figure 11d illustrates common issues in metallic components, like voids, corrosion, material inclusions, and both parallel and perpendicular cracks. Together, these diagrams provide a clear visual overview of defect types in aerospace materials. The following sections will analyse three representative examples of fabricated specimens with artificially induced defects to simulate real-world scenarios. Each example includes details on the material type, geometry, defect fabrication methods, and inspection techniques used by researchers.