Materials, Processes, and Non-Destructive Testing
Materials, Processes, and Non-Destructive Testing tests alloy selection, heat treatment, corrosion control and inspection method choice. Questions often ask which NDT method detects a given defect in a given material.
Materials and processes
2024 aluminium is high-strength with copper; 7075 uses zinc and is stronger but less corrosion-resistant. Titanium is used for high-temperature and composite interfaces; composites are anisotropic and sensitive to delamination, impact and moisture. Galvanic corrosion needs an electrolyte and dissimilar metals; isolation, sealant and plating break the couple.
NDT methods
Visual inspection finds surface damage. Dye penetrant detects surface-breaking defects in non-porous materials. Magnetic particle inspection works only in ferromagnetic materials and finds surface and near-surface defects. Eddy current detects surface and subsurface cracks in conductive materials. Ultrasonic detects internal defects and measures thickness. Radiography shows internal density changes but is directional and a radiation hazard.
Worked example
Ultrasonic thickness measurement: longitudinal velocity 5,900 m/s, round-trip time 2.0 µs. Thickness = v × t / 2 = 5,900 × 2.0 × 10^-6 / 2 = 0.0059 m = 5.9 mm. If the display reads 5.4 mm against a 5.0 mm minimum, the remaining margin is 0.4 mm; if corrosion is local, the minimum reading governs.
Traps
Using MPI on aluminium or composites (it does not work), using dye penetrant on porous surfaces, and assuming radiography detects tight delaminations. Heat treatment and anodising are not interchangeable corrosion protections.