Summary of Aerospace Materials and Inspection Basics

## Introduction Aircraft maintenance and inspection ensure that aircraft remain safe, reliable, and airworthy. This guide covers specific inspection techniques, common defects, measurement tools, fastener tightening, and material behavior relevant to structural inspection and maintenance tasks. ## Visual and Magnified Inspection ### Visual inspection Perform routine visual checks for surface defects, corrosion, and improper welds. > A magnifying glass of at least ten power is recommended for performing a detailed visual inspection of a weld. ### Magnified inspection example - Use a 10x magnifier to inspect weld toes, edges, and small cracks that are not visible to the naked eye. - Practical application: During a post-repair check, a 10x loupe helps detect undercut, cold laps, or fine hairline cracks. ## Welding Defects: Cold Welds ### What is a cold weld? > A cold weld is a weld with improper penetration and cold laps caused by insufficient heat; it appears rough and irregular, and its edges are not feathered into the base metal. Characteristics: - Improper penetration into the base metal - Presence of cold laps (folded-over metal) and lack of fusion - Rough, irregular appearance - Edges not feathered into the base metal Practical example: If a fillet weld on a bracket looks grainy and separates under light hammering, it may be a cold weld and must be removed and re-welded with proper heat. ## Non-Destructive Inspection (NDI) Methods ### Magnetic Particle Inspection (MPI) Principles: - Magnetize the part - Apply ferromagnetic particles to the surface area to be inspected - Discontinuities disturb magnetic flux lines; opposite magnetic poles appear on either side of the discontinuity, attracting particles and revealing the defect When to use: - Detects surface and slightly subsurface cracks in ferromagnetic materials - Common on landing gear components, shafts, and structural fittings Practical tip: Ensure part is clean and properly magnetized; use visible or fluorescent particles based on lighting. ### Dye Penetrant Inspection (DPI) Principles: - Apply a penetrant (a colored or fluorescent liquid) to a clean surface - Penetrant enters surface openings and remains in those openings - After dwell time, remove excess penetrant and apply developer to draw out penetrant, revealing defects When to use: - Detects surface-breaking defects on non-porous materials (metals, composites, glass) - Common for detecting cracks, porosity, and fine flaws on non-ferromagnetic parts Practical example: After machining a bolt hole, use DPI to inspect for hairline cracks before installation. ## Corrosion: Filiform Corrosion ### Appearance and cause > Filiform corrosion typically appears as thread-like lines of puffiness under a film of polyurethane or other dense finish system topcoats. Characteristics: - Looks like worm- or thread-like pathways beneath paint - Common under painted surfaces where moisture is present at primer/paint interface Practical prevention: Control moisture ingress, use proper primers, and remove contaminated coatings before repainting. Fun fact: Filiform corrosion often starts at a small coating defect and can spread under the topcoat without obvious surface blistering. ## Material Identification Codes ### Steel and Aluminum designation systems - Steel: Structural steels commonly use the SAE numerical index to identify chemical composition. - Wrought aluminum: Uses a four-digit index system divided into groups that identify alloy series and variations. Practical application: When replacing a structural member, reference the SAE or aluminum index to match material composition and mechanical properties. ## Measurement Tools: Micrometer Calipers - A micrometer (often called a micrometer caliper) measures outside dimensions of shafts, thickness of sheet stock, drill diameters, and similar features with high accuracy. Usage tips: - Clean surfaces before measuring - Use consistent measuring pressure (ratchet or