Aircraft Materials - Composite and Non-Metallic
Core concepts
A composite is reinforcement plus matrix. Reinforcements are glass (cheap, insulating, radar-transparent, moderate strength), aramid such as Kevlar (tough, good in tension, poor in compression, absorbs moisture) and carbon (stiffest and strongest, electrically conductive, brittle in impact). Matrices are usually epoxy, with phenolic used where low smoke and toxicity matter and polyester in light structures.
Know the lay-up language: warp and weft directions, bias, ply orientation, quasi-isotropic stacking, and sandwich construction with honeycomb (Nomex or aluminium) or foam cores. Understand pot life versus cure time, exotherm in thick laminates, vacuum bagging and autoclave cure, and the moisture absorption that demands dry storage of prepreg.
Traps in questions
Composites do not yield. There is no visible bending before failure, and a low-energy impact can leave barely a mark while halving residual strength - that is barely visible impact damage. Delamination and disbond are internal and need tap testing or NDT. Carbon fibre is conductive, forms a galvanic couple with aluminium and needs isolation and lightning-strike protection on outer skins. Drilling composites with a blunt or wrong-geometry bit causes delamination. Aramid is difficult to cut and machine. Glass is the choice for radomes because it is transparent to radar.
Worked example
A wing skin shows a small paint crack after a dropped tool. It is carbon/epoxy sandwich. Do not simply fill and paint: coin-tap the area for a change in sound indicating disbond, then compare the finding with the manufacturer's damage-tolerance limits before any repair decision.
Rule of thumb
If the fibre is cut, the strength is gone - never judge composite damage by appearance alone.