What are composite materials and why are they used in aerospace?

Composite materials combine two or more constituents: Reinforcement - provides strength and stiffness (carbon, glass, aramid fibers); Matrix - binds fibers, transfers loads, protects fibers (epoxy, BMI, PEEK thermoplastics). Types: Carbon Fiber Reinforced Polymer (CFRP) - highest performance, most common in aerospace; Glass Fiber Reinforced Polymer (GFRP) - lower cost, radar transparent; Aramid (Kevlar) - impact resistant, used in containment structures. Advantages: Excellent strength-to-weight (30-50% lighter than aluminum), Tailorable properties (directional stiffness), Fatigue resistant, Corrosion resistant, Complex shapes in single piece. Disadvantages: High material and processing costs, Damage detection more difficult, Repair complexity, and Moisture absorption effects. Modern aircraft (787, A350) are 50%+ composite by weight, primarily in fuselage and wing structures.