Composite Materials

How Composites Work

The fundamental principle of composite materials is that the combination of a strong, stiff reinforcement with a tough, formable matrix produces a material that exploits the best properties of both constituents while compensating for their individual weaknesses. Glass fibers are strong but brittle and cannot hold a structural shape on their own. Polyester resin is tough but relatively weak and flexible. Combined as fiberglass, the glass fibers carry the structural loads while the resin matrix transfers stress between fibers, protects them from damage, and holds the component in its designed shape.

The rule of mixtures provides a first approximation for predicting composite properties. For a load applied parallel to continuous fibers, the composite modulus equals the fiber volume fraction times the fiber modulus plus the matrix volume fraction times the matrix modulus. A carbon fiber reinforced polymer (CFRP) with 60 percent fiber volume fraction, where the carbon fibers have a modulus of 230 gigapascals and the epoxy matrix has a modulus of 3.5 gigapascals, achieves a composite modulus of approximately 139 gigapascals along the fiber direction. This is lower than steel (200 gigapascals) but the composite density is only about 1.6 grams per cubic centimeter compared to 7.8 for steel, giving CFRP a specific stiffness roughly five times higher.

Composites are inherently anisotropic, meaning their properties depend on direction. A unidirectional carbon fiber laminate is extremely strong and stiff in the fiber direction but much weaker perpendicular to the fibers, where the softer matrix carries most of the load. Engineers address this by stacking multiple layers (laminae) at different angles, creating a laminate with tailored properties in all relevant directions. A quasi-isotropic layup with equal proportions of 0, 45, minus 45, and 90 degree plies produces roughly equal in-plane properties in all directions, though at a lower performance level than a unidirectional laminate loaded along its fibers.

Polymer Matrix Composites

Polymer matrix composites (PMCs) are the most widely used composite category, combining polymer resins with glass, carbon, or aramid fibers. Glass fiber reinforced polymer (GFRP), commonly called fiberglass, accounts for the largest volume of composite production. E-glass fibers (alumino-borosilicate glass) cost roughly 1 to 3 dollars per kilogram and provide a tensile strength of about 3,400 megapascals. GFRP is used for boat hulls, wind turbine blades, automotive body panels, pipes, tanks, and building cladding.

Carbon fiber reinforced polymer (CFRP) offers significantly higher specific strength and stiffness than GFRP but at 5 to 20 times the cost. Standard modulus carbon fibers (230 gigapascals modulus, 3,500 megapascals tensile strength) are produced by pyrolyzing polyacrylonitrile (PAN) precursor fiber at temperatures above 1,000 degrees Celsius in an inert atmosphere. The resulting graphitic structure, with carbon atoms arranged in hexagonal layers oriented along the fiber axis, gives carbon fiber its extraordinary axial properties. CFRP is the primary structural material for modern commercial aircraft, with the Boeing 787 and Airbus A350 using over 50 percent composite by weight. Racing cars, high-performance bicycles, tennis rackets, and satellite structures also exploit CFRP advantages.

Aramid fiber composites (Kevlar is the best-known brand) excel in impact resistance and are used for ballistic protection, helicopter rotor blades, and pressure vessels. Aramid fibers absorb energy through fibrillation, splitting into many small fibrils during impact rather than fracturing cleanly, which spreads the impact energy over a larger area and longer time.

Metal and Ceramic Matrix Composites

Metal matrix composites (MMCs) use a metallic matrix, typically aluminum, titanium, or magnesium, reinforced with ceramic particles, whiskers, or fibers. Silicon carbide particle reinforced aluminum (Al-SiCp) is the most commercially important MMC, used for automotive brake rotors, electronic packaging, and aerospace structural components. The ceramic reinforcement increases stiffness and wear resistance while the metal matrix provides toughness and thermal conductivity. Alumina fiber reinforced aluminum is used in the Toyota Altezza engine block cylinder liners, where the composite provides wear resistance at the piston ring contact surfaces that would rapidly degrade in unreinforced aluminum.

Ceramic matrix composites (CMCs) address the fundamental brittleness of monolithic ceramics by embedding ceramic fibers in a ceramic matrix. The key to CMC toughness is a carefully engineered interface coating (typically boron nitride or carbon) between the fiber and matrix that allows the fiber to debond and pull out of the matrix when a crack approaches, absorbing energy and preventing catastrophic failure. Silicon carbide fiber reinforced silicon carbide (SiC/SiC) composites are the most advanced CMCs, now operating in the hottest sections of GE and Safran jet engines at temperatures above 1,300 degrees Celsius while weighing one-third as much as the nickel superalloy components they replace.

Natural and Sustainable Composites

Nature produces sophisticated composites that inspire engineering design. Wood is a composite of cellulose fibers (providing tensile strength) embedded in a lignin matrix (providing compressive strength and moisture resistance), with the fibers oriented along the grain direction. Bone combines hydroxyapatite mineral crystals (providing stiffness and compressive strength) with collagen protein fibers (providing toughness and tensile strength) in a hierarchical structure optimized across multiple length scales, from the nanometer arrangement of mineral and protein to the millimeter-scale organization of osteons.

Natural fiber composites using flax, hemp, jute, or kenaf fibers in polymer matrices are growing in automotive applications, where they offer lower weight and cost compared to glass fiber composites for semi-structural interior panels, door linings, and trunk covers. European automakers use over 80,000 tonnes of natural fiber composites annually. While natural fibers cannot match the mechanical properties of glass or carbon, they offer lower density, better vibration damping, reduced environmental impact, and lower cost per unit volume.

Manufacturing and Joining

Composite manufacturing methods vary with the matrix type and component requirements. Autoclave curing uses heat and pressure (typically 180 degrees Celsius and 700 kilopascals for aerospace epoxy composites) to consolidate pre-impregnated fiber sheets (prepregs) into high-quality laminates with low porosity and consistent properties. This remains the standard for primary aircraft structures but is expensive and slow. Resin transfer molding (RTM) places dry fiber preforms in a closed mold and injects liquid resin under pressure, offering faster cycle times and lower cost for medium-volume production. Filament winding wraps resin-coated fibers around a rotating mandrel to create axially symmetric structures like pressure vessels, rocket motor cases, and pipes.

Joining composite structures presents unique challenges. Mechanical fastening (bolting and riveting) is reliable but requires drilling holes that cut fibers and create stress concentrations. Adhesive bonding distributes load over a larger area and avoids fiber damage, but quality depends heavily on surface preparation and is difficult to inspect non-destructively. The Boeing 787 uses both approaches extensively, with thousands of titanium fasteners supplementing adhesive bonds in critical joints.

Testing and Quality Control

Composite quality assurance requires specialized testing methods because internal defects like delaminations, porosity, and fiber misalignment are invisible from the surface. Ultrasonic inspection is the primary non-destructive evaluation technique, using sound waves to detect delaminations and porosity by measuring changes in signal amplitude or time-of-flight through the composite thickness. Automated ultrasonic scanning of large aircraft panels produces C-scan images that map defect locations across the entire component area, with resolution sufficient to detect delaminations smaller than 6 millimeters in diameter.

Mechanical testing of composites follows standards developed by ASTM and other organizations. Tensile testing uses tabbed specimens to measure strength and modulus in specific directions. Compression testing is particularly challenging because thin composite specimens tend to buckle before reaching their true compressive strength, requiring specialized anti-buckling fixtures. Interlaminar shear strength, measured by short beam shear tests, evaluates the quality of the bond between layers. Impact testing using drop-weight impactors at standardized energy levels assesses damage tolerance, measuring the residual compressive strength after impact, one of the most critical design properties for aircraft composite structures.

Fiber volume fraction, one of the most important quality parameters, is typically measured by matrix digestion (dissolving the resin in acid and weighing the remaining fibers) or by image analysis of polished cross-sections viewed under a microscope. Thermal analysis techniques including differential scanning calorimetry (DSC) verify that the resin has fully cured by measuring the residual heat of reaction, while dynamic mechanical analysis (DMA) measures the glass transition temperature to confirm adequate cure and thermal performance margins.