A composite combines two or more chemically distinct, insoluble phases whose performance beats the individual ingredients. Oldest example: straw in mud bricks (4000 B.C.); also rebar in concrete. Used in aircraft, spacecraft, piping, automobiles, boats, sporting goods.

Structure of Reinforced Plastics (FRP)

Fibers in a plastic matrix:

Fiber forms: chopped (short L/D ≈ 20–60, long ≈ 200–500) or continuous layers / fabric mats.
Fibers are strong, stiff, brittle, lack toughness, and can degrade in the environment — the matrix fixes these.
Fiber volume fraction usually 10–65%.
Hybrid composites mix fiber types for local needs.

Reinforcing Fibers

Glass (drawn through a platinum die): E (calcium aluminoborosilicate, most common), S (higher properties, costlier), E-CR (best, high-T).
Graphite (pyrolysis of organic yarns): carbonizing ~1500°C, graphitization ~3000°C; carbon fibers 80–95% C, graphite > 99% C.
Aramids (e.g. Kevlar): toughest, some plastic deformation before fracture; ballistic armor.
Boron (deposited on tungsten fibers).

Matrix Materials

Functions: support fibers & transfer stress, protect them, slow crack propagation (matrix is more ductile). Common thermosets: epoxies, polyesters, polyimides (high-T, with graphite). Thermoplastics, ceramics, and metals can also serve as matrices.

Properties of Reinforced Plastics

Depend on fiber type, orientation, length, volume fraction, and the fiber–matrix bond (improved by coatings / coupling agents). Highest stiffness/strength when fibers align with the tension force — but that unidirectional composite is anisotropic.

Load is shared by fibers and matrix:

$P_c = P_f + P_m \;\Rightarrow\; \sigma_c A_c = \sigma_f A_f + \sigma_m A_m$

With total area $A_c = A_f + A_m$ and fiber fraction

$x = \frac{A_f}{A_c} = \frac{V_f}{V_c}$

the composite stress is

$\sigma_c = x\,\sigma_f + (1-x)\,\sigma_m$

Fibers and matrix share the same strain (iso-strain), so

$e_c = e_f = e_m \;\Rightarrow\; \frac{\sigma_c}{E_c} = \frac{\sigma_f}{E_f} = \frac{\sigma_m}{E_m}$

Applications

Aircraft, rockets, helicopter blades, auto bodies, pipes, ladders, sporting goods, helmets, boat hulls. Notes: ~90% of the Voyager craft is carbon-reinforced plastic; metal helicopter blades → S-glass/epoxy (high stiffness, fatigue & ballistic resistance); fiberglass ladders are preferred by electricians (non-conductive, unlike wet wood or aluminum).

Key Notes

Two main drawbacks: anisotropy and possible environmental attack on fibers (water absorption) — reduce via random fiber dispersion and protective coatings.
Fibers carry the load only if the bond transfers stress to them.
Metal-matrix composites beat reinforced plastics at higher temperatures and are tougher/more ductile.
Small indentations distinguish matrix vs reinforcement hardness; a large Brinell indent gives an overall value.