Composite structures

A composite is strong fibers held in a matrix of resin, usually epoxy. The fibers carry the load; the resin holds them in place and passes the load between them.

• Fiberglass: cheap and easy to work, heavier than the others. Doesn't conduct electricity.
• Aramid (Kevlar): very tough and resists impact, but hard to cut and drill cleanly, and it soaks up water at its edges.
• Carbon (graphite): very stiff and strong for its weight. It conducts and sits far from aluminum in the galvanic series, so carbon touching aluminum corrodes the aluminum; a layer of fiberglass or sealant keeps them apart.

Woven cloth has warp threads running along the roll and fill (weft) threads across it. Bias is 45° to both. Fibers are only strong along their length, so each ply is laid at the angle the drawing gives, and a repair must copy the original's ply directions.

Fiber comes dry (wet with resin by hand) or as prepreg, already wet with resin and kept frozen; it has a limited out-time at room temperature. Parts are cured under heat and pressure: a vacuum bag squeezes the plies together and pulls out air, and heat comes from blankets, an oven or an autoclave (a pressurized oven). Sandwich panels have two thin skins bonded to a light honeycomb or foam core: very stiff for their weight.

Damage and repair

Composites often hide damage: an impact can leave a barely visible dent with a delamination (layers separated) or disbond (skin off the core) underneath. Find it with the tap test, ultrasonic or thermography. A typical repair removes the damage and sands a long, shallow taper around it (a scarf), then lays new plies, each a little bigger than the last, at the original orientations, and cures them. The SRM gives the taper, the plies and the cure.

Resin, hardeners and sanding dust are hazards: gloves, eye protection and a respirator, and good ventilation.