"Composite" defines a structure made from more than one material. Plywood is a composite made of wood held together with adhesive. Concrete is also a composite.

Advanced composite is a narrower term used to describe structures or parts that use one of the four main reinforcements along with a resin: S glass, Kevlar, carbon fiber and ceramics. Kevlar is a registered brand of DuPont. Aramid is essentially the same as Kevlar but manufacturered by another company. S glass is a variation of traditional fiberglass. S-glass is stronger than E-glass (fiberglass) but not as strong as the remaining fabrics. Kevlar has excellent abrasion resistance, low CTE (coefficient of thermal expansion) and is the only fabric that acts like a metal. This means that as Kevlar nears or exceeds it's load limit, it will bend instead of fracture and shatter apart like other advanced composite materials, especially carbon fiber. Carbon fiber is stronger and stiffer than Kevlar. It is also brittle compared to Kevlar. A composite of carbon fiber can be engineered to be less brittle through various means however. Ceramics are generally used for application that require the part to be near or part of very high temperatures. Typically this is 600F up to or past 3000F. The ceramic tiles on the underside of the space shuttle are an example of ceramic's in action.

Advanced composites combine one or more reinforcement types with a resin (adhesive). Resins come in hundreds of types but three main types are: polyester, vinyl ester and epoxy. The first two have the typical "fiberglassing" smell while epoxy has very little odor. They increase in strength as listed, epoxy being the strongest, when used properly. Epoxy has better stiction to most reinforcements as well. Epoxy normally has higher inherent thermal properties too. Epoxy is typically much more expensive than the other two types, costing $100 to $250+ per gallon. This can be as much as 12-13 times more expensive than a polyester resin.

Careful consideration needs to be applied when combining all of these raw materials in combinations. Some materials will not work well with others. Epoxy is almost always used with carbon fiber and Kevlar applications where those fabrics are being used for their strength and rigidity. The epoxy merely adds to the parts optimization characteristics. Certain resins do not stick well to Kevlar and carbon fiber, while most epoxies do very well. Polyester resins change shape (slightly) as they cure. The use of carbon fiber is normally in situations where low CTE and therefore very stable parts are required. Polyester resin is therefore not a good choice in that situation and is one reason so many carbon fiber parts use epoxy.

Fiberglass fabrics are generally white with a tinge of green to them. Kevlar is typically yellow. Carbon fiber is normally charcoal black. Each of the fabrics can be purchased in numerous "weights." This describes, generally, the thickness of the fabric and size of the weave pattern. There are dozens of main weave patterns from plain weaves to twills to unidirectional fabrics.

Unidirectional fabrics are inherently much stronger than plain weave or even twill weaves, in the longitudinal direction, because they have almost all of their fibers running in one direction. This also means they are much weaker in the opposite direction, perpindicular to their long strands. Many applications will lay down multiple layers of unidirectional fabric by varying the angle of each layer. The inner most layer at 0 degrees, the next at 90 degrees and the outermost layer at 0 degrees again. The final structure will have more strength in the 0 degree direction, say the length of the tube, but by adding the layer at 90 degrees (perpindicular to the other two layers) it will have added strength in that direction as well. This helps the tube resist compression (hoop strength).

You can tell from the previous layup example that a part can be engineered by the fabrics used and the direction in which the fabrics are laid up. In addition to this design freedom there are also fabrication methods that influence the final parts thickness, strength and overall characteristics.

Hand-layup is describes laying up fabric and wetting it out by hand. If no other heat or pressure is being applied to the part, it leaves the thickest wall for a given part because it has the highest amount of resin. The strength of any part is in its fibers, not in the resin. The exception to this being hand-lay up that does not employ vacuum bagging, autoclave or some other form of force that helps squeeze out excess resin. The more resin you have in a part, the more your part will act like the properties of the resin, instead of the fibers. The resin type does matter in numerous ways, but lowering the resin content of the part will increase the strength of the final part, generally much more than simply using a "stronger" resin, like epoxy.

Vacuum bagging is one technique that applies vacuum pressure to the part. This compresses the part, thus reducing the resin content, which makes it stronger. The process also gets rid of air bubbles, moisture and pushes the fibers closer together. Resins are heavy as well, so decreasing the resin amount produces three main affects: the part's thickness is reduced, it has less resin which means it is lighter and it is stronger because the fibers are closer together and the strength is relying more on the strong fibers versus relying on the weaker resin.

Aerospace usually produces parts with resin contents (by weight of the final part) anywhere from 25% to 50%.

Many aerospace companies use a variation on vacuum bagging as well as applying heat to the part. Many use autoclaves that generally mimic a pressure cooker. The part is exposed to much higher forces (using pressure as well as vacuum) than typical vacuum bagging alone can achieve. This can reduce the resin amount even further than vacuum bagging but more often it is done for other reasons. Those reasons can include: consolidation, consistency, even forces over the entire lay up (irregardless of the parts shape) and slightly reduced expertise needed during lay up. Low resin contents can be achieved with vacuum bagging alone. Dream has produced parts with as low as 16.137% resin content (by weight) using E-glass and vacuum bagging only. Autoclaves are extremely expensive and therefore make the final products produced with this technique also extremely expensive. There are a few other techniques that can achieve pressures at or higher than typical autoclaves.

Autoclaves are typically used in combination with prepregs. The purchaser buys the prepreg with the resin already impregnated into the reinforcement. The prepreg has been wet out with a smaller amount of resin than (typical) hand-lay up would have. Many prepregs have higher resin contents than vacuum bagging can achieve. Prepregs usually contain around 40-60% resin (by weight). Although this is better than hand-layup alone (without vacuum or pressure and for a typical fabrication house), it is still a fairly high percentage.

The resin is very evenly applied. Prepregs also speed up production because a separate worker does not have to mix the resin matrix while another wets out the fabric. Many prepregs require refrigeration, so a walk-in cooler is required to store the prepreg supply, near or below 40F. The prepreg material is laid up, bagged and then inserted into the autoclave where it is exposed to the pressures and temperatures mentioned above. The heat causes the resin matrix to react and cure.

Infusion is becoming more and more preferred by the industry. You can see Dream's testing of the infusion technique here. Although the technique makes lay up and wetting out much easier, set up time is longer and to date Dream has not been able to produce a fiberglass sample panel with less than 28.5% resin by weight. Conventional vacuum bagging techniques allow Dream to attain resin contents as low as 16.137% with fiberglass, almost half that of infusion thus far. Testing will continue.

Filament winding is a fabrication technique that is most easily described as a loom combined with a CNC machine. Single or multiple tows (yarns) are wound around a mandrel. The computer controls the pattern the tow(s) make and the tows eventually make a "fabric" or weave. Filament winding is most recognized by its diamond-like pattern but there are many, many other patterns. This type of manufacturing is expensive but offers a product that can be highly engineered (by controlling the exact direction, placement of each tow and fiber utilitized) and therefore stronger than typical fabrication techniques using prepregs or wet lay up with vacuum bagging. The tows are not crimped like they are in woven goods either so they are inherently stronger than woven goods as well.

Pultrusion is yet another fabrication technique. Pultrusion is akin to Playdo being pushed through a template. It will take on the shape of that template. As it comes out the other side of the template it is exposed to controlled heat. This rapidly cures the part. So quickly that after passing out of the heating chamber it is fully cured and solid. It is most often used for small diameter tubes in varying profiles. From round to square to "I" beam shapes. Normally pultrusion tubes are not available in sizes larger than 4-5 inches in diameter. They can be done but price jumps exponentially as sizes go larger and larger. Smaller sizes, 1/2" diameter and less, can be very reasonable in all materials (fiberglass or carbon fiber) but as sizes rise past 1/2" OD, the costs jump considerably. The poles used for most tents are pultruded carbon fiber tubes. All of the fibers are longitudinal and have enormous strength in the axial direction. Transverse or hoop strength is very low though.

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