Long before humans were making cob nature had produced composite structures. The best example is also the oldest composite building material used by man: wood. Wood is composed of long fibers made from the cell walls of the original tree bound together by a glue-like substance called lignin. In this case lignin is the matrix and the fibers are the reinforcement. The strength of wood comes from the fibers which are supported and held together by lignin. In the direction the fibers run, called the grain, the wood is stronger than in any other direction. However, because the lignin soaked fibers are more stiff than the lignin itself, against the grain the wood is more flexible.
Materials like wood are very different from those like metal or stone. Metal and stone have nearly the same properties in all directions, given the term isotropic. Wood is an anisotropic material, meaning that its properties depend on what direction it is assembled and loaded in. Composites like wood, which act differently in different directions, can often act in ways that seem counter-intuitive to people who aren't accustomed to dealing with them.
Modern composites are very much like wood, they are composed of fibers supported by a resin matrix. Composites are formed of thin layers of combined fiber/resin bound together by resin. They are strongest in their fiber direction (called the principle direction), weaker in the other direction within a ply (called the transverse direction), and weakest between plies (the normal direction). In a manufactured composite the fibers can be either long, like in wood, or short, like in concrete.
In long, or continuous, fiber composites the fibers extend the entire length of the part. In this case it is the fiber carrying the load and the matrix serves to bind fibers together and ensure they don't bend or buckle. Fibers are strong but they are also flexible and will bend rather than support a load in compression. Matrix material gives the fiber their rigidity and holds them in shape. Typically long fiber composites are nonisotropic, since they are strongest in their fiber direction. In the other direction they act more like a short fiber composite, where the fibers provide reinforcement but the matrix has to carry the load between them.
Fibers can be produced in many ways. Terms for composite fibers are borrowed from textiles, since that's where many of the processes that produce them came from:
- filament - a single fiber, much longer than it is wide.
- yarn - a bundle filaments that are lightly wound together, may or may not be twisted
- tow - a large (3K - 12K) bundle of loosely bound filaments that are not twisted together
- fabric - a large flat weave of filaments, yarn, or tows interlaced
- mat - a random assortment of short fibers laid flat
Lay-up is the process of shaping a composite part. Typically parts are laid up around a mold, or tool. Most tools are either male, meaning the part is laid up over the tool, or female, meaning the part is laid up in the tool. Two-part tools squeeze the lay-up between male and female halves. When laying up the part in or around a mold there are several options for getting the fiber and matrix where you want them.
Anisotropic lamina can be used in specific situations but they must be very carefully analyzed to anticipate their reactions. For example the X-29 experimental plane used the fact that its composite wings would tend to bend under load to limit the angle of attack. As lift increased the wings bent forward, which reduced the lift. This ensured that the plane would remain under control. For most purposes, though, designers still prefer isotropic laminates.
There are cases where fiber and matrix plies are not the only components that must be included in a structural composite. Parts that need to resist bending or buckling work best when their strongest components are removed from the center, the reason why I beams have their shape. In some cases parts need to have a specific cross-section for aero- or hydro-dynamic reasons. In both cases we want to increase the area of the part without adding too much weight. The easiest way to do that is to separate the faces using some sort of filler core material, creating a sandwich structure with the two face sheets as the bread and the filler in the center.
One of the most common fillers is honeycomb core, composed of thin walled, mostly empty cells. The core is light but surprisingly strong and good at resisting compression in the cell direction. Face sheets are bonded to both sides and cured in place, with the core present. Honeycomb supports the face sheets and provides the shape desired without much additional weight.
An everyday example of a honeycomb sandwich material is corrugated cardboard, the faces are flat cardboard but between them is a thin walled paper material. The combined structure is surprisingly strong for its light weight and cheap construction. Core can be made from a number of materials, including Nomex, aluminium, and even paper. Most core only bends in one direction, and resists bending in the other. If the face is punctured core cells can also fill with water, dramatically increasing weight.
Potting compound is a porous, putty like bonding agent that can be used on its own or between sections of other core material. Potting compound is easy to shape and fairly strong but tends to be heavier in bulk sections that either foam or honeycomb. Typically potting is used as a way of filling in between other core sections or building up a non structural surface.
Laminate cross section can be varied through a part by adding or removing plies and core material. This can be done to reinforce high stress areas or to build up a shape for other reasons. It's rare that we want core material to be exposed to the environment; it should be completely covered. Stepping down and tapering the cross-section is one way of doing this. Another is to wrap material around the edge of a part to cover the exposed core at the edge.
After we make the composite parts we need to assemble them into the final product. Early on when designers didn't have much experience with composite parts they would simply replace existing aluminum pieces with the new, lighter, carbon fiber parts. These black aluminum designs didn't use many of the biggest advantages of composite design and often encountered problems that the aluminum parts didn't have, because carbon fiber isn't aluminum and doesn't act like it.
When attaching two composite pieces together, or when attaching a composite part to a metal one, there are two methods that can be used. One is to use fasteners, like bolts and rivets, as is frequently done with metal parts. Installing a fastener means drilling a hole through the part, disrupting its fibers and creating a location of increased stress. Fasteners often pull against the laminate, which pushes some plies against their fiber direction. Over tightening a fastener causes additional damage as it crushes the laminate and any core material, creating unbonds that grow out away from the fastener. Under certain circumstances fasteners can be torn completely through the laminate, causing significant damage where they pull out, even if the part is held in place by other fasteners.
Of course there are also reasons why you might need to fasten things. We already said there's a limit to how big parts can be made, so breaking a structure down into smaller parts has to be done so you can build it at all. Smaller parts are easier to replace during maintenance. Sometimes it just isn't possible to have a composite part do something, so metal parts have to be attached to the structure. There are always reasons why assembly is needed, but in general it's something that should be reduced as much as possible.
Once a part is built it can be damaged by use or carelessness. Since matrix is the weakest component it is typically where failure occurs. Adhesive failure occurs when the adhesive itself breaks, but remains attached to the fibers, the adhesive itself it weaker that its bond to the fiber. Bond failure occurs when the adhesive fails by releasing the fiber or ply it is attached to, but remains intact. Usually we see some combination of the two, but talk about whichever it was that started the break as the source of failure.
Fiber failure is less common, but can occur since the matrix can often by more flexible than the fibers are when the part is under tension. When the part begins to fail it stretches, when it stretches the fiber breaks first, then without the fiber support the matrix fails. Fiber pullout occurs, with broken fibers sticking out of broken matrix.
When a crack grows through a ply we call it in-plane fracture. This means that it is the adhesive in a single play that failed, either by releasing the fiber or by breaking itself. These cracks look very much like the cracks that grow through a piece of wood, rapidly expanding along the matrix between fibers.
Even if the face sheets are undamaged it is still possible to see core damage occur, especially when the core is weaker than the face material. It is also possible to see both undamaged but the adhesive between them allow skin to core unbond. This frequently happens when the core is poorly shaped or placed, leaving a gap between the face and core, and when impact damaged crushes the core slightly, without damaging the face sheet. It's important to remember that every part of a composite material has different properties so each part wants to do something the others don't, which creates stress when they are all held together.
It's often difficult to spot damage to a composite structure, because the damage is often below the surface while surface plies look completely undamaged. Spotting damage early can become very important since composite materials are often more brittle than metal components. Athough ripping a piece apart will definitely spot any damage, we want to keep parts in service if we can. So we need some form of non-destructive testing (NDT) for composites that will let us find sub-surface damage.
The first step in scanning a part for damage is a simple visual inspection. Trained technicians can sometimes spot signs of damage below the surface of the part, even though the surface is undamaged. Damage can appear in the form of light spots, sudden color changes, or changes in light transmission through a part.
Another simple inspection technique is the coin tap test. Disbonds below the surface of a laminate often sound hollow when tapped. So by tapping a suspect area with a coin, key, small hammer, or similar item an inspector can quickly check for near surface failures. Both of these methods are simple, easy, and cheap. They are also imprecise and prone to human error.
Since voids in the laminate have different sound transmission than the undamaged laminate sound is an obvious choice for inspection. Ultrasonic testing (UT) uses high frequency sound transmitted through a part for a check. A receiver listens to the sound emitted by a transmitter and reports changes. The probe listens for either the echo of the initial pulse, reflection, or is placed on the opposite side and listens for the through transmission, attenuation.
Fibers are thicker and more dense than matrix. So the fibers block more electromagnetic radiation than the matrix. Just like bone is more dense than muscle, and for the same reason X-rays can spot damage. Many fabrics and tapes also include by design additional, nonstructural, fibers that are made specifically to be visible to X-ray, called tracers. X-ray inspection involves taking X-ray photographs of a part, looking for regions of less dense material or flaws in the tracer direction. The downside of X-rays is that the image is two dimensional. Multiple layers stacked above each other can create a confused image, because the fibers of multiple layers are superimposed on the same image.
Once damage is identified we'll want to fix it. Most repairs that we conduct on a part are either cosmetic, temporary, or permanent. Cosmetic repairs are surface deep, repairing damage that doesn't affect the load bearing ability of a piece. This can be done not just for looks, but also to ensure a surface will be the right shape to fit when it is assembled or to produce a smooth, low drag part or to seal the core to prevent water from getting in. Temporary repairs are intended only to last until the next regular maintenance cycle a part goes through. Though the repaired part might not have the same profile it once did, it should be as close as possible to its original strength. Permanent repairs attempt to restore a part as close as possible to its original properties. Once a permanent repair is done the part should be like new, though the repair will usually be slightly weaker than the original part was.
When a scratch or depression on the part doesn't get through the topmost layer a very easy surface repair can be done. In this repair the defect is filled with adhesive, cured, and then sanded smooth. The repair doesn't add any strength, it just smoothes out the part. It is a purely cosmetic repair. In principle is isn't much different from patching a hole in a wall with stucco, it seals the hole but is much weaker than the original material.
When simple repairs are not enough a patch over the damaged area is needed. Patch repairs work just like the patches on clothes. By covering the tear with undamaged fabric you can keep the hole from getting any bigger. Patches bridge the load gap the hole creates, carrying the stress around the tear instead of to it.
A simple patch is just layers of material bonded over the top of a damaged area. Though relatively easy and quick such a patch also adds thickness and weight to the final part. This changes the profile of the part, which can hurt its fit in place or increase drag. The added weight is also usually undesirable. Simple patches are often done as temporary repairs, to hold until other facilities are available.
If the core itself is damaged then the only repair is to remove it. A core unbond can be repaired by a drill and fill operation, but damaged core requires a core replacement. In this operation the technician sands through the face of a part down to the core, then drills or cuts out the damaged core section. Another section of core material is cut that fits in the hole left by removing the damaged section and fixed in place with adhesive. Potting compound can be added to fill in any gaps. Then a step or taper sand repair replaces the removed face plies. Sometimes the damage penetrates the whole laminate. In this case on face is removed to do the core replacement. Then the other face is removed in a step or taper sand.