Composites

Composites brought to you by AnnMarie and Nathan
 * **What is a composite?**

Technically a composite is a material made by combining two or more distinct materials that don’t chemically react. Concrete, for instance, is a composite material, which commonly consists of cement and gravel. Wood is a natural composite consisting of cellulose fibers embedded in lignin. As it turns out most modern engineered composites are more like wood than concrete. They consist of strong fibers embedded in another material which is referred to as the matrix. || A composite material || __Fiber types__

Fibers made from glass are flexible, lightweight, and inexpensive and are the most common type of fiber used in low-cost applications.
 * Glass fibers**

Fiberglass is a composite consisting of a polyester matrix reinforced with glass fibers. It is often used to make lightweight boats and is used to make the body of the Chevrolet Corvette.

Another interesting fiberglass composite uses an epoxy resin as the matrix. Epoxies start as two liquids that when mixed will gradually harden. This allows you to take glass fibers that have been woven into a matt and essentially glue them to a surface. If you choose an epoxy that that has an index of refraction close to the index of the glass fiber used then after the epoxy hardens it produces a clear coating that will protect and strengthen the underlying material. Also known as Graphite fibers, these fibers are very strong and lightweight but also very expensive. They have managed to work their way into applications in which people are willing to pay extra for lighter weight products. Most notably hoods for hot rods, sporting goods, and aircraft parts.
 * Carbon fiber**

[|Kevlar], Boron, Silicon carbide, alumina are other common fibers used in composites.
 * Other fibers**

Fibers are used because most materials are much stronger as fibers than as bulk materials. Glass fiber mentioned above is a good example. Normally we would not think of glass as being a good structural material since it is so breakable. We might have similar thoughts about ceramics. Both types of materials are brittle and easy to break because lots of cracks exist in the surface of these materials that can easily spread through the material. However, the process of making small fibers of glass or ceramics naturally reduces the number of surface cracks in the material so they are stronger and less brittle. Furthermore once a crack starts growing in a large piece of glass it can easily spread through the whole piece, but a crack that grows in a small fiber cannot spread to the other fibers. So a material made of fibers is less likely to fail catastrophically.
 * Why Fibers?**

Similarly metal fibers are stronger than large pieces because the fibers have fewer defects (known as dislocations).

Polymers are also stronger as fibers because the polymer chains are aligned along the fiber length rather than the randomly entangled arrangement found in the bulk polymer. Random arrangement of polymer chains found in a bulk polymer ||
 * [[image:file:///C:/Users/Berad/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg width="247" height="243"]][[image:random_polymer.jpg width="324" height="319"]]

The polymer chains are lined up in a polymer fiber. || The matrix serves several purposes. Even though the fibers are very strong they are not rigid so they cannot be used for structural applications by themselves. So, one purpose of the matrix is to hold the fibers in a rigid structure. The matrix can also transfers load from one fiber to nearby fibers, thereby make the load more uniformly distributed across the material. There is also friction between the matrix and the fibers. If a fiber breaks it is possible that this friction will keep it from slipping so that the fiber can still support some of the weight.
 * Role of the Matrix**

Metals, ceramics, and polymers can be used for the matrix material. When choosing a matrix several criteria must be satisfied. The matrix cannot react with the material. For instance if one were to pour liquid metal around a carbon fibers to produce a metal matrix, the hot metal would react with the carbon fibers destroying the fibers. The material must also have good adhesion to the fiber so that there is a lot of friction between the fiber and the matrix. In practice polymers such as polyester resins, vinyl ester resins, and epoxy resins are the most common matrix materials because they are the easiest to work with.
 * Types of Matrices**

The main advantage of composites is that they are stiffer and stronger compared to an equal weight of a conventional material. The use of composites is an effective way to reduce the weight of a product. They can also be highly customized for the application. Many matrices (especially polymers) make it easy to form the material into complex shapes needed for a particular part. The properties of the composite can also be customized. Obviously one could make a composite stronger or weaker by changing the number of fibers, but you can also customize the composite so that it is stronger in one direction than another.
 * Advantages of Composites**

Interesting composite activity []
 * The picture to the right shows a composite with a 3-deminsional arrangement of fibers. There is the same number of fibers in each dimension so this composite will have the same strength in each dimension. || [[image:3d_composite.jpg width="267" height="265"]][[image:file:///C:/Users/Berad/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg width="178" height="176"]] ||
 * What if someone did not need the composite to be strong in every direction? What if most of the load is in one direction so they need the composite to be very strong in that direction but it does not matter if it is strong an any other direction. In an extreme case you could align all the fibers along the one axis in which the high strength is needed as shown to the right. In this case one would be able to increase the strength in the one direction without changing the weight of the material used. || [[image:1D_composite.jpg width="200" height="499"]][[image:file:///C:/Users/Berad/AppData/Local/Temp/msohtmlclip1/01/clip_image004.jpg width="134" height="332"]] ||