3.1 Introduction and Background
The basic definition of a composite would be a material comprised of two elements. Composite materials comprise fibers and matrices, which form interlinking layers that create multi-layered composite. Fibers are the basic structural element of composites, they provide firmness, while the matrices keep the fibers together and has a vital role in the transfer of pressure, provides the outside structure of the composite and defines its behavior in respect to atmospheric and other elements [9].
3.1.1 Types and classification of composite materials
In most cases, fibers are made of carbon, glass, aramid (such as Kevlar) and various types of metal. Out of the entire volume of a composite, fibers tend to constitute roughly 60 to 70%, and the rest are the matrices [4]. Figure 3a shows the properties of several common composites comprised of 60% fiber volume (insert figure 3-1 from the sample text). Matrices can be polymeric, carbon, metal or ceramic based. It is important to stress that fibers, in addition to the most common configuration – long fibers – can be formed as short fibers (whiskers), spherical and similar [4]. Whiskers, for example, are short fibers, very firm and up to 10µm in girth. Another type of composite is the so called pre-impregnated composite, which consists of fibers that are woven and immersed into the matrix (usually a polymer matrix) [11].
As a rule of thumb the pre-impregnated composites are used in the production of components in autoclave manufacturing. This technology is common in the airline production industry where it is used in the manufacture of the composite elements of airplane construction. The concept of nano-composites has been classified as a completely new type of composite materials [15]. These are composed of carbon based nano-tubes in which the two dimensional basal longitudinal sides are folded in a tubular shape, and their form depends on the folding process. They can comprise one or more sides. In most cases, nano-tubes are up to 10µm long, and are extremely firm and rigid. Under laboratory conditions, nano-tubes achieved the elasticity modulus of up to 1 TPa, and firmness of 100GPa and more [10].
These figures vary significantly in real world application, mainly due to impurities of the material which accumulate during processing. The fundamental problem for the application of nano-tubes on a wider scale is their economic value. Because of the high production cost, and the inability to mass produce them, they are no widely used in construction and engineering projects [6]. However, the material shows tremendous prospect for the future and is the subject of intensive research [1]. Due to these constraints, however, nano-tubes will not be discussed in length. The figures 3b-3d show the properties of various types of composites [7]. (Insert figure 3.2, 3.3 and 3.4 ).
3.1.2 The fiber phase
The high mechanical properties of fibers are the result of strong atomic forces that prevail in materials with low atomic numbers and low density (for example Si, Al, B, C). Fibers can be constructed from these elements, their polymers or a combination of compounds with oxygen or nitrogen [2]. These materials have very little plastic flow, such as it is in metals. Instead, fracturing occurs due to the integral effect of the activity of microscopic tears, whose number reduces proportionally with the dimensions of the fiber itself. Any fiber material has very small volume per length, which means that the probability of a tear is reversely proportional to the volume of the material (according to the Weibull theory). However, these materials have a much stronger deviation of properties in comparison to values of references [14]. Figure 3e shows the impact of girth on tensile strength [9] [13]
This distortion of the mechanic properties is a significant issue, especially in calculation and production of composite constructions. The number of tears can be further reduced by the use of protective coating around the fiber. For example, the speed of molecule accumulation and the cooling speed have significant influence on the structure of fibers, where very high cooling speeds produce very intricate molecular structure that cannot be achieved with non-fiber materials. Furthermore, the procedures that include pulling or other forms of reinforcement have a positive effect on the atomic structures within fibers, mainly due to the tensile pull within the fiber itself [11] [8].
The most widespread type of fibers today are carbon fibers, especially in avionics where the mechanical pressure is the greatest. It must be pointed out that the common identification of carbon fibers with graphite fibers is wrong. Although graphite is a form of carbon, it has a different atomic structure and is used for different purposes. Its main property is that of friction reduction. This is due to the very low Van der Waals force within the 3 dimensional structure of the basal plain of the molecules [12] [4].
The first carbon fibers were made of rayon, but the process was scrapped soon after it was developed because of the low carbon yield, and inadequate mechanical properties. The majority of modern carbon fibers, especially those used in avionics and other advanced constructions are made from Polyacrylonitrile fibers (PAN) or various types of pitch through the process of carbonization. There are significant differences in these processes. For example, the use of PAN produces fibers that have a 50% share of carbon, while pitch based fibers may contain up to 80% carbon [7].
According to their mechanical properties, fibers can be classified as HM (high modulus), IM (intermediate modulus), HS (high strength) or UHM (ultra high modulus). The fibers produced from pitch tend to have higher modulus of elasticity, but are not as firm when compared to PAN fibers which are more porous, which adds to their strength. Pitch based fibers can have very high electrical conductivity and tend to be somewhat thicker than PAN fibers (5-10 µm for PAN compared to 10-11 µm for pitch) [6] [7].
Glass fibers are usually produced from Silica (Silicium dioxide - SiO2), through a variety of procedures. This type of fiber tends to have worse mechanical properties than carbon based fiber. However, they are widely used in constructions that are not as advanced, mainly because they are much cheaper to manufacture than carbon fiber. There is a variety of types, for example A, C, E, S. The rarest type is the A glass. It used to be the norm, however newer iterations have made it exceedingly rare in modern industry. The C type has increased resistance to acids. The E type is the most frequently used type, it has increased resistance to humidity and some chemical compounds. The S type is the most durable, it has increased strength and modulus elasticity. It is used in constructions which require higher resistance to stressors and high density of material [6] [7].
Polymer fiber is the only type of fiber that is completely synthetic in origin. Within this group Kevlar and polyimides are the most common type. Polyimides are very resistant, they can endure temperatures up to and exceeding 572°F. Polymer fiber are comprised of polymer chains which are elongated horizontally

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