A composite material is made from the combination of two components, the matrix and fibers. The first is basically a binder for all fibers of the laminate. These are the compounds in charge of the laminate strength. They can be disposed in several ways as unidirectional, bi-directional and multi-directional. Usually fibers are bonded to the matrix in order to form a ply or a lamina. The coupling agents and lubricants, added in order to increase the ply wet-ability, are also part of the ply. Hence, a consolidated stack of many plies forms a laminate. In terms of material engineering, composite materials are based in two different parts or elements, these are the constituent and the dispersed phases. The main characteristics of these is that, they are different and recognizable. This article proposes a overview of the main types of fibers and their characteristics.

Constituent phases

The constituent phase can be composed by matrix, particles or fillers, fibers and pores. The matrix is considered a continuous phase, while the other components are considered dispersed. The pores indicates the influence of voids into the material. The idea behind their use as fillers is that, they are a possible case of a dispersed phase. They represent a lack of matrix with respect to its presence. By definition, composites are made by different constituents, thus they are heterogeneous. Regarding the spatial arrangement of constituent and dispersed phases, this characterizes the composite materials as anisotropic, because they are different. This is valid even in cases which the constituent phases are isotropic.

Dispersed phases

The dispersed phase has this definition, because it is dispersed through the matrix. It is characterized by different aspects:

• Morphology;
• Target;
• Origin of elements;
• Composition.

These are the morphology, target, origin and composition. The first defines if this phase is composed by fibers, particles and/or whiskers. The target of a dispersed phase can be functional or structural. In the first case, the element added has the objective to conceive a new functionality to the material, while in the structural target, the addition of a new component aims to improve the mechanical properties of the matrix, for instance. The origin of the elements can be natural or synthetic, in this case they are developed in laboratories in order to have a specific function or performance. Then, depending on the considerations made regarding these aspects, it is possible to develop different dispersed phase roles, which are listed below:

• Reinforcements;
• Functional;
• Cheap extender;
• Pore.

The first one, also called structure role, has the objective to improve performance, thus the mechanical properties of the matrix. It is commonly used in high-performance fibers. These are added into the matrix in order to to provide a stiffer material with respect to the original matrix. The idea behind a functional dispersed matrix, is to provide special properties and functions. For instance, matrices normally are electroconductive. Then, the functional role means special features like that and lubricants, pigment, anti-oxidant that can be added in the same way. The pigment are encapsulated in particles in order to conceive a aesthetic aspect to the fiber. In addition, it can also be used a safety device, since when cracks break those particles, those pigments give a different color from the matrix. This feature could not be useful if it is being considered carbon fibers, since they are black. However, it can be useful for glass fibers. The cheap extender is a second phase applied in order to reduce the overall cost of the laminate. It should be added with some criteria to avoid the loss of mechanical properties. Regarding pores, these could be considered defects, since solvents and water leaves a circular voids when they evaporate. However, this also represents lightweight features, because pores are a way to have a lower mass for the same volume. In addition, they have a good improvement in the acoustic insulation.

Fiber overview

The fiber is the main constituent of the reinforcement phase, it gives the strength to resist the stresses applied to the component. The main fibers used in racing car applications are the glass, the carbon and the aramid fibers. Usually, they are supplied in tows of many single fibers. Actually, an unique tow can have filaments formed by 3,000 to 24,000 fibers. The typical diameter of glass and carbon fibers vary between 10 to 12 µm and 6 to 8 µm, respectively. The filaments are not straight, actually they are twisted as can be seen in Figure 2. This helps in the fabric organization. The fiber twisting is necessary to avoid the fiber breakage during the production process. The twist types are Z and S ones, they are standard for general purposes. However, in the aerospace field, twisting should be avoided. Hence, in the description of the fibers in technical sheets there is a specific letter, that indicates the presence of twisting or not in the base material. Certainly, if the twist is considered, in the case of long fibers arranged one beneath the other or disordered short fibers, it is possible observe that twisting modifies a bit the quality of the material itself, specially tenacity (Figure 3).

This keeps the loads and prevent the fiber breakage between fibers. The more they are twisted, the worst become the mechanical properties. In the racing field though, twisting arrange is used. The focus is more on the performance, but the components and chassis have an expected life of 3-4 years. On the other hand, aircrafts need to fly for 10 years, in some cases 30. The quality requirements are completely different.

Types

Normally, the main fibers used in the racing field are the glass, the carbon and the aramid fibers. The desirable characteristics for that application are the elastic modulus, the fracture toughness and the density. Hence, to be used in a racing car chassis, the fibers must be light, stiff and tough. However, none of those types of fibers have all these characteristics together. As a result, the application requires some trade-offs. The carbon fiber is the one with the highest mechanical properties, but it is expensive with respect to the glass and aramid fibers. The glass fiber is the most used one for general industrial applications. In the racing field they are more used for body work, because these are components which are not heavily requested in terms of loads. The carbon fibers are used for the monocoque construction, suspension components and gearbox cases. The aramid fibers also exhibit lower mechanical properties, with respect to carbon ones, but they have the lowest density. Then they are the lightest fiber and the toughest one. In other words, they are quite resistant against impacts. For this reason they are commonly used as intrusion panels in monocoques. Aramids are elastic enough to absorb significant energy during impacts. Therefore, the application of those fibers depend on the requirements of the component.

Glass fibers

Glass fibers are the most common fibers for general purposes. There are glass fibers from the military field to the ordinary industrial field. The main reason for that is its cost and its reasonable mechanical properties. Actually, the glass fibers have a good tensile strength, chemical resistance and insulation properties. This justify its wide range of applications. However, when the requested performance is very high, mainly in terms of weight, it normally does not reach the requirements. The reason is that, glass fibers have a poor tensile modulus and high density with respect to carbon and aramid fibers. In addition, it is quite sensitive to abrasion when it is being handled. As a result, it is a material that tends to wear. It is a quite hard fiber, thus inapt for conditions that require high fatigue capabilities. Usually, this also wears or damages the molding die or cutting tools.

There are two basic variations of glass fibers, the S-glass and the E-glass ones. The E-glass fibers are the conventional type, which is used for the most general applications. The S-glass fibers are a class of high performance glass fibers used for military purposes. Since these are not shared between other industrial fields, it is developed a secondary S-glass fibers. These are sometimes mentioned as S-2 glass fibers. The E-glass fibers are the most affordable ones, while the S-glass fibers have the highest manufacturing cost between these two. The S-2 fibers have similar strength and modulus with respect to S-ones, but their specifications are different from S-glass fibers.

In any class, the main component of a glass fiber is the silicon oxide (SiO2). Certainly there are other compounds added in order to provide essential features to the final composition. The other ingredients are B2O3, Al2O3, Na2O and K2O. Usually, the last two are reduced in order to improve the water resistance of the fibers. Al2O3 and B2O3 are added to improve the workability and the network structure, respectively. The glass fibers are considered amorphous materials, because they are not ordered at long range. They are ordered at short range and disordered at the long one. In addition, they are defined by a specific glass transition temperature Tg.

The structure of the glass fibers are composed by silicon with four covalent bonds of oxygen atoms. Hence, the structure is a 3D one, a tetrahedron composed by these elements. The other compounds are randomly arranged. This structure can also vary in terms of organization, which can be more ordered or disordered one. The glass transition temperature Tg vary according to the structure. If the silicon is more ordered, the structure is a more crystalline ones. Therefore, the glass fiber characteristics are defined by the composition, the structure, the degree and rate of crystalization.

Manufacturing

The production of fiber glasses began in 1930s and is composed of different steps. These are the batching, the melting, the fiberization, the coating, the drying and the packing. The main point regarding the process is that, it starts from a raw powder materials, which are melted together. In this part, the process suffered a lot from contamination, since there were no standards to control it. Nowadays, the production of the glass fibers, even though it is more affordable than carbon ones, it is automated. The glass fibers melting requires powerful furnaces since glass has a very high melting point. This requests from the furnaces the capability to reach temperatures like 1000 – 2000 °C. Since glass fibers reach very high temperatures, 1000 °C, the standard tests DSC and DMA are not capable to retrieve information regarding the fiber melting. The reason is its temperature range, which beyond the one from those experiments. The problem is that, to keep furnaces for long time on the melting temperature range (≈ 1400 °C / 2552 °C) it is extremely expensive in terms of energy. Hence, the raw materials are now increasing their costs due to the energy problems.

Once the glass becomes in very fluid condition, there is a refining process, that gradually reduces the temperature in order to precipitate impurities and contaminant. After the refinement is done, it is time to realize the fibers. The fiberization is performed through a series of platinum bushings, which the material is pushed inside them while occurs a slowly cool down. Hence, it is obtained an amorphous or more crystalline silica according to the rate of cooling. Moreover, a small tension is applied at beginning of the extrusion and when tensioned by the winder. This is quite important, because in some cases it is possible to have defects due to residual tensions on the material. The objective is to avoid very abrupt failures at low values of tensions.

The next step is the coating or size effect. Even though the fibers are pretty new, they are quite fragile, easily abrade and friction with other filaments. Then, the coating is performed. It is the application of a protective layer of chemical agents that lubricates the filaments. This is performed before the filaments are wound on a drum. The agents avoid the abrasion and the static friction between the filaments and apply a binder. This last one is in the very last step before the winder. It packs the filaments in order to form a strand. The coating also prepare the fibers with coupling agents that promotes a good penetration of the fiber into the matrix. The next steps of the glass fiber production are more related to the roving, inspections and packaging.

Carbon fibers

The automotive racing market changed after an outstanding material was introduced, the carbon fiber. The range of performance of this one goes from low to high tensile modulus, 207 to 1035 GPa, respectively.

Characteristics

A general application carbon fiber exhibit low modulus, lower cost, lower weight, higher tensile strength, higher compressive strength and higher tensile strain at failure than high performance one. Malick didaticaly described the advantages and disadvantages of carbon fibers. The advantages are their very strength to weight ratio, very high tensile modulus to weight ratio, very low coefficient of linear thermal expansion, high fatigue strengths and high thermal conductivity. As can be noticed, the advantages suggest a very expensive, strong and light material. For this reason, carbon fibers are barely used in road going cars. Actually, they are only used in applications that the weight is more important than the cost. Hence, it is almost exclusively used in some high performance and racing cars. However, carbon fibers also have disadvantages. These are the low strain at failure, the low impact resistance and the high electrical conductivity. Then, carbon fibers are not good in energy absorption on impacts, which is a required characteristic for mass production cars. In the racing field, this is mitigated by using other components of the chassis to absorb energy. For this reason, the driver must be fit.

Manufacturing process and precursors

The manufacturing process of carbon fibers is not so different from glass fiber ones. The main steps are the extrusion (spinning), oxidation or thermalstabilization, carbonization and graphitization. The raw material inputed in the beginning of the process is basically from two variations, the polyacrylonitrile (PAN) or the pitch, which a petroleum residue. These are called precursors and are named by textile or pitch precursors, respectively, in some literatures. PAN is the most common carbon fiber due to its high mechanical properties, while pitch results in fragile final material (Souto, F. et al, 2018). For this reason, pitch carbon fiber are basically banned for the racing field.

Once the material is melted, it is submitted to an extrusion (spinning) at very high temperatures. There are many variations of the extrusion process, they applied accounting to the design requirements. This graphitizes the polymer and is the main point of carbon fiber manufacturing. The reason is that, depending on the process, temperature and speed of the production chain, different final materials are obtained. The carbonization and graphitization steps are determinant for the carbon fiber structure. In the first, the graphene sheets are stacked in a disordered way, thus randomly folded. In the graphitization, the graphene sheet are more ordered. The main objective of this production process is to transform graphene sheet structure into a more graphitic one.

Usually, carbonization and graphitization steps are performed in an inert atmosphere. They remove all non-organic elements from the fibers (Souto, F. et al, 2018), while defined its structure. If the carbonization was performed in a nitrogen environment at an intermediate temperature, it would be possible to reach an elastic modulus about 2000-3000 GPa. This characterizes a high stress carbon fiber, but it is not an usual case. The graphitization can reach temperatures about 2500-3000 °C, but the elastic modulus is improved at about 500-700 GPa.

Actually, the ideal structure of the carbon fiber would be composed by a simple layer of graphene. From this one, it is possible to obtained the carbon nanotubes. Hence, a structure full of graphene would be the ideal carbon fiber structure. However, this is not possible due to the extraordinary thermal and electrical properties. In addition, the oxygen stability of this material would be very poor. Hence, the possible process is a more disordered one with respect to the structure. This is the turbostratic one, which the graphene sheets are folded in a semi-disordered way. This is done on the carbonization step. After that, the material is heated to above 2000 °C, thus it is obtained a graphitic structure that enhancers the material characteristics. Usually, the result is a quite high elastic modulus, but very fragile.

An important detail regarding carbon fiber manufacturing is the temperature at which the material is heated. At the extrusion, both PAN and pitch are exposed to temperature about 200-300 °C. Actually, pitch has also a heat treatment before melt spinning process in which it reaches about 300-500 °C. However, the last processes, carbonization and graphitization occur at 1000-2000 °C and 2000 °C, respectively. Hence, this material can not be tested through DSC experiments, because its temperature range is out of one from this test. Since DSC can reach polymer composition, if it goes to very high temperature, thus the process for the graphitic carbon would start from the polymer, performing a controlled graphitization.

References

• This article was based on the lecture notes written by the author during the Composite Materials course attended at Università di Modena e Reggio Emilia;
• Mallick, P.K. Fiber-Reinforced Composite: materials, manufacturing and design. Edition 3, CRC Press, 2008;
• B. Vessal. Amorphous Solids. C.R.A. Catlow, Computer Modeling in Inorganic Crystallography, Academic Press, 1997, 295-332, ISBN 9780121641351. Available online at: https://doi.org/10.1016/B978-012164135-1/50013-7 (https://www.sciencedirect.com/science/article/pii/B9780121641351500137);
• Felipe Souto et al 2018 Mater. Res. Express 5 072001.