Composite Materials: Definition and How its Classified?

Welcome to Explore Automotive, In this blog we will see the composite materials in details like what is Composite Materials? How Composite Materials classified? And many more problems which arises in our mind regarding composite materials, here I try to solve and answer the questions in this topic.

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The development of composite materials as well as the related design and manufacturing technologies is one of the most important advances in the history of materials. 

Composites are multifunctional materials having unprecedented mechanical and physical properties which can be tailored to meet the requirements of a particular application. 

Many composites also exhibit great resistance to wear, corrosion, and high-temperature exposure. These unique characteristics which are helping to provide the mechanical engineer with the design opportunities which is not possible with the conventional monolithic (unreinforced) materials. 

Composites technology can also makes the possible use of an entire class of the solid materials, ceramics, in an application for which monolithic versions are not suitable because of their great strength scatter and poor resistance to mechanical and thermal shocks. 

Further, many manufacturing processes for composites are well adapted to the fabrication of large, complex structures, which allows consolidation of parts, reducing manufacturing costs. 

Composites are important materials which are now used widely, not only in the aerospace industry, but also in a large and increasing number of commercial mechanical engineering applications, such as internalcombustion engines; machine components; thermal management and electronic packaging; automobile, train, and aircraft structures and mechanical components, such as brakes, drive shafts, flywheels, tanks, and pressure vessels; dimensionally stable components; process industries equipment requiring resistance to high-temperature corrosion, oxidation, and wear; offshore and onshore oil exploration and production; marine structures; sports and leisure equipment; ships and boats; and biomedical devices. 

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It should be noted that biological structural materials occurring in nature are typically some type of composite. Common examples are wood, bamboo, bone, teeth, and shell. 

Further, use of an artificial composite material is not new thing. Straw-reinforced mud bricks were employed in biblical times. Using modern terminology, discussed later, this material would be classified as an organic fiber-reinforced ceramic matrix composite.

Classes and Characteristics of Composite Materials

Solid materials can be divided as into four categories given as—polymers, metals, ceramics, and carbon, which we consider as a separate class because of its unique characteristics. 

We found both the reinforcements and matrix materials in those all four categories. This gives us the ability to create a limitless number of new material systems which have unique properties that cannot be obtained with any single monolithic material. 

                        
                                    Table 1 Types of Composite Materials

Reinforcement	Matrix
	Polymer     Metal    Ceramic    Carbon
Polymer	x            x                  x                  x
Metal	x             x                  x                  x
Ceramic	x                x                      x                      x
Carbon	x                x                      x

Table 1 shows the types of material combinations which are now in use. Composite materials are usually classified by the type of material used for the matrix. 

The four primary categories of composites are polymer matrix composites (PMCs), metal matrix composites (MMCs), ceramic matrix composites (CMCs), and carbon matrix composites (CAMCs). Carbon–carbon composites (CCCs) are the most important subclass of CAMCs. 

At this time, PMCs are by far the most widely used type of composites. However, there are important applications of the other types of composites which are indicative of their great potential in mechanical engineering applications. 

Figure 1. Types of Reinforcements

In Figure 1 which showing the main types of reinforcements used in composite materials, aligned continuous fibers, discontinuous fibers, whiskers (elongated single crystals), particles, and numerous forms of fibrous architectures which are produced by textile technology, such as fabrics and braids. 

Carbon nano tubes are similar to discontinuous fibers. Two-dimensional fabrics are shown. There are also a wide range of three-dimensional woven and braided reinforcements. 

A common way to represent the fiber-reinforced composites which are able to show the fiber and matrix separated by using a slash. For example, carbon fiber-reinforced epoxy is typically written carbon/epoxy, or in abbreviated form, C/Ep. 

We represent particle reinforcements by enclosing them in parentheses followed by “p.” Using this convention, silicon carbide (SiC) particle-reinforced aluminum appears as (SiC) p/Al. 

Composites are strongly heterogeneous materials. That is, the properties of a composite material varying in a considerably manner from point to point in the material, depending on which material phase the point is located in. 

Monolithic ceramics and metallic alloys are usually considered to be isotropic materials to a first approximation, although rolled aluminum alloys have anisotropic strength properties. 

Many artificial composites, especially those which are reinforced with fibers, are anisotropic in nature, which means their properties vary with the direction while the properties of isotropic materials are the same in every direction. This is a characteristic that they shared with a widely used natural fibrous composite material, wood. 

As for wood, when structures made from artificial fibrous composites are required to carry load in more than one direction, they are used in laminated form (plywood). Particulate composites can be effectively isotropic if the reinforcements are equiaxed, that is, have roughly the same dimensions in three orthogonal directions (think of sand particles). 

Many fiber-reinforced composites, especially PMCs, MMCs, and CAMCs, do not display plastic behavior as metals do, which makes them more sensitive to stress concentrations. 

However, the absence of plastic deformation does not mean that composites are brittle materials like monolithic ceramics. The heterogeneous nature of composites results in complex failure mechanisms which impart toughness. 

Fiber-reinforced materials have been found that to produce a durable and reliable structural component in countless applications. The special characteristics of composite materials, especially anisotropy, required the use of special designing methods.

      

Types of Matrices

Generally, composite materials are typically classified on basis of matrix which is main constituent. The major composite classes are including that Organic Matrix Composites (OMCs), Metal Matrix Composites (MMCs) and Ceramic Matrix Composites (CMCs). 

The term organic matrix composite is generally assumed to include that two classes of composites, namely Polymer Matrix Composites (PMCs) and carbon matrix composites commonly referred to as carbon-carbon composites.

There are three types of matrices produced three common types of composites as follows:

1. Polymer matrix composites (PMCs), of which GRP is the best-known example, use ceramic fibers in a plastic matrix.

2. Metal-matrix composites (MMCs) mostly used silicon carbide fibers embedded in a matrix made from an alloy of aluminum and magnesium, but other matrix materials such as titanium, copper, and iron are increasingly being used. 
Specially, the applications of MMCs include bicycles, golf clubs, and missile guidance systems; an MMC made from silicon carbide fibers in a titanium matrix is currently being developed for use as the skin means its fuselage material for the US National Aerospace Plane.

3. Ceramic-matrix composites (CMCs) are the third major type of composites and examples include silicon carbide fibers fixed in a matrix made from a borosilicate glass. 

The ceramic matrix makes them particularly suitable for the purpose of use in lightweight, high-temperature components, such as parts for airplane jet engines.

Polymer Matrix Composites (PMC)/Carbon Matrix Composites/Carbon-Carbon Composites (CCC)

Polymer makes an ideal materials as they can be processed easily which possess light in weight, and desirable mechanical properties. It follows that high temperature resins are extensively used in aeronautical applications. 

There are two main kinds of polymers are thermosets and thermoplastics. Thermosets have some different qualities such as a well-bonded three-dimensional molecular structure after curing up. 

They decompose instead of melting on hardening. Simply changing the basic composition of the resin is good enough to alter the conditions suitably for curing and determine its other characteristics. 

They can be retained in a partially cured condition too over a prolonged period of time, rendering Thermosets very flexible. 

Thus, they are most suitable as matrix bases for the advanced conditions fiber reinforced composites. 

Thermosets are found that a wide ranging applications in a chopped fiber composites form particularly when a premixed or moulding compound with fibers of specific quality and the aspect ratio happens to be in starting material as in epoxy, polymer and phenolic polyamide resins etc. 

Thermoplastics have one or two-dimensional molecular structure and they tends to at an elevated temperature and show exaggerated melting point. 

Another advantage is that the process of softening at an elevated temperatures can reversed to regain its properties during cooling phase, facilitating applications of conventional compressing techniques to mould the compounds. 

Resins reinforced with thermoplastics and get comprised an emerging group of composites. The structure of most of the experiments in this area to improve the base properties of the resins and extract the greatest functional advantages from them in new avenues, including attempts to replace metals in die-casting processes. 

In a crystalline thermoplastic, the reinforcement affects the morphology to a considerable extent, prompting the reinforcement to empower the nucleation. 

Whenever, a crystalline or amorphous, these resins possess the facility to alter their creep over a considerable range of temperature. 

But this range includes that the point at which the usage of resins is constrained, and the reinforcement in such systems can increase the possibility of the failure load as well as creep resistance. Figure 2 shows kinds of thermoplastics.

                

Figure 2. Types of Thermoplastics

A small quantum of shrinkage and the tendency of the shape to retain into its original form are also to be accounted but reinforcements can change this condition too. 

The advantages of thermoplastics systems over the thermosets are that there are no chemical reactions get involved, which often result in the release of gases or heat. 

Manufacturing is limited by the time required for heating, shaping and cooling the structures.

Thermoplastics resins are sold as moulding compounds. Fiber reinforcement is apt for these resins. Since the fibers are randomly dispersed, the reinforcement will be almost isotropic. However, when subjected to moulding processes, they can be aligned directionally.

There are a few options to increase the heat resistance in thermoplastics. Addition of fillers in thermoplastics raises the heat resistance. 

But all thermoplastic composites tends lose their strength at an elevated temperatures. However, their compensating qualities like rigidity, toughness and ability to repudiate creep, place thermoplastics in the important composite materials bracket. They are used in automotive control panels/ dashboards, electronic products encasement etc.

Newer developments augur the broadening of the scope of applications of thermoplastics. In market, huge sheets of reinforced thermoplastics are now available and they only required sampling and heating to be moulded into the required shapes. This has facilitated easy fabrication of bulky components, doing away with the more cumbersome moulding compounds.

Thermosets are the most popular material of the fiber composite matrices without which, research and development in structural engineering field could get dependant. 

Mostly in aerospace components, automobile parts, defense systems etc., use a great deal of this type of fiber composites. Epoxy matrix materials are used in printed circuit boards and other similar electric and electronic field. Figure 3 shows some kinds of thermosets.

Figure 3. Types of Thermosets

Direct condensation polymerization followed by rearrangement reactions to form heterocyclic entities is the method generally used to produce thermoset resins. Water, a product for the reaction, in both the methods, hinders production of void-free composites. 

These voids have a negative effect on properties of the composites in terms of strength and dielectric properties. Polyesters phenolic and Epoxies are the two important classes of thermoset resins.

Epoxy resins are widely used in filament-wound composites and are suitable for moulding prepress. They are reasonably stable to chemical attacks and are excellent adherents having slow shrinkage during curing and no emission of volatile gases. 

These advantages, however, make the use of epoxies rather expensive. Also, they cannot be expected beyond the temperature limit of 140ºC. Their use in high technological areas where service temperatures are gets higher, as a result, is ruled out.

Polyester resins on the other hand which are quite easily accessible, cheap and find in use of a wide range of fields. 

Liquid polyesters are stored at room temperature for couple of months; sometimes for years and the addition of a catalyst can be cure the matrix material within a short period of time. They are widely used in automobiles components and structural applications.

The cured polyester is usually in a form of rigid or flexible as the case may be a transparent. Polyesters withstand against the variations of the environment and stable against chemical reactions. 

Depending upon the formulation of the resin or service requirement of the application, they can be used up to about 75ºC or higher than 75°C. Other advantages of the polyesters include that easy compatibility with few glass fibers and can be used with verified version of reinforced plastic accoutrey.

Aromatic Polyamides are the most seeking after candidates as the matrices of advanced fiber composites for the structural applications which demanding a long duration exposure for continuous service at around 200 to 250ºC.

Metal Matrix Composites (MMC)

Metal matrix composites, in today’s condition its generating a wide interest in research fraternity, are not as widely in use as their plastic counterparts. 

High strength, fracture toughness and stiffness are getting offered by the metal matrices than those which are offered by their polymer counterparts. 

They can withstand at an elevated temperature in the corrosive environment than the polymer composites. 

Most of the metals and alloys could be used as matrices and they required reinforcement materials which need to be stable over a range of temperature and non-reactive too. 

However the guiding aspect for the choice actually depends essentially on the matrix material. Light metal forms the matrix for the temperature application and the reinforcements in addition to the some reasons are characterized by high moduli.

Most of the metals and alloys make good matrices. However, practically, the choices for low temperature applications are not so many. Only light metals are responsive in nature, with their low density proving an advantage. 

Such as Titanium, Aluminium and magnesium are the popular matrix metals currently in vogue, which are particularly useful for the aircraft applications. 

If the metallic matrix materials have to offer high strength, they required high modulus reinforcements. The strength-to-weight ratios of the resulting composites can be higher than most of the alloys.

The melting point, physical and mechanical properties of the composite material at various temperatures to determine the service temperature of composites. 

Most of the metals, ceramics and compounds can be used with the matrices of low melting point alloys. The choice of reinforcements becomes more stunted with increase in the melting temperature of the matrix materials.

Ceramic Matrix Materials (CMM)

Ceramics can be described as the solid materials which exhibits very strong ionic bonding in general and in few cases covalent bonding shown. 

High melting points, good corrosion resistance and stability at an elevated temperatures and high compressive strength, rendered ceramic-based matrix materials avails for the applications which requiring a structural material that doesn’t give any other option at the temperature above 1500ºC. 

Naturally, ceramic matrices are the obvious best choice for the high temperature applications.

High modulus of elasticity and low tensile strain, which most of the ceramics posses, have combined to cause the failure of attempts to add reinforcements to obtain improvement in the strength. This is because of the stress levels at which ceramics ruptured, there is insufficient elongation of the matrix produced which keeps the composite from transferring to an effective quantum of load to the reinforcement and the composite may fail unless the percentage of the fiber volume is getting high enough. 

A material is reinforcement to utilize the higher tensile strength of the fiber, to produce an increase in the load bearing capacity of the matrix. In addition to this high-strength fiber to a weaker ceramic has not always been successful and often the resultant composite has proved to be a weaker.

Use of reinforcement with high modulus of elasticity may take care of the problem to some amount and presents pre-stressing of the fiber in the ceramic matrix is being increasingly resorted to as an option.

When ceramics having a higher thermal expansion coefficient than the reinforcement materials, the resultant composite is getting unlikely to have a superior level of strength. 

In that case, the composite material will develop the strength within the ceramic at the time of cooling resulting in microcracks extending from fiber to fiber within the matrix. Microcracking can result in a composite with lower tensile strength than that of the matrix.

Role of matrix materials

The choice of a matrix alloy for MMC is dictated by the several considerations of particular importance is whether the composite is to be continuously or discontinuously reinforced. 

The use of continuous fibers as a reinforcement may results in the transfer of most of the load to the reinforcing filaments and hence composite strength will becomes primarily by the fiber strength. 

The primary roles of the matrix alloys are to provide efficient transfer of load to the fibers and to the blunt cracks in the event that fiber failure occurs and so the matrix alloy for continuously reinforced composites may be chosen more for toughness than the strength. 

On that basis, lower strength, more ductile, and tougher matrix alloys can be utilized in continuously reinforced composites. For discontinuously reinforced composites, the matrix may covers the composite strength. 

Then, the choice of matrix will be impact by the consideration of the required composite strength and higher strength matrix alloys will be required.

Some additional considerations in the choice of the matrix includes that potential reinforcement, matrix reactions, either during processing or in service stage, which might be result in degraded composite performance; thermal stresses due to thermal expansion mismatching between the reinforcements and the matrix; and the influence of matrix fatigue behavior on the cyclic response of the composite material. 

Actually, the behavior of composites under cyclic loading conditions is an area requiring the special consideration. 

In composites, intended use at an elevated temperature, an additional consideration is the difference in melting temperatures between the matrix and the reinforcements. 

Due to large melting temperature difference may results in matrix creep while the reinforcements remain elastic and even at temperatures approaching the matrix melting point. 

Wherever, creep in both the matrix and reinforcement must be taken into consideration when there is a small melting point difference in the composite.

Functions of a Matrix

In composite material, the matrix material which plays the following functions:

1. Matrix holds the fibres together with strong bonding.

2. Matrix protects the fibres from environmental impact.

3. Matrix helping to distributes the load evenly between the fibres so that all fibres are subjected to the same amount of strain.

4. Matrix enhances the transverse properties of a laminate.

5. It improves the impact and fracture resistance of a component.

6. It helps to avoid propagation of crack growth through the fibres by providing alternate failure path along the interface between the fibres and the matrix.

7. It carries inter laminar shear.

8. Matrix plays a minor role in the tensile load-bearing capacity of a composite structure. 

However, the selection of a matrix has a major influence on the inter-laminar shear as well as in-plane shear properties of the composite material. Inter-laminar shear strength is an important for the design consideration point of view specially for the structures under the bending loads, whereas in-plane shear strength is important under torsion loads. 

The matrix provides lateral support against the possibility of fibre buckling under the compression loading, thus influencing to some extent the compressive strength of the composite material. 

The interaction between fibres and matrix is also important in designing the damage tolerant structures. Finally, the processability and defects in a composite material depends strongly on the physical and thermal characteristics, such as viscosity, melting point, and curing temperature of the matrix.

Advantages and Limitations of Composites Materials

Advantages of Composites

List of the advantages exhibited by the composite materials, which are of significant use in aerospace industry are as follows:

1. Composite materials have high resistance to fatigue and corrosion degradation.

2. Composite materials having high ‘strength or stiffness to weight’ ratio. As enumerated above, weight savings are significant ranging from 25-45% of the weight of conventional metallic designs which are to be made.

3. Due to greater reliability of composites, there are fewer inspections and structural repairs occurred.

4. Composite’s directional tailoring capabilities to meet the design requirements. The fibre pattern can be formed in such a manner that will customize the structure to efficiently bear the applied loads.

5. Composite material has fibre to fibre redundant load path.

6. It improved dent resistance is normally achieved. Composite panels do not sustain the damage as easily as thin gauge sheet metals.

7. For composite material, it is easier to achieve smooth aerodynamic profiles for drag reduction. Complex double-curvature parts with a smooth surface finish can be made only in one manufacturing operation.

8. Composite materials offer an improved torsional stiffness. This implies that high whirling speeds, reduced number of intermediate bearings and supporting to the structural elements. The overall part count and manufacturing & assembly costs are thus gets reduced.

9. Composites have high resistance to impact damage.

10. Polymer composite materials such as thermoplastics having a rapid process cycle, which making them attractive for high volume commercial applications that traditionally have been the domain of sheet metals. Moreover, thermoplastics can also be reformed.

11. As comparing with metals, thermoplastics have indefinite shelf life.

12. Composite materials are dimensionally stable that is they have low thermal conductivity and low coefficient of thermal expansion. Composite materials can be customizing to comply with the broad range of thermal expansion design requirements and to minimize the thermal stresses.

13. For composite material, manufacture and assembly are simplified because of its part integration such as joint, fastener reduction because of that reducing the cost.

14. Improved weatherability of composites in a marine application environment as well as their corrosion resistance property and durability reduced at the down time for maintenance.

15. In composites, close tolerances can be achieved without machining.

16. Wastage of material is reduced while production because composite parts and structures are frequently built to shape rather than machined to the required configuration, as is common with metals.

17. Due to excellent heat sink properties of composites, especially Carbon-Carbon, combined with their lightweight has extended their use for aircraft brakes.

18. Composites have improved friction and wear properties.

19. Composites have ability to tailor the basic material properties of a Laminate has allowed new approaches to the design of aeroelastic flight structures.

Advantages which are given above translate not only use into airplane, but also into common implements and equipment such as a graphite racquet that has inherent damping, and causes less fatigue and pain to the user of it.

Limitations of Composites

Some of the associated disadvantages of advanced composites are as follows:

1. For composites, it has high cost of raw materials and fabrication.

2. Composite materials are more brittle than the wrought metals and thus they are more easily damaged.

3. Transverse properties of composite materials may be weak.

4. Composite Matrix is weak in strength, therefore, low toughness.

5. Composite materials reuse and disposal may be difficult and difficult to attach.

Repair introduces new problems, for the following reasons:

1. Composite Materials required a refrigerated transport and storage because it has limited shelf life.

2. In some composite materials hot curing is necessary in many cases which required special tooling. And hot or cold curing takes time.

3. Analysis of composite material is difficult.

4. Composite materials matrix is subjected to environmental degradation.

However, proper design and material selection can affect many of the above disadvantages.

New technology is to be provided a variety of reinforcing fibres and matrices those can be combined with to form the composites which having a wide range of exceptional properties. 

Since, the advanced composite materials are capable to provide the structural efficiency at lower weights as compared to the equivalent metallic structures; they have been emerged as the primary materials for the future use.

In aircraft application, advanced fibre reinforced composites are now being used in many structural applications, for example, floor beams, engine cowlings, flight control surfaces, landing gear doors, wing-to-body fairings, etc., and also major load carrying structures including the vertical and horizontal stabilizer main torque boxes.

Composite materials are also taken into consideration for use in improvements into civil infrastructures, for examples, earthquake proof highway supports; power generating wind mills, long span bridges, etc.

Comparison with Metals

Requirements commanding the choice of materials which is apply to both metals and reinforced plastics.

Composites offer significant weight saving over existing metals. Composites can provide structures which are 25% to 45% lighter than the conventional aluminium structures which designed to meet the same functional requirements. This is due to the lower density of the composites.

Depending on material form, composite densities range from 1260 to 1820 kg/in³ (0.045 to 0.065 lb/in³) as compared to 2800 kg/in³ (0.10 lb/in³) for aluminum. Some other applications to be required thicker composite sections to meet the strength/stiffness requirements, however, weight savings will still result.

Unidirectional fibre composites have specific tensile strength (ratio of material strength to density) about 4 to 6 times greater than that of steel and aluminium.

Unidirectional composite materials have specific –modulus that is the ratio of the material stiffness to density of about 3 to 5 times greater than that of steel and aluminium.

Fatigue endurance limit of composite materials can approach about 60% of their ultimate tensile strength. For steel and aluminium, this value is lower while taken into consideration.

Fibre composites are more versatile in nature than the metal, and it can be customized so as to meet the performance needs and complex design requirements such as aero-elastic loading on the wings and the vertical & horizontal stabilizers of aircraft.

Fibre reinforced composites can be designed with excellent structural damping features. While comparing with metals, they are less noisy and provide lower vibration transmission than metals.

High corrosion resistance of fibre composite materials mainly contributes to reduce the life- cycle cost.

Composite materials offer reduced manufacturing cost principally by significantly reducing the number of detailed parts and expensive technical joints which required to form the large metal structural components. 

Alternatively we can say that, composite parts can eliminate joints or fasteners thereby providing parts simplification and integrated design.

Long term service experience environment and durability behaviour of composite material is limited in comparison with the metals.