Composites

A composite is considered to be any multi-phase material that exhibits a significant proportion of the properties of both constituent phases such that a better combination of properties is realised.

According to this principle of combined action, better property combinations are fashioned by the judicious combination of two or more distinct materials.
There are also a number of composites that occur in nature. For example:
- - - 1. wood consists of strong and flexible cellulose fibres surrounded and held together by a stiffer material called lignin.
      2. bone is a composite of the strong yet soft protein collagen and the hard, brittle mineral apatite.

The constituent phases of a composite must be chemically dissimilar and separated by a distinct interface.
Most composites have been created to improve combinations of mechanical characteristics such as stiffness, toughness, and ambient and high-temperature strength.
Many composite materials are composed of just two phases; one is termed the matrix, which is continuous and surrounds the other phase, often called the dispersed phase.
The properties of composites are a function of the properties of the constituent phases, their relative amounts, and the geometry of the dispersed phase.

The three main divisions of composite materials are

1.1. particle-reinforced

2.2. fibre-reinforce

3.3. structural composites

The dispersed phase for particle-reinforced composites is equi-axed (i.e., particle dimensions are approximately the same in all directions); for fibre-reinforced composites, the dispersed phase has the geometry of a fibre (i.e., a large length-to-diameter ratio). Structural composites are combinations of composites and homogeneous materials.

Fig. Classification of Composite material

Particle-Reinforced Composites:

Large-particle and dispersion-strengthened composites are the two sub classifications of particle-reinforced composites. The distinction between these is based upon reinforcement or strengthening mechanism.The term “large” is used to indicate that particle–matrix interactions cannot be treated on the atomic or molecular level; rather, continuum mechanics is used. For most of these composites, the particulate phase is harder and stiffer than the matrix.
These reinforcing particles tend to restrain movement of the matrix phase in the vicinity of each particle. The matrix transfers some of the applied stress to the particles, which bears a fraction of the load.
The degree of reinforcement or improvement of mechanical behaviour depends on strong bonding at the matrix–particle interface. For dispersion-strengthened composites, particles are normally much smaller, with diameters between 0.01 and 0.1 m (10 and 100 nm). Particle–matrix interactions that lead to strengthening occur on the atomic or molecular level.
The mechanism of strengthening is similar to that for precipitation hardening.Whereas the matrix bears the major portion of an applied load, the small dispersed particles hinder or impede the motion of dislocations. Thus, plastic deformation is restricted such that yield and tensile strengths, as well as hardness, improve.

LARGE–PARTICLE COMPOSITES:

Particles can have quite a variety of geometries, but they should be of approximately the same dimension in all directions (equi-axed). For effective reinforcement, the particles should be small and evenly distributed throughout the matrix. The volume fraction of the two phases influences the behaviour; mechanical properties are enhanced with increasing particulate content.
Two mathematical expressions have been formulated for the dependence of the elastic modulus on the volume fraction of the constituent phases for a two-phase composite. These rule of mixtures equations predict that the elastic modulus should fall between an upper bound represented by

E_c(u) = EmVm + EpVp

and a lower bound, or limit,

where E and V denote the elastic modulus and volume fraction, respectively, whereas the subscripts c, m, and p represent composite, matrix, and particulate phases. Large-particle composites are utilised with all three material types (metals, polymers, and ceramics).

DISPERSION-STRENGTHENED COMPOSITES:

Metals and metal alloys may be strengthened and hardened by the uniform dispersion of several volume percent of fine particles of a very hard and inert material.The dispersed phase may be metallic or non-metallic; oxide materials are often used.
The strengthening mechanism involves interactions between the particles and dislocations within the matrix, as with precipitation hardening. Examples:

1. 1. The high-temperature strength of nickel alloys may be enhanced significantly by the addition of about 3 vol% of thoria (ThO₂) as finely dispersed particles; this material is known as thoria-dispersed (or TD) nickel.
  2. A very thin and adherent alumina coating is caused to form on the surface of extremely small (0.1 to 0.2 m thick) flakes of Aluminium, which are dispersed within an Aluminium metal matrix; this material is termed sintered Aluminium powder (SAP).

Fibre-Reinforced Composites:

Design goals of fibre-reinforced composites often include high strength and/or stiffness on a weight basis.
These characteristics are expressed in terms of specific strength and specific modulus parameters, which correspond, respectively, to the ratios of tensile strength to specific gravity and modulus of elasticity to specific gravity.
Fibre-reinforced composites with exceptionally high specific strengths and module have been produced that utilise low-density fibre and matrix materials.

INFLUENCE OF FIBRE LENGTH

The mechanical characteristics of a fibre-reinforced composite depend on the properties of the fibre, the degree to which an applied load is transmitted to the fibres by the matrix phase.
Load transmittance is the magnitude of the inter-facial bond between the fibre and matrix phases.
Under an applied stress, this fibre–matrix bond ceases at the fibre ends, yielding a matrix deformation pattern.
There is no load transmittance from the matrix at each fibre extremity. Some critical fibre length is necessary for effective strengthening and stiffening of the composite material.
This critical length L_c is dependent on the fibre diameter d and its ultimate (or tensile) strength (sigma) and on the fibre–matrix bond strength (Tau) (or the shear yield strength of the matrix, whichever is smaller) according to

For a number of glass and carbon fibre–matrix combinations, this critical length is on the order of 1 mm, which ranges between 20 and 150 times the fibre diameter. The maximum fibre load is achieved only at the axial centre of the fibre. As fibre length l_c increases, the fibre reinforcement becomes more effective.

Fig. The deformation pattern in the matrix surrounding a fibre that is subjected to an applied tensile load.

Fibres for which L> Lc (normally L>>15Lc) are termed continuous; discontinuous or short fibres have lengths shorter than this.
For discontinuous fibres of lengths significantly less than the matrix deforms around the fibre such that there is virtually no stress transference and little reinforcement by the fibre.
To affect a significant improvement in strength of the composite, the fibres must be continuous.

INFLUENCE OF FIBRE ORIENTATION AND CONCENTRATION

The arrangement or orientation of the fibres relative to one another, the fibre concentration, and the distribution all have a significant influence on the strength and other properties of fibre-reinforced composites.
With respect to orientation, two extremes are possible:

(1) a parallel alignment of the longitudinal axis of the fibres in a single direction

(2) a totally random alignment.

Continuous fibres are normally aligned, whereas discontinuous fibres may be aligned, randomly oriented, or partially oriented.Better overall composite properties are realised when the fibre distribution is uniform.

Elastic Behaviour—Longitudinal Loading

Expression for the modulus of elasticity of a continuous and aligned fibrous composite in the direction of alignment (or longitudinal direction), as

since the composite consists of only matrix and fibre phases; that is Vm + Vf =1. Thus, is equal to the volume-fraction weighted average of the moduli of elasticity of the fibre and matrix phases. Other properties, including density, also have this dependence on volume fractions.

It can also be derived ,for longitudinal loading,that the ratio of the load carried by the fibres to that carried by the matrix is

Elastic Behavior—Transverse Loading

E_ct is the modulus of elasticity in the transverse direction.

THE FIBRE PHASE:

On the basis of diameter and character, fibres are grouped into three different classifications: whiskers, fibres, and wires.
Whiskers are very thin single crystals that have extremely large length-to-diameter ratios.
As a consequence of their small size, they have a high degree of crystalline perfection and are virtually flaw free, which accounts for their exceptionally high strengths; they are among the strongest known materials.
In spite of these high strengths, whiskers are not utilised extensively as a reinforcement medium because they are extremely expensive.
It is difficult and often impractical to incorporate whiskers into a matrix. Whisker materials include graphite, silicon carbide, silicon nitride, and aluminium oxide.
Materials that are classified as fibres are either polycrystalline or amorphous and have small diameters; fibrous materials are generally either polymers or ceramics (e.g., the polymer aramids, glass, carbon, boron, aluminium oxide, and silicon carbide).
Fine wires have relatively large diameters; typical materials include steel, molybdenum, and tungsten.
Wires are utilised as a radial steel reinforcement in automobile tires, in filament-wound rocket casings, and in wire-wound high-pressure hoses.

THE MATRIX PHASE:

The matrix phase of fibrous composites may be a metal, polymer, or ceramic.
Commonly, metals and polymers are used as matrix materials because some ductility is desirable; for ceramic-matrix composites, the reinforcing component is added to improve fracture toughness.
For fibre-reinforced composites, the matrix phase serves several functions.
First, it binds the fibres together and acts as the medium by which an externally applied stress is transmitted and distributed to the fibres; only a very small proportion of an applied load is sustained by the matrix phase.
The matrix material should be ductile.
The elastic modulus of the fibre should be much higher than that of the matrix.
The second function of the matrix is to protect the individual fibres from surface damage as a result of mechanical abrasion or chemical reactions with the environment.
Such interactions may introduce surface flaws capable of forming cracks, which may lead to failure at low tensile stress levels.
Finally, the matrix separates the fibres and, by virtue of its relative softness and plasticity, prevents the propagation of brittle cracks from fibre to fibre, which could result in catastrophic failure; in other words, the matrix phase serves as a barrier to crack propagation.
Even though some of the individual fibres fail, total composite fracture will not occur until large numbers of adjacent fibres, once having failed, form a cluster of critical size. It is essential that adhesive bonding forces between fibre and matrix be high to minimise fibre pull-out.
Bonding strength is an important consideration in the choice of the matrix–fibre combination.
The ultimate strength of the composite depends to a large degree on the magnitude of this bond; adequate bonding is essential to maximise the stress transmittance from the weak matrix to the strong fibres.

POLYMER-MATRIX COMPOSITES:

Polymer-matrix composites (PMCs) consist of a polymer resin as the matrix, with fibres as the reinforcement medium.
These materials are used in the greatest diversity of composite applications, as well as in the largest quantities, in light of their room-temperature properties, ease of fabrication, and cost.

Examples:

Glass Fibre-Reinforced Polymer (GFRP) Composites: Fibreglass is simply a composite consisting of glass fibres, either continuous or discontinuous, contained within a polymer matrix. fibre diameters normally range between 3 and 20 m.

¨ In spite of having high strengths, they are not very stiff and do not display the rigidity that is necessary for some applications (e.g., as structural members for airplanes and bridges).

Most fibreglass materials are limited to service temperatures below at higher temperatures, most polymers begin to flow or to deteriorate.
Service temperatures may be extended to approximately by using high-purity fused silica for the fibres and high-temperature polymers such as the polyimide resins.
Applications: automotive and marine bodies, plastic pipes, storage containers, and industrial floorings.
The transportation industries are utilising increasing amounts of glass fibre-reinforced plastics in an effort to decrease vehicle weight and boost fuel efficiencies.
A host of new applications are being used or currently investigated by the automotive industry.

Carbon Fibre-Reinforced Polymer (CFRP) Composites:

Carbon is a high-performance fibre material that is the most commonly used reinforcement in advanced (i.e.,non-fibreglass) polymer-matrix composites.
Carbon fibres are not totally crystalline, but are composed of both graphitic and noncrystalline regions; these areas of non-crystallinity are devoid of the three-dimensional ordered arrangement of hexagonal carbon networks that is characteristic of graphite.
Three different organic precursor materials are used: rayon, polyacrylonitrile (PAN), and pitch.
Carbon fibres are normally coated with a protective epoxy size that also improves adhesion with the polymer matrix.
Applications: Carbon-reinforced polymer composites are currently being utilised extensively in sports and recreational equipment (fishing rods, golf clubs), filament-wound rocket motor cases, pressure vessels, and aircraft structural components—both military and commercial,fixed wing and helicopters (e.g.,as wing,body,stabiliser,and rudder components).

Aramid Fibre-Reinforced Polymer Composites: They are especially desirable for their outstanding strength-to-weight ratios, which are superior to metals.

Chemically, this group of materials is known as poly(paraphenylene terephthalamide). There are a number of aramid materials; trade names for two of the most common are Kevlar™ and Nomex™.
This material is known for its toughness, impact resistance, and resistance to creep and fatigue failure.
Even though the aramids are thermoplastics, they are, nevertheless, resistant to combustion and stable to relatively high temperatures; the temperature range over which they retain their high mechanical properties is between -200°C and 200°C.
Chemically, they are susceptible to degradation by strong acids and bases, but they are relatively inert in other solvents and chemicals.
The aramid fibres are most often used in composites having polymer matrices; common matrix materials are the epoxies and polyesters.
Since the fibres are relatively flexible and somewhat ductile, they may be processed by most common textile operations.
Applications: ballistic products (bulletproof vests and armor), sporting goods, tires, ropes, missile cases, pressure vessels, and as a replacement for asbestos in automotive brake and clutch linings, and gaskets.

Other Fibre Reinforcement Materials:

Glass, carbon, and the aramids are the most common fibre reinforcements incorporated in polymer matrices.
Boron fibre-reinforced polymer composites have been used in military aircraft components, helicopter rotor blades, and some sporting goods.
Silicon carbide and aluminium oxide fibres are utilised in tennis rackets, circuit boards, military armor, and rocket nose cones.

POLYMER MATRIX MATERIALS :

The matrix often determines the maximum service temperature, since it normally softens, melts, or degrades at a much lower temperature than the fibre reinforcement.
The most widely utilised and least expensive polymer resins are the polyesters and vinyl esters; these matrix materials are used primarily for glass fibre-reinforced composites.
The epoxies are more expensive and,I n addition to commercial applications, are also utilised extensively in PMCs for aerospace applications; they have better mechanical properties and resistance to moisture than the polyesters and vinyl resins.
For high-temperature applications, polyimide resins are employed; their continuous-use, upper-temperature limit is approximately 230°C.

Finally, high-temperature thermoplastic resins offer the potential to be used in future aerospace applications; such materials include polyetheretherketone (PEEK), poly(phenylene sulfide) (PPS), and polyetherimide (PEI).

METAL-MATRIX COMPOSITES:

Metal-matrix composites (MMCs) the matrix is a ductile metal.
These materials may be utilised at higher service temperatures than their base metal counterparts; the reinforcement may improve specific stiffness, specific strength, abrasion resistance, creep resistance, thermal conductivity, and dimensional stability.
Some of the advantages of these materials over the polymer-matrix composites include higher operating temperatures, non-flammability, and greater resistance to degradation by organic fluids.
Metal-matrix composites are much more expensive than PMCs and, therefore, their (MMC) use is somewhat restricted.
The superalloys, as well as alloys of aluminium, magnesium, titanium, and copper, are employed as matrix materials.
The reinforcement may be in the form of particulates, both continuous and discontinuous fibres, and whiskers; concentrations normally range between 10 and 60 vol%.
Continuous fibre materials include carbon, silicon carbide, boron, aluminium oxide, and the refractory metals.
Discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibres of aluminium oxide and carbon, and particulates of silicon carbide and aluminium oxide.
Some matrix–reinforcement combinations are highly reactive at elevated temperatures.
Composite degradation may be caused by high-temperature processing or by subjecting the MMC to elevated temperatures during service.
This problem is commonly resolved either by applying a protective surface coating to the reinforcement or by modifying the matrix alloy composition.
Normally the processing of MMCs involves at least two steps: consolidation or synthesis (i.e., introduction of reinforcement into the matrix), followed by a shaping operation.
A host of consolidation techniques are available, some of which are relatively sophisticated; discontinuous fibre MMCs are amenable to shaping by standard metal-forming operations (e.g., forging, extrusion, rolling).
Applications: Automobile manufacturers have recently begun to use MMCs in their products. For example, some engine components have been introduced consisting of an aluminium-alloy matrix that is reinforced with aluminium oxide and carbon fibres; this MMC is light in weight and resists wear and thermal distortion.
Metal-matrix composites are also employed in driveshafts (that have higher rotational speeds and reduced vibrational noise levels), extruded stabiliser bars, and forged suspension and transmission components.
The aerospace industry also uses MMCs. Structural applications include advanced aluminium alloy metal-matrix composites; boron fibres are used as the reinforcement for the Space Shuttle Orbiter, and continuous graphite fibres for the Hubble Telescope.
The high-temperature creep and rupture properties of some of the superalloys (Ni- and Co-based alloys) may be enhanced by fibre reinforcement using refractory metals such as tungsten.
Excellent high-temperature oxidation resistance and impact strength are also maintained. Designs incorporating these composites permit higher operating temperatures and better efficiencies for turbine engines.

CERAMIC-MATRIX COMPOSITES:

Ceramic materials are inherently resilient to oxidation and deterioration at elevated temperatures.

The fracture toughnesses of ceramics can be improved significantly by the development of a new generation of ceramic-matrix composites (CMCs)— particulates, fibres, or whiskers of one ceramic material that have been embedded into a matrix of another ceramic.
This improvement in the fracture properties results from interactions between advancing cracks and dispersed phase particles.
Crack initiation normally occurs with the matrix phase, whereas crack propagation is impeded or hindered by the particles, fibres, or whiskers.
Ceramic-matrix composites may be fabricated using hot pressing ,hot isostatic pressing, and liquid phase sintering techniques.

Applications: SiC whisker-reinforced aluminas are being utilised as cutting tool inserts for machining hard metal alloys; tool lives for these materials are greater than for cemented carbides.

CARBON–CARBON COMPOSITES:

Both reinforcement and matrix are carbon.
Their desirable properties include high-tensile moduli and tensile strengths that are retained to temperatures in excess of 2000°C, resistance to creep, and relatively large fracture toughness values.
Carbon–carbon composites have low coefficients of thermal expansion and relatively high thermal conductivities; these characteristics, coupled with high strengths give rise to a relatively low susceptibility to thermal shock.
Their major drawback is a propensity to high temperature oxidation.
Applications: The carbon–carbon composites are employed in rocket motors, as friction materials in aircraft and high-performance automobiles, for hot-pressing moulds in components for advanced turbine engines, and as ablative shields for re-entry vehicles.

HYBRID COMPOSITES:

Hybrid composites are obtained by using two or more different kinds of fibres in a single matrix.
Hybrids have a better combination of properties than composites containing only a single fibre type.
A variety of fibre combinations and matrix materials are used but in the most common system, both carbon and glass fibres are incorporated into a polymeric resin.
The carbon fibres are strong and relatively stiff and provide a low-density reinforcement; however,they are expensive.
Glass fibres are inexpensive and lack the stiffness of carbon. The glass–carbon hybrid is stronger and tougher, has a higher impact resistance, and may be produced at a lower cost than either of the comparable all-carbon or all-glass reinforced plastics.
There are a number of ways in which the two different fibres may be combined, which will ultimately affect the overall properties.
For example, the fibres may all be aligned and intimately mixed with one another; or laminations may be constructed consisting of layers each of which consists of a single fibre type, alternating one with another.
All hybrids the properties are anisotropic.

Applications: lightweight land, water, and air transport structural components, sporting goods, and lightweight orthopedic components.

STRUCTURAL COMPOSITES:

A structural composite is normally composed of both homogeneous and composite materials, the properties of which depend not only on the properties of the constituent materials but also on the geometrical design of the various structural elements.
Laminar composites and sandwich panels are two of the most common structural composites.

LAMINAR COMPOSITES:

A laminar composite is composed of two-dimensional sheets or panels that have a preferred high-strength direction such as is found in wood and continuous and aligned fibre-reinforced plastics. The layers are stacked and subsequently cemented together such that the orientation of the high-strength direction varies with each successive layer. For example, adjacent wood sheets in plywood are aligned with the grain direction at right angles to each other.

Laminations may also be constructed using fabric material such as cotton, paper, or woven glass fibres embedded in a plastic matrix.

Thus a laminar composite has relatively high strength in a number of directions in the two-dimensional plane; however, the strength in any given direction is lower than it would be if all the fibres were oriented in that direction.

Fig. The stacking of successive oriented, fibre-reinforced layers for a laminar composite

SANDWICH PANELS :

Sandwich panels, considered to be a class of structural composites, are designed to be light-weight beams or panels having relatively high stiffnesses and strengths.

A sandwich panel consists of two outer sheets, or faces, that are separated by and adhesively bonded to a thicker core.
The outer sheets are made of a relatively stiff and strong material typically aluminium alloys, fibre-reinforced plastics, titanium, steel, or plywood; they impart high stiffness and strength to the structure, and must be thick enough to withstand tensile and compressive stresses that result from loading.
The core material is light weight, and normally has a low modulus of elasticity. Core materials typically fall within three categories: rigid polymeric foams (i.e.,phenolics,epoxy,polyurethanes),wood (i.e.,balsa wood),and honeycombs.
Structurally,the core serves several functions.

1. It provides continuous support for the faces.
2. It must have sufficient shear strength to withstand transverse shear stresses, and also be thick enough to provide high shear stiffness (to resist buckling of the panel). (It should be noted that tensile and compressive stresses on the core are much lower than on the faces.)

Core consists of a “honeycomb”structure—thin foils that have been formed into interlocking hexagonal cells, with axes oriented perpendicular to the face planes.
The honeycomb material is normally either an aluminium alloy or aramid polymer.
Strength and stiffness of honeycomb structures depend on cell size, cell wall thickness, and the material from which the honeycomb is made.
Sandwich panels are used in a wide variety of applications including roofs, floors, and walls of buildings; and, in aerospace and aircraft (i.e., for wings, fuselage, and tailplane skins).