Many of our modern technologies require materials with combinations of properties that cannot be met by conventional metals, alloys, ceramics and polymeric materials. This is especially the case of aerospace industries, underwater and transportation applications. A composite is considered to be any multiphase material that exhibit a significant proportion of properties of both constituent phases such that a better combination of properties is realized. Most composites have been created to improve combinations of mechanical characteristics such as stiffness, toughness and ambient temperature and high temperature strength.
Many composite materials are composed of just two phases one is termed the matrix, which is continuous and surrounds other phase often called dispersed phase. The properties of composites are a function of properties of constituent phases, their relative amounts and geometry of dispersed phase. Dispersed phase geometry means the shape of particles an particle size, distribution and orientation. The special property exhibited by the composite materials is that they exhibit directional properties, strongly only where required. Thus the designer can achieve optimum weight. This is also called as selective strengthening. Apart from Aircraft industry composite materials are also used in
Fibres like boron, glass or carbon has greater tensile strength, stiffness than High speed steel. To use the superior property of fibres, composite materials are made with a two phase structure.
Classification of composites:
| 1 | Particle reinforced |
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| 2 | Fibre reinforced |
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| 3 | Structural composites |
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Dispersion phase for particle reinforced composites is equiaxed. Fibre reinforced composites the dispersed phase has geometry of a fibre (Large length to diameter ratio). Structure composite are combination of composites and homogenous materials.
1. Particle reinforced composites:
Large particle and dispersion strengthened composites are classification of particle reinforced composites. Distinction between these is based upon reinforcement or strengthen mechanism. The term large is used to indicate that particle matrix interaction cannot be treated on atomic level or molecular level rather continuous mechanics is used. For most of these composites the particulate phase is harder and stiffer than matrix. These reinforcing particles tend to restrain movement of matrix phase of applied stress to particle which bear a friction of load, the degree of reinforcement or improvement of behavior depends on strong bonding at matrix particle interface E.g.. Concrete.
For dispersion strengthened composites particles are normally much smaller having diameters between 0.01 and 0.1 x 10-6 m. Particle matrix interactions that lead to strengthening occur on atomic level. The matrix bears the major portion of an applied load the small dispersed particles hinder or impede motion of dislocations. Thus plastic deformation is restricted such that yield and tensile strength as well as hardness improve E.g.. Thoria dispersed nickel.
2. Fibre-reinforced composites:
Technologically important composites are those in which dispersed phase is in the form of fibre. Design goals are to include high strength or stiffness on a weight basis. These characteristics are expressed in terms of specific strength i.e.. Ratio of modulus of elasticity to specific gravity. Fibre reinforced composites with exceptionally high specific strength and moduli having been produced that utilize low density fibres and matrix materials.
2a. Fibre phase:
On the basis of diameter and character fibres are grouped into three different classifications as
Whiskers are thin single crystals that have extremely large length to diameter ratios. As a consequence of their small size that have extremely high degree of crystalline perfection and are virtually are strongest known materials. In spite of these high strengths whiskers are not utilize extensively as a reinforcement medium because they are very expensive. Moreover it is difficult and often impractical to incorporate whiskers into matrix. Whisker material include graphite, silicon carbide, silicon nitride.
Materials that are classified as fibres are either polycrystalline or amorphous and have small diameters. Fibrous materials are generally either polymers or ceramics (Glass, carbon, Boron etc.) Fine wires have relatively large diameters. Typical material include steel, molybdenum, tungsten. Great strength of structural materials offers great strength and high load carrying capacity. Good ductility prevents sudden and catastrophic failures. However good strength and ductility are generally incompatible. Composite materials provide a balance of both properties (Strength and ductility). The stiff reinforcing fibres fibres are responsible for carrying load and ductility, toughness is offered by matrix. Fracture of fibres in brittle manner is retarded by soft matrix. Thus combination of matrix and reinforcement offers strength as well as toughness which cannot be separately attained by either components.
2b. Matrix phase:
It is the phase of fibrous composites may be a metal, polymer or ceramic. In general metals and polymers are used as matrix material because some ductility is desirable. For ceramics matrix composites the reinforcing component is added to improve fracture roughness. Matrix materials should be ductile. In addition the elastic modulus of fibres should be much higher than that of matrix. Secondly the function of matrix is to protect the individual fibres from surface damage as a result of mechanical abrasion or chemical reactions with environment. Such interactions may introduce surface flows capable of forming cracks, Which may lead to failure at low tensile stress levels. Finally the matrix separates the fibres and 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.
2c. Polymer-Matrix composites:
It consists of polymer resin as matrix with fibre as reinforcement medium. These materials are used in greatest diversity of composite application due to ease of fabrication and cost. Various classification are discussed according to the type of reinforcement.
Glass fibre-reinforced polymer composites:
It consists of glass fibres either continuous or discontinuous contained within a polymer matrix fibre diameter normally in the range between 3 and 20 x 10-6 m. Glass is used because of the following advantages.
Carbon fibre reinforced polymer composites:
Carbon is a high performance fibre material most commonly sued reinforcement in advanced polymer matrix composites. Following are the important characteristics.
Aramid fibre reinforced polymer composites:
Aramid fibres are high strength, high modulus materials. This group of materials is known as polyparaphenylene tere pthalamide (Trade name is Kevlar). There are various grades, with various mechanical properties. Even though Aramids are thermoplastics they are resistant to combustion and stable to relatively high temperatures. The temperature range over which they retain their high mechanical properties is between -200 to 200 degree centigrade. They are cheaper than carbon fibres. They are 50% cheaper than carbon fibre for the same weight, and thickness. For a heavily loaded structures a hybrid combination of Kevlar and carbon fibres are used. Boron fibre reinforced composite is used mainly in the manufacture of Helicopter blades.
Metal matrix composites:
These materials may be used at higher temperatures than their base metal counterparts. The reinforcement may improve specific stiffness, specific strength, abrasion resistance, creep superior with respect to strength, stability, better hazardous environment conditions.
Carbon-carbon composite:
One of the most advanced and promising engineering materials is carbon fibre reinforced carbon matrix composite. Both reinforcement and matrix are carbon. They are expensive and are not used extensively. Their desirable properties include high tensile moduli, tensile strength which are retained even at 2000o C. These materials have a high resistance to creep and relatively large fracture toughness values and low coefficient of thermal expansion and relatively high thermal conductivity. The biggest drawback of this material is the high temperature oxidation.
2d. Processing of fibre reinforced composites:
To fabricate continuous fibre reinforced plastics that meet design specification the fibres should be uniformly distributed within plastic matrix and in most instances all oriented in virtually same direction. Various process are
3. Structural composites:
It is normally composed of both homogenous and composite materials. The properties of which depend not only on properties of constituent materials but along on the geometrical design of various structural elements.
3a. Laminar composites:
It is made of two dimensional sheets or panels that have a preferred high strength direction such as in wood and continuous and aligned fibre reinforced plastics. Layers are stacked and subsequently cemented together such that the orientation of high strength direction varies with each successive layer laminations may also be constructed using fabric material such as cotton, paper etc. Thus a laminar composite has relatively high strength in a number of directions. However the strength in any given direction is of course lower than it would be if all fibres were oriented in that direction.
3b. Sandwich panels:
Consists of two strong outer sheets, or faces separated by a layer of less dense material or core which has lower stiffness and lower strength. The faces bear most of the in-plane loading and also any traverse bending stresses. Typical face materials include Aluminum alloys, fibre reinforced plastics, Titanium, steel core serve two functions. First it separates the faces and resists deformation perpendicular to face plane.
Secondly it provides certain degree of shear rigidity along planes which are perpendicular to faces. Various materials and structures are utilized for cores, including foamed polymers, synthetic rubber, inorganic cement. Another popular core consist of honey comb structure. Here thin foils that have been formed into interlocking hexagonal cells with axes oriented perpendicular to the face planes. Material of honey comb is made my be similar to face material.
Advantages of using composite:
Difficulties associated with composites:
The conditions for machining are different from metals. There is a limit to the temperature that can be attained while machining otherwise the curing temperature will be reached ( or exceeded ) and the materials gets destroyed. Hence maintaining dimensional accuracy is difficult. Specials tools and drills are to be used for this purpose.
Lastly updated on Sunday, December 21, 2003 , 06:13 PM