Composite material is a material system composed of two or more dissimilar constituents, differing in forms, insoluble in each other, physically distinct and chemically inhomogeneous. The resulting product possesses properties much different from the properties of constituent materials.
Classification of Composite Materials
Composite materials, also referred as composites, are broadly classified as
- Agglomerated composite materials,
- Laminated composite materials, and
- Reinforced composite materials.
Lamina and Laminate: Laminated materials also referred as laminates are layered composites made-up of many laminae. A lamina also known as a ply or a layer is very thin, about 0.1 mm to 1 mm thick. A single lamina is unsuitable for any purposeful application. They are, therefore, joined or glued together to form a laminate of desired thickness. Thus a laminate is made up of an arbitrary number of lamina.
Number of lamina, in a laminate, can be few to many tens. A few examples of common laminates are the following.
- Metal to metal laminate viz. cladded metals
- Sheet moulding compounds (SMC)
- Bulk moulding compounds (BMC)
- Linoleum etc..
Now we shall discuss main among them, one by one.
Laminate and delamination: A laminate is a stack of lamina with various orientations of the directions of the principal materials in the lamina. Laminates can be built up with plates or plies of different materials or of the same material, such as glass fibers.
Shear stresses are always present between the layers of a laminate because each layer tends to deform independently of its neighboring layers due to each layer having different properties. These shear stresses, including the transverse normal stresses, are a cause of delamination.
Bulk molding compounds (BMCs) are a premixed material of short fibers (chopped-glass strands) pre-impregnated with resin and various additives. A dough molding compound (DMC) is an alternative term for a BMC. Some thermoset resins are quite thick and are called moulding doughs. Parts made by BMCs are limited to about 400 mm in their longest dimension due to problems with separation of the components of moulding compound during molding.
Sheet moulding compounds (SMC) are nonmetallic plastic-rein-forced-composite laminates made-up by pressing together many unidirectional (U/D) laminae one over the other. Laminate of desired properties may be prepared by placing U/D laminae in different orientations. Generally a prepreg is used for that.
Prepreg SMC is an intermediate compound between raw material and the final product. Its pot life is 3 to 4 days, and cannot be used after that. SMC is a blend of resin, hardener, fibers, catalyst and accelerator. Its curing time can be further decreased by adding more than specified quantity of hardener.
Reinforced Composite Materials
Reinforced composites are made-up of two basic constituents viz.
- Matrix or body constituent, and
- Reinforcing constituent.
The matrix constituent comprises of the following materials.
- Elastomers, and
Commonly used reinforcing agents are the following.
- Metals such as steel in cement concrete.
- Organic fibers such as carbon, graphite, kevlar etc.
- Inorganic fibers such as glass, ceramic etc.
Classification of Reinforced Composite Materials
Based on various considerations, the reinforced composites are classified as follows.
Based on the nature:
- Organic Composites e.g. polymeric based
- Inorganic Composites e.g. reinforced cement concrete (RCC)
Based on the matrix material used:
- Polymer Matrix Composites (PMC)
- Metal Matrix Composites (MMC)
- Ceramic Matrix Composites (CMC)
- Cement Matrix Composites.
Based on the type of reinforcing material used:
- Particulate Composites
- Fiber Reinforced Composites (FRC)
- Flake Reinforced Composites or Flake Composites
- Whisker Reinforced Composites
- Hybrid Composites
- Sandwich Composites.
Based on the existence of fiber:
- Natural Fibers Composites
- Synthetic Fibers Composites
Based on the aspect ratio of fiber:
- Continuous Fiber Composites
- Short Fiber Composites
Based on specialized material system:
- Fiber-Fiber Composites
- Toughened Composites, such as rubber-toughened-plastics
- Fiber Reinforced Glass (FRG)
- Glass Ceramic Matrix (GCM) Composites
- Polymer Concretes (PCs)
- Fiber Reinforced Concrete (FRC)
- Polymer Cement Concrete (PCC)
- Asbestos Reinforced Plastics (ARP)
- Nylon Reinforced Elastomers (NRE)
- Fiber Reinforced Metals (FRM)
- Wood-Plastics Composite
Based on the arrangement of fiber lay:
- Unidirectional (U/D) Composites
- Bidirectional or Cross-plied Composites
- Angle-plied Composites
- Off-axis Composites
- Randomly Oriented Composites
We shall now discuss some main composites out of the above classification.
Particulate composites have one or two dimensional macroscopic additive constituents randomly embedded in the matrix. The size, nature and function of these particles vary widely. The particles are 1µm or more in size and having volume concentrations of 20 to 40 percent.
Small particles of uniform size with proper orientation exhibit a more strengthening effect. Elastic modulus of a particulate composite may be obtained by simple rule of mixture.
Applications: Particulate composites are made by sintering which is a powder metallurgy technique. The W-Ni-Fe and W-Ni-Cu systems are the examples of particulate composites. Cermets described below are similar to particulate composites.
Cermets: Cermet is made of cer and met. Here ‘cer’ stands for ceramic and `met’ for metal. Cermet is a combination of ceramic and metal. A cermet consists of ceramic matrix and the reinforcing metal particles. They are generally made by powder metallurgy technique.
We know from that the refractories (a type of ceramics) are high temperature resisting, brittle materials. Contrary to this, metals are ductile and have melting points varying from low to very high. They also have higher mechanical strengths.
A composition of ceramics and metals in different proportions gives desired thermal and mechanical properties in cermets. They are very good wear and abrasion resistant materials, and possess high hardness.
Applications: Some notable applications of cermets are the following.
- Rotary drills in mining industries.
- Cutting tools in metal cutting industries.
- Shaping tools for refractories.
- Cemented carbide in high speed cutting.
- Components in satellites and space-going vehicles.
These are composites of two-dimensional nature, and are preferred when planer isotropy is also desired in components of structures and machines. It should be noted that the composites generally possess orthotropy; or anisotropy and not isotropy. Moreover, flakes of two-dimensional geometry can be more closely packed than the fibers.
These qualities make them suitable for various applications. Mica flakes-glass matrix composites are easily machinable, and are used in heat and electrical insulating applications. Silver flakes are used where good conductivity is desired.
Whisker Reinforced Composites
Whiskers are a form of materials possessing extremely high strength and moduli. Strength and moduli of whiskers are much superior than the bulk and fiber form of the same material.
Hence their reinforcement in composites will impart too higher strengths and moduli. Whisker reinforced composites are in the initial stage of development. It is a likely material to be used in the near future.
Fiber composites are made up of only a single type of fiber such as glass, carbon or kevlar etc. Each type of fiber has its own limitations in terms of strength, cost and other material properties. These limitations can be eliminated by using a combination of two or more types of fibers in the same matrix.
Mixing of two or more different-types of continuous fibers in the same matrix is called hybridization and the resulting product is called hybrid composite. Improvement in properties of such composites is due to the hybrid effect.
Characteristics: Above Figure shows an idealized stress-strain characteristic curve of two fiber system. The initial slope OA gives initial modulus and slope BC gives the final modulus. The fall AB in the curve is due to failure of first fiber system at strain LA.
The second fiber system continues to take the load until final fracture occurs at strain Ec. In a typical case, the first and second fiber systems may be those of glass and carbon.
Types of Hybrid Composites: Hybrid composites are subdivided into following four types:
- Interply hybrid composites
- Intraply hybrid composites
- Inter-Intraply composites
- Super hybrid.
The interply hybrid composite consists of alternate lamina (layer or ply) of the same matrix but different fibers. Intraply hybrid composite contains each lamina having two or more kinds of fibers system. The inter-intraply composite is a combination of the above two types.
Applications: Practical utilization of hybrid composites have been widely made. Out of these some important applications are given below.
- Antenna dishes of CFRP (carbon fiber reinforced plastics) and aluminum honeycomb.
- CFRP and CRP leaf springs and drive shafts for automobiles.
- Busbars of aluminum, reinforced by CFRP.
- Squash racquets and golf clubs with shafts of CFRP/GRP/wood hybrid.
- Helicopter rotors and thin-walled tubes of CFRP and GRP.
- Coil springs of glass, aramid and carbon fiber hybrid.
- Artificial limbs and external bracing systems having CFRP base.
A sandwich composite is constructed by sandwiching foam core between two skins of FRP laminates as shown in Figure given below. The thickness of the skin is ts kept up to 3 mm, and the thickness of core tc, is kept deeper.
The core is either foamed or made of honeycomb material so that its density is very less. As tc is deep (tc>> ts), the area moment of inertia of the cross-section is enhanced too much; due to which the flexural rigidity of sandwich beam becomes more. This higher flexural rigidity construction along with very lightweight makes the sandwich composite most suitable as a beam.
Honeycomb Materials: Sandwich constructions are widely employed as beam component of structures in airplanes, spacecrafts, satellites etc. A non-sandwiched, FRP composite beam will be much heavier than a sandwiched beam of same dimensions.
Advantages and of Composites
- They posses combination of excellent mechanical, chemical, structural, electrical, optical and other desired properties.
- They are lightweight materials possessing higher specific strength and specific modulus than the conventional materials.
- Power by weight ratio in aeroplanes is approximately 5 with the conventional materials while it is about 16 with composites. This will require prime mover of reduced power resulting in fuel economy, or more pay-load carrying capacity. Alternately it helps in weight reduction.
- Composites can be moulded to any shape and size and according to any desired specification.
- They possess excellent anti-chemical and anti-corrosion properties.
- Making, repairing and fabricating of composites are easier than the metals and RCC.
- Assembling and de-assembling of components is easy and quick. Efficient utilization of material may be done. The fibers may be oriented in such a way so as to provide greatest strength and stiffness in the desired direction.
- Seepage and weathering problems are negligible.
- Composites may be designed to obtain aesthetic appearance.
Disadvantages of Composites
- They have low flash and fire points.
- They may develop undesired biological effects seen in polymers.
- Polymeric composites are not suitable for high temperature applications.
- Cost of composites is still higher than many conventional materials.
- On prolonged exposure to sunlight, the colors of composites generally fade-out.
Types of Fibers
The fibers can be classified as under:
- Natural and synthetic fibers.
- Organic and inorganic fibers.
- Continuous and short fibers.
Natural fibers are obtained from natural sources such as plants, animals and minerals. For example, fibers of jute, cotton and silk.
Synthetic fibers are produced in industries. They are cheaper and more uniform in cross section than the natural fibers. For example, fibers of glass, boron and carbon.
Organic fibers such as carbon and graphite fibers are light in weight, flexible, elastic, and heat senstive. Commericial carbon fibers are available by the trade names such as Hyfil, Grafil, PAN etc.
Inorganic fibers, have high strength, low fatigue resistance and good heat resistance. Their examples are glass, tungsten, ceramic.
Continuous and short fibers: The strength of composite increases when it is made of long continuous fibers. A smaller diameter of fibers also enhances the overall strength of composite. Fibers of various cross-sections such as square, rectangular, circular, hollow circular, hexagonal, irregular cross-section are employed in composite constructions.
Advanced fibers and composites: Composites using kevlar, graphite and boron fibers are termed as advanced composites.
Aspect Ratio of Fibers
Composites may be called as paniculate composite, continuous fiber composite, or short fiber composite depending on the aspect ratio (l/d) given below.
- l/d = 1 for particulate composite.
- l/d = 10 to 100 for short fiber composite.
- l/d = infinity for continuous fiber composite
Where l is the length and d the diameter, or shorter dimension in a non-circular section fiber. Filament wound continuous fibers are used for fabrication of bodies of revolution.
Glass fibers are made by molten ‘glass drop’ through minute orifices and then lengthening them by air jet. The standard glass fiber used in glass-reinforced composite materials is E-glass, a borosilicate type of glass.
The glass fibers produced, with diameters from 5 to 25 µm, are formed into strands having a tensile strength of 5 GPa. Chopped glass used as a filler material in polymeric resins for moulding consists of glass fibers chopped into very short lengths.
E-glass is the first glass developed for use as continuous fibers. It is composed of 55% silica, 20% calcium oxide, 15% aluminum oxide, and 10% boron oxide. It is the standard grade of glass used in fiberglass and has a tensile strength of about 3.5 GPa and high resistivity, fiber diameters range between 3 and 20 urn.
S-glass was developed for high-tensile-strength applications in the aerospace industry. It is about one-third stronger than E-glass and is composed of 65% silicon dioxide, 25% aluminum oxide, and 10% magnesium oxide.
Types of glass fibers: Glass fibers classified as A, E, S etc. have particular fields of applications. These are
- A — glass fiber for acid resistance.
- C— glass fiber for improved acid resistance.
- D— glass fiber for electronic applications.
- E— glass fiber for electrical insulation.
- S— glass fiber for high strength.
Boron fibers are composites of the substrates tungsten, silica coated with graphite, or carbon filaments upon which boron is deposited by a vapor-deposition process (CVD).
The final boron fiber has specific gravity of about 2.6, a diameter between 0.01 and 0.15 mm, a tensile strength of about 3.5 GPa, and a tensile modulus of around 415 GPa.
Boron is more expensive than graphite and requires expensive equipment to place the fibers in a resin matrix with a high degree of precision,
Carbon and Graphite Fibers
Carbon is a nonmetallic element. Black crystalline carbon, known as graphite, has a specific gravity of 2.25. The terms carbon and graphite are often used interchangeably. However, a line of demax cation has been established between them in terms of modulus and carbon content.
Carbon fiber usually has a modulus of less than 345 GPa and a carbon content between 80% and 95%. Graphite fibers have a modulus of over 345 GPa and a carbon content of 99% or greater.
Another distinguishing feature is the pyrolyzing temperatures. For carbon, this temperature is around 1315°C, and for graphite it is around 1900° to 2480°C. Pyrolysis is the thermal decomposition of a polymer.
Kevlar, an organic fiber is an aramid or aromatic polyamide fiber. This organic fiber is melt-spun from a liquid polymer solution. The aromatic ring structure results in high thermal stability. The rod-like nature of the molecules classifies Kevlar as a liquid-crystalline polymer characterized by its ability to form ordered domains in which the stiff, rodlike molecules line up in parallel arrays.
There are three grades of Kevlar. These are:
- Kevlar 29
- Kevlar 49, and
- Kevlar 149
Kevlar 29 provides high toughness with a tensile strength of about 3.4 GPa for use where resistance to stretch and penetration are important. Kevlar 49 has a high-tensile-strength modulus of 130 GPa and is used with structural composites. Kevlar 149 has an ultrahigh-tensile-strength modulus of 180 GPa.
The development of both continuous and discontinuous ceramic fibers based on oxide, carbide, and nitride compositions was undertaken due to the need for high-temperature reinforcing fibers in composites for the aerospace industry. Most oxide fibers are compositions of Al203, and SiO2, although a few are almost pure oxide of aluminum and silicon (alumina and silica). The average properties of continuous oxide fibers are as follows:
- Density 3 g/cm3
- Tensile strength 2 GPa
- Useable temperature 1300°C
- Diameter 12µm
- Tensile modulus 20 Gpa
High Performance Polymeric Fibers
Included in the high-performance category are fibers based on polyester, nylon, aramid, and polyolefm. Fibers such as aramid (kevlar and nomex), polybenzimidazole (PBI), sulfar, and spectra have increased the range of choices for materials engineers in designing materials with tailor-made properties.
However, most do not possess the thermal properties found in ceramic and metal based fibers.
Polymeric fibers are long chains of molecules aligned in longitudinal direction. This imparts directional properties to the fiber. The elastic modulus and strength are much higher in the longitudinal direction than in the other transverse directions.
These fibers may be further sub-grouped as follows.
- Natural fibers such as wool, cellulose, cotton, silk etc.
- Synthetic fibers such as nylon, terylene (dacron), rayon, orlon, kevlar etc.
Cellulose fibers are flexible and strong in tension. Cotton clothes shrink due to the presence of wrinkled molecules. Sanforized cotton clothes are non-shrinking as long chains are made to align in such clothes.
Polyester fibers: Now-a-days, synthetic fibers find too much use due to their durability, dimensional stability and uniformity etc. Synthetic polymeric fibers belong to the family of both, the thermosets and the thermoplastics. Polyester fibers (a thermoset) such as terylene, is characterized by
linkage. Oxygen provides flexibility to polyester. It, therefore, becomes soft more easily with increasing temperature.
Nylon fibers: Contrary to this, nylon 66 (a thermoplast) which is a polyamide fiber does not soften easily with increasing temperatures. The polyamide fibers are characterized by
Configurations of Reinforcing Fibers
The fibers alone are not suitable for any kind of reinforcement. They are likely to be subjected to twisting and abrasion, resulting in wearing out of their reinforcements. Moreover, because of their negligible weight and micro-dimensions, getting a desired reinforcement pattern may be a problem. Hence fibers are transformed to other forms for reinforcement. Usually the transformed forms are Strands, and Yarns.
Forms of end products: The strands and yarns are then converted into following forms of end products.
- Tapes and cloth
- Fabrics: Plain weave form,Tnidirectional form, square weave form, twill weave form, twill weave form. The satin weave may be of (a) 4 Harness type, and (b) 8 harness type.
- Rovings: filament winding, preform, weaving grade, translucent sheeting, chopping grade.
- Woven roving fabris
- Chopped strands
- Mats: Continuous strand type or swirl mat, weave mat, surfacing mat, needled chopped strand mat.
- A complex form: These may have arrangements which is a combination of two or more of the above forms.
Rovings are available in densities of 280 gm/m2, 360 gm/m2 and 600 gm/m2 and widths of 1 m and 1.5 m. Mats have densities ranging from 300 gm/m2 to 450 gm/m2.
Forms of Fibers
Strands: In order to minimize self-abrasion and mechanical damage to the fibers, they are collected and size (starch) is applied. Then sized fibers are binded together to form a strand. Thus a strand is combination of large number of fibers which can be termed as unified multi-fibers.
Yarn: Yarn is produced from sized cake by first twisting-the strand then followed by plying a number of twisted strands together to form a doubled balm-feed yarn. It is designated by its ‘Count’ which is expressed in the unit of ‘Tex’. One Tex is a measure of linear density, and is defined as weight in gms per km. Yarns are used for manufacturing woven fabrics.
Mylar: A Form of Flake
Mylar, a polyester, is used as thin flake (sheet of 25 to 150 µm thickness) in magnets and as an insulating material. Aluminum diboride is a new fascinating material for planer reinforcement in flaked composites. Planer stiffness of its flake measures 265 GPa which is four times higher than the cross-plied planer reinforced graphite fibers.
The flakes have excellent damage tolerance, are easy to process, and holes can be drilled into them for attachments. However, they suffer from limited stretchability under application of pressure.
Generalized Hooke’s Law
Generalized Hooke’s law is different from Hooke’s law. Isotropic materials obey Hooke’s law whereas non-isotropic materials like composites follow Generalized Hooke’s law. Generalized Hooke’s law can be expressed mathematically as under:
σL = A11 εL + A12 εT + A13 εT’ + A14 γLT’ + A15 γTT’ + A16 γLT
σT = A21 εL + A22 εT + A23 εT’ + A24 γLT’ + A25 γTT’ + A26 γLT
σT’ = A31 εL + A32 εT + A33 εT’ + A44 γLT’ + A55 γTT’ + A66 γLT
τ LT’ = A41 εL + A42 εT + A43 εT’ + A44 γLT’ + A45 γTT’ + A46 γLT
τ TT’ = A51 εL + A52 εT + A53 εT’ + A54 γLT’ + A55 γTT’ + A56 γLT
τ LT = A61 εL + A62 εT + A63 εT’ + A64 γLT’ + A65 γTT’ + A66 γLT
Where coefficients A11, A12,…….A23,….A44,….A55,….A66 are elastic constants. Some ot these elastaic constants are Youngs moduli, some are shear moduli, many are Poisson’s ratios and other are coupling constants.
Different Moduli and Coupling Coefficients: Considering first row and first column, we can see that A11 is Young’s modulus in longitudinal direction as σL/εL = A11 is Young’s modulus for uniaxial case.
Considering fourth row and fourth column, we notice that A44 is shear modulus for LT’ plane because τLT/γLT = A44 which is shear modulus in uniaxial case.
Now consider third row and sixth column which shows that a shear strain γLT is produced in LT’ plane on application of direct tensile stress σT’ in T’-direction. Here effect of direction T’ has reached to plane LT due to coupling effect. Thus A36 is a coupling coefficient.
Major and Minor Poisson’s Ratio: From first row and second column, we observe that a longitudinal stress σL causes a transverse strain εT. Similarly, first column of second row reveals that stress in transverse direction σr produces a longitudinal strain εL. Thus A12 and A21 are Poisson’s ratios.
Of these A12 is called major Poisson’s ratio and A21 is known as minor Poisson’s ratio. Numerical value of major Poisson’s ratio may be less than that of minor Poisson’s ratio.
Number of Dependent and Independent Elastic Constants: Number of elastic constants in a highly anisotropic composite may be 81 which reduces to 54 due to symmetry of strains, and reduces further to 36 on considering symmetry of stresses. Details of these reductions are omitted.
The summary details of non-zero elastic constants and independent elastic constants for different class of materials are illustrated in Table given below. Many non-zero elastic constants bear relation among themselves, that is why the number of independent elastic constants are less than the non-zero constants.
|Types of composite material||Number of non-zero elastic constants||Number of independent elastic constants|
|Three dimensional case|
|Two dimensional case|
Applications of Composite Materials
Now-a-days, composites are used almost everywhere. Some important fields of applications are space vehicles and satellites, aircraft, rockets, automobiles, pressure vessels and heat exchanger, sports, music and amusements, building constructions, machine components, electronic and computer components etc.
Some notable applications of composite materials are given below.
- The roof of Montreal Olympic stadium was built with kevlar fabric reinforced composite.
- The air-conditioned, Nagar Mahapalika’s underground market of London is fully made of composites.
- Boeing 747 contains 929 m2 surface area made of composites.
- Fighter aeroplane F-18 has about 10% structure made of graphite-epoxy (Gr-Ep) composite that results in a weight saving of about 25%.
- Moon landing mission Apollo 15,16 and 17 had tubular drill-bore stem made of boron-epoxy (B-Ep) composite.
- Turbine and engine shafts, discs and bearings are made of carbon fiber composites.
- Satellites employ composites to work in the temperature range of —160°C to +95°C.
- Voyager spacecraft used 3.7 m diameter Gr-Ep flight antenna.
- Commercial acoustic guitars are made of graphite-epoxy composites.
- Natural gas vehicle fuel cylinder and fiber reinforced aluminum pistons are used in Toyota cars.
- Printed circuit boards, switch-gears, body of computers etc. are made of composites.
- G-10 is a laminated thermosetting glass fiber composite used in magnets and cryostats as a high strength material where metals are unsuitable.
- Carbon-fiber reinforced composites are used in biomedical applications such as total hip replacement and fracture fixation.
- Electro-rheological fluids like zeolite in silicone oil, starch in corn oil are used to fill-in the graphite-epoxy cantilever beam. They give damping and variable stiffness to beams.
Mechanical Properties of Composites
Composites possess the nature of orthotropy and anisotropy. They can be homogeneous or heterogeneous, and obey generalized Hooke’s law. The stress-strain behavior of various composites and of the fibers impregnated in them is discussed here. These are shown in Figures given below.
The tensile stress-strain curve of various fibers in longitudinal direction, shown in Figure (a), is linear. High modulus (HM) graphite fiber is stiffest among them. Figure (b) depicts tensile longitudinal behavior of U/D boron-epoxy and graphite-epoxy composites. Based on the volume fraction of fiber in the matrix of linear or nonlinear nature, the curves may assume the shapes as shown in Figures (e) and (d). The stress-strain curve of a laminate is shown in Figure (e) in which the successive failures of different plies are marked by 1, 2, 3,… etc.
Composite Manufacturing Methods
Polymeric composites can be manufactured by various methods. These methods are different for thermoset composites and thermoplast composites. These are listed below.
Manufacturing processes for thermoset composites
- Prepreg lay-up process or autoclave processing or vacuum bagging process
- Hand lay-up (or wet lay-up) process
- Spray-up process
- Filament winding process
- Pultrusion process
- Resin transfer moulding process
- Structural reaction injection moulding (SRIM) process
- Compression moulding process
- Roll wrapping process
- Injection moulding process
Manufacturing processes for thermoplastic composites
- Thermoplastic tape winding process
- Thermoplastic pultrusion process
- Compression moulding of glass mat thermoplastic (GMT)
- Hot press technique
- Autoclave processing
- Diaphragm forming process
- Injection moulding process.
For more information, you may visit Wikipedia.