5 Composite Material

Fiber Reinforced Concrete

COMPOSITE MATERIALS

Can you think of any examples of where composites are used?

COMPOSITES2IntroductionComposite materials are engineered materials made from two or more constituent materials that remain separate and distinct while forming a single component.Generally, one material forms a continuous matrix while the other provides the reinforcement.The two materials must be chemically inert with respect to each other so no interaction occurs upon heating until one of the components melts.Examples:Plastic molding compounds containing fillers Rubber mixed with carbon blackWood (a natural composite as distinguished from a synthesized composite) Composites can be found in:-The aerospace industry Automotive parts (panels, frames, dashboards, body repairs)Cement buildings, bridges.Why Composites are ImportantComposites can be very strong and stiff, yet very light in weight, so ratios of strengthtoweight and stiffnesstoweight are several times greater than steel or aluminum Fatigue properties are generally better than for common engineering metals Toughness is often greater too. Composites can be designed that do not corrode like steel Possible to achieve combinations of properties not attainable with metals, ceramics, or polymers alone. There is unabated thirst for new materials with improved desired properties. All the desired properties are difficult to find in a single material. For example, a material which needs high fatigue life may not be cost effective. Types of reinforcementThe reinforcements in a composite material come in various forms. Fiber:Fiber is an individual filament of the material. A filament with length to diameter ratio above 1000 is called as a fiber. The fibrous form of the reinforcement is widely used. The fibers can be in the following two forms:Continuous fibres:If the fibres used in a composite are very long and unbroken or cut then it forms a continuous fibre composite. A composite, thus formed using continuous fibres is called asfibrous composite. The fibrous composite is the widely used form of composite.Short/chopped fibres: The fibres are chopped into small pieces when used in fabricating a composite. A composite with short fibres as reinforcements is called asshort fibre composite.Particulate:The reinforcement is in the form of particles which are of the order of a few microns in the diameter. The particles are generally added to increase the modulus and decrease the ductility of the matrix materials. In this case, the load is shared by both particles and matrix materials. The composite with reinforcement in particle form is called as particulate composite.Flake:Flake is a small, flat, thin piece or layer (or a chip) that is broken from a larger piece. Since these are two dimensional in geometry, they impart almost equal strength in all directions of their planes. Thus, these are very effective reinforcement components. The flakes can be packed more densely when they are laid parallel, even denser than unidirectional fibres and spheres. Whiskers:These are nearly perfect single crystal fibres. These are short, discontinuous and polygonal in cross-section.

Classification of composites based on reinforcement type

Classification of fibre composite materials Classification of fibre composite materialsThere are various reasons because of which the reinforcement is made in thin fibre form. These reasons are given below.Wires of same diameter with shorter length showed higher tensile strength than those longer lengths.

The reason for this is the fact that the number of flaws in a shorter length of wire is small as compared longer length. Further, it is well known that the strength of a bulk material is very less than the strength of the same material in wire form. The same fact has been explored in the composites with reinforcement in fibre form. As the fibres are made of thin diameter, the inherent flaws in the material decrease. Hence, the strength of the fibre increases as the fibre diameter decreases. Why is the reinforcement made in thin fibre form?

b) The quality of load transfer between fibre and matrix depends upon the surface area between fibre and matrix. If the surface area between fibre and matrix is more, better is the load transfer. It can be shown that for given volume of fibres in a composite, the surface area between fibre and matrix increases if the fibre diameter decreases. Let D be the average diameter of the fibres,L be the length of the fibres and Nbe the number of fibres for a given volume of fibres in a composite. Then the surface area available for load transfer is

The volume of these fibres in a composite is Now, let us replace the fibres with a smaller average diameter ofdsuch that the volume of the fibres is unchanged. Then the number of fibres required to maintain the same fibre volume isThe new surface area between fibre and matrix isThus, for a given volume of fibres in a composite, the area between fibre and matrix is inversely proportional to the average diameter of the fibres.

c) The fibres should be flexible so that they can be bent easily without breaking. Thisproperty of the fibres is very important for woven composites. In woven composites the flexibility of fibres plays an important role. Ultra thin composites are used in deployable structures.The flexibility is simply the inverse of the bending stiffness. From mechanics of solids study the bending stiffness is EI, whereEis Youngs modulus of the material andIis the second moment of area of the cross section of the fibre. For a cylindrical fibre, the second moment of area is

Thus, from the above equation it is clear that if a fibre is thin, that is, small in diameter it is more flexible. Thus, flexibility Contd.. These are the main load carrying constituents.The reinforcing materials have significantly higher desired properties.It transfers the strength and stiffness to the matrix material. function of a reinforcing agent The matrix material holds the fibres together.The matrix plays an important role to keep the fibres at desired positions.It transfers the load uniformly between fibers.It provides protection to fibers from environmental effects.It provides better finish to the final product.The matrix material enhances some of the properties of the resulting material and structural component (that fibre alone is not able to impart) function of a matrix material Types of fiber Fibres that are used in the fabrication of a composite can be divided into two broad categories as follows:1.Natural fibresand 2. Advanced fibresNatural fibres:-These are divided into following three sub categories.Animal fibers:silk, wool, camel hair, etc.Plant/vegetable fibers:cotton, jute, bamboo, sugarcane, banana etc.Mineral fibers: asbestos, basalt, mineral wool, glass wool.Advanced fibers: An advanced fibre is defined as a fibre which has a high specific stiffness (that is, ratio of Youngs modulus to the density of the material) and a high specific strength (that is the ratio of ultimate strength to the density of the material).Eg:-fibres made from following materials are the advanced fibres.Carbon or Graphite 2. Glass fibers3. Alumina 4. Silicon carbide Types of Matrix Materials Matrix MaterialsMetalpolymerThermosetsCarbon and GraphiteThermoplasticsCeramicAluminumTitaniumCopperSilicon carbideSilicon nitride Polymer Matrix Materials Polymers make ideal materials as they can be processed easily, possess lightweight, and desirable mechanical properties. It follows, therefore, that high temperature resins are extensively used in aeronautical applications. Two main kinds of polymers are thermoplastics and thermosets . Thermoplastic: which soften upon heating and can be reshaped with heat and pressure.Eg:- 1. Polypropylene 2. Polyvinyl chloride 3. Nylon 4. Polyurethane 5. Polyphenylene sulfide (PPS),Key features of the thermoplastic matrix materials are: 1. Higher toughness 2. High volume 3. Low cost processing 4. The use temperature range is upto 2250CThermoset which become cross linked during fabrication and does not soften upon reheating. They decompose instead of melting on hardening. Merely changing the basic composition of the resin is enough to alter the conditions suitably for curing and determine its other characteristics. They are most suited as matrix bases for advanced conditions fiber reinforced composites.The thermoset matrix materials are: 1. Polyesters 2. Epoxies 3. PolyimidesKey features of Polyesters are:Used extensively with glass fibers InexpensiveLight weightTemperature range upto 1000CResistant to environmental exposures

Contd.. Key features of Epoxy are:ExpensiveBetter moisture resistanceLower shrinkage on curingUse temperature is about 1750C

Key features of Polymide are:Higher use temperature about 3000CDifficult to fabricate

Limited temperature range.Susceptibility to environmental degradation due to moisture, radiation, atomic oxygen.Low transverse strength.High residual stress due to large mismatch in coefficients of thermal expansion between fiber and matrix.

Contd.. Problems with the use of Polymer Matrix MaterialsThermoplasticsThermosetsSoften upon heat and pressureDecompose upon heatingHence, can be repairedDifficult to repairHigh strains are required for failureLow strains are required for failureCan be re-processedCan not be re-processedIndefinite shelf lifeLimited shelf lifeShort curing cyclesLong curing cyclesNon tacky and easy to handleTacky and therefore, difficult to handleExcellent resistance to solventsFair resistance to solventsHigher processing temperature is required.Lower processing temperature is required. Comparison Between Thermoplastics and Thermosets Properties of the constituent materials.Properties of other phases present, like additives, fillers and other reaction phases.Length of the fibre.Orientation of the fibres (with respect to the loading direction).Cross sectional shape of the fibre.Distribution and arrangement of the fibres in the matrix material.Proportions of the fibre and matrix material, that is, volume fractions of the constituent materials Factors affecting composite properties(a) A hexagonal cell honeycomb core, (b) can be joined to two face sheets by means of adhesive sheets, (c) producing an exceptionally lightweight yet stiff, strong honeycomb sandwich structure.

Aramid-aluminum laminate(layers joined by adhesives)Lightning strike resistance

Fatigue resistance

Longitudinal directionTransverse directionThrough-thickness directionUnidirectional composite Fiber OrientationMaximum strength is obtained when long fibers are oriented parallel to the applied load.The effect of fiber orientation and strength can be seen in the plot

Effect of fiber orientation on the tensile strength of E-glass fiber-reinforced epoxy composites.

Contd.. 24The properties of fiber composites can be tailored to meet different loading requirements.By using combinations of different fiber orientation quasi-isotropic materials may be produced

Figure (a) shows a unidirectional arrangementFigure (b) shows a quasi-isotropic arrangement Contd.. 25A three dimensional weave is also possible.This could be found when fabrics are knitted or weaved together

Contd.. 26In a composite material with a metal matrix and ceramic fibers, the bulk of the energy would be transferred through the matrix.In a composite consisting of a polymer matrix containing metallic fibers, the energy would be transferred through the fibers.When the fibers are not continuous or unidirectional, the simple rule of mixtures may not apply.For example, in a metal fiber-polymer matrix composite, electrical conductivity would be low and would depend on the length of the fibers, the volume fraction of fibers and how often the fibers touch one another. Rule of Mixtures27FRP stands for Fiber Reinforced Plastic. FRP is used in structural shapes, repair materials or as reinforcement for concrete. Fiber reinforced composites provide improved strength, fatigue resistance, Youngs modulus and strength to weight ratio over the constituent materials.This is achieved by incorporating strong, stiff, yet brittle fibers into a more ductile matrix.Fiber supplies the strength and stiffness while the matrix binds the fibers together and provides a means of transferring the load between fibers.The matrix also provides protection for the fibers.Materials used in FRP as a fiber are-Glass, Poly-vinyl Alcohol, Carbon.Materials used in FRP as a matrix(Resin) are-Epoxy and polyester.

Fiber Reinforced Composites / FRP28Many factors must be considered when designing a fiber-reinforced composite including the length, diameter, orientation, amount and properties of the constituents, and the bonding between them.The method used to produce the final product is also very important as it indicates the type of properties just mentioned as well as the quality of the product. Fiber dimensions are characterized by their aspect ratio l/d where l is the fiber length and d is the diameter.The strength improves when the aspect ratio is large.Typical fiber diameters are from 10 mm to 150 mm. Fibers often fracture because of surface imperfections.Making the diameter small reduces its surface area, which has fewer flaws.Long fibers are preferred because the ends of the fiber carry less of the load. Thus the longer the fiber, the fewer the ends and the higher the load carrying capacity of the fibers.

Characteristics of Fiber Reinforced Composites29As can be seen from this plot, the strength of the composite increases as the fiber length increases (this is a chopped E-glass-epoxy composite)

Contd.. 30 Applications of FRPReinforcement bars for Concrete.Prestressing Tendons for Concrete Members.FRP sheets can be used to increase flexural strength in weakened or under designed members.Aerospace much of the structural weight of todays airplanes and helicopters consist of advanced FRPs.Automotive Continued use of low-carbon sheet steel in cars is evidence of its low cost and ease of processing

Advantages of FRPWill Not Corrode In Field Conditions.Lightweight.Strong in Tension.Methods of Construction Same as Steel Reinforcement. Disadvantages of FRPLow Moduli of Elasticity.Cannot be Shaped in the Field.More Expensive than Steel.Coefficients of Thermal Expansion are Different than Those of Steel or Concrete.Glass fiberis a material consisting of numerous extremely fine fibers ofglass. Individual filaments are small in diameters, isotropic and very flexible as the diameter is small.Due to the relatively inexpensive cost glass fibers are the most commonly used reinforcement.Has roughly comparable properties to other fibers such as polymers andcarbon fiber. Although not as strong or as rigid as carbon fiber, it is much cheaper and significantly less brittle.Properties of Glass Fiber are: High strength-to-weight ratio;High modulus of elasticity-to-weight ratio;Good corrosion resistance;Good insulating properties;Low thermal resistance (as compared to metals and ceramics). Glass fiber is commonly used as an insulating material. It is also used as a reinforcing agent for manypolymer products; to form a very strong and lightfiber-reinforced polymer (FRP)composite material calledglass-reinforced plastic (GRP), popularly known as "fiberglass". Glass Fibers 32Types of Glass FiberTypes of glass fibre with key featuresE glass- The most popular and inexpensive glass fibers. The designation letter E means electrical ( E-Glass is excellent insulator). Properties are high strength and high resistivity.S glass- Stronger than E-Glass fibers (the letter S means strength). High strength glass is generally known as S-type glass in the United States, R-Glass in Europe and T-Glass in Japan. Properties are high strength, modulus and stability under extreme temperature and corrosive environment. Used in military applications and in aerospace.C glass- corrosion and chemical resistant glass fibers. To protect against water erosion, a moisture-resistant coating such as a silane compound is coated onto the fibers during manufacturing. Adding resin during composite formation provides additional protection. C-Glass fibers are used for manufacturing storage tanks, pipes and other chemical resistant equipment. D glasslow dielectric constant with superior electrical properties. However, its mechanical properties are not so good as E-or S-glass. It is available in limited quantities.Carbon fiber reinforced plastic (CFRP or CRP) is a light-weight, high strength, composite material. Depending on the task, carbon fiber reinforced plastic is made with different kind of plastics and fibers.For low volume parts, it can be made by putting carbon fiber fabric and resin into a mold and allowing to harden in a vacuum. Carbon fibers have gained a lot of popularity in the last two decades due to the price reduction.Carbon fiber composites are five times stronger than steel yet five times lighter. In comparison to aluminum, carbon fiber composites are seven times stronger and two times stiffer yet still 1.5 times lighter.Initially used exclusively by the aerospace industry they are becoming more and more common in fields such as automotive, civil infrastructure, and paper production. CARBON FIBER34low density (40% lower than aluminum).high strength (as strong as high-strength steels).high stiffness (stiffer than titanium, yet much lower in density).good fatigue resistance (a virtually unlimited life under fatigue loading).low friction coefficient and good wear resistance.corrosion resistance (impervious to corrosion). Easier to mold and work.Longer lifetime.Less maintenance.Lesser material used.Reduces pollution.Reduces weight

Attractive properties of carbon fiber polymer-matrix composites Specific stiffness and specific strength:The composite materials have high specific stiffness and strengths. Thus, these material offer better properties at lesser weight as compared to conventional materials. Due to this, one gets improved performance at reduced energy consumption.Tailor able design:A large set of design parameters are available to choose from. Thus, making the design procedure more versatile. The available design parameters are:Choice of materials (fiber/matrix), volume fraction of fiber and matrix, fabrication method, layer orientation, no. of layer/lamina in a given direction, thickness of individual layers, type of layers stacking sequence.A component can be designed to have desired properties in specific directions.Fatigue Life:The composites can with stand more number of fatigue cycles than that of aluminum. The critical structural components in aircraft require high fatigue life. The use of composites in fabrication of such structural components is thus justified.

Advantages of composite material Corrosion Resistance:Polymer and ceramic matrix material used to make composites have high resistance to corrosion from moisture, chemicals.Cost Effective Fabrication:The components fabricated from composite are cost effective with automated methods like filament winding and tape laying. There is a lesser wastage of the raw materials as the product is fabricated to the final product size unlike in metals.Conductivity:The conductivity of the composites can be achieved to make it a insulator or a highly conducting material. For example, Glass/polyesters are non conducting materials. These materials can be used in space ladders, booms etc. where one needs higher dimensional stability, whereas copper matrix material gives a high thermal conductivity. Dimensional Stability:Strain due to temperature can change shape, size, increase friction, wear and thermal stresses. The dimensional stability is very important in application like space antenna. For composites, with proper design it is possible to achieve almost zero coefficient of thermal expansion.

Contd.. Some fabrics are very hard on tooling.Hidden defects are difficult to locate.Inspection may require special tools and processes.Filament-wound parts may not be repairable. Repairing may introduce new problems.High cost of raw materials.High initial cost of tooling, production set-up, etc.Labour intensive.Health and safety concerns.Training of the labour is essential.Reuse of the materials is difficult.The composites, in general, are brittle in nature and hence easily damageable.The matrix material is weak and hence the composite has low toughness.The transverse properties of lamina or laminate are, in general, weak. Disadvantages of composite material