COMPOSITES

COMPOSITES

Composites are formed from two or more types of materials.Composites are used because overall properties of the composites are superior to those of the individual components. For example: polymer/ceramic composites have a greater modules than the polymer component, but aren't as brittle as ceramics.

composite materials intro (video)

Advantages of Composites

Lower densityNon-corrosiveNon-conductiveFlexible, will not dentLow maintenanceLong lifeDesign flexibilityEasy to MachineLow cost

disAdvantages of CompositesNot often environmentally friendlyLow recyclabilityCan be damagedLow reusabilityAnisotropic Properties

Application of compositesAircraftBoats and marineSporting equipment(Golf shafts, tennis rackets, surfboards, hokey sticks, etc.)Automotive componentsWind turbine bladesBody armorBuilding materialsWater pipesBridgesTool handlesLadder rails

Composite Materials and Manufacturing (video)

Composite Material

MatrixReinforcement

Matrix ReinforcementPolymerFiberMetallic ParticalCeramics Flake

Matrix Composites

Classification of Composite Materials by MatrixPolymer Matrix Composites (PMC)Metal Matrix Composites (MMC)Ceramic Matrix Composites (CMC)

1.Polymer matrix composites or Fiber Reinforced Polymer (FRP)

Fibers generally glass, carbon or KevlarMatric can be:Thermoplastics: PE, Nylon, PS, PP, PC, PVCThermosets: Epoxy, polyester, phenolicHave high strength and stiffness to weight ratioApplication : Aerospace, sporting goods, marine

ContExamples: GFRP fiberglass (polyester or epoxy and glass), CFRP (polyester or epoxy and carbon), KFRP (polyester or epoxy and Keflar)

2.Metal Matrix Composites (MMC)Metal matrix composites (MMCs), as the name implies, have a metal matrix. Examples of matrices in such composites include aluminum, magnesium, and titanium. Typical bers include carbon and silicon carbide.Metals are mainly reinforced to increase or decrease their properties to suit the needs of design. For example, the elastic stiffness and strength of metals can be increased, and large coefcients of thermal expansion and thermal and electric conductivities of metals can be reduced, by the addition of bers such as silicon carbide.

Advantages of MMCsMetal matrix composites are mainly used to provide advantages over monolithic metals such as steel and aluminum. These advantages include higher specic strength and modulus by reinforcing low-density metals, such as aluminum and titanium. lower coefcients of thermal expansion by reinforcing with bers with low coefcients of thermal expansion, such as graphite; and maintaining properties such as strength at high temperatures.

Advantages of MMCs (cont)Advantages over polymer matrix composites. These include higher elastic properties; higher service temperature; insensitivity to moisture; higher electric and thermal conductivities; and better wear, fatigue, and aw resistances. The drawbacks of MMCs over PMCs include higher processing temperatures and higher densities.

Properties of composites

Fabrication Method of MMcFabrication methods for MMCs are varied. One method of manufacturing them is diffusion bonding (Figure 1.28), which is used in manufacturing boron/aluminum composite parts. A ber mat of boron is placed between two thin aluminum foils about 0.05 mm thick. A polymer binder or an acrylic adhesive holds the bers together in the mat. Layers of these metal foils are stacked at angles as required by the design. The laminate is rst heated in a vacuum bag to remove the binder. The laminate is then hot pressed with a temperature of about 500C and pressure of about 35 MPa in a die to form the required machine element.

Applications of MMCs

Space: The space shuttle uses boron/aluminum tubes to support its fuselage frame. In addition to decreasing the mass of the space shuttle by more than 145 kg, boron/aluminum also reduced the thermal insulation requirements because of its low thermal conductivity. The mast of the Hubble Telescope uses carbon-reinforced aluminum.

ContMilitary: Precision components of missile guidance systems demand dimensional stability that is, the geometries of the components cannot change during use. Metal matrix composites such as SiC/aluminum composites satisfy this requirement because they have high microyield strength. In addition, the volume fraction of SiC can be varied to have a coefcient of thermal expansion compatible with other parts of the system assembly.

ContTransportation: Metal matrix composites are nding use now in automotive engines that are lighter than their metal counterparts. Also, because of their high strength and low weight, metal matrix composites are the material of choice for gas turbine engines.

3. Ceramic Matrix Composites (CMC)Ceramic matrix composites (CMCs) have a ceramic matrix such as alumina calcium alumino silicate reinforced by bers such as carbon or silicon carbide.For use in high temperature and severe stress applicationExample: Automobile & aircraft gas turbine emgine.

Advantages of CMCs

High strengthHardnessHigh service temperature limits for ceramicsChemical inertnessLow density. However, ceramics by themselves have low fracture toughness. Under tensile or impact loading, they fail catastrophically.

Cont..Reinforcing ceramics with bers, such as silicon carbide or carbon, increases their fracture toughness because it causes gradual failure of the composite. This combination of a ber and ceramic matrix makes CMCs more attractive for applications in which high mechanical properties and extreme service temperatures are desired.

Mechanical properties of cmc

Manufacturing Method of CMCOne of the most common methods to manufacture ceramic matrix composites is called the hot pressing method. Glass bers in continuous tow are passed through slurry consisting of powdered matrix material, solvent such as alcohol, and an organic binder (Figure). The tow is then wound on a drum and dried to form prepreg tapes. The prepreg tapes can now be stacked to make a required laminate. Heating at about 500C burns out the binder. Hot pressing at high temperatures in excess of 1000C and pressures of 7 to 14 MPa follows this.

Reinforcement Composites

Carbon - Carbon CompositesCarboncarbon composites use carbon bers in a carbon matrix. These composites are used in very high-temperature environments of up to 3315C, and are 20 times stronger and 30% lighter than graphite bers.

Advantages of C-C CompositesCarbon is brittle and aw sensitive like ceramics. Reinforcement of a carbon matrix allows the composite to fail gradually and also gives advantages such as ability to withstand high temperatures, low creep at high temperatures, low density, good tensile and compressive strengths, high fatigue resistance, high thermal conductivity, and high coefcient of friction.

Mechanical properties of c-c Composites

Processing a cc compositeLow-pressure carbonizationA graphite cloth is taken, impregnated by resin (such as phenolic, pitch, and furfuryl ester), and laid up in layers. It is laid in a mold, cured, and trimmed. The part is then pyrolized, converting the phenolic resin to graphite.

The composite is then impregnated by furfuryl alcohol. The process drives off the resin and any volatiles. The process is repeated three or four times until the level of porosity is reduced to an acceptable level. Each time, this process increases its modulus and strength. Because carboncarbon composites oxidize at temperatures as low as 450C, an outer layer of silicon carbide may be deposited.

Applications of c-c compositesSpace shuttle nose cones: As the shuttle enters Earths atmosphere, temperatures as high as 1700C are experienced.Aircraft brakes: The carboncarbon brakes cost $970/kg, which is several times more than their metallic counterpart; however, the high durability (two to four times that of steel), high specic heat (2.5 times that of steel), low braking distances and braking times (three-quarters that of berylium), and large weight savings of up to 450 kg on a commercial aircraft such as Airbus A300-B2K and A300-B4 are attractive.

Cont..Mechanical fasteners: Fasteners needed for high temperature applications are made of carboncarbon composites because they lose little strength at high temperatures.

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