*Designed Materials that have one or more properties that can be

significantly changed in a controlled fashion by external stimuli, such as

stress, temperature, moisture, pH, electric or magnetic fields.

SMART MATERIALS

3

What is a composite Material?

Two or more chemically distinct materials which when combined have improved properties over the individual materials.

Composites could be natural or synthetic.

Wood is a good example of a natural composite, combination of cellulose fiber and lignin.

The cellulose fiber provides strength and the lignin is the "glue" that bonds and stabilizes the fiber.

4

What is a composite Material?

****The Sword of Tipu Sultan

Bamboo is a very efficient wood composite structure. The components are cellulose and lignin, as in all other wood, however bamboo is hollow.

**This results in a very light yet stiff structure.

5

Composites

Composites are combinations of 2 materials in which one of the material is called the reinforcing phase, is in the form of fibers, sheets, or particles, and is embedded in the other material called the matrix phase.

Typically, reinforcing materials (strong with low densities) while the matrix is (ductile or tough material).

Reinforcement: fibers

GlassCarbonOrganicBoronCeramicMetallic

Matrix materials

PolymersMetalsCeramics

Interface

Bonding surface

Components of composite materials

• For uniaxial loading (e.g., tension in one direction only): = E

1

E

Stress

Strain,

u

y

Rupture

Young's modulus of elasticity (E) is a measure of the stiffness of the material. It is defined as the slope of the linear portion of the normal stress-strain curve of a tensile test conducted on a sample of the material.

Yield strength, y, and ultimate strength, u, are points shown on the stress-strain curve below.

Strength of Materials

Strength of Materials

• Normal stress is the state leading to expansion or contraction. The formula for computing normal stress is:

Where, (stress), P is (applied force); and A (cross-sectional area). The units of stress are N/m2 or Pascal. Tension is +ve and compression is -ve.

• Normal strain is related to the deformation of a body under stress. The normal strain, , is defined as the change in length of a line, L, over it’s original length, L.

A

P

L

L

PP

L L

A

The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of the composite will differ markedly from that of the component materials.

8

Composites – Polymer MatrixPolymer matrix composites (PMC) and fiber reinforced plastics (FRP) are referred to as Reinforced Plastics.

Common fibers used are glass (GFRP), graphite (CFRP), boron, and aramids (Kevlar).

These fibers have high specific strength (strength-to-weight ratio) and specific stiffness (stiffness-to-weight ratio)

Matrix materials are usually thermoplastics or thermosets; polyester, epoxy (80% of reinforced plastics), fluorocarbon, silicon, phenolic.

9

Composites – Polymer Matrix

Reinforcing fibers

Glass – most common and the least expensive, high strength, low stiffness and high density. GFRP consists 30-60% glass fibers by volume.

– highest specific strength, toughest fiber, undergoes plastic deformation before fracture, but absorbs moisture, and is expensive.Aramids (Kevlar)

– boron fibers consist of boron deposited on tungsten fibers, high strength and stiffness in tension and compression, resistance to high temperature, but they are heavy and expensive.

Boron

– more expensive than glass fibers, but lower density and higher stiffness with high strength. The composite is called carbon-fiber reinforced plastic (CFRP).

Graphite (99% carbon) or Carbon (80-95% carbon)

The average diameter of fibers used is usually less than .0004 inch (.01 mm). The tensile strength of a glass fiber could be as high as 650 ksi (bulk glass Su = 5-150 ksi)

10

Applications of Reinforced Plastics1920: Phenolic /asbestos fibers

(acid-resistant tank)

1940s boats were made of fiberglass. More advanced developments started in 1970s.

, recreational vehicles, bathwear, and sporting goods. In many cases, the cosmetic finish is an in-mold coating known as gel coat.

Consumer Composites (1970)

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Applications of Reinforced Plastics

The aerospace industry, including military and commercial aircraft of all types, is the major customer for advanced composites.

These materials have also been adopted for use in sporting goods, where high-performance equipment such as golf clubs, tennis rackets, fishing poles, and archery equipment, benefits from the light weight – high strength offered by advanced materials.

There are a number of exotic resins and fibers used in advanced composites, however, epoxy resin and reinforcement fiber of aramid, carbon, or graphite dominates this segment of the market.

Advanced Composites

12

Composites – Metal Matrix

The metal matrix composites offer higher modulus of elasticity, ductility, and resistance to elevated temperature than polymer matrix composites. But, they are heavier and more difficult to process.

13

Composites – Ceramic Matrix

Ceramic matrix composites (CMC) are used in applications where resistance to high temperature and corrosive environment is desired. CMCs are strong and stiff but they lack toughness (ductility)

Matrix materials are usually silicon carbide, silicon nitride and aluminum oxide, and mullite (compound of aluminum, silicon and oxygen). They retain their strength up to 3000 oF.

Fiber materials used commonly are carbon and aluminum oxide.

Applications are in jet and automobile engines, deep-see mining, cutting tools, dies and pressure vessels.

Ken Youssefi Mechanical Engineering Dept. 14

15

Application of Composites

Pedestrian bridge in Denmark, 130 feet long

(1997)

Swedish Navy, Stealth (2005)

Lance Armstrong’s 2-lb. Trek bike, 2004 Tour de France

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Advantages of Composites

Composites have an advantage over other materials because they can be molded into complex shapes at relatively low cost. This gives designers the freedom to create any shape or configuration. Boats are a good example of the success of composites.

Design flexibility

Composites products provide long-term resistance to severe chemical and temperature environments. Composites are the material of choice for outdoor exposure, chemical handling applications, and severe environment service.

Corrosion Resistance

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Advantages of Composites

One reason the composites industry has been successful is because of the low relative investment in setting-up a composites manufacturing facility. This has resulted in many creative and innovative companies in the field.

Low Relative Investment

Composite products and structures have an exceedingly long life span. Coupled with low maintenance requirements, the longevity of composites is a benefit in critical applications. In a half-century of composites development, well-designed composite structures have yet to wear out.

Durability

In 1947 the U.S. Coast Guard built a series of forty-foot patrol boats, using polyester resin and glass fiber. These boats were used until the early 1970s when they were taken out of service because the design was outdated. Extensive testing was done on the laminates after decommissioning, and it was found that only 2-3% of the original strength was lost after twenty-five years of hard service.

18

Application of Composites in Aircraft Industry

19

Disadvantages of Composites

The experience and intuition gained over the years about the behavior of metallic materials does not apply to composite materials.

properties in composites vary from point to point in the material. Most engineering structural materials are homogeneous.

Composites are heterogeneous

Composites are highly anisotropic

The strength in composites vary as the direction along which we measure changes (most engineering structural materials are isotropic). As a result, all other properties such as, stiffness, thermal expansion, thermal and electrical conductivity and creep resistance are also anisotropic. The relationship between stress and strain (force and deformation) is much more complicated than in isotropic materials.

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Disadvantages of Composites

Composites materials are difficult to inspect with conventional ultrasonic, eddy current and visual NDI methods such as radiography.

American Airlines Flight 587, broke apart over New York on Nov. 12, 2001 (265 people died). Airbus A300’s 27-foot-high tail fin tore off. Much of the tail fin, including the so-called tongues that fit in grooves on the fuselage and connect the tail to the jet, were made of a graphite composite. The plane crashed because of damage at the base of the tail that had gone undetected despite routine nondestructive testing and visual inspections. 

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Disadvantages of Composites

In November 1999, America’s Cup boat “Young America” broke in two due to debonding face/core in the sandwich structure.

A nanocomposite is as a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm)

NANOCOMPOSITES

Constituents have at least one dimension in the nanometer scale.

– Nanoparticles (Three nano-scale dimensions)

– Nanofibers (Two nano-scale dimensions)

– Nanoclays (One nano-scale dimensions)

NANOCOMPOSITES

Properties of Nanocomposites

• Tiny particels with very high aspect ratio, and hence larger surface area.

• Larger surface area enables better adhesion with the matrix/surface.

• Improvement in the mechanical performance of the parent material.

• Better transparency due to small size(>wavelength of light).

Nanoclays

• Silicates layers separated by an interlayer or gallery.

• Silicates layers are ~ 1 nm thick, 300 nm to microns laterally.

• Polymers as interlayers.

• Tailor structural, optical properties

Nanofibers/Nanotubes

• Nanotubes in metal, metal oxide and ceramic matrix have also been fabricated.

• Nanotubes in polymer matrices by mixing, then curing.

• Most important filler category in nanocpomposites

Modulus ~1000 GPa (SWCNT)~1200 GPa (MWCNT)

Tensile Strength ~ 100 GPa

ThermalConductivity

2000 W/m/K

Density 1300 –1400 kg/cm3

Length up to microns

Nano composites are found in nature also. It is found in abalone

(small or very large-sized edible sea snail) and bones.

Bio-Nanocomposite

Synthesis of Nanocomposites

• Chemical Synthesis: 1. Gas Phase Synthesis

2. Chemical Vapor Condensation

3. Combustion Flame Synthesis

4. Liquid Phase Synthesis

Gas Phase Synthesis(Synthesis of ultra pure metal powders/metal oxides(ceramics)

• The nano powder formed normally has the same composition as the starting material.

• * The starting material, which may be a metallic or inorganic material is vaporized using some source of energy

• * The metal atoms that boil off from the source quickly loose their energy.

• These clusters of atoms grow by adding atoms from the gas phase and by coalescence

• A cold finger is a cylindrical device cooled by liquid nitrogen. The nano particles collect on the cold finger

• The cluster size depends on the particle residence time and is also influenced by the gas pressure, the kind of inert gas, evaporation rate of the starting material.

• The size of the nano particle increases with increasing gas pressure, vapor pressure and mass of the inert gas used.

Gas Phase Synthesis(Synthesis of ultra pure metal powders/metal oxides(ceramics) )

Nanocomposites

Chemical Vapor Condensation • The precursor vapor is passed through a hot walled reactor.

• The precursor decomposes and nano particles nucleate in the gas phase.

• The nano particles are carried by the gas stream and collected on a cold finger.

• The size of the nano particles is determined by the particle residence time, temperature of the chamber, precursor composition and pressure.

Nanocomposites by Mechanical Alloying/Ball milling)

*Originally invented to form small-particle (oxide, carbide, etc.) dispersion-strengthened metallic alloys.

*Repeated breaking up and joining of the component particles.

By this process one can prepare highly metastable structures such as amorphous alloys and nanocomposite structures with high flexibility.

Nanocomposites by mechanical alloying

Liquid Phase Synthesis • Two chemicals are chosen such that they react to

produce the material we desire

• An emulsion is made by mixing a small volume of water in a large volume of the organic phase.

• A surfactant is added. The size of the water droplets are directly related to the ratio of water to surfactant.

• The surfactant collects at the interface between the water and the organic phase.

• If more surfactant were to be added, smaller drops would be produced and therefore, as will become apparent, smaller nano-particles.

Combustion Flame Synthesis • Energy to decompose the precursor may be supplied by burning a fuel-air

mixture with the precursor.

• In order to reduce agglomeration of the particles in the flame, the flame is specially designed to be low pressure.

• If you have observed the flame of a candle, you would have noticed that the flame consist of a blue center and a yellow to red periphery.

• This is because the temperature in the flame varies with position in the flame.

• Such a variation in the temperature profile of the flame would cause nanoparticles of different sizes to grow in the different regions of the flame.