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Composite Materials

ISSUES TO ADDRESS...

• What are the classes and types of composites?

• Why are composites used instead of metals, ceramics, or polymers?

• How do we estimate composite stiffness & strength?

• What are some typical applications?

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Pole-vaulting

Lightweight - low densityBuckling resistance - stiffnessStrong - yield strengthMinimal twistingCost

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Boeing 757-200

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Composites

Combine materials with the objective of getting a more desirable combination of properties Ex: get flexibility & weight of a polymer plus the

strength of a ceramic

Principle of combined action Mixture gives “averaged” properties

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Terminology/Classification• Composites: -- Multiphase material w/significant proportions of each phase.

• Dispersed phase: -- Purpose: enhance matrix properties. MMC: increase sy, TS, creep resist. CMC: increase Kc

PMC: increase E, sy, TS, creep resist. -- Classification: Particle, fiber, structural

• Matrix: -- The continuous phase -- Purpose is to: - transfer stress to other phases - protect phases from environment -- Classification: MMC, CMC, PMC

metal ceramic polymer

Reprinted with permission fromD. Hull and T.W. Clyne, An Introduction to Composite Materials, 2nd ed., Cambridge University Press, New York, 1996, Fig. 3.6, p. 47.

woven fibers

cross section view

0.5mm

0.5mm

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Factors influence Composite properties

Properties of the reinforcement material

Matrix–reinforcement

bonding/adhesion

Properties of the matrix material

Mode of fabrication

Ratio of matrix to

reinforcement

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Functions of the continuous matrix phase: Bind fibers together, applied stress distributed

among the fibers Protect the fibers from being damaged Separate the fibers, inhibit crack propagation

Characteristics of fiber and matrix: Matrix phase – ductile, soft, low elastic modulus Fiber phase – stiff, strong, brittle

Strong interfacial bond between fiber-matrix: maximize stress transmittance minimize fiber pull-out and failure

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Composite Survey

L arg e-p artic le

D isp ers ion -s tren g th en ed

P artic le -re in fo rced

C on tin u ou s(a lig n ed )

A lig n ed R an d om lyorien ted

D iscon tin u ou s(sh ort)

F ib er-re in fo rced

L am in a tes S an d w ichp an e ls

S tru c tu ra l

C om p os ites

Adapted from Fig. 16.2, Callister 7e.

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Composite Survey: Particle-I

• Examples:Adapted from Fig. 10.19, Callister 7e. (Fig. 10.19 is copyright United States Steel Corporation, 1971.)

- Spheroidite steel

matrix: ferrite (a)(ductile)

particles: cementite (Fe3C)

(brittle)60 mm

Adapted from Fig. 16.4, Callister 7e. (Fig. 16.4 is courtesy Carboloy Systems, Department, General Electric Company.)

- WC/Co cemented carbide

matrix: cobalt (ductile)

particles: WC (brittle, hard)Vm:

10-15 vol%! 600 mmAdapted from Fig. 16.5, Callister 7e. (Fig. 16.5 is courtesy Goodyear Tire and Rubber Company.)

- Automobile tires

matrix: rubber (compliant)

particles: C (stiffer)

0.75 mm

Particle-reinforced Fiber-reinforced Structural

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Composite Survey: Particle-II

Concrete – gravel + sand + cement - Why sand and gravel? Sand packs into gravel voids

Particle-reinforced Fiber-reinforced Structural

• Concrete consists of an aggregate of particles that are bonded together by a cement.

• Limitations of concrete: weak and brittle material large thermal expansions - changes in temperature may crack when exposed to freeze-thaw cycles

• Three reinforcement strengthening techniques:1. reinforcement with steel wires, rods, etc2. reinforcement with fine fibers of a high modulus material3. introduction of residual compressive stresses by prestressing or

posttensioning.

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Composite Survey: Particle-III

• Elastic modulus, Ec, of composites: -- two approaches.

For equal volume of fibers:• Upper limit: isostrain conditions, higher modulus• Lower limit: isostress conditions, lower modulus

Adapted from Fig. 16.3, Callister 7e. (Fig. 16.3 is from R.H. Krock, ASTM Proc, Vol. 63, 1963.)

lower limit:1

Ec= Vm

Em+

Vp

Ep

c m m

upper limit:E = V E + VpEp

“rule of mixtures”

Particle-reinforced Fiber-reinforced Structural

Data: Cu matrix w/tungsten particles

0 20 40 60 80 100

150

200

250

300

350

vol% tungsten

E(GPa)

(Cu) (W)

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Composite Survey: Fiber-I

• Fibers very strong– Provide significant strength improvement to

material– Ex: fiber-glass

• Continuous glass filaments in a polymer matrix• Strength due to fibers• Polymer simply holds them in place

Particle-reinforced Fiber-reinforced Structural

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Why fiberglass-reinforced composites are utilized extensively?

inexpensive to producecomposites have relatively high specific

strengthschemically inert in a wide variety of

environmentsLimitations of these composites?care in handling the fibers, susceptible to

surface damagelacking in stiffness in comparison to other

fibrous compositeslimited maximum temperature use

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• Characteristics:

• an inorganic, synthetic, multifilament material• are the most common of all reinforcing fibers

for polymeric (plastic) matrix composites • strong, low in cost, nonflammable,

nonconductive (electrically), and corrosion resistant.

Glass fibers

• Glass fibers are amorphous (noncrystalline) and isotropic (equal properties in all directions) and are a long, three-dimensional network of silicon, oxygen, and other atoms arranged in a random fashion.

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Glass fibers

• low tensile modulus• relatively high specific gravity (among the

commercial fibers) • sensitivity to abrasion with handling (which

frequently decreases tensile strength)• relatively low fatigue resistance• high hardness (which causes excessive wear on

molding dies and cutting tools)

• Disadvantages:

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Glass fibers

E-glass High-strength glass

S-glass

S-2 glass

S-2 hollow glass fiber

C-glass

Categories

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• Applications:– E-glass has the lowest cost of all

commercially available reinforcing fibers, which is the reason for its widespread use in the fiber-reinforced plastics (FRP) industry.

– S-glass, originally developed for aircraft components and missile casings, has the highest tensile strength among all fibers in use. However, the compositional difference and higher manufacturing cost make it more expensive than E-glass.

Glass fibers

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Composite Survey: Fiber-II

• Fiber Materials– Whiskers - Thin single crystals - large length to diameter ratio

• graphite, SiN, SiC• high crystal perfection – extremely strong, strongest known• very expensive

Particle-reinforced Fiber-reinforced Structural

– Fibers• polycrystalline or amorphous• generally polymers or ceramics• Ex: Al2O3 , Aramid, E-glass, Boron, UHMWPE

– Wires• Metal – steel, Mo, W

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Fiber Alignment

alignedcontinuous

aligned randomdiscontinuous

Adapted from Fig. 16.8, Callister 7e.

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Composite Survey: Fiber-III

• Aligned Continuous fibers• Examples:

From W. Funk and E. Blank, “Creep deformation of Ni3Al-Mo in-situ composites", Metall. Trans. A Vol. 19(4), pp. 987-998, 1988. Used with permission.

-- Metal: g'(Ni3Al)-a(Mo) by eutectic solidification.

Particle-reinforced Fiber-reinforced Structural

matrix: a (Mo) (ductile)

fibers: g’ (Ni3Al) (brittle)

2 mm

-- Ceramic: Glass w/SiC fibers formed by glass slurry

Eglass = 76 GPa; ESiC = 400 GPa.

(a)

(b)

fracture surface

From F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, 2000. (a) Fig. 4.22, p. 145 (photo by J. Davies); (b) Fig. 11.20, p. 349 (micrograph by H.S. Kim, P.S. Rodgers, and R.D. Rawlings). Used with permission of CRCPress, Boca Raton, FL.

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Composite Survey: Fiber-IV

• Discontinuous, random 2D fibers• Example: Carbon-Carbon -- process: fiber/pitch, then burn out at up to 2500ºC. -- uses: disk brakes, gas turbine exhaust flaps, nose cones.

• Other variations: -- Discontinuous, random 3D -- Discontinuous, 1D

Adapted from F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, 2000. (a) Fig. 4.24(a), p. 151; (b) Fig. 4.24(b) p. 151. (Courtesy I.J. Davies) Reproduced with permission of CRC Press, Boca Raton, FL.

Particle-reinforced Fiber-reinforced Structural

(b)

fibers lie in plane

view onto plane

C fibers: very stiff very strong

C matrix: less stiff less strong

(a)

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Composite Survey: Fiber-V

• Critical fiber length for effective stiffening & strengthening:

• Ex: For fiberglass, fiber length > 15 mm needed

Particle-reinforced Fiber-reinforced Structural

c

f d

15length fiber

fiber diameter

shear strength offiber-matrix interface

fiber strength in tension

• Why? Longer fibers carry stress more efficiently!Shorter, thicker fiber:

c

f d

15length fiberLonger, thinner fiber:

Poorer fiber efficiency

Adapted from Fig. 16.7, Callister 7e.

c

f d

15length fiber

Better fiber efficiency

s(x) s(x)

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Composite Strength: Longitudinal Loading

Continuous fibers - Estimate fiber-reinforced composite strength for long continuous fibers in a matrix

Longitudinal deformation

c = mVm + fVf but c = m = f

volume fraction isostrain\ Ece = Em Vm + EfVf longitudinal (extensional)

modulus

mm

ff

m

f

VE

VE

F

F f = fiber

m = matrix

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Composite Strength: Transverse Loading

In transverse loading the fibers carry less of the load - isostress

c = m = f = c= mVm + fVf

f

f

m

m

ct E

V

E

V

E

1transverse modulus

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Composite Strength

• Estimate of Ec and TS for discontinuous fibers:

-- valid when

-- Elastic modulus in fiber direction:

-- TS in fiber direction:

efficiency factor:-- aligned 1D: K = 1 (aligned )-- aligned 1D: K = 0 (aligned )-- random 2D: K = 3/8 (2D isotropy)-- random 3D: K = 1/5 (3D isotropy)

(aligned 1D)

Values from Table 16.3, Callister 7e. (Source for Table 16.3 is H. Krenchel, Fibre Reinforcement, Copenhagen: Akademisk Forlag, 1964.)

c

f d

15length fiber

Particle-reinforced Fiber-reinforced Structural

(TS)c = (TS)mVm + (TS)fVf

Ec = EmVm + KEfVf

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Yarns

Chopped strands

Woven fabric

Fiber forms

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Matsbidirectional unidirectional

X-mat

Roving

Fiber forms

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Composite Production Methods-I

Pultrusion Continuous fibers pulled through resin tank,

then preforming die & oven to cure

Adapted from Fig. 16.13, Callister 7e.

Performs of desired shape,Establishes the resin/fiber ratio

Precision machine toimpart the final shape

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Pultrusion

The advantages are:Process may be automatedHigh production ratesVariety of shapes,constant cross-sections,

long pieces may be producedThe disadvantage – shapes are limited to

those having a constant cross-section.

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Composite Production Methods-II

Filament Winding Ex: pressure tanks Continuous filaments wound onto mandrel to form a hollow (usually cylindrical) shape

Adapted from Fig. 16.15, Callister 7e. [Fig. 16.15 is from N. L. Hancox, (Editor), Fibre Composite Hybrid Materials, The Macmillan Company, New York, 1981.]

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Filament Winding

The advantages are: Process may be automatedVariety of winding patterns are possibleHigh degree of control over winding

uniformity and orientation The disadvantage – variety of shapes is

limited

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Composite Survey: Structural

• Stacked and bonded fiber-reinforced sheets -- stacking sequence: e.g., 0º/90º -- benefit: balanced, in-plane stiffness

Adapted from Fig. 16.16, Callister 7e.

Particle-reinforced Fiber-reinforced Structural

• Sandwich panels -- low density, honeycomb core -- benefit: small weight, large bending stiffness

honeycombadhesive layer

face sheet

Adapted from Fig. 16.18,Callister 7e. (Fig. 16.18 isfrom Engineered MaterialsHandbook, Vol. 1, Composites, ASM International, Materials Park, OH, 1987.)

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Composite Benefits

• CMCs: Increased toughness

fiber-reinf

un-reinf

particle-reinfForce

Bend displacement

• PMCs: Increased E/r

E(GPa)

G=3E/8K=E

Density, r [mg/m3].1 .3 1 3 10 30

.01

.1

1

10102

103

metal/ metal alloys

polymers

PMCs

ceramics

Adapted from T.G. Nieh, "Creep rupture of a silicon-carbide reinforced aluminum composite", Metall. Trans. A Vol. 15(1), pp. 139-146, 1984. Used with permission.

• MMCs: Increased creep resistance

20 30 50 100 20010-10

10-8

10-6

10-4

6061 Al

6061 Al w/SiC whiskers s(MPa)

ess (s-1)

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Summary

• Composites are classified according to: -- the matrix material (CMC, MMC, PMC) -- the reinforcement geometry (particles, fibers, layers).• Composites enhance matrix properties: -- MMC: enhance sy, TS, creep performance -- CMC: enhance Kc

-- PMC: enhance E, sy, TS, creep performance• Particulate-reinforced: -- Elastic modulus can be estimated. -- Properties are isotropic.• Fiber-reinforced: -- Elastic modulus and TS can be estimated along fiber dir. -- Properties can be isotropic or anisotropic.• Structural: -- Based on build-up of sandwiches in layered form.