fiber and composite notez

7/27/2019 Fiber and Composite Notez

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Questions

1. What are the two phase types for composite materials?1. What are the two phase types for composite materials?

2. Which factor may2. Which factor mayNOTNOTinfluence properties of composites?influence properties of composites?A. shape of dispersed phase B. amount of dispersed phase

C. distribution of dispersed phase D. color of disp. phase

3. What are the strengthening mechanism for large3. What are the strengthening mechanism for large--particleparticle

composites and dispersion strengthened composites?composites and dispersion strengthened composites?

Large-particle composites:

larger than atomic/molecular level

Dispersion strengthened composite:

on atomic/molecular level


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2

Continuous & aligned fiber composites

Stress-strain behavior for longitudinal loading

Schematic stressstrain curve for

an aligned fiber

reinforced composite that is exposed to a uniaxialstress applied in the direction of alignment

Stage I:

Both fibers and matrix deformelastically, so the stress-strain

curve is linear.

Stage II:The fibers continue to stretch

elastically, the matrix yields and

deforms plastically.

The onset of composite failure

begins as the fibers start to

fracture, which corresponds to a

strain off


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Elastic behavior for longitudinal loadingc = mVm + fVf and c = m = f

Volume fraction

Ecl = EmVm + EfVf Ecl = longitudinal modulus

c : composite

f : fiberm : matrix

mm

ff

m

f

VE

VE

F

F

Ratio of the load carried by fibers to that by matrix:


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In transverse loading the fibers carry less of the load

c= mVm + fVf and c = m = f =

f

f

m

m

ct EV

EV

E1

Ect = transverse modulus

c = composite

f = fiberm = matrix

isostress

Ect

EmEf

VmEf VfEm


Elastic behavior for transverse loading


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Example Problem 1


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Example Problem 2

Compute the elastic modulus of the composite material

described in Example Problem 1, but assume that the stress is

applied perpendicular to the direction of fiber alignment.


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Longitudinal Tensile Strength

fffmcl VV*'* )1(

Longitudinal strength of the

composite

: stress in the matrix at fiber

failure;

: fiber tensile strength

'

m

*

f


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Transverse Tensile StrengthThe strengths of continuous and aligned fibrous

composites are highly anisotropic.

Along the longitudinal direction, have high TS.

Along the transverse direction, have low TS.


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Discontinuous & aligned fiber composites

Even though reinforcement efficiency is

lower for discontinuous fibers, discontinuous

and aligned fiber composites are becoming

increasingly more important in the

commercial market.

Chopped glass fibers are used most

extensively; however, carbon and aramid

discontinuous fibers are also employed.

These short fiber composites can be

produced hav ing modul i o f e last ic ity

and tensile strengths that approach 90% and

50%, respectively, of their continuous

fiber counterparts.


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Discontinuous & Random fiber composites

efficiency factor:

-- aligned: K = 1 (aligned parallel)

-- aligned: K = 0 (aligned perpendicular)

-- random 2D: K = 3/8 (2D isotropy)

-- random 3D: K = 1/5 (3D isotropy)

Ecd = EmVm + KEfVf


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16.13 Assume that the composite described in Problem 16.8

has a cross-sectional area of 480 mm2 (0.75 in.2) and is

subjected to a longitudinal load of 53,400 N (12,000 lbf).

(a) Calculate the fibermatrix load ratio.

(b) Calculate the actual loads carried by both fiber and matrix

phases.

(c) Compute the magnitude of the stress on each of the fiberand matrix phases.

(d) What strain is experienced by the composite?

Example Problem 3


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Fiber Types (diameter, character)

Whiskers - thin single crystals - large length to diameter

ratios graphite, silicon nitride, silicon carbide

high crystal perfection extremely strong, strongestknown

very expensive and difficult to disperse

Fibers (small diam.)

polycrystalline or amorphous

generally polymers or ceramics Ex: alumina, aramid(), glass, carbon

Wires (large diam.)

metals steel, molybdenum, tungsten

The fiber phase


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The matrix phase

Matrix phase:

The matrix phase of fibrous composites may be a metal,

polymer, or ceramic. In general, metals and polymers are

used as matrix materials because some ductility is desirable;for ceramic-matrix composites, the reinforcing component is

added to improve fracture toughness.

-- Functions are to:- transfer stress to dispersed phase

- protect dispersed phase from environment

- separate fibers and prevent crack propagation

-- Types:

Metal-matrix

Ceramic-matrix

Polymer-matrix


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Bonding strengthBonding strength between fiber and matrix is an

important consideration in the choice of the matrix

fiber combination.

The ultimate strength of the composite depends to a large

degree on the magnitude of this bond; adequate bonding isessential to maximize the stress transmittance from the weak

matrix to the strong fibers.

The matrix phase


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Polymer-matrix composites

Polymer matrix + fiber reinforcement phase

used most widely in largest quantities

Polymer-matrix compositesPolymer-matrix compositesPolymer-matrix compositesPolymer-matrix compositesPolymer-matrix compositesPolymer-matrix composites

aramid fiber

carbon fiber

Polymer-matrix composites

glass fiber


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-Glass fiber reinforced polymer composites

Merits of glass fibers

1. easily drawn into high-strength fibers

2. composites be fabricated economically3. relatively strong fiber

4. used in various corrosive environments

Limitations

1. poor stiffness

2. service temp: < 200 C

Applications

automotive and marine bodies,

industrial floorings


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Most commonly used in advanced polymer-matrix

composites.

Merits of carbon fibers1. highest specific modulus and specific strength

2. retain high tensile modulus at high temp.

3. good chemical stability4. a variety of physical and mechanical properties

5. relatively inexpensive and cost effective

Applicationssports & recreational equipment,

aircraft structural components

-Carbon fiber reinforced polymer composites


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Typical applications:

sporting goods, tires, ropes

-Aramid fiber reinforced polymer composites


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Metal-matrix composites

Advantages

higher operating temp.

nonflammability greater resistance to degradation by organic fluids

more expensive than PMCs

Applications

automotive engine component, driveshafts

aerospace industry


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The fracture toughness maybe improved significantly

by introducing particles, fibers and whiskers.

Ceramic-matrix composites

Carbon-carbon compositesOne of the most advanced and promising materials.

Matrix & reinforcement: carbon

Desirable properties

high-tensile modulus & strength at high temp.

high fracture toughness

high thermal conductivity, low thermal expansion

expensive

limited: rocket motor, friction materials in aircraft and high-

performance automobiles


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Core Problems:

Self-help Problems:

Hybrid composites

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