Composites and

carbon fibers
Reading assignment
 Askeland and Phule, “The Science and Engineering of
Materials”, 4th Edition, Ch. 16.
 Shakelford, “Introduction to Materials Science for
Engineers”, 6th Edition, Ch. 14.
 Chung, “Composite Materials”, Ch. 2.
 Chung, “Carbon Fiber Composites”, Ch. 1, 2 and 3.
Hull made of a sandwich composite
Exterior: Kevlar fiber epoxy-matrix composite
Interior: Polyvinyl chloride foam
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 (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
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Glass fibers
A carbon
fiber tow

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Glass fiber polymer-matrix composite
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A three-dimensional weave for fiber-

reinforced composites.
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 (a) Tapes containing aligned fibers can be joined to

produce a multi-layered different orientations to
produce a quasi-isotropic composite. In this case,
a 0°/+45°/90° composite is formed.
 Longitudinal
Unidirectional direction
composite
Transverse
direction

Through-
thickness
direction
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license.

epoxy composites.
strength of E-glass fiber-reinforced
Effect of fiber orientation on the tensile
Size distribution of particles used as
reinforcement
Single fiber tensile
strength
Carbon fiber 3.5 GPa
Kevlar fiber 3.6 GPa
E-glass fiber 3.4 GPa
Steel 1.3 GPa
 Specific strength
Carbon fiber 2.00 GPa
Kevlar fiber 2.50 GPa
E-glass fiber 1.31 GPa
Steel 0.17 GPa
 Single fiber tensile
modulus
Carbon fiber 230 GPa
Kevlar fiber 60 GPa
E-glass fiber 22 GPa
Steel 210 GPa
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modulus of fibers versus metals and polymers.

 Comparison of the specific strength and specific
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 The structure of KevlarTM. The fibers are joined by

secondary bonds between oxygen and hydrogen
atoms on adjoining chains.
Some examples of composite materials: (a) plywood is a laminar
composite of layers of wood veneer, (b) fiberglass is a fiber-
reinforced composite containing stiff, strong glass fibers in a
softer polymer matrix ( 175), and (c) concrete is a particulate
composite containing coarse sand or gravel in a cement matrix
(reduced 50%).
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The effect of clay on the properties of

polyethylene.
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 The influence of volume percent boron-coated SiC

(Borsic) fibers on the properties of Borsic-reinforced
aluminum parallel to the fibers
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 Increasing the length of chopped E-glass fibers in

an epoxy matrix increases the strength of the
composite. In this example, the volume fraction of
glass fibers is about 0.5.
 Critical stress
Tensile stress for fiber failure

Short fiber

Long fiber
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A comparison of the specific modulus and specific

strength of several composite materials with those
of metals and polymers.
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several composites and metals.

The specific strength versus temperature for
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A comparison of the specific strength of various

carbon-carbon composites with that of other high-
temperature materials relative to temperature.
Elastic modulus (slope of stress-strain curve)

Unidirectional composite
 Isostrain
condition

Longitudinal direction
Pc = Pm + Pf
 c Ac =  m Am +  f A f
For isostrain condition ( c =  m =  f )
Ec c Ac = Em m Am + E f  f A f
Am Af
Ec = E m + Ef
Ac Ac
Ec =  m E m +  f E f
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 The stress-strain curve for a fiber-reinforced

composite. At low stresses (region l), the modulus of
elasticity is given by the rule of mixtures. At higher
stresses (region ll), the matrix deforms and the rule
of mixtures is no longer obeyed.
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 X c =  m X m + f X f

Rule of Mixtures (ROM)

 Fraction of load carried by fibers

Pf  f Af E f  f Af E f
= = = f
Pc  c Ac Ec c Ac Ec
Isostress
condition

Transverse
direction
Isostress condition

c = m =  f
Lc = Lm + L f
Lc Lm L f
= +
Lc Lc Lc
Lm = Am Lc
L f = Af Lc
Lc Am Lm A f L f
= +
Lc Lm Lf
 c =  m m +  f  f
For isostress condition ( = Ec c = Em m = E f  f ),
σ  
= m + f
Ec Em Ef
1 m  f
= +
Ec E m E f
Em E f
Ec =
 m E f +  f Em
 XmX f
Xc =
 m X f + f X m
 E =  t E + h E
n
c l
n n
h

n = 1 Isostrain
n = -1 Isostress

Rule of Mixtures (ROM)

 Fiber-matrix
debonding
Silver-copper alloy matrix
Carbon fiber reinforcement
Fracture surface observation
(fractography)
 Fiber-matrix
debonding

Polymer matrix
Glass fiber reinforcement
Poor bonding Good bonding
Failure mechanisms
 Failure in fibers (ductile-matrix composites,
e.g., polymer-matrix and metal-
matrix composites), so a high
interfacial strength is desired.
 Failure in matrix (brittle-matrix composites,
e.g., ceramic-matrix and carbon-
matrix composites), so a low
interfacial strength is desired (to allow
cracks to deflect along fiber-matrix
interface, thereby allowing fibers to
pull out for the purpose of increasing
the toughness)
Matrix Fiber Fiber pull-out
 Two failure modes in ceramic-ceramic composites:
(a) Extensive pull-out of SiC fibers in a glass matrix
provides good composite toughness (x20). (b)
Bridging of some fibers across a crack enhances
the toughness of a ceramic-matrix composite
(unknown magnification).
Fracture toughness
Increased by reinforcement
Specific strength = strength/density
Reasons for fiber-matrix
interface engineering

To control fiber-matrix

bond strength (shear bond
strength)
To improve wetting of
matrix precursor on fiber
To improve fiber dispersion
Table 2.2 Effects of various surface treatments on properties of high-modulus carbon fibers and their epoxy-
matrix composites. All liquid treatments at reflux temperature.
Fiber properties Composite properties
Fiber treatment Wt. loss Tensile strength Flexural strength ILSS gain (%)
(%) loss (%) loss (%)
400ºC in air (30 min) 0 0 0 18
500ºC in air (30 min) 0.4 6 12 50
600ºC in air (30 min) 4.5 50 Too weak to test -
60% HNO3 (15 min) 0.2 0 8 11
5.25% NaOCl (30 min) 0.4 1.5 5 30
10-15% NaOCl (15 min) 0.2 0 8 6
15% HClO4 (15 min) 0.2 0 12 0
5% KMnO4/10% NaOH (15 min) 0.4 0 15 19
5% KMnO4/10% H2SO4 (15 min) 6.0(+) 17 13 95
10% H2O2/20% H2SO4 (15 min) 0.1 5 14 0
42% HNO3/30% H2SO4 (15 min) 0.1 0 4(+) 0
10% NaClO3/15% NaOH (15 min) 0.2 0 12 12
10% NaClO3/25% H2SO4 (15 min) 0.2 2 5(+) 91
15% NaClO3/40% H2SO4 (15 min) 0.7 4 15 108
10% Na2Cr2O7/25% H2SO4 (15 min) 0.3 8 15(+) 18
15% Na2Cr2O7/40% H2SO4 (15 min) 1.7 27 31 18
 Types of
polymer-matrix
composites

Thermoplastic-matrix
composites
Thermoset-matrix composites
Lower manufacturing cost of
thermoplastic-matrix
composites
 no cure
 unlimited shelf-life
 reprocessing possible (for repair and recycling)
 less health risks due to chemicals during
processing
 low moisture content
 thermal shaping possible
 weldability (fusion bonding possible)
 Better performance of
thermoplastic-matrix
composites

 high toughness (damage tolerance)

 good hot/wet properties
 high environmental tolerance
Disadvantages of
thermoplastic-matrix composites

 limitations in processing methods

 high processing temperatures
 high viscosities
 prepreg (collection of continuous fibers aligned to form a sheet
which has been impregnated with the polymer or polymer
precursor) being stiff and dry when solvent is not used (i.e., not
drapeable or tacky)
 fiber surface treatments less developed
Attractive properties of carbon fiber
polymer-matrix composites

 low 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)
 good creep resistance
 Attractive properties of carbon fiber
polymer-matrix composites

 low friction coefficient and good wear resistance (a 40 wt.% short carbon
fiber nylon-matrix composite has a friction coefficient nearly as low as
Teflon and unlubricated wear properties approaching those of lubricated
steel)
 toughness and damage tolerance (can be designed by using laminate
orientation to be tougher and much more damage tolerant than metals)
 chemical resistance (chemical resistance controlled by the polymer matrix)
 corrosion resistance (impervious to corrosion)
 Attractive properties of carbon
fiber polymer-matrix composites

 dimensional stability (can be designed for zero coefficient of thermal

expansion)
 vibration damping ability (excellent structural damping when compared
with metals)
 low electrical resistivity
 high electromagnetic interference (EMI) shielding effectiveness
 high thermal conductivity
Limitation of
polymer-matrix
composites
Inability to resist high
temperatures
Carbon-matrix
composites
Ability to resist high
temperatures
Carbon-carbon (C/C)
composites

Carbon fiber
Carbon matrix
Carbon matrix made
from pitch or polymer
Carbon matrix
precursors
Pitch
Resins
Carbonaceous gases
 Properties of graphite
 Anisotropic
 Easy shear between carbon layers
limiting the strength
 High electrical and thermal
conductivity and high modulus in the
plane of the carbon layers
Carbon nanotube

Hybrid of graphite and

fullerene
Crystal forms of carbon

Graphite
Diamond
Fullerene
Nanofiber group
morphology

Intertwined
Parallel
 Fabrication of
carbon nanofibers
Catalytic growth from
carbonaceous gas
Arc discharge
Laser evaporation
 Catalytic method
Carbonaceous gases: acetylene,
ethylene, methane, natural gas,
benzene, etc.
Catalyst: iron, nickel, etc.
(particles typically 10 nm, from
salts or organometallics)
Reducing gases: CO, hydrogen
 Methods of making carbon-
carbon
Carbonization, composites
followed by impregnation of pitch
or resin, and repeating the carbonization-
impregnation process again and again until
sufficient density has been attained.
Chemical vapor infiltration (CVI) using a
carbonaceous gas, i.e., CVD under a
temperature/pressure gradient so as to prevent
crust formation, thereby allowing complete
infiltration; CVI can be an extra step that follows
carbonization-impregnation for the purpose of
filling the pores.
 Table 2.3 Pitch properties.

Carbon yield (%)

Pitch Molecular 0.1 MPa 10 MPa
weight
A 726 45.2 85.9
B 782 54.4 86.4
C 931 84.5 89.8
Metal-matrix
composites
Better temperature resistance than
polymer-matrix composites
Lower temperature resistance than
ceramic-matrix composites
High fabrication cost compared to
polymer-matrix composites
Low fabrication cost compared to
ceramic-matrix composites
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 The steps in producing a silver-tungsten electrical

composite: (a) Tungsten powders are pressed, (b) a low-
density compact is produced, (c) sintering joins the
tungsten powders, and (d) liquid silver is infiltrated into the
pores between the particles.
Ceramic-matrix composites

Ceramic-ceramic composites
(ceramic-fiber ceramic-matrix
composites)
Better oxidation resistance than
carbon-carbon composites
Technology less matured than
carbon-carbon composite
technology
 Examples of ceramic
matrices
Silicon carbide
Silicon nitride
Alumina (aluminum oxide
Al2O3)
Mullite (Al2O3-SiO2)
Glasses