AskelandPhuleNotes-CH16Printable.ppt

1
The Science and Engineering
of Materials, 4th ed
Donald R. Askeland – Pradeep P. Phulé
Chapter 16 – Composites: Teamwork
and Synergy in Materials

2
Objectives of Chapter 16
 Study different categories of composites:
particulate, fiber, and laminar
 Focus on composites used in structural or
mechanical applications.

3
Chapter Outline
 16.1 Dispersion-Strengthened Composites
 16.2 Particulate Composites
 16.3 Fiber-Reinforced Composites
 16.4 Characteristics of Fiber-Reinforced
Composites
 16.5 Manufacturing Fibers and Composites
 16.6 Fiber-Reinforced Systems and
Applications
 16.7 Laminar Composite Materials
 16.8 Examples and Applications of Laminar
Composites
 16.9 Sandwich Structures

4
Figure 16.1 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%).

5
 A special group of dispersion-strengthened
nanocomposite materials containing particles 10 to 250
nm in diameter is classified as particulate composites.
 Dispersoids - Tiny oxide particles formed in a metal
matrix that interfere with dislocation movement and
provide strengthening, even at elevated temperatures.
Section 16.1
Dispersion-Strengthened Composites

6
©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
Figure 16.2
Comparison of the yield
strength of dispersion-
strengthened sintered
aluminum powder
(SAP) composite with
that of two
conventional two-phase
high-strength
aluminum alloys. The
composite has benefits
above about 300°C. A
fiber-reinforced
aluminum composite is
shown for comparison.

8
Figure 16.3 Electron micrograph
of TD-nickel. The dispersed ThO2
particles have a diameter of 300
nm or less ( 2000). (From Oxide
Dispersion Strengthening, p. 714,
Gordon and Breach, 1968. ©
AIME.)

9
Suppose 2 wt% ThO2 is added to nickel. Each ThO2 particle
has a diameter of 1000 Å. How many particles are present
in each cubic centimeter?
Example 16.1 SOLUTION
The densities of ThO2 and nickel are 9.69 and 8.9 g/cm3,
respectively. The volume fraction is:
Example 16.1
TD-Nickel Composite

10
Example 16.1 SOLUTION (Continued)
Therefore, there is 0.0184 cm3 of ThO2 per cm3 of
composite. The volume of each ThO2 sphere is:

11
Section 16.2
Particulate Composites
 Rule of mixtures - The statement that the properties of a
composite material are a function of the volume fraction
of each material in the composite.
 Cemented carbides - Particulate composites containing
hard ceramic particles bonded with a soft metallic
matrix.
 Electrical Contacts - Materials used for electrical contacts
in switches and relays must have a good combination of
wear resistance and electrical conductivity.
 Polymers - Many engineering polymers that contain
fillers and extenders are particulate composites.

12
Figure 16.4 Microstructure of
tungsten carbide—20% cobalt-
cemented carbide (1300). (From
Metals Handbook, Vol. 7, 8th Ed.,
American Society for Metals,
1972.)

13
A cemented carbide cutting tool used for machining contains 75
wt% WC, 15 wt% TiC, 5 wt% TaC, and 5 wt% Co. Estimate the
density of the composite.
First, we must convert the weight percentages to volume
fractions. The densities of the components of the composite
are:
Example 16.2
Cemented Carbides

14
From the rule of mixtures, the density of the composite is

15
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure 16.5 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.

16
A silver-tungsten composite for an electrical contact is produced
by first making a porous tungsten powder metallurgy compact,
then infiltrating pure silver into the pores. The density of the
tungsten compact before infiltration is 14.5 g/cm3. Calculate
the volume fraction of porosity and the final weight percent of
silver in the compact after infiltration.
From the rule of mixtures:
Example 16.3
Silver-Tungsten Composite

17
After infiltration, the volume fraction of silver equals the
volume fraction of pores:

18
Figure 16.6 The effect of clay on the properties of polyethylene.

19
Design a clay-filled polyethylene composite suitable for injection
molding of inexpensive components. The final part must have a
tensile strength of at least 3000 psi and a modulus of elasticity of
at least 80,000 psi. Polyethylene costs approximately 50 cents per
pound and clay costs approximately 5 cents per pound. The
density of polyethylene is 0.95 g/cm3 and that of clay is 2.4 g/cm3.
Example 16.4
Design of a Particulate Polymer Composite
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a
trademark used herein under license.
Figure 16.6 The effect of
clay on the properties of
polyethylene.

20
In 1000 cm3 of composite parts, there are 350 cm3 of
clay and 650 cm3 of polyethylene in the composite, or:
The cost of materials is:

21
Suppose that weight is critical. The composite’s density is:
If we use only 0.2 volume fraction clay, then (using the
same method as above) we find that we need 1.06 lb
clay and 1.67 lb polyethylene.
The cost of materials is now:
The density of the composite is:

22
Figure 16.7 Microstructure of an
aluminum casting alloy reinforced with
silicon carbide particles. In this case, the
reinforcing particles have segregated to
interdendritic regions of the casting
( 125). (Courtesy of David Kennedy,
Lester B. Knight Cost Metals Inc.)

23
 The Rule of Mixtures in Fiber-Reinforced Composites
 Strength of Composites - The tensile strength of a fiber-
reinforced composite (TSc) depends on the bonding
between the fibers and the matrix.
Section 16.3
Fiber-Reinforced Composites

24
Figure 16.8 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.

25
Example 16.5
Rule of Mixtures for Composites:
Stress Parallel to Fibers
Derive the rule of mixtures (Equation 16.5) for the modulus of
elasticity of a fiber-reinforced composite when a stress ( ) is
applied along the axis of the fibers. We use the symbol ‘‘ ’’ for
stress to distinguish it from the symbol used for conductivity.
The total force acting on the composite is the sum of the forces
carried by each constituent:
Fc = Fm + Ff


Since F = σA:

26
If the fibers have a uniform cross-section, the area
fraction equals the volume fraction f :
If the fibers are rigidly bonded to the matrix, both the
fibers and the matrix must stretch equal amounts (iso-
strain conditions):
From Hooke’s law, σ = εE. Therefore:

27
Example 16.6
Modulus of Elasticity for Composites:
Stress Perpendicular to Fibers
Derive the equation for the modulus of elasticity of a fiber-
reinforced composite when a stress is applied perpendicular to the
axis of the fiber (Equation 16-7).
The strains are no longer equal; instead, the weighted sum of the
strains in each component equals the total strain in the composite,
whereas the stresses in each component are equal (iso-stress
conditions):

28
Boron coated with SiC(or Borsic) reinforced aluminum
containing 40 vol% fibers is an important high-
temperature, lightweight composite material. Estimate the
density, modulus of elasticity, and tensile strength parallel
to the fiber axis. Also estimate the modulus of elasticity
perpendicular to the fibers.
Example 16.7
Boron Aluminum Composites

29
Figure 16.9 The influence of volume percent boron-coated
SiC (Borsic) fibers on the properties of Borsic-reinforced
aluminum parallel to the fibers (for Example 16.7).

30
The properties of the individual components are shown
below.
From the rule of mixtures:
Perpendicular to the fibers:

31
Boron coated with SiC(or Borsic) reinforced aluminum
containing 40 vol% fibers is an important high-
temperature, lightweight composite material. Estimate the
density, modulus of elasticity, and tensile strength parallel
to the fiber axis. Also estimate the modulus of elasticity
perpendicular to the fibers.
The modulus of elasticity for each component of the
composite is:
Eglass = 10.5  106 psi Enylon = 0.4  106 psi
Example 16.8
Nylon-Glass Fiber Composites

32
Both the nylon and the glass fibers have equal strain if
bonding is good, so:
Almost all of the load is carried by the glass fibers.

33
Section 16.4
Characteristics of Fiber-Reinforced
Composites
 Many factors must be considered when designing a fiber-
reinforced composite, including the length, diameter,
orientation, amount, and properties of the fibers; the
properties of the matrix; and the bonding between the
fibers and the matrix.
 Aspect ratio - The length of a fiber divided by its
diameter.
 Delamination - Separation of individual plies of a fiber-
reinforced composite.

34
Figure 16.10 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.

35
©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
Figure 16.11 Effect of
fiber orientation on the
tensile strength of E-
glass fiber-reinforced
epoxy composites.

36
Figure 16.12 (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.

37
Figure 16.13 A three-dimensional weave for fiber-
reinforced composites.

39
©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
Figure 16.14
Comparison of the
specific strength and
specific modulus of
fibers versus metals
and polymers.

40
Figure 16.15 The structure of KevlarTM. The fibers are joined
by secondary bonds between oxygen and hydrogen atoms on
adjoining chains.

41
We are now using a 7075-T6 aluminum alloy (modulus of
elasticity of 10  106 psi) to make a 500-pound panel on a
commercial aircraft. Experience has shown that each pound
reduction in weight on the aircraft reduces the fuel consumption
by 500 gallons each year. Design a material for the panel that
will reduce weight, yet maintain the same specific modulus, and
will be economical over a 10-year lifetime of the aircraft.
let’s consider using a boron fiber-reinforced Al-Li alloy in the T6
condition. The specific modulus of the current 7075-T6 alloy is:
Example 16.9
Design of an Aerospace Composite

42
If we use 0.6 volume fraction boron fibers in the
composite, then the density, modulus of elasticity, and
specific modulus of the composite are:
If the specific modulus is the only factor influencing
the design of the component, the thickness of the part might
be reduced by 75%, giving a component weight of 125
pounds rather than 500 pounds. The weight savings would
then be 375 pounds, or (500 gal/lb)(375 lb) = 187,500 gal
per year. At about $2.00 per gallon, about $375,000 in fuel
savings could be realized each year, or $3.75 million over
the 10-year aircraft lifetime.

43
Figure 16.16 Scanning
electron micrograph of
the fracture surface of a
silver-copper alloy
reinforced with carbon
fibers. Poor bonding
causes much of the
fracture surface to
follow the interface
between the metal
matrix and the carbon
tows ( 3000). (From
Metals Handbook,
American Society for
Metals, Vol. 9, 9th Ed.,
1985.)

44
Section 16.5
Manufacturing Fibers and Composites
 Chemical vapor deposition - Method for manufacturing
materials by condensing the material from a vapor onto
a solid substrate.
 Carbonizing - Driving off the non-carbon atoms from a
polymer fiber, leaving behind a carbon fiber of high
strength. Also known as pyrolizing.
 Filament winding - Process for producing fiber-reinforced
composites in which continuous fibers are wrapped
around a form or mandrel.
 Pultrusion - A method for producing composites
containing mats or continuous fibers.

45
Figure 16.17 Methods for producing (a) boron and (b) carbon
fibers.

46
Figure 16.18 Photomicrographs of two fiber-reinforced
composites: (a) In Borsic fiber-reinforced aluminum, the fibers
are composed of a thick layer of boron deposited on a small-
diameter tungsten filament ( 1000). (From Metals Handbook,
American Society for Metals, Vol. 9, 9th Ed., 1985.) (b) In this
microstructure of a ceramic-fiber–ceramic-matrix composite,
silicon carbide fibers are used to reinforce a silicon nitride
matrix. The SiC fiber is vapor-deposited on a small carbon
precursor filament ( 125). (Courtesy of Dr. R.T. Bhatt, NASA
Lewis Research Center.)

47
©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
Figure 16.19 The
effect of heat-
treatment
temperature on the
strength and modulus
of elasticity of carbon
fibers.

48
Figure 16.20 A scanning electron micrograph of a
carbon tow containing many individual carbon
filaments (x200).

49
Figure 16.21 Production of fiber tapes by encasing fibers
between metal cover sheets by diffusion bonding.

50
Figure 16.22 Producing composite shapes in dies by (a)
hand lay-up, (b) pressure bag molding, and (c) matched die
molding.

51
Figure 16.23 Producing composite shapes by filament winding.

52
Figure 16.24 Producing composite shapes by pultrusion.

53
Section 16.6
Fiber-Reinforced Systems and
Applications
 Advanced Composites - The advanced composites
normally are polymer–matrix composites reinforced with
high-strength polymer, metal, or ceramic fibers.
 Metal-Matrix Composites - These materials,
strengthened by metal or ceramic fibers, provide high-
temperature resistance.
 Ceramic-Matrix Composites - Composites containing
ceramic fibers in a ceramic matrix are also finding
applications.

54
Figure 16.25 A comparison of the specific modulus and specific
strength of several composite materials with those of metals
and polymers.

56
©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
Figure 16.26 The
specific strength
versus temperature
for several composites
and metals.

57
Figure 16.27 The manufacturer of composite super-conductor
wires: (a) Niobium wire is surrounded with copper during
forming. (b) Tim is plated onto Nb-Cu composite wired. (c)
Tin diffuses to niobium to produce the Nb3Sn-Cu composite.

58
©2003
Brooks/Cole,
a
division
of
Thomson
Learning,
Inc.
Thomson
Learning
™
is
a
trademark
used
herein
under
license.
Figure 16.28 A
comparison of the
specific strength of
various carbon-carbon
composites with that
of other high-
temperature materials
relative to
temperature.

60
Figure 16.29 Two failure modes in ceramic-ceramic
composites: (a) Extensive pull-out of SiC fibers in a
glass matrix provides good composite toughness
(x20). (From Metals Handbook, American Society for
Metals, Vol. 9, 9th Ed., 1985.) (b) Bridging of some
fibers across a crack enhances the toughness of a
ceramic-matrix composite (unknown magnification).
(From Journal of Metals, May 1991.)

61
Design a unidirectional fiber-reinforced epoxy-matrix
strut having a round cross-section. The strut is 10 ft long
and, when a force of 500 pounds is applied, it should
stretch no more than 0.10 in. We want to assure that the
stress acting on the strut is less than the yield strength
of the epoxy matrix, 12,000 psi. If the fibers should
happen to break, the strut will stretch an extra amount
but may not catastrophically fracture. Epoxy costs about
$0.80/lb and has a modulus of elasticity of 500,000 psi.
Example 16.10
Design of a Composite Strut

62
For high modulus carbon fibers, E = 77  106 psi; the
density is 1.9 g/cm3 = 0.0686 lb/in.3, and the cost is about
$30/lb. The minimum volume fraction of carbon fibers
needed to give a composite modulus of 14.5  106 psi is:
The volume fraction of epoxy remaining is 0.817. An area
of 0.817 times the total cross-sectional area of the strut
must support a 500-lb load with no more than 12,000 psi
if all of the fibers should fail:

63
Although the carbon fibers are the most expensive, they permit
the lightest weight and the lowest material cost strut. (This
calculation does not, however, take into consideration the costs
of manufacturing the strut.) Our design, therefore, is to use a
0.255-in.-diameter strut containing 0.183 volume fraction high
modulus carbon fiber.

64
Section 16.7
Laminar Composite Materials
 Rule of Mixtures - Some properties of the laminar
composite materials parallel to the lamellae are
estimated from the rule of mixtures.
 Producing Laminar Composites - (a) roll bonding, (b)
explosive bonding, (c) coextrusion, and (d) brazing.

65
Figure 16.30 Techniques for producing laminar composites:
(a) roll bonding, (b) explosive bonding, and (c) coextrusion,
and (d) brazing.

66
Section 16.8
Examples and Applications of
Laminar Composites
 Laminates - Laminates are layers of materials joined by
an organic adhesive.
 Cladding - A laminar composite produced when a
corrosion-resistant or high-hardness layer of a laminar
composite formed onto a less expensive or higher-
strength backing.
 Bimetallic - A laminar composite material produced by
joining two strips of metal with different thermal
expansion coefficients, making the material sensitive to
temperature changes.

67
Figure 16.31 Schematic diagram of an aramid-aluminum
laminate, Arall, which has potential for aerospace
applications.

68
Section 16.9
Sandwich Structures
 Sandwich - A composite material constructed of a
lightweight, low-density material surrounded by dense,
solid layers. The sandwich combines overall light weight
with excellent stiffness.
 Honeycomb - A lightweight but stiff assembly of
aluminum strip joined and expanded to form the core of
a sandwich structure.

69
Figure 16.32 (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.

70
Figure 16.33 In the corrugation method for producing a
honeycomb core, the material (such as aluminum) is
corrugated between two rolls. The corrugated sheets are
joined together with adhesive and then cut to the desired
thickness.

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