Materials
Composites
Introduction
Introduction
•
The major problem in the application of
polymers to engineering is their low stiffness
and strength compared to steel.
–
Moduli are 100 times lower
–
Strengths are 5 times lower
Introduction
•
Two methods are used to overcome these
deficiencies
–
Use of shape (moment of inertia)
•
Ribs
•
Gussets
–
The addition of reinforcing fibers to form a
composite material
Introduction
•
A good reinforcing additive has the following
properties
–
It is stiffer and stronger than the polymer matrix
–
It has good particle size, shape, and surface
character for effective mechanical coupling to the
matrix
–
It preserves the desirable qualities of the polymer
matrix
Introduction
•
The best reinforcement in any application is
the one that achieves the designers objective
at the lowest cost
Mechanism of Fiber
Reinforcement
Mechanism of Fiber Reinforcement
•
We have a single reinforcing fiber embedded
in a polymer matrix and perfectly bonded to
it.
•
The particle is stiffer than the matrix and
deform less, causing the matrix strain to be
reduce overall
–
The strain is much less at the interface
Mechanism of Fiber Reinforcement
•
The reinforcing fiber achieves its restraining
effect on the matrix entirely through the fiber
-
matrix interface
•
The strength of the composite depends on the
strength of bond between fiber and matrix,
and the area of the bond.
Mechanism of Fiber Reinforcement
•
A useful parameter for characterizing the
effectiveness of the reinforcement is the ratio
of surface area of the reinforcement to the
volume of reinforcement.
•
We want the area to volume ratio to be as
high as possible.
•
We define the aspect ratio (a) as the ratio of
length to diameter
Mechanism of Fiber Reinforcement
•
The figure on the next slide show a plot of
aspect ratio(a) vs area to volume ratio.
•
It show the optimum shapes for a cylindrical
reinforcement to be:
–
a>>1, a fiber
–
a<<1, a platelet
Mechanism of Fiber Reinforcement
Mechanism of Fiber Reinforcement
•
Two main classes of reinforcement are fibers
and platelets.
•
Examples of fibers:
–
Glass fibers
–
Carbon fibers
–
Carbon nanotubes
•
Examples of platelets
–
Mica
–
Talc
Forming Reinforced Plastics
Forming Reinforced Plastics
•
Reinforced thermoplastics are usually formed
using extrusion or injection molding.
•
Alignment of the fibers is caused by drag on
the particle by the flowing viscous polymer.
–
Usually aligned in the direction of flow.
–
But the flow field varies greatly and we end up
with random fiber alignment.
•
The damage done to the fiber must also be
taken into account.
How Molecular Orientation Occurs
Forming Reinforced Plastics
•
Thermoset resins can be formed by
compression molding.
•
The fiber and resin are premixed before being
loaded into a heated mold which causes the
resin to crosslink.
•
Many forms of premix are available, making a
variety of fiber arrangements possible.
Forming Reinforced Plastics
•
Many other forming processes:
•
Pultrusion
–
Continuous fibers are pulled through a bath of
resin, then through a shaping die.
–
The resin is then crosslinked.
–
Produces a long fiber with uniaxial alignment.
Forming Reinforced Plastics
•
Filament winding
–
Continuous fibers are pulled through a bath of
resin, then wound onto a driven mandrel.
–
Then the resin is crosslinked.
–
This method is used for making pipe and other
shapes
Forming Reinforced Plastics
•
Pultrusion and Filament winding
Forming Reinforced Plastics
•
Hand Layup
–
The fiber is laid down by hand in the required
arrangement and shape, then resin is applied with
a brush.
–
The resin then crosslinks.
•
Hand Spray Layup
–
Fibers are fed to a spray gun which chops and
sprays the fibers at a panel where the
reinforcement is needed.
–
Resin is then applied with a brush.
–
The resin then crosslinks.
Physical Properties
Physical Properties
Physical Properties
•
Density
•
The density of the composite differs from that
of the polymer
•
A mass (m) of composite occupies a volume
(V)
–
m
f
of fibers occupies
V
f
–
m
m
of matrix (polymer) occupies
V
m
–
m = m
f
+ m
m
–
V =
V
f
+
V
m
Physical Properties
•
The proportion of fibers and matrix in the
composite are expressed as fractions of the
total volume they occupy.
v
v
f
f
v
v
m
m
1
f
m
Physical Properties
•
The density(
ρ
) of the composite with no voids
is:
m
f
f
f
*
)
1
(
Physical Properties
•
In practice, composite materials contain voids.
–
A void is a source of weakness
•
Over 2% voids indicates poor fabrication.
•
Less than 0.5% voids indicates “aircraft
quality” fabrication.
Mechanics of Fiber
Reinforcement
Mechanics of Fiber Reinforcement
•
Accurately predicting the mechanical properties
of a composite material is not easy
•
The differences between properties of the
reinforcing particle and the polymer matrix cause
complex distributions of stress and strain at the
microscopic level, when loads are applied.
•
By using simplified assumptions about stress and
strain, reasonably accurate predictions can be
made
Mechanics of Fiber Reinforcement
•
Consider the case of the fibers that are so long
that the effects of their ends can be ignored.
Mechanics of Fiber Reinforcement
•
The equation for the Composite Modulus (E)
in the 1 direction is:
•
The equation for the Composite Modulus (E)
in the 2 direction is:
m
f
f
f
m
f
E
E
E
E
E
*
*
)
1
(
*
2
m
f
f
f
E
E
E
*
)
1
(
*
1
Mechanics of Fiber Reinforcement
•
Poisson’s ratio (
ν
), the elastic constant of the
composite in the 1,2 direction is:
•
Poisson’s ratio (
ν
), the elastic constant of the
composite in the 2,1 direction is:
m
f
f
f
12
12
12
*
)
1
(
*
1
2
12
21
*
E
E
Mechanics of Fiber Reinforcement
•
When a shear stress acts parallel to the fibers,
the composite deforms as if the fibers and
matrix are coupled is series.
•
The shear Modulus (G
12
) is:
m
f
f
f
m
f
G
G
G
G
G
*
*
)
1
(
*
12
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