Materials Chapter 2. Plastic Composites - SSU Carbon Nanotube ...

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




