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

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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