CHAPER THREECHAPER THREE

COMPOSITES MATERIALS

REINFORCED POLYMER REINFORCED POLYMER IntroductionThe major problem in the application of polymers in

engineering is their low stiffness and strength compared to metals.

Stiffness and strength are improved by two means. 1. Improve of design and shape. (Ribs and box

sections)2. The addition of reinforcing particles or fibers to

form a composite material.

EXAMPLE OF COMPOSITE EXAMPLE OF COMPOSITE MATERIALSMATERIALS

1. Small boats (epoxy resin and continuous Kevlar and Carbon fibre).

2. Carbon fibre reinforced nylon tennis racquet.

3. Rubber car tire. (Carbon black, and polyester fibre and steel wires).

PRINCIPLE OF POLYMER PRINCIPLE OF POLYMER REINFORCEMENT.REINFORCEMENT.

A good reinforcing additive has the following attributes:1. It is stiffer and stronger than the polymer matrix.2. It has good particles size, shape and surface

character for effective mechanical coupling to the matrix 3. It preserves the desirable qualities of the polymer

matrix4. Lowest net cost.

MECHANISM OF REINFORCEMENT MECHANISM OF REINFORCEMENT Consider the case of a single

cylindrical reinforcing particles embedded in a block of polymer matrix and perfectly bonded to it. 

Figure (a) shows imagined horizontal lines drawn in the block before load is applied

Figure (b) demonstrate the strain distribution which develops under load.

Figure a Figure b

ASSUMPTIONS MADE FOR THIS ASSUMPTIONS MADE FOR THIS MECHANISMMECHANISM

1. The particle is stiffer than the material and deforms less, causing the martial strain to be reduced overall

2. The particle achieves its restraining effect on the martial via the particle-matrix interface.

The effectiveness of reinforcement is characterized by the the ratio of surface area of reinforcement to volume of the reinforcement. (A/v)

Therefore this ratio should be as high as possible.

For a cylindrical particlesFor a cylindrical particles

dl2

dA

2

4

ldV

2

dLV

A 42

3

13

231

22

aaV

a

and

dLa if (aspect

ratio)

A plot of A/V against a is shown in Fig. 3

a >> 1 the reinforcement is fibre.

a << 1 the reinforcement is platelet.

Figure 3. Surface to area ratio A/V of a cylindrical particle of given volume plotted versus aspect ratio a=l/d

•Examples of fibre reinforcement are glass, carbon and Kevlar•Examples of platelet reinforcement are mica and talc.

But, the most critical parameters of reinforcement are the cost of the material and processing

The cost of the materials depends on whether the decisive factor is cost per unit mass on cost per unit volume.

Since the reinforcement additive has a different density from that of the polymer matrix. Thus the density of the composite differs from that of polymer.

Consider the fibre-reinforced polymer shown is Figure 4

The mass of the composite is m, and the volume of the composite is V.

It contains a mass mf of fibers occupying a volume Vf.

and a mass mm of matrix occupying a volume Vm.

If we assume no voids. m = mf + mm 3.4a

V = Vf + Vm 3.4b

Figure 4

By expressing the properties of fibre and matrix in the composite by the fractions of the total volume.

V

Vff

Dividing equation 2.4a by V

1V

V

VV

V

V

V

f

m

mf

mmm

3.5

Combine equation 3.4b and 3.5

fm 1

mfff 1*

Example 3.1 Example 3.1

Thank YouThank You

See You Next Lecture