CHAPER THREE COMPOSITES MATERIALS REINFORCED POLYMER l Introduction

CHAPER THREE COMPOSITES MATERIALS

REINFORCED POLYMER l Introduction l The major problem in the application of polymers in engineering is their low stiffness and strength compared to metals. l Stiffness and strength are improved by two means. l 1. Improve of design and shape. (Ribs and box sections) l 2. The addition of reinforcing particles or fibers to form a composite material.

EXAMPLE OF COMPOSITE MATERIALS l 1. Small boats (epoxy resin and continuous Kevlar and Carbon fibre). l 2. Carbon fibre reinforced nylon tennis racquet. l 3. Rubber car tire. (Carbon black, and polyester fibre and steel wires).

PRINCIPLE OF POLYMER REINFORCEMENT. l. A good reinforcing additive has the following attributes: l 1. It is stiffer and stronger than the polymer matrix. l 2. It has good particles size, shape and surface character for effective mechanical coupling to the matrix l 3. It preserves the desirable qualities of the polymer matrix l 4. Lowest net cost.

MECHANISM OF REINFORCEMENT Consider the case of a single cylindrical reinforcing particles embedded in a block of polymer matrix and perfectly bonded to it. l Figure (a) shows imagined horizontal lines drawn in the block before load is applied l Figure (b) demonstrate the strain distribution which develops under load. l Figure a Figure b

ASSUMPTIONS MADE FOR THIS MECHANISM l 1. The particle is stiffer than the material and deforms less, causing the martial strain to be reduced overall l 2. The particle achieves its restraining effect on the martial via the particle-matrix interface. l The effectiveness of reinforcement is characterized by the ratio of surface area of reinforcement to volume of the reinforcement. (A/v) l Therefore this ratio should be as high as possible.

For a cylindrical particles and lif l(aspect ratio)

A plot of A/V against a is shown in Fig. 3 l a >> 1 the reinforcement is fibre. l a << 1 the reinforcement is platelet. l 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.

l But, the most critical parameters of reinforcement are the cost of the material and processing l The cost of the materials depends on whether the decisive factor is cost per unit mass on cost per unit volume. l 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.

l l l l 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. 4 a V = Vf + Vm 3. 4 b Figure 4

l By expressing the properties of fibre and matrix in the composite by the fractions of the total volume. 3. 5 l. Combine equation 3. 4 b and 3. 5 l. Dividing equation 2. 4 a by V

Example 3. 1

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