Page 185 - Plastics Engineering

CHAPTER 3 - Mechanical Behaviour of Composites

3.1 Deformation Behaviour of Reinforced Plastics
It was mentioned earlier that the stiffness and strength of plastics can be
increased significantly by the addition of a reinforcing filler. A reinforced plastic
consists of two main components; a matrix which may be either thermoplastic
or thermosetting and a reinforcing filler which usually takes the form of fibres.
A wide variety of combinations are possible as shown in Fig. 3.1. In general,
the matrix has a low strength in comparison to the reinforcement which is
also stiffer and brittle. To gain maximum benefit from the reinforcement, the
fibres should bear as much as possible of the applied stress. The function of
the matrix is to support the fibres and transmit the external loading to them by
shear at the fibrdmatrix interface. Since the fibre and matrix are quite different
in structure and properties it is convenient to consider them separately.

3.2 Qp of Reinforcement
The reinforcing filler usually takes the form of fibres but particles (for example
glass spheres) are also used. A wide range of amorphous and crystalline
materials can be used as reinforcing fibres. These include glass, carbon,
boron, and silica. In recent years, fibres have been produced from synthetic
polymers-for example, Kevlar fibres (from aromatic polyamides) and PET
fibres. The stress-strain behaviour of some typical fibres is shown in Fig. 3.2.
Glass in the form of fibres is relatively inexpensive and is the principal
form of reinforcement used in plastics. The fibres are produced by drawing off
continuous strands of glass from an orifice in the base of an electrically heated
platinum crucible which contains the molten glass. The earliest successful
glass reinforcement had a calcium-alumina borosilicate composition developed
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