Introduction to fibre—polymer composites

Summary

This chapter sets the scene for the rest of the book. It introduces the main types of behaviour of fibre-polymer composites, with most emphasis on mechanical behaviour, and describes briefly the main methods for making products based on polymer-fibre composites.

One of the basic building blocks for composites is the unidirectional array of stiff strong fibres held together by a polymeric matrix. We shall call this a ‘unidirectional ply’ (or ‘lamina’). We describe the need for a bond between polymer and fibre, the effect of using fibres of different lengths, the properties of representative polymers and fibres, and the influence of fibre volume fraction. In addition, the stiffnesses and strengths of such a lamina are described in terms of the properties of fibre and matrix and the volume fraction of fibres. Hooke’s law, and some simple concepts of strength, are stated for the basic lamina loaded in-plane along and transverse to the fibre direction. These concepts are extended to a sheet which is loaded in-plane but at some angle to the fibre direction (the off-axis loading problem).

It becomes clear that the usefulness of the unidirectional lamina is severely limited by the poor transverse and shear properties, which are usually little better than those of the unreinforced polymer, and some strengths may be worse.

Stacks of bonded plies or laminae, called laminates, are used to compensate for the poor transverse properties of a single lamina.

A description is given of the stiffnesses and strengths of some simple laminates which are widely used in commerce, particularly the crossply laminate, the angle-ply laminate, and the random mat laminate. The importance of symmetry in laminate construction is emphasized. The discussion concludes with a note that a check should always be made of the interlaminar shear behaviour of a laminate subject to bending because of the lack of reinforcement through the thickness of the laminate.

Interspersed with the discussion about the mechanics of composites are descriptions of the processes commonly used to make products from (mainly long) fibre polymer composites.

To make the single unidirectional ply, or versions of it, techniques such as calendering, pultrusion and the winding of prepreg are used. Plies containing random in-plane arrangements of fibres wetted out by resin are also available as Sheet Moulding Compound (SMC).

For laminates based on unidirectional plies, various precision lay-up tech-niques are used and also filament winding. The use of woven or knitted cloth lends itself to hand lay-up (contact moulding) operations. Random fabrics can be processed by contact moulding, or by various resin-injection techniques together with closed moulds. Many laminates based on SMC are processed by compression moulding. Dough (Bulk) moulding compounds may be processed using either compression or (more commonly) the faster injection moulding processes.

Topics outside the scope of this introductory chapter include the buckling and fracture of fibre polymer composites, edge effects, and environmental effects. For detailed discussion of these topics and those covered in the chapter, the reader should refer to the further reading section.