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1994 | Buch

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Engineering with Fibre-Polymer Laminates

verfasst von: Peter C. Powell

Verlag: Springer Netherlands

Enthalten in: Professional Book Archive

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This book has its recent origins in a Master's course in Polymer Engineering at Manchester. It is a rather extended version of composite mechanics covered in about twenty five hours within a two-week intensive programme on Fibre Polymer Composites which also formed part of the UK Government and Industry-sponsored Integrated Graduate Development Scheme in Polymer Engineering. The material has also been used in other courses, and in teaching to students of engineering and of polymer technology both in the UK and in mainland Europe. There are already many books describing the analysis of and mechanical behaviour of polymer/fibre composites, so why write another? Most of these excellent books appear to be aimed at readers who already have a substantial understanding of stress analysis for linear elastic isotropic materials, who are thoroughly at home with mathematical analysis, and who seem often not to need much of the reassurance which numerical examples and illustrated applications can offer. In teaching the mechanics of composites to many groups of scientists, technologists and engineers, I have found that most of them need and seek an introduction before consulting the advanced texts. This book is intended to fill the gap. Throughout this text is interspersed a substantial range of examples to bring out the practical implications of the basic principles, and a wide range of problems (with outline solutions) to test the reader and extend understanding.

Inhaltsverzeichnis

Frontmatter

1. 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.

Peter C. Powell

2. Introduction to solid body mechanics

Abstract

The main preoccupations of solid body mechanics are seeking answers (for a particular product) to two types of questions, typified by: (a) will it deform within acceptable limits under the applied loads?; and (b) under what values of loads or deformations will it break? Most of this chapter, and indeed the book as a whole, is concerned with (a) not least because it is more straightforward and therefore easier to discuss at the introductory level of the book. Strength is a complicated, and often rather controversial, topic, and we shall leave the main ideas to Chapter 6.

Peter C. Powell

3. Stiffness behaviour of single ply

Overview

The purpose of this chapter is threefold. First, to introduce the nomenclature and language commonly used in the composites literature. Because many composites are used in the form of flat or curved plates, it is convenient to use concepts such as force resultant per unit width and moment resultant per unit width in addition to the more familiar concept of stress.

Peter C. Powell

4. Laminates based on isotropic plies

Abstract

The discussion of the behaviour of plates under in-plane or bending loads has so far focused on a single sheet of an isotropic linear elastic material, or a single unidirectional ply. Before we discuss the similar behaviour for a laminate based on a bonded stack of unidirectional plies orientated at different angles to a reference direction, it will be helpful to examine the mechanics for laminates based on a bonded stack of isotropic linear elastic plies. This will provide the helpful stepping stone of enabling the reader to appreciate the essential concepts of laminates before tackling the more complicated effects of varying the directionality through the thickness.

Peter C. Powell

5. Laminates based on unidirectional plies

Abstract

We noticed in Chapter 3 that one major limitation of unidirectional plies is the difference in stiffnesses in the two principal directions of up to one or two orders of magnitude. In Chapter 4 we looked at the stiffness properties of laminates based on stacks of different isotropic plies. We are now in the position where we can explore (and confirm) the usefulness of laminates based on unidirectional plies for solving many defined stiffness problems.

Peter C. Powell

6. Strength of polymer/fibre composites

Abstract

We have already seen that the stiffness(es) of unidirectional plies, and laminates based on them, depend greatly on the directions in which they are measured or described. The rules governing calculation of these stiffnesses from a given set of data for the individual plies are relatively straightforward, and we have discussed them and their implications in some detail in the previous four chapters.

Peter C. Powell

7. Effect of change of temperature

Abstract

How will a ply or a laminate behave when it undergoes a change of temperature? Will clearances in the plane, or curvatures causing out-of-plane displacements, be acceptable? Will the product warp? Will it warp too much? What modes of deformation might we expect to see in a given ply or laminate? How does laminate construction affect the deformation response to a change of uniform temperature or a temperature gradient through the thickness of the ply or laminate? For example, which of the following midplane strain and curvature responses in Figure7.1 is likely from which initially-flat single ply or laminate constructions under a change of uniform temperature? What types of stress will be set up in the plies and what effect will this have on the strength of the laminate, bearing in mind the simultaneous application of mechanical loads?

Peter C. Powell

8. Stiffness of thin-walled structures

Abstract

The earlier chapters of this book have examined in detail an introduction to how flat panels behave under in-plane loads or moments. The thin flat plate, and (to an approximation) the thin singly-curved panel, is a two-dimensional structural element. It is clearly the fundamental building block for composite structures, and it is therefore essential that the behaviour of the flat plate is well-understood before looking at more complicated structures. Even with the detail studied in the previous chapters, we have not covered all possible types of loading situations — transverse loading of plates is obviously missing — but to cover this would require a much deeper knowledge of the theory of plates and shells than we have space to cover within the scope of Chapter 2 for isotropic materials and in the rest of this book for composites: for further details the reader is advised to consult more advanced texts cited in the bibliography.

Peter C. Powell

Backmatter

Titel: Engineering with Fibre-Polymer Laminates
verfasst von: Peter C. Powell
Verlag: Springer Netherlands
Electronic ISBN: 978-94-011-0723-5
Print ISBN: 978-0-412-49620-2
DOI: https://doi.org/10.1007/978-94-011-0723-5

Springer Professional

Über dieses Buch

Inhaltsverzeichnis

Frontmatter

1. Introduction to fibre—polymer composites

2. Introduction to solid body mechanics

3. Stiffness behaviour of single ply

4. Laminates based on isotropic plies

5. Laminates based on unidirectional plies

6. Strength of polymer/fibre composites

7. Effect of change of temperature

8. Stiffness of thin-walled structures

Backmatter