Mechanics of Sheet Metal Forming Mechanics of Sheet Metal Forming Z. Marciniak The Technical University of Warsaw, Poland J.L. Duncan The University of Auckland, New Zealand S.J. Hu The University of Michigan, USA OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Butterworth-Heinemann An imprint of Elsevier Science Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801-2041 First published by Edward Arnold, London, 1992 Second edition published by Butterworth-Heinemann 2002 Copyright 2002 S.J. Hu, Z. Marciniak, J.L. Duncan All rights reserved The right of S.J. Hu, Z. Marciniak and J.L. Duncan to be identified as the authors of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress ISBN 0 7506 5300 0 For information on all Butterworth-Heinemann publications visit our website at www.bh.com Typeset by Laserwords Private Limited, Chennai, India Printed and bound in Great Britain Contents Preface to the second edition ix Preface to the first edition xi Disclaimer xii Introduction xi 1 Material properties 1 1.1 Tensile test 1 1.2 Effect of properties on forming 10 1.3 Other mechanical tests 12 1.4 Exercises 13 2 Sheet deformation processes 14 2.1 Introduction 14 2.2 Uniaxial tension 14 2.3 General sheet processes (plane stress) 16 2.4 Yielding in plane stress 17 2.5 The flow rule 22 2.6 Work of plastic deformation 24 2.7 Work hardening hypothesis 25 2.8 Effective stress and strain functions 26 2.9 Summary 27 2.10 Exercises 27 3 Deformation of sheet in plane stress 30 3.1 Uniform sheet deformation processes 30 3.2 Strain distributions 31 3.3 Strain diagram 31 3.4 Modes of deformation 33 3.5 Effective stress–strain laws 36 3.6 The stress diagram 39 3.7 Principal tensions or tractions 41 3.8 Summary 43 3.9 Exercises 43 v 4 Simplified stamping analysis 45 4.1 Introduction 45 4.2 Two-dimensional model of stamping 46 4.3 Stretch and draw ratios in a stamping 57 4.4 Three-dimensional stamping model 57 4.5 Exercises 59 5 Load instability and tearing 61 5.1 Introduction 61 5.2 Uniaxial tension of a perfect strip 62 5.3 Tension of an imperfect strip 64 5.4 Tensile instability in stretching continuous sheet 67 5.5 Factors affecting the forming limit curve 75 5.6 The forming window 79 5.7 Exercises 80 6 Bending of sheet 82 6.1 Introduction 82 6.2 Variables in bending a continuous sheet 82 6.3 Equilibrium conditions 84 6.4 Choice of material model 85 6.5 Bending without tension 86 6.6 Elastic unloading and springback 92 6.7 Small radius bends 96 6.8 The bending line 100 6.9 Bending a sheet in a vee-die 104 6.10 Exercises 106 7 Simplified analysis of circular shells 108 7.1 Introduction 108 7.2 The shell element 108 7.3 Equilibrium equations 110 7.4 Approximate models of forming axisymmetric 111 shells 7.5 Applications of the simple theory 112 7.6 Summary 115 7.7 Exercises 116 8 Cylindrical deep drawing 117 8.1 Introduction 117 8.2 Drawing the flange 117 8.3 Cup height 123 8.4 Redrawing cylindrical cups 123 8.5 Wall ironing of deep-drawn cups 125 8.6 Exercises 127 vi Contents 9 Stretching circular shells 129 9.1 Bulging with fluid pressure 129 9.2 Stretching over a hemispherical punch 132 9.3 Effect of punch shape and friction 134 9.4 Exercises 135 10 Combined bending and tension of sheet 136 10.1 Introduction 136 10.2 Stretching and bending an elastic, perfectly 136 plastic sheet 10.3 Bending and stretching a strain-hardening sheet 142 10.4 Bending a rigid, perfectly plastic sheet under 144 tension 10.5 Bending and unbending under tension 145 10.6 Draw-beads 150 10.7 Exercises 151 11 Hydroforming 152 11.1 Introduction 152 11.2 Free expansion of a cylinder by internal pressure 153 11.3 Forming a cylinder to a square section 155 11.4 Constant thickness forming 159 11.5 Low-pressure or sequential hydroforming 161 11.6 Summary 163 11.7 Exercises 163 Appendix A1 Yielding in three-dimensional stress state 165 Appendix A2 Large strains: an alternative definition 168 Solutions to exercises 176 Index 205 Contents vii Preface to the second edition The first edition of this book was published a decade ago; the Preface stated the objective in the following way. In this book, the theory of engineering plasticity is applied to the elements of common sheet metal forming processes. Bending, stretching and drawing of simple shapes are analysed, as are certain processes for forming thin-walled tubing. Where possible, the limits governing each process are identified and this entails a detailed study of tensile instability in thin sheet. To the authors’ knowledge, this is the first text in English to gather together the mechanics of sheet forming in this manner. It does, however, draw on the earlier work of, for example, Swift, Sachs, Fukui, Johnson, Mellor and Backofen although it is not intended as a research monograph nor does it indicate the sources of the models. It is intended for the student and the practitioner although it is hoped that it will also be of interest to the researcher. This second edition keeps to the original aim, but the book has been entirely rewritten to accommodate changes in the field and to overcome some earlier deficiencies. Professor Hu joined the authors and assisted in this revision. Worked examples and new problems (with sample solutions) have been added as well as new sections including one on hydro- forming. Some of the original topics have been omitted or given in an abbreviated form in appendices. In recent years, enormous progress has been made in the analysis of forming of complex shapes using finite element methods; many engineers are now using these systems to analyse forming of intricate sheet metal parts. There is, however, a wide gulf between the statement of the basic laws governing deformation in sheet metal and the application of large modelling packages. This book is aimed directly at this middle ground. At the one end, it assumes a knowledge of statics, stress, strain and models of elastic deformation as contained in the usual strength of materials courses in an engineering degree program. At the other end, it stops short of finite element analysis and develops what may be called ‘mechanics models’ of the basic sheet forming operations. These models are in many respects similar to the familiar strength of materials models for bending, torsion etc., in that they are applied to simple shapes, are approximate and often contain simplifying assumptions that have been shown by experience to be reasonable. This approach has proved helpful to engineers entering the sheet metal field. They are confronted with an ix industry that appears to be based entirely on rules and practical experience and they require some assistance to see how their engineering training can be applied to the design of tooling and to the solution of problems in the stamping plant. Experienced sheet metal engineers also find the approach useful in conceptual design, in making quick calculations in the course of more extensive design work, and in interpreting and understanding the finite element simulation results. Nevertheless, users of these models should be aware of the assumptions and limitations of these approximate models as real sheet metal designs can be much more complex than what is captured by the models. The order in which topics are presented has been revised. It now follows a pattern developed by the authors for courses given at graduate level in the universities and to sheet metal engineers and mechanical metallurgists in industry and particularly in the automotive field. The aim is to bring students as quickly as possible to the point where they can analyse simple cases of common processes such as the forming of a section in a typical stamping. To assist in tutorial work in these courses, worked examples are given in the text as well as exercises at the end of each chapter. Detailed solutions of the exercises are given at the end of the text. The possibility of setting interesting problems is greatly increased by the familiarity of students with computer tools such as spread sheets. Although not part of this book, it is possible to go further and develop animated models of processes such as bending, drawing and stamping in which students can investigate the effect of changing variables such as friction or material properties.