STRUCTURAL IMPACT Second Edition Structural Impact is concerned with the behaviour of structures and components subjected to large dynamic, impact and explosive loads which produce inelastic deformations and is of interest for safety calculations, hazard assessments and energy-absorbing systems throughout industry. The first five chapters of this book introduce the rigid plastic methods of analysis for the static behaviour and the dynamic response of beams, plates and shells. These chapters develop the key features of the subject from an engineering viewpoint and are followed by several chapters on various phenomena of importance to structural impact. The influence of transverse shear, rotatory inertia, finite displacements and dynamic material properties are introduced and studied in some detail. Dynamic progressive buckling, which develops in several energy-absorbing systems, is then examined, and the phenomenon of dynamic plastic buckling is introduced in the penultimate chapter. The last chapter on the scaling laws is important for relating the response of smallscale experimental tests to the dynamic behaviour of full-scale prototypes. This text is invaluable to undergraduates, graduates and professionals who want to learn more about the behaviour of structures subjected to large impact, dynamic and blast loadings producing an inelastic response. Dr is an Emeritus Professor of Engineering at the University of Liverpool, UK. He was previously the A. A. Griffith Professor of Engineering, headed the University s Department of Mechanical Engineering (1982 1990) and served as Director of the Impact Research Centre (1985 2005). Prior to that, he was a Professor of Ocean Engineering at MIT in the United States. He has published more than 300 papers, principally on many aspects of the response of structures subjected to dynamic, impact and blast loadings which produce large inelastic strains. He is the honorary editor-in-chief of the International Journal of Impact Engineering and was editor (1983 87) and editor-in-chief (1988 2008), and he is the author of the first edition of Structural Impact (Cambridge University Press, 1989). Professor Jones is honorary professor at Huazhong and Taiyuan Universities of Science and Technology in China, a Fellow of the Royal Academy of Engineering (London) and a Foreign Fellow of the Indian National Academy of Engineering.
Structural Impact Second Edition University of Liverpool
cambridge university press Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Tokyo, Mexico City Cambridge University Press 32 Avenue of the Americas, New York, NY 10013-2473, USA Information on this title: /9781107010963 C 1989, 2012 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 1989 First paperback edition 1997 Second edition published 2012 Printed in the United States of America A catalog record for this publication is available from the British Library. Library of Congress Cataloging in Publication data Jones, Norman, 1938 Structural impact /. 2nd ed. p. cm. Includes bibliographical references and indexes. ISBN 978-1-107-01096-3 1. Impact. 2. Structural dynamics. 3. Girders. 4. Plates (Engineering). 5. Shells (Engineering). I. Title. TA654.2.J66 2012 624.1 76 dc23 ISBN 978-1-107-01096-3 Hardback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party Internet Web sites referred to in this publication and does not guarantee that any content on such Web sites is, or will remain, accurate or appropriate.
To Jenny, Alison and Catherine, William and Georgina
Contents Preface to the Second Edition Preface to the First Edition page xi xiii 1 Static Plastic Behaviour of Beams...1 1.1 Introduction 1 1.2 Basic Equations for Beams 2 1.3 Plastic Collapse Theorems for Beams 5 1.4 Static Plastic Collapse of a Cantilever 8 1.5 Static Plastic Collapse of a Simply Supported Beam 10 1.6 Static Plastic Collapse of a Fully Clamped Beam 12 1.7 Static Plastic Collapse of a Beam Subjected to a Concentrated Load 13 1.8 Static Plastic Collapse of a Partially Loaded Beam 14 1.9 Experiments on Beams 17 1.10 Final Remarks 19 problems 19 2 Static Plastic Behaviour of Plates and Shells...21 2.1 Introduction 21 2.2 Generalised Stresses and Strains 21 2.3 Basic Concepts 23 2.4 Plastic Collapse Theorems 26 2.5 Basic Equations for Circular Plates 27 2.6 Static Plastic Collapse Pressure of Circular Plates 30 2.7 Basic Equations for Rectangular Plates 33 2.8 Static Plastic Collapse Pressure of Rectangular Plates 34 2.9 Basic Equations for Cylindrical Shells 41 2.10 Static Collapse Pressure of a Long Reinforced Cylindrical Shell 45 2.11 Static Plastic Collapse of a Ring-Loaded Cylindrical Shell 48 2.12 Experiments on Plates and Shells 51 2.13 Final Remarks 55 problems 57 vii
viii Contents 3 Dynamic Plastic Behaviour of Beams...59 3.1 Introduction 59 3.2 Governing Equations for Beams 61 3.3 Simply Supported Beam, p c p 0 3p c 62 3.4 Simply Supported Beam, p 0 > 3p c 66 3.5 Simply Supported Beam Loaded Impulsively 78 3.6 Fully Clamped Beam, p c p 0 3 p c 83 3.7 Fully Clamped Beam, p 0 > 3 p c 85 3.8 Impact of a Mass on a Fully Clamped Beam 86 3.9 Impact of a Cantilever Beam 94 3.10 Final Remarks 99 problems 103 4 Dynamic Plastic Behaviour of Plates...105 4.1 Introduction 105 4.2 Governing Equations for Circular Plates 107 4.3 Annular Plate Loaded Dynamically 107 4.4 Simply Supported Circular Plate Loaded Dynamically, p c p 0 2p c 115 4.5 Simply Supported Circular Plate Loaded Dynamically, p 0 > 2p c 120 4.6 Fully Clamped Circular Plate Loaded Impulsively 131 4.7 Governing Equations for Rectangular Plates 133 4.8 Simply Supported Square Plate Loaded Dynamically, p c p 0 2p c 135 4.9 Simply Supported Square Plate Loaded Dynamically, p 0 > 2p c 140 4.10 Final Remarks 146 problems 149 5 Dynamic Plastic Behaviour of Shells...151 5.1 Introduction 151 5.2 Governing Equations for Cylindrical Shells 152 5.3 Long Cylindrical Shell 153 5.4 Long Reinforced Cylindrical Shell 158 5.5 Fully Clamped Short Cylindrical Shell 162 5.6 Elastic, Perfectly Plastic Spherical Shell Subjected to a Spherically Symmetric Dynamic Pressure 170 5.7 Shallow Shells 190 5.8 Some Comments on Spherical Shells 196 5.9 Influence of Pressure Pulse Characteristics 197 5.10 Final Remarks 203 problems 205 6 Influence of Transverse Shear and Rotatory Inertia...206 6.1 Introduction 206 6.2 Governing Equations for Beams 208
Contents ix 6.3 Transverse Shear Effects in a Simply Supported Beam Loaded Impulsively 209 6.4 Impact of a Mass on a Long Beam 225 6.5 Transverse Shear Effects in a Simply Supported Circular Plate 231 6.6 Transverse Shear Effects in Cylindrical Shells 243 6.7 Final Remarks 251 problems 265 7 Influence of Finite Displacements...267 7.1 Introduction 267 7.2 Static Plastic Behaviour of a Beam Subjected to a Concentrated Load 269 7.3 Static Plastic Behaviour of Circular Plates 281 7.4 Static Plastic Behaviour of Rectangular Plates 283 7.5 Dynamic Plastic Behaviour of Rectangular Plates 291 7.6 Dynamic Plastic Behaviour of Beams 302 7.7 Dynamic Plastic Behaviour of Circular Plates 304 7.8 Dynamic Plastic Behaviour of a Circular Membrane 309 7.9 Mass Impact Loading of Plates 311 7.10 Final Remarks 316 problems 326 8 Strain-Rate-Sensitive Behaviour of Materials...327 8.1 Introduction 327 8.2 Material Characteristics 329 8.3 Constitutive Equations 340 8.4 Theoretical Solutions of Idealised Models 347 8.5 Theoretical Behaviour of Strain-Rate-Sensitive Structures 363 8.6 Final Remarks 374 problems 375 9 Dynamic Progressive Buckling...377 9.1 Introduction 377 9.2 Static Axial Crushing of a Circular Tube 381 9.3 Dynamic Axial Crushing of a Circular Tube 385 9.4 Static Axial Crushing of a Square Tube 392 9.5 Dynamic Axial Crushing of a Square Tube 396 9.6 Comparison of the Axial Crushing Characteristics of Circular and Square Tubes 397 9.7 Some Comments on Energy Absorption Systems 399 9.8 Structural Crashworthiness 406 9.9 Structural Protection 417 9.10 Final Remarks 419 problems 423
x Contents 10 Dynamic Plastic Buckling...425 10.1 Introduction 425 10.2 Dynamic Elastic Buckling of a Bar 427 10.3 Dynamic Plastic Buckling of a Bar 437 10.4 Dynamic Plastic Buckling of a Circular Ring Subjected to an External Impulse 442 10.5 Dynamic Axial Plastic Buckling of a Long Cylindrical Shell 457 10.6 Critical Impulsive Radial Velocity for Collapse of a Cylindrical Shell without Buckling 472 10.7 Final Comments 473 problems 478 11 Scaling Laws...479 11.1 Introduction 479 11.2 Introduction to Geometrically Similar Scaling 479 11.3 Phenomena Which Do Not Scale Geometrically 484 11.4 Dimensional Analysis 486 11.5 Crack Propagation in Elastic Structures 492 11.6 Ductile Brittle Fracture Transitions 495 11.7 Experimental Results on the Scaling of Structures Loaded Dynamically 499 11.8 Final Comments 508 problems 510 APPENDIX 1: Principle of Virtual Work...511 APPENDIX 2: Path-Dependence of an Inelastic Material...514 APPENDIX 3: Principle of Virtual Velocities...516 APPENDIX 4: Consistent Sets of Equilibrium Equations and Geometrical Relations...517 APPENDIX 5: Buckingham Π-Theorem...528 APPENDIX 6: Quasi-Static Behaviour...532 APPENDIX 7: Martin s Upper Bound Displacement Theorem...538 References 541 Answers to Selected Problems 571 Author Index 575 Subject Index 580
Preface to the Second Edition The general field of structural impact has expanded significantly since the preparation of the first edition of this book more than 20 years ago. This expansion is driven partly by the quest for the design of efficient structures which require more accurate safety factors against various types of dynamic loadings causing large plastic strains. The enhancement of safety in many industries, including transportation, has become more prominent in recent years, as well as the protection of structures and systems against terrorist attacks. In tandem with these developments and enhanced requirements, rapid advances have occurred in numerical analyses, which have outpaced, in many ways, our understanding of structural impact. Nevertheless, numerical schemes are used throughout design offices. This book emphasises the basic mechanics of structural impact in order to gain some insight into its broad field. It is important that an engineering designer has a good grasp of the mechanics which underpin this highly nonlinear and complex engineering field. The book attempts to achieve this aim through an analysis of simple models which expose the basic aspects of the response, an understanding of which will pay dividends when interpreting the results emanating from both experimental studies and numerical calculations. For example, the issues raised in Chapters 8 and 11, on material strain rate sensitivity and scaling, respectively, are certainly important for both numerical calculations as well as experimental programmes. In some cases, the equations presented in this book are suitable for preliminary design purposes, particularly when bearing in mind frequent uncertainties in the input data. For example, the values of coefficients and form of dynamic constitutive equations are often approximate, and there are difficulties in specifying the correct details for the boundary conditions at joints, etc., and in obtaining the characteristics of the external dynamic loadings which arise from impact, explosive and large dynamic loadings. The first five basic chapters of this book remain largely unchanged, except for some slight improvements here and there to aid clarity and the addition of two appendices, one on quasi-static behaviour and the other giving the proof of a displacement bound theorem. Recent developments on these topics have been confined largely to solutions for special cases and numerical studies. Considerable research effort has been expended, over the last few decades, on the topics studied in the last six chapters. Therefore, these chapters of the book have been updated xi
xii Preface to the Second Edition selectively. However, more research is required, particularly for Chapters 8 to 11, before structural impact is a fully mature subject. The author wishes to thank Mr Peter Gordon of Cambridge University Press for his assistance and his invitation to prepare this second edition. I also wish to thank the many people who have suggested improvements since the publication of the paperback edition, particularly Professor M Alves and Dr Q. M. Li. I also wish to thank Mrs I. Arnot for assistance with the new figures, and last but not least to acknowledge the valuable support of my wife, Jenny. February 2011
Preface to the First Edition Impact events occur in a wide variety of circumstances, from the everyday occurrence of striking a nail with a hammer to the protection of spacecraft against meteoroid impact. All too frequently, we see the results of impact on our roads. Newspapers and television report spectacular accidents which often involve impact loadings, such as the collisions of aircraft, buses, trains and ships, together with the results of impact or blast loadings on pressure vessels and buildings due to accidental explosions and other accidents. The general public is becoming increasingly concerned about safety, including, for example, the integrity of nuclear transportation casks in various accident scenarios involving impact loads. Clearly, impact is a large field which embraces both simple structures (e.g., nails) and complex systems, such as the protection of nuclear power plants. The materials which are impacted include bricks, concrete, ductile and brittle metals, and polymer composites. Moreover, on the one hand, the impact velocities may be low and give rise to a quasi-static response, or, on the other hand, they may be sufficiently large to cause the properties of the target material to change significantly. In this book, I have concentrated on the impact behaviour of ductile structures and, in particular, beams, plates and shells. Most complex engineering systems are constructed largely of these simple structural members, so that an understanding of their response is an essential prerequisite for revealing the dynamic behaviour of a more complex system. The topic remains a large one, and so I have specialised it further by focusing on large impact loads producing plastic strains which dominate the elastic effects. A dynamic load causes elastic and plastic stress waves to propagate through the thickness of this class of structures, as well as produces an overall structural response. The propagation of stress waves through the structural thickness can cause failure by spalling when the shock or impact loads are sufficiently severe. This phenomenon occurs in the same order of time that it takes a stress wave to propagate through the thickness of the structure. Thus, this type of failure usually occurs within microseconds of initial impact, and is sometimes referred to as the early time response to distinguish it from the gross structural behaviour which occurs at later times. The early time response of structural members is not considered further. xiii
xiv Preface to the First Edition This book focuses on the long-term behaviour of structures (typically of the order of milliseconds for small structures), for which the external dynamic load is assumed to impart momentum instantaneously to the middle surface of a structure (i.e., transverse wave propagation is disregarded). It is customary practice to uncouple the early time wave propagation behaviour from the long-time or gross structural response because the time durations of these two phenomena usually differ by a few orders of magnitude. Obviously, a gross structural analysis cannot be used to predict the detailed behaviour through the structural thickness. It is necessary, therefore, to establish separately whether or not failure due to spalling can occur in a given case. Although the static plastic behaviour of structures was first studied last century (e.g., J. A. Ewing, The Strength of Materials, Cambridge University Press, 1899), systematic investigations on the dynamic plastic behaviour are much more recent. Serious studies appear to have commenced during the Second World War, when, for example, J. F. Baker designed the Morrison air raid shelters to protect people from falls of masonry in their own homes, G. I. Taylor studied the dynamic response of thin plates, and Pippard and Chitty examined the dynamic behaviour of cylindrical shells for research into submarine hulls. Considerable research activity and progress have been reported over the past forty years, some of which is discussed in this book or cited in the references. Generally speaking, this body of work seeks the response of a structural member when subjected to a known impact load. However, the theoretical solutions may also be used for diagnostic or forensic purposes. Lord Penney and his colleagues, for example, estimated the nuclear explosive yields at Hiroshima and Nagasaki by calculating the impact loads required to cause the permanent damage which was observed for bent or snapped poles, squashed empty drums or cans, tops of office cabinets pushed in by the blast, etc. Coupling between the external impact loading and the structural response is a difficult topic, which is not well understood, and is disregarded in this book. Despite restricting our attention to the impact behaviour of beams, plates and shells subjected to large impact loads, the field is still large, active and growing rapidly. The results of studies in this area are being used to guide the development of rational design procedures which avoid the destructive action of earthquakes on buildings and to improve the collision protection of passengers in automobiles, trains, buses and aircraft. The collision protection of automobiles and buses is achieved by improving the interior and exterior energy-absorbing capabilities of the vehicles and incorporating the principles of structural crashworthiness into the design of highway safety systems and roadside furniture. Theoretical methods have been used to design impact absorbers of various types, as well as to assess the safety of reactor tubes subjected to violent transient pressure pulses, which can arise in certain circumstances in sodium-cooled fastbreeder reactors. The slamming damage which is sustained by the bottom plating of ships and hydrovehicles has been estimated using these methods which have also been employed to design buildings to withstand internal gaseous explosions. The response of re-entry vehicles, structural crashworthiness of offshore platforms, safety calculations for industrial plants, various military applications, interpretation of constitutive equations from dynamic ring tests, and even the denting of aircraft surfaces due to hail has been studied using the various methods discussed in this book. Many other practical applications have been made, and, no doubt, more
Preface to the First Edition xv will be found as engineers strive to design new and efficient structures which must be as light and safe as possible, yet withstand the large dynamic loads arising in many practical situations without catastrophic failure, or absorb the external dynamic energy in a controlled and predictable fashion. It is the aim of this book to equip the reader with a clear understanding of the impact behaviour of some simple structures. The dynamic response of the particular cases studied may be adequate to predict the response of various practical problems, particularly when recognising the lack of information on the impact loading characteristics and the shortage of data on the properties of materials under dynamic loads. If simple methods of analysis are inadequate for a particular problem, then the understanding gained from this book provides a foundation for a reader to make further progress with other solution strategies. In particular, this understanding and insight are indispensable for the efficient use and interpretation of numerical codes which play an increasingly important role in engineering design. It is assumed that a reader has not studied previously the static plastic behaviour of structures, though a knowledge of elementary strength of materials is a prerequisite. Thus, Chapter 1 introduces some basic concepts of plasticity theory, from an engineering viewpoint, including the limit theorems of plasticity, and examines the static plastic collapse behaviour of several beams. Yield conditions for biaxial stress states are introduced in Chapter 2, together with the important normality condition which requires the generalised strain rate vector to remain normal to the associated portion of the yield curve during plastic flow. Theoretical solutions are presented for the static collapse behaviour of circular plates, rectangular plates and cylindrical shells. These two chapters contain all the basic elements from the theory of static plasticity which are required for the remainder of the book. The rigid-plastic approximations, which have been developed for the static plastic behaviour of structures in Chapters 1 and 2, are also used to obtain the dynamic plastic response of structures. The static plastic collapse load is the largest possible external load which may act on a perfectly plastic structure according to the limit theorems of plasticity. Thus, a structure is not in static equilibrium for larger external loads, so that inertia forces are generated, and motion commences. This motion continues until all the external energy has been consumed by internal plastic work. It is evident that the permanent displacements and response duration are of particular interest for dynamic loads. Chapters 3 to 5 examine, respectively, the dynamic plastic behaviour of beams, plates and shells, many of which were studied in Chapters 1 and 2 for static loads. The energy consumed in the plastic deformation dominates the elastic energy, so that a rigid-plastic method of analysis, which neglects elastic effects, is suitable. The yield conditions in Chapters 1 to 5 ignore the influence of transverse shear forces which were retained in the equilibrium equations. However, transverse shear forces are more important for dynamic loads than for similar problems subjected to static loads. In fact, failures may occur due to excessive transverse shear forces such as at hard points in structures loaded dynamically. Thus, Chapter 6 retains the transverse shear force in the yield conditions for dynamically loaded beams, circular plates and cylindrical shells, and contains some comments on the influence of rotatory inertia.
xvi Preface to the First Edition The theoretical solutions in Chapters 1 to 6 have been developed for infinitesimal displacements, since the equilibrium equations were derived in the initial undeformed configuration. This appears to be an unreasonable simplification when elastic effects are neglected and the external dynamic loads produce plastic strains and permanent deformations. However, there are some structural problems for which the displacements may be taken as infinitesimal without loss of accuracy. Nevertheless, there are other problems for which the influence of finite displacements plays a significant role in the dynamic response. The first part of Chapter 7 examines the influence of finite displacements, or geometry changes, on the static plastic behaviour of beams, circular plates and rectangular plates. An approximate kinematic method of analysis is also introduced in Chapter 7, and is used to examine the dynamic plastic response of beams, circular plates, rectangular plates and circular membranes. Comparisons are made with the corresponding experimental results, and a simple procedure is introduced to estimate the threshold impact energy which is required to cause structural failure due to material failure. The properties of many materials under dynamic loading conditions are different from the corresponding static values. In particular, the stress-strain relations are sensitive to the speed of a test, a phenomenon which is known as strain rate sensitivity, or viscoplasticity. The strain-rate-sensitive properties of various materials, under several dynamic loadings, are discussed in Chapter 8. The well-known Cowper- Symonds constitutive equation is introduced, and theoretical solutions are obtained for several structural problems, using various simplifications and approximations. The theoretical solutions in Chapters 1 to 8 have been developed for structures undergoing a stable response. However, an unstable response may develop in many practical problems. Thus, we study, in Chapter 9, the behaviour of a circular tube subjected to a dynamic axial load. This produces many axisymmetric convolutions, or wrinkles, in the tube, and gives rise to a fluctuating resistance about a mean crushing force. This phenomenon is known as dynamic progressive buckling, because the deformations form progressively with time from one end of a tube. The inertia forces in a tube are not significant, and are neglected, but the phenomenon of material strain rate sensitivity must be retained for a strain-rate-sensitive material. The mode of deformation is taken to be the same as for static loads. It transpires that considerable energy may be absorbed in an axially crushed tube before bottoming out. Thus, some comments on structural crashworthiness are given in Chapter 9. The phenomenon of dynamic progressive buckling in Chapter 9 develops, typically, at tens of metres per second. At higher-impact velocities, the inertia forces in a tube become important, and the mode of deformation may change into a more highly wrinkled form. This phenomenon is known as dynamic plastic buckling, and is studied in Chapter 10 for axially loaded columns, rings and cylindrical shells. Finally, Chapter 11 examines similitude, or geometrically similar scaling, which is important for relating the results of impact experiments on small-scale models to the response of geometrically similar full-scale prototypes. The various phenomena associated with structural impact are introduced in this book in a simple yet rigorous manner, as far as possible. The book should be useful, therefore, as a textbook for undergraduate and postgraduate students in universities
Preface to the First Edition xvii and polytechnics who are pursuing upper-level courses, projects and research investigations into structural impact, dynamic plasticity or advanced strength of materials. However, the book has been written with the designer in mind, so that it should be of value for a wide range of industries which have an interest in, and responsibility for, safety assessment and the evaluation of the response of structures subjected to dynamic loadings. Many postgraduate students and visitors who have worked with me over the past twenty years at the Massachusetts Institute of Technology and the University of Liverpool have contributed in various ways to this book. In particular, I wish to acknowledge the valuable assistance of Professor W. Abramowicz, Dr R. S. Birch, Mrs S. E. Birch, Dr J. C. Gibbings, Dr W. S. Jouri, Dr Jianhui Liu, Dr R. A. W. Mines, Mr Wen He Ming, Dr J. G. de Oliveira, Mr Jiang Ping, Professor W. Q. Shen, Dr W. J. Stronge, Professor H. Vaughan, Professor T. Wierzbicki, Professor Jilin Yu and Professor T. X. Yu. I take full responsibility for any errors in this book, and I should be grateful if readers would inform me of any they find. Thanks are also due to Dr George Abrahamson, Mr E. Booth, Professor W. Johnson, Professor S. B. Menkes and again Dr R. S. Birch for their assistance with the photographs, and to the American Society of Mechanical Engineers, Pergamon Press and Her Majesty s Stationery Office, for permission to reproduce some photographs. I wish also to record my appreciation to Mr F. Cummins, Mr H. Parker and Mrs A. Green for their assistance with the figures; to Mr R. Coates for some assistance with computing; and last, but not least, to my secretary, Mrs M. White, who typed the drafts and final copy over the extended gestation period of this book. May 1988 Preface to the Paperback Edition The author is grateful to all those readers who have pointed out misprints in the first edition and who have suggested improvements to the text. In particular, I am indebted to Mr Jiang Ping, Professor Jilin Yu and Professor Wang LiLi, who read it most thoroughly when preparing the Chinese edition; to my Ph.D. students, Dr M. Alves, Mr Q. M. Li and Mr C. C. Yang; and to Mr M. Moussouros. February 1997
STRUCTURAL IMPACT Second Edition