Introduction to Impact Dynamics E-Book

T. X. Yu

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 415

Veröffentlichungsjahr: 2017

Beschreibung

Fundamental guidance--including concepts, models, and methodology--for better understanding the dynamic behavior of materials and for designing for objects and structures under impact or intensive dynamic loading This book introduces readers to the dynamic response of structures with important emphasis on the material behavior under dynamic loadings. It utilizes theoretical modelling and analytical methods in order to provide readers with insight into the various phenomena. The content of the book is an introduction to the fundamental aspects, which underpin many important industrial areas. These areas include the safety of various transportation systems and a range of different structures when subjected to various impact and dynamic loadings, including terrorist attacks. Presented in three parts--Stress Waves in Solids, Dynamic Behaviors of Materials Under High Strain Rate, and Dynamic Response of Structures to Impact and Pulse Loading--Introduction to Impact Dynamics covers elastic waves, rate dependent behaviors of materials, effects of tensile force, inertial effects, and more. The book also features numerous case studies to aid in facilitating learning. The strength of the book is its clarity, balanced coverage, and practical examples, which allow students to learn the overall knowledge of impact dynamics in a limited time whilst directing them to explore more advanced technical knowledge and skills. * Considers both the dynamic behavior of materials and stress waves, and the dynamic structural response and energy absorption, emphasizing the interaction between material behavior and the structural response * Provides a comprehensive description of the phenomenon of impact of structures, containing both fundamental issues of wave propagation and constitutive relation of materials, and the dynamic response of structures under impact loads * Based on the authors' research and teaching experience as well as updated developments in the field Introduction to Impact Dynamics is the perfect textbook for graduate and postgraduate students, and will work as a reference for engineers in the fields of solid mechanics, automotive design, aerospace, mechanical, nuclear, marine, and defense.

Leseprobe

Introduction to Impact Dynamics

T.X. Yu

The Hong Kong University of Science and TechnologyKowloon, Hong Kong

XinMing Qiu

Tsinghua UniversityBeijing, China

This edition first published 2018 by John Wiley & Sons Singapore Pte. Ltd under exclusive licence granted by Tsinghua University Press for all media and languages (excluding simplified and traditional Chinese) throughout the world (excluding Mainland China), and with non‐exclusive license for electronic versions in Mainland China.© 2018 Tsinghua University Press

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The right of T.X. Yu and XinMing Qiu to be identified as the authors of this work has been asserted in accordance with law.

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Library of Congress Cataloging‐in‐Publication Data

Names: Yu, T. X. (Tongxi), 1941– author. | Qiu, XinMing, author.Title: Introduction to impact dynamics / by T.X. Yu, Prof. XinMing Qiu.Description: Hoboken, NJ ; Singapore : John Wiley & Sons, 2018. | Includes bibliographical references and index. |Identifiers: LCCN 2017033421 (print) | LCCN 2017044755 (ebook) | ISBN 9781118929858 (pdf) | ISBN 9781118929865 (epub) | ISBN 9781118929841 (cloth)Subjects: LCSH: Materials–Dynamic testing.Classification: LCC TA418.34 (ebook) | LCC TA418.34 .Y8 2018 (print) | DDC 620.1/125–dc23LC record available at https://lccn.loc.gov/2017033421

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Preface

Various impact events occur every day and everywhere in the physical world, in engineering and in people’s daily lives. Our universe and planet were formed as a result of a series of impact and explosion events. With the rapid development of land vehicles, ships, and aircraft, traffic accidents have become a serious concern of modern society. Landing of spacecraft, safety in nuclear plants and offshore structures, as well as protection of human bodies during accidents and sports, all require better knowledge regarding the dynamic behavior of structures and materials.

Obviously, impact dynamics is a big subject, which looks at the dynamic behavior of all kinds of materials (e.g. metals, concrete, polymers, and composites), and the structures under study range from small objects (e.g., a mobile phone being dropped on the ground) to complex systems (e.g., a jumbo jet or the World Trade Center before 9/11). The impact velocity may vary from a few meters per second (as seen in ball games) to several kilometers per second (as seen in military applications). Driven by the needs of science and engineering, the dynamic response, impact protection, crashworthiness, and energy absorption capacity of materials and structures have attracted more and more attention from researchers and engineers. Numerous research papers and monographs have appeared in the literature, and it is not possible for anyone to condense the huge amount knowledge out there into a single book.

This book is mainly meant as a textbook for graduate students (and probably also for senior undergraduates), aiming to provide fundamental knowledge of impact dynamics. Instead of covering all aspects of impact dynamics, the contents are organized so as to consider only its three major aspects: (i) wave propagation in solids; (ii) materials’ behavior under high‐speed loading; and (iii) the dynamic response of structures to impact. The emphasis here is on theoretical models and analytical methods, which will help readers to understand the fundamental issues raised by various practical situations. Numerical methods and software are not the main topic of this textbook. Readers who are interested in numerical modeling related to impact dynamics will have to consult other sources for the relevant knowledge.

The audience for this textbook may also include those engineers working in the automotive, aerospace, mechanical, nuclear, marine, offshore, and defense sectors. This textbook will provide them with fundamental guidance on the relevant concepts, models, and methodology, so as to help them face the challenges of selecting materials and designing/analyzing structures under intensive dynamic loading.

The contents of this textbook have been used in graduate courses at a number of universities. The first author (T.X. Yu) taught Impact Dynamics as a credit graduate course at Peking University, UMIST (now University of Manchester), and the Hong Kong University of Science and Technology. In recent years, he has also used part of the contents to deliver a short course for graduate students in many universities, including Tsinghua University, Zhejiang University, Wuhan University, Xi’an Jiaotong University, Taiyuan University of Technology, Hunan University, and Dalian University of Technology. The second author (X.M. Qiu) has also taught Impact Dynamics as a credit graduate course at Tsinghua University over recent years.

Using the content developed for these graduate courses, we authored a textbook in Chinese, entitled Impact Dynamics and published by Tsinghua University Press in 2011. Although the current English version is mainly based on this Chinese version, we have made many changes. For instance, some contents in Chapters 3 and 4 have been rewritten, and Chapter 10 containing case studies is entirely new for this English version.

As a textbook, we have adopted much content from relevant monographs, such as Meyer (1994) (for Chapters 3 and 4) and Stronge and Yu (1993) (for Chapter 6), including a number of figures. This is because that content clearly elaborated the respective concepts and methods with carefully selected examples and illustrations, which are particularly suitable for a graduate course. Those monographs have been cited accordingly in the relevant places, and we would like to express our sincere gratitude to the original authors.

We would also like to thank Ms. Lixia Tong of Tsinghua University Press, who gave us a great deal of help in preparing this book.

T.X. Yu and XinMing QiuJuly 2017

Introduction

With the rapid development of all kinds of transport vehicles, the lives lost and high cost of traffic accidents are of serious concern to modern society. The public is becoming increasingly aware of the safe design of components and systems with the objective of minimizing human suffering as well as the financial burdens on society. At the same time, many other issues in modern engineering, e.g., nuclear plants, offshore structures, and safety gear for humans, also require us to understand the dynamic behavior of structures and materials.

Driven by the needs of engineering, the dynamic response, impact protection, crashworthiness, and energy absorption capacity of various materials and structures have attracted more and more attention from researchers and engineers. As a branch of applied mechanics, impact dynamics aims to reveal the fundamental mechanisms of large dynamic deformation and failure of structures and materials under impact and explosive loading, so as to establish analytical models and effective tools to deal with various complex issues raised from applications.

In the classical theories of elasticity and plasticity, usually only static problems are of concern, in which the external load is assumed to be applied to the material or structure slowly, and the corresponding deformation of the material or structure is also slow. The acceleration of material is very small and thus the inertia force is negligible compared with the applied external load; hence the whole deformation process can be analyzed under an equilibrium state.

However, it is known that the material behavior and structural response under dynamic loading are quite different from those under quasi‐static loading. In engineering applications, the external load may be intensive and change rapidly with time, termed intense dynamic loading; consequently, the deformation of material or of a structure has to be quick enough under intense dynamic loading. Some examples are given in the following.

The collision of vehicles. Cars, trains, ships, aircraft, and other vehicles may collide with each other or with surrounding objects during accidents. These accidents will lead to the failure or deformation of the structures as well as personnel casualties, resulting in serious economic losses. As the number of cars has rapidly increased in many countries, car accidents have become the number one cause of death in the world. Collisions between ships and collisions between ships and rocks/bridges all cause huge economic loss as well as environmental pollution. Along with the development of high‐speed rail transportation, the safety of occupants is also of greater public concern. It is very dangerous if a bird impinges on the cockpit or engine of an airplane, as the relative velocity between bird and airplane could be high even though the speed of the bird is not great. More and more space debris has been produced as a result of human activities, and the relative velocity of space debris to spacecraft can be as high as 10 km/s, so there would be great damage in the case of a collision.

Damage effects of explosive. Buildings, bridges, pipelines, vehicles, ships, aircrafts, and protective structures could be subjected to intensive explosion loading, due to industrial accident, military action, or terrorist attack. Typically, these structures would be suddenly loaded by a shock wave propagating in the air.

The effects of natural disasters. Natural disasters, such as earthquakes, tsunamis, typhoons, floods and so on will produce intensive dynamic loads to structures, e.g., dams, bridges, and high‐rise buildings. These intense dynamic loads are likely to cause damage to the structures.

The strong dynamic loads caused by the local rupture of the storage structures. In nuclear power plants or chemical plants, if there is local damage to a pipeline, the jet of high‐pressure liquid that would escape from the broken section exerts a lateral reaction force (the blowdown force) on the broken pipe, causing rapid acceleration and large deformation, termed “pipe whip”. After local damage, the consequences from a pressure vessel or a dam could be disastrous.

Load of high‐speed forming. During a dynamic metal forming process, such as explosive forming and electromagnetic forming, the work‐piece is subjected to intensive dynamic loading and deforms rapidly. Similar situations take place in the process of forging or high‐speed stamping.

Impact or collision in daily life and sports. For example, falling objects, falling on the ice, collision between moving people, a football or golf ball hitting the head or body with high speed.

All kinds of the above‐mentioned problems encountered in engineering or daily life require the understanding and study of the behavior of the solid materials and structures subjected to intensive dynamic loads. First of all, why is the dynamic behavior of materials and structures usually different from the quasi‐static behavior? This is the result of three major attributes in mechanics, as briefly illustrated here.

Stress wave propagation in material and structure. When a dynamic load is applied to the surface of a solid, the stress and generalized deformation will propagate in the form of stress wave. If the disturbance is weak, it is an elastic wave; but if the stress level of wave is higher than the yield strength of the material, it will be plastic wave.

Suppose a solid medium has a characteristic scale of L, and the wave speed of its material is c. It is subjected to an external dynamic loading that has a characteristic time tc, e.g., the time period for the external load to reach its maximum value or time duration of the impulse. If , the stress and deformation distribution in this solid are not uniform; hence, the effect of stress wave propagation must be considered. For example, the characteristic scale of the crust is very large, so the effects of earthquake or underground explosion are mainly presented in the form of stress waves.

In a piling machine or a split Hopkinson pressure bar device (SHPB for short – an important experimental technique in studying the dynamic properties of materials; refer to Chapter 3 for more details), the perturbation is along the longitudinal (large scale) direction of a long bar rather than in the radial (small scale) direction. Hence wave reflection, transmission, and dispersion are important factors that need to be analyzed carefully.

By contrast, some other structural components that are widely used in engineering, such as beams, plates, and shells, are usually subjected to lateral loads along their thickness direction, which is the smallest scale direction of the structure. The elastic wave speed in metals is usually in the order of several kilometers per second (e.g., 5.1 km/s for steel). Therefore, in several micro‐seconds all the particles in the thickness direction of the structure will be affected by the external disturbance, and then the entire section of structure will be accelerated and then move together. This global motion of the whole cross‐sections of the structure is classified as the elastic‐plastic dynamic response of the structure; this is discussed in detail in Part 3 of this book. This subsequent global structural response may last several milliseconds or even several seconds, depending on the type of structure and loading, before the structure reaches its maximum deformation.

Because the effective time of stress wave propagation is usually several orders of magnitude smaller than that of the long‐term structural response, the total response of the structure can be divided into two decoupled separate stages. That is, in the analysis of wave propagation, the structure is assumed to remain in its original configuration, which is regarded as the reference frame for geometric relations and equations of motion, while in the analysis of structural response, the early time wave propagation is disregarded and only its global deformation is considered.

Rate‐dependency of a material’s properties. The material in a solid or structure will deform rapidly under intensive dynamic loading. Depending on the microscopic deformation mechanism, the resistance of material to rapid deformation is generally higher than that to slow deformation, as revealed by numerous experiments on materials. For example, the mechanism of plastic deformation of metals is mainly attributed to the movement of dislocations. The resistance to the dislocation motion will be much higher when the dislocation passes through the metal lattice at a high speed than at a low speed, and this will lead to the higher yield stress and the high flow stress of metals during high‐speed deformation.

An important task in the study of dynamic properties of materials is to summarize the effect of strain rate on the stress–strain relationship, based on the experimental data, so as to establish the strain rate‐dependent constitutive relation of materials. As the strain history and instantaneous strain rate of the material elements inside a structure vary with position and time, the dynamic constitutive relation has to be simplified to a large extent when it is applied to dynamics analysis of structures.

Inertia effect in structural response. In the analysis of dynamic response of a structure, usually both elastic deformation and plastic deformation exist, and the boundary between elastic‐plastic regions changes with time. Therefore, different constitutive relations should be employed in different regions. Further, the complicated moving boundary has to be dealt with. In order to reduce the complexity, the constitutive relation of material also needs to be simplified in the theoretical modeling of structural dynamics. The most successful idealization commonly adopted in theoretical modeling is to assume that the structure is made of a rigid‐perfectly plastic material, which neglects all the effects of elasticity, strain hardening and strain rate.

The basis of the hypothesis is that the structure usually experiences considerable large plastic deformation under intensive dynamic loading, and thus most of the work done by the external load will be dissipated by plastic deformation, while only a small amount of external work will be stored in the form of elastic deformation energy. Therefore, to ignore the elastic deformation and the corresponding energy will only result in minor influence over the final deformation and failure mode of the structure. As will be seen in the book, this idealization largely simplifies the analysis.

Due to the similarity of material idealization, the dynamic analysis of rigid‐perfectly plastic structures is closely related to the limit analysis of structures, especially in its concept and methodology. For example, the widely accepted concepts in limit analysis in the kinematically admissible velocity field can be successfully extended to construct dynamic deformation mechanisms containing stationary or traveling plastic hinges.

At the same time, it should be noted that the main difference between dynamic analysis and limit analysis lies with the intervention of the inertia effect in dynamic analysis. The limit analysis based on the theory of plasticity reveals that a structure made of rigid‐perfectly plastic material under external load must have a limit state, i.e., if the external load reaches a certain limit value, the structure will become a mechanism and lose the load‐carrying capacity. On the other hand, from the dynamic analysis of the same structure, it is found that if the dynamic load exceeds the static limit load, i.e., the collapse load, the structure will be accelerated. According to the D’Alembert principle, it is the inertia force of the structure that is in equilibrium with the external load and that resists deformation. The greater the external load, the greater the acceleration, so the greater the inertia force. Thus, the structure can bear a much higher external load than the static limit load in a short time, which is a notable feature of the structural dynamic response and is different from the static limit analysis.

Generally speaking, dynamic loading and dynamic response always become significant when accidents occur. Nowadays, alongside the development of computing capability and software, different kinds of numerical tools and methods have been developed very rapidly, so they have wider and wider applications. Therefore, some researchers think that it is appropriate to employ numerical simulations in handling problems of impact dynamics. However, even for numerical simulations, a proper understanding of the basic principles, concepts, and theoretical models used in dynamic analyses is crucial if simulation methods and models are to give reliable results. Furthermore, the numerical simulations will result in a huge amount data, so our understanding of impact dynamics will greatly help us to digest the data and discover the underlying physical significance and engineering implications.

This textbook aims to demonstrate the fundamental features of the dynamic behavior of materials and structures, to clearly illustrate the widely applicable theoretical models and analytical methods, and to highlight the most important factors that affect dynamic behavior. The textbook is highly relevant to education programs at both graduate and senior undergraduate levels. For those engineers who are working in the automotive, aerospace, mechanical, nuclear, marine, offshore, and defense sectors, the book will also provide fundamental guidance on relevant concepts, models, and methodology, to help them face the challenges of understanding the dynamic behavior of materials and of analyzing and designing structures under various types of intensive dynamic loading.

Part 1Stress Waves in Solids