Springer Japan KK Toshiro Kobayashi Strength and Toughness of Materials With 313 Figures Springer Toshiro Kobayashi Vice-President Toyohashi University of Technology 1-1 Hibarigaoka, Tempaku-cho, Toyohashi Aichi 441-8580, Japan Library ofCongress Cataloging-in-Publication Data applied for. ISBN 978-4-431-67973-8 ISBN 978-4-431-53973-5 (eBook) DOI 10.1007/978-4-431-53973-5 This English translation is based on the Japanese original: Strength and Toughness of Materials by T. Kobayashi Published by AGNE Gijutsu Center © 2000 Toshiro Kobayashi Printed on acid-free paper © Springer Japan 2004 Originally published by Springer-Verlag Tokyo in 2004 This work is subject to copyright. AII rights are reserved, whether the whole or part of the material is concemed, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. The use ofregistered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. SPIN: 10958810 Preface Materials are very important elements supporting various technological fields. However, students do not necessarily want to study in this field. It may be that they can not conceptualize a concrete image of materials. Even in the limited field of engineering, development of each technology depends mainly on materials. Materials often become a key problem also in design and R&D. The most basic theme in those efforts seems to exist in materials. Recently used materials, including ceramics and polymers, reach a very wide range and a shift from metal age is underway. However, most advanced materials have inherent brittleness; therefore their practical use is limited in many cases. In this respect, metal is superior in toughness and is used largely in various fields. What is toughness? In short, it represents a resistance to fracture; moreover, high strength is implicitly expected. Material having high strength and fracture resistance is demanded in new applications: metal is used widely for such reasons. Notwithstanding, we must find a new way to develop high temperature properties, which have reached a point of saturation in metallic materials. Ceramics and intermetallic compounds are very attractive in this respect. However, it will be difficult to utilize them immediately as structural materials for their brittleness. Moreover, strength and toughness show a generally contradictory tendency. It is a mission of material engineers to solve such a contradiction and to realize practical utilization. To realize high strength and toughness, it is necessary to clarify the fracture mechanism of materials first. Fracture mechanics initiated in the 1950's were very effective to prevent fracture accidents. This is one field of continuum mechanics; it treats mechanical behavior of materials macroscopically. On the other hand, material engineers generally seek to develop high fracture toughness from controlling of microstructure of materials. Therefore there is the necessity to explain macroscopic fracture behavior based on a microscopic fracture mechanism. The scale problem in fracture, therefore, has been presented before. It is not an easy problem, but continued study must be done and some efforts have been attempted in preparation for this book. Strength and toughness are everlasting themes in materials and material scientists and mechanical engineers must cooperate to solve these problems. The VI author has built a laboratory for materials assurance. This is an interdisciplinary laboratory between materials science and fracture mechanics. No such reference book like this book has ever been found. This book was summarized based on research work carried out in the laboratory. It forms a basis of laboratory's principle. The book will be most suitable to graduate students as a text, but it is also beneficial for undergraduate students, general engineers and researchers in the fields of materials and mechanics. Each chapter is edited mainly on the author's published review papers in Japanese. The author would like to thank such publishers and also Prof. Emeritus Seiki Nishi (Nagoya University, Toyohashi University of Technology) and the late Hideyo Maniwa (formerly Central Research Laboratory, Fuji Electric Co., Ltd.) for their guidance to this field and hearty support. The author expresses his thanks also to coworkers Mr. Koichi Takai (formerly Central Research Laboratory, Fuji Electric Co., Ltd.), Prof. Mitsuo Niinomi and Prof. Hiroyuki Toda (Toyohashi University of Technology) for their helpful discussion and assistance. Finally, the author would like to thank related graduate students of Nagoya University and Toyohashi University of Technology for their experimental work and assistance. The English version of this book was permitted by AGNE Technical Center (Tokyo). Prof. Lei Wang of Northeastern University of P. R. China and Dr. Zheng Ming Sun of AIST Tohoku assisted translation from Japanese. Critical reading on English was also assisted by Mr. Brad Fast of Fastec, Ltd. . They are deeply appreciated. This publication was supported by the 2003 Grant-in Aid for Publication of Scientific Research Results of Japan Society for the Promotion of Science. The author would like to thank the society. Finally, I dedicate this book to my wife Fumiko. Toshiro Kobayashi Contents Preface............................................................ V 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Development of Materials and their Characteristics .............. 1 1.2 Fracture and Damage ....................................... 5 1.3 Rise of Fracture Mechanics and Strengthening and Toughening.... 10 2 Basic Concepts of Fracture Mechanics. . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.1 Fracture Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.1.1 General Concepts of Fracture Toughness from an Energy Criterion ........................................... 17 2.1.2 Linear Elastic Fracture Mechanics in a Crack-tip Stress Field 19 2.1.3 Plastic Zone at Crack-tip. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 23 2.2 Elastic-Plastic Fracture Mechanics. . . . . . . . . . . . . . . . . . . . . . . . . . .. 24 2.3 Measurement of Fracture Toughness. . . . . . . . . . . . . . . . . . . . . . . . . .. 26 2.4 Application of Fracture Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28 3 Principles of Strength and Toughness. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33 3.1 Classical Fracture Theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33 3.2 Microstructure and Fracture Mechanism ....................... 37 3.3 Inexpensive Toughness Evaluation Method-Instrumented Charpy Impact Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 41 3.4 Specimen Size Effect and J-Q Theory. . . . . . . . . . . . . . . . . . . . . . . . .. 48 4 Steels..... ... ........ ... ........ ... .. ..... ..... ..... ... ... .... 53 4.1 Solid Phase Transformation in Steels ..... . . . . . . . . . . . . . . . . . . . .. 54 4.1.1 Precipitation of Proeutectiod Ferrite. . . . . . . . . . . . . . . . . . . .. 56 4.1.2 Pearlitic Transformation .............................. 57 4.1.3 Bainitic Transformation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 59 4.1.4 Martensitic Transformation. . . . . . . . . . . . . . . . . . . . . . . . . . .. 60 4.2 Correlations among Strength, Fracture and Microstructures ....... 62 VIII Contents 4.3 Strengthening and Toughening of Practical Steels. . . . . . . . . . . . . . .. 68 4.3.1 Ferritic-Pearlitic Steel ................................ 68 4.3.2 Bainitic and Martensitic Steels. . . . . . . . . . . . . . . . . . . . . . . .. 71 4.3.3 Maraging Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 71 4.3.4 TRIP Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 72 4.3.5 Dual Phase Steel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 77 4.3.6 Controlled Rolling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 78 4.4 Degradation in Steels ....................................... 80 4.5 Strength and Fracture of Carburized Steel . . . . . . . . . . . . . . . . . . . . .. 82 5 Ductile Cast Iron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.1 Fracture Mechanism in Ductile Cast Iron. . . . . . . . . . . . . . . . . . . . . . . 90 5.2 Evaluation of Fracture Toughness. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 92 5.2.1 Definition of a Crack Initiation Point. . . . . . . . . . . . . . . . . . .. 92 5.2.2 Ductile-Brittle Transition Behavior. . . . . . . . . . . . . . . . . . . . . 94 5.3 Influence of Microstructure on Fracture Toughness .............. 96 5.3.1 The Effect of Matrix Microstructure .................... 96 5.3.2 Effects of Morphology and Distribution of Graphite . . . . . .. 99 5.4 Strengthening and Toughening of Ductile Cast Iron .............. 101 5.4.1 Austempered Ductile Cast Iron ......................... 101 5.4.2 Strengthening and Toughening Based on Traditional Matrix Phases ............................................. 105 5.5 Fatigue Characteristics of Ductile Cast Iron ..................... 106 6 Wrought Aluminum Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 111 6.1 Aluminum Alloys and their Features at Deformation . . . . . . . . . . . .. 111 6.2 Microstructure and the Fracture Mechanism .................... 115 6.2.1 General Relationship between Strength and Fracture in Aluminum Alloys .................................... 115 6.2.2 Formation of Voids and Secondary Phase Particles in Aluminum Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 116 6.2.3 Growth and Coalescence Processes of Voids .............. 120 6.3 Ductile Fracture Details .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 122 6.3.1 Classification of Deformation and Fracture Mechanisms for Age Hardening-type Alloys ......................... 122 6.3.2 Ductile Fracture Theories ............................. 125 6.4 Testing Methods for Fracture Toughness of Aluminum Alloys-R Curves Method ............................................ 130 6.5 Toughness of Aluminum Alloys and the Metallurgical Factors . . . .. 134 6.5.1 AI-Li Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 134 6.5.2 Other Wrought Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 137 Contents IX 7 Cast Aluminum Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 141 7.1 Aluminum Alloy Casting and Solidification .................... 141 7.2 Solidification Microstructure and Fracture Toughness ............ 144 7.2.1 Secondary Phase Particle and Fracture .................. 144 7.2.2 Influence of Dendrite Arm Spacing ..................... 146 7.2.3 Effects of Gas Content and Impurities . . . . . . . . . . . . . . . . . .. 147 7.2.4 Influence of Modification Treatment ..... '" ............ 150 7.2.5 Influence of Casting Defects ........................... 154 7.3 Fatigue Characteristics ...................................... 154 8 Metal Matrix Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 163 8.1 Key Points of Composite Materials. . . . . . . . . . . . . . . . . . . . . . . . . . .. 163 8.2 General Deformation and Fracture Mode ....................... 166 8.2.1 Formation of Microdamage Caused by Deformation ....... 166 8.2.2 Fracture Process ................................... " 171 8.2.3 Crack Growth Mode under Monotonic Loading. . . . . . . . . .. 172 8.3 Improvement of Fracture Characteristics by Controlling MMC Microstructure ........................................... " 175 8.3.1 Microstructural Factor of Reinforcement. .............. " 175 8.3.2 Microstructural Factors About Interfaces ................ 178 8.3.3 Microstructural Factors About the Matrix. . . . . . . . . . . . . . .. 179 8.4 Fatigue Fracture Behavior ................................... 182 8.4.1 Short Fatigue Crack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 183 8.4.2 Long Fatigue Crack .................................. 184 9 Titanium Alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 189 9.1 Mechanical Characteristics of Titanium Alloys ................ " 189 9.1.1 Mechanical Properties of Titanium Alloys ............... 189 9.1.2 Classification of Titanium Alloys and their Mechanical Properties .......................................... 191 9.2 Influence of Microstructure on Fracture Toughness .............. 192 9.2.1 Equiaxed a Microstructure .......................... " 192 9.2.2 Acicular a Microstructure ............................. 194 9.2.3 Microstructural Units Controlling Crack Propagation Initiation Toughness ................................ " 198 9.3 Micromechanism of Crack Initiation and Crack Propagation. . . . . .. 199 9.4 Embrittlement and Strengthening of Titanium Alloys by Hydrogen. 203 9.4.1 Embrittlement. ...................................... 203 9.4.2 Strengthening ....................................... 204 9.5 Strain Induced Transformation and Mechanical Properties ........ 205 10 Intermetallic Compounds ..... ............. '" .................. 209 10.1 Application of Fracture Mechanics Testing . . . . . . . . . . . . . . . . . . . .. 211 10.1.1 Effect of Specimen Size ............................... 211 10.1.2 Notched Specimens .................................. 212 X Contents 10.1.3 Detection of Crack Initiation Point. ..................... 213 10.2 Influence of Alloying ....................................... 214 10.3 Influence of Microstructure Control ........................... 216 10.3.1 Ti3Al-based Alloy ................................... 216 10.3.2 TiAl-based Alloys ................................... 217 10.3.3 Composite Materials ................................. 221 10.4 Environmental Embrittlement .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 222 10.4.1 Hydrogen Absorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 222 10.4.2 Hydrogen Embrittlement .............................. 223 11 Ceramics...................................................... 227 11.1 Characteristics of Strength and Toughness in Ceramics . . . . . . . . . .. 227 11.1.1 Linear Elastic Fracture and Non-linear Fracture ........... 227 11.1.2 Influence of Various Material Science and Mechanical Factors on Fracture Toughness . . . . . . . . . . . . . . . . . . . . . . . .. 232 11.1.3 Strengthening and Toughening for Ceramics ............. 237 11.2 Evaluation Methods for Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . .. 239 11.2.1 Analysis Method of Absorbed Energy by Instrumented Charpy Testing Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 239 11.2.2 Dynamic Fracture Toughness Testing ................... 242 12 Polymers...................................................... 247 12.1 Characteristics and Deformation Mechanisms of Polymers. . . . . . .. 247 12.2 Mechanical Properties of Polymers. . . . . . . . . . . . . . . . . . . . . . . . . . .. 251 12.2.1 Fracture Toughness .................................. 251 12.2.2 Instrumented Charpy Impact Testing .................... 252 12.2.3 Fatigue Crack Propagation Characteristics ............... 259 12.2.4 Usual Fatigue and Impact Fatigue Tests .................. 260 SI Units and Conversion Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 265 Index ............................................................. 269 1 Introduction This chapter generalizes main points of strength and toughness to state the background of the book constitution and the outline. 1.1 Development of Materials and their Characteristics The phrases 'new material' and 'advanced material' have been used frequently since the 1980s. Such various advanced materials are stealing the limelight in the age of the traditional metal zenith. When a substance that exists in nature is modified by some processing and is used by human beings, it acquires the status of a "material". In the present age of high-technology, requirements for high-quality materials have become stronger. However, when targets become a ceramic, intermetallic compound or composite material, it is never easy to put this to practical application. It seems that a cheap material like steel, which has both good strength and toughness, will be very difficult to develop further. Figure 1.1 shows a rough summary of the change of materials in mankind's long history [1]. Cutting tools that civilizations possessed before 2000 BC were stone. Before metal was used, the main materials were polymers (wood, straw and skin). It is said that civilization began to make use of bronze from about 1500 BC, and to use steel from about 1850. From that time, the importance of steel increased remarkably. That prominence reached a peak at around 1960. However, conditions changed from the 1980s, introducing an age of coexistence of four kinds of materials (three main kinds of materials of metal, ceramics, polymers, and their composites) including new metals. Figure 1.2 shows the model of the degree of maturity in the market for materials [2]. It can be predicted that traditional metals have saturated their markets, while polymers, ceramics, composite materials and new metals will increase rapidly. In the metallic materials field, requirements for light weight, high strength, and good heat resistance have become more and more stringent. Recently, from the viewpoint of harmony with the earth's environment, ecomaterials have also become important. When characteristics of the three major kinds of the materials were considered from the viewpoint of the atomic bond, polymers are formed with covalent bonds T. Kobayashi, Strength and Toughness of Materials © Springer Japan 2004