Strength and Toughness of Materials

276 Pages·2004·10.065 MB·English

by  Toshiro Kobayashi (auth.)

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
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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