氏名　Akhmad Ardian Korda
学位の種類　博士（工学）
学位記番号　博甲第385号
学位授与の日付　平成18年8月31日
学位論文題目　EFFECT OF PEARLITE MORPHOLOGY ON FATIGUE CRACK GROWTH BEHAVIOR IN FERRITE-PEARLITE STEELS
論文審査委員
　主査　システム安全系　教授　武藤　睦冶
　副査　教授　古口　日出男
　副査　教授　岡崎　正和
　副査　システム安全系実務家　教授　永田　晃則
　副査　助教授　井原　郁夫

• 論文目次
• 論文要旨
• 論文審査の結果の要旨

［平成18（2006）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

TABLE OF CONTENTS

Abstract　p.ii
Dedication　p.iv
Abstract　p.ii
Dedication　p.iv
Acknowledgments　p.v
Table of Contents　p.vi
List of figures　p.ix
List of tables　p.xvi
Nomenclature　p.xvii

Chapter1- Introduction and scope of work　p.1

　1.1- Introduction to structural steel plate　p.2
　1.2- Introduction to fatigue crack growth　p.4
　1.2.1- Fracture mechanics approach　p.4
　1.2.2- Fatigue crack growth curve　p.5
　1.3- Problem statement　p.6
　1.4- Literature review　p.7
　1.4.1- Effect of microstructure on fatigue crack growth behavior　p.7
　1.4.2- Fatigue crack closure　p.8
　1.4.3- Crack tip stress shielding phenomena　p.10
　1.5- Research motivation & objectives　p.12
　1.6- Work limitations　p.16
　1.7- Dissertation outline　p.17
　1.8- References　p.18

Chapter2- Effect of pearlite morphology on fatigue crack behavior of ferrite-pearlite steels　p.25
　2.1- Introduction　p.26
　2.2- Experimental procedures and materials　p.27
　2.2.1- Materials　p.27
　2.2.2- Fatigue test specimen　p.29
　2.2.3- Fatigue crack growth testing　p.30
　2.3- Results and discussion　p.32
　2.3.1- Fatigue crack growth curve　p.32
　2.3.2- Crack growth path　p.32
　2.3.3- Crack closure behavior　p.38
　2.3.4- Effect of pearlite morphology　p.41
　2.4- Conclusions　p.43
　2.5- References　p.44

Chapter3- In situ observation of constant-ΔK fatigue crack growth behavior in ferrite-pearlite steels　p.46
　3.1- Introduction　p.47
　3.2- Experimental procedures and materials　p.48
　3.2.1- Materials　p.48
　3.2.2- Fatigue crack growth testing　p.49
　3.3- Results and discussion　p.53
　3.3.1- Constant-ΔK fatigue crack growth test　p.53
　3.3.2- Correlation between crack growth rates and crack growth paths　p.54
　3.3.3- Fracture surfaces　p.65
　3.3.4- Crack closure and crack tip stress shielding behavior　p.69
　3.4- Conclusions　p.73
　3.5- References　p.74

Chapter4- Effect of pearlite band spacing on fatigue crack growth behavior in ferrite-pearlite steels　p.76
　4.1- Introduction　p.77
　4.2- Experimental procedures and materials　p.79
　4.3- Results and discussion　p.82
　4.3.1- Fatigue crack growth testing　p.82
　4.3.2- In situ SEM observation constant-ΔK fatigue test　p.85
　4.3.3- Crack closure and crack tip stress shielding behavior　p.88
　4.4- Conclusions　p.92
　4.5- References　p.93

Chapter5- Effects of pearlite morphology on plastic zone size　p.94
　5.1- Introduction　p.95
　5.2- Experimental procedures and materials　p.96
　5.2.1- Materials　p.96
　5.2.2- Fatigue crack growth testing　p.97
　5.2.3- Strain controlled fatigue testing　p.98
　5.3- Results and discussion　p.101
　5.3.1- Observed plastic zone size　p.101
　5.3.2- Strain controlled fatigue test　p.104
　5.3.3- Plastic zone size and fatigue crack growth behavior　p.108
　5.4- Conclusions　p.112
　5.5- References　p.113

Chapter6- Crack tip stress shielding in ferrite-pearlite steels　p.114
　6.1- Introduction　p.115
　6.2- Experimental procedures and materials　p.117
　6.3- Results and discussion　p.120
　6.3.1- constant-ΔK fatigue crack growth test　p.120
　6.3.2- Effective stress intensity factor range　p.123
　6.3.3- Crack tip effective stress intensity factor tip range　p.124
　6.4- Conclusions　p.127
　6.5- References　p.128

Chapter7- Effect of specimen thickness on fatigue crack growth behavior　p.130
　7.1- Introduction　p.131
　7.2- Experimental procedures and materials　p.132
　7.2.1- Materials　p.132
　7.2.2- Fatigue test specimen　p.134
　7.2.3- Fatigue crack growth testing　p.134
　7.3- Results and discussion　p.136
　7.3.1- Fatigue crack growth curve　p.136
　7.3.2- Crack growth path　p.138
　7.3.3- Crack closure behavior　p.140
　7.4- Conclusions　p.142
　7.5- References　p.142

Chapter8- General conclusions and recommendations　p.144
　8.1- General conclusions　p.145
　8.2- Recommendations for further work　p.149

Besides of strength as a basic requirement of steel structures, fracture resistance of the structural steel is also extremely required since it is known that failure problems of structures and machines reported are mainly caused by fatigue. Unfortunately, the fatigue crack growth （FCG） rate is nearly the same regardless of material strength. Therefore, the improvement of fatigue strength and fatigue crack growth （FCG） resistance of structural materials as well as development of safety design and fabrication processes is strongly required. For this purpose, structural steel plates with excellent resistance to FCG have been newly developed to improve structural integrity. It is well known that the microstructure has significant influence on FCG behavior in threshold region whereas it is less in Paris regime. These regions have received the most attention since they dominate the crack propagation life. The effect of microstructure in threshold region has been widely reported. However, only limited information is available on detailed influence of microstructure on FCG resistance in Paris regime. The understanding on microstructure contributions to FCG resistance in structural steels is significantly required to produce high FCG resistance steels. Therefore, crack propagation mechanisms which can improve FCG resistance become an important key.
　Chapter 1 introduces structural steel, FCG and microstructural aspects of FCG behavior. Problem statement and a brief literature review of topics relevant to this dissertation are presented. In this chapter, the motivation and objectives are also specifically described. Work limitations and dissertation outline are also described.
　In chapter 2, in situ SEM observations of FCG behavior in ferrite-pearlite structural steels plates with different pearlite morphologies was investigated. The steels plates with islands of pearlite colonies （Steel N）, uniformly distributed pearlite （Steel D） and banded pearlite （Steel B） microstructure were used in this study. Stress intensity factor DK decreasing/increasing FCG tests have been carried out. The results showed that the role of pearlite in encountering the crack propagation depends on the morphology of the pearlite. The same crack growth behavior was observed in the Steel D in long and short transverse direction. High FCG resistance was observed in the Steel D and Steel B. The difference in FCG behavior is explained by using crack closure. High crack closure level in steels with banded and distributed pearlite encountered the crack propagation were promoted by roughness-induced closure due to large angle crack deflections at the interface of ferrite-pearlite.
　In chapter 3, in situ SEM constant-DK FCG tests of ferrite-pearlite steels were carried out. The tests were performed to understand the contribution of pearlite morphology to the fatigue crack growth resistance in detail especially in Paris regime. Crack growth rate versus crack length were plotted along with microstructures of the steels. In situ observation revealed that FCG rates decreased due to crack deflection and crack branching. Steel B induces more large-angled crack deflections and crack branchings which then indicates higher crack growth resistance. Steel D induces large-angled crack deflections. Steel N indicates lower FCG resistance. The differences in FCG behavior are explained by using crack closure mechanism and crack tip stress shielding phenomena.
　In chapter 4, effect of pearlite band spacing on FCG behavior in ferrite-pearlite structural steels was investigated. In situ scanning electron microscope （SEM） observations of crack growth behavior were carried out during the tests. Crack closure was also monitored to discuss the fatigue crack growth resistance in detail. The results show that the banded ferrite-pearlite steel with narrow pearlite band spacing has superior FCG resistance compared to the banded ferrite-pearlite steel with large pearlite band spacing. The difference in FCG behavior is explained by using crack closure and crack tip stress shielding phenomena. High crack closure level in the narrow banded ferrite-pearlite steel was promoted by roughness-induced closure due to frequent large angle crack deflections at the interface of ferrite-pearlite. The crack tip stress shielding introduced by the frequent crack branching and large angle crack deflection is suggested to be the reason for the higher FCG resistance in the narrow banded ferrite-pearlite steel.
　In chapter 5, effect of pearlite morphology on plastic zone size was investigated. In situ SEM observations on the plastic deformation at crack tip vicinity and along the crack path of fatigue crack growth tests under a constant-DK in the Paris regime were carried out. The results show that the Steel D and Steel B inhibited the extent of plastic deformation around the crack tip compared to that in the Steel N. The observed plastic zone size was found to be smaller than the estimated plastic zone size. The constraint of plastic deformation due to the uniformly distributed pearlite and banded pearlite microstructure, which results in smaller plastic zone size, may also contribute to significant crack closure. The rate of crack growth can be roughly correlated with the size of plastic zone. The smaller the plastic zone size, the slower the FCG.

　In chapter 6, crack tip stress shielding phenomena in the ferrite-pearlite structural steels was investigated. The crack growth curves of the Steel N and Steel D arranged by using DKeff, where crack closure behavior is taken into consideration, didn't coincide, which indicated that factors other than crack closure, such as crack tip stress shielding phenomena, would play an important role. Ktip was experimentally evaluated by measuring the crack mouth opening displacement. The crack growth curves arranged by using DKeff,tip, where crack tip shielding behavior as well as crack closure are taken into consideration, successfully coincided on the one line.
　In chapter 7, effect of specimen thickness on FCG behavior in the ferrite-pearlite was investigated. DK-decreasing/increasing FCG test was carried out by using a standard compact tension （CT） specimen with 6 mm thickness and a single edge cracked plate tension （SECT） specimen with 1.5 mm thickness. The results showed that the specimen thickness had significant effect on the rate of FCG. Fatigue crack growth path of thin and thick specimens in both the steels was observed to have the same manner. Fatigue crack growth rate of both the steels increased as the specimen thickness increased. The thin specimen indicated higher crack closure level compared to the thick specimen. The high crack closure level results in more retardation of crack growth. Therefore, crack growth rate would be decelerated with decreasing specimen thickness.
　In chapter 8, the study of the effect of pearlite morphology on FCG behavior in ferrite-pearlite structural steels is summarized. The most significant research results and their significance are described here. Future research topics are also presented.

　本論文は、「Effect of Pearlite Morphology on Fatigue Crack Growth Behavior in Ferrite-Pearlite Steels（フェライト・パーライト鋼の疲労き裂伝ぱ挙動に及ぼすパーライト形態の影響）」と題し、8章より構成されている。
　第1章「緒論」では、構造用鋼の疲労、疲労と組織の関係などに関する従来の研究の概要を示すとともに、本研究の目的と範囲を述べている。
　第2章「疲労き裂伝ぱに及ぼすパーライト形態の影響」では、3種類のパーライト形態の材料について疲労き裂伝ぱ試験を行い、き裂伝ぱ特性に及ぼすパーライト形態について検討し、パーライト粒が一様に分散した組織および層状に分散した組織が、ネットワーク状に分散した組織に比べ、伝ぱ抵抗が優れていること、き裂開閉口挙動が顕著に求められることなどを明らかにしている。
　第3章「ΔK一定下でのき裂伝ぱ挙動のその場観察」では、第1章で得られた結果をさらに詳細に検討するため、電子顕微鏡を用い、ΔK一定下でのき裂伝ぱ挙動のその場観察を行っている。その結果、き裂のブランチングおよびインターロッキングがき裂伝ぱ抵抗を高めていること、前者は層状組織の場合顕著であり、後者は均一分散の場合に顕著であることなどを明らかにしている。
　第4章「パーライト層間隔の疲労き裂伝ぱ挙動に及ぼす影響」では、層状組織材のパーライト層間隔を変化させた試料を用意し、ΔK一定のその場観察疲労き裂伝ぱ試験を行い、層間隔が小さい方が、細かなブランチングを多量に生じ、その結果、伝ぱ抵抗を高めていることなど、を明らかにしている。
　第5章「パーライト形態が塑性域寸法に及ぼす影響」では、同じΔKで、破壊力学的には同一の塑性域となる場合でも、実際の塑性域は、パーライト形態の影響を顕著に受け、均一分散の場合や層状組織の場合はかなり小さくなることなどを示している。
　第6章「き裂先端応力遮蔽効果」では、インターロッキングやブランチングによる、き裂先端での応力拡大係数の遮蔽効果を考慮した、新たな力学パラメータ、ΔKeff,tip、を提案し、き裂開口量の計測から破壊力学的に評価する方法を用い、その有効性を実証している。
　第7章「疲労き裂伝ぱに及ぼす試験片厚さの影響」では、第6章までの、板厚1.5mmの試験片を用いて得られた結果が、厚板の場合にも、同様に有効であることを示している。
　第8章「結論」では、本論文で得られた結果を要約するとともに、本論文に基づき、今後の展開についても論じている。
　よって、本論文は工学上及び工業上貢献するところが大きく、博士（工学）の学位論文として十分な価値を有するものと認める。