氏名　MOHAMMAD SUKRI BIN MUSTAPA
学位の種類　博士（工学）
学位記番号　博甲第551号
学位授与の日付　平成22年6月30日
学位論文題目　Effect of Second Phase Particle on Fatigue Crack Growth Behavior in Microstructure Controlled Steels　（微細組織制御鋼の疲労き裂進展挙動に及ぼす第二相粒子の影響）
論文審査委員
　主査　教授　武藤　睦治
　副査　実務家教授　永田　晃則
　副査　教授　岡崎　正和
　副査　准教授　井原　邦夫
　副査　准教授　宮下　幸雄

• 論文目次
• 論文要旨
• 論文審査の結果の要旨

［平成22（2010）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

Acknowledgements　p.iii
Abstruct　p.iv
Table of Contents　p.vi
List of Figures　p.viii
List of Tables　p.x
Chapter 1　INTRODUCTION　p.1
　1.1　Introduction of TMCP steel　p.1
　1.2　The application of TMCP steel　p.2
　1.3　Fracture mechanic approach　p.7
　1.4　Fatigue crack growth behavior　p.8
　1.5　Factors that Influences fatigue crack growth rate　p.10
　1.6　Crack closure　p.10
　1.7　Crack tip stress shielding mechanisms　p.19
　1.8　Literature review　p.21
　1.9　Significance and objectives of study　p.24
Chapter 2　Effects of size and spacing of second phase particles on fatigue crack growth
behavior of ferrite-pearlite steels　p.28
　2.1　Abstract　p.28
　2.2　Introduction of TMCP steel　p.29
　2.3　experimental procedures　p.30
　2.3.1　Materials　p.30
　2.3.2　Fatigue crack growth test　p.33
　2.3.3　Evaluation of crack tip effective stress intensity factor range　p.35
　2.4　Results and discussion　p.38
　2.4.1　In-situ observations of fatigue crack growth behavior　p.38
　2.4.2　Effect of crack closure phenomena　p.43
　2.4.3　effect of crack tip stress shielding　p.44
　2.4.4　Effects of pearlite particle size and spacing　p.44
　2.5　Conclusions　p.49
Chapter 3　Effect of volume fraction of second phase particle on fatigue crack growth
behavior of ferrite-pearlite and ferrite-bainite steels　p.50
　3.1　Abstract　p.50
　3.2　Introduction　p.51
　3.3　Materials and experimental procedures　p.52
　3.4　Results and discussion　p.55
　3.4.1　In-situ observations of fatigue crack growth behavior　p.59
　3.4.2　Effect of crack closure phenomena　p.60
　3.4.3　Effect of crack tip stress shielding　p.63
　3.4.4　Effect of volume Fraction of Second Phase　p.66
　3.5　Conclusions　p.66
Chapter 4　Effect of hardness of second phase particle on fatigue crack growth
behavior of ferrite-pearlite, ferrite-bainite and ferrite-martensite steels　p.67
　4.1　Abstract　p.67
　4.2　Introduction　p.68
　4.3　Experimental procedures　p.69
　4.4　Results and discussion　p.72
　4.4.1　Crack path and fatigue crack growth behavior　p.72
　4.4.2　Effect of crack closure and stress shielding phenomena　p.73
　4.4.3　Effect of hardness　p.79
　4.5　Conclusions　p.85
Chapter 5　Conclusions and recommendations for future work　p.86
　5.1　Introduction　p.86
　5.2　Conclusions　p.86
　5.3　Recommendations for future work　p.87
References　p.89

The fracture resistance of the structural steel is extremely required since it is known that failure problems of structures and machines reported are mainly caused by fatigue. 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. In contrast, 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.
In the present study,　using a new parameter “crack tip stress shielding”, a series of ?K-constant fatigue crack growth tests in Paris regime were carried out to investigate the effect of second phase particle on fatigue crack growth behavior of microstructure controlled steels with uniformly distributed hard particles. Three kinds of materials were used in this study, ferrite matrix with pearlite particles （FP）, ferrite matrix with bainite particles （FB） and ferrite matrix with martensite particles （FM）. The content of the present dissertation are as follows;

In Chapter 1, a brief introduction about the background of structural steels of thermomechanical controlled process （TMCP） is explained. Literature review related to FCG behavior of structural steels has been presented. The significance and objectives of study have been addressed.

In Chapter 2, the effects of size and spacing of uniformly distributed pearlite particles on fatigue crack growth behavior in ferrite-pearlite steels have been investigated. The result showed that the large size and spacing of pearlite particles exhibited the higher FCG resistance compared to the small size and spacing of pearlite particles. The results were discussed based on crack closure and crack tip stress shielding phenomena.

In Chapter 3, the effect of volume fraction of hard particle on fatigue crack growth behavior in ferrite-pearlite and ferrite-bainite steels has been investigated. The result indicated the higher volume fraction of hard particle in the ferrite-bainite and ferrite-pearlite steels have significantly increased the fatigue crack growth resistance, respectively. The comparison of FCG behavior between FB and FP steels were also discussed.

In Chapter 4, the effect of hardness of hard particle on fatigue crack growth behavior in ferrite-pearlite （FP）, ferrite-bainite （FB） and ferrite-martensite （FM） steels has been investigated. The comparison of FCG rate of FP, FB and FM steels, the FM steel showed that the lower fatigue crack growth rate compared to the FP and FB steels. The results were discussed based on the crack closure, crack tip stress shielding and plastic constrained deformation phenomena.

In Chapter 5, the most significant results of the present study have been addressed. Also the recommendation of further works has been stated.

　本論文は、「Effect of Second Phase Particle on Fatigue Crack Growth Behavior in Microstructure Controlled Steels（微細組織制御鋼の疲労き裂進展挙動に及ぼす第二相粒子の影響）」と題し、5章より構成されている。
第1章「Introduction」では、本研究に関わる研究動向を概説するとともに、本研究の目的と範囲を述べている。
　第2章「Effects of size and spacing of second phase particle on fatigue crack growth behavior of ferrite-pearlite steels」では、フェライトマトリックスにパーライト粒子が一様に分散している組織において、パーライト粒子の寸法と粒子間距離が疲労き裂進展挙動に及ぼす影響を調べ、大きな粒子寸法と粒子間隔は顕著なき裂閉口とき裂先端での応力遮蔽挙動を引き起こし、疲労き裂進展抵抗を向上させることなどを明らかにしている。
　第3章「Effect of volume fraction of second phase particle on fatigue crack growth behavior of ferrite-pearlite and ferrite-bainite steels」では、フライトマトリックス中にパーライトあるいはベイナイト粒子が一様に分散している組織において、粒子の体積含有率が疲労き裂進展挙動に及ぼす影響について検討し、体積含有率が高い方が、き裂閉口ならびにき裂先端の応力遮蔽効果が顕著となり、き裂進展抵抗も向上することなどを明らかにしている。
　第4章「Effect of hardness of second phase particle on fatigue crack growth behavior of ferrite-peearlite, ferrite-bainite and ferrite-martensite steels」では、第2相粒子の硬度に着目し、フェライトマトリックス中に、パーライト、ベイナイトおよびマルテンサイト粒子を一様に分散した組織について、疲労き裂進展挙動を調べ、高硬度粒子ほど、き裂先端の応力遮蔽挙動が顕著となり、き裂進展抵抗が向上することなどを明らかにしている。
　第5章「Conclusions and recommendations for future work」では、以上の研究の結果を総括的にまとめるとともに、将来の展望について述べている。
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。