氏名　Duangporn Ounpanich
学位の種類　博士（工学）
学位記番号　博甲第475号
学位授与の日付　平成20年6月30日
学位論文題目　Fatigue behavior of automotive steels after plastic forming　（塑性加工した自動車用鋼の疲労破壊挙動）

論文審査委員
　主査　教授　武藤　睦治
　副査　教授　福澤　康
　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20（2008）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

Contents
1　Introduction　p.1
　1.1　Material and fabricating process　p.2
　1.1.1　Introduction of automotive steel　p.2
　1.1.2　Fabricating process of automotive components　p.4
　1.1.2.1　Discontinuous fabricating process　p.5
　1.1.2.2　Continuous fabricating process　p.6
　1.1.3　Fabricating process of automotive wheel　p.8
　1.1.3.1　Rolling stage　p.9
　1.1.3.2　Forming stage　p.12
　1.1.3.3　Cold spinning stage　p.13
　1.2　Fatigue behavior of fabricated component　p.15
　1.2.1　Plain fatigue behavior　p.15
　1.2.2　Fretting fatigue behavior　p.17
　1.3　Problem statement　p.19
　1.4　Dissertation outline　p.23
2　Effect of fabricated surface on plain fatigue behavior　p.25
　2.1　Introduction　p.26
　2.2　Experimental procedure　p.27
　2.2.1　Material　p.27
　2.2.2　Specimen and fabricating process　p.28
　2.2.3　Four-point bending fatigue test　p.30
　2.2.4　Fatigue crack propagation test　p.31
　2.3　Results　p.34
　2.3.1　Microstructure　p.34
　2.3.2　Microhardness　p.34
　2.3.3　Fabricated surface observation　p.37
　2.3.4　Residual stress measurements　p.40
　2.3.5　Fatigue behavior　p.40
　2.3.6　Fatigue crack propagation　p.43
　2.4　Discussion　p.45
　2.4.1　Effect of surface condition　p.45
　2.4.2　Effect of surface defect geometry　p.48
　2.5　Conclusion　p.49
3　Effect of surface defect shape on fatigue crack growth behavior　p.51
　3.1　Introduction　p.52
　3.2　Experimental procedures　p.52
　3.2.1　Material　p.52
　3.2.2　Specimen preparation for short fatigue crack propagation test　p.54
　3.2.3　Small fatigue crack propagation test　p.56
　3.3　Results and discussion　p.59
　3.3.1　Through-the-thickness edge crack　p.59
　3.3.2　Semicircular surface crack　p.60
　3.4　Conclusion　p.63
4　Fatigue strength prediction of fabricated component with surface defect　p.64
　4.1　Introduction　p.65
　4.2　Experimental procedure　p.66
　4.2.1　Material and specimen preparation　p.66
　4.2.2　Specimen surface observation　p.67
　4.2.3　Hardness distribution　p.69
　4.2.4　Fatigue test　p.72
　4.2.5　Fatigue crack propagation test　p.73
　4.3　Results and Discussion　p.74
　4.3.1　Fatigue behavior　p.74
　4.3.2　Fatigue crack propagation behavior　p.77
　4.3.3　Estimation of fatigue crack propagation life　p.78
　4.3.4　Estimation of fatigue strength　p.80
　4.4　Conclusion　p.84
5　Effect of fabricated surface on fretting fatigue behavior　p.86
　5.1　Introduction　p.87
　5.2　Experimental procedure　p.88
　5.2.1　Material and specimen　p.88
　5.2.2　Microstructure observation　p.90
　5.2.3　Microhardness test　p.91
　5.2.4　Surface observation and roughness measurement　p.91
　5.2.5　Residual stress measurement　p.93
　5.2.6　Plain fatigue and fretting fatigue test　p.93
　5.3　Results and Discussion　p.95
　5.3.1　Microstructure observation　p.95
　5.3.2　Fabricated surface morphology and roughness measurement　p.96
　5.3.3　Hardness distribution　p.96
　5.3.4　Residual stress measurement　p.97
　5.3.5　Plain fatigue strength and fracture behavior　p.100
　5.3.6　Fretting fatigue strength　p.100
　5.3.7　Fretting fatigue fracture surface observations　p.103
　5.3.8　Variation of surface roughness in the fretted area　p.104
　5.4　Conclusion　p.106
6　General conclusion and future work　p.107
　6.1　General conclusion　p.108
　6.2　Future work　p.110
References　p.122

Since automotive wheels are classified as safety components, they are subjected to particular attention during design and fabrication in order to ensure the proper in-service durability as well as the need of a lightweight design. In order to achieve high strength and lightweight, multi piece steel wheels, which consisting of wheel rim and disc, are widely used. Plastic forming during various stages of fabricating process for automotive wheel have been not only increased surface hardness but also induced surface defects. Such defects can result in the premature failure of components in service. Therefore, understanding of fatigue behavior and crack propagation behavior as well as life prediction of fabricated wheel, where small surface cracks exist, are important to enable safety design of wheel. In the present study, influence of fabricated surface after plastic forming on plain and fretting fatigue behavior is examined. Based on fracture mechanics approach using the surface hardness value and the surface defect depth observed on the fabricated surface, new procedure for predicting fatigue strength of fabricated component after plastic forming was developed.

Chapter 1 Introduction: A brief introduction on automotive steel as well as fabricating process for automotive components and wheel have been described. Short introductions on plain fatigue and fretting fatigue behavior of fabricated component have been described. Problem statement encouraged undertaking the present study has been presented.

Chapter 2 Effect of fabricated surface on plain fatigue behavior: The plain fatigue strength and fatigue crack growth tests of specimens sectioned from the fabricated wheel were carried out in order to understand the effect of fabricated surface on plain fatigue behavior. The fatigue test results indicated that strain hardening enhanced fatigue strength, however, surface defects induced during fabricating process strongly degraded fatigue strength. The results of fatigue crack growth test indicated that fabricating process influenced crack growth behavior in the threshold region, while the fabrication process did not affect the fatigue crack growth behavior in Paris regime.

Chapter 3 Effect of surface defect shape on fatigue crack growth behavior: Influence of fabricated surface defect shape on fatigue crack growth behavior was investigated in order to confirm influence of surface crack shape on fatigue propagation performance. The growth behavior of short fatigue cracks has been investigated by generating two typical surface cracks i.e. the through-the-thickness surface crack and semi-elliptical surface crack. The fatigue crack growth rate for the through-the-thickness surface crack was higher than that for semi-elliptical surface crack at the same ΔK-value. Since the defects induced during the fabricating process could be assumed as the through-the-thickness surface crack, the shape of defect might influence the fatigue strength. It has been confirmed that defects induced during the fabricating process in the present study are the edge through-the-thickness crack like defects.

Chapter 4 Life prediction of fabricated component with surface defect: Life prediction of fabricated component after plastic forming based on fracture mechanics approach using the surface hardness value and the surface defect depth observed on the fabricated surface has been carried out. New procedure for predicting the fatigue strength of fabricated component after plastic forming was developed. The estimated fatigue strength for fabricated components with surface defects agreed with the experimental results.

Chapter 5 Effect of fabricated surface on fretting fatigue behavior: In order to understand the influence of fabricated surface after plastic forming on fretting fatigue behavior, the fretting fatigue tests of specimens sectioned from actual fabricated wheel after cold spinning were carried out. Cold spinning increased surface hardness and enhanced fatigue strength. However, cold spinning also increased surface roughness and induced surface defects, which significantly degraded plain fatigue strength. In fretting fatigue, stress concentration at fretting contact edge was a dominant factor to control fretting fatigue behavior more than that of surface defect. Therefore, fretting fatigue strengths for grinding, milling and cold spinning surfaces of fretting fatigue specimens almost coincided regardless of surface condition.

Chapter 6 General conclusion and future work: The conclusions of the current work were summarized. Recommendation for further work was also presented.

　本論文は、「Fatigue Behavior of Automotive Steels after Plastic Forming（塑性加工した自動車用鋼の疲労破壊挙動）」と題し、6章より構成されている。
　第1章「緒言」では、自動車に関連した材料と製造加工プロセス、特にホイールの加工について詳細に説明するとともに、製造加工後の部材の疲労強度に関するこれまでの研究を概説し、本論文の目的と意味を述べている。
　第2章「通常疲労挙動に及ぼす製造加工表面の影響」では、自動車部品として製造加工されたホイールの表面特性を、欠陥などの形状的損傷、硬度、残留応力などの点から明らかにし、冷間スピン加工後の表面には、残留応力はきわめて小さいが、深さ数10μmのき裂上の欠陥を生じるとともに、高硬度を示すこと、硬度は疲労強度の向上をもたらすが、観察されたような微小な表面欠陥でも疲労強度を低下させることなどを明らかにしている。
　第3章「疲労き裂伝ぱ挙動に及ぼす表面欠陥形態の影響」では、半円状の表面き劣と微小深さの片側貫通き裂を導入した試験片を用意し、それらの疲労き裂伝ぱ挙動を詳細に調べ、き裂長さが130μm以下になるとき裂はそこから進展せず、他の部分から疲労破壊を生じること、したがって、加工により生じた欠陥は、むしろ片側貫通き裂とみなせることなどを明らかにしている。
　第4章「表面損傷を有する製造加工材の疲労強度推定」では、加工により変化し、疲労強度に影響を及ぼすパラメータが、硬さと表面欠陥であることから、疲労強度を硬さの関数として定式化するとともに、北川・高橋線図とEl Haddadの微小き裂補正式を組み合わせ、表面欠陥の寸法から疲労強度を推定する手法を提案し、その有効性を示している。
　第5章「フレッティング疲労挙動に及ぼす製造加工表面の影響」では、実際の製造部材で問題となるフレッティング疲労の問題を取り上げ、通常疲労の場合とは異なり、フレッティング接触部端部の応力集中は、製造加工表面に存在する表面欠陥による応力集中よりもきわめて高く、したがって、疲労破壊起点とはならず、フレッティング疲労強度にもほとんど影響を及ぼさないことなどを明らかにしている。
　第6章「結論」では、本論文で得られた結論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。