氏名　Zainuddin Bin Sajuri
学位の種類　博士（工学）
学位記番号　博甲第336号
学位授与の日付　平成17年3月25日
学位論文題目　Study on Fatigue Behavior of Magnesium Alloys （マグネシウム合金の疲労特性に関する研究）
論文審査委員
　主査　教授　武藤　睦治
　副査　教授　古口　日出男
　副査　教授　岡崎　正和
　副査　助教授　井原　郁夫
　副査　富山大学　工学部教授　石原　外美

• 論文目次
• 論文要旨

［平成16（2004）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

Contents

Chapter 1　Introduction
　1.1　General View　p.1
　1.2　Magnesium Alloys Characteristics　p.4
　1.2.1 Corrosion properties　p.4
　1.2.2 Mechanical properties　p.6
　1.2.3 Creep properties　p.7
　1.2.4 Fatigue properties　p.7
　1.3　Scope of the Present Work　p.9

Chapter 2　Development of a Fatigue Testing Method for Magnesium Alloys
　2.1　Introduction　p.20
　2.2　Materials and Methods　p.22
　2.3　Fatigue test specimen shape　p.27

Chapter 3　Effects of Temperature and Humidity on Fatigue Behavior of Magnesium Alloy
　3.1　Introduction　p.31
　3.2　Materials and Experimental Procedures　p.33
　3.2.1 Material　p.33
　3.2.2 Experimental Procedures　p.35
　3.3　Results and Discussion　p.36
　3.3.1 Tensile properties　p.36
　3.3.2 Fatigue strength　p.37
　3.3.3 Fracture surface observation　p.40
　3.3.4 Formation of corrosion pit and crack initiation　p.44
　3.4　Conclusions p.48

Chapter 4　Effects of Frequency on Fatigue Strength and Fatigue Crack Growth Behavior of Magnesium Alloy at High Humidity Environment
　4.1　Intrduction　p.51
　4.2　Experimental Prcedures　p.53
　4.2.1 Fatigue test　p.53
　4.2.2 Fatigue crack growth test　p.53
　4.2.3 Fatigue pre-crack preparation　p.56
　4.2.4 Crack closure determination　p.56
　4.3　Results and Discussion　p.60
　4.3.1 Effect of frequency on fatigue strength　p.60
　4.3.2 Fracture surface observations　p.61
　4.3.3 Effect of frequency on fatigue crack growth behavior　p.64
　4.3.4 Effect of humidity on fatigue crack growth behavior　p.69
　4.3.5 Fatigue life prediction for extruded material with corrosion pits　p.70
　4.4　Conclusions　p.76

Chapter 5　Effect of Stress Ratio on Fatigue Crack Growth Behavior of Magnesium Alloys
　5.1　Introduction　p.79
　5.2　Experimental Procedures　p.81
　5.3　Results and Discussion　p.82
　5.3.1 Effect of R ratio and crack closure on fatigue crack growth behavior of AZ61 magnesium alloy　p.82
　5.3.2 Fracture surface observations　p.89
　5.4　Conclusions　p.96

Chapter 6　Effect of Manufacturing Processes on Fatigue Behavior of Magnesium Alloys
　6.1　Introduction　p.99
　6.2　Materials and Experimental Procedures　p.100
　6.3　Results and Discussions　p.103
　6.3.1 Microstructure and casting defect　p.103
　6.3.2 Tensile properties　p.108
　6.3.3 Fatigue strength　p.109
　6.3.4 Fracture surface observations　p.112
　6.3.5 Fatigue life prediction of the as-cast and die-cast magnesium alloys　p.114
　6.4 Conclusions　p.118

Chapter 7　Effects of Surface Roughness and Notch on Fatigue Behavior of Magnesium Alloy
　7.1　Introduction　p.121
　7.2　Experimental Procedures　p.123
　7.2.1Test material and specimen preparation　p.123
　7.2.2Fatigue test　p.127
　7.3　Results and Discussion　p.127
　7.3.1Influence of surface roughness　p.127
　7.3.2Influence of notch　p.135
　7.4　Conclusions　p.138

Chapter 8　Effects of Texture on Fatigue Strength and Fatigue Crack Growth Behavior of Magnesium Alloy
　8.1　Introduction　p.142
　8.2　Experimental Procedures　p.144
　8.2.1 Material　p.144
　8.2.2 Texture analysis　p.146
　8.2.3 Tensile test　p.147
　8.2.4 Fatigue test　p.147
　8.2.5 Short and long fatigue crack growth tests　p.148
　8.3　Results and Discussions　p.151
　8.3.1 Tensile properties　p.151
　8.3.2 Fatigue strength　p.154
　8.3.3 Interrupted fatigue test　p.158
　8.3.4 Short crack growth behavior　p.162
　8.3.5 Long crack growth behavior　p.167
　8.4　Conclusions　p.172

Chapter 9　Summary
　9.1　General Conclusion　p.176
　9.2　Future Prospects　p.181

Magnesium alloys is one of the best structural materials for the purpose of weight reduction due to its low density and high specific strength. It is very important to understand the fatigue behavior and fracture mechanisms of magnes-ium alloys before it can be used as reliable structural materials. However, information about the fatigue behavior, fatigue crack initiation and growth mechanisms are very limited. In the present study, the effects of environment, fre-quency, stress ratio, manufacturing processes, surface roughness, and tex-ture on fatigue and fatigue crack growth behavior of magnesium alloys have been investigated. This thesis consist of nine chapters:
Chapter 1 "Introduction": General view of the present study and magnesium alloys characteristics were presented. Research works related to this study were reviewed and the objectives of this study were addressed.
Chapter 2 "Development of a Fatigue Testing Method for Magnesium Alloys":A reliable fatigue testing methods in order to obtain a reliable basic fatigue data for magnesium alloys was proposed. A compact environmental chamber, which can control both temperature and humidity, was specially designed and fabricated so that it could be installed into a fatigue test machine to conduct fatigue tests under certain humidity and temperature levels.
Chapter 3 "Effects of Temperature and Humidity on Fatigue Behavior of Magnesium Alloy": Since magnesium alloys are very sensitive to environment, effect of environment on fatigue behavior magnesium alloy was investigated at the temperatures of 20℃ and 50℃ and at the relative humidity （RH） levels of 55% and 80%. Fatigue test at 150℃ was also carried out. The fatigue limits at 20℃ and 50℃ under 55%RH were in the range of 140－150MPa. However, at 150℃,the fatigue strength was drastically reduced. A clear effect of humidity was seen at stress amplitudes below the fatigue limit under 80%RH, where the fati-gue fracture were originated from the corrosion pit formed at the surface. The observed reduction in fatigue strength due to the formation of corrosion pit and its growth to criticalsize for fatigue crack initiation and propagation is attributed to the effects associated with fatigue-environment interaction.
Chapter 4 "Effects of Frequency on Fatigue Strength and Fatigue Crack Growth Behavior of Magnesium Alloy at High Humidity Environment": Two different fracture behaviors in AZ61 magnesium alloy were identified; time dependent behavior at stresses below the fatigue limit and cyclic dependent behavior at stresses above the fatigue limit. The fatigue crack growth behavior of 1 Hz frequency under 50℃-80%RH demonstrated higher threshold stress intensity fac-tor and fatigue crack growth resistance compared to that of 10 Hz. High humid-ity did not influencethe fatigue crack growth behavior under high loading fre-quency. Further, the fatigue AZ61 under high humidity was predicted follows the fracture mechanics law.The prediction showed a good agreement with the experim-ental results.
Chapter 5 "Effect of Stress Ratio on Fatigue Crack Growth Behavior of Magnesium Alloys": Investigation on the effect of stress ratio R showed that fatigue crack growth rates increases with increasing R. Consequently, the threshold stress intensity factor range ΔKth for both alloys tends to decrease with an increase in R. The difference of the crack growth rate between da/dN-ΔKeff and da/dN-ΔK curves is larger at the near-threshold region, while it becomes smaller with increasing ΔK. This was due to the crack closure effect, which was more pronounced at near threshold and the crack opening ratio that increases with increasing ΔK. Regardless of stress ratio, the da/dN-ΔKeff curves for all stress ratio conditio-ns lay in a narrow band.
Chapter 6 "Effects of Manufacturing Processes on Fatigue Behavior of Magnesium Alloys": Fatigue behavior of different processes; as-cast, die-cast and extruded magnesium alloys were investigated. The size and distribution of casting defect influenced the tensile and fatigue properties of these alloys. The shrinkage and gas pores in the as-cast and die-cast alloys served as a stress concentration site for fatigue crack initiation so that the fatigue strength of the cast alloys was significantly low compared to the extruded alloys.
Chapter 7 "Effects of Surface Roughness and Notch on Fatigue Behavior of Magnesium Alloy": Surface roughness and stress concentration were also found to give significant effects on the fatigue strength of magnesium alloys. Fatigue strength decreased with increasing surface roughness and stress concentration factor. However, it is worst to note that AZ91D alloy has better surface rough ness and notch sensitivity compared to A7075-T6 alloy. By assuming that the ma-ximum surface roughness Ry as an initial crack, fatigue limit predict-ion results based on fracture mechanics approach showed a good agreement with the experimental results for both AZ91D and A7075-T6 alloys.
Chapter 8 "Effects of Texture on Fatigue Strength and Fatigue Crack Growth Behaviors of an Extruded AZ61 Magnesium Alloy": Effect of texture on fatigue strength was clearly observed: the longitudinal specimen, where the basal plane is parallel to the loading axis, indicated high fatigue limit compared to the 45-degree and transverse specimens. A fatigue crack nucleated from inclusions in the early stage of fatigue life. High stress concentration around inclusion due to pile-up of slips results in cracking of matrix for fatigue crack nucleation. The easiness of slip deformation in the 45-degree and transverse specimens would degrade the fatigue limit. The inclusions at the fatigue crack nucleation site were mainly consisted of manganese and aluminum. Effect of texture of fatigue crack growth behavior was also clearly observed in the near-threshold region: the longitudinal specimen indicated high fatigue crack growth resistance and hi-gh threshold value. The effect of texture will be mainly due to the difference of crack closure in different crack growth orientation.
Chapter 9 "Summary": The general conclusion and future prospects are given.