| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系 === 89 === During the application of graphite cast iron, resonant vibration may be encountered to accelerate failure. Therefore, the mechanism of resonant vibration fracture of cast iron and how to improve its vibration fracture resistance by metallurgical factors are necessary to be explored.
The experimental results indicate that the deflection amplitude can be classified into two regions as a function of vibration cycle during resonant test. The deflection amplitude maintains a constant value for a certain period of vibration cycles (region I) and then decreases significantly with increasing the vibration cycles (region II). The feature of resonant vibration fracture behavior of cast iron can be concisely divided into four steps, namely (1) crack initiation, (2) crack linking, (3) the major cracks formation, and (4) deepening of the major cracks in the through-thickness direction. The initial three steps correspond to the region I period of the D-N curves, and the fourth step occurs in the region II period. Based on the tensile test results, the yield strength of specimen decreases slightly during region I period, but significantly decreases during region II period. Therefore, the apparent vibration fracture resistance can be defined based on the D-N curves, specifically, the deflection amplitude value dropped to just 10% with the dwelling cycle of numbers beyond region I. As compared to the S-N curve can be used to realize the fatigue behavior, the D-N curve (Deflection amplitude vs. Number of vibration cycles) can be used to comprehend the resonant vibration fracture behavior of cast iron. Not only the vibration fracture resistance but also the initial deflection amplitude can be realized from the feature of D-N curve.
In current study, the applied vibration stress of specimen can be determined from the initial deflection amplitude, which is mainly influenced by the applied vibration force, vibration frequency and damping capacity. Increasing the applied vibration force can increase the initial deflection amplitude, but the increasing trend is slightly when the g value surpassed 2g. The initial deflection amplitude reaches the maximum value at resonant frequency of the specimen. The resonant frequency in this experimental system is mainly influenced by the graphite nodularity, when the graphite shape is spheroidal, the differences in the microstructure have slightly influence on resonant frequency. This study concluded that the damping capacity of cast iron is mainly effected by the ability of plastic deformation. Not only the stress concentration effect which causing by graphite but also the deformation resistance which causing by matrix structure influences the damping capacity of cast iron.
The vibration fracture resistance of cast iron is mainly influenced by the damping capacity and crack propagation resistance. Lowering the graphite nodularity can enhance the damping capacity, but owing to the higher stress concentration coefficient and the shorter graphite spacing, the cracks easily initiated and propagated. Therefore, the vibration fracture resistance decreased with lowering graphite nodularity. When the graphite shape is spheroidal, the crack propagation behavior is effected by the graphite nodules and the concentrated eutectic cell wall inclusions. Notably, the cell wall particles can be dispersed by increasing solidification rate or lowering the silicon contents. On the other hand, as compared to the variation of graphite phase, the variation of matrix structure significantly influence the vibration fracture behavior of spheroidal graphite cast iron. Increasing the silicon contents can increase the yield strength of ferritic spheroidal graphite cast iron, but decreasing the vibration fracture resistance. The spheroidal graphite cast iron with ferrite and pearlite dual phase structure shows the different manner with the full ferritic matrix specimen. Excepting the full pearlitic matrix sample, increasing the pearlite structure increases not only the deflection amplitude but also the vibration fracture resistance. It is because the ferrite structure which around graphite nodules can lowering the deflection amplitude, and the pearlite which far from graphite nodules can arrest the crack linking, thus increasing the vibration fracture resistance.
Since the initial deflection amplitude and crack propagation resistance mainly influences the vibration fracture resistance, thus lowering the initial deflection amplitude and increasing the crack propagation resistance can increase the vibration fracture resistance. In summary, the vibration fracture resistance of cast iron can be enhanced by the following methods, (a) decreasing the carbon and silicon contents, (b) increasing the solidification rate and graphite nodularity, and (c) the microstructure with bull’s structure.
中文摘要 I
英文摘要 III
總目錄 VI
表目錄 X
圖目錄 XII
第一章 前言 1
第二章 文獻回顧 6
2-1 共振狀態 6
2-1-1 共振頻率 6
2-1-2 振動所引起之損害 6
2-1-3 阻泥(damping)之種類 8
2-1-4 阻泥(damping)的量測方法 9
2-2 鑄鐵之基本特性 11
2-2-1 鑄鐵之材質特徵種類 11
2-2-2 鑄鐵的破壞行為 12
第三章 實驗程序 22
3-1 材料準備及微觀組織觀察 22
材料熔煉及試片準備 22
拉伸及硬度測試 23
電解腐蝕方法 24
振動破壞實驗 24
共振頻率之量測 24
共振破壞測試 25
對數衰減率(值)的量測 25
微觀組織以及裂紋傳播路徑之定量化解析 26
第四章 實驗結果與討論 37
4-1 振動試驗條件與鑄鐵振動破壞之間關係 37
4-1-1 前言 37
4-1-2 實驗方法提要 37
4-1-3 實驗結果 38
4-1-3-1 振動頻率、振動推力與微觀組織變化對振動偏移量之影響 38
4-1-3-2 D-N Curve與材料劣化之間關係 38
4-1-4 討論 41
4-1-4-1 影響振動振幅要因之探討 41
4-1-4-2 振動破壞行為與D-N Curve之相關性 43
4-1-5 結論 47
4-2 球狀石墨相對肥粒體基球墨鑄鐵振動破壞行為之影響 65
4-2-1 前言 65
4-2-2 實驗方法提要 65
4-2-3 實驗結果 66
4-2-3-1 碳含量變化對振動破壞行為之影響 66
4-2-3-2 凝固速率變化之效應 67
4-2-4 討論 67
4-2-4-1 共振頻率之檢討 67
4-2-4-2 球狀石墨相變化對振動破壞阻抗之影響 68
4-2-4-3 共晶胞界介在物分佈型態之檢討 71
4-2-4-4 改善肥粒體基球墨鑄鐵晶胞界脆性之可能性探討 72
4-2-5 結論 74
4-3 基地組織變化對振動破壞特性之影響 96
4-3-1 前言 96
4-3-2 實驗方法提要 96
4-3-3 實驗結果 97
4-3-3-1 共振頻率與振動偏移量 97
4-3-3-2 D-N 曲線 97
4-3-3-3 振動裂紋傳播之觀察 98
4-3-4 討論 99
4-3-4-1 共振頻率之檢討 99
4-3-4-2 基地組織變化對振動破壞行為之影響 99
4-3-5 結論 104
4-4 矽固溶強化效應對肥粒體基球墨鑄鐵振動破壞行為的影響 120
4-4-1 前言 120
4-4-2 實驗方法提要 120
4-4-3 實驗結果 121
4-4-3-1 微觀組織解析及機械性質測試 121
4-4-3-2 振動初始偏移量之矽含量效應 121
4-4-3-3 振動裂紋傳播之矽含量效應 121
4-4-4 討論 122
4-4-5 結論 124
第五章 球狀石墨鑄鐵拉伸性質與振動破壞特性之間關係檢討 132
5-1 前言 132
5-2 實驗方法提要 132
5-3 D-N Curve之變化與振動變形破壞之間關係檢討 133
5-4 鑄鐵試料振動初始偏移量之微觀組織效應 137
5-4-1 石墨相變化對振動初始偏移量之影響 138
5-4-2 基地組織變形阻抗之變化效應 139
5-4-3 影響振動初始偏移量之主導性要因 140
5-5 球狀石墨鑄鐵拉伸性質與耐振動破壞阻抗之相互關係檢討 143
5-6 提升鑄鐵耐振動破壞阻抗之微觀組織控制方法 147
第六章 總結論 166
第七章 參考文獻 168
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