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| 中文摘要： | |
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本文主旨在探討負荷應變比、頻率及波形效應對AISI 347不銹鋼應變控制之低週腐蝕疲勞的影響，分析在空氣、NaCl 及H2SO4水溶液中低週疲勞壽命之差異。此外，亦利用掃描式電子顯微鏡(SEM)觀察破斷面，以了解疲勞破壞機制。 實驗結果顯示，在各應變比下，高應變振幅區中腐蝕環境並不會使材料產生過早的裂縫起始，此乃為腐蝕溶解量不足的原因所致，且由SEM 的觀察可發現在腐蝕環境中，其破壞模式仍由機械破壞所引起。然而，在低應變振幅區，NaCl溶液中的疲勞壽命隨應變比的提高而降低，而在H2SO4環境中此一趨勢僅發生於高應變比(R = 0.5及0.8)，此乃因為張平均應力會產生不可回復之滑移階而形成一幾何不連續的位置，而在此一幾何不連續的位置易形成濃度集中的現象進而促進材料的腐蝕。由實驗結果亦可發現，在NaCl環境中其腐蝕破壞的程度高於H2SO4 水溶液，其原因為在NaCl 水溶液中，其腐蝕破壞形態與表面蝕孔有關的缺陷，此一缺陷易形成有效之應力集中而導致裂縫的提早形成。另外，在頻率及波形效應方面，僅有H2SO4水溶液在10秒的張應變持時條件下，其環境效應有高於1 Hz三角波的現象，其原因為鈍化膜的溶解使得再鈍化的效果較低，因而造成整體腐蝕溶解量的上升。 在應變比為-1的低週疲勞試驗中，三環境在高應變區均呈現循環硬化的現象，造成此現象的原因為應變硬化的結果，然而在低應變振幅區則呈現循環軟化的現象。在R = 0.5，ea = 0.7% 及R = 0.8 之各振幅其塑性區有縮小的現象，此一現象可能與麻田散鐵的形成有關。另外，在腐蝕環境中其應力振幅均大於空氣中的應力振幅(R = -1，ea = 0.3%、0.4% 除外)，其可能原因為氫幫助差排的形成與移動，因而加強硬化的效果。 此外，本研究亦提出修正型Manson and Halford 及修正型SWT 關係式，迴歸AISI 347 不銹鋼在各環境中、不同應力比的低週疲勞壽命值以得到適用的壽命評估模式。
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| Abstract： | |
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The aim of this study is to investigate the influence of strain ratio, frequency, and waveform on low cycle corrosion fatigue behavior of AISI 347 stainless steel in different environments, namely, air, NaCl, and H2SO4 solutions. Fractography analyses with scanning electron microscopy (SEM) were conducted to investigate the corrosion fatigue crack initiation mechanism. Results showed that at high strain amplitude region for each strain ratio the fatigue lives in three environments were comparable. These results implied that the environmental effects were not distinguishable as a result of insufficient amount of local dissolution. This was supported by SEM observations in which the fatigue fracture was found to be dominated by large plastic deformation from mechanical loading. However, at low strain amplitude region, the fatigue lives in NaCl solution were decreased with an increase in strain ratio, while in H2SO4 solution similar tendency only occurred at R = 0.5 and 0.8. The strain-ratio effects might be explained by the influence of a concentrated environment on the geometrical discontinuities generated by a tensile mean stress. The concentrated environment at such areas would enhance corrosion rate to dissolve fresh surface produced in the next loading cycle. It was also found that the environmental effects were more pronounced in NaCl solution than in H2SO4 one due to the formation of sharp fissures to accelerate crack initiation in NaCl solution. With regard to the effect of frequency and waveform on LCCF, the environmental effect was only enhanced at the testing condition with a 10-s hold time at peak strain in H2SO4 solution due to less repairment of passive film and more local dissolution during a loading cycle. LCF specimens in the given three environments exhibited cyclic hardening at high strain amplitude region as a result of strain hardening, while at low strain amplitude region cyclic softening was observed. Furthermore, the plastic strain amplitudes for R = 0.5 with an applied strain amplitude of 0.7% and for all applied strain amplitudes at R = 0.8 were slightly smaller than the corresponding ones at other strain ratios due to a possible martensitic transformation. Cyclic hardening in both aqueous solutions was more pronounced than that in air due to the influence of hydrogen. The LCF life data in the given three environments obtained for AISI 347 stainless steel under various strain ratios could be well correlated by a modified SWT model as well as a modified Manson-Halford one.
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