中文摘要：
	本研究的主旨在探討平均應力及頻率效應對T6、T73兩種熱處理條件下7050鋁合金在3.5 wt% NaCl溶液中的腐蝕疲勞性質之影響，同時量測不同受力條件下的裂縫開啟應力值，以期瞭解頻率及平均應力效應與腐蝕產物所誘發之裂縫閉合效應之間的關係。另一方面藉由扣除裂縫閉合效應之影響，瞭解應力腐蝕機制與機械疲勞機制在不同受力條件下對該等鋁合金腐蝕疲勞性質影響的程度。此外，再利用光學顯微鏡與掃瞄式電子顯微鏡，對金相組織及破斷面進行觀察，以瞭解7050鋁合金腐蝕疲勞裂縫的生成及延伸的模式。 實驗結果顯示，在平均應力的影響方面，負荷平均應力的提升將導致7050鋁合金在鹽水中的裂縫成長速率變快以及疲勞壽命的降低。在頻率效應的影響方面，低頻作用下的腐蝕疲勞週期壽命較高頻作用下來的短；而低頻的裂縫成長速率在中、高△K區域雖然較高頻快，在低△K區卻較高頻慢，造成此現象主要是由於高、低頻率所產生的裂縫閉合效應隨著△K的降低而逐漸拉大差距的緣故。扣除裂縫閉合效應後，平均應力的提升以及頻率的降低都會造成腐蝕疲勞行為受應力腐蝕機制影響程度的變大，加速疲勞裂縫的成長。 相較於在低平均應力、高頻率負荷受力條件下的7050-T73有較7050-T6短的疲勞壽命，在氫脆作用較為嚴重的高平均應力、低頻率負荷受力條件下的7050-T73由於其耐氫脆性質的凸顯，導致其裂縫成長速率明顯慢於7050-T6，並且在整體壽命上超越7050-T6。 由金相及SEM的觀察中得知，應力比R = 0.5、0.1，頻率1 Hz、20 Hz四種受力組合下的7050-T6及7050-T73之裂縫延伸方式皆為穿晶模式，而各種受力狀態下的裂縫起始都跟蝕孔有關。
Abstract：
	This study investigated the influence of mean stress and loading frequency on the corrosion fatigue properties of 7050-T6 and 7050-T73 in 3.5 wt% NaCl solution. Crack opening levels were measured under different loading conditions in order to understand the effects of mean stress and loading frequency on the oxide-induced crack closure levels. Furthermore, the roles of SCC and mechanical fatigue mechanisms in influencing the corrosion fatigue properties of such alloys were investigated by subtracting the crack closure effect. Fractograph and microstructure analyses with scanning electron microscoy (SEM) and optical microscoy (OM) were conducted to determine the corrosion fatigue crack initiation and propagation mode. Results showed that an increase in mean stress level of cyclic loading resulted in faster crack growth rate and shorter fatigue life for 7050 aluminum alloys tested in salt water. The corrosion fatigue life was reduced as a result of decreasing loading frequency. However, as the loading frequency decreased, the fatigue crack growth rate increased at intermediate and high stress intensity factor ranges (△Ks) and decreased at low △K. The reverse frequency dependence of the fatigue crack growth rate is attributed to the pronounced crack closure effect difference between low and high loading frequencies when △K is reduced. By subtracting the crack closure effect, the results showed that both increasing the mean stress level and decreasing the loading frequency enhanced the degree of influence of SCC mechanism on the corrosion fatigue damage, leading to a greater crack growth rate. In contrast to the shorter fatigue lives of 7050-T73 over 7050-T6 smooth specimens under low mean stress and high loading frequency conditions, 7050-T73 smooth specimens were superior to 7050-T6 ones in terms of fatigue life and crack growth rate under high mean stress and low loading frequency. This reverse trend in resistance to corrosion fatigue for smooth specimen between 7050-T6 and 7050-T73 can be attributed to the greater SCC resistance possessed by 7050-T73 under high mean stress and low loading frequency. Fractography analyses indicate that corrosion fatigue cracks of 7050-T6 and 7050-T73 initiated mostly from the corrosion pits and propagated transgranularly under various loading conditions applied in this study.