本研究以Ti-6Al-4V鈦合金為實驗材料，分析超載負荷及裂縫閉合對疲勞裂縫成長之影響，並量測可用以準確評估疲勞裂縫成長壽命之材料參數。藉掃瞄式電子顯微鏡之觀察以瞭解材料的疲勞破壞機構。研究結果顯示，超載負荷造成裂縫成長速率下降，需要多次的週期數才能穿越塑性區並回到原始裂縫成長速率。超載負荷比愈大，裂縫成長延遲現象也愈明顯。在施加超載負荷的期間，裂縫前緣會產生新月形的損傷區域。Ti-6Al-4V鈦合金在Wheeler模式中的延遲形狀參數為3.3。在裂縫閉合方面，應力比或應力強度因子範圍愈力，則裂縫閉合應力愈高。在低應力比時，Ti-6Al-4V的裂縫閉合係數約為0.83，遠高於鋼鐵材料的0.5。亦即Ti-6Al-4V受到裂縫閉合的影響較小，若採用鋼鐵材料的裂縫閉合經驗參數值進行裂縫成長評估，會嚴重高估其疲勞裂縫成長壽命。本研究進一步得到裂縫閉合係數與應力比之關係式。;In this study, Ti-6Al-4V titanium alloy was used as experimental material to analyze the effects of overload load and crack closure on fatigue crack growth. The fatigue failure mechanism of materials was studied by scanning electron microscope. The results show that overload load reduces fatigue crack growth rate. It takes more load cycles to go through the plastic zone and return to the original crack growth rate. The larger the overload load ratio, the more obvious the crack growth retardation phenomenon. During the loading period, crescent-shaped areas of damage were produced at the crack front. The delay shape parameter of Ti-6Al-4V titanium alloy in Wheeler model is 3.3. In terms of crack closure, the greater the stress ratio or the stress intensity factor, the higher the crack closure stress. At low stress ratio, the crack closure coefficient of Ti-6Al-4V is about 0.83, which is much higher than 0.5 of steels. That is to say, Ti-6Al-4V is less affected by crack closure. If fatigue crack growth evaluation is carried out with the empirical crack closure coefficient of steels, the fatigue crack growth life will be apparently overestimated. Furthermore, the relationship between crack closure coefficient of Ti-6Al-4V and stress ratio was obtained.
