中文摘要：
	本研究的目的在探討基地組織對沃斯回火球墨鑄鐵疲勞裂縫成長性質的影響，調查的項目包括沃斯回火熱處理、不同沃斯回火恆溫處理條件、平均應力效應及應力誘起的麻田散鐵相變態對該等材質疲勞裂縫成長性質的影響。實驗結果顯示，鑄態與沃斯回火球墨鑄鐵兩者之疲勞破壞機構不盡相同，造成在較低的應力強度因子範圍△K時，鑄態球墨鑄鐵較沃斯回火球墨鑄鐵具有較低的疲勞裂縫成長速率；但在較高的△K時，鑄態球墨鑄鐵則具有較快的疲勞裂縫成長速率。沃斯回火球墨鑄鐵材質ADI-I(300℃, 3 hr沃斯回火恆溫處理)與ADI-II(360℃, 2 hr沃斯回火恆溫處理)在疲勞裂縫成長過程中，均有應力誘起麻田散鐵相變態現象，且此相變態容易發生於基地中的不穩態塊狀之殘留沃斯田鐵，經過較高溫的沃斯回火恆溫處理，其基地中含有較高的不穩態塊狀殘留沃斯田鐵，有較大的麻田散鐵相變態效應，即ADI-II比ADI-I有較大的麻田散鐵相變態效應，使得在應力比為0.1時，ADI-II的裂縫成長速率較ADI-I低。此外，不同應力比(R = 0.1, 0.5, 0.7)對於ADI-I與ADI-II的疲勞裂縫成長速率之影響，不能僅以裂縫閉合效應解釋之，此乃因為在扣除閉合效應後，應力比的增加會提升ADI-I及ADI-II的疲勞裂縫成長速率。在麻田散鐵相變態的觀察方面，X-ray繞射結果顯示ADI-I與ADI-II均有麻田散鐵相變態，SEM的觀察也發現，沃斯回火球墨鑄鐵基地中的不穩態塊狀之殘留沃斯田鐵受應力誘起麻田散鐵相變態之作用，造成在試片表面接近裂縫附近高應力區之不穩態塊狀之殘留沃斯田鐵中有凸起或龜裂的現象。
Abstract：
	The purpose of this study is to characterize the factors that influence the fatigue crack growth (FCG) properties of austempered ductile iron (ADI).  These factors include the heat treatment conditions, mean stress effect, and stress-induced martensitic transformation.  FCG tests using compact tension (CT) specimens were conducted at three load ratios (R = 0.1, 0.5, and 0.7) for as-cast and two austempered ductile irons.  Fractography and microstructure were analyzed by scanning electron microscopy (SEM).  X-ray diffraction was conducted to determine the volume fraction of retained austenite. Experimental results show that as-cast ductile iron has lower FCG rate than ADI at low △K region.  However, as-cast ductile iron has higher FCG rate than ADI at high △K region.  Stress-induced martensitic transformation has been observed to occur at the blocky retained austenite near the main crack area for ADI specimens.  As ADI-II (austemped at 360℃, 2 hr) has a larger volume fraction of retained austenite than ADI-I (austemped at 300℃, 3 hr), more amounts of martensitic transformation have been observed to exist in the ADI-II specimens.  Such difference in the extent of martensitic transformation resulted in the greater crack closure effect and lower FCG rate at a load ratio of R = 0.1 for ADI-II in comparison to ADI-I.  At higher load ratios (R = 0.5 and 0.7), the superiority of ADI-II over ADI-I in FCG resistance became less evident due to the absence of crack closure effect from stress-induced martensitic transformation.  However, the difference in FCG rates at various load ratios for a given ADI can’t not be explaied simply by the crack closure effect.  This is inferred from the fact that when the FCG rates are correlated with the effective stress intensity range, the FCG rate is increased with an increase in load ratio.