中文摘要：
本研究主旨在探討不同熱處理及應變比對Custom 450不銹鋼在應變控制下低週疲勞行為之影響，並嘗試以各種平均應力對壽命影響的關係式來評估Custom 450在不同應變比作用下應變振幅與壽命的關係。此外，並利用掃描式電子顯微鏡(SEM)對破斷面加以觀察分析，以了解其疲勞破壞機構。實驗結果顯示，應變比為-1的低週疲勞試驗，經由三種熱處理之材料在高應變振幅區均會發生循環軟化現象，而在低應變振幅區經SA及H900熱處理的材料軟化的程度會變小，反而經H1150熱處理的材料會發生循環硬化現象。在應變比R=-1狀態下，經H900熱處理的材料低週疲勞壽命較長，而隨著應變比的增加至R=0及0.5，經H900熱處理過的循環壽命就會和經SA及H1150熱處理的疲勞壽命差不多，原因是因H900受平均應力效應之影響較大。就任一熱處理狀態比較其在不同應變比(R=-1、0及0.5)之低週疲勞壽命，由於受平均應力和緩現象的影響，發現在高應變振幅區，三不同應變比下之疲勞壽命皆十分相近，而在低應變振幅區應變比為-1狀態下的疲勞壽命才會明顯優於應變比為0及0.5的狀態。此外，本研究亦提出修正型Manson and Halford及修正型SWT關係式，將Custom 450在不同熱處理及應變比組合作用下的低週疲勞壽命，迴歸得到適用的壽命預估公式。另由SEM的觀察中得知，裂縫在穩定成長區內，基地較為平坦屬於劈裂穿晶方式的脆性破壞，同時在裂縫慢速成長區可觀察到羽毛狀的河流條紋從試棒表面之一點向外擴張，此點即為裂縫起始的地方。在快速破裂區內，基地在短時間內發生破斷，所以破裂的表面起伏較大，則可發現許多類似延性破壞的酒渦狀組織。
Abstract：
This study investigated the effects of mean stress on the low-cycle fatigue (LCF) strength of Custom 450 stainless steels in different tempers.  Uniaxial LCF tests were conducted under strain control with three strain ratios R=-1, 0 and 0.5 at several strain amplitudes in cyclic loading.  LCF specimens were prepared in three different tempers, namely solution-annealed (SA), peak-aged (H900), and overaged (H1150) conditions.  The effects of aging treatment on the LCF behavior are also discussed.  Fractography analysis with scanning electron microscopy (SEM) was applied to determine the LCF failure mechanisms and fracture initiation sites.  Results show that LCF specimens in these three tempers all exhibited cyclic softening at high strain amplitudes under a strain ratio of R=-1.  At low strain amplitudes, the cyclic softening is less evident for SA and H900 temper while H1150 temper exhibits cyclic hardening.  Under a strain ratio of R=-1, Custom 450 in H900 temper has longer LCF life than those in SA and H1150 tempers.  However, this advantage for H900 over SA and H1150 tempers disappeared at higher load ratios (R=0 and 0.5) due to the greater sensitivity to mean stress effect in H900 temper.  For a given temper at high strain amplitudes, the LCF lives among the three applied strain ratios did not show significant differences as a result of the mean stress relaxation effect.  However, at low strain amplitudes, cyclic loading at R=-1 generated the lowest mean stress levels and longest LCF lives as compared to R=0 and 0.5.  The LCF life data obtained for this precipitation-hardening stainless steel under various combinations of heat treatment conditions and strain ratios could be well correlated by a modified Manson and Halford approach and a modified SWT approach.