軌道工程中藉由螺栓來連結軌道與電路端子，產生電路迴路並確保路段之通車安全，而列車行駛時輪軌之間可能的接觸碰撞，間接使螺栓組合件產生反覆的應力變化，進而產生疲勞破壞的風險。為了適當評估螺栓之疲勞壽命，本文使用ABAQUS有限元素法軟體，建立兩種電路螺栓組合件之三維數值分析模型，並考慮螺栓與其他零件之彈塑性力學性質，將組合件分別以不同扭矩值鎖固後，於軌道面施加反覆的橫移量，比較兩種組合件螺栓在前斷面缺口處、中斷面光滑處、後斷面螺紋凹槽處的應力分布情形與應力集中效應，了解特定點之應力幅度變化，再藉由S-N曲線來評估螺栓疲勞破壞的可能性。由分析結果可得知，若組合件之零件間的初始空隙大小分布不一，會使螺栓在鎖固後產生撓曲變形，並使其斷面應力分布不均，容易產生較大的局部應力值；當軌道面有較大橫移量時，發生最大應力位置有轉移現象。在較低的鎖固扭矩情形下，夾緊件之間容易受軌道面橫移量作用而產生無法回復的空隙，並產生較大的應力幅度值，導致其完全反覆應力幅度可能接近或超過其疲勞極限值。在較高的鎖固扭矩情形下，雖然能提升夾緊件之間的貼合程度，但在應力集中效應較大處，材料塑性變形程度較大。
若螺栓組合件之幾何配置對稱且良好貼合，能避免鎖固後螺栓內部應力分布不均，而合理的鎖固扭矩值，會使夾緊件之間較不易因軌面橫移量鬆動或產生過大應力幅度值，能適當避免疲勞破壞的風險與過多的塑性變形。

Bolted joints are widely used in mechanical and civil engineering applications. When bolted joints suffer from vibration or cyclic loading, they may finally damage due to fatigue failure. This study built the three-dimensional finite element models for two bolted joints to carry out the numerical simulations of the bolted joints during tightening and under cyclic loadings. The elastic-plastic material properties were considered into the simulation, so that the concentrated stress near the thread and bolt head can be computed accurately. The result shows that high tightening torque allows the jointed parts to contact each other well even at cyclic loadings, so that the stress amplitude can be lower than the fatigue limit of stainless steel and the fatigue life of the bolt exceeds 106 cycles. However, when the tightening torque reaches 90N-m, a large portion of the bolt at the thread yields, hence the risk of ductile failure increases. To reduce bolt failure and increase its fatigue life, the appropriate tightening torque based on our simulations is suggested. It is noted that conformal contact between the jointed parts is the key to more uniform stress distribution and less stress amplitude of the bolted joint.

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