不同应力比下U75V轨道钢疲劳裂纹扩展行为研究

对高速铁路常用轨道钢U75V进行了不同应力比下的疲劳裂纹扩展门槛值测试和疲劳裂纹扩展试验。试验结果表明，应力比对U75V轨道钢的裂纹扩展行为有着显著影响。利用分级降载法测得的疲劳裂纹扩展门槛值随应力比的增大而减小,裂纹扩展速率随应力比的增大而增大。对裂纹断口微观形貌进行了细致观察分析，发现整个疲劳断口分布有凸条纹棱线。疲劳门槛值附近出现沿晶体学平面的穿晶小刻面；低扩展速率区凸条纹棱线平行排列，棱线方向与珠光体片层方向一致；高扩展速率区的棱线出现弯曲及碎裂，同时伴有解理断面及二次裂纹。

A Study of Fatigue Crack Growth Behavior of U75V Rail Steels under Different R-ratios

The fatigue crack growth threshold values of  U75V  rail steels under different stress ratios were measured and the fatigue crack propagation tests were  carried out.It is shown that R-ratio has significant effects on the crack propagation behavior.The fatigue crack growth threshold values acquired by the stepped unloading method decrease with increasing the stress ratio,while the crack growth rates increase with increasing stress  ratio.The fracture surface morphologies are carefully studied after the experiments.It is  found that ridges cover the fracture surface.Transgranular fracture occurring in crystallographic plane is observed around the near-threshold regions;the lower growth rate regime appears to be parallel arrays of ridges, orientated at the contours of the pearlite lamellae; curving and fragmental ridges accompanied with cleavage fracture as well as secondary cracks are found in the higher growth rate regime.