【作者】 闫桂玲；

【导师】 高庆；

【作者基本信息】 西南交通大学 ， 固体力学， 2005， 硕士

【摘要】 已有许多研究成果显示,材料在10⁷周次以上超高周疲劳阶段仍然发生疲劳断裂,值得注意的是各种动力机械的构件在服役期实际承受的疲劳循环周次已远高于10⁷周次,甚至达到了10⁹～10¹⁰周次,因此用传统的10⁷周次的疲劳试验数据进行疲劳强度设计并不安全。近二十年来,借助于超声疲劳试验技术开展的超长寿命疲劳研究（10⁸周次以上）倍受人们关注,在国际上得到了较快发展,成为了疲劳研究的热点问题。近二十年来发表的文献,材料超高周范围内的疲劳研究主要集中在对称拉压加载下材料疲劳S-N曲线形状特征、疲劳断口观察、超高周疲劳行为和疲劳断裂机理的研究上,然而对于实际工程构件,常常是在非对称载荷下工作的,常规疲劳试验表明,平均应力对构件的疲劳性能有很大的影响。因此,对材料进行10⁷周次以上的非对称加载下的疲劳性能研究尤为重要。 本文对现有的对称拉压超声疲劳试验系统进行了改进,开发了三点弯曲超声疲劳试验系统和非对称拉压超声疲劳试验系统;利用超声疲劳试验系统对50钢进行了含有平均应力的三点弯曲疲劳试验和非对称拉压疲劳试验,研究超声频率加载下弯曲平均应力和拉压平均应力对其疲劳性能的影响,得到超声频率加载下描述平均应力影响的模型;用扫描电子显微分析方法（Scanning Electron Microscopy）和X射线能谱分析方法（Energy Dispersive Atomic X-ray）对疲劳断口形貌进行显微分析,研究了50钢的疲劳断裂行为和疲劳断裂机理,确定夹杂的成分。同时,对高温回火和中温回火50钢进行了对称拉压试验,研究了热处理工艺对其超高周疲劳性能的影响。 50钢10⁴～10¹⁰周次的非对称拉压疲劳试验和10⁶～10⁹周次的三点弯曲疲劳试验结果显示:超声频率加载下平均应力对50钢疲劳性能有显著的影响,在应力幅相同的情况下,随着平均应力σ_m的增大,疲劳寿命降低;常规疲劳用于描述平均应力影响的Goodman关系仍能很好的描述超声频率加载下一定寿命的应力幅值和平均应力的配合;常规疲劳用于描述平均应力影响的Morrow关系虽能描述高周（<10⁷周次）范围内拉伸平均应力对50钢疲劳性能的影响,但是不能用来描述超高周范围拉伸平均应力和弯曲平均应力对50钢疲劳性能的影响。 50钢疲劳断口显微分析显示:疲劳裂纹存在两种裂纹萌生机制,在寿命小于10⁷周次的低周高周阶段,疲劳裂纹在试样表面萌生;在寿命大于10⁷周次的超高周阶段,疲劳裂纹从内部夹杂（缺陷）处萌生。夹杂周围都有黑区,黑区更多还原

【Abstract】 Many findings indicate that metals fatigue failure can be observed in ultra-long cycle regime beyond 10⁷ cycles. It is important that many structural components are working beyond 10⁷ cycles in service, even at 10¹⁰ cycles. So the design of fatigue strength based on the fatigue datum of 10⁷ cycles do not guarantee the security to industrial components. In recent years, it is interesting to us to develop the study of ultra-high cycle fatigue （UHCF） by means of ultrasonic fatigue testing technology. The technology develops rapidly in international and becomes a focus problem. But up to now, the ultra-long cycle fatigue is interested in the characteristic of S-N curve, the observation of fatigue failure surface, the behavior and fracture mechanism of ultra-long cycle fatigue. The experiments are carried out at symmetric cycle stress. But the industrial components and structures are often subjects to cyclically varying loads with mean stresses. It is well known that the fatigue process is sensitive to superposed tensile mean stress in both low-cycle and high-cycle fatigue regimes. So it is important to study the fatigue behavior at non-symmetric loading beyond 10⁷ cycles.The three-points bending and non-symmetric tension-compression ultrasonic fatigue testing system have been developed based on symmetric tension-compression ultrasonic fatigue testing system. The fatigue behavior of 50 steel with mean stress is tested by these experiments of ultrasonic fatigue at 20kHz. The effect of bending mean stress and tensile mean stress on fatigue behavior of 50 steel is discussed in this paper and achieve the model that describe the effect of mean stress under ultrasonic frequency loading. The fracture surfaces are analyzed using the scanning electron microscopy （SEM） and energy dispersive atomic X-ray （EDAX）, and study the fatigue behavior and fracture mechanism of 50 steel. In addition, the specimens of medium tempering and high tempering of 50 steel are tested by the symmetric tension-compression fatigue system, and study the effect of heat treatment on ultra-long fatigue behavior.The results of non-symmetric tension-compression fatigue testing in 10⁴¹0¹⁰ cycles regime and three-point bending fatigue testing in 10⁶¹0⁹ cycles regime show that mean stress remarkable effect on fatigue strength of 50 steel under ultrasonic frequency loading. Along with increasing of mean stress, fatigue life decreases on更多还原