【作者】 王弘；

【导师】 高庆；

【作者基本信息】 西南交通大学 ， 固体力学， 2004， 博士

【摘要】 在现代工业机械设备中，许多零部件在低应力超长寿命条件下工作，服役期内需要承受循环载荷的作用达到10⁹～10¹⁰循环周次。目前通用的疲劳强度设计规范和寿命预测模型一般都建立在10⁷（或10⁸）循环周次以下的疲劳试验数据基础上。为提高超长寿命条件下工作的零部件强度设计的可靠性和精确性，保障结构运行的安全，有关材料在10⁷循环周次以上的疲劳性能和疲劳破坏行为的研究已引起工程界的高度重视。 用现有的常规疲劳试验方法完成10⁷～10¹⁰超高周次范围内疲劳试验要耗费大量的时间和费用，为此，本文对一种加速疲劳试验技术（超声疲劳试验技术）进行了开发研究，在对称拉压超声疲劳试验装置基础上，开发研制了非对称拉压超声疲劳试验装置和三点弯曲超声疲劳试验装置，分析了影响超声疲劳试验过程和试验精度的因素，这些研究成果对超声疲劳试验技术的推广应用和试验的标准化、规范化工作具有指导意义。在超声疲劳试验技术研究和开发的基础上，本文采用超声疲劳试验方法，结合扫描电镜断口微观分析，对40Cr钢和50车轴钢光滑试样和缺口试样在超高周疲劳范围内的疲劳性能和疲劳断裂机制进行了研究，获得了以下研究成果： 对40Cr钢和50车轴钢光滑试样和缺口试样在10⁵～10¹⁰循环周次范围内的S-N曲线测定结果显示，调质热处理的40Cr钢在10⁵～10¹⁰循环周次范围内的S-N曲线呈现“连续下降型”特征，在10⁷循环周次附近不存在传统概念上的水平平台，在超过10⁷循环周次后，试样仍然发生疲劳断裂。正火热处理的50车轴钢在10⁵～10¹⁰循环周次范围内的S-N曲线呈现“阶梯下降型”特征，S-N曲线在10⁶～10⁸周次范围内出现一段水平平台，超过10⁸循环周次，S-N曲线第二次下降，表明在平台对应的应力幅以下超高周疲劳范围内，50钢试样仍然会发生疲劳断裂。试样断口显微分析显示，在10⁷周次以下的疲劳断裂，疲劳裂纹在试样表面萌生；而在10⁷以上超高周范围内的疲劳断裂，疲劳裂纹主要在试样内部或次表面材料夹杂处萌生。表明40Cr钢和50车轴钢的疲劳断裂存在疲劳裂纹表面萌生和疲劳裂纹内部萌生两种机制，分别对应不同的S-N曲线，通过两种机制对应的S-N曲线在试验研究范围内的位置关系可以描述材料的S-N曲线的形状特征，本文将这种描述方法称为“双曲线模型”。 对缺口试样疲劳性能的研究结果显示，在10⁵～10¹⁰循环周次范围内，第日页西南交通大学博士研究生学位论文缺口应力集中对40Cr钢和50钢疲劳性能的影响呈现“阶段性特征”，疲劳缺口系数随疲劳循环周次的变化在107循环周次附近存在一个临界循环周次Nc（或临界范围），对应一个最大的疲劳缺口系数;当疲劳循环断裂周次玛<Nc时，疲劳缺口系数随循环周次的增加呈上升趋势;当汉户Nc时，疲劳缺口系数随循环周次的增加呈下降趋势。分析表明缺口应力集中对疲劳性能的影响呈现出的这种“阶段性特征”与两种疲劳裂纹萌生机制的转换有关。 通过对超声疲劳试验结果与常规旋转弯曲疲劳试验结果的比较分析显示，超声疲劳载荷频率对40Cr钢和50钢的疲劳性能存在影响，超声高频载荷使材料疲劳性能提高。分析结果显示，可以用一个加载频率修正系数来修正超声高频疲劳试验结果与常规疲劳试验结果之间的差异，且加载频率修正系数可通过不同应变速率下的材料断裂强度的比值来近似确定。 综合试验研究结果，本文认为疲劳裂纹内部萌生过程是裂纹形核和核心长大成宏观概念上的可扩展裂纹的过程。其中疲劳裂纹内部萌生的核心是材料中的第二相粒子、夹杂物或微空洞，称为微裂纹;而微裂纹的长大是材料中间歇原子或空位等点缺陷在微裂纹尖端富集沉淀的过程，微裂纹的长大速率受到点缺陷富集扩散速率和疲劳载荷的双重影响。本文将这种描述疲劳裂纹内部萌生过程的微观模型称为“点缺陷沉淀”机理。并根据该微观机理建立了疲劳裂纹内部萌生寿命的表达式，分析了影响疲劳裂纹内部萌生寿命的因素，提出了提高疲劳裂纹内部萌生寿命的途径。关键词:40Cr钢，50车轴钢，超声疲劳试验，S一N曲线，缺口应力集中， 疲劳性能，疲劳断裂，疲劳裂纹萌生，裂纹萌生微观机理，载荷 频率的影响更多还原

【Abstract】 The concept of a fatigue limit has been the basis of design against fatigue failure since the late 19th century and is still in use today. Ferrite steels, in particular, were considered to possess a well-defined fatigue limit corresponding to threshold stress amplitude below which fatigue life was believed to be infinite. However, in conventional fatigue testing, the range of numbers of cycles investigated is usually limited to about 107 or, at best, 108 cycles, since performing experiments in the very high cycle regime is extremely time consuming and expensive. Hence, the assertion that fatigue life would be infinite at stress amplitudes below the classical fatigue limit was actually an unproved assumption. With technical development in modern industry, in many applications, the lifetime of the mechanical components have to endure up to 108 cycles of loading without failure. A number of recent studies, extending into the ultra-high-cycle fatigue (UHCF) or gigacycle fatigue range, showed clearly for different steels and other metallic materials that, even at stress amplitudes below the classical fatigue limit, fatigue life was finite. Moreover, it was reported that, whereas fatigue failure in the conventional high-cycle fatigue (HCF) range (<106-107 cycles) occurred at the surface, fatigue failures in the UHCF range generally originated from internal defects. As a result, the endurance limit determined by the conventional fatigue tests cannot provide the safety design data of the mechanical structures. Thus the fatigue property of metallic materials in the UHCF range tends to be an important subject in the mechanical design to ensure the long term safety of the mechanical structures.Since performing experiments of the ultra-high-cycle regime in the range of 108~1010 cycles using a conventional fatigue testing method is very time consuming and expensive, in this paper, a kind of acceleration fatigue test technique, called the ultrasonic fatigue testing, was developed and studied. In the study, based on the symmetrical pull-push ultrasonic fatigue testing machines, the methods of unsymmetrical pull-push ultrasonic fatigue load and the three points bending ultrasonic fatigue load are been developed, and the factor influencing testing process and accuracy is analyzed. These studies will give direction to the method of the ultrasonic fatigue testing.In this study, fatigue behavior of the 40Cr and 50 axles steels including smoothand notched specimens was studied with the ultrasonic fatigue testing technique, with a loading frequency of 20kHz. The fracture surface of specimens was examined by scanning electron microscopy (SEM).Experimental results showed that the S-N curve of 40Cr steel quenched at 850 and tempered at 560, displays the characteristic of "continually decreasing type" up to 1010 cycles and exhibit no traditional horizontal plateau beyond 106 cycles. And the S-N curve of 50 axles steel displays the characteristic of "the multi-stage type", and two decreasing curve and separated by a horizontal step in the region of 106~10 cycles. Over 107 cycles and up to 1010 cycles, fatigue failure in 40Cr and 50 steels do occurs.The observation of the fracture surface showed that fatigue cracks initiate from surface of specimen for short lives (<107 cycles) at high stress levels, and from internal inclusion for very long lives at low stress levels. This indicates that crack initiates mechanism of 40Cr steel and 50 axles steel are of two kinds, and each mechanism corresponds to the respective S-N curves. The S-N curve can be described by the relative position of these two curves and is called "double S-N curve model", in this paper.Study of fatigue behavior on the notched specimen of 40Cr steel and 50 axles steel showed that the influence of notch stress concentration on fatigue properties exhibits characteristic of "different stage", and near 107 cycles, exist a critical cycle Nc (or a critical range of cycle) to correspond to maximum fatigue-notch factor or maximum fatigue-notch sensitivity factor. As number of cycles to f更多还原