【作者】 张继旺；

【导师】 鲁连涛；

【作者基本信息】 西南交通大学 ， 载运工具运用工程， 2008， 硕士

【摘要】 随着机械设备向轻量化发展的需求,高强度钢被大量地用于机械设备和工程结构。过去疲劳研究者一直认为,钢铁材料仅萌生表面裂纹,随着疲劳载荷的降低,通常在10⁷周次以前,表面裂纹出现闭口想象,因此材料出现疲劳极限。尽管很多的机械设备和工程结构在服役期间将承受10⁷甚至10¹⁰周次以上的载荷,由于在10⁷周次以前,钢铁材料具有疲劳极限,因此,疲劳试验和研究仅考虑到10⁷周次的载荷作用即可满足设计要求。可是最近十几年的研究却发现高强度钢在承受低于疲劳极限的交变载荷作用,当加载超过10⁷周次的超长寿命区域（Giga-fatigue regime）,发生由材料内部夹杂物等缺陷引起的疲劳破坏。本课题的研究目标就是揭示高强度钢内部裂纹萌生和扩展行为。本研究以高碳铬轴承钢GCr15为研究对象,用常规旋转弯曲试验机和超声疲劳试验机下进行的10⁷～10¹⁰周次的超长寿命疲劳试验得到的断口,进行了GCr15钢内部裂纹萌生和扩展特性的仿真分析,并综合多种分析手段进行了内部裂纹形成机理的分析和研究,同时定量的分析了内部夹杂尺寸对疲劳寿命的影响,具体内容如下:使用扫描电子显微镜对试验所得的疲劳断口进行了详细观察并发现,两种试验系统下的破坏机制均可以分为:高应力、短寿命下的表面裂纹萌生机制和低应力、长寿命下的内部裂纹萌生机制。用FRASTA法进行了内部裂纹萌生和扩展特性的仿真分析,结果表明:在两种加载系统下得到的超长寿命区带有GBF的断口的内部疲劳破坏的过程是:在夹杂周围首先产生微小的不连续的裂纹,随着裂纹的成长,裂纹逐渐贯通形成GBF区域,然后发生连续扩展。另一方面,在短寿命区没有GBF区的断口的内部疲劳破坏的过程是:首先从夹杂的边缘出现裂纹,然后裂纹发生连续扩展。对GBF区域的详细观察和分析发现,该区域具有:大的粗糙度,区域内凸起部分的面积大小和组织中的碳化物面积大小基本相同,区域内具有高浓度的碳元素分布等特征,综合分析结果显示GBF区域是材料内部夹杂物周围的球形碳化物从组织中离散地剥离后出现的微小裂纹成长形成的。通过使用旋转弯曲试验获得的S-N曲线数据推定了内部裂纹成长率,并基于内部裂纹成长率,定量分析了材料内部夹杂物的尺寸对超长寿命疲劳寿命的影响。对超声加载系统下得到的断口进行详细观察后发现,在从内部夹杂开裂的断口的夹杂周围有明显的发热现象,即超声试验过程的冷却不够充分,超声频率加载下温度升高对材料的疲劳性能有一定的影响。更多还原

【Abstract】 Along with the light requirement of machine, a lot of high strength steels are used in machine components and structures. In the past the fatigue researchers considered that only the surface crack can initiate in the steel materials and the materials show the fatigue limit duing to crack closure before 10⁶ cycles with the reduction of stress amplitude. Although lots of machine components and structure are performed over 10⁷, even 10¹⁰ loading cycles, it is considered that the loading stress at 10⁷ cycles can satisfy the requirement of design when doing the experiment and research. However, in recent teen years it is found that the high strength steel can also fracture from the defect as internal inclusion in the very high cycle regime above 10⁷ loading cycles at the stress amplitude below the convention fatigue limit. The purpose of this paper is revealing the subsurface crack initiation and propagation property in GCr15 steel.The object of this research is high-carbon-chromium bearing steel, GCr15. The simulation analysis of subsurface crack initiation and propagation property in GCr15 steel was carried out by using the fracture surface obtained from the fatigue tests performed by four-axis cantilever-type rotary bending fatigue machine and ultrasonic fatigue machine. Based on different kinds of methods subsurface crack initiation mechanism was discussed, meanwhile, the size of internal inclusion’s influence on fatigue life was quantitatively analyzed. The particular contents are as follows:From detailed observation of the fracture surface by scanning electron microscopy, it was discovered that the materials fractured from surface at high-stress amplitude and low cycles, whereas fatigue failure occured at small internal defects at low stress amplitude in a high-cycle region.The simulation analysis of subsurface crack initiation and propagation property was carried out by the FRASTA method. The result shows that on the fracture surface with GBF area, multiple dispersive micro-cracks formed around the inclusion and coalesced each other to form the GBF area. After that, the crack grew continually. On the other hand, on the fracture surface without GBF area the fatigue crack was initiated around the inclusion and then propagated continually. The fracture surface obtained from the two kind of fatigue test system had the same results.Detail observation and detection of GBF area indicated that the roughness of this area was large, the area of convex particles in it corresponded to those of carbide particles in the microstructure of the tested materials and there was rich carbon distribution in the area. According to these results it can be concluded that the GBF area was formed by the micro-cracks growth owing to the carbide particles around the inclusion dispersively desquamated from the matrix.The subsurface crack growing speed was calculated based on the S-N data obtained from the rotary bending fatigue test. The size of internal inclusion’s influence on fatigue life was quantitatively analyzed according to this speed.The detailed observation on fracture surface which was obtained from the fatigue test performed by ultrasonic fatigue machine shows that on the internal inclusion induced fracture surface there is burned area around the inclusion owing to the deficient cooling. The temperature induced by the ultrasonic frequency makes influence on the fatigue property of the materials.更多还原