【作者】 张真源；

【导师】 王弘；

【作者基本信息】 西南交通大学 ， 工程力学， 2007， 硕士

【摘要】 传统疲劳设计理念认为，载荷循环周次在10⁷以上的结构构件具有无限疲劳寿命，通常以10⁷循环周次所对应的疲劳试验数据做为强度设计依据。但是随着机械设备向高速大型化发展，许多机械和工程结构，如高速铁路的机车车辆结构及零部件，核电站散热器管道，发动机零部件等在承受10⁷～10¹⁰周次的应力循环载荷后的超高周阶段，仍然发生疲劳破坏。传统的疲劳设计规范和寿命预测方法已经不能满足超高周区域机械设备的使用要求，所以有必要对常用的结构钢种进行超高周疲劳试验，得到超高周疲劳数据，对现有设施以及以后待建项目的疲劳强度设计、安全评估及寿命预测都有非常重要的工程意义。本文以16Mn结构钢，304不锈钢，42CrMo钢为研究对象，用超高周疲劳试验方法，进行了10⁴-10¹⁰寿命范围内的超高周疲劳行为的研究。通过超高周疲劳试验得到三种材料在试验下的S-N曲线，分析了S～N曲线的特征，用扫描电子显微分析方法（Scanning Electron Microscopy）研究了三种结构钢的疲劳断裂行为和疲劳断裂机理，分析了裂纹萌生和裂纹扩展机制。研究发现，三种结构钢S-N曲线均具有阶梯型下降特征，不存在传统意义上的疲劳极限，16Mn结构钢在10⁷循环周次以下高周疲劳阶段和10⁷循环周次以上超高周疲劳阶段，疲劳裂纹源均在试样表面萌生。而304不锈钢，42CrMo钢在10⁷循环周次以下高周疲劳阶段，疲劳裂纹从试样表面萌生，10⁷循环周次以上超高周疲劳阶段疲劳裂纹多在试样次表面缺陷处萌生。16Mn结构钢和42CrMo钢疲劳裂纹的扩展机制对高周和超高周疲劳而言没有本质的区别。而304不锈钢第一扩展区形貌与传统定义的第一扩展形貌有着明显的不同，进而定义了304不锈钢超高周疲劳的第一扩展区。研究了加载频率对16Mn结构钢的疲劳性能的影响，发现16Mn结构钢内部组织结构存在塑性很差的渗碳体是导致超高周疲劳性能降低的主要原因。通过扫描电子显微分析方法（Scanning Electron Microscopy）和X衍射分析（X-ray diffractometer）发现在304不锈钢的超高周试验中存在裂尖应变诱发马氏体相变，分析了产生相变的原因。推导了304不锈钢管道流固耦合方程，并对管道使用寿命进行了预测。为管道疲劳设计提供一种可行的方法。通过比较分析40Cr钢，42CrMo4钢的超高周疲劳S～N曲线，发现合金元素Mo提高了材料超高周疲劳性能。并结合Mo在合金化中的作用，分析提高材料超高周疲劳性能的微观机理。从微观角度分析本文三种结构钢没有出现典型内部萌生机制的原因。更多还原

【Abstract】 Conventional fatigue design point considers that structure components have boundless fatigue life-span above 10⁷ loading circulation cycle, corresponding design of fatigue strength based on the fatigue datum of 10⁷ cycles. But with the high-speed and large-scale development of modern mechanism in recent years,mang mechanism and engineering structure such as rolling stock structure and Component and part in high speed railway , nuclear power station radiator pipeline ,and engine component and part and so on can take place fatigue failure in ultra-long life regime exceeding 10⁷-10¹⁰ cycles. So conventional fatigue design standard and life prediction method already don’t meet the activity acquire of mechanism equipment in ultra-long life regime. So it is necessary that testing some structural steel in common use by using ultrasonic fatigue test , and gain the data of ultrasonic fatigue test. It has very important project meaning for fatigue strength designing and safety appraise and life-span forecast.Fatigue behavior in ultra-long life regime (about 10⁴-10¹⁰ cycles) of 16 Mn structural steel , 304 stainless steel , 42 CrMo steel is studied by using ultrasonic fatigue test. Gaining three fatigue S-N curves of the three Materials in testing by ultrasonic fatigue test, and analyse the characteristics of three fatigue S-N curves. The fracture surfaces are analyzed using the scanning electron microscopy （SEM） and study the fatigue behavior and fracture mechanism of the three structure steels . analyzing crack initiation mechanism and crack propagation mechanism. The result of the experiment indicated The curves of the three structure steels are the stepwise shape, and no exit tradition significance fatigue limit. All of the fatigue crack initiated from the surface of specimen in 16 Mn structural steel , but fatigue crack initiated from the surface of specimen below 10⁷ circulation cycle and fatigue crack initiated from the flaws of second surface above 10⁷ circulation cycle in 304 stainless steel and 42 CrMo steel. The crack propagation mechanism is consistent from high-cycle fatigue regimes to ultra-long life regime in 16 Mn structural steel and 42 CrMo steel . The first propagation regime shape is obvious diversity to tradition definition of shape in 304 stainless steel,Then definiting the first propagation regime of it . Comparison of ultrasonic fatigue properties with conventional fatigue properties of 16 Mn structural steel showed that the ultrasonic loading frequency effect on fatigue property of 16 Mn structural steel,. discovering there is existing a very bad plasticity of cementite in framework of organization in the insides of 16 Mn structural steel is to lead to the main cause reducing ultrasonic fatigue property .Using the scanning electron microscopy （SEM） and X-ray diffractometer discover the austenite-martensite transformation occurs in crack pointed end of 304 stainless steel during the ultrasonic fatigue test and analyse cause of austenite-martensite transformation .The equation of the solid-liquid coupling vibration of pipe conveying fluid is deduced , and life time has carried out a forecast on the pipeline .Provides one kind of feasible method to the pipeline fatigue design.By analysing 40 Cr steel comparatively, 42 CrMo4 steel’s ultrasonic fatigue test S-N curves, discover alloying element Mo having raised material surpassing the ultrasonic fatigue property . Effect in being deltamax-rization combining with Mo and, analyses the microcosmic mechanism raising material ultrasonic fatigue property.Microscopic analysis of this from the perspective of three structural steel not typical for internal initiation mechanism更多还原