应力比和残余应力对Ti-6Al-4V高周疲劳断裂模式的影响

对钛合金Ti-6Al-4V进行室温空气环境下的高周疲劳试验,研究消应力和未消应力试样在多个应力比下的疲劳强度,分析和检测表面残余应力在不同应力比和载荷水平下随循环次数而松弛情况,探讨表面残余应力随循环次数而松弛的规律以及残余应力和应力比对Ti-6Al-4V疲劳强度影响的机理。结果表明,随着应力比的提高,残余压应力对Ti-6Al-4V疲劳强度的提升影响逐步减小并消失;与消除残余应力后疲劳破坏源于表面相比,表面残余压应力使得低应力比下疲劳源于内部;当应力比R=-1.0时,疲劳循环导致表面残余压应力松弛且保持稳定,其产生的表面裂纹闭合是疲劳强度提高的主要机制;当R=-0.6~0.1时,未见疲劳循环导致表面残余应力松弛,由于表面层的平均应力减小使得疲劳破坏主要为内部缺陷和局部应力集中控制。     

48MnV钢拉压疲劳过程中的磁记忆信号变化

为探索金属磁记忆信号在材料疲劳过程中的变化规律，选用48MnV钢为材料，以其疲劳极限为负载作标准拉压疲劳试验，并采用金属磁记忆诊断仪检测材料疲劳过程中磁记忆信号的变化。试验结果表明，磁记忆信号在整个疲劳过程中表现出振荡、突变和收敛三大规律，即：磁记忆信号随疲劳循环次数的增加而上下振荡；在500万次疲劳循环附近，磁记忆信号存在一个突变点；该突变点前后，磁记忆信号随疲劳循环次数增加均有明显的收敛现象。     

钝缺口试样疲劳短裂纹形成与扩展

对两种晶粒尺寸的中碳钢钝缺口试样进行了对称拉疲劳试验，应用复型方法观察了疲劳理解纹沿缺口根部的早期形成与扩展过程；测量了表面裂纹形状随裂纹长度的变化规律。结果表明，试样的破坏是由形成于缺口根部表面的短裂纹扩展、裂纹间连接形成穿透裂纹，穿透裂纹扩则引起的。     

20 kHz频率下高强度钢超高周疲劳研究

应用超声疲劳试验技术，对两种高强度钢在20kHz频率下10^5～10^10循环周次的疲劳性能进行了测试分析。结果表明：应力比R=-1时，D38MSV5S钢与100C6钢在10^5～10^10循环周次之间，试样仍会发生疲劳断裂，疲劳强度随循环次数的增加而下降；R=0．1时，D38MSV5S钢在10^6～10^7循环周次之问会出现疲劳强度。与30Hz频率下常规疲劳试验数据比较发现，频率对100C6钢的疲劳性能影响不大；R=-1时，D38MSV5S钢20kHz频率下的SN曲线略低于30Hz频率下的SN曲线，当R=0．1时，频率对D38MSV5S钢疲劳性能的影响更为明显。在超高周疲劳循环下，疲劳破坏主要起源于试样内部夹杂物。     

基于固有频率的两级加载下拉剪点焊结构疲劳损伤分析

对两个焊点的拉剪电阻点焊试样,进行了低-高和高-低非等平均载荷的两级加载,分析了点焊结构的疲劳损伤累积规律。测量了疲劳破坏过程中,动态响应固有频率随循环数的增加的变化。结果表明:对于低-高加载,实验结果存在"锻炼效应",随着较低载荷循环数的增多,疲劳寿命有变长的趋势;对于高-低加载,在较低载荷作用下疲劳寿命变长,存在过载延迟现象。使用固有频率可以很好地监测"锻炼效应"和过载延迟。     

316L钢高温疲劳蠕变规律研究

通过316L奥氏体不锈钢在420、550和600℃下非间断应变疲劳和有保持时间的应变疲劳试验，对316L钢高温环境下疲劳、蠕变规律进行了研究。结果表明：材料的疲劳寿命随温度的升高而降低；保持时间增加，试样中蠕变损伤增大，循环应力松弛更多，裂纹扩展机制由穿晶方式向沿晶方式转变。     

基于红外和声发射的复合材料疲劳损伤实时监测

采用红外热像和声发射技术,实时同步监测2D C/SiC复合材料带孔板的拉-拉疲劳损伤过程,并利用疲劳过程中模量的变化、声发射信号和试样表面温度变化,讨论了带孔板疲劳损伤演变情况.结果表明:当外加应力低于疲劳极限时,疲劳损伤在循环初始阶段产生,损伤随循环数增加逐渐增大,最后趋于稳定;当外加应力超过疲劳极限时,疲劳损伤在短...     

LY12CZ铝合金拉扭复合载荷下疲劳损伤律实验研究

对LY12CZ铝合金薄壁圆管试样进行多种路径下按高—低和低—高顺序作用的两级变幅载荷下的疲劳损伤累积实验。分析两级变幅循环载荷下各种路径的疲劳累积损伤规律,以及第一级载荷作用循环次数对试件总的疲劳寿命的影响。实验结果表明,在多轴拉扭复合加载的情况下,累积损伤规律呈现出非线性,且损伤程度受第一级载荷作用循环次数影响。     

基于固有频率变化的两焊点接头的疲劳损伤参量

使用有限元软件ABAQUS分析两焊点拉剪试样在疲劳裂纹扩展过程中的动态响应,研究固有频率随裂纹面积的变化.将裂纹截面积作为损伤面积,确定损伤与固有频率变化率的关系.利用损伤力学中损伤随寿命的演化规律,对疲劳损伤达到一定程度的试样进行疲劳寿命预测.结果表明,此损伤参量能较好地描述双焊点结构的疲劳破坏过程.     

疲劳载荷作用下应力集中部位金属的磁记忆现象

研究了含有缺口的18CrNi4A渗碳钢试样在疲劳载荷作用下的磁记忆现象。结果表明:在应力集中部位磁记忆信号Hp(y)变化明显,且随循环加载次数的增加呈现规律性变化;在试验过程中,当循环加载到一定次数后,Hp(y)曲线过零点,在随后循环中零点的位置变化很小,直到试样产生裂纹、断裂;磁记忆检测对18CrNi4A渗碳钢构件的疲劳损伤具有较准确的判断能力。     

Q235钢试样低周疲劳过程中表面三维形貌的参数演化规律

为揭示金属低周疲劳过程中表面三维形貌参数演化规律,选取Q235钢试样开展低周疲劳实验,并使用3D轮廓仪获取在同一加载条件、经历不同循环次数的试样表面高度信息,进而对其表面三维形貌进行描述,获取二维粗糙度Ra及三维粗糙度Sa.Vvc.Sal参数特征。结果表明:疲劳过程中表面二维粗糙度Ra及三维粗糙度参数Sa、Vvc、Sal随着疲劳循环次数的增加而增加,且Ra、Sa和Vvc参数在初始与断裂阶段增加幅度较大,中间过程增速较缓。该研究方法及结论可为后续开展形貌相关疲劳寿命评估研究提供基础支撑。     

工程机械金属结构疲劳可靠性分析的响应面法

研究了响应面法在分析工程机械金属结构疲劳可靠性时的应用。求解疲劳可靠性时若将结构的材料、几何尺寸、载荷等视为基本随机变量，功能函数不能表达为基本随机变量的显式函数。本文运用商业有限元软件对结构进行计算，由响应面法构造功能函数二次多项式，再结合JC法或改进的一次二阶矩法求解疲劳可靠度。文中对160t铁路救援起重机伸缩臂结构其进行了疲劳可靠度计算。     

Fatigue strength depending on position of cracks for weldments

This is a study of fatigue strength of weld deposits with transverse cracks in plate up to 50 mm thick. It is concerned with the fatigue properties of welds already with transverse cracks. A previous study of transverse crack occurrence, location and microstructure in accordance with welding conditions was published in the Welding Journal (Lee et al., 1998). A fatigue crack develops as a result of stress concentration and extends with each load cycle until fatigue occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal. Fatigue specimens were machined to have transverse cracks located on the surface and inside the specimen. Evaluation of fatigue strength depending on location of transverse cracks was then performed. When transverse cracks were propagated in a quarter-or half-circle shape, the specimen broke at low cycle in the presence of a surface crack. However, when the crack was inside the specimen, it propagated in a circular or elliptical shape and the specimen showed high fatigue strength, enough to reach the fatigue limit within tolerance of design stresses.     

Effect of plastic media blasting method on mechanical properties of Al 2024-T6 alloy

We investigated the effect of Plastic media blasting (PMB) method on mechanical properties of Al 2024-T6 alloy for aircraft body coatings. Painting-stripping process with three cycles was performed using PMB for three different thicknesses. The relationship between surface morphology-deformation and defects was characterized. PMB affects the fatigue life and surface morphology of specimens. The fatigue life decreases when the specimen thickness decreases. In contrast, this method does not significantly affect the fatigue life of thick specimens. The surface roughness was between 0.30-0.65 Ra for processed specimens. The study will determine proper sheet metal thickness which is not affected seriously because of the method for Al 2024-T6 alloy.

     

Influence of Thermal Fatigue on the Wear Behavior of Brake Discs Sliding against Organic and Semimetallic Friction Materials

ABSTRACT

In this article, brake discs are exposed to high thermal stress, causing thermal fatigue damage. The aim of this work is to study the evolution of the wear behavior of brake disc materials, such as cast iron, chromium steel, and metal matrix composites, under the influence of thermal fatigue. The brake disc specimens are heated and then cooled rapidly. Then, wear tests are carried out using a pin-on-disc-type tribometer. Organic and semimetallic friction materials are used for all wear tests. The results show that thermal fatigue affects the structure of the contact surfaces of all of the disc specimens by increasing their roughness. Furthermore, the wear rate of the friction materials increased, except a reduction of the wear rate is noted for the semimetallic friction material rubbing against cast iron. Moreover, thermal fatigue has no significant influence on the coefficient of friction. The worn surface of the metal matrix composite sliding against semimetallic friction material is characterized by abrasive and adhesive wear mechanisms.     

轿车用薄板焊接结构疲劳性能研究

薄板焊接结构的抗疲劳设计是轿车底盘结构件设计开发中的主要难题,充分理解并掌握薄板焊接结构的疲劳性能是进行焊缝抗疲劳设计的基础。通过一系列的薄板焊接结构疲劳性能试验研究,总结出了焊缝疲劳性能受焊接母材材料强度、母材厚度、焊接制造工艺等因素影响的普遍规律,从而为薄板焊接结构的抗疲劳设计打下了基础。     

Influence of cavitation erosion on corrosion fatigue and the effect of surface coatings on resistance

The fracture of S35C test pieces under cavitation erosion in 3% salt water occurs in the area of corrosion as in the case of corrosion fatigue without cavitation erosion. However, erosion fatigue strength decreases more than corrosion fatigue strength owing to the formation of a macrogalvanic cell between the erosion and corrosion areas. When the surface of the test piece is coated with either a less noble or a more noble metal than that of the matrix, its fatigue strength is recovered. The effects of different materials, test liquids and distances between the disc and test piece are also considered.     

基于非线性超声调制的疲劳裂纹识别方法

由于应力作用、撞击以及周期载荷等因素的影响,金属结构中不可避免地产生疲劳裂纹损伤。若结构存在边界非线性,传统的非线性调制方法将无法有效识别疲劳裂纹损伤。针对该问题,提出一种能够避免边界非线性干扰的非线性超声调制方法。该方法采用正弦脉冲信号和持续正弦信号作为激励,通过识别信号之间的非线性调制现象来进行损伤检测。分别以铝制裂纹梁和完整梁为实验对象,粘贴两个压电片作为作动器和传感器,利用短时傅里叶变换对响应信号进行时频分析,提取非线性调制信号成分,对疲劳裂纹损伤进行有效的识别。     

Prediction of high-cycle fatigue life reliability of aluminum cast alloy from statistical characteristics of defects at meso-scale

In this paper, a growth rate law of a small fatigue crack of an arbitrary metal and a method for predicting a size distribution of spheroidal defects cut by a specimen are proposed. And then, by combining the crack growth rate law and the prediction method for the surface defect size distribution, a method for predicting a high-cycle fatigue life reliability of a metal with defects is proposed. Rotating bending fatigue tests of plain specimens with/without a small blind hole (0.3 mm in diameter, 0.15 mm in depth) are carried out on an aluminum cast alloy JIS AC4B-T6 with eutectic Si, Fe compounds and porosities. By comparing analyzed results with experimental ones, the validity of the present method is examined.     

金属断口图像句法模式识别与分类方法

针对金属断口图像，提出了一种基于句法模式识别理论的分类方法。该方法通过提取断口图像上反映出的形貌特征。并将这些特征量化成特征参数，分别定义不同类型断口的全图特征参数和典型特征块特征参数，可以较好地将不同类型的断口区分开来。分别对等轴塑坑、拉长塑坑、河流花样、腐蚀疲劳、韧性沿晶和韧性疲劳六类断口进行分类识别研究。实验结果表明，这种方法具有较高的识别率。