表面粗糙度对三维应力集中系数及疲劳寿命的影响

利用半椭圆微缺口表征表面粗糙度,建立了平板表面形貌的三维有限元模型,总结了微缺口参数对表面应力集中系数的影响规律。建立了表面应力集中系数与表面粗糙度之间的经验公式,预测了不同表面粗糙度下平板的疲劳寿命。研究结果表明：应力集中系数与表面粗糙度成正相关关系;当缺口数大于10时,应力集中系数随缺口数的增加而恒定;当间宽比大于5时,多缺口已经失去缓和应力集中的作用;疲劳寿命随着表面粗糙度的增加而缩短。     

表面微观形貌特征对二次挤压强化孔疲劳性能影响

建立了孔的二次挤压强化疲劳仿真模型,探究孔壁表面微观形貌特征对强化孔疲劳性能的影响。采用Abaqus有限元软件及MSC. Fatigue疲劳分析软件,对孔的二次挤压强化过程进行了模拟并计算了强化孔的疲劳寿命。选取孔壁微元,用半椭圆微观缺口代表零件表面微观轮廓,通过有效应力集中系数K_f修正材料S-N曲线,探究了表面微观轮廓对二次挤压强化孔疲劳性能的影响。结果表明,二次挤压强化后孔壁产生残余压应力层,试验件疲劳寿命得到提高;孔壁微观缺口数目增加可以有效缓解应力集中系数,应力集中系数K_t与深宽比b/a及间宽比d/(2a)成正相关;二次挤压强化可以降低孔壁表面粗糙度21%～49%,能有效缓解缺口应力集中现象,提高孔的疲劳性能。     

复杂曲面表面形貌的评定及其应力集中分析

为合理评定圆棒试样缺口底部复杂曲面的表面加工质量,提出小波变换并结合摄动法对其进行定量分析的方法。对测量的原始表面形貌进行小波变换提取出表面粗糙度信息,并进行表面粗糙度参数分析。采用摄动法对提取出的表面粗糙度信息进行应力集中分析,提出计算复杂曲面表面应力集中系数和表面相对应力梯度的方法。使用NPFLEX白光干涉仪测量平板试样和圆棒试样缺口底部复杂曲面的表面形貌。针对平板试样的表面形貌,使用所提方法对其表面形貌进行评定,验证该方法的准确性。对圆棒试样缺口底部复杂曲面的表面形貌进行评定及应力集中分析,为圆棒试样缺口底部复杂曲面表面加工质量的评定提供参考方法。     

构件不同部位粗糙度对疲劳性能影响差异分析

为验证构件不同部位粗糙度影响的差异性,以某型铝合金为研究对象,采用疲劳试验与理论分析相结合的方法分析不同部位粗糙度组合下构件疲劳性能的差异。试验设计中,为区分不同部位粗糙度造成的影响,着重对试样角部、面部和曲面部做不同的表面处理,同时设计棒试样及两类厚度的板试样以说明试验结果的普适性,并在试验结果处理中通过观测试样的断口形貌定义面断裂(曲面断裂)与角断裂两种断裂形式。为进一步定量分析不同部位粗糙度影响差异性的大小,采用有限元软件分析不同部位微缺口附近的应力分布特性,另外,针对本研究的特殊性提出一种适用于微尺寸缺口处应力计算的改进场强法,并采用此方法分别计算试样表面各类微缺口处的有效破坏应力及有效应力集中系数。通过试验结果与理论计算结果对比发现,对于同样表面粗糙度的试样,不同部位微缺口处的有效应力集中系数存在显著差异,即角部微缺口处集中系数明显高于面部,这一结论反映到宏观上表现为试样边角处的粗糙度对试样疲劳性能的影响明显高于试样面部粗糙度的影响,因此传统方法中将两者做等同处理缺乏一定的合理性。     

一种磨削表面微观形貌应力集中系数的计算方法

针对一种磨削表面微观形貌的应力集中问题提出了一种应力集中系数的精确计算方法。通过实验观察磨削工件表面微观形貌在进给方向具有沟壑状形貌特征,采用缺口模型分析计算磨削表面微观形貌应力集中系数;提出基于二维表面轮廓数据的缺口表征参数计算方法,建立无限大平面缺口模型并采用迭代法对模型进行修正,并用有限元方法进行对比验证。针对磨削表面微观形貌的应力集中系数与缺口模型特征参数-缺口的宽度、深度和谷底半径的定量关系,提供了对应的经验公式。研究工作为磨削表面微观形貌与应力集中系数关联规律研究提供一种定量计算方法,对分析磨削工件表面微观形貌的疲劳性能具有基础作用。     

大型组合机架的疲劳寿命分析

针对大型模锻压机机架的疲劳失效形式,在承受拉、压复合作用下,以等效应力作为裂纹寿命估算的依据,并采用有限元进行疲劳寿命数值分析。分别计算不同应力集中系数、过载荷、残余应力、表面粗糙度取值下的疲劳寿命,并进行相应的敏度分析。通过仿真分析,得到影响疲劳寿命的关键因素变化趋势,为改进疲劳寿命提供直接依据。     

冲击值与金属材料其他力学性能关系的研究

通过试验研究与理论分析,确定缺口应力集中系数,找出缺口应力集中系数和应力强化系数之间的关系;通过40CrMnSiMoA数字化冲击试验数据分析结果,找出冲击值与材料缺口敏感性的关系、冲击裂纹形成功与疲劳裂纹形成寿命之间的关系;从而得出结论:对于高强度和超高强度材料,冲击功值对缺口试样疲劳裂纹形成寿命影响更大,且冲击功值较高的材料比冲击功值较低的材料在等应力条件下疲劳寿命长。     

表面微观形貌对疲劳强度影响的定量分析

利用原子力显微镜对表面定量测量的结果,通过建立有限元模型,分析了表面微观形貌表征参数对微应力集中的影响,研究了四种不同表面处理方式对lCrllNi2w2MoV不锈钢疲劳性能的影响。结果表明:各表征参数包括微缺陷温度、微缺陷的谷底半径和单峰平均间距对微压力集中都有影响,仅靠表面粗糙度参数无法真实反映表面微观形貌对微应力分布的影响;不同表面微观形貌所引起疲劳性能的变化与其表征参数的变化是一致的,利用表面形貌参数可以定量分析不同表面所导致的疲劳性能的改变。     

汽车用CNG钢瓶应力与疲劳寿命分析

通过有限元分析法,验证了计算选用的CNG钢瓶在热-固耦合作用下应力满足GB 150要求,在无表面划痕情况下疲劳寿命满足GB 17258要求。在此前提下,对表面存在划痕的钢瓶进行数值模拟研究,结果表明：缺口应力随缺口深度增加而明显增加,受缺口长度的影响并不显著;钢瓶的疲劳寿命随环向缺口的加深而递减,当缺口加深至1.0 mm后,疲劳寿命耗用系数仍小于1,满足疲劳寿命要求;钢瓶的疲劳寿命随纵向缺口加深而锐减,其影响程度远大于环向缺口,当缺口深度大于0.2 mm后,疲劳寿命耗用系数大于1,不再满足疲劳寿命要求。     

疲劳寿命影响因素的试验研究

考虑应力集中、表面加工及尺寸三方面影响因素设计10种疲劳试样,进行拉压疲劳试验,得到了不同形式试样的疲劳寿命。应用三参数Weibull分布描述了疲劳寿命分布的特性。试验结果分析表明:同时存在应力集中和表面加工影响的零件,应力集中对疲劳寿命的影响起主导作用;对于缺口试件,纯粹意义上的尺寸效应是不起作用的,危险点周围材料的应力梯度是造成尺寸效应的关键。     

Experimental study of surface integrity and fatigue life in the face milling of inconel 718

The Inconel 718 alloy is widely used in the aerospace and power industries. The machining-induced surface integrity and fatigue life of this material are important factors for consideration due to high reliability and safety requirements. In this work, the milling of Inconel 718 was conducted at different cutting speeds and feed rates. Surface integrity and fatigue life were measured directly. The effects of cutting speed and feed rate on surface integrity and their further influences on fatigue life were analyzed. Within the chosen parameter range, the cutting speed barely affected the surface roughness, whereas the feed rate increased the surface roughness through the ideal residual height. The surface hardness increased as the cutting speed and feed rate increased. Tensile residual stress was observed on the machined surface, which showed improvement with the increasing feed rate. The cutting speed was not an influencing factor on fatigue life, but the feed rate affected fatigue life through the surface roughness. The high surface roughness resulting from the high feed rate could result in a high stress concentration factor and lead to a low fatigue life.     

The stress and strain concentrations of out-of-plane bending plate containing a circular hole

The elastic stress and strain fields of a finite thickness plate containing a circular hole subjected to out-of-plane bending are systematically investigated using the finite element method. It is found that the stress and strain concentration factors of the finite thickness plate are different except at the notch root of the free surface even if the plate is in elasticity state. The through-thickness distributions of strain components are not linear with the distance from the mid-plane in the stress concentration region. The nonlinearity of these distributions is very severe near the free surface especially in thick plate. The Euler-Bernoulli hypothesis and Kane-Mindlin's plate theory may not be reasonable to be used in the stress concentration region especially near the free surface. The maximum stress and strain do not always occur on the free surface and their locations depend on the moment ratio and the plate thickness. The maximum stress and strain concentration factors occur on the free surface only in thin plates of small moment ratio. The differences between the maximum value and surface value of stress concentration factor increase with the plate thickness and the moment ratio. This relation of strain concentration factor is similar to the one of stress concentration factor. But the difference magnitude of stress concentration factor is larger than that of strain concentration factor in same plate.     

The Effect of Surface Roughness on Contact Fatigue Behavior Using Mesoscopic Approach

The effect of surface roughness on the contact fatigue was investigated in this study. To accomplish this goal, contact analysis based on the influence functions and the rectangular patch solutions was performed to obtain the subsurface stress. Mesoscopic multiaxial fatigue criterion is then applied to predict fatigue damage. Rainbow counting method and damage rule were used to evaluate the fatigue life under random loadings caused by the rough surface contact. As a result of the fatigue analysis, the relationship among the fatigue life, the crack initiation depth, and the surface roughness were revealed. It is observed that the fatigue life has hardly changed and a crack is initiated at subsurface for the surface with the surface roughness below a critical value. However, if the surface roughness exceeds the critical value, fatigue life is remarkably reduced and the site of crack initiation moves to the near surface.     

Study of Friction and Transfer Layer Formation in Copper-Steel Tribo-System: Role of Surface Texture and Roughness Parameters

In the present investigation, experiments were conducted on a tribological couple—copper pin against steel plate—using an inclined pin-on-plate sliding tester to understand the role of surface texture and roughness parameters of the plate on the coefficient friction and transfer layer formation. Two surface characteristics of the steel plates—roughness and texture–were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture of the plate. The plowing component of friction was highest for the surface texture that promotes plane strain conditions while it was lowest for the texture that favors plane stress conditions at the interface. Dimensionless quantifiable roughness parameters were formulated to describe the degree of plowing and hence the plane strain/stress type deformations taking place at the asperity level.     

Distributions of load stresses and residual stresses at notches

The fatigue strengh reduction factor K, can be mitigated or eliminated by suitable surface treatments. Analysis of these affects requires the knowledge of the distributions of load stresses and of residual stresses below the surface of notches. This paper describes a simple, approximate formula to determine load stress distributions and residual stress distributions at notches. The load stress distributions by the present approach were compared with finite element analysis under tension, bending and torsion loading. Residual stress distributions by the simple formula were compared with measured date by shot peening. An example of optimization in surface treatments by such analysis is shown.     

表面粗糙度对滑动电接触磨损率的影响

在电气化铁路弓网系统中,磨损率是衡量列车运行状态与接触导线使用状态的重要指标。为了充分模拟弓网系统中磨损率情况,利用自行搭建的滑动电接触摩擦磨损试验机对滑板和接触导线进行摩擦磨损试验,分析滑板表面粗糙度、法向压力、接触电流与运行速度对磨损率的影响。得出结论:滑板磨损率随滑板初始表面粗糙度、接触电流、法向压力、运行速度的增加而增加,而高载荷下粗糙度对于磨损率的影响降低;滑板摩擦从磨合期进入稳定摩擦期存在一个临界表面粗糙度,当滑板初始表面粗糙度值等于临界粗糙度值时,其磨损率最低;不同初始表面粗糙度的滑板在跑合期内磨损过程不同,在稳定摩擦期内磨损过程趋于一致,且摩擦试验后滑板表面粗糙度也接近。     

模具粗糙度对板材拉延件摩擦特性的影响

金属板材拉延是一个复杂的塑性变形过程,广泛应用在各种大型汽车覆盖件等零件生产中。介绍了金属带料弯曲拉伸试验机的工作原理和试验方法。该试验机能很好地模拟凹模圆角区的摩擦场变化规律;通过选用不同的模具表面粗糙度,改变带料拉伸力、变形速度和成形头平均面压,测出摩擦因数的变化规律,用以研究模具表面粗糙度等对板材拉延件摩擦特性的影响。研究表明,适当降低模具表面粗糙度,有利于发展边界润滑,减少摩擦热的产生和作用。该研究方法把拉延过程中的摩擦、磨损和润滑有机地结合起来,为优化拉延件加工的摩擦条件奠定了理论基础。     

Improving surface integrity in finish machining of Inconel 718 alloy using intelligent systems

In machining of hard materials, surface integrity is one of the major customer requirements which comprise the study of the changes induced to the workpiece. Surface roughness and residual stress are often considered as the most significant indications of surface integrity. Inducing tensile residual stress during the machining processes is a critical problem which should be avoided or minimized to obtain better service quality and component life. This problem becomes more evident in the presence of rough machined surface because fatigue life of manufactured components might be decreased significantly. Inconel 718 superalloy is one of the hard materials used extensively in the aerospace industries. It is prone to tensile residual stress in machined surface. Thus, controlling and optimizing residual stress and surface roughness in machining of Inconel 718 are so needed. Intelligent techniques based on the predictive and optimization models can be used efficiently for this purpose. In this study, the optimal machining parameters including cutting speed, depth of cut, and feed rate were accessed by intelligent systems to evaluate the state of residual stress and surface roughness in finish turning of Inconel 718. The results of experiments and analyses indicated that implemented techniques in this work provided a robust framework for improving surface integrity in machining of Inconel 718 alloy. It was shown that cutting speed has more effect on surface integrity than other investigated parameters. Also, depth of cut and feed rate were found in the moderate range to obtain satisfactory state of tensile residual stress and surface roughness.