考虑接触刚度的燃气轮机拉杆转子动力特性研究

基于弹塑性理论对具有粗糙表面的长方微元体进行有限元接触分析,根据受力和变形关系得到了不同载荷作用下的法向和切向界面接触刚度。将微元体界面接触刚度与宏观结构应力分析结果相结合,给出了考虑接触刚度的组合结构动力特性研究方法,分析了压气段轮盘接触刚度对某重型燃气轮机拉杆转子固有振动频率的影响。结果表明,界面接触刚度导致转子固有频率降低,随着接触刚度的增加,其对固有频率的影响逐渐减小。接触刚度对转子各阶固有频率的影响不同,法向刚度对第一阶弯曲频率影响较大,切向刚度对第二阶弯曲频率影响较大。     

粗糙度和预紧力对拉杆转子轮盘接触面静摩擦系数的试验研究

为探究拉杆转子轮盘接触面间静摩擦系数的变化规律,根据分形接触理论对拉杆转子轮盘结合面的实际接触状态进行理论分析。针对不同粗糙度等级和不同拉杆预紧力影响下的拉杆转子轮盘接触面静摩擦系数进行数值仿真计算和试验研究工作。试验研究发现,静摩擦系数随法向载荷的增大而减小。试验测量轮盘端面的粗糙度,发现粗糙度较小或较大时,静摩擦系数的大小与粗糙度成反比;当粗糙度位于某一范围时,静摩擦系数的大小与粗糙度成正比。但分形接触理论认为,结合面间的静摩擦系数随法向载荷的增大而增大。所以分形接触理论不适用于粗糙度较大、法向力较小的场合,接触面静摩擦系数的机理有待进一步研究,分形接触理论有待完善。     

拉杆转子扭转振动研究

以往对拉杆转子的研究多以连续整体转子对待,模型精度不高,为此,将拉杆转子非连续界面上的接触效应考虑在内,运用键合图法建立了拉杆转子扭转振动的动力学模型,并对接触面当量扭转刚度的影响因素作了相关分析。结合一实验拉杆转子对其扭转振动进行了理论计算与实测对比,分析了不同预紧力作用下拉杆转子与连续整体转子扭转振动的固有频率,并将预紧力为4000N时拉杆转子与连续转子的模态振型进行了对比。结果表明,该拉杆转子扭振模型不仅具有更高的精度,而且它通过接触面的扭转变形从本质上体现了拉杆转子与连续转子的区别,为今后对拉杆转子的进一步研究奠定了基础。     

干气密封动静环摩擦界面热弹法向接触刚度分形模型

基于分形理论,根据重新建立的微凸体接触模型,并考虑微凸体弹性变形以及热应力变形,建立了干气密封两摩擦界面热弹性法向接触刚度计算模型,并对其影响因素进行了数值分析。研究结果表明:热法向接触刚度、弹性法向接触刚度均随无量纲真实接触面积、分形维数增大而增大,而随特征尺寸增大而减小,其中分形维数、特征尺寸对弹性法向接触刚度影响较为显著;摩擦因数对热法向接触刚度和弹性法向接触刚度的影响相反,摩擦因数增大,热法向接触刚度变大,而弹性法向接触刚度变小;热法向接触刚度随着转速以线性方式增大。摩擦界面热弹法向接触刚度分形模型的建立与分析,将为研究考虑热效应的干气密封摩擦振动奠定一定基础。     

基于Persson接触理论的转子轮盘接触刚度的计算

目前对于分布式拉杆转子轮盘接触刚度的研究较多使用的是Hertz接触理论。Hertz接触理论把轮盘间的接触等效为微观微凸体弹性半球之间的接触过于理想化,不能精确地模拟轮盘接触的实际情况;且Hertz接触理论中弹性半球接触角大于30°时计算结果就会出现很大误差。基于Persson接触理论提出一种新的接触模型,优化了分布式拉杆转子轮盘接触刚度的求解方法。     

拉杆结构中弹簧刚度和有限元模型刚度融合修正方法研究

为建立更加准确的航空发动机高压转子的有限元模型,对表征高压转子拉杆结构的盘与盘、拉杆与盘之间接触的力学模型进行研究,重构了接触部位的有限元模型。提出弹簧刚度矩阵和有限元模型刚度矩阵的融合修正方法,建立了包含接触的有限元刚度整体优化模型。运用拉直算子将优化模型中需要修正的参数分离出来,结合实验模态分析得到的模态参数对有限元刚度矩阵进行了修正。实例证明修正后的有限元模型真实反映了拉杆联接的航空发动机高压转子的接触状态。     

基于分形理论的两粗糙表面接触的黏滑摩擦模型EI北大核心CSCD

两粗糙表面的接触本质上是大量微凸体的接触,具有复杂的力学行为,连接界面的力学建模是重要的科学问题。从微观角度出发,对单个微凸体进行接触分析,并考虑了微凸体相互作用造成的基底面的下降,根据分形理论积分,建立了整个接触面的法向接触模型。利用该模型,可确定在给定法向预紧载荷下微接触截面积的概率密度函数;根据Mindlin模型、Masing准则和分形理论,建立了两粗糙表面接触的切向载荷与切向位移的关系,并研究了不同参数对系统能量耗散的影响。数值仿真结果表明,能量耗散随分形维数D增大而增大,随分形粗糙度参数G及法向预紧力增大而降低。     

机械结合面切向接触刚度的三维分形理论建模

将微凸体的弹塑性变形区进一步划分,并考虑三维结合面形貌的W-M函数,推导了三维机械结合面切向分形接触刚度的理论模型。数值模拟了结合面的三维切向接触刚度随着分形维数D、分形尺度系数G、材料特征参数间的变化趋势,以及二维分形和三维分形间的对比分析。仿真结果显示:机械结合面的三维切向接触刚度与法向载荷和材料特征参数成单调递增关系,与分形尺度系数成单调递减关系;而其与分形维数之间以D=2.5为界,依次成递增与递减关系;三维分形下的结合面切向接触刚度大于二维分形下的结合面切向接触刚度。切向接触刚度模型的构建可为后续粗糙表面接触非线性动力学及整机动力学模型的建立提供基础。     

级间接触耦合的失谐叶盘模态局部化问题研究

基于有限元实体建模方法,建立了燃气轮机拉杆转子中两级叶盘系统有限元模型,分析比较了有无级间接触耦合作用下叶盘系统的模态特性,重点研究了轮盘级间接触耦合效应对叶盘系统频率转向特性的影响。研究表明,级间接触耦合作用下出现了叶盘耦合振动的复杂振动形式,振动能量不再局限于单级叶盘;级间耦合作用导致叶盘系统的频率转向特性发生改变,频率转向区内的模态不再高度密集,降低了叶盘系统对失谐的敏感度。建立了叶片刚度失谐的两级叶盘系统有限元模型并进行模态分析,结果显示失谐叶盘系统模态局部化程度较低。     

基于粗糙表面分形表征新方法的结合面法向接触刚度模型

如何有效预测结合面的接触刚度,是机械结构设计研究的一个重要课题。结合面接触刚度模型主要分为基于统计学特征参数的和基于分形参数的两类。前者依赖于粗糙表面形貌的测量尺度,后者与测量尺度无关。然而,多数研究者在利用分形理论进行建模时,以对应于微凸体接触面积a的尺寸l=a~(1/2)作为微凸体基底尺寸描述微凸体初始轮廓,给出了错误的微凸体变形机制和结合面接触刚度模型。提出了一种基于D,G和最大微凸体高度的粗糙表面轮廓分形表征新方法,探讨了微凸体接触变形机制,建立了与测量尺度无关的粗糙表面接触力学分形模型,揭示了接触刚度的变化规律。研究表明:接触载荷可用表达式F_c=F(E,D,G,h,a_L)描述;当结合面上的接触压力小于其屈服强度时,无论微凸体发生何种变形,结合面均因存在有弹性变形的接触点而具有一定的法向接触刚度。     

预紧力与粗糙度对拉杆弯曲振动特性影响的研究

微小型涡喷发动机的离心叶轮轴向长度相比转轴长度不可忽略,可将其看作拉杆转子结构。为研究该拉杆转子结构的弯曲振动特性,建立了包含连接面的简支梁数学模型,应用结合面接触分形理论,推导了预紧力、粗糙度对拉杆弯曲振动特性的数学关系。通过算例研究,得出了预紧力、粗糙度对拉杆弯曲振动特性的影响趋势。结果表明:连接面所需的最小预紧力随着粗糙度的增加呈指数增长;仅考虑连接面刚度变化的情况下,拉杆固有频率与对数坐标下的预紧力之间呈S型曲线关系,粗糙度值越大,S曲线的潜伏期越长;同时考虑预紧力对连接面刚度和拉杆抗弯刚度的影响,当粗糙度较小时,拉杆固有频率随预紧力上升而上升;粗糙度较大或预紧力较大时,拉杆固有频率随预紧力上升而下降。     

Experimental investigation of the contact mechanics of rough fractal surfaces

The nonstationary character of roughness is a widely recognized property of surface morphology and suggests modeling several solid surfaces by fractal geometry. In the field of contact mechanics, this demands novel investigations attempting to clarify the role of multiscale roughness during physical contact. Here we review the results we recently obtained in the characterization of the contact mechanics of fractal surfaces by depth-sensing indentation. One class of experiments was conducted on organic thin films, load-displacement curves being acquired by atomic force microscopy using custom-designed tips. Another class of experiments focused on well-defined crystalline and mechanically polished ceramic substrates probed by a traditional nanoindenter. We observed the first-loading cycle to be considerably affected by surface roughness. Plastic failure was found to dominate incipient contact while contact stiffness increased on decreasing fractal dimension and roughness. Our findings suggest fractal parameters to drive contact mechanics whenever the penetration depth is kept below the interface width.     

On the temperature in the wheel–rail rolling contact*

The flash temperature in an asperity of the rail due the wheel–rail rolling contact is investigated. First the contact configuration and the total heat produced are explained. Then approaches to heat partitioning between the two contacting bodies are discussed. A new heat partitioning factor is derived for the case of frictional heating and heating due to plastic deformation, thereby taking into account the roughness of the contacting surfaces by a contact intensity factor distribution as well as the local pressure intensification by individual asperities. Several roughness distributions are studied. Finally, an explicit calculation is outlined of the asperity flash temperature at the end of the contact, expressed by two temperatures and two factors contributing to the partitioning of heat between the contacting bodies.     

The contact mechanics of fractal surfaces

The role of surface roughness in contact mechanics is relevant to processes ranging from adhesion to friction, wear and lubrication. It also promises to have a deep impact on applied science, including coatings technology and design of microelectromechanical systems. Despite the considerable results achieved by indentation experiments, particularly in the measurement of bulk hardness on nanometre scales, the contact behaviour of realistic surfaces, showing random multiscale roughness, remains largely unknown. Here we report experimental results concerning the mechanical response of self-affine thin films indented by a micrometric flat probe. The specimens, made of cluster-assembled carbon or of sexithienyl, an organic molecular material, were chosen as prototype systems for the broad class of self-affine fractal interfaces, today including surfaces grown under non-equilibrium conditions, fractures, manufactured metal surfaces and solidified liquid fronts. We observe that a regime exists in which roughness drives the contact mechanics: in this range surface stiffness varies by a few orders of magnitude on small but significant changes of fractal parameters. As a consequence, we demonstrate that soft solid interfaces can be appreciably strengthened by reducing both fractal dimension and surface roughness. This indicates a general route for tailoring the mechanical properties of solid bodies.     

裂纹拉杆转子连接界面非线性建模与多参数演变规律研究

针对含拉杆裂纹的转子非线性振动现象,基于Paris裂纹扩展规律,建立了裂纹拉杆刚度模型;以界面接触刚度表征拉杆预紧作用,建立了轮盘接触刚度模型;采用Capone短轴承模型,完成了含拉杆裂纹的转子连接界面非线性动力学建模。通过系统Poincare截面分析发现,在转速与偏心多因素影响下得到的e-n分岔集中,系统在倍周期分岔与拟周期分岔之间交替变化;在裂纹扩展与转速的η-n分岔集中,解的分岔点分布不连续;不同转速下,位移随裂纹扩展的分岔轨迹比较复杂,拉杆裂纹的存在会加速或减缓分岔点的出现。结果表明:拉杆裂纹对转子稳定性作用机理受系统参数集的影响。研究结论对拉杆转子的参数设计与故障识别有一定意义。     

The role of van der Waals forces in adhesion of micromachined surfaces

Interfacial adhesion and friction are important factors in determining the performance and reliability of microelectromechanical systems. We demonstrate that the adhesion of micromachined surfaces is in a regime not considered by standard rough surface adhesion models. At small roughness values, our experiments and models show unambiguously that the adhesion is mainly due to van der Waals dispersion forces acting across extensive non-contacting areas and that it is related to 1/Dave2, where Dave is the average surface separation. These contributions must be considered because of the close proximity of the surfaces, which is a result of the planar deposition technology. At large roughness values, van der Waals forces at contacting asperities become the dominating contributor to the adhesion. In this regime our model calculations converge with standard models in which the real contact area determines the adhesion. We further suggest that topographic correlations between the upper and lower surfaces must be considered to understand adhesion completely.     

法向接触刚度对装配体振动模态影响的研究

在对装配体进行模态分析时,装配体接触面间的法向接触刚度对计算结果具有很大影响,但在实际中由于处理困难却往往被省略。基于有限元建模方法分析了装配体接触面法向接触刚度对装配体振动模态的影响。首先,根据接触力学理论推导了法向接触刚度与接触表面粗糙度和接触应力之间的关系;然后,探讨了通过改变层单元弹性模量来模拟法向接触刚度的有限元建模方法;最后,通过算例分析了考虑与不考虑接触刚度时振动模态的差异。研究结果表明,考虑法向接触刚度的有限元等效模型计算结果与解析解非常接近,不考虑接触刚度的整体模型计算结果与解析解异较大,一般超过了10%以上,且频率越低差异越大,另外振型也有明显差异。本文建立的有限元建模方法可用于研究法向接触刚度对装配体振动模态的影响。