固体火箭发动机推力向量控制系统动态性能分析

以某型号固体火箭发动机推力向量控制系统为研究对象,采用多体系统动力学计算方法,以作用在系统活动体上的轴向载荷为工作阻力,作动器指令位移为原动件的已知运动规律,接头内引入并联非线性弹簧阻尼器表征各构件间的接触,建立该型号推力向量控制系统动力学模型,进行多体动力学计算,得到接头内构件间的实时运动规律、接触力及力矩。理论计算与系统冷摆试验相结合,分析作动力矩与摆角、频率的关系,计算接触变形产生的力矩增量,分析接头压痕产生的主要原因,并提出改进措施。     

MoS2/Ti复合固体润滑膜的力学性能与摩擦特性

采用非平衡磁控溅射法在9Cr18轴承钢基底上制备了厚度约3μm的MoS2/Ti复合固体润滑膜,基于球形压头纳米压痕试验,采用连续刚度法对MoS2/Ti复合固体润滑膜的力学性能进行研究,探究MoS2/Ti复合固体润滑膜力学性能随压痕深度的变化规律;根据压痕试验载荷-位移曲线,采用Hertz接触理论计算MoS2/Ti复合固体润滑膜的弹性模量并与试验结果进行对比;利用CSM摩擦试验机对低速、低载下MoS2/Ti复合固体润滑膜的摩擦特性进行研究;基于压痕试验提出了一种能够更准确计算钢球加载时MoS2/Ti复合固体润滑膜接触应力的方法,并计算了摩擦试验不同载荷下的接触应力。结果表明:MoS2/Ti复合固体润滑膜的力学性能和摩擦特性都会受到表面形貌的影响;除表面初始压入阶段外,MoS2/Ti复合固体润滑膜的弹性模量和接触刚度都随着压痕深度的增大而增大;滑动速度和载荷共同影响MoS2/Ti复合固体润滑膜的摩擦特性。     

Determination of parameters for an upsetting-sliding test to identify friction coefficients in real contact conditions

The upsetting-sliding test where a rigid contact tool induces localised plastic deformation on the surface of a test piece was developed to identify the friction coefficients and lubrication failures encountered in metal-forming processes. The test has two consecutive stages. In the first, upsetting phase, the contact tool comes into contact with the workpiece and makes an indentation of some hundredths of a millimetre. In the second, sliding phase, the contact tool slides along the workpiece surface. A methodology, based on mechanical analysis of this test using the finite element method, was devised to determine the parameters needed to simulate pre-defined mechanical magnitudes at the contact tool/workpiece interface during the sliding phase. The present work shows that applying this methodology to the upsetting phase of the test reduces the number of numerical simulations required and, consequently, the cost of such a determination, whilst maintaining the test's high level of representativeness.     

A numerical model of friction between rough surfaces

This paper describes a computational method to calculate the friction force between two rough surfaces. In the model used, friction results from forces developed during elastic deformation and shear resistance of adhesive junctions at the contact areas. Contacts occur between asperities and have arbitrary orientations with respect to the surfaces. The size and slope of each contact area depend on external loads, mechanical properties and topographies of surfaces. Contact force distribution is computed by iterating the relationship between contact parameters, external loads, and surface topographies until the sum of normal components of contact forces equals the normal load. The corresponding sum of tangential components of contact forces constitutes the friction force. To calculate elastic deformation in three dimensions, we use the method of influence coefficients and its adaptation to shear forces to account for sliding friction. Analysis presented in Appendix A gives approximate limits within which influence coefficients developed for flat elastic half-space can apply to rough surfaces. Use of the method of residual correction and a successive grid refinement helped rectify the periodicity error introduced by the FFT technique that was used to solve for asperity pressures. The proposed method, when applied to the classical problem of a sphere on a half-space as a benchmark, showed good agreement with previous results. Calculations show how friction changes with surface roughness and also demonstrate the method's efficiency.     

Dynamic simulation of a displacement-controlled total knee replacement wear tester

This paper presents a dynamic finite element method (FEM) model of a commercial displacement-controlled total knee replacement (TKR) wear tester. The first goal of the study was to validate the model, which included both the wear tester and the TKR components. Convergence simulations and experimental testing were performed. These included a novel experimental determination of the coefficient of friction and an evaluation of predicted joint contact areas by comparing simulation results with experimental data collected using pressure-sensitive film. The second goal of this study was to develop a procedure for implementing force-based testing protocols on a displacement-controlled TKR wear tester. A standard force-based cyclic wear-testing protocol was simulated using the FEM model and resulting displacement waveforms were extracted. These were used as control inputs to the physical wear tester and an experimental wear test was performed. Reaction loads on the tibial components were measured and compared with the simulated results. The model was capable of accurately predicting the tibial loads throughout the test cycle, verifying the model's contact mechanics. The study demonstrated the use of computational modelling to convert a force-based testing protocol into displacement-based control parameters for use in a displacement-controlled mechanical testing system.     

某型号固体火箭发动机推力矢量控制系统接触承载性能分析

某型号固体火箭发动机推力矢量控制系统的结构设计,必须保证在承受极端工作载荷时,喷管不能有大的轴向位移,阴、阳球与滚动体间具有一定的接触强度,同时不产生过量的塑性变形,属于复杂结构在复杂载荷作用下的弹塑性摩擦接触问题,无法解析计算且无法试验测量。为解决决定系统结构设计和功能发挥的接触承载性能这一关键问题,模拟系统冷试车试验,充分考虑材料表面强化层,建立各构件间的弹塑性摩擦接触模型。基于计算精度高的三维摩擦接触问题的Lagrange乘子法,解决了与弹塑性耦合的有限元计算问题。计算分析喷管位移,阴球、阳球与滚动体间的接触应力、摩擦应力、变形分布及材料破坏机理。通过与冷试车试验结果的对比分析,检验系统接触承载性能、有限元法及结果的正确性。为该型号推力矢量控制系统的设计提供一种更为高效、精确的计算方法。     

Electronic and phononic friction of solids at low temperatures

It has recently been shown both experimentally and theoretically that there is no static friction in a contact of atomically flat crystalline solids provided the periods of their lattices are incommensurate and their interaction does not exceed some critical value. The only mechanisms of friction in this case are phonon generation and excitation of conducting electrons. It is shown that, at low temperatures, the phonon contribution to the coefficient of friction can be very small by virtue of the quantum mechanical nature of the elementary excitations in a solid. Incommensurate dielectric crystals could therefore slip at low temperatures practically without friction. In metals, on the contrary, the excitation of electrons leads to a finite dynamic friction force at any temperature. Presently, both phonon and electron contributions to the friction force are estimated (the latter both in the normal and the superconducting state of metal).     

点接触弹流摩擦副动特性研究

弹流油膜具有显著的弹簧、阻尼特性,对高副机械零件和系统的动特性具有重要影响。研究弹流摩擦副的动特性,揭示弹流摩擦副动力学特性的变化规律,对改进和提升整个机械系统的动力学设计具有重要意义。基于弹流润滑理论和机械振动学,建立点接触弹流摩擦副的摩擦学-动力学耦合模型,采用数值方法求解弹流摩擦副在简谐激励下的振动响应;通过简谐激励下弹流摩擦副的阻尼环识别出弹流摩擦副的刚度和阻尼,用参数控制的方法研究载荷、速度、材料参数及椭圆度等对弹流摩擦副刚度和阻尼的影响。结果表明：在研究的速度和载荷范围内,摩擦副的刚度和阻尼随载荷和椭圆度的增大而增大,随速度和材料参数的增大而减小,其中载荷对点接触EHL摩擦副的刚度和阻尼的影响最为显著,相比阻尼,摩擦副的刚度随载荷、速度和材料参数的变化幅度要大得多。在数值算例的基础上,给出弹流摩擦副的刚度和阻尼关于载荷、速度和材料参数的拟合公式。数值比较结果表明,给出的拟合公式具有满意的精度,可快速计算弹流状态下点接触摩擦副的刚度和阻尼。     

高速金属切削的摩擦分析及有限元模拟

基于更新的拉格朗日方程,模拟了高速条件下金属正交切削的加工过程,并在刀-屑接触表面上分别建立了库仑摩擦模型和粘结-滑移摩擦模型,通过将切削力、吃刀抗力、切屑厚度和刀-屑接触长度的模拟预测值与相关文献的试验结果比较表明,粘结-滑移摩擦模型更符合实际的摩擦模型,即在金属切削过程中,刀-屑接触表面上同时存在滑移摩擦和粘结摩擦。     

Dissipative function of coulomb friction on elliptical contact area

The problem of calculation of the moduli of the resultant vector and the basic moment of the forces of the dry rolling friction of a solid body against a rough plane surface in the presence of spinning around a vertical axis is solved in quadratures. The elementary forces of sliding friction are directed opposite to the points of the elliptical contact area fulfilling plane motion. The friction coefficient is accepted to be independent of velocity. The proposed method of computing the resultant vector and the basic moment of friction forces on the elliptical contact area makes it possible to find with a preset accuracy the forces and the friction torque in the contact problems by using numerical procedures. In particular, it is used to determine the tangential forces arising during the motion of a rail vehicle. With increasing vertical load on the wheels, the contact spot area in which the friction forces of sliding play the main role becomes larger. The proposed method can also be used with some modifications, e.g., by introducing a corrective factor, to calculate the contact spots having a configuration different from elliptical but restricted by a closed curve.     

PEEK450 FC30与SiC陶瓷在海水润滑下的摩擦磨损特性研究

碳纤维增强聚醚醚酮PEEK450 FC30与工程陶瓷SiC软硬组合作为海水柱塞泵关键摩擦副备选材料,利用MCF 10摩擦磨损试验机对其在海水润滑下的摩擦磨损特性进行试验研究,探讨接触压力、滑动转速对材料磨损率和摩擦系数的影响规律。试验结果表明：在一定范围内的滑动速度、接触压力下,该摩擦副呈现出较小的磨损率和摩擦系数。当滑动速度在0.5~1.5 m/s之间,接触压力为1.33 MPa时,磨损率最小。通过扫描电子显微镜观察摩擦副磨损表层发现,在海水润滑下,SiC磨损并不明显,而PEEK450 FC30的磨损主要是以塑性涂抹为特征的粘着和SiC表面粗糙峰引起的机械犁耕。研究结果对水液压元件的选材具有十分重要的指导作用。     

Acoustic emission method for research and control of friction pairs

One of the new physical methods that provides much information about the processes going on in a friction contact zone in real time is acoustic emission (AE). Basic experimental results of AE in friction are presented and analysed. Using AE signals we can get an indication about the state of the friction processes, about the quality of solid and liquid layers on the contacting surfaces in real time, sometimes about intensity of wear, changing of wear and friction regimes and so on. The influence of sliding velocity, load, roughness and some physical and mechanical properties of the body on the parameters of AE are also given. Some attention to the practical implementation of these techniques for control of capability for work of friction pairs in real conditions are given. Modern methods and devices enable condition monitoring of sliding and rolling bearings.     

Bridging the Gap Between the Atomic-Scale and Macroscopic Modeling of Friction

A short survey of a modern view on the problem of friction from the physical viewpoint is presented. An atomically thin lubricant film confined between two substrates in moving contact has been studied with the help of molecular dynamics (MD) based on Langevin equations with coordinate- and velocity-dependent damping coefficient. Depending on model parameters, the system may exhibit either the liquid sliding regime, when the lubricant film melts during sliding (the “melting-freezing” mechanism of stick-slip motion), the “layer-over-layer” sliding regime, when the film keeps a layered structure at sliding, or the solid sliding regime, which may provide an extremely low friction (“superlubricity”). Atomic-scale MD simulations of friction, however, lead to a “viscosity” of the thin film, as well as to the critical velocity of the transition from stick-slip to smooth sliding, which differ by many orders of magnitude from the values observed in macroscopic experiments. This contradiction can be resolved with the help of the earthquakelike (EQ) model with a continuous distribution of static thresholds. The evolution of the EQ model is reduced to a master equation which can be solved analytically. This approach describes stick-slip and smooth sliding regimes of tribological systems within a framework which separates the calculation of the friction force from the atomic-scale studies of contact properties.     

Thermal and thermomechanical effects in dry sliding

Whenever friction occurs in dry sliding of mechanical components, mechanical energy is transformed into heat through surface and volumetric processes in and around the real area of contact. This frictional heating, and the thermal and thermomechanical phenomena associated with it, can have a very important influence on the tribological behavior of the sliding components, especially at high sliding velocities. Significant developments in the study of these phenomena are reviewed in this paper. Among the topics reviewed are mechanisms of frictional heating and the distribution of heat during sliding friction, the measurement and analysis of surface and nearsurface temperatures resulting from frictional heating, thermal deformation around sliding contacts and the changes in contact geometry caused by thermal deformation and thermoelastic instability, and the thermomechanical stress distribution around the frictionally heated and thermally deformed contact spots. The paper concludes with a discussion of the influence of the thermal and thermomechanical contact phenomena on wear, thermocracking and other modes of failure of sliding mechanical components.     

Effect of high hydrostatic pressure on the external friction coefficient

Experiments are carried out to determine the molecular and mechanical components of the specific friction force under the effect of hydrostatic pressure of up to 140 MPa. The molecular component of the friction coefficient declines by up to two times under the effect of the hydrostatic pressure in various fluids. It is found that the combined influence of the temperature and hydrostatic pressure on the mechanical properties and the contact pressure leads to considerable variations in the deformation component of the static friction coefficient in plastic contact at temperatures of up to 200°C and under pressures of up to 140 MPa. The dependence of the hardness of structural materials on the hydrostatic pressure is analyzed to predict the effect of the latter on the deformation component of friction. It is shown that with increasing pressure within the above range the hardness grows in proportion to the square of the pressure and is inversely proportional to the initial hardness. The formula for calculating the dependence of the indentation depth of a spherical indenter in elastic contact on the hydrostatic pressure is derived.     

摩擦引起的模态耦合不稳定性分析

模态耦合是摩擦系统振动不稳定性的重要机理之一,它会受到系统摩擦副的结构阻抗特性和摩擦接触特性的显著影响。针对一类由一个摩擦副部件的不同方向模态导致耦合不稳定性的现象,建立2自由度摩擦振动动力学模型,推导在摩擦副部件机械阻抗特性相差极大的条件下,系统摩擦耦合模态不稳定性及其对摩擦力方向的依赖性,以及研究了不稳定区域对摩擦副结构和摩擦接触特性参数的敏感性;同时,也分析摩擦副部件的自身约束模态频率和系统接触模态频率对系统摩擦耦合模态区域和频率的影响。这为指导以避免模态耦合的摩擦振动系统的结构参数和摩擦接触参数匹配,以及识别摩擦振动不稳定的摩擦副模态来源提供了理论依据。     

Optimal Energy Dissipation in Sliding Friction Simulations

Non-equilibrium molecular dynamics simulations, of crucial importance in sliding friction, are hampered by arbitrariness and uncertainties in the removal of the frictionally generated Joule heat. Building upon general pre-existing formulation, we implement a fully microscopic dissipation approach which, based on a parameter-free, non-Markovian, stochastic dynamics, absorbs Joule heat equivalently to a semi-infinite solid, and harmonic substrate. As a test case, we investigate the stick?Cslip friction of a slider over a two-dimensional Lennard-Jones solid, comparing our virtually exact frictional results with approximate ones from commonly adopted dissipation schemes. Remarkably, the exact results can be closely reproduced by a standard Langevin dissipation scheme, once its parameters are determined according to a general and self-standing variational procedure.     

Mechanism of energy dissipation in mechanical system with dry friction

Determination of the energy dissipative mechanism in a mechanical system composed of two elastic structures in dry contact is presented. The analysis is based on the measurement of displacement ratio of the contacting elastic structures as a function of frequency due to light impulse excitation at a single point on any of the two elastic structures. The theoretical analysis depends on a very simple model of a two-degree-of-freedom system where two solid friction models are adopted in the analysis of the mathematical model. Several experiments are presented to illustrate the dominant friction mechanism of contacting surfaces within the micro slip regime in a frequency range of oscillation up to 400 Hz. It was shown experimentally that the solid friction model behaves in a way that is described as structural (hysteretic) damping. In other words, the energy dissipated due to dry friction during micro slip regime does not depend on the relative velocity between the two contacting surfaces but it is proportional to their relative displacements. The determination of the contact stiffness and damping loss factor in addition to their variation with the applied normal load was also shown.     

Friction Reduction in Elastohydrodynamic Contacts by Thin-Layer Thermal Insulation

Reducing friction is of utmost importance to improve efficiency and lifetime of many products used in our daily lives. Thin hard coatings like diamond-like carbon (DLC) have been shown to reduce friction in full-film-lubricated contacts. In this work, it is shown that contrarily to common belief, the friction reduction stems mainly from a thermal phenomenon and not only a chemical/surface interaction one. It is shown that a few micrometer-thin DLC coating can significantly influence the thermal behavior in a lubricated mechanical system. The presented simulations, validated by experiments, show that applying a thin DLC coating to metal surfaces creates an insulating effect that due to the increased liquid lubricant film temperature at the center of the contact, locally reduces lubricant viscosity and thus friction. The results of the investigation show that the addition of thin insulating layers could lead to substantial performance increases in many applications. On a component level, the contact friction coefficient in some common machine components like gears, rolling element bearings, and cam followers can potentially be reduced by more than 40 %. This will most likely open up the way to new families of coatings with a focus on thermal properties that may be both cheaper and more suitable in certain applications than DLC coatings.     

Adhesion of cleaned nanoscopic metal contacts

Nanoscopic metal contact in the context used in this paper means atomic force microscope (AFM) tips in contact with cleaned, flat, smooth surfaces in ultrahigh vacuum. Such idealised contact conditions are of more and more relevance for technical friction systems despite the fact that for the latter the loads and the velocities are usually higher, the materials used are impure (e. g. alloys) and contaminants or lubricants are present between the sliding surfaces. Sliding contacts in micro‐system technology, which is unequivocally a key technology of the future, will presumably be compared with such idealised conditions. Another important reason for the investigation of nanoscopic contacts is the strong interest in the understanding of the elementary processes and mechanisms causing friction. The investigation of friction systems with a reduced number of influencing parameters offers a chance to gain insight into elementary processes and to compare the results with theoretical models based on physical principles, such as molecular dynamics simulation.