陶瓷髋关节异响的研究进展和展望

分别从假体外科植入因素、假体设计因素和假体的摩擦振动因素3个方面,评述了近10年来国内外学者在陶瓷髋关节异响的产生机理及其影响因素等方面所做的研究工作和取得的进展。目前,在关节异响研究领域的主要进展是认识到髋关节植入假体的磨损和假体设计的缺陷会导致关节异响,但这些认识不能完全解释关节异响的物理现象。此外,一些学者已经注意到假体摩擦自激振动可能是关节异响的发生机理,但研究工作才刚起步。关节异响的发生机理尚不明确,许多问题还有待深入研究。为了更好地探究关节异响的发生机理,建议从摩擦自激振动与髋关节体内动力学交叉的角度来研究和分析关节异响。     

摩擦与滚阻对被动行走器步态影响的研究

本文研究了圆弧足被动行走器支撑足与地面间的摩擦系数和滚阻系数对被动行走器步态的影响. 首先分别利用扩展的 赫兹接触力模型和LuGre摩擦模型描述了支撑足与地面接触点处的法向支撑力和切向摩擦力,并考虑了行走过程中支撑足 所受的滚动摩阻;其次利用第二类Lagrange方程推导出了该系统的动力学方程,并通过与已有成果的对比确定 了合适的LuGre摩擦模型参数;最后仿真分析了摩擦系数和滚阻系数对被动行走器步态的影响. 研究发现:摩擦系数的改变 虽然对被动行走器行走的平均速度、步幅,以及支撑足接触点处的最大法向接触力的影响较小,但摩擦系数的减小 会改变其行走步态类型,如发生倍周期分岔甚至混沌现象;然而,滚阻系数的改变会对行走器行走的 平均速度、步幅,以及支撑足接触点处的最大法向接触力的影响较大,尚未发现滚阻系数的改变会引起其行走步态的变化.      

时变接触特性表面对界面摩擦学行为的影响

通过在锻钢基材表面沟槽中分别填充灰铸铁HT300、Mn-Cu合金和Mn-Cu阻尼合金材料获得具有时变接触特性的表面,并对锻钢光滑表面试样和时变接触特性表面进行摩擦学试验,研究不同时变接触特性表面对界面摩擦学行为(摩擦噪声、摩擦振动以及磨损行为)的影响. 结果表明:填充HT300的时变接触特性表面缓解了界面磨损,有效延续摩擦系统的稳定状态,抑制摩擦振动和噪声的产生;相反,填充Mn-Cu合金和填充Mn-Cu阻尼合金的时变接触特性表面加剧了界面磨损,加速了摩擦系统不稳定状态的出现,进而激发出高强度的摩擦振动和噪声. 在本研究中,摩擦系统失稳引起摩擦振动和噪声主要归因于摩擦磨损过程中黏着撕裂和犁削等界面作用,填充材料的阻尼特性未能起到减振降噪的效果.      

盘-销摩擦系统摩擦接触力测试与特性分析

测量分析动态摩擦接触力是研究摩擦振动与噪声发生机理的关键.本文中建立了盘-销系统摩擦尖叫试验台架,成功再现了摩擦尖叫.采用三向力传感器对有无摩擦尖叫条件下的动态摩擦接触力进行了测量,并利用小波信号分解、概率密度函数、功率谱密度函数、时频分析等方法进行了分析和讨论.研究发现:在发生摩擦尖叫时,动态摩擦力和法向力发生高频波动,是系统噪声的激励源;无摩擦尖叫时的摩擦力和法向力的动态分量为典型的白噪声随机过程,呈非高斯分布;有摩擦尖叫时的摩擦力和法向力为窄带高频类谐波信号,摩擦力呈非高斯分布,而法向力近似为高斯分布;模态耦合是导致盘-销系统发生动态接触力高频波动以及摩擦尖叫的原因.     

Friction-induced vibrations in the framework of dynamic substructuring

Nonlinear Dynamics - In complex vibrating systems, contact and friction forces can produce a dynamic response of the system (friction-induced vibrations). They can arise when different parts of the...     

A minimal model for studying properties of the mode-coupling type instability in friction induced oscillations

A minimal two degree of freedom model is used to clarify from an intuitive perspective the physical mechanisms underlying the mode-coupling instability of self-excited friction induced oscillations. It is shown that simultaneous out-of-phase oscillations of friction force and displacement tangential to the friction force may lead to energy transfer from the frictional system to vibrational energy. Also it is shown that the friction force acts like a cross-coupling force linking motion normal to the contact surface to motion parallel to it and that a necessary condition for the onset of instability is that these friction-induced cross-coupling forces balance the corresponding structural cross-coupling forces of the system. Finally the origin and the role of phase shifts between oscillations normal and parallel to the contact surface is clarified with respect to the mode-coupling instability. It may be expected that the intuitive picture gained will be of considerable help for practical design purposes.     

The frequency response of dynamic friction: Enhanced rate-and-state models

The prediction and control of friction-induced vibration requires a sufficiently accurate constitutive law for dynamic friction at the sliding interface: for linearised stability analysis, this requirement takes the form of a frictional frequency response function. Systematic measurements of this frictional frequency response function are presented for small samples of nylon and polycarbonate sliding against a glass disc. Previous efforts to explain such measurements from a theoretical model have failed, but an enhanced rate-and-state model is presented which is shown to match the measurements remarkably well. The tested parameter space covers a range of normal forces (10–50 N), of sliding speeds (1–10 mm/s) and frequencies (100–2000 Hz). The key new ingredient in the model is the inclusion of contact stiffness to take into account elastic deformations near the interface. A systematic methodology is presented to discriminate among possible variants of the model, and then to identify the model parameter values.     

Friction-induced limit cycling in flexible rotor systems: An experimental drill-string set-up

Friction-induced limit cycling deteriorates system performance in a wide variety of mechanical systems. In this paper, we study the way in which essential friction characteristics affect the occurrence and nature of friction-induced limit cycling in an experimental drill-string set-up. This study is performed on the level of a Lyapunov-based stability analysis and on the level of both numerical and experimental bifurcation analyses. The synthesis of these results confirms that friction-induced limit cycling is due to a subtle balance between negative damping at lower velocities and viscous friction at higher velocities. Moreover, it is shown how these essential friction characteristics depend on physical conditions such as temperature and normal forces in the frictional contact in the experimental set-up.This work was performed while affiliated to the Eindhoven University of Technology.     

接触共振频率与摩擦噪声频率之间关系的试验研究

在金属往复滑动试验机上用2种尺寸不同和结构略有差别的球-平面摩擦副系统,对接触共振与摩擦噪声的相关性进行了试验研究.结果表明:摩擦系统的接触共振频率非常接近于摩擦噪声频率,两者相对误差不大于2%~5%.对发生尖叫噪声的磨痕形貌进行SEM扫描电镜分析发现:摩擦噪声的发生与摩擦过程中微凸体之间的锤击相互作用有着密切的联系,通过接触共振试验可以辨识实际滑动摩擦系统中最容易发生摩擦噪声的模态频率.     

含摩擦与碰撞平面多刚体系统动力学线性互补算法

基于接触力学理论和线性互补问题的算法, 给出了一种含接触、碰撞以及库伦干摩擦, 同时具有理想定常约束(铰链约束) 和非定常约束(驱动约束) 的平面多刚体系统动力学的建模与数值计算方法. 将系统中的每个物体视为刚体, 但考虑物体接触点的局部变形, 将物体间的法向接触力表示成嵌入量与嵌入速度的非线性函数,其切向摩擦力采用库伦干摩擦模型. 利用摩擦余量和接触点的切向加速度等概念, 给出了摩擦定律的互补关系式; 并利用事件驱动法, 将接触点的黏滞-滑移状态切换的判断及黏滞状态下摩擦力的计算问题转化成线性互补问题的求解. 利用第一类拉格朗日方程和鲍姆加藤约束稳定化方法建立了系统的动力学方程, 由此可降低约束的漂移, 并可求解该系统的运动、法向接触力和切向摩擦力, 还可以求解理想铰链约束力和驱动约束力. 最后以一个类似夯机的平面多刚体系统为例, 分析了其动力学特性, 并说明了相关算法的有效性.      

Passive control of vibration of thin-walled gears: advanced modelling of ring dampers

Vibrations on gears are mainly induced by the gear mesh contact. Resonance conditions of the gear may occur during service if the mesh frequency is close to the natural frequencies of the system at the designed speed of the shaft. Since detuning is not always possible in gears, the response level must be reduced by increasing the damping of the system. In this paper, a passive approach based on the application of a ring damper to reduce the vibration level is presented. The ring damper is placed in a groove underneath the outer rim of the gear. The contact is guaranteed by the preload due to the elasticity of the ring damper itself and above all by the centrifugal force that presses the damper against the groove during rotation. The relative motion of the two components at the contact interface dissipates energy by friction, and hence damping is generated. The vibration amplitude is reduced by optimizing the material and geometrical properties of the ring damper. One of the most important parameters in the determination of the amount of damping due to friction phenomena is the static normal load at the contact, which depends on the mass, the shape, and the material of the ring damper. A numerical method is presented, which couples the static and dynamic equilibrium equations of the assembly. The core of the proposed method is the contact element that takes into account local stick–slip–lift off of the contact and determines the contact forces in terms of static and dynamic loads, which are then used to solve the coupled static and dynamic equilibrium. Since the ring damper has a cut that breaks its continuous circular shape in order to be fitted on the groove, the hypothesis of cyclic symmetry for the gear/ring–damper assembly fails. As a consequence, an appropriate reduced-order modeling is presented to allow the forced response calculations. The algorithm is applied to a dummy bevel gear and to a ring damper having a flat punch contact area. The forced response calculations are performed to highlight the nonlinear interaction between the gear and damper by varying the parameters that mainly affect the amount and distribution of the contact forces and therefore the response level.     

A modified LuGre friction model for an accurate prediction of friction force in the pure sliding regime

We propose a new friction model based on the well known LuGre friction model that can accurately describe the nature of friction force in the gross sliding regime. The modification is based on the responses observed from a single degree-of-freedom friction-induced vibration system. Numerical analysis shows that the friction curve in the gross sliding regime can only show counter clockwise hysteretic loops without violating other essential features. We then develop a new friction model by modifying the LuGre friction model that can describe both clockwise as well as counter clockwise hysteretic loops in the pure sliding domain.     

Analytical approximations for stick-slip vibration amplitudes

The classical “mass-on-moving-belt” model for describing friction-induced vibrations is considered, with a friction law describing friction forces that first decreases and then increases smoothly with relative interface speed. Approximate analytical expressions are derived for the conditions, the amplitudes, and the base frequencies of friction-induced stick-slip and pure-slip oscillations. For stick-slip oscillations, this is accomplished by using perturbation analysis for the finite time interval of the stick phase, which is linked to the subsequent slip phase through conditions of continuity and periodicity. The results are illustrated and tested by time-series, phase plots and amplitude response diagrams, which compare very favorably with results obtained by numerical simulation of the equation of motion, as long as the difference in static and kinetic friction is not too large.     

Comparison and analysis of two Coulomb friction models on the dynamic behavior of slider-crank mechanism with a revolute clearance joint

The objective of this study is to investigate the effects of the Coulomb dry friction model versus the modified Coulomb friction model on the dynamic behavior of the slider-crank mechanism with a revolute clearance joint. The normal and tangential forces acting on the contact points between the journal and the bearing are described by using a Hertzian-based contact force model and the Coulomb friction models, respectively. The dynamic equations of the mechanism are derived based on the Lagrange equations of the first kind and the Baumgarte stabilization method. The frictional force is solved via the linear complementarity problem (LCP) algorithm and the trial-and-error algorithm. Finally, three numerical examples are given to show the influence of the two Coulomb friction models on the dynamic behavior of the mechanism. Numerical results show that due to the stick friction, the slider-crank mechanism may exhibit stick-slip motion and can balance at some special positions, while the mechanism with ideal joints cannot.     

Micromechanical theoretical and computational modeling of energy dissipation due to nonlinear vibration of hard ceramic coatings with microstructural recursive faults

Engine failures due to high-cycle fatigue during severe dynamic vibration have cost the US Air Force an estimated $400 million dollars per year over the past two decades. Therefore, structural materials that exhibit high damping capacities are desirable for mechanical vibration suppression and acoustic noise attenuation. Few experimental studies suggested that hard ceramic coatings, which are commonly used as thermal barrier coatings (TBCs) to protect engine components from high temperatures and corrosion, can also serve as passive dampers due to their unique microstructure which consists of several layers of splats with inter- and intra-microstructural recursive faults (micro-cracks). Therefore, the focus of this study is on the development of a fundamental understanding of the unique microstructural features and mechanisms responsible for this observed energy dissipation in ceramic coatings under nonlinear vibration through the development of a micromechanical computational framework. Inter- and intra-fatigue damage and internal friction is simulated through the development of thermodynamic-based nonlinear cohesive laws that consider interfacial degradation, debonding, plastic sliding, and Coulomb/contact friction between the interfaces of microstructural faults. Representative volume element-based micromechanical simulations are conducted in order to assess the main micromechanical mechanisms responsible for the experimentally observed nonlinear (amplitude- and frequency-dependent) damping in plasma sprayed hard ceramic coatings. It is concluded that the major part of energy dissipation is achieved through contact friction which results from sliding of the splat interfaces along the microstructural recursive faults. Energy dissipation due to progressive decohesion and evolution of new micro-cracks is not that significant as compared to energy dissipated due to increased friction from existing and new created faults. Therefore, internal friction is the main mechanism that makes TBCs effective dampers.     

Nonlinear vibration analysis in precision motion stage with PID and time-delayed feedback controls

Nonlinear Dynamics - We investigate the control of friction-induced vibration in a precision motion stage under the effect of the LuGre friction dynamics. We consider a lumped parameter model of...     

A friction contact stiffness model of fractal geometry in forced response analysis of a shrouded blade

To investigate the nonlinear vibration behavior of a shrouded blade with friction dynamic contact interface, a friction contact stiffness model is proposed to describe the friction force at different rough interfaces and different normal loads. In the proposed model, the friction contact interface is discretized to a series of friction contact pairs and each of them can experience stick, slip, or separate states. Fractal geometry is used to simulate the topography of contact surfaces. The contact stiffness is calculated using the Hertz contact theory and fractal geometry, which is related to contact interfaces parameters including normal load, roughness, Young??s modulus, and Poisson??s ratio. The trajectory tracking method is used to predict the friction force and it is not necessary to judge the transition condition among stick, slip, and separate states. It is suitable for complicated periodic motion of the contact interfaces. The forced response of a real shrouded blade is predicted using the proposed model and the multi-harmonic balance method. The effect of surface roughness, initial normal load, and contact area on the forced response of a shrouded blade is studied. It is shown that contact stiffness increases with normal load and fractal dimension. The resonant amplitude is sensitive to the initial normal load and contact surface roughness. The response can be influenced by the contact area, which is an important parameter for blade designers.     

The frequency response of dynamic friction: A new view of sliding interfaces

Any attempt to predict friction-induced vibration, such as brake squeal in vehicle systems, requires as input some information about the dynamic friction force at the sliding interface. If the concern is only with the threshold of instability based on linearised theory, the quantity required is a type of frictional frequency response function over the range of possible unstable frequencies. This quantity has not previously been measured, and this paper reports the development and testing of a novel tribometer to measure it. Based on the concept of a pin-on-disc machine, controlled fluctuations in sliding speed are applied using an actuator and the resulting fluctuations in friction force are monitored in amplitude and phase. Sample results are shown, revealing parameter dependence, which is in some cases strikingly different from that of the mean friction force. The device has potential for providing necessary data on friction materials and also for discriminating between rival proposed theoretical models for dynamic friction.     

Flow-induced vibration of curved cylinder arrays subject to loose support

In this paper, an improved model is developed aimed at analyzing the fluidelastic vibration of a single flexible curved tube which is surrounded by rigid cylinders and subjected to cross-flow and loose support. Based on the previous model, the axial extension of the curved tube described by von Karman nonlinearity has been accounted for in the current research. Simulations are performed to explore the effect of quasi-steady fluid force model and velocity-limited friction model on the post-instability behavior. Numerical results show that the out-of-plane response is confined to a limit cycle by the von Karman nonlinearity and the in-plane vibration is induced by the out-of-plane vibration through the nonlinear coupling. When the loose support comes into play, the nonlinear impact forces become dominate. The results are presented; comparisons are made to analyze the parameters influencing the fretting-wear damage, such as normal work rate, contact ratio and impact force level.     

Development of models correlating vibration excitation forces to dynamic characteristics of two-phase flow in a tube bundle

Recent experiments revealed significant quasi-periodic forces in both the drag and lift directions in a rotated triangular tube bundle subjected to two-phase cross-flow. The quasi-periodic drag forces were found to be related to the momentum flux fluctuations in the main flow path between the cylinders. The quasi-periodic lift forces, on the other hand, are mostly correlated to the oscillation in the wake of the cylinders. In this paper, we develop semi-analytical models for correlating vibration excitation forces to dynamic characteristics of two-phase flow in a rotated triangular tube bundle for a better understanding of the nature of vibration excitation forces. The relationships between the lift or drag forces and the dynamic characteristics of two-phase flow are established through fluid mechanics momentum equations. A model has been developed to correlate the void fraction fluctuation in the main flow path and the dynamic drag forces. A second model has been developed for correlating the oscillation in the wake of the cylinders and the dynamic lift forces. Although still preliminary, each model can predict the corresponding forces relatively well.