Influence of the alignment of load and oscillation on the frictional shakedown of an elastic rolling contact with Coulomb friction

We examine frictional shakedown of a three dimensional elastic rolling contact. Slight oscillatory rolling of one contacting body varies the normal pressure distribution. In turn this causes incremental sliding processes and a macroscopic rigid body motion. We consider two settings: tangential force and rolling direction aligned parallel and perpendicular to each other. In both cases, the slip ceases after the first few periods and a safe shakedown occurs if the oscillation is sufficiently small. Otherwise ratcheting occurs and the accumulated slip leads to a continuing rigid body motion.Numerical simulations with Kalker’s and Vollebregt’s software CONTACT show that the rolling direction leads to differences in the contact region and the traction distribution. Using the method of dimensionality reduction we derive the analytical shakedown limits for the tangential load and the oscillation amplitude. The results show strong agreement with experimental data and allow the accurate prediction of the shakedown displacement and the maximum tangential load capacity in the shakedown state. It shows that a perpendicular alignment of force and rolling direction increases the final displacement in case of shakedown as well as the incremental shift in case of ratcheting.     

Shakedown and induced microslip of an oscillating frictional contact

Using the method of reduction of dimensionality, we calculate the microslip motion of a tangentially loaded frictional contact between an elastic sphere and a rigid base. An oscillating rotation of the sphere with a small amplitude leads to a creep motion of the rigid base. Depending on the amplitude and the tangential force, two possible scenarios may occur. For oscillation amplitudes smaller than a critical value, the rigid body shakes down in the sense that the frictional slip ceases after a limited number of rotation cycles. Otherwise, the rigid base starts to slip with a constant mean velocity, which depends on the static displacement and the rotational amplitude.     

Parametric instabilities of a viscous liquid layer covered by a thin elastic plate under vertical periodic motion

Parametric instabilities of a horizontal liquid layer with a finite depth covered by a thin elastic plate under a vertical periodic motion are investigated with account taken of the viscosity of the liquid layer. The primary regions and the secondary ones of dynamic instabilities are determined by using the equation of a thin elastic plate including the normal component of the viscous stress, but not the tangential component of it. The critical amplitude of the imposed oscillation, beyond which a parametric instability occurs (that is, the neutral stability curves) is found in the space of the frequency and amplitude of the imposed vertical oscillation. These results are confirmed by experimental ones for a liquid layer of glycerine covered with a thin rubber plate.     

A numerical study on stick-slip motion of a brake pad in steady sliding

A numerical model for an elastic brake pad sliding under constant load and with constant velocity over a rigid surface is investigated by finite element analysis. The geometry is taken to be two-dimensional, the contact is assumed to follow the laws of continuum mechanics and temporal and spatial resolution are such that dynamical effects localized at the interface are resolved. It turns out that at the contact interface localized slip events occur either in the form of long-lasting slip pulses, or in the form of brief local relaxations. Macroscopically steady sliding, macroscopic stick-slip motion or slip-separation dynamics occurs, depending on the macroscopic relative velocity. While structural oscillations of the brake pad do not seem to play a significant role during steady sliding at least one structural oscillation mode becomes synchronized with the interfacial dynamics during stick-slip or slip-separation motion. Assuming a given friction law for the interface, the macroscopically observed friction coefficient depends considerably on the underlying dynamics on the interface.     

横观各向同性的半无限弹性体的若干问题

一.引言 作者在前文1里求得了横观各向同性体的弹性力学空间问题的通解,并且作了初步应用。本文的目的在应用前文的通解来具体地讨论横观各向同性的半无限体(假定物体的各向同性面与它的表面平行)的若干问题。这些问题是实用上比较重要的问题,包括:(i)已知半无限体表面上的载荷;(ii)已知表面上各点的位移;(iii)已知表面上各点的法应力和切向位移;(iv)已知表面上各     

Contact mechanics analysis of oscillatory sliding of a rigid fractal surface against an elastic–plastic half-space

Oscillatory sliding contact between a rigid rough surface and an elastic–plastic half-space is examined in the context of numerical simulations. Stick-slip at asperity contacts is included in the analysis in the form of a modified Mindlin theory. Two friction force components are considered – adhesion (depending on the real area of contact, shear strength and interfacial adhesive strength) and plowing (accounting for the deformation resistance of the plastically deformed half-space). Multi-scale surface roughness is described by fractal geometry, whereas the interfacial adhesive strength is represented by a floating parameter that varies between zero (adhesionless surfaces) and one (perfectly adhered surfaces). The effects of surface roughness, apparent contact pressure, oscillation amplitude, elastic–plastic properties of the half-space and interfacial adhesion on contact deformation are interpreted in the light of numerical results of the energy dissipation, maximum tangential (friction) force and slip index. A non-monotonic trend of the energy dissipation and maximum tangential force is observed with increasing surface roughness, which is explained in terms of the evolution of the elastic and plastic fractions of truncated asperity contact areas. The decrease of energy dissipation with increasing apparent contact pressure is attributed to the increase of the elastic contact area fraction and the decrease of the slip index. For a half-space with fixed yield strength, a lower elastic modulus produces a higher tangential force, whereas a higher elastic modulus yields a higher slip index. These two competing effects lead to a non-monotonic dependence of the energy dissipation on the elastic modulus-to-yield strength ratio of the half-space. The effect of interfacial adhesion on the oscillatory contact behaviour is more pronounced for smoother surfaces because the majority of asperity contacts deform elastically and adhesion is the dominant friction mechanism. For rough surfaces, higher interfacial adhesion yields less energy dissipation because more asperity contacts exhibit partial slip.     

考虑风剪切的漂浮式风力机气动特性研究

波浪与风载的共同作用下,海上风电机组平台存在多自由度运动,同时伴随着海上复杂的风况,其气动特性变化较为复杂。以NREL5MW风电机组为研究对象,在风剪切来流下,依据波浪和风载的作用规律,研究平台纵摇和纵荡运动对机组气动特性和绕流场细节的影响。结果表明:平台运动会造成风电机组气动性能的周期性波动,而风剪切作用使得风电机组平均发电量降低并加剧功率和推力的波动。风剪切会增大纵摇运动过程中展向截面的法向载荷幅值且波动加剧,但会降低与功率输出有关的切向受力;风剪切作用推迟了纵荡运动过程中展向截面法向和切向载荷峰值出现时刻,加剧载荷的波动。外叶展截面的法向和切向力系数曲线出现平台,加剧了叶片疲劳载荷,减小了功率输出。     

Influence of wall motion on particle sedimentation using hybrid LB-IBM scheme

We integrate the lattice Boltzmann method (LBM) and immersed boundary method (IBM) to capture the coupling between a rigid boundary surface and the hydrodynamic response of an enclosed particle laden fluid. We focus on a rigid box filled with a Newtonian fluid where the drag force based on the slip velocity at the wall and settling particles induces the interaction. We impose an external harmonic oscillation on the system boundary and found interesting results in the sedimentation behavior. Our results reveal that the sedimentation and particle locations are sensitive to the boundary walls oscillation amplitude and the subsequent changes on the enclosed flow field. Two different particle distribution analyses were performed and showed the presence of an agglomerate structure of particles. Despite the increase in the amplitude of wall motion, the turbulence level of the flow field and distribution of particles are found to be less in quantity compared to the stationary walls. The integrated LBM-IBM methodology promised the prospect of an efficient and accurate dynamic coupling between a non-compliant bounding surface and flow field in a wide-range of systems. Understanding the dynamics of the fluid-filled box can be particularly important in a simulation of particle deposition within biological systems and other engineering applications.     

Contact charging between surfaces of identical insulating materials in asymmetric geometries

The charging that occurs when a pair of insulating surfaces of identical chemical composition are rubbed (i.e. triboelectric charging) remains poorly understood. It is believed that asymmetry in contact plays an important role in this charging. To study this phenomenon, we have developed an experimental methodology that asymmetrically rubs two surfaces by contacting a rotating cylinder with a stationary cylinder – the rubbing is asymmetric in that the contacting area is much greater on the rotating cylinder than on the stationary cylinder. We find that the charge transfer occurs with a spatial distribution of charge, in terms of magnitude and polarity, on the contacted area. The direction of the average charge transfer is material dependent: for Teflon–Teflon contact, the surface with the larger contacting area charges positively, but for Nylon–Nylon contact the surface with the larger contacting area charges negatively. This difference is interpreted as being due to a negatively-charged species transferred in the case of Teflon (electrons or negative ions), but a positively-charged species transferred in the case of Nylon (positive ions).     

Coupled motion of [10−10] tilt boundaries in magnesium bicrystals

Magnesium bicrystals were grown with symmetric and asymmetric tilt boundaries about the [10–10] axis using the vertical Bridgman technique. Isothermal constant load tensile tests were conducted on these bicrystals in the temperature range 300–500°C and relative displacements of the two grains were measured to obtain an appreciation for grain boundary motion characteristics. Coupled grain boundary motion was noted in almost all cases with the degree of tangential motion versus migration changing with tilt misorientation, temperature and applied stress. Specifically, within the family of symmetric bicrystals evaluated, a minimum in grain boundary displacement in the specimen plane was observed at a tilt misorientation of 20°. In specific stress/temperature regimes, rigid body sliding was observed for the particular case of a 35° asymmetric tilt misorientation. The ease of basal and prism slip in magnesium at the temperatures considered and the consequential impingement of intragranular dislocations on the bicrystal boundary and their decomposition and motion along the boundary are thought to play an important role in the observed coupled motion of these tilt boundaries.     

Dynamics of an array formed by three neutrally buoyant cylindrical cantilevers subjected to tensile follower forces

The flexural-rotational coupled motion of three identical flexible cylindrical cantilevers joined symmetrically to a central head is investigated. Effects of the tensile follower forces and inertia parameters on the natural frequencies of the system are studied. The analysis suggests two types of inplane motion: one corresponding to the oscillation of the cantilevers without any rotation of the central body, while the other involves coupled motion of the array. The former corresponds to the repeated eigenvalues which are identical to those of a single cantilever having the same axial tension parameter, P. Three sets of eigenvalues govern the out-of-plane motion: (a) the central head remaining stationary with no rolling motion of the array; (b) vertical motion of the central body without any rolling motion of the array; and (c) rigid body rolling motion without any vertical motion of the central head. There is a possibility of dynamic instability for small inertia parameters and large axial tension.     

限位液滴瞬时失重自激振荡

为探索重力瞬变引起的约束液滴自激振荡机理, 本文利用落塔装置模拟短时微重力环境并借助高速CCD记录圆形限位基片上液滴整个过程的运动情况. 自激振荡是微重力下液滴形态的重整恢复过程, 边界的限位作用使得液滴在整个运动过程接触线钉扎不变, 具体可分为两个阶段: 首先是振荡的高低点位置高度渐进上升的液滴形态变化阶段, 与重力环境渐进变化有关; 而后是平衡位置附近的阻尼衰减振荡阶段, 此时振荡的频率恒定, 振幅衰减类似孤立黏性液滴的指数衰减过程. 对于第二阶段, 在高低点等位置处存在高度不变过程, 高度起伏变化时液滴振荡模式类似自由液滴二阶振荡, 高度不变时振荡模式类似自由液滴三阶振荡. 此外, 对于本实验体系的恒定接触面积的钉扎约束, 液滴的体积量不同时, 内驱振荡的阶段和模式不变, 但具体的振荡过程有所不同. 对于大体积液滴, 会在初始振荡的中间位置出现高度不变现象, 并且随振荡逐渐消失; 而小液滴中间位置则不存在此现象, 波形较一致; 第二阶段小体积液滴振幅衰减的阻尼率更大, 无量纲频率也更高.     

DSPI strain measurement on an externally reinforced bending beam: A comparison of step-by-step addition and pixel shift correlation

A small-scale concrete beam reinforced with an adhesively bonded carbon fiber reinforced polymer (CFRP) plate was subjected to four-point bending. Finite element analyses (FEA) of the bending deformations were carried out to predict strain gradients near the end of the CFRP plate. In order to measure these strains, phase-stepping 3D-digital speckle pattern interferometry was employed. To avoid speckle decorrelation due to the inevitable rigid body motion of the specimen, the load was increased in small increments. Two evaluation schemes for the electronic speckle pattern interferometry phase maps are compared: summing up the measured displacement components load step-by-load step versus regain of the correlation by shifting the final image by an integer number of pixels. Measured strain values are evaluated using a polynomial fit to the measured in-plane displacements and are compared to the FE predicitions. It can be concluded that pixel shift correlation is preferable to summing up load steps for cases of large rigid body motion.     

Elastic convection in vibrated viscoplastic fluids

We observe a new type of behavior in a shear-thinning yield stress fluid: freestanding convection rolls driven by vertical oscillation. The convection occurs without the constraint of container boundaries, yet the diameter of the rolls is spontaneously selected for a wide range of parameters. The transition to the convecting state occurs without hysteresis when the amplitude of the plate acceleration exceeds a critical value. We find that a nondimensional stress, the stress due to the inertia of the fluid normalized by the yield stress, governs the onset of the convective motion.     

Local resonant characteristics of a layered cylinder embedded in the elastic medium

Three kinds of resonant modes of a single layered circular elastic cylinder embedded in the elastic medium are analysed by considering the oscillation of the scatter＇s core, based on the fact that the core moves as a rigid body when the shell material is very compliant. The resonant frequencies of the single resonator acquired by our method are in good agreement with those calculated by the local interaction simulation approach （LISA） for the local resonant phononic crystal. Therefore, the local resonant characteristics of a single layered circular elastic cylinder can be used to evaluate the resonant frequencies of the phononic crystal. The effects of the geometrical and physical parameters of the shell and the core are also studied in details. This work is significant for designing the locally resonant phononic crystal based on the local resonant characteristics of the single resonator, and the resonant frequencies can be tuned by selecting the geometrical sizes and the materials.     

Development of an accelerometer-based underwater acoustic intensity sensor

An underwater acoustic intensity sensor is described. This sensor derives acoustic intensity from simultaneous, co-located measurement of the acoustic pressure and one component of the acoustic particle acceleration vector. The sensor consists of a pressure transducer in the form of a hollow piezoceramic cylinder and a pair of miniature accelerometers mounted inside the cylinder. Since this sensor derives acoustic intensity from measurement of acoustic pressure and acoustic particle acceleration, it is called a p-a intensity probe. The sensor is ballasted to be nearly neutrally buoyant. It is desirable for the accelerometers to measure only the rigid body motion of the assembled probe and for the effective centers of the pressure sensor and accelerometer to be coincident. This is achieved by symmetric disposition of a pair of accelerometers inside the ceramic cylinder. The response of the intensity probe is determined by comparison with a reference hydrophone in a predominantly reactive acoustic field.     

Transverse shear oscillator investigation of boundary lubrication in weakly adhered films

We investigate the boundary lubrication in weakly adhered molecularly thin films deposited between a sphere and a plane, below the sliding threshold. The shear contact stiffness and interfacial dissipation at the micrometer scale are determined with a high-frequency quartz oscillator. Two distinct behaviors are found as a function of the shear oscillation: a linear viscoelastic response at low amplitude and a nonlinear frictional microslip at high amplitude. A friction model is proposed to analyze the data, which allows evaluating the shear strength, the friction coefficient, and the interfacial viscosity at different solid interfaces under low load.     

倾斜均质表面上非等径液滴的聚合特性

对倾斜均匀表面上非等径液滴的聚合过程及特性进行了可视化实验研究,获得了液滴半径和表面倾角等参数对液滴聚合过程中液滴液桥半径、接触角和接触线变化特性的影响规律,进一步说明了倾斜表面上液滴聚合可以加快液滴的运动.     

A novel split-belt apparatus: the stress distribution and performance of its tangent split die

A novel high pressure device with a tangent split cylinder was investigated on the basis of the belt-type apparatus. The cylinder is split into several blocks and the dividing line is tangent to the inner hole. This structure of the cylinder can eliminate the tangential tensile stress due to the friction and squeeze each other on the contact surface between the divided bodies. In this paper, different split numbers of the cylinders were choosen to study the stress distribution and the pressure-bearing capacity, and compare them with the traditional belt-type die (BTD). Simulation results show that the more the number of divided bodies, the smaller the stress and the higher the pressure-bearing capacity. The experiments also indicate that the tangent split die can bear higher pressure than the BTD. The tangent split tungsten carbide cylinder has the advantages of easy manufacture, strong bearing capacity and replaceable performance.     

Analysis of the vibroacoustic field in an extended layer and in the layer–semispace structure

We analyze the acoustic field excited by dipole vibration sources in an extended layer. Two formulations of the problem are considered. In the first case, it is assumed that the layer boundaries are stiff in the normal direction, but the particles oscillating at the boundary may slip in the direction tangential to the walls. In another case, a rigid contact of a “patch” type takes place at the boundary with the host medium. A pattern of the space distribution of the field amplitude both inside and outside the layer is given. The vibration field of the sources operated in pulsed mode in the wave hodograph plane is presented. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 2, pp. 151–163, February 2009.