Dynamics of fingertip contact during the onset of tangential slip

Through highly precise perceptual and sensorimotor activities, the human tactile system continuously acquires information about the environment. Mechanical interactions between the skin at the point of contact and a touched surface serve as the source of this tactile information. Using a dedicated custom robotic platform, we imaged skin deformation at the contact area between the finger and a flat surface during the onset of tangential sliding movements in four different directions (proximal, distal, radial and ulnar) and with varying normal force and tangential speeds. This simple tactile event evidenced complex mechanics. We observed a reduction of the contact area while increasing the tangential force and proposed to explain this phenomenon by nonlinear stiffening of the skin. The deformation''s shape and amplitude were highly dependent on stimulation direction. We conclude that the complex, but highly patterned and reproducible, deformations measured in this study are a potential source of information for the central nervous system and that further mechanical measurement are needed to better understand tactile perceptual and motor performances.     

Influence of contact configuration on fretting fatigue behavior of Ti–6Al–4V under independent pad displacement condition

Fretting fatigue tests were conducted, using cylindrical pad and flat pad with rounded edges, at various applied pad displacements and at two normal forces on the pad under a constant bulk stress amplitude condition. The evolution of tangential force was independent of the contact configuration at a given normal force. The ratio of the tangential force to normal force increased and stabilized to a certain value with increasing applied pad displacement. The minimum fretting fatigue life was observed at the relative slip range between 50 and 60 μm and it was independent of both contact configuration and applied normal force. With increase in the applied pad displacement the response of the tangential force (Q) and the relative slip (δ) showed different fretting conditions, i.e. stick, stick-slip and gross slip. The gross slip condition was characterized by rectangular shape of the Q–δ curve with or without monotonically increasing value of Q with increasing fretting fatigue cycles. Surface profile on the fretting scar was affected by the contact configurations. For cylinder-on-flat contact, the profile showed surface damage (e.g. material loss or wear) along the entire contact area. However, the fretting damage in flat-on-flat (with rounded edges) contact was concentrated on the edge, not affecting much of the flat portion of the fretting scar.     

旋转型行波超声电机理论模型的仿真研究

建立了旋转型行波超声电机定、转子间的摩擦驱动模型，不仅考虑了接触界面上的纵向分布力，而且分析接触力沿周向和径向上的分量，在此基础上结合定子和转子的动力学模型得到了整个电机的机电耦合模型。文章分析了预压力影响电机驱动频率的本质原因，说明该频率不能直接通过分析自由定子得到，最后通过数值计算分析了接触界面力对电机工作频率的影响，取得了与实验一致的结果。     

Analysis of the energy and damage evolution rule for sandstone based on the particle flow method

We use the particle flow code PFC3D to simulate the triaxial compression of sandstone under various radial stresses and loading strain rates to determine the triaxial stress-strain curves, crack propagation path, and contact forces to investigate the failure process of sandstone. We analyze the energy and damage evolution during triaxial compression. The results indicate that the tension and shear-induced cracks increase with the increase of radial stress under the same loading strain rate. Both normal and tangential contact forces exhibit strong anisotropy and increase with radial stress and strain rate. The normal contact force has an approximately symmetrical distribution with respect to the horizontal plane, whereas the tangential contact force has an approximately symmetrical distribution with respect to the axis. For the characteristics of the energy evolution, the boundary energy density, strain energy density, and dissipated energy density all increase linearly with the radial stress, and the boundary energy density increases at the fastest rate, followed by the strain energy density and dissipated energy density. In the post-peak stage the primary energy consumption is the dissipated energy. After that, in the remaining stage the strain energy decreases gradually. By analyzing the evolution of the damage variables in the prepeak area we observed that the damage variable followed an exponential relationship with the axial strain. When the loading strain rate is constant, the damage variable corresponding to the same strain value decreases with increase of radial stress. The results indicate that the increase in radial stress delays the damage acceleration. In contrast, the effect of the loading strain rate on the damage variable is small. The findings reveal the internal structural evolution of rocks during deformation and failure.

     

Effect of skin hydration on the dynamics of fingertip gripping contact

The dynamics of fingertip contact manifest themselves in the complex skin movements observed during the transition from a stuck state to a fully developed slip. While investigating this transition, we found that it depended on skin hydration. To quantify this dependency, we asked subjects to slide their index fingertip on a glass surface while keeping the normal component of the interaction force constant with the help of visual feedback. Skin deformation inside the contact region was imaged with an optical apparatus that allowed us to quantify the relative sizes of the slipping and sticking regions. The ratio of the stuck skin area to the total contact area decreased linearly from 1 to 0 when the tangential force component increased from 0 to a maximum. The slope of this relationship was inversely correlated to the normal force component. The skin hydration level dramatically affected the dynamics of the contact encapsulated in the course of evolution from sticking to slipping. The specific effect was to reduce the tendency of a contact to slip, regardless of the variations of the coefficient of friction. Since grips were more unstable under dry skin conditions, our results suggest that the nervous system responds to dry skin by exaggerated grip forces that cannot be simply explained by a change in the coefficient of friction.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

Statistical properties of a 2D granular material subjected to cyclic shear

This work focuses on the evolution of structure and stress for an experimental system of 2D photoelastic particles that is subjected to multiple cycles of pure shear. Throughout this process, we determine the contact network and the contact forces using particle tracking and photoelastic techniques. These data yield the fabric and stress tensors and the distributions of contact forces in the normal and tangential directions. We then find that there is, to a reasonable approximation, a functional relation between the system pressure, P, and the mean contact number, Z. This relationship applies to the shear stress τ, except for the strains in the immediate vicinity of the contact network reversal. By contrast, quantities such as P, τ and Z are strongly hysteretic functions of the strain, ε. We find that the distributions of normal and tangential forces, when expressed in terms of the appropriate means, are essentially independent of strain. We close by analyzing a subset of shear data in terms of strong and weak force networks.     

The Effects of Debonding on the Low-Velocity Impact Response of Steel-CFRP Fibre Metal Laminates

The effect of metal-composite debonding on low-velocity impact response, i.e. on contact force–central deflection response, deformation profiles and strains on the free surfaces was studied. We focused on type 2/1 fibre metal laminate specimens made of stainless steel and carbon fibre epoxy layers, and tested them with drop-weight impact and quasi-static indentation loadings. Local strains were measured with strain gauges and full-field strains with a 3-D digital image correlation method. In addition, finite element simulations were performed and the effects of debonding were studied by exploiting cohesive elements. Our results showed that debonding, either the initial debonding or that formed during the loading, lowers the slope of the contact force–central deflection curve during the force increase. The debonding formation during the rebound phase was shown to amplify the rebound of the impact side, i.e. to lower the ultimate post-impact deflection. The free surface strains were studied on the laminate’s lower surface at the area outside the debond damage. In terms of in-plane strains, debonding formation during impact and indentation, as well as the initial debonding, lowered the peripheral strain and resulted in a positive change in the radial strain.     

Linear-frictional contact model for 3D discrete element simulations of granular systems

The linear-frictional contact model is the most commonly used contact mechanism for discrete element (DEM) simulations of granular materials. Linear springs with a frictional slider are used for modeling interactions in directions normal and tangential to the contact surface. Although the model is simple in two dimensions, its implementation in 3D faces certain subtle challenges, and the particle interactions that occur within a single time step require careful modeling with a robust algorithm. The paper details a three-dimensional algorithm that accounts for the changing direction of the tangential force within a time step, the transition from elastic to slip behavior within a time step, possible contact sliding during only part of a time step, and twirling and rotation of the tangential force during a time step. Without three of these adjustments, errors are introduced in the incremental stiffness of an assembly. Without the fourth adjustment, the resulting stress tensor is not only incorrect but it is also no longer a tensor. The algorithm also computes the work increments during a time step, both elastic and dissipative.     

轮对运动状态对轮轨滚动接触应力的影响

分析计算了锥型踏面轮对沿轨道滚动接触时轮轨接触几何参数和不同运动状态下的轮轨之间的刚性蠕滑率.根据确定的轮轨接触几何参数和轮轨接触界面之间的蠕滑率,利用非Hertz滚动接触理论分析计算了锥型轮对和钢轨滚动接触斑作用力的分布.再利用弹性力学中Bossinesq-Cerruti力/位移计算公式并借助Gauss数值积分方法,确定了轮轨滚动接触时体内的弹性位移、应变和应力随轮对运动状态变化情况.数据结果为轮轨强度设计提供了重要的参考依据.     

回转窑滚圈与托轮接触应力解析

 滚圈与托轮接触应力的计算对回转窑滚动接触疲劳寿命预测起到至关重要的作用。考虑滚圈与托轮在牵引滚动接触状态下相互之间传递法向力和切向摩擦力的综合作用,将其简化为二维平面应变问题,然后从经典Hertz理论中弹性半空间在分布法向力和切向力作用下受载区应力分量的积分表达式出发,对接触区对称平面上各点的应力进行分析与计算,得出受载区各主应力的解析表达式,并采取最优函数逼近法求得半解析解,确定了最大主剪应力及其位置,能满足工程应用的要求。     

钢管混凝土桁肋内栓钉相贯节点受力行为试验研究

进行了主管内壁设置栓钉的钢管混凝土K形相贯节点（内栓钉节点）受力行为试验,同时进行了无内栓钉节点的对比试验。试验时在主管上施加压力,在两根支管上分别施加等量同步的压力和拉力。试验结果表明,该文节点的破坏为受压支管屈曲,荷载-受压支管变形曲线（N-S 曲线）可分为弹性段和弹塑性段。无论有无内栓钉,主管焊接区域的应变都大于其他区域,它与荷载的关系曲线可分为弹性段和弹塑性段,而其余区域钢管均处于弹性状态。内栓钉不会改变主管钢管应变的分布规律,但是内栓钉的设置会减小主管钢管应变,表明内栓钉能将外力从钢管传递到管内混凝土,并且对应变集中的部位效果更显著。在主管轴向,应变主要集中在距离节点中心截面两侧各约2.7倍支管直径范围内;在主管环向,应变主要集中在圆心角-60°~60°区间。建议将此轴向与环向范围的应变集中区作为内栓钉的主要布设区域,区域外可以少布设或不布设。     

旱寒区输水渠道防渗抗冻研究进展与前沿

Winkler冻土地基模型无法考虑土弹簧间相互作用,且需要预先假定切向冻结力分布。为克服这些不足之处,在现有模型基础上引入Pasternak剪切层反映冻土-衬砌间法向相互作用,引入切向弹簧组成的接触界面层反映冻土-衬砌间切向相互作用,构建考虑双重剪切的梯形渠道Pasternak双参数冻土地基模型。以塔里木灌区某渠道为例计算衬砌冻胀变形,并与材料力学法、Winkler模型计算值及观测值进行对比。结果表明：与材料力学法、Winkler模型相比,该文模型计算值更接近观测值。根据该模型估算的衬砌板易开裂范围也与灌区现场调查结果相符,表明模型适用性。通过参数研究分析了界面切向刚度k_x对各处切向位移及切向冻结力分布的影响。结果表明：k_x越小时,切向位移与切向冻结力趋于线性分布;k_x较大时,两者均偏离线性分布且k_x越大偏离越明显,线性分布假设不再适用。该模型可较充分地反映冻土与衬砌板间接触界面相互作用的基本特征,从而有效提高模型的计算精度。

     

Analysis of the apparent friction of polymeric surfaces

The apparent friction coefficient is the ratio between the tangential force and the normal load applied to moving body in contact with the surface of a material. This coefficient includes a so-called “true local friction” at the interface and a “geometrical friction” which is the ploughing effect. The material underneath a moving tip may display various types of behaviour: elastic, elastic–plastic where elastic and plastic strain are present in the contact area, or fully plastic. As is usual in polymers, the material behaviour is time and temperature dependent and may exhibit strain hardening. A surface flow line model of a scratching tip which links the apparent friction to the local friction and contact geometry was recently proposed. An inverse analysis is used in the present work to estimate the local friction from the measured apparent friction and a knowledge of the contact area and tip shape. The polymer true friction coefficient displays temperature and sliding speed dependency, which may be attributed to the surface thermodynamics. It is shown that the local friction depends on the level of strain in the polymer at the contact interface.     

Contact properties determination of macroscopic fine disperse glass particles via compression tests in normal direction

The paper describes the deformation behavior of spherical, dry and non-porous particles during a single particle compression test in normal direction. Therefore a compression tester was built. Industrial used soda lime glass particles with two macroscopic fine disperse sizes (d_1,50,3 = 284.30 μm and d_2,50,3 = 513.20 μm) were applied as model material to investigate the micromechanical contact behavior. In order to influence the elastic-plastic contact properties of particles, the surfaces were altered with chemical modification by means of silanization.The determination of various micromechanical contact properties (e.g. adhesion force, modulus of elasticity and contact stiffness) was carried out model-based with the contact model ‘stiff particles with soft contacts’ by means of a back-calculation.It could be shown that the model-based determination of material properties was a good alternative compared to the comprehensive tensile tests and pull-off force measurements.In addition to the gained normal force-displacement data in normal direction, the friction limits for tangential loading and rolling with the load-dependent adhesion force were model-based determined.     

In-plane shear investigation of biaxial carbon non-crimp fabrics with experimental tests and finite element modeling

In-plane shear is one of the basic deformation mechanisms in forming fabrics on complicated shapes. In this paper, the in-plane shear behavior of non-crimp fabrics (NCFs), including NCFs based on T300 carbon fibers with chain or tricot-chain stitches, was characterized by picture frame and bias extension tests. It was found that the stitching yarns’ strained condition depending on loading direction influences the shear behavior of NCFs. Further, the results of these two tests were compared by normalizing the shear force. It was observed that the normalized results of these two tests for shear force are consistent with each other in the direction of shear of the stitching, while deviations in other directions are attributed to the different strain mechanisms, as a result of the clamping way of the sample in the test. Finally, a mesoscopic finite element (FE) model was established to simulate the picture frame and bias extension tests for the selected T300 NCF with chain stitches. The model’s validity was checked by comparing the simulated results with the experimental ones. Although some improvements are still needed, the model provides encouraging results and good foundations to predict the shear behavior for NCFs’ forming.     

DEM investigation on the evolution of microstructure in granular soils under shearing

The mechanical behaviors of granular soils at different initial densities and confining pressures in the drained and undrained triaxial tests are investigated micromechanically by three-dimensional discrete element method (DEM). The evolutions of the microstructure in the numerical specimen, including coordination number, contact force and anisotropies of contact normal and contact force, are monitored during the shearing. The typical shear behaviors of granular soils (e.g. strain softening, phase transformation, static liquefaction and critical state behavior) are successfully captured in the DEM simulation. It is found that the anisotropies of contact normal, normal and tangential contact forces comprise the shear resistance and show different evolution features during shearing. After large strain shearing, the microstructure of the soil will finally reach a critical state, although the evolution path depends on the soil density and loading mode. Similar to the macroscopic void ratio $e$ and deviatoric stress $q$ , the coordination number and anisotropies of contact normal and contact force at the critical state also depend on the mean normal effective stress $P^{\prime }$ at the critical state.     

Transformation of sliding motion to rolling during spheres collision

In this research, we have investigated the three-dimensional elastic collision of two balls, based on friction in the tangential plane. Our aim is to offer analytical closed form relations for post collision parameters such as linear and angular velocities, collision time and tangential and normal impulse in three dimensions. To simplify the problem, stick regime is not considered. In other words, balls have a low tangential coefficient of restitution. Sliding, sliding then rolling, and rolling at the beginning of contact are three cases that can occur during impact which have been considered in our research. The normal interaction force is described by the Hertz contact force and dimensionless analysis is used for investigating normal interaction force; furthermore, Coulomb friction is considered during sliding. Experimental data for collisions show when sliding exists through the impact, tangential impulses can be taken as frictional impulses using the Coulomb law if the dynamic regime is not stick regime. To identify transformation of sliding motion to rolling or sticking during the impact process, linear and trigonometric functions are considered as an approximation for the normal interaction force. Afterwards, we have obtained the condition for the possibility of this transformation; moreover, we can estimate the duration of sliding and rolling or sticking. We have obtained an analytical solution for maximum force and deformation, collision time, impulses and post-collision linear and angular velocities in three dimensions.     

Fabric rates of elliptical particle assembly in monotonic and cyclic simple shear tests: a numerical study

This paper aims to investigate the evolutions of microscopic structures of elliptical particle assemblies in both monotonic and cyclic constant volume simple shear tests using the discrete element method. Microscopic structures, such as particle orientations, contact normals and contact forces, were obtained from the simulations. Elliptical particles with the same aspect ratio (1.4 and 1.7 respectively for the two specimens) were generated with random particle directions, compacted in layers, and then precompressed to a low pressure one-dimensionally to produce an inherently anisotropic specimen. The specimens were sheared in two perpendicular directions (shear mode I and II) in a strain-rate controlled way so that the effects of inherent anisotropy can be examined. The anisotropy of particle orientation increases and the principal direction of particle orientation rotates with the shearing of the specimen in the monotonic tests. The shear mode can affect the way fabric anisotropy rate of particle orientation responds to shear strain as a result of the initial anisotropy. The particle aspect ratio exhibits quantitative influence on some fabric rates, including particle orientation, contact normal and sliding contact normal. The fabric rates of contact normal, sliding contact normal, contact force, strong and weak contact forces fluctuate dramatically around zero after the shear strain exceeds 4 % in the monotonic tests and throughout the cyclic tests. Fabric rates of contact normals and forces are much larger than that of particle orientation. The particle orientation based fabric tensor is harder to evolve than the contact normal or contact force based because the reorientation of particles is more difficult than that of contacts.     

Strong force networks in granular mixtures

Using the results of 3D discrete element method simulations we study the force transmission through binary mixtures of sand and silt sized spheres under one-dimensional compression. Three types of contact are categorized depending on the size of the two spheres in contact. The contributions of each contact type to the deviator stress are dependent on the proportion of silt sized spheres. We demonstrate that the magnitude of the deviator stress is solely due to the normal and tangential forces at contacts transmitting normal forces greater than a characteristic normal force, which is generally slightly greater than the average normal force. The maximum packing efficiency was obtained with the mixture of 30 % silt sized spheres and this mixture corresponds to a transition point when there are enough silt sized particles to start to separate the sand sized particles from each other and establish contacts between silt sized spheres that contribute to the deviator stress.