Micro sliding friction model considering periodic variation stress distribution of contact surface and experimental verification

Micro sliding phenomenon widely exists in the operation process of mechanical systems,and the micro sliding friction mechanism is always a research hotspot.In this work,based on the total reflection method,a measuring device for interface contact behavior under two-dimensional(2D)vibration is built.The stress distribution is characterized by the light intensity distribution of the contact image,and the interface contact behavior in the 2D vibration process is studied.It is found that the vibration angle of the normal direction of the contact surface and its fluctuation affect the interface friction coefficient,the tangential stiffness,and the fluctuation amplitude of the stress distribution.Then they will affect the change of friction state and energy dissipation in the process of micro sliding.Further,an improved micro sliding friction model is proposed based on the experimental analysis,with the nonlinear change of contact parameters caused by the normal contact stress distribution fluctuation taken into account.This model considers the interface tangential stiffness fluctuation,friction coefficient hysteresis,and stress distribution fluctuation,whose simulation results are consistent well with the experimental results.It is found that considering the nonlinear effect of a certain contact parameter alone may bring a greater error to the prediction of friction behavior.Only by integrating multiple contact parameters can the accuracy of friction prediction is improved.

Effects of vertical excitation on the seismic performance of a seismically isolated bridge with sliding friction bearings

A finite element model is constructed for a sliding friction bearing in a seismically isolated bridge under vertical excitation with contact/friction elements. The effects of vertical excitation on the seismic performance of a seismically isolated bridge with sliding friction bearings and different bearing friction coefficients and different stiffness levels （pier diameter） are discussed using example calculations, and the effects of excitation direction for vertical excitation on the analysis results are explored. The analysis results shows that vertical excitation has a relatively large impact on seismic performance for a seismically isolated bridge with sliding friction bearings, which should be considered when designing a seismically isolated bridge with sliding friction bearings where vertical excitation dominates.

Coupled effects of particle overall regularity and sliding friction on the shear behavior of uniformly graded dense sands

Particle morphology has been regarded as an important factor affecting shear behaviors of sands,and covers three important aspects,i.e.global form(overall shape),local roundness(large-scale smoothness),and surface texture(roughness)in terms of different observation scales.Shape features of different aspects can be independent of each other but might have coupled effects on the bulk behavior of sands,which has been not explored thoroughly yet.This paper presents a systematic investigation of the coupled effects of the particle overall regularity(OR)and sliding friction on the shear behavior of dense sands using three-dimensional(3D)discrete element method(DEM).The representative volume elements consisting of ideal spheres and irregular clumps of different mass proportions are prepared to conduct drained triaxial compression simulations.A well-defined shape descriptor named OR is adopted to quantify particle shape differences of numerical samples at both form and roundness aspects,and the particle sliding friction coefficient varies from 0.001 to 1 to consider the surface roughness effect equivalently in DEM.The stress-strain relationships as well as peak and critical friction angles of these assemblies are examined systematically.Moreover,contact network and anisotropic fabric characteristics within different granular assemblies are analyzed to explore the microscopic origins of the multi-scale shape-dependent shear strength.This study helps to improve the current understanding with respect to the influence of the particle shape on the shear behavior of sands from different shape aspects.

Numerical Analysis of a Sliding Frictional Contact Problem with Normal Compliance and Unilateral Contact

This paper represents a continuation of [1] and [2]. Here, we consider the numerical analysis of a non-trivial frictional contact problem in a form of a system of evolution nonlinear partial differential equations. The model describes the equilibrium of a viscoelastic body in sliding contact with a moving foundation. The contact is modeled with a multivalued normal compliance condition with memory term restricted by a unilateral constraint and is associated with a sliding version of Coulomb’s law of dry friction. After a description of the model and some assumptions, we derive a variational formulation of the problem, which consists of a system coupling a variational inequality for the displacement field and a nonlinear equation for the stress field. Then, we introduce a fully discrete scheme for the numerical approximation of the sliding contact problem. Under certain solution regularity assumptions, we derive an optimal order error estimate and we provide numerical validation of this result by considering some numerical simulations in the study of a two-dimensional problem.

Role of particle stiffness and inter-particle sliding friction in milling of particles

Discrete element method （DEM） has been used to investigate the effects of particle elastic modulus and coefficient of inter-particle sliding friction on milling of mineral particles. An autogeneous mill of 600 mm diameter and 320 mm length with 14,500 particles has been selected for the simulation. Various mill performance parameters, for example, particle trajectories, collision frequency, collision energy and mill power have been evaluated to understand the effects of particle elastic modulus and inter-particle sliding friction during milling of particles. For the given model, it has been concluded that at high energy range, as the elastic modulus and particle sliding friction increase the energy dissipated among the particles increases. The collision frequency increases with the increase in elastic modulus, however, this trend is not clearly observed with increasing inter-particle sliding friction. The power draw of the mill increases with the increase in fraction of mill critical speed.

Effect of surface roughness on sliding friction of micron-sized glass beads

In order to understand the contact phenomena of micron‐sized particles,which have a tremendous impact on a variety of applications in industry and technology,direct access to the loads as well as the displacements accompanying such contacts are mandatory.Typical particle ensembles show a size variation ranging from the nanometer to the tenths of micron scale.Especially the contact behavior of particles featuring radii of several up to several tenths of microns is scarcely studied as these particles are typically too large for atomic force microscopy(AFM)based approaches and too small for conventional macroscopic testing setups.In this work a nanoindenter based approach is introduced to gain insight into the contact mechanics of micron‐sized glass beads sliding on rough silicon surfaces at various constant low normal loads.The results are analyzed by a simple modified Coulomb friction law,as well as Hertz,JKR,and DMT contact theory.

Altered age-hardening behavior in the ultrafine-grained surface layer of Mg-Zn-Y-Ce-Zr alloy processed by sliding friction treatment

To gain insight into the ageing behavior of ultrafine grain(UFG)structure,the precipitation phenomena and microstructural evolutions of Mg-6 Zn-1 Y-0.4 Ce-0.5 Zr(wt.%)alloy processed by sliding friction treatment(SFT)were systematically studied using hardness texting,transmission electron microscopy(TEM)equipped with high-angle annular dark-field scanning(HADDF-STEM),X-ray diffraction(XRD)and XRD line broadening analysis.The microhardness of the SFT-processed(SFTed)sample initially decreases from 109.6 HV to 104.8 HV at ageing for 8 h,and then increases to the peak-ageing point of 115.4 HV at16 h.Subsequently,it enters the over-aged period.The un-SFTed sample,as the counterpart,follows a regular ageing behavior that increases from 89.9 HV to 99.6 HV when ageing for 12 h,and then drops.A multi-mechanistic model is established to describe the strengthening due to grain refinement,dislocation accumulation,precipitation etc.The analysis reveals that the temperature sensitive UFG structure has an obvious grain coarsening effect,which arouses the soft phenomenon in the early ageing stage.But precipitation hardening provides an excellent hardness enhancement for overcoming the negative influence and helping to reach the peak-aged point.In our microstructural observations,a lot of equilibrium ultrafine Mg Zn2 precipitates precipitate along dislocations because defects can provide the favorable conditions for the migration and segregation of solute atoms.

A numerical model to simulate the transient frictional viscoelastic sliding contact

Sliding motion has always been one of the major concerns when it comes to the analysis of viscoelastic contact problems.A new model simulating the transient sliding contact of smooth viscoelastic surfaces is developed in this paper.By taking the dry contact friction and the coupling between shear tractions and normal pressure into account,the effect of the early partial slip period,which is often neglected in the study of viscoelastic sliding contact problems,is investigated numerically.Compared with solutions based on the frictionless assumption,the steady-state pressure profile is found to be slightly different under the effect of the partial slip regime,including a lower peak pressure and the shift of the contacting region in the direction opposite to the sliding motion.Furthermore,the time required for the viscoelastic contact to reach its steady state is delayed owing to the partial slip period preceding the global sliding motion.

Friction and Wear Mechanism of Unlubricated 304L Austenitic Stainless Steel at Room Temperature

To explore wear mechanism of stainless steel used in nuclear pump, the wear properties and the worn surface characteristics of unlubricated 304L austenitic stainless steel on itself were investigated in air at room temperature. The experimental results demonstrated that the wear rate of the material decreased with the increase of the wear time. The friction coefficient fluctuated severely when the applied load was 120 N. At 120 N the wear rate was much higher than that of the applied load of 70 N. At 70 N the wear rate did not show much difference from that of 30 N. The wear mechanism was adhesive and abrasive wear under different load at the initial stage of the wear test. Then, the main wear mechanism changed with the wearing time and the applied load.

Energy corrugation in atomic-scale friction on graphite revisited by molecular dynamics simulations

Although atomic stick–slip friction has been extensively studied since its first demonstration on graphite,the physical understanding of this dissipation-dominated phenomenon is still very limited. In this work, we perform molecular dynamics（MD） simulations to study the frictional behavior of a diamond tip sliding over a graphite surface. In contrast to the common wisdom, our MD results suggest that the energy barrier associated lateral sliding（known as energy corrugation） comes not only from interaction between the tip and the top layer of graphite but also from interactions among the deformed atomic layers of graphite. Due to the competition of these two subentries, friction on graphite can be tuned by controlling the relative adhesion of different interfaces.For relatively low tip-graphite adhesion, friction behaves normally and increases with increasing normal load. However,for relatively high tip-graphite adhesion, friction increases unusually with decreasing normal load leading to an effectively negative coefficient of friction, which is consistent with the recent experimental observations on chemically modified graphite. Our results provide a new insight into the physical origins of energy corrugation in atomic scale friction.

Novel algorithm of gait planning of hydraulic quadruped robot to avoid foot slidingand reduce impingement

In order to solve kinematic redundancy problems of a hydraulic quadruped walking robot,which include leg dragging,sliding,impingement against the ground,an improved gait planning algorithm for this robot is proposed in this paper.First,the foot trajectory is designated as the improved composite cycloid foot trajectory.Second,the landing angle of each leg of the robot is controlled to satisfy friction cone to improve the stability performance of the robot.Then with the controllable landing angle of quadruped robot and a geometry method,the kinematic equation is derived in this paper.Finally,agait planning method of quadruped robot is proposed,a dynamic co-simulation is done with ADAMS and MATLAB,and practical experiments are conducted.The validity of the proposed algorithm is confirmed through the co-simulation and experimentation.The results show that the robot can avoid sliding,reduce impingement,and trot stably in trot gait.

Decohesion of graphene from a uniaxially-stretched substrate:Failure analysis of a frictional adhesive interface

Composite structures consisting of two-dimensional(2D)materials deposited on elastic substrates have a wide range of potential applications in flexible electronics.For such devices,robust 2D film/substrate interfacial adhesion is essential for their reliable performance when subjected to external thermal and mechanical loads.To better understand the strength and failure behavior of the 2D film/substrate interfaces,two types of graphene/polymer samples with distinct interfacial adhesion properties are fabricated and tested by uniaxially stretching the substrates.Depending on the interfacial adhesion,two drastically different debonding rates are observed,i.e.,rapid snap-through debonding and more progressive crack propagation.Motivated by the experimental observation,we propose an improved shear-lag model with a trapezoidal-shaped cohesive zone to derive an analytical solution for the decohesion behavior.The theoretical model reveals that the decohesion behavior of the frictional adhesive interface is governed by three dimensionless parameters.Particularly,the dimensionless length of the film essentially determines the decohesion rate;while the other two parameters affect the critical substrate strain to initiate debonding.By fitting the experimental data with the theoretical model,the intrinsic adhesion properties of the two samples are obtained with physically meaningful values.This work offers an analytical solution to describing the decohesion behavior of general thin film/substrate systems with a frictional adhesive interface,which is beneficial for characterizing and optimizing the mechanical properties of various thin film/polymer devices.

Effect of mechanical degradation of laminated elastomeric bearings and shear keys upon seismic behaviors of small-to-medium-span highway bridges in transverse direction

Laminated elastomeric bearings have been widely used for small-to-medium-span highway bridges in China, in which concrete shear keys are set transversely to prohibit large girder displacement. To evaluate bridge seismic responses more accurately, proper analytical models of bearings and shear keys should be developed. Based on a series of cyclic loading experiments and analyses, rational analytical models of laminated elastomeric bearings and shear keys, which can consider mechanical degradation, were developed. The effect of the mechanical degradation was investigated by examining the seismic response of a small-to-medium-span bridge in the transverse direction under a wide range of peak ground accelerations（PGA）. The damage mechanism for small-to-medium-span highway bridges was determined, which can explain the seismic damage investigation during earthquakes in recent years. The experimental results show that the mechanical properties of laminated elastomeric bearings will degrade due to friction sliding, but the degree of decrease is dependent upon the influencing parameters. It can be concluded that the mechanical degradation of laminated elastomeric bearings and shear keys play an important role in the seismic response of bridges. The degradation of mechanical properties of laminated elastomeric bearings and shear keys should be included to evaluate more precise bridge seismic performance.

Wear and Corrosion Properties of Mo Surface-modified Layer in TiNi Alloy Prepared by Plasma Surface Alloying

In order to improve the wear resistance and restrain nickel release of TiNi alloys,the Mo modified layers on TiNi substrates were obtained using the double glow plasma surface alloying technique.Scanning electron microscopy（SEM）,glow discharge optical emission spectroscopy（GDOES） and X-ray diffraction（XRD） were employed to investigate the morphology,composition and structure.Microhardness test and scratch test were performed to analyze the microhardness and coating/substrate adhesion.Tribological and electrochemical behaviors of the Mo modified layers on TiNi were tested by the reciprocating wear instrument and electrochemical measurement system.The Ni concentrations in Hanks’ solution where surface electrochemical tests took place were measured by mass spectrometry.The surface-modified layer contained a Mo deposition layer and a Mo diffusion layer.The X-ray diffraction analysis revealed that the modified layers were composed of Mo,MoTi,Mo Ni,and Ti2Ni.The microhardnesses of the Mo modified layers treated at 900 ℃ and 950 ℃ were 832.8 HV and 762.4 HV,respectively,which was about 3 times the microhardness of the TiNi substrate.Scratch tests indicated that the modified layers possessed good adhesion with the substrate.Compared with as-received TiNi alloy,the modified alloys exhibited significant improvement of wear resistance against Si3N4 with low normal loads during the sliding tests.Mass spectrometry displayed that the Mo alloy layers had successfully inhibited the Ni release into the body.

Numerical analysis of submarine landslides using a smoothed particle hydrodynamics depth integral model

Submarine landslides can cause severe damage to marine engineering structures. Their sliding velocity and runout distance are two major parameters for quantifying and analyzing the risk of submarine landslides.Currently, commercial calculation programs such as BING have limitations in simulating underwater soil movements. All of these processes can be consistently simulated through a smoothed particle hydrodynamics（SPH） depth integrated model. The basis of the model is a control equation that was developed to take into account the effects of soil consolidation and erosion. In this work, the frictional rheological mode has been used to perform a simulation study of submarine landslides. Time-history curves of the sliding body＇s velocity, height,and length under various conditions of water depth, slope gradient, contact friction coefficient, and erosion rate are compared; the maximum sliding distance and velocity are calculated; and patterns of variation are discussed.The findings of this study can provide a reference for disaster warnings and pipeline route selection.

Experimental study on the mechanism of non-synchronism of seismo-electromagnetic radiation precursors

In order to investigate the physical mechanism of non-synchronism of seismo-electromagnetic radiation precursors, we have made shear fracture and frictional sliding tests of rock samples by loading them in biaxial compression to model the activity of tectonic fault zones in seismogenic region. By installing antennae of different frequency responses and acoustic transducers around the fracture surface, the signals of electromagnetic radiation and acoustic emission in different directions and of different frequencies produced by rock samples during their shear fracturing and frictional sliding have been recorded by an automatic, high-speed and continuous observation system. Some implications given by the experimental results are as follows. (1) During the process of shear fracturing and frictional sliding of rock samples under biaxial compression, large amounts of electromagnetic signals are produced; their frequencies range from several hundred Hz to several thousand Hz. (2) Signals received by antennae in different directions and of different frequencies are non-synchronous. They differ in amplitude from one another, with the signal received by antennae nearest to the fracture surface being the maximum. (3) Electric signals and magnetic signals do not appear synchronously; among them, electric signals appear more frequently and are of larger amplitude. The authors hold that electric signals (E) and magnetic signals (M) are of different genetic mechanisms: electric signals from inside the rock sample are produced by the piezoelectric effect of rock-forming crystals and net electric charge produced by the newly created surfaces due to rock fracture, whereas magnetic signals are produced by the high-speed motion of charged rock fragments and the ionization of ambient air excited by electrons emitted by rock in fracturing. Analysis shows that in some moderately strong earthquakes the electric and magnetic signals also showed the phenomenon of non-synchronism: the electric signal was earlier than the magnetic signal. If the same station observes simultaneously the electric field (underground) and magnetic field (air), the efficiency of earthquake prediction by seismo-electromagnetic radiation precursors would be raised effectively.

Dry Sliding Tribological Properties of a Dendrite-reinforced Zr-based Bulk Metallic Glass Matrix Composite

In-situ dendrite-reinforced metallic glass matrix （MGM） composites with the composition of Zr58.5Ti14.3Nb5.2Cu6.1Ni4.9Be11.0 were prepared with a vacuum arc melter by the copper mold suction casting. Effect of different normal loads and sliding velocities on the tribological properties of MGM composites was studied. The results showed that the friction coefficient and wear rate of composites initially descended with increasing the normal load and reached a minimum of 0.339 and 1.826 × 10^-4 mm^3/（N m） at 10 N, respectively, then ascended. Similarly, the friction coefficient and wear rate of composites initially decreased with the increase in the sliding velocity and reached a minimum of 0.330 and 2.389 × 10^-4 mm^3/（N m） at 0.4 m/s and 0.3 m/s, respectively, then raised. The wear mechanism of composites was mainly adhesive wear accompanied by abrasive wear at lower normal load and sliding velocity. However, the wear mechanism of composites was abrasive wear and adhesive wear as well as delamination at higher normal load and sliding velocity due to the nucleation and propagation of surface and subsurface cracks during the wear process. The flake-like and particle-like wear debris was the dominant shapes of debris observed.

The research works of Coulomb and Amontons and generalized laws of friction

The paper is devoted to the contributions of Coulomb and Amontons to the physics of friction from the viewpoint of current discussions and attempts to formulate generalized laws of friction.

Impact of bending dies with different friction forms on forming force and quality of tubes manufactured by free bending technology

In the 3 D free bending forming system,the bending die can be designed either in a sliding type or rolling friction type.Bending die-based sliding friction type is often called normal bending dies;however,the bending dies-based rolling friction type includes bending die-based roller type and ball type in structure.In the current study,the impact of three bending dies on the forming force,and the bent tube quality was investigated.The obtained results showed that the tangential stresses and strains of the tubes formed by the bending die-based roller type were the smallest among the three bending dies.Besides,the spherical bearing force PUwas reduced drastically after using the roller type and ball type compared to the sliding friction type.Moreover,the uniformity of the wall thickness distribution of the tubes formed by the roller type and ball type was better than those obtained from the sliding friction type.In addition,the cross-section distortion rate was reduced by 2.8%using the roller type,and 1.8%using ball-type compared to the sliding friction type.

Surface structuring of case hardened chain pins by cold-sprayed microparticles to modify friction and wear properties

This paper presents the results of the application of a cold spray technique for structuring metallic surfaces with microparticles. The resulting changes in surface properties were characterized to observe their influences on the tribological behavior of the structured surface. The spray technique was applied to a technical component, a 16MnCr5 steel chain pin, designed to be mounted in a linear reciprocating tribometer. TiO2 microparticles were used to structure the surface with a homogeneous distribution of singly dispersed particles, rather than a homogeneous closed coating on the surface. Tribometer tests were performed to directly compare structured and unstructured chain pins, and a significantly reduced sliding friction coefficient was observed for the structured pin. The pins were characterized in detail by surface analysis prior to and after application of the tribological load to set the surface parameters and surface chemistry, even on the microscale. It was confirmed that the particle structuring induced changes in the surface properties, and the durability of the changes after tribological loading was evaluated.