Friction In Systems Research Articles

On Friction, Heat Input, and Material Flow Initiation during Friction Stir Welding: Tool and Process Optimization

The Friction Stir Welding (FSW) process depends entirely upon mechanical contact between the tool and the workpiece. As a result of this, all process phenomena and process outcomes such as weld geometry and mechanical properties are governed by FSW’s frictional system. The following work characterizes this system with a focus on process initialization, heat input and material flow. For this purpose, an experimental program for the isolated investigation of the frictional system was carried out. Short-term effects such as contact initiation, run-in behavior and frictional transitions are considered as well as the influences of process parameters and geometry. The system and its behavior are analyzed quantitatively and qualitatively by experiments altering the normal pressure, relative velocity, and tool geometry. The experiments demonstrate a self-similar behavior of the process, including an important wear transition which initiates the material flow, and a subsequent equilibrium of forces, heat balance, and temperatures. The interaction between the tool and the welded material is described, as is the link between the frictional interface and material flow initialization. Based on these findings, recommendations are provided for process optimization and tool design.

Motion control of hydraulic actuators with nonlinear friction compensation: Applied to variable valve systems of diesel engine

Friction force plays an important role in motion control systems. Research shows that friction force is strongly nonlinear in most applications. And the nonlinear friction force has great influence on control performance, especially in motion control systems of hydraulic actuators. To improve control performance, many model-based controllers have been developed. In the past years, LuGre model has been widely used in most hydraulic actuators. However, for systems with one-way seals whose friction force is asymmetric between instroke and outstroke stage, LuGre model is no longer suitable. The paper developed a friction force test bench and studied the transient friction force of a hydraulic actuator which includes one-way seals. Experiment results show that friction force at instroke stage is bigger than that of outstroke stage for the variable valve actuator. Based on the experiment results, a bristle direction hypothesis is proposed. The bristle of the friction surface is thought to have an initial direction. For actuators with two-way seals or without seals, the bristle is thought to be perpendicular to the friction surface. And for actuators with one-way seals, the angle between bristles and friction surface is not 90° anymore. To apply the bristle direction hypothesis to controllers, a bristle direction coefficient is introduced to LuGre model to explain asymmetric friction behaviors. The simulation results have good agreements with experiment results. Based on the improved LuGre model, a backstepping controller is developed to compensate the nonlinear friction force. The friction force and system matched uncertainty are estimated by an extended state observer. Finally, some comparative experiments are implemented to verify the advantages of the improved LuGre model. The experiment results show that the improved LuGre model can help to reduce the engine valve lift tracking errors. At outstroke stage (engine valve close), the maximum absolute errors can be reduced by 40.6% to 65.0%, and the mean absolute errors can be reduced by 21.2% to 59.4%.

From Molecular to Multiasperity Contacts: How Roughness Bridges the Friction Scale Gap

The tangential force required to observe slip across a whole frictional interface can increase over time under a constant load, due to any combination of creep, chemical, or structural changes of the interface. In macroscopic rate-and-state models, these frictional aging processes are lumped into an ad hoc state variable. Here we explain, for a frictional system exclusively undergoing structural aging, how the macroscopic friction response emerges from the interplay between the surface roughness and the molecular motion within adsorbed monolayers. The existence of contact junctions and their friction dynamics are studied through coupled experimental and computational approaches. The former provides detailed measurements of how the friction force decays, after the stiction peak, to a steady-state value over a few nanometers of sliding distance, while the latter demonstrates how this memory distance is related to the evolution of the number of cross-surface attractive physical links, within contact junctions, between the molecules adsorbed on the rough surfaces. We also show that roughness is a sufficient condition for the appearance of structural aging. Using a unified model for friction between rough adsorbed monolayers, we show how contact junctions are a key component in structural aging and how the infrajunction molecular motion can control the macroscopic response.

Investigation of the Tribological Behaviour of Various AMC Surfaces against Brake Lining Material.

AlSi7Mg/SiCp aluminium matrix composites (AMCs) with a high ceramic content (35 vol.%) that were produced by using the field-assisted sintering technique (FAST) were subjected to tribological preconditioning and evaluated as a potential lightweight material to substitute grey cast iron brake discs. However, since an uncontrolled running-in process of the AMC surface can lead to severe wear and thus to failure of the friction system, AMC surfaces cannot be used directly after finishing and have to be preconditioned. A defined generation of a tribologically conditioned surface (tribosurface) is necessary, as was the aim in this study. To simulate tribological conditions in automotive brake systems, the prepared AMC samples were tested in a pin-on-disc configuration against conventional brake lining material under dry sliding conditions. The influence of the surface topography generated by face turning using different indexable inserts and feeds or an additional plasma electrolytic treatment was investigated at varied test pressures and sliding distances. The results showed that the coefficient of friction remained nearly constant when the set pressure was reached, whereas the initial topography of the samples studied by SEM varied substantially. A novel approach based on analysing the material ratio determined by 3D surface measurement was developed in order to obtain quantitative findings for industrial application.

Study on the boundedness, stability and dynamic characteristics of friction system based on fractal and chaotic theory

Friction signals under different working conditions are obtained, the phase trajectories and the recurrence plots are constructed to identify the wear states in the Friction System (FS). The phase trajectory moves in limited space without exceeding the critical range, the recurrence plots represent different wear states, the boundedness, stability and dynamics are studied using the chaotic parameters of correlation dimension D, recurrence degree RR and the proposed saturation radius rs. The rs has the evolution law of "decrease-stability- increase" variation while D and RR have the opposite law. Therefore, it is significant to describe the complex friction behavior through chaotic representation methods, constantly explore the boundedness, stability and dynamic characteristics to realize the state identification, the wear prediction and quantitative design control.

Comparative Evaluation of the Tribological Properties of Polymer Materials with Similar Shore Hardness Working in Metal-Polymer Friction Systems.

This article presents comparative tests of contact strength and tribological wear resistance of polymer sliding materials of the polyamide group. The aim of this work was to study Shore hardness, indentation hardness, modulus, creep, relaxation, Martens hardness and sliding wear resistance of two commercial materials. One of these materials was produced with the recycling process in mind. Abrasion tests were performed against a stainless-steel ball (100CRr6) on a normal load of 5 N for 23,830 friction cycles. The samples were tested under dry friction conditions and taking into account the hydrothermal factor, the presence of which was assumed in the anticipated operating conditions. It was distilled water at a temperature of 50 °C. The volumetric wear of the samples under various environmental conditions was assessed and related to the mechanical properties, in particular, Shore hardness. This mechanical size, which characterizes the surface, was considered the most frequently used by engineers selecting polymeric materials for tribological applications in industry. The Shore hardness of both materials was similar, which may indicate similar tribological performance properties. However, research and analysis indicate the need to use measures that directly correspond to tribological wear. The friction and wear of both materials varied. The coefficient of friction in hydrothermal conditions was lower and the wear was higher than in the dry friction test. It seems that it was not hardness that determined the suitability in the anticipated operating conditions, but the ability to form a sliding layer on the friction surface. The properties of the material that has been envisaged as a replacement may be appropriate for the intended uses.

Self-healing solitonic slip pulses in frictional systems.

A prominent spatiotemporal failure mode of frictional systems is self-healing slip pulses, which are propagating solitonic structures that feature a characteristic length. Here, we numerically derive a family of steady state slip pulse solutions along generic and realistic rate-and-state dependent frictional interfaces, separating large deformable bodies in contact. Such nonlinear interfaces feature a nonmonotonic frictional strength as a function of the slip velocity, with a local minimum. The solutions exhibit a diverging length and strongly inertial propagation velocities, when the driving stress approaches the frictional strength characterizing the local minimum from above, and change their character when it is away from it. An approximate scaling theory quantitatively explains these observations. The derived pulse solutions also exhibit significant spatially-extended dissipation in excess of the edge-localized dissipation (the effective fracture energy) and an unconventional edge singularity. The relevance of our findings for available observations is discussed.

Stick–Slip Characteristic Analysis of High-Speed Train Brake Systems: A Disc–Block Friction System with Different Friction Radii

Inspired by the difference in the friction radii of the pads from the high-speed train brake system, stick–slip experiments for a disc–block friction system with different friction radii were carried out via a test device. Based on the test results, the stick–slip vibration characteristics of the disc–block friction system with variation in the friction radius were analyzed, and the corresponding Stribeck model parameters in exponential and fractional forms were identified. The experimental results show that with an increase in the friction radius the vibration amplitude first increased and then decreased and the frequency of stick–slip vibration increased. The identified Stribeck model parameters show that the decay factors increased, the static friction coefficient decreased, and the dynamic friction coefficient decreased first and then increased as the friction radius increased. Moreover, the identified Stribeck model in an exponential form can more accurately reflect the stick–slip characteristics of a disc–block friction system than the model in a fractional form. It can be further applied in the investigation of the dynamic behaviors of high-speed train brake systems.

Обґрунтування механізму функціонування мехатронної системи склоочищення автомобіля

The four-link mechanism of the car's mechatronic windshield cleaning system is substantiated using different ratios of links and analysis of their kinematic and dynamic characteristics. A mathematical model of the glass cleaning mechanism in the Matlab/Simulink environment is presented and its kinematic synthesis is carried out. A mathematical model of the movement of the brush on the glass was developed for the case of close to uniform pressure distribution along the length of the brush, taking into account the unevenness of the law of movement associated with the operation of the transmission mechanism. The influence of the material properties of the rubber element on the quality of glass cleaning and the obtained values of the values characterizing the elastic and dissipative properties of the neck in the limit modes of the system are considered. The possibility of regeneration self-oscillations is shown when the dissipative properties of the neck are reduced as a result of wear or an aggressive environment. The processes of dry friction and hydraulic movement resistance occurring in the system are clarified, the limits of possible visually noticeable self-oscillations depending on the parameters of the friction mode and system operation are shown. It was found that the presence in the system of the possibility of regenerative self-oscillations in critical operating modes requires the introduction of micro-oscillations into the law of movement of the brush on the glass, which can effectively reduce the amplitudes of these self-oscillations. Analytical dependencies are proposed for calculating the parameters of the software, which implements the management of the dynamic characteristics of the system by superimposing the law of motion of micro-oscillations. The necessary hardware and software requirements for the controller are shown, as well as the possibility of saving the windshield wiper control unit as a functional unit of the mechatronic system for ensuring the comfort of movement.

On the discontinuous dynamics of a class of 2-DOF frictional vibration systems with asymmetric elastic constraints

<abstract><p>In this paper, the discontinuous dynamic behavior of a two-degree-of-freedom frictional collision system including intermediate elastic collision and unilateral elastic constraints subjected to periodic excitation is studied by using flow switching theory. In this system, given that the motion of each object might have a velocity that is either greater than or less than zero and each object experiences a periodic excitation force that has negative feedback, because the kinetic and static friction coefficients differ, the flow barrier manifests when the object's speed is zero. Based on the discontinuity or nonsmoothness of the oscillator's motion generated by elastic collision and friction, the motion states of the oscillator in the system are divided into 16 cases and the absolute and relative coordinates are used to define various boundaries and domains in the oscillator motion's phase space. On the basis of this, the G-function and system vector fields are used to propose the oscillator motion's switching rules at the displacement and velocity boundaries. Finally, some dynamic behaviors for the 2-DOF oscillator are demonstrated via numerical simulation of the oscillator's stick, grazing, sliding and periodic motions and the scene of sliding bifurcation. The mechanical system's optimization designs with friction and elastic collision will benefit from this investigation's findings.</p></abstract>

Discontinuous rigidity transition associated with shear jamming in granular simulations.

We investigate the rigidity transition associated with shear jamming in frictionless, as well as frictional, disk packings in the quasi-static regime and at low shear rates. For frictionless disks, the transition under quasi-static shear is discontinuous, with an instantaneous emergence of a system spanning rigid clusters at the jamming transition. For frictional systems, the transition appears continuous for finite shear rates, but becomes sharper for lower shear rates. In the quasi-static limit, it is discontinuous as in the frictionless case. Thus, our results show that the rigidity transition associated with shear jamming is discontinuous, as demonstrated in the past for isotropic jamming of frictionless particles, and therefore a unifying feature of the jamming transition in general.

Optimization of Dynamically Loaded Nonlinear Technical Systems

Technological progress is impossible without new innovative developments, discoveries, inventions. Modern nonlinear technical systems cannot do without the use of friction units, which are various gears, connections, as well as complex nodes and devices. The geometrical parameters of the railway-rolling stock system were optimized when moving in curves of small radius. A unique roller rig was used to carry out model tests using the method of physical and mathematical modeling. It is proposed a method for increasing the efficiency of friction systems, which is based on fundamental research in the field of mechanics of contact friction interaction of solids. One of the technical solutions implemented on the basis of this study is the development of technology and technological equipment for increasing the traction force of a locomotive and increasing the service life of wheelsets. The theoretical basis of this technology is the anisotropy of friction junctions formed by thermos metal plating of the wheel working surfaces.

INVESTIGATION OF THE INFLUENCE OF SLIDING SPEED ON THERMOELASTIC PROBLEM IN THE FRICTIONAL CLUTCH SYSTEM WHEN APPLYING A CONSTANT HEAT GENERATION

There are many demands that are considered essential to obtain a successful design of frictional clutches. The main function of such a mechanical part is to transfer torque between driving and driven shafts. The time of engagement process for the clutch system should be as short as possible to reach the desired speed with the minimum losses of power. Owing to the high level of contact stresses that are generated between the sliding components of the frictional clutch, high thermal fields will be produced during the engagement period. The thermoelastic problem during the sliding time of the frictional system (single disc) employing the finite element method (FEM) was investigated deeply. In the present research paper, the effect of variable sliding speeds on the thermoelastic performance during applying a constant initial heat generation was explored. The new numerical models have been developed based on finite element axisymmetric simulation for the engagement process of the dry frictional clutch components. It was found that the sliding speed is more significant than the applied pressure when applying a constant frictional heat generation.

Friction Reduction by Laser Irradiation for a Friction System Using Bearing Steel and Aluminum Alloy in Engine Oil

Aluminum alloy sliding components are widely used in internal combustion engines. However, aluminum easily adheres to the countersurface, resulting in high friction. Hence, laser irradiation was used to oxidize the aluminum surface and promote tribofilm formation using molybdenum dithiocarbamate and zinc dialkyldithiophosphate (ZnDTP). This study investigated laser irradiation to reduce friction between steel balls and aluminum disks in fully formulated engine oil. Laser scanning was used to create 100-μm-pitch concentric circles on the disks. The friction behavior was classified into two modes: no friction reduction (mode-I, μ: 0.09–0.12) and friction reduction after the high-friction period (mode-II, μ: 0.06–0.09). Friction mode transition occurred when the laser energy density exceeded the critical value (Ec). Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy showed that, for mode-I, aluminum adhered to the ball, and no sulfur-containing tribofilm formed. For mode-II, the ZnDTP-derived phosphate film formed on the disk suppressed aluminum adhesion, and a sulfur-containing tribofilm formed on the ball. Micro-Vickers tests and X-ray diffraction showed that an amorphous/nanocrystalline structure formed in the unirradiated area owing to heat diffusion under high-energy-density laser irradiation (>Ec). Results suggest that metallographic structural change in the unirradiated area promotes ZnDTP reactions, causing the mode transition.

Periodic solution and stability analysis of dry friction system based on an alternate state-space shooting algorithm

Periodic solution and stability analysis of dry friction system based on an alternate state-space shooting algorithm

Unraveling the friction response from selective hydrogenation of textured amorphous carbon surface

Ultra-low friction of amorphous carbon (a-C) film can be achieved by binding to hydrogen atoms or texturing conformations. However, it remains unclear how selective surface hydrogenation affects the friction behavior of textured a-C film. In particular, the corresponding transformation of interfacial structure and the movement of hydrogen atoms have not yet been reported because of the limitation of in-situ experimental characterization. Here, textured a-C films with selective hydrogenated surfaces were prepared, and the corresponding friction response and the transformation of interfacial structure were investigated systematically using reactive molecular dynamics simulation. Results showed that introducing hydrogen atoms to the selective bump sites of textured a-C surfaces significantly reduced the friction coefficient; however, its efficiency was closely sensitive to the hydrogen content, which was related to the interfacial passivation and the repulsion effect induced by H atoms. Most importantly, the separation of –CH clusters from textured a-C surface during sliding process and their re-bonding with the naked mating a-C surface played a key role in further enhancing the repulsive effect between contacted a-C surfaces, thereby improving the anti-friction behavior, which has not been mentioned in previous studies. These results suggest a new approach to develop the high-efficient a-C friction system for applications.

Mathematical analyses and experimental verification of elimination of measurement error in UMT-2 rotating friction system

This paper aims to investigate and eliminate the measurement errors of coefficient of friction (COF) in the rotating friction system of UMT-2 and the corresponding solutions are proposed. Three main factors leading to measurement errors were discussed: the displacement of the friction center (Δx), the tilt of the sample holder, and the tilt angle of the friction disc (angle α). Analysis based on Abbé error/offset shows that a smaller displacement Δx leads to a smaller measurement error, and ensuring that Δx is equal to the spacing between the axis of rotation and the trajectory of the friction center can reduce the measurement error. In addition, the verification tests show that the measurement error was greater than 5 % when the friction force was greater than 0.87 N for the selected sensor and the measurement error due to the tilt of sample holder is proportional to the friction force. Therefore, this verification method is recommended to determine the applicable measurement range of selected sensors, avoiding friction force exceeding the reliable measurement range of selected sensor. Pin-on-disc friction tests carried out in clockwise and counterclockwise were conducted under load of 9 N and speed of 7.85 mm/s to examine the effectiveness of the leveling device proposed in this work. The results show that the tilt angle α of the disc was reduced from 0.135° to 0.005°, a reduction of 96.3 % with the help of the leveling device. The relative error due to the angle α was as low as 0.8 % at a COF of 0.001 magnitude, which fundamentally improved the measurement accuracy of the friction test at ultra-low COF. In addition, future research is proposed on how to eliminate the measurement error of friction, which can help researchers and instrument designers to improve the measurement accuracy.

Frictional sliding of infilled planar granite fracture under oscillating normal stress

This paper investigates the frictional behavior of the infilled rock fracture under dynamic normal stress. A series of direct shear tests were conducted on saw-cut granite fractures infilled with quartz using a self-developed dynamic shear apparatus, and the effects of normal load oscillation amplitude, normal load oscillation period and sliding velocity were studied. The test results reveal that the shear response can be divided into three stages over a whole loading-unloading process, characterized by different time spans and stress variations. Generally, a smaller oscillation amplitude, a longer oscillation period and a fast shear velocity promote the stability of the friction system, which is also confirmed by the Coulomb failure criterion calculated based on the observed periodic apparent friction coefficient. The dynamic strengthening/weakening phenomenon is dependent on the oscillation amplitude and product of sliding velocity and oscillation period (vT). Also, the rate and state friction law incorporating the parameter α that characterizes the normal stress variation is employed to describe the dynamic friction coefficient but exhibits an incompetent performance when handling intensive variation in normal stress. Finally, the potential seismicity induced by oscillating normal stress based on the observed stress drop is analyzed. This work helps us understand the sliding process and stability evolution of natural faults, and its benefits for relative hazard mitigation.

Cascade architecture for nonlinear control of transient and stationary regimes of friction-induced vibrations

This paper presents a new control approach for mitigating friction-induced vibration (FIV) issued from the mode-coupling mechanism. The key idea is to put the friction system onto a cascade of independent subsystems by using a first-stage control. Then, a second-stage control is defined and proven to be much more efficient for controlling both the transient and stationary regimes of the FIV. Asymptotic stabilization is formally demonstrated for the whole control scheme. Moreover, by numerical simulations, it is shown that the performances of the closed-loop systems present an interesting robustness with respect to parameter uncertainty and to the saturation phenomenon which opens up promising practical perspectives for the proposed control technique.

Beyond the main order sensitivity analysis for a frictional system: is the eXtended FAST algorithm applicable?

Beyond the main order sensitivity analysis for a frictional system: is the eXtended FAST algorithm applicable?