Dry Contact Research Articles

A Wearable In-Ear EEG Device for Emotion Monitoring.

For future healthcare applications, which are increasingly moving towards out-of-hospital or home-based caring models, the ability to remotely and continuously monitor patients’ conditions effectively are imperative. Among others, emotional state is one of the conditions that could be of interest to doctors or caregivers. This paper discusses a preliminary study to develop a wearable device that is a low cost, single channel, dry contact, in-ear EEG suitable for non-intrusive monitoring. All aspects of the designs, engineering, and experimenting by applying machine learning for emotion classification, are covered. Based on the valence and arousal emotion model, the device is able to classify basic emotion with 71.07% accuracy (valence), 72.89% accuracy (arousal), and 53.72% (all four emotions). The results are comparable to those measured from the more conventional EEG headsets at T7 and T8 scalp positions. These results, together with its earphone-like wearability, suggest its potential usage especially for future healthcare applications, such as home-based or tele-monitoring systems as intended.

Scaling of wetting and prewetting transitions on nanopatterned walls.

We consider a nanopatterned planar wall consisting of a periodic array of stripes of width L, which are completely wet by liquid (contact angle θ=0), separated by regions of width D which are completely dry (contact angle θ=π). Using microscopic density functional theory, we show that, in the presence of long-ranged dispersion forces, the wall-gas interface undergoes a first-order wetting transition, at bulk coexistence as the separation D is reduced to a value D_{w}∝lnL, induced by the bridging between neighboring liquid droplets. Associated with this is a line of prewetting transitions occurring off coexistence. By varying the stripe width L, we show that the prewetting line shows universal scaling behavior and data collapse. This verifies predictions based on mesoscopic models for the scaling properties associated with finite-size effects at complete wetting including the logarithmic singular contribution to the surface free energy.

Wear behaviour of cast aluminium silicon (Al-Si) alloy in Dot 4 brake fluid

The frictional sliding behaviour of cast Al-Si alloy was investigated with and without DOT 4 hydraulic brake fluid lubrication/environments. Cast Al-Si specimen was produced from wrought alloy and the specimen surface was metallurgically prepared. The as-cast product was characterized with aid of micro-hardness tester, the chemical composition was analysed by X-ray fluorescence (XRF), while X-ray diffraction (XRD) was used to identify phases present, microstructures and surface examinations were done on a high resolution metallurgical Optical Microscope. The properties of the polished and worn sample surfaces were examined by Scanning electron microscope (SEM) equipped with Energy dispersive X-ray (EDS) facilities. A 6mm diameter alumina ball under varying normal loads (3-10 N) was rubbed against the surface of the cast Al-Si coupon specimen. The dry and wet sliding wear behaviour was assessed based on the CoF-sliding time behaviour. From the results, the average CoF values of 0.9064 for dry and 0.2038 for wet contacts were obtained. Comparatively, 1.03E-06 and 4.21E-07 minimum wear intensities; and 467290 and 132170 wear resistances were obtained for dry and wet contacts respectively. Behaviour of cast Al-Si in DOT4 were compared with its performance in DOT3 regarding the hydraulic brake system application.

Effect of shoe braking on wear and fatigue damage of various railway wheel steels for high speed applications

The effect of shoe braking on three railway wheel steels was studied by means of bi-disc tests, in order to evaluate the applicability of shoe braking as an emergency stop braking system in high-speed passenger trains. Disc-shaped specimens, extracted from real wheels, were initially paired with discs extracted from cast iron brake blocks, in condition of rolling and sliding such to reach the typical temperature of the surface of a wheel rim during stop braking. Subsequently, the wheel discs were coupled with rail steel discs in rolling and sliding dry contact. Some of them were cut and analysed; other ones were subsequently subjected to rolling and sliding contact with rail specimens in wet condition, with various test duration. Measurements of the coefficient of friction and of the weight loss were carried out during the tests; at the end of the tests, hardness measurements and microstructure observation were made on the disc cross-sections. The main damage phenomena observed in the specimens subjected to wheel-brake contact and dry wheel-rail contact were ratcheting, wear and nucleation of surface cracks. Traces of cast iron, previously transferred from the brake specimens to the wheel ones, were detected. The main damage phenomenon observed in the specimens subjected to wheel-brake, dry wheel-rail and wet wheel-rail contact was rolling contact fatigue, developing by the propagation of surface cracks due to the pressurization of the water entrapped inside the cracks. All these phenomena were found to be similar to those observed in wheel specimens subjected to wheel-rail contact only. The effect of the cast iron traces on surface crack propagation was evaluated by means of finite element simulations and it was found to be limited. Therefore, the application of shoe braking as emergency stop braking system to high speed trains did not show any relevant contraindications.

Влияние силы адгезии и коэффициента деформационного упрочнения материала на интенсивность адгезивного износа в сухом тангенциальном контакте с трением

Влияние силы адгезии и коэффициента деформационного упрочнения материала на интенсивность адгезивного износа в сухом тангенциальном контакте с трением

Improvements in the mixed elastohydrodynamic lubrication and in the efficiency of hypoid gears

In this paper, the influence of the manufacturing parameters on the conditions of mixed elastohydrodynamic lubrication is investigated. On the basis of the obtained results, recommendations are formulated to improve the mixed EHL and the efficiency of face-milled hypoid gears. A full numerical analysis of the mixed EHL in hypoid gears is applied. The equation system and the numerical procedure are unified for a full coverage of all the lubrication regions, including the full film, mixed, and boundary lubrication. In the hydrodynamically lubricated areas, the calculation method employed is based on the simultaneous solution of the Reynolds, elasticity, energy, and Laplace's equations. In the asperity contact areas, the Reynolds equation is reduced to an expression equivalent to the mathematical description of dry contact problem. The real geometry and kinematics of the gear pair based on the manufacturing procedure are applied; thus, the exact geometrical separation of the mating tooth surfaces is included in the oil film shape, and the real velocities of these surfaces are used in the Reynolds and energy equations. The transient nature of gear tooth mesh is included. The oil viscosity variation with respect to pressure and temperature and the density variation with respect to pressure are included. The non-Newtonian behaviour of the lubricant is considered. Using this model, the pressures, film thickness, temperatures, and power losses in the mixed lubrication regime are predicted. The effectiveness of the presented method is demonstrated by using hypoid gear examples.

Comparison of Meshing Strategies in THR Finite Element Modelling.

In biomechanics and orthopedics, finite element modelling allows simulating complex problems, and in the last few years, it has been widely used in many applications, also in the field of biomechanics and biotribology. As is known, one crucial point of FEM (finite element model) is the discretization of the physical domain, and this procedure is called meshing. A well-designed mesh is necessary in order to achieve accurate results with an acceptable computational effort. The aim of this work is to test a finite element model to simulate the dry frictionless contact conditions of a hip joint prosthesis (a femoral head against an acetabular cup) in a soft bearing configuration by comparing the performances of 12 common meshing strategies. In the simulations, total deformation of the internal surface of the cup, contact pressure, and the equivalent von Mises stress are evaluated by using loads and kinematic conditions during a typical gait, obtained from a previous work using a musculoskeletal multibody model. Moreover, accounting for appropriate mesh quality metrics, the results are discussed, underlining the best choice we identified after the large amount of numerical simulations performed.

Drop-on-Demand Printing as Novel Method of Oil Supply in Elastohydrodynamic Lubrication

The feasibility of drop-on-demand printing as a novel method for oil supply in elastohydrodynamic lubrication was tested for a single ball-on-disk contact to allow precise lubrication control at a minimal level of lubricant. It is shown that it is feasible to jet droplets of 3.3 pL for an oil of 46 mPa s viscosity at a controlled frequency which, for the conditions considered, results in droplets on the surface with a radius significantly smaller than the Hertzian contact radius. Experimental results of the film thickness evolution over time are presented from dry contact to fully flooded contact with different supply frequencies and a total oil usage of approximately 40 nL. By means of numerical simulation, the details of droplet ingestion into the contact are illustrated. It is shown that the central film thickness over time can accurately be predicted. The theoretical and experimental results provide a very strong incentive for further investigation of drop-on-demand printing as method to achieve optimal (re)lubrication at minimum level in the realistic setting of a rolling bearing.

Low friction of metallic multilayers by formation of a shear-induced alloy

During sliding of metallic surfaces, the near surfaces undergo significant changes in terms of topography, composition and microstructure. Since friction and wear behavior of the materials are strongly influenced by sub-surface deformations, it is fundamental to investigate these effects. Therefore, the present study aims towards a better understanding of the behavior of friction depending on well-defined initial microstructures. By performing sliding experiments on Au-Ni multilayer samples under ultrahigh vacuum (UHV) conditions, we observe that the individual layer thickness of multilayer systems has a strong influence on friction behavior due to the transition in the dominant deformation mechanism near the surface. The experiments reported here provide a new route for lowering the friction force of metallic material systems in dry contact by providing more stable microstructures and alloy formation. Through ultrafine grains present in the alloy formed by mechanical mixing the number of grain boundaries strongly increases and hence, grain boundary-mediated deformation results in the low friction coefficient.

Evaluation of Tribological Properties of Hard Coatings Obtained on Steel C45 by Electro-Spark Alloying

It was shown that under dry contact conditions, under normal load of to 2 MPa, all coatings demonstrated a significant increase in wear resistance compared to that of the substrate. However, among them, the Mo coating showed the highest wear resistance: ~20 times higher than that of the uncoated steel. That was caused not only by the Mo high microhardness and the lowest initial roughness, but also by the structure of this coating. Meanwhile, the Ti + SiC samples displayed the highest microhardness among investigated coatings. A correlation was established between the microhardness of the coating and the friction coefficient: the larger the microhardness of the coating, the higher is the coefficient of friction. An X-ray analysis of the coatings obtained by ESA on steel with compositions (Ti + Al + C), (Ti + AlN) and (Ti + SiC) revealed phases of titanium carbide, titanium nitride, intermetallic compound AlFe3, and small amounts of aluminum nitride, silicon dioxide and titanium dioxide. This could explain the high microhardness (from 6.8 up to 13.8 GPa) of the obtained coatings.

Numerical and Experimental Research on Identifying a Delamination in Ballastless Slab Track.

This paper aims to adopt the total focusing method (TFM) and wavenumber method for characterizing a delamination in ballastless slab track. Twelve dry point contact (DPC) transducers located at the upper surface of the slab track compose a linear array. These transducers are employed to actuate shear waves, which are suitable for identifying the delamination. The technique of removing the surface wave has been implemented for only retaining the scattered wave caused by the delamination and the reflected wave from the bottom of bed plate. Numerical and experimental results demonstrate that the delamination and bottom of the bed plate can be identified by the proposed methods. Furthermore, the near-surface pseudomorphism is distinctly restrained after removing the surface wave. Compared to TFM, the wavenumber method has the great advantages of improving computational performance and lateral resolution. However, they have no significant difference in the longitudinal resolution. Furthermore, it has been confirmed that the lateral resolution can be affected by the amount of transducers. This paper can provide valuable suggestions on improving the computational performance and the imaging accuracy when we identify a delamination in ballastless slab track.

Numerical and experimental study on the effect of considering plastic and elastoplastic deformation of each asperity in dry contact of rough surfaces

Contact is one of the main means to transmit force between bodies, and a detailed study of contact mechanics provides the required knowledge toward a realistic prediction of the behavior of contacting bodies. In this study, an approach based on the Greenwood-Williamson model is developed in which all of the contact points on the contacting surfaces are observed momentarily. The surface roughness is experimentally measured. The surface asperities are treated as spline functions, and their properties such as radius of curvature are obtained by differentiating these functions. The response of each individual contact point is evaluated by solving contact equations related to a specific asperity in elastic, elastoplastic, and plastic regimes. The method is therefore called spatially resolved Greenwood-Williamson (SRGW) model. To validate the model, a new technique is used which is based on indentation. A series of indentation experiments are conducted on two contacting surfaces, and the elastic and plastic deformations of the asperities under different loads are measured. The percentage error of the proposed approach is lower compared to other methods such as GW and ZMC, which shows the higher accuracy of this new method. The results show that the presented model can predict experimental results with reasonable accuracy under different contact conditions.

Attenuation of a Slow Subsonic A0 Mode Ultrasonic Guided Wave in Thin Plastic Films

The ultrasonic testing technique using Lamb waves is widely used for the non-destructive testing and evaluation of various structures. For air-coupled excitation and the reception of A0 mode Lamb waves, leaky guided waves are usually exploited. However, at low frequencies (<100 kHz), the velocity of this mode in plastic and composite materials can become slower than the ultrasound velocity in air, and its propagation in films is accompanied only by an evanescent wave in air. To date, the information about the attenuation of the slow A0 mode is very contradictory. Therefore, the objective of this investigation was the measurement of the attenuation of the slow A0 mode in thin plastic films. The measurement of the attenuation of normal displacements of the film caused by a propagating slow A0 mode is discussed. The normal displacements of the film at different distances from the source were measured by a laser interferometer. In order to reduce diffraction errors, the measurement method based on the excitation of cylindrical but not plane waves was proposed. The slow A0 mode was excited in the polyvinylchloride film by a dry contact type ultrasonic transducer made of high-efficiency PMN-32%PT strip-like piezoelectric crystal. It was found that that the attenuation of the slow A0 mode in PVC film at the frequency of 44 kHz is 2 dB/cm. The obtained results can be useful for the development of quality control methods for plastic films.

Boundary lubrication properties and contact mechanism of carbon/MoS2 based nanolubricants under steel/steel contact

Lubrication mechanism of some carbon-based nanoparticles is unresolved and needs proper attention. In this work, friction and wear properties of carbon/MoS2 solid lubricants are tested in the boundary lubrication regime and further compared with oil-based binary and ternary nanolubricants. Results show that MWCNT and fullerene have high values of friction coefficient and wear rate in a dry medium. However, in lubricated medium with fomblin oil, they comply with the boundary lubrication mechanism and provide low friction and wear. The binary and ternary nanolubricants of oil-graphene, oil-MoS2, and oil-graphene-MoS2, oil-graphene-fullerene have provided enhanced wear and friction performance due to a build-up of a tribofilm between the contact zones. The interfacial contact mechanism depends on aggregation and particle rolling for dry and lubrication contact, respectively.

Molecular Origin of Superlubricity between Graphene and a Highly Hydrophobic Surface in Water.

Graphene is efficient to provide ultralow friction after the formation of an incommensurate interface but is limited to dry contact conditions and specific lattice structures. In this Letter, a new strategy is proposed to achieve the superlubricity of graphene through the creation of a sliding interface between graphene and a highly hydrophobic surface of self-assembled fluoroalkyl monolayers (SAFMs) in water. A superlow friction coefficient of μ = 0.0003 was obtained, demonstrating the extremely low shear stress between graphene and hydrophobic SAFMs in water. Molecular dynamics (MD) simulation shows that a nanometer-thick water layer is intercalated between graphene and hydrophobic SAFMs, and the weak interactions between water molecules and graphene provide a small energy barrier for water molecules sliding on graphene, which contributes to superlubricity. This finding reveals how to form a superlubricity interface by water intercalation, which has implications for minimizing the friction of layered materials and hydrophobic surfaces in water.

Adhesive wear mechanisms in the presence of weak interfaces: Insights from an amorphous model system

Engineering wear models are generally empirical and lack connections to the physical processes of debris generation at the nanoscale to microscale. Here, we thus analyze wear particle formation for sliding interfaces in dry contact with full and reduced adhesion. Depending on the material and interface properties and the local slopes of the surfaces, we find that colliding surface asperities can either deform plastically, form wear particles, or slip along the contact junction surface without significant damage. We propose a mechanism map as a function of material properties and local geometry, and confirm it using quasi-two-dimensional and three-dimensional molecular dynamics and finite-element simulations on an amorphous, siliconlike model material. The framework developed in the present paper conceptually ties the regimes of weak and strong interfacial adhesion together and can explain that even unlubricated sliding contacts do not necessarily lead to catastrophic wear rates. A salient result of the present paper is an analytical expression of a critical length scale, which incorporates interface properties and roughness parameters. Therefore, our findings provide a theoretical framework and a quantitative map to predict deformation mechanisms at individual contacts. In particular, contact junctions of sizes above the critical length scale contribute to the debris formation.

Surface topography and friction coefficient evolution during sliding wear in a mixed lubricated rolling-sliding contact

Wear is a common phenomenon which most mechanical components suffer from. Available studies are mainly focused on the wear performance after contact pairs operate for a period of time. The studies on evolution of surface topography and friction coefficient resulting from the wear process under mixed lubrication conditions are still limited. Further study of this issue may have a significant influence on wear reduction and lubricating performance improvement. In the present study, surface topography and friction coefficient evolution during sliding wear in a mixed lubricated rolling-sliding contact are investigated by the method based on the numerical approach developed by Zhu.et al. [1]. The numerical results are compared with experiment results to demonstrate the efficiency and accuracy of the present wear model. The evolutions of wear track, root mean square (RMS) roughness, and friction coefficient are observed systematically during wear process. The influences of load, slide-to-roll ratio and surface orientation on these characteristics are studied as well. The simulation results are in good agreement with experimental observations. The wear track is quite different from the one under dry contact, due to the effect of oil and the contact deformation. The friction coefficient drops rapidly at first and then it keeps stable with small fluctuations with respect to wear evolution. It indicates that load and slide-to-roll ratio have a remarkable impact on wear rate, though the effect of surface topography is limited. Further, the results show that the RMS roughness deceases slightly at first and then it increases gradually during the wear process. • A point wear model of real, engineering rough surfaces has been introduced in this paper. • In the early running-in process, the wear tracks on both sides are deeper than in the middle, which is different from that under dry contact. • In the running-in process, the friction coefficient drops rapidly first and then keeps stable with small fluctuations. • Increasing load and slide-to-roll ratio can increase the wear rate.

Phase Division and Critical Point Definition of Electromagnetic Railgun Sliding Contact State

Sliding contact of electromagnetic railgun affects launch accuracy, launch efficiency, and rail life. The research on sliding contact of electromagnetic railgun is very important to the theory and engineering design. However, the whole process of electromagnetic launch is high speed and big current and high heat. The state of sliding contact is complicated. The whole process is also transient process. The stage division of sliding contact is vague, and there is no definite definition about stage division. In this paper, the theoretical analysis of sliding electric contact process is introduced, and the design test is carried out to verify it. The whole launch process of electromagnetic railgun is divided into three stages: dry friction contact, liquefied layer contact, and high-speed unstable contact. The line density 15 kA/mm is defined as the critical point between dry friction stage and liquefaction stage. The armature speed 1300 m/s is defined as critical point between the liquefied contact layer and the high-speed unstable contact stage, which provides reference for the study of sliding contact in the future.

Time change for the propagation of ultrasonic vibrations in concrete under changing the conditions of measurements

Problem statement . The determination of physicomechanical characteristics of the material of the structures in operation using non-destructive testing methods has significant limitations. These restrictions are caused by differences in the composition of concrete, the parameters of the construction of the structure as well as the conditions of measurement. The human factor depends on the level of training of the specialist who carries out the measurements. Р urpose . To evaluate the impact of ultrasonic devices pressing during the dry contact and the need to comply with the requirements of device manufacturers regarding the size of the force of pressing the device. Methods . The measurement of the propagation time of ultrasonic vibrations was carried out using the «Pulsar 1.1» device with a change in the force of pressing the device to the concrete of the sample in the range recommended by the device manufacturer and less. Statistical processing of the results obtained (with visualization) was performed using the EXEL software package. Results. Studies have confirmed the need to take into account the magnitude of the force pressing the device to the concrete surface when measuring the propagation time of ultrasonic vibrations. It has been established that with a change in the magnitude of the pressing force of the device to the concrete surface, several intervals can be distinguished for which the dependence “propagation time of ultrasonic vibrations − the pressing force of the device” is approximated linearly. The dimensions and interposition of such intervals substantially depend on changes in the surface parameters of the concrete structure, change in the composition of the concrete mix and its properties, conditions of concrete molding and hardening. For all tests (when viewing the entire series of samples), the sample size is larger than the limits of the confidence interval. When considering each of the samples separately from the rest of the sample size is within the limits of the confidence interval. Conclusions. Studies have confirmed the effect on the measurement results of the pressure of the device to the surface, the composition and properties of the concrete mix, the molding conditions and the hardening of the concrete. Improving the accuracy of determining the physicomechanical characteristics of the material of the structures is possible taking into account the mentioned above factors.

Modelling cascaded caloric refrigeration systems that are based on thermal diodes or switches

Solid state cooling based on caloric materials has a large potential, because there is no need for fluid refrigerants. The state of the art system approach relies on a heat transfer fluid, which is pumped back and forth through a regenerator of caloric material. This system approach has fundamental limitations regarding the cycle frequency. Different system approaches are considered. One is the utilization of fast thermal diodes or switches. The aim of this work is to provide a universal model to describe a cooling system based on caloric materials using thermal diodes or switches. The heat capacitance of the caloric material is defined to be constant, allowing a lumped elements representation. An analytical model based on an analogy to an electrical low pass filter is introduced. The analytical model is compared with good agreement to lumped elements simulations and measurement data from the literature. It is shown, that the newly introduced cut-off frequency as merit for the potential cycle frequency is proportional to the heat transfer coefficient of the diodes/switches in flux direction and inverse proportional to the length scale of the caloric material in heat flux direction. Different heat transfer mechanisms are discussed as basis for such thermal diodes and switches in combination with different caloric effects. To increase the cycle frequency compared to state of the art systems, latent heat transfer is a promising candidate for all caloric effects, whereas for example a dry press contact for a magnetocaloric system presents many challenges.