Showing papers on "Contact area published in 2014"

Harvesting water wave energy by asymmetric screening of electrostatic charges on a nanostructured hydrophobic thin-film surface

Abstract: Energy harvesting from ambient water motions is a desirable but underexplored solution to on-site energy demand for self-powered electronics. Here we report a liquid-solid electrification-enabled generator based on a fluorinated ethylene propylene thin film, below which an array of electrodes are fabricated. The surface of the thin film is charged first due to the water-solid contact electrification. Aligned nanowires created on the thin film make it hydrophobic and also increase the surface area. Then the asymmetric screening to the surface charges by the waving water during emerging and submerging processes causes the free electrons on the electrodes to flow through an external load, resulting in power generation. The generator produces sufficient output power for driving an array of small electronics during direct interaction with water bodies, including surface waves and falling drops. Polymer-nanowire-based surface modification increases the contact area at the liquid-solid interface, leading to enhanced surface charging density and thus electric output at an efficiency of 7.7%. Our planar-structured generator features an all-in-one design without separate and movable components for capturing and transmitting mechanical energy. It has extremely lightweight and small volume, making it a portable, flexible, and convenient power solution that can be applied on the ocean/river surface, at coastal/offshore areas, and even in rainy places. Considering the demonstrated scalability, it can also be possibly used in large-scale energy generation if layers of planar sheets are connected into a network.

Classical shear cracks drive the onset of dry frictional motion

Abstract: The transition between ‘static’ and ‘dynamic’ friction in a model system is found to be quantitatively captured by the same theoretical framework as is used to describe brittle fracture, but deviations from this correspondence are observed as the rupture velocity approaches the speed at which sound waves propagate along the interface. That the processes of friction and fracture might share some common features is not a new idea, but there is still much to be learned about the nature of these connections. Ilya Svetlizky and Jay Fineberg have turned their attention to the sliding interface between dry brittle polymer blocks, and investigate the transition between 'static' and 'dynamic' friction in this model system. They find that this transition can for the most part be quantitatively captured by the same theoretical framework used to describe brittle fracture, but they also see deviations from this correspondence as the rupture velocity approaches the speed at which sound waves propagate along the interface. The coupling between friction and fracture is important in various fields including materials sciences, engineering and the study of earthquake dynamics. Frictional processes entail the rupture1,2 of the ensemble of discrete contacts defining a frictional interface3,4. There are a variety of views on how best to describe the onset of dry frictional motion. These range from modelling friction with a single degree of freedom, a ‘friction coefficient’3,5, to theoretical treatments using dynamic fracture5,6,7,8 to account for spatial and temporal dynamics along the interface. We investigated the onset of dry frictional motion by performing simultaneous high-speed measurements of the real contact area and the strain fields in the region surrounding propagating rupture tips within the dry (nominally flat) rough interfaces formed by brittle polymer blocks. Here we show that the transition from ‘static’ to ‘dynamic’ friction is quantitatively described by classical singular solutions for the motion of a rapid shear crack5,9,10,11,12,13. We find that these singular solutions, originally derived to describe brittle fracture, are in excellent agreement with the experiments for slow propagation, whereas some significant discrepancies arise as the rupture velocity approaches the Rayleigh wave speed. In addition, the energy dissipated in the fracture of the contacts remains nearly constant throughout the entire range in which the rupture velocity is less than the Rayleigh wave speed, whereas the size of the dissipative zone undergoes a Lorentz-like contraction as the rupture velocity approaches the Rayleigh wave speed. This coupling between friction and fracture is critical to our fundamental understanding of frictional motion and related processes, such as earthquake dynamics.

Dynamics of fingertip contact during the onset of tangential slip

Abstract: 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.

The Hertz-Type and Adhesive Contact Problems for Depth-Sensing Indentation

Abstract: Connections between the Hertz-type contact problems and depth-sensing indentation of materials are studied. Formulations of Hertz-type contact problems with various boundary conditions within the contact area are discussed in detail. The problems under investigations can be subdivided into two large groups: self-similar problems for anisotropic materials with various rheological properties and adhesive contact problems for arbitrary bodies of revolution or for power-law shaped blunt indenters. Specific features of indentation problems are described and the common methods for extracting elastic and adhesive properties of materials are briefly reviewed. The basic formulae are extended to the case of nonslipping boundary conditions between a probe and the material. The main formulae of the JKR theory of adhesion are extended to any material with rotational symmetry of the elastic properties. These materials include not only isotropic or transversely isotropic elastic solids but also homogeneously prestressed isotropic or transversely isotropic nonlinear elastic materials. The BG method introduced for extracting adhesive and elastic properties of isotropic elastic materials from depth-sensing diagrams of spherical indenters, is described and extended to linear or linearized materials with rotational symmetry of the elastic properties.

Study on the resistance distribution at the contact between molybdenum disulfide and metals.

Abstract: Contact resistance hinders the high performance of electrical devices, especially devices based on two-dimensional (2D) materials, such as graphene and transition metal dichalcogenide. To engineer contact resistance, understanding the resistance distribution and carrier transport behavior at the contact area is essential. Here, we developed a method that can be used to obtain some key parameters of contact, such as transfer length (Lt), sheet resistance of the 2D materials beneath the contacting metal (Rsh), and contact resistivity between the 2D materials and the metal electrode (ρc). Using our method, we studied the contacts between molybdenum disulfide (MoS2) and metals, such as titanium and gold, in bilayer and few-layered MoS2 devices. Especially, we found that Rsh is obviously larger than the sheet resistance of the same 2D materials in the channel (Rch) in all the devices we studied. With the increasing of the back-gate voltage, Lt increases and Rsh, ρc, Rch, and the contact resistance Rc decrease in all the devices we studied. Our results are helpful for understanding the metal–MoS2 contact and improving the performances of MoS2 devices.

Anisotropic Friction of the Ventral Scales in the Snake Lampropeltis getula californiae

Abstract: Since the ventral body side of snakes is in almost continuous contact with the substrate during undulating locomotion, their skin is presumably adapted to generate high friction for propulsion and low friction to slide along the substrate In this study, the microstructure of ventral scales was analyzed using scanning electron microscopy, atomic force microscope and confocal laser scanning microscopy Dynamic friction was investigated by a microtribometer The ventral scales demonstrated anisotropic frictional properties To analyze the role of the stiffness of underlying layers on the frictional anisotropy, two different types of scale cushioning (hard and soft) were tested To estimate frictional forces of the skin surface on rough substrates, additional measurements with a rough surface were performed Frictional anisotropy for both types of scale cushioning and rough surfaces was revealed However, for both types of surface roughness, the anisotropy was stronger expressed in the soft-cushioned sample This effect could be caused by (1) the stronger interaction of the microstructure with the substrate in soft-cushioned samples due to larger real contact area with the substrate and (2) the composite character of the skin of this snake species with embedded, highly ordered fiber-like structures, which may cause anisotropy in material properties

Experimental analysis by measurement of surface roughness variations in turning process of duplex stainless steel

Abstract: The objective of the investigation was to identify surface roughness after turning with wedges of coated sintered carbide. The investigation included predic ting the average surface roughness in the dry machining of Duplex Stainless Steel (DSS) and the determination of load curves together with roughness profiles for various cutting conditions. The load curves and roughness profiles for various cutting wedges and variable cutting parameters were compared. It has been shown that dry cutting leads to a decrease in friction for lubricated surfaces, providing a small initial contact area where the surface is contacted. The st udy has been performed within a production facility during the production of electric motor parts and deep-well pumps. Keywords: turning, coatings, friction-reducing, optical microscopy, surface roughness analysis and models. © 2014 Polish Academy of Sciences. All rights reserved 1. Introduction Engineering surfaces, particularly those generated using multi-step manufacturing processes and intended for tribological applications such as bearings and gears, rarely if ever have perfectly normal distributed elevations [1]. Surface roughness measurements of any workpiece are among the most important ones in length and angle metrology, both in theory and practice. According to Wieczorowski et al. [2], there are great discrepancies in these measurements because of the large variety of instruments for surface roughness analysis. Hence, three-dimensional surface topography parameters are necessary for assessing the surface roughness characteristics more effectively [3]. According to Mahovic Poljacek et al. [4], a precise characterization of roughness and surface topography is of prime importance in many engineering industries because certain functional properties of the materials are often determined by the surface structure and characteristics. Estimation of the magnitude of surface roughness under the given cutting conditions resulting from metal removal operations is one of the major goals in this area [5, 6]. According to Benardos and Vosniakos [7], surface roughness is a widely used index of product quality and in most cases a technical requirement for mechanical products. Achieving the desired surface quality is of great importance for the functional behaviour of a part. Surface profilometry is for many years a well-known method of topography inspection [8–12]. Topography parameters represent surface properties is much better than 2D ones. Using the surface parameters can be determined functions describing surface behaviour. The workpiece material is duplex stainless steel because this stainless steel is widely used for many industrial applications due to its unique properties. Cabrera et al. [13] and Park et al. [14] consider that the good combination of their mechanical properties (high strength and toughness) and corrosion resistance makes them of great interest for a wide range of

The effects of surface energy and roughness on the hydrophobicity of woven fabrics

Abstract: The wetting behavior of a hydrophobic rough surface is investigated on a surface fabricated by applying low surface tension materials such as silicone or fluoropolymer to polyester woven fabric consisting of multifilament yarns. The roughness factor of various woven fabrics can be calculated by Wenzel’s and Cassie–Baxter’s equations. For the fabrics treated with silicone or fluoropolymer, the Cassie–Baxter model was applied, showing a level of agreement for the fabric specimens non-textured filament fibers between the predicted contact angles and the measured values. More precisely, the fabrics treated with silicone or fluoropolymer represent the transitional state between the Wenzel type and the Cassie–Baxter type; that is, the fractional contact area between the water and air f2 is greater than zero, and the sum of the fractional contact areas for solid–water f1 and air–water f2 is greater than 1. A surface with lower energy and higher roughness gave f1 + f2 close to 1 with smaller f1 and larger f2, whi...

Use of biomimetic hexagonal surface texture in friction against lubricated skin

Abstract: Smooth contact pads that evolved in insects, amphibians and mammals to enhance the attachment abilities of the animals' feet are often dressed with surface micropatterns of different shapes that act in the presence of a fluid secretion. One of the most striking surface patterns observed in contact pads of these animals is based on a hexagonal texture, which is recognized as a friction-oriented feature capable of suppressing both stick–slip and hydroplaning while enabling friction tuning. Here, we compare this design of natural friction surfaces to textures developed for working in similar conditions in disposable safety razors. When slid against lubricated human skin, the hexagonal surface texture is capable of generating about twice the friction of its technical competitors, which is related to it being much more effective at channelling of the lubricant fluid out of the contact zone. The draining channel shape and contact area fraction are found to be the most important geometrical parameters governing the fluid drainage rate.

Rolling friction of adhesive microspheres

Abstract: The rolling friction of adhesive microspheres is an important quantity as it determines the strength and stability of larger aggregates. Current models predict rolling forces that are 1 to 2 orders of magnitude smaller than observed experimentally. Starting from the well-known Johnson–Kendall–Roberts (JKR) contact description, we derive an analytical theory for the rolling friction based on the concept of adhesion hysteresis, e.g. a difference in apparent surface energies for opening/closing cracks. We show how adhesion hysteresis causes the pressure distribution within the contact to become asymmetrical, leading to an opposing torque. Analytical expressions are derived relating the size of the hysteresis, the rolling torque, and the rolling displacement, ξ. We confirm the existence of a critical rolling displacement for the onset of rolling, the size of which is set by the amount of adhesion hysteresis and the size of the contact area. We demonstrate how the developed theory is able to explain the large rolling forces and particle-size dependence observed experimentally. Good agreement with experimental results is achieved for adhesion hysteresis values of (Δγ/γ) ≃ 3 for polystyrene, and (Δγ/γ) ≃ 0.5 for silicates, at crack propagation rates of 0.1 µm s−1 and 1–10 µm s−1, respectively.

Dry friction of microstructured polymer surfaces inspired by snake skin

Abstract: The microstructure investigated in this study was inspired by the anisotropic microornamentation of scales from the ventral body side of the California King Snake (Lampropeltis getula californiae). Frictional properties of snake-inspired microstructured polymer surface (SIMPS) made of epoxy resin were characterised in contact with a smooth glass ball by a microtribometer in two perpendicular directions. The SIMPS exhibited a considerable frictional anisotropy: Frictional coefficients measured along the microstructure were about 33% lower than those measured in the opposite direction. Frictional coefficients were compared to those obtained on other types of surface microstructure: (i) smooth ones, (ii) rough ones, and (iii) ones with periodic groove-like microstructures of different dimensions. The results demonstrate the existence of a common pattern of interaction between two general effects that influence friction: (1) molecular interaction depending on real contact area and (2) the mechanical interlocking of both contacting surfaces. The strongest reduction of the frictional coefficient, compared to the smooth reference surface, was observed at a medium range of surface structure dimensions suggesting a trade-off between these two effects.

Light controlled friction at a liquid crystal polymer coating with switchable patterning

Abstract: We describe a new methodology that enables dynamical control of motion through modulating the friction at coating surfaces by exposing to UV light. The principle is based on reversible switching of the surface topographies of the coating by light. The coating surface transfers from flat in the dark to corrugated in the presence of UV by forming regular ridge-like line gratings. Both the static and the kinetic friction coefficient are investigated in a dynamic manner by switching between the off (flat surface) and the activated (with ridges) state. By dynamically changing the friction, we are able to bring the sample from a static state into motion via UV exposure. When in motion, the friction coefficient can be altered further by modulating the light conditions. For example, a smooth sliding can transfer into an interlocking state, or vice versa. Moreover, we can dynamically reduce the contact area in the interface and thus lowering friction forces.

Grain trajectory and grain workpiece contact analyses for modeling of grinding force and energy partition

Abstract: To achieve controlled stress grinding and controlled grinding of the depth of modificative layer, coupled analysis of grinding force and grinding heat is required. Therefore, this paper investigated grinding force and energy partition to lay a foundation for the coupled analysis. Firstly, a new grinding force model based on the analyses of grain trajectory and grain workpiece contact. In the modeling of grinding force, critical grain indention depths for plowing and cutting were calculated and the grinding force models of a single grain were established. This model can analyze the contributions of sliding, plowing, and cutting to total grinding forces. Secondly, an energy partition model was established based on the analyses of grain trajectory and grain workpiece contact. In the modeling of energy partition, the real contact area ratio and the grain contact radius were calculated. Finally, experiments were pursued to validate the grinding force model by comparing the experimental measurements to the theoretical results. Comparisons showed reasonable agreement quantitatively.

Effects of Substrate Hardness and Spray Angle on the Deposition Behavior of Cold-Sprayed Ti Particles

Abstract: In this study, finite element analysis combined with experimental observation was conducted to clarify the effects of substrate hardness and spray angle on the deposition behavior of cold-sprayed Ti particles. It is found that metallurgical bonding is highly possible to occur between the Ti particle and Cu substrate due to the intensive metal jet at the rim of the interface which helps to remove the cracked oxides. Because metallurgical bonding and large interfacial contact area can guarantee high adhesion strength, the thick Ti coating is achieved after deposition on the Cu substrate. As for the soft Al substrate, the first layer Ti particles are embedded in and then trapped by the soft substrate material, which results in the occurrence of mechanical interlock at the interface. As a consequence, the final coating thickness is also relatively large. When using hard stainless steel as the substrate, the essential conditions for forming the mechanical interlock are lacked due to the high hardness of the substrate material. In addition, the metal jet at rim of the interface is less prominent and also the interfacial contact area is smaller in comparison with the Ti-Cu case. Therefore, the particle-substrate bonding strength and the consequent coating thickness are relatively low. Besides, it is also found that the particle deformation and coating quality are significantly affected by the spray angle. The deformation of the particle localizes at only one side due to the additional tangential momentum. Also, such localized deformation becomes increasingly intensive with decreasing the spray angle. Moreover, the coating thickness is found to reduce with the decrease in spay angle, but the coating porosity shows a reverse trend.

Physicochemical processes of frictional healing: Effects of water on stick‐slip stress drop and friction of granular fault gouge

Abstract: Understanding the micromechanical processes that dictate the evolution of fault strength during the seismic cycle is a fundamental problem in earthquake physics. We report on laboratory experiments that investigate the role of water during repetitive stick-slip frictional sliding, with particular emphasis on the grain-scale and atomic-scale mechanisms of frictional restrengthening (healing). Our experiments are designed to test underlying concepts of rate and state friction laws. We sheared layers of soda-lime glass beads in a double direct shear configuration at a constant normal stress of 5 MPa. Shear stress was applied via a constant displacement rate from 0.3 to 300 µm/s. During each experiment, relative air humidity (RH) was kept constant at values of 5, 50, or 100%. Our data show a systematic increase in maximum friction (μmax), stick-slip friction drop (Δμ), and frictional healing rate, with increasing RH. The highest values of interevent dilation occur at 100% RH. Postexperiment scanning electron microscope observations reveal details of contact junction processes, showing a larger grain-to-grain contact area at higher RH. We find that the evolution of contact area depends inversely on slip velocity and directly on RH. Our results illuminate the fundamental processes that dictate stick-slip frictional sliding and provide important constraints on the mechanisms of rate and state friction.

Experimental determination of the tool–chip thermal contact conductance in machining process

Abstract: Tool–chip contact is still a challenging issue that affects the accuracy in numerical analysis of machining processes. The tool–chip contact phenomenon can be considered from two points of view: mechanical and thermal contacts. Although, there is extensive published literature which addresses the friction modeling of the tool–chip interface, the thermal aspects of the tool–chip contact have not been investigated adequately. In this paper, an experimental procedure is adopted to determine the average thermal contact conductance (TCC) in the tool–chip contact area in the machining operation. The tool temperature and the heat flux in tool–chip contact area were determined by inverse thermal solution. Infra-red thermography was also used to measure the average chip temperature near the tool–chip interface. To investigate the effects of the work piece material properties on the tool–chip TCC, AISI 1045, AISI 304 and Titanium materials were considered in the machining experiments. Effects of the cutting parameters such as cutting velocity and feed rate on TCC were also investigated. Evaluating the tool–chip thermal contact conductance for the tested materials shows that TCC is directly proportional to the thermal conductivity and inversely proportional to the mechanical strength of the work piece. The thermal contact conductance presented in this paper can be used in the future numerical and analytical modeling of the machining process to achieve more accurate simulations of the temperature distribution in the cutting zone and better understanding of the tool–chip contact phenomena.

Possibility of Steering of Product Surface Layers Properties in Burnishing Rolling Process

Abstract: In this study the process of burnishing rolling is considered as a geometrical and physical boundary and initial value problem, with unknown boundary conditions in the contact area. 3D dynamic explicit method for burnishing rolling process with taking into account surface after turning (as previous treatment) under ANSYS/LS-DYNA environment was established. The analysis covered surfaces characterized by vertical angles of the asperities in range: 60°÷150°. The simulation results (i.e. surface deformation, states of strain and stresses, depth of stress deposition) were evaluated. The influence of vertical angle of the asperities after turning process on the states of stress and strain and the depth of its deposition are presented.

Specimen-specific predictions of contact stress under physiological loading in the human hip: validation and sensitivity studies.

Abstract: Hip osteoarthritis may be initiated and advanced by abnormal cartilage contact mechanics, and finite element (FE) modeling provides an approach with the potential to allow the study of this process. Previous FE models of the human hip have been limited by single specimen validation and the use of quasi-linear or linear elastic constitutive models of articular cartilage. The effects of the latter assumptions on model predictions are unknown, partially because data for the instantaneous behavior of healthy human hip cartilage are unavailable. The aims of this study were to develop and validate a series of specimen-specific FE models, to characterize the regional instantaneous response of healthy human hip cartilage in compression, and to assess the effects of material nonlinearity, inhomogeneity and specimen-specific material coefficients on FE predictions of cartilage contact stress and contact area. Five cadaveric specimens underwent experimental loading, cartilage material characterization and specimen-specific FE modeling. Cartilage in the FE models was represented by average neo-Hookean, average Veronda Westmann and specimen- and region-specific Veronda Westmann hyperelastic constitutive models. Experimental measurements and FE predictions compared well for all three cartilage representations, which was reflected in average RMS errors in contact stress of less than 25 %. The instantaneous material behavior of healthy human hip cartilage varied spatially, with stiffer acetabular cartilage than femoral cartilage and stiffer cartilage in lateral regions than in medial regions. The Veronda Westmann constitutive model with average material coefficients accurately predicted peak contact stress, average contact stress, contact area and contact patterns. The use of subject- and region-specific material coefficients did not increase the accuracy of FE model predictions. The neo-Hookean constitutive model underpredicted peak contact stress in areas of high stress. The results of this study support the use of average cartilage material coefficients in predictions of cartilage contact stress and contact area in the normal hip. The regional characterization of cartilage material behavior provides the necessary inputs for future computational studies, to investigate other mechanical parameters that may be correlated with OA and cartilage damage in the human hip. In the future, the results of this study can be applied to subject-specific models to better understand how abnormal hip contact stress and contact area contribute to OA.

Rough viscoelastic sliding contact: theory and experiments.

Abstract: In this paper, we show how the numerical theory introduced by the authors [Carbone and Putignano, J. Mech. Phys. Solids 61, 1822 (2013)] can be effectively employed to study the contact between viscoelastic rough solids. The huge numerical complexity is successfully faced up by employing the adaptive nonuniform mesh developed by the authors in Putignano et al. [J. Mech. Phys. Solids 60, 973 (2012)]. Results mark the importance of accounting for viscoelastic effects to correctly simulate the sliding rough contact. In detail, attention is, first, paid to evaluate the viscoelastic dissipation, i.e., the viscoelastic friction. Fixed the sliding speed and the normal load, friction is completely determined. Furthermore, since the methodology employed in the work allows to study contact between real materials, a comparison between experimental outcomes and numerical prediction in terms of viscoelastic friction is shown. The good agreement seems to validate---at least partially---the presented methodology. Finally, it is shown that viscoelasticity entails not only the dissipative effects previously outlined, but is also strictly related to the anisotropy of the contact solution. Indeed, a marked anisotropy is present in the contact region, which results stretched in the direction perpendicular to the sliding speed. In the paper, the anisotropy of the deformed surface and of the contact area is investigated and quantified.