Showing papers on "Contact area published in 2016"

The evolving quality of frictional contact with graphene

Abstract: Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction. The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction. Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases, and was observed only when the two-dimensional material was loosely adhering to a substrate. This layer-dependent transient phenomenon has not been captured by any simulations. Here, using atomistic simulations, we reproduce the experimental observations of layer-dependent friction and transient frictional strengthening on graphene. Atomic force analysis reveals that the evolution of static friction is a manifestation of the natural tendency for thinner and less-constrained graphene to re-adjust its configuration as a direct consequence of its greater flexibility. That is, the tip atoms become more strongly pinned, and show greater synchrony in their stick-slip behaviour. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. Moreover, the effects can be tuned by pre-wrinkling. The evolving contact quality is critical for explaining the time-dependent friction of configurationally flexible interfaces.

Stimuli-Responsive Reversible Two-Level Adhesion from a Structurally Dynamic Shape-Memory Polymer

Abstract: A shape-memory adhesive has been prepared that exhibits two levels of reversible adhesion. The adhesive is a semicrystalline cross-linked polymer that contains dynamic disulfide bonds. Melting of the crystalline regions via heat causes a drop in the modulus of the material facilitating wetting of the substrate as well as enhancing the surface contact area with the substrate, which result in the formation of an adhesive bond. Exposure to higher heat or UV light results in dynamic exchange of the disulfide bonds, which yields a further drop in the modulus/viscosity that improves surface wetting/contact and strengthens the adhesive bond. This improvement in adhesion is shown to apply over different substrates, contact forces, and deformation modes. Furthermore, the adhesive acts as a thermal shape-memory material and can be used to create joints that can reposition themselves upon application of heat.

Surface Texture Manufacturing Techniques and Tribological Effect of Surface Texturing on Cutting Tool Performance: A Review

Abstract: The tribological characteristics of sliding surfaces have been remarkably improved by surface texturing. Surface texturing can be beneficial in many ways; for example, it can reduce friction and wear, increase load carrying capacity, and increase fluid film stiffness. The design process for surface texturing is highly correlated to the particular functions of any application for which texturing is required. Texture quality is greatly affected by manufacturing methods, therefore, it is important to have a detailed understanding of the related parameters of any technique.The use of surface texturing to improve the cutting performance of tools is a relatively new application. These textures improve cutting performance by enhancing lubricant availability at the contact point, reducing the tool-chip contact area, and trapping wear debris. Reductions in crater and flank wear, friction force, cutting forces, and cutting temperature are the main benefits obtained by this technique. To date, surface textur...

A Smart Washer for Bolt Looseness Monitoring Based on Piezoelectric Active Sensing Method

Abstract: Piezoceramic based active sensing methods have been researched to monitor preload on bolt connections However, there is a saturation problem involved with this type of method The transmitted energy is sometimes saturated before the maximum preload which is due to it coming into contact with flat surfaces When it comes to flat contact surfaces, the true contact area will easily saturate with the preload The design of a new type of bolt looseness monitoring sensor, a smart washer, is to mitigate the saturation problem The smart washer is composed of two annular disks with contact surfaces that are machined into convex and concave respectively, to eliminate the complete flat contact surfaces and to reduce the saturation effect One piezoelectric patch is bonded on the non-contact surface of each annular disk These two mating annular disks form a smart washer One of the two piezoelectric patches serves as an actuator to generate an ultrasonic wave that propagates through the contact surface; the other one serves as a sensor to detect the propagated waves The wave energy propagated through the contact surface is proportional to the true contact area which is determined by the bolt preload The time reversal method is used to extract the peak of the focused signal as the index of the transmission wave energy; then, the relationship between the signal peak and bolt preload is obtained Experimental results show that the focused signal peak value changes with the bolt preload and presents an approximate linear relationship when the saturation problem is experienced The proposed smart washer can monitor the full range of the rated preload

Tibiofemoral Contact Mechanics with Horizontal Cleavage Tear and Resection of the Medial Meniscus in the Human Knee

Abstract: Background: The meniscus is known to increase the contact area and decrease contact pressure in the tibiofemoral compartments of the knee. Radial tears of the meniscal root attachment along with partial resections of the torn meniscal tissue decrease the contact area and increase pressure; however, there is a lack of information on the effects of a horizontal cleavage tear (HCT) and partial leaf meniscectomy of such tears on tibiofemoral contact pressure and contact area. Methods: Twelve fresh-frozen human cadaveric knees were tested under 10 conditions: 5 serial conditions of posterior medial meniscectomy (intact meniscus, HCT, repaired HCT, inferior leaf resection, and resection of both inferior and superior leaves), each at 2 knee flexion angles (0° and 60°) under an 800-N axial load. Tekscan sensors (model 4000) were used to measure the contact pressure and contact area. Results: HCT and HCT repair resulted in small changes in the contact area and an increase in contact pressure compared with the intact condition. Resection of the inferior leaf resulted in significantly decreased contact area (to a mean 82.3% of the intact condition at 0° of flexion and 81.8% at 60° of flexion; p < 0.05) and increased peak contact pressure (a mean 36.3% increase at 0° flexion and 43.2% increase at 60° flexion; p < 0.05) in the medial compartment. Further resection of the remaining superior leaf resulted in additional significant decreases in contact area (to a mean 60.1% of the intact condition at 0° of flexion and 49.7% at 60° of flexion; p < 0.05) and increases in peak contact pressure (a mean 79.2% increase at 0° of flexion and 74.9% increase at 60° of flexion; p < 0.05). Conclusions: Resection of meniscal tissue forming the inferior leaf of an HCT resulted in substantially decreased contact area and increased contact pressure. Additional resection of the superior leaf resulted in a further significant decrease in contact area and increase in contact pressure in the medial compartment. Clinical Relevance: Repair or minimal resection of meniscal tissue of an HCT may be preferred to complete leaf resection to maintain knee tibiofemoral contact mechanics.

Analysis of Contact Phenomena and Heat Exchange in the Cutting Zone Under Minimum Quantity Cooling Lubrication conditions

Abstract: The paper critically investigates about the influ- ence of emulsion mist cooling on the conditions of heat absorption from the machining zone. The cooling conditions under which the total number of drops falling on the hot sur- faces of the machining zone evaporate have been studied. The state of cutting wedges made of P25 sintered carbide after finish turning of two-phase pearlite-ferrite AISI 1045 steel with the presence of an anti-seizure and anti-wear addi- tive has been subjected to scanning inspection. In the contact area, the content of surface active compounds is much larger as compared to the areas beyond the contact. It has been observed that the concentration of active compounds on the

Contact area of rough spheres: Large scale simulations and simple scaling laws

Abstract: We use molecular simulations to study the nonadhesive and adhesive atomic-scale contact of rough spheres with radii ranging from nanometers to micrometers over more than ten orders of magnitude in applied normal load. At the lowest loads, the interfacial mechanics is governed by the contact mechanics of the first asperity that touches. The dependence of contact area on normal force becomes linear at intermediate loads and crosses over to Hertzian at the largest loads. By combining theories for the limiting cases of nominally flat rough surfaces and smooth spheres, we provide parameter-free analytical expressions for contact area over the whole range of loads. Our results establish a range of validity for common approximations that neglect curvature or roughness in modeling objects on scales from atomic force microscope tips to ball bearings.

Direct Laser Interference Patterning: Tailoring of Contact Area for Frictional and Antibacterial Properties

Abstract: Surface functionalization by topographic micro- and nano-structures in order to achieve unique properties, like super-hydrophobicity or ultrahigh light absorption, is a common strategy in nature. In this paper, direct laser interference patterning (DLIP) is presented as a promising tool allowing for the generation of such surface patterns on technical surfaces in order to mimic these biological surfaces and effects. Friction optimization and antibacterial effects by DLIP are exemplarily described. Topographic surface patterns on the micro- and nano-scale demonstrated a significant reduction in the coefficient of friction and bacterial adhesion. It was shown that in both cases, the control of the contact area between surfaces or between surface and bacteria is of utmost importance.

Surface strain measurements of fingertip skin under shearing.

Abstract: The temporal evolution of surface strain, resulting from a combination of normal and tangential loading forces on the fingerpad, was calculated from high-resolution images. A customized robotic device loaded the fingertip with varying normal force, tangential direction and tangential speed. We observed strain waves that propagated from the periphery to the centre of the contact area. Consequently, different regions of the contact area were subject to varying degrees of compression, stretch and shear. The spatial distribution of both the strains and the strain energy densities depended on the stimulus direction. Additionally, the strains varied with the normal force level and were substantial, e.g. peak strains of 50% with a normal force of 5 N, i.e. at force levels well within the range of common dexterous manipulation tasks. While these observations were consistent with some theoretical predictions from contact mechanics, we also observed substantial deviations as expected given the complex geometry and mechanics of fingertips. Specifically, from in-depth analyses, we conclude that some of these deviations depend on local fingerprint patterns. Our data provide useful information for models of tactile afferent responses and background for the design of novel haptic interfaces.

Towards accuracy improvement in single point incremental forming of shallow parts formed under laser assisted conditions

Abstract: Single point incrementally formed parts with a low wall angle geometry typically exhibit a manufactured geometry that significantly deviates from the design surface due to accumulated unwanted bulging deformation. Development of the bulge on the bottom of the part might result in wrinkling of the sheet at the bulged region which leads to higher forming forces and can even cease the forming process. In this study, the geometric inaccuracy of low angled parts is investigated by means of both Finite element analysis and an experimental campaign on a conical geometry. Deformation mechanisms in shallow sloped parts have been studied in detail and the tool-sheet contact area has been characterized both for low and high angled geometries. In a second phase, the laser assisted single point incremental forming process and its potential for improving accuracy are investigated. To obtain suitable process parameters for a warm forming condition, a transient heat transfer analysis is developed to simulate the laser movement on the conical geometry. Based on the simulated and experimentally determined tool-sheet contact zone, different laser spot positioning strategies have been used while the accuracy of the part and forming forces were measured. It has been observed that overforming of the cone wall is due to the continuous deformation of the sheet outside the contact zone which changes into underforming upon laser treatment. By selection of a proper laser positioning strategy a reduction of 42 % in bulge height is observed. This shows its effect in reducing radial forming forces.

Superlubricity in quasicrystalline twisted bilayer graphene

Abstract: The unique atomic positions in quasicrystals lead to peculiar self-similarity and fractal-like structural morphology. Accordingly, many of the material properties are supposed to manifest exceptional characteristics. In this Rapid Communication, we explain through numerical simulations the fundamental and peculiar aspects of quasicrystals wearless friction manifested in a 30\ifmmode^\circ\else\textdegree\fi{} twisted bilayer graphene system. In particular, the sliding force exhibits a fractal structure with distinct area correlations due to the natural mixture between both periodic and aperiodic lateral modulations. In addition, zero power scaling of the sliding force with respect to the contact area is demonstrated for a geometric sequence of dodecagonal elements.

Effect of thickness and boundary conditions on the behavior of viscoelastic layers in sliding contact with wavy profiles

Abstract: In this work, the sliding contact of viscoelastic layers of finite thickness on rigid sinusoidal substrates is investigated within the framework of Green's functions approach. The periodic Green's functions are determined by means of a novel formalism, which can be applied, in general, to either 2D and 3D viscoelastic periodic contacts, regardless of the contact geometry and boundary conditions. Specifically, two different configurations are considered here: a free layer with a uniform pressure applied on the top, and a layer rigidly confined on the upper boundary. It is shown that the thickness affects the contact behavior differently, depending on the boundary conditions. In particular, the confined layer exhibits increasing contact stiffness when the thickness is reduced, leading to higher loads for complete contact to occur. The free layer, instead, becomes more and more compliant as thickness is reduced. We find that, in partial contact, the layer thickness and the boundary conditions significantly affect the frictional behavior. In fact, at low contact penetrations, the confined layer shows higher friction coefficients compared to the free layer case; whereas, the scenario is reversed at large contact penetrations. Furthermore, for confined layers, the sliding speed related to the friction coefficient peak is shifted as the contact penetration increases. However, once full contact is established, the friction coefficient shows a unique behavior regardless of the layer thickness and boundary conditions.

Geometric Effect of Asperities on Shear Mechanism of Rock Joints

Abstract: Three-dimensional tracking of changes of asperities is one of the most important ways to illustrate shear mechanism of rock joints during testing. In this paper, the changes of the role of asperities during different stages of shearing are described by using a new methodology for the characterization of the asperities. The basis of the proposed method is the examination of the three-dimensional roughness of joint surfaces scanned before and after shear testing. By defining a concept named ‘tiny window’, the geometric model of the joint surfaces is reconstructed. Tiny windows are expressed as a function of the x and y coordinates, the height (z coordinate), and the angle of a small area of the surface. Constant normal load (CNL) direct shear tests were conducted on replica joints and, by using the proposed method, the distribution and size of contact and damaged areas were identified. Image analysis of the surfaces was used to verify the results of the proposed method. The results indicated that the proposed method is suitable for determining the size and distribution of the contact and damaged areas at any shearing stage. The geometric properties of the tiny windows in the pre-peak, peak, post-peak softening, and residual shearing stages were investigated based on their angle and height. It was found that tiny windows that face the shear direction, especially the steepest ones, have a primary role in shearing. However, due to degradation of asperities at higher normal stresses and shear displacements, some of the tiny windows that do not initially face the shear direction also come in contact. It was also observed that tiny windows with different heights participate in the shearing process, not just the highest ones. Total contact area of the joint surfaces was considered as summation of just-in-contact areas and damaged areas. The results of the proposed method indicated that considering differences between just-in-contact areas and damaged areas provide useful insights into understanding the shear mechanism of rock joints.

Strain gradient plasticity analysis of elasto-plastic contact between rough surfaces

Abstract: From a microscopic point of view, the real contact area between two rough surfaces is the sum of the areas of contact between facing asperities. Since the real contact area is a fraction of the nominal contact area, the real contact pressure is much higher than the nominal contact pressure, which results in plastic deformation of asperities. As plasticity is size dependent at size scales below tens of micrometers, with the general trend of smaller being harder, macroscopic plasticity is not suitable to describe plastic deformation of small asperities and thus fails to capture the real contact area and pressure accurately. Here we adopt conventional mechanism-based strain gradient plasticity (CMSGP) to analyze the contact between a rigid platen and an elasto-plastic solid with a rough surface. Flattening of a single sinusoidal asperity is analyzed first to highlight the difference between CMSGP and J2 isotropic plasticity. For the rough surface contact, besides CMSGP, pure elastic and J2 isotropic plasticity analysis is also carried out for comparison. In all cases, the contact area A rises linearly with the applied load, but with a different slope which implies that the mean contact pressures are different. CMSGP produces qualitative changes in the distributions of local contact pressures compared with pure elastic and J2 isotropic plasticity analysis, furthermore, bounded by the two.

Drop impact on natural porous stones

Abstract: Drop impact and spreading on three natural porous stones are experimentally determined using high-speed imaging and compared with spreading over an impermeable steel surface. The dynamic non-wetting behavior during spreading and the hydrophobic contact angle >90° is attributed to the presence of an air layer between the droplet and the porous substrate. As the contact line pins at maximum spreading on the porous stone, the maximum spreading determines the liquid contact area on such substrate. The droplet gets pinned when the air layer is broken at the contact line and capillary forces develop in fines pores at the droplet edge, pinning the droplet. Maximum spreading on porous stones increases with impact velocity but does not scale with Weber number at low impact velocity. It is demonstrated that dynamic wetting plays an important role in the spreading at low velocity and that the dynamic wetting as characterized by the dynamic contact angle θD has to be taken into account for predicting the maximum spreading. Correcting the maximum spreading ratio with the dynamic wetting behavior, all data for porous stones and non-porous substrate collapse onto a single curve.