Elastic Contact Model Research Articles

A New Model for Predicting Fiber Clumping Phenomenon in Bio-Inspired Dry Adhesives

Self-adhesion, or clumping, primarily occurs in bio-inspired dry-adhesives when two or more adjacent fibers become in contact and adhere to each other due to large van der Waals (vdW) intermolecular forces. Studying self-adhesion is of primary interest as dry-adhesives drastically lose their adhesion strength after few load cycles if clumping occurs. An optimization procedure to select material properties, shape, and geometry of the fibers is therefore highly desirable for both improving the adhesion and reducing the probability of clumping. Few numerical models have been utilized to attain optimal geometry and material parameters. These models are developed based on the small-deflection beam theory and use an over-simplified elastic-contact model to determine adhesion forces. In this work, a model suitable to simulate large deformations with distributed intermolecular forces is proposed. The model is based on the finite element method (FEM) and uses an adaptive meshing procedure to accurately calculate adhesion forces. By using this model, clumping is studied and predictions are compared against experimental results obtained from the literature. In addition to the FEM model, an analytical model that considers large-deflection reactions of fibers is proposed to corroborate finite element results. Results from analytical and finite element models show precise clumping predictions.

Two-dimensional elastic contact model for rubber covered rollers

A model for the contact between a rubber covered roller and a rigid roller was developed using analytical functions. The model, being two-dimensional, connects the line load, geometric properties (roller radii, rubber thickness) and the shear modulus of the rubber to nip properties in the roller contact. The output from the model is the indentation of the rigid roller into the rubber, the nip maximum pressure, the nip width and the surface strain in the centre of the nip. Moreover, the shape of the pressure distribution is also given as an analytical expression. The basic assumption relies on the work of Parish (−58 and −61), but a development of this work was performed, showing the influence of the rubber shear modulus and also how the surface strain in the nip can be described. The functional relationships are based on least squares fitting of analytical functions, depending on two master variables, to a large number of finite element analysis results.

Fixture performance improvement by an accelerated integral method of fixture layout and clamping force plan

Deformation under external and clamping forces is an important factor affecting fixture performance. To reduce the deformation of the workpiece–fixture system and improve the performance of the fixture, an optimisation method for the fixture layout and clamping force plan is constructed in this article. First, the workpiece–fixture is illustrated as an elastic–elastic contact model with friction in which the workpiece is modelled by the finite element model, while the fixel can be modelled by either the spring model or finite element model. To accelerate the computing speed of solving the contact problem, a matrix size reducing method for the finite element model stiffness matrix is proposed, utilising less computer memory. Based on the same idea, a clamping force optimisation method considering the friction effect is presented to achieve the optimal clamping force of a special fixture layout within very few finite element model computing processes. Then, based on these, an integral fixture layout and clamping force optimisation algorithm are built by genetic algorithm. At the end of this article, numerical examples are taken to prove the performance of the methods. The results show that the accelerating method yields sufficient performance, and the optimisation algorithms of both the clamping force plan and the fixture layout design achieve favourable convergence.

Surface Properties and Interaction Forces of Biopolymer-Doped Conductive Polypyrrole Surfaces by Atomic Force Microscopy

Surface properties and electrical charges are critical factors elucidating cell interactions on biomaterial surfaces. The surface potential distribution and the nanoscopic and microscopic surface elasticity of organic polypyrrole-hyaluronic acid (PPy-HA) were studied by atomic force microscopy (AFM) in a fluid environment in order to explain the observed enhancement in the attachment of human adipose stem cells on positively charged PPy-HA films. The electrostatic force between the AFM tip and a charged PPy-HA surface, the tip-sample adhesion force, and elastic moduli were estimated from the AFM force curves, and the data were fitted to electrostatic double-layer and elastic contact models. The surface potential of the charged and dried PPy-HA films was assessed with Kelvin probe force microscopy (KPFM), and the KPFM data were correlated to the fluid AFM data. The surface charge distribution and elasticity were both found to correlate well with the nodular morphology of PPy-HA and to be sensitive to the electrochemical charging conditions. Furthermore, a significant change in the adhesion was detected when the surface was electrochemically charged positive. The results highlight the potential of positively charged PPy-HA as a coating material to enhance the stem cell response in tissue-engineering scaffolds.

Significance of grain and grain boundary characteristics of ultrananocrystalline diamond films and tribological properties

Tribological properties of plasma chemistry dependent ultrananocrystalline diamond (UNCD) films deposited by microwave plasma enhanced chemical vapor deposition system are studied using 100Cr6 steel ball as a sliding counter element. UNCD films synthesized with 1.5% H2 in Ar/CH4 plasma consist of large number of clustered diamond grains with small volume fraction of grain boundaries. Decrease in grain boundary volume fraction resulted in simultaneous decline in trans-polyacetylene (t-PA) chain and sp2 bonded amorphous carbon (a-C) phase fraction. However, UNCD films grown in Ar/CH4 plasma shows formation of UNCD grains with high volume fraction of grain boundaries, large amount of sp2 bonding, a-C and network of t-PA chains. The t-PA passivates the dangling bonds and sp2/a-C phase that externally reduces sliding shear resistance, thereby, resulting in ultra-low friction behavior of UNCD films deposited from Ar/CH4 plasma medium. Load dependent frictional behavior of these films was investigated in present study. These films tested under both low and high normal loadings showed high friction coefficient. Capillary dominated surface interaction and plastic deformation based models were found to be appropriate for describing friction behavior under low and high loads, respectively. However, at specific and intermediate normal loads, both low and ultra-low friction coefficients could be explained through elastic contact model that is accounted by the presence of lubricious phase of sp2 and a-C. In addition, low friction coefficient is also governed by surface passivation mechanism. This mechanism is explained when friction test is carried out in controlled atmospheric condition. Investigating the tribological behavior of these films against steel ball is useful for implementing reliable micro-sliding based device applications.

A model for the contact behaviour of weakly orthotropic viscoelastic materials

Fibre reinforced elastomers behave anisotropically as well as viscoelastically. Yang and Sun (1982) developed an elastic contact model for anisotropic materials which, in the present work, is extended to account for viscoelastic effects. The developed viscoelastic contact model uses the creep compliance function of the material in the direction of indentation. The results of the model agree with experimental results obtained on short fibre reinforced EPDM. Furthermore, a parameter study of the coefficients of the creep compliance function on the real contact area has been made. The results show that, at short time scales, the viscoelastic real area of contact can be significantly smaller than when assuming fully elastic behaviour. At long time scales the results of the viscoelastic contact model equal those of the elastic model of Yang and Sun.

Interfacial Mechanical Properties of $n$ -Alkylsilane Monolayers on Silicon Substrates

The interfacial properties of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n$</tex></formula> -alkylsilane monolayers on silicon were investigated by normal force spectroscopy, lateral force measurements, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Monolayers of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\hbox{CH}_{3}{( \hbox{CH}_{2})}_{n - 1}\hbox{SiCl}_{3})$</tex></formula> with chain lengths <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n = 5$</tex></formula> , 8, 12, and 18 were prepared and NEXAFS spectra were used to compute the dichroic ratio, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$R_{\rm I}$</tex></formula> . As <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n$</tex> </formula> decreased from 18 to 5, the film structures change from ordered <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(R_{ \rm I} = 0.41)$</tex></formula> to disordered <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(R_{\rm I} = 0.12)$</tex></formula> states. Normal force spectroscopy data were analyzed with a modified elastic adhesive contact model to extract Young's modulus, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$E_{\rm film}$</tex></formula> , and the work of adhesion, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$w$</tex></formula> , of the film; <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$E_{\rm film}$</tex></formula> decreased from 1.2 to 0.67 GPa, and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$w$</tex></formula> increased from 48.6 to 60.1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{mJ}\cdot\hbox{m}^{-2}$</tex></formula> as <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n$</tex></formula> decreased from 18 to 5. Lateral force measurements quantified the reduction in friction via an interfacial shear strength, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\tau$</tex> </formula> , and a lateral deformation analog, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\eta$</tex></formula> . Monolayer adsorption reduced <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\tau$</tex></formula> from 3500 MPa for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{SiO}_{2}$</tex></formula> to less than 50 MPa for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n = 12$</tex></formula> and 18 alkylsilanes and was dependent on contact pressure. Conversely, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\eta$</tex></formula> was pressure invariant, with values of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\approx$</tex></formula> 3500 MPa for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n = 5$</tex></formula> and 8 and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\approx$</tex></formula> 1000 MPa for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n = 12$</tex></formula> and 18. <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hfill$</tex> </formula> [2012-0165]

DEM Model of Wheat Grains in Storage Considering the Effect of Moisture Content in Direct Shear Test

Discrete Element Method (DEM) modeling was conducted for predicting strength properties of stored wheat grains in different levels of moisture contents, to extend the knowledge of grain storage beyond current experimental studies in the future. The main features of agricultural and food materials that make them different from mineral materials are strong influence of Moisture Content (MC) on mechanical behavior. Published data on grain and bulk properties of wheat relevant to DEM modeling were reviewed by considering the effect of MC. The shape of grains was modeled by Multi Sphere Method (MSM) and the interaction between grains was represented by linear and nonlinear frictional elastic contact models. The effect of MC was considered in simulation through an innovative procedure. The initial test was performed to choose the best models to perform the simulation. Statistical comparisons with relevant experimental result were conducted on simulation data. It has been proved that the DEM is able to capture the variation of strength properties of wheat with MC of grains. It is observed the simplest shape of grain models would allow us to obtain quite closely prediction of wheat strength parameters. Also the linear elastic contact model has more capability in representing the form of strength properties variation with MC. This suggests that the modeling is a useful tool in the study of mechanical behavior of wheat and other cereal grains during storage processes as well as provision of data necessary in the future design of appropriate machinery and structure.

Role of Exponential and Power Law formulations in Contact Stress

Analyzing contact stress in rails and wheels are very important in mechanical and railway engineering. In this paper, new formulations of the contact stress are presented for two rolling bodies by exponential and power law forms semi- analytically. Innovative elastic wheel-rail contact models and FE Modeling are proposed. The present model assumes the collection of rail and wheel as elastic deformable bodies and needs numerical and novel analytical solutions. Results of this work are close to the Hertz Stress, previous published work and FEM results, in which good agreements are found among the results. So, we can rely on this method and their results. With this approach, suitable results will be achieved. Important novelty of this research is presentation of new analytical formulations in the Power Law (PL) and Exponential Forms (EF) for obtaining contact stress in the rolling bodies.

A COMPRESSIBLE LAGRANGIAN FRAMEWORK FOR MODELING THE FLUID–STRUCTURE INTERACTION IN THE UNDERWATER IMPLOSION OF AN ALUMINUM CYLINDER

We propose a fully Lagrangian monolithic system for the simulation of the underwater implosion of cylindrical aluminum containers. A variationally stabilized form of the Lagrangian shock hydrodynamics is exploited to deal with the ultrahigh compression shock waves that travel in both air and water domains. The aluminum cylinder, which separates the internal atmospheric-pressure air from the external high-pressure water, is modeled by a three-node rotation-free shell element. The cylinder undergoes fast transient deformations, large enough to produce self-contact along it. A novel elastic frictionless contact model is used to detect contact and compute the non-penetrating forces in the discretized domain between the mid-planes of the shell. Mesh quality in the vicinity of the cylinder is guaranteed by regenerating the mesh in the air and water domains when large displacements occur. A monolithic fluid–structure interaction (FSI) system is then solved. Two schemes are tested, implicit using the predictor/multi-corrector Bossak scheme, and explicit, using the forward Euler scheme. The results of the two simulations are compared with experimental data.

Self - Excited Vibration Arising from Tribology - Dynamic Interaction

An elastic contact model of a pad/disc system is developed which can give some explanations on self-excited vibration induced during continuous dry friction. Based on this model, it is possible to show that establishement and sustainability of self-excited vibration in disc brake system are linked to the interface properties as well as the physical properties of the disc brake components. It is shown using theoretical and experimental analysis that the regime of self-excited vibrations can be triggered when approaching any of the modified resonant frequencies of either pad or disc system. These frequencies are dependant upon contact stiffness value during self-excitation regime. Experimental investigations have also shown that contact stiffness during sustained self-excitation may increase monotonically upto 35% of its initial value due to continuous wear and sliding. One of the main findings in this work, is that the resonance of any of the system frequencies is linked mainly to the established value of contact stiffness due to wear, load and speed. Also, for each self-excited regime, there is a limiting frequency after which the self-excited regime either sustains at this frequency or change to another regime. Sudden changing of load or speed at this limiting frequency may cause the transition from one regime of self-excitation to another.

Semi Analytical Solutions of Elastic-Plastic Contact between Two Surfaces

A problem related to widespread accepting GW model is that there is no analytical expression for the parameters of interest at the interface. The probability density of asperity heights for closely 90% of engineering surfaces tends to be Gaussian. Taking GW model simple exponential probability density to approximate the Gaussian one is not explicit for some conditions. Many practical closed form analytical expressions were derived for the contact load, contact area and contact spot number for GW elastic contact model.

Permeability of Fractured Media under Confining Pressure: A Simplified Model

An elastic-plastic model was developed to describe the deformation and permeability of fractured rocks under confining pressure. Elastic and elastic-plastic contact models were used in the model to analyze the deformations of a representative element containing a single fracture under various confining pressures. The present work utilized elements from contact mechanics and viscous flows to derive an expression for permeability of fractured rocks under a range of confining pressures. The elastic-plastic model predictions for coal permeability under loading and unloading confining pressure were compared with the experimental data. The new model exhibited trends similar to the data for the range of confining pressures used in the experiments. The model can be used to predict the permeability of other fractured media with different material properties.

Discrete Element Simulation of Bending Beam Rheometer Tests for Asphalt Binder

This study presents a discrete element modeling approach to bending beam rheometer (BBR) tests for control and nanomaterial modified asphalt binders. In the discrete element model, a linear elastic contact model was used to simulate force displacement relations among adjacent elements, while a slip model and two bonding models were used to simulate the strength properties at each contact. The two bonding models include the contact bond model and parallel bond model, which were utilized simultaneously. In order to compute contributions of the two bonding models, a coefficient of α was introduced. A compressive study found that discrete element simulation results were acceptable when α was close to 1.0. Since asphalt materials exhibit time dependent behaviors and cannot be directly simulated with an elastic model, the time domain was represented by a few key time points that were simulated individually to account for the time dependency of asphalt materials. In order to obtain inputs for the discrete element model, BBR testing results of control and nanomaterial modified asphalt binder were fitted with the five-parameter Generalized Maxwell model, which represents the laboratory testing data well. A careful analysis of the discrete element simulation results indicates that 1) the discrete element model in this study can simulate a BBR test of asphalt binder, and 2) the stress and displacement distributions within the beam model can be virtualized and demonstrated.

Practical Expressions of Elastoplastic Contact between Rough Surfaces

A difficulty associated with widespread acceptance GW model is that there is no analytic expression for the parameters of interest at the interface. The probability density of asperity heights for nearly 90% of engineering surfaces tends to be Gaussian. Applying GW model simple exponential probability density to approximate the Gaussian one is not explicit for some cases. The evaluation of parabolic cylinder function was carried out by employing software Maple. Some practical closed form analytic expressions were derived for the contact load, contact area and contact spot number for both GW elastic contact model and CEB elastoplastic contact model by the generalized exponential probability density fitting the Gaussian one. Some main digital features of the two contacting rough surfaces were given in the tabulating form.

Distributed Contact Model for Indeterminate Grasp Force Analysis towards Heavy-Duty Manipulation

This paper presents a new contact model to describe the contact constraints for heavy-duty manipulation. This contact model considers the contact area as a surface, which can translate three-dimension forces and torques. The contact deformations which are resulting from the large contact forces cannot be ignored. Distributed linear elastic contact model is adopted to describe the relationship between contact force and contact deformation. A case study is carried out in this paper based on this contact model for the grasp force analysis of heavy-duty manipulators.

Mechanical Model’s of Conical Valve’s Spool-Seat Contact Pairs Based on the Hertz Theory

Conical valve’s spool-seat contact pairs is consisted of two surfaces of different curvature radius, and form closed circular contact line. It can be considered as the model of elastic hemi-space contact by the Hertz Theory. The numerical simulation and experiment show: The model is suitable for the analysis of conical valve’s spool-seat contact pairs. The conical valve’ sealing performance is very credible and its performance is very good. Maximum stress is smaller than allowable stress of the seat’s material. The consistency of experiment and simulation proves the validity and practicability of the hemi-space contact model of the valve’s spool-seat contact pairs.

Wheel-Rail Multi-Point Contact Method for Railway Turnouts

A calculation method of wheel-rail multi-point contact based on the elastic contact model is introduced. Moreover, the simulation calculation of vehicles passing through branch lines of No.18 turnouts is carried out. The result showed that the acute change of wheel-rail normal force caused by the transfers of wheel-rail contact point between two rails can be avoid by wheel-rail multi-point contact method, and the transfers of wheel-rail normal force between two rails is smoother. The validity of wheel-rail multi-point contact method is verified.

Structure–property relationships for methyl-terminated alkyl self-assembled monolayers

Abstract Structure–property relationships for methyl-terminated alkyl self-assembled monolayers (SAMs) are developed using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and atomic force microscopy (AFM). NEXAFS C K-edge spectra are used to compute the dichroic ratio, which provides a quantitative measure of the molecular structure. AFM data are analyzed with an elastic adhesive contact model, modified by a first-order elastic perturbation method to include substrate effects, to extract the monolayer mechanical properties. Using this approach, the measured mechanical properties are not influenced by the substrate, which allows universal structure–property relationships to be developed for methyl-terminated alkyl SAMs.

Adhesive contact in the context of multi-asperity interaction

In order to analyse the scale effect of roughness and adhesion in contact between solids, we introduce in this work a 3D model of elastic contact combined with the adhesive theory. The model of roughness with different fractal dimension is introduced to study the adhesive contact in the elastic state at different length scales. The results of this study show that the scale of roughness modifies the distribution law of the contact pressure and stiffness. The introduction of the adhesive force and fractal property of roughness clearly shows the combined influence of roughness scale and adhesive force on contact stiffness.