Adhesive Contact Model Research Articles

Dynamic Simulation and Parameter Analysis of Contact Mechanics for Mimicking Geckos’ Foot Setae Array

According to the dynamic characteristics of the adhesion desorption process between gecko-like polyurethane setae and the contact surface, the microcontact principle of an elastic sphere and plane is established based on the Johnson–Kendall–Robert model. On this basis, combined with the cantilever beam model, microscale adhesive contact models in the case of a single and an array of setae are obtained. The contact process is numerically simulated and verified by the adhesion desorption test. After that, the effects of external preload, the elastic modulus of setae material, the surface energy, and the surface roughness on the contact force and depth during the dynamic contact process of setae are studied. The results show that the error between the simulation and test is 15.9%, and the simulation model could reflect the real contact procedure. With the increase in preload, the push-off force of the setae array would grow and remain basically constant after reaching saturation. Increasing the elastic modulus of setae material would reduce the contact depth, but have little effect on the maximum push-off force; with the increase in the surface energy of the contact object, both the push-off force between the objects and the contact depth during desorption would increase. With the increase in wall roughness, the push-off force curve of the setae array becomes smoother, but the maximum push-off force would decrease. By exploring the dynamic mechanical characteristics of the micro angle of setae, the corresponding theoretical basis is provided for the numerical simulation of the adsorption force of macro materials.

An adhesive Gurtin-Murdoch surface hydrodynamics theory of moving contact line and modeling of droplet wettability on soft substrates

In this work, by extending the Gurtin-Murdoch surface elasticity theory to a surface hydrodynamics theory, we developed an adhesive surface hydrodynamics theory of moving contact line (MCL), which is essentially a hybrid theory of a deformable fluid interface membrane model and an adhesive moving contact line model. The proposed adhesive surface hydrodynamics theory combines a surface Newtonian hydrodynamics theory with a coarse-grained adhesive contact model, so that it couples molecular scale adhesive interaction with the motion of macroscale fluid flow. By doing so, we achieved the following three goals: (1) the intermolecular adhesive force (van der Waals force) separates and levitates the liquid droplet from the supporting elastic substrate, so that the proposed adhesive surface hydrodynamics theory of moving contact line avoids the shear stress singularity at the moving contact line front; (2) the adhesive surface hydrodynamics theory provides a framework to relate the mesoscale adhesive traction to the balance of surface tension and slip boundary condition so that it allows the difference of the surface energies and surface stresses propelling droplet spreading naturally, and (3) the proposed adhesive surface hydrodynamics theory of moving contact line takes into account the viscous friction force during the droplet dynamic contact process.To validate the proposed moving contact line theory of the adhesive surface hydrodynamics, we applied it to simulate liquid droplet spreading over various soft elastic substrate and capillary motion. The numerical simulation results obtained by using the present method are compared well with the experimental and molecular dynamics results reported in the literature.

Viscoelastic adhesive contact between a sphere and a two-dimensional nano-wavy surface

For contact involving viscoelastic materials with nanoscale roughness surfaces, both the viscoelasticity and the adhesive interaction play significant roles, and an in-depth understanding of this viscoelastic adhesive contact behavior is essential. This paper establishes a numerical viscoelastic adhesive contact model between a smooth sphere and a two-dimensional nano-wavy surface based on the Derjaguin approximation and elastic–viscoelastic correspondence principle, and the wavy surface is considered by introducing the height distribution into the surface gap equation. Influences of approach speed, contact position and wavy surface parameters on the viscoelastic adhesive contact behavior are quantitatively analyzed. It is shown that the viscoelasticity can be ignored at large approach speed, while when the speed is small, viscoelasticity significantly increases the maximum contact area and pull-off force. Variations of maximum contact area and pull-off force with contact position or wavy surface parameters are not completely consistent, where the contact positions with relatively large maximum contact area may exhibit small pull-off force. The pull-off force changes monotonically with increasing wavy parameter, while the maximum contact area changes nonmonotonically. This study can offer guidance for the application of wavy surface in viscoelastic adhesion regulation, and provide basis for the surface design and parameter determination of viscoelastic materials.

Mechanical models for the adhesive friction of nano- and micro-particles

Friction is common in human’s daily life and production that may be either detrimental or beneficial. The friction law at microscale or even smaller scales nevertheless is lacking. In the present work, on the basis of JKR model for adhesive contact and by analogy with the emission of dislocations at a crack tip, the critical tangential force has been obtained for the onset of sliding. When the tangential force in adhesive contact induces a torque at the interface, a mechanical model for rolling adhesion has been presented on the basis of the assumption that it is governed by adhesion hysteresis. The critical torque and tangential force for rolling onset have been determined, which may be taken as measure for the rolling friction force. Comparisons with relevant experiments and models have been made to support the present concept. A kind of friction duality in small sized particles can thus be predicted.

Adhesive Boundary Element Method Using Virtual Crack Closure Technique

In this study, a new adhesive contact model is built upon a boundary element method (BEM) model developed by Pohrt and Popov (2015). The strain energy release rate (SERR) on the edge of the bonding interface is evaluated using Virtual Crack Closure Technique (VCCT) which is shown to have better accuracy and weaker mesh-size dependency than the closed-form SERR formula derived by Pohrt and Popov. A composite delamination criterion is proposed for crack nucleation and propagation. Numerical results predicted by the present model are in good agreement with the analytical solutions of two classic problems, namely, the axisymmetric parabolic contact and the sinusoidal waviness contact in the plane strain condition. The model of Pohrt and Popov can achieve a similar accuracy for the axisymmetric parabolic contact where the mesh grid is non-conforming to the crack front. Once the conforming mesh grid is used, the accuracy of their model is significantly deteriorated, especially at high work of adhesion and high mesh density. In both BEM models, however, the crack nucleation is found to be mesh-dependent which may be solved by introducing an upper limit for the tensile normal traction.

Investigations on the Adhesive Contact Behaviors between a Viscoelastic Stamp and a Transferred Element in Microtransfer Printing

Microtransfer printing is a sophisticated technique for the heterogeneous integration of separately fabricated micro/nano-elements into functional systems by virtue of an elastomeric stamp. One important factor influencing the capability of this technique depends on the adhesion between the viscoelastic stamp and the transferred element. To provide theoretical guidance for the control of adhesion in the transfer printing process, a finite element model for the viscoelastic adhesive contact between a polydimethylsiloxane (PDMS) stamp and a spherical transferred element was established, in which the adhesive interaction was modeled by the Lennard-Jones surface force law. Effects of the unloading velocity, preload, and thermodynamic work of adhesion on the adhesion strength, characterized by the pull-off force, were examined for a loading-dwelling-unloading history. Simulation results showed that the unloading path deviated from the loading path due to the viscoelastic property of the PDMS stamp. The pull-off force increased with the unloading velocity, and the increasing ratio was large at first and then became low. Furthermore, the influence of the preload on increasing the pull-off force was more significant under larger unloading velocity than that under smaller unloading velocity. In addition, the pull-off force increased remarkably with the thermodynamic work of adhesion at a fixed maximum approach.

A mathematical model for evaluating adhesion contact of an elastomeric seal-on-seal structure

For low impact docking systems (LIDS) developing for rendezvous and docking of spacecraft, the main interface docking seal (MID seal) is one of the key components, and its seal and adhesion performances are crucial for mating LIDS-adapted spacecrafts. An elastomeric seal-on-seal structure is one of the mainstream designs of the MID seal and is generally made of silicone rubbers that can adapt to complex space environments. For the MID seal of seal-on-seal structure, the adhesion performance has been confirmed to have significant effects on the separation reliability of mating spacecrafts. By analyzing the sealing and adhesive mechanisms of the MID seal that is an elastomeric seal-on-seal structure, an adhesive contact model of single rough peak is derived on the grounds of Johnson, Kendall and Roberts (JKR) theory. Utilizing the asperities model and the adhesive contact model of single rough peak, an adhesive contact model of the elastomeric seal-on-seal structure is further proposed. The experiments were performed to verify the adhesion model, and the satisfied consistencies were presented in the comparative studies of the experimental data and the calculated data. Based on the proposed mathematical model, the simulation analyses were performed to disclosure adhesive performances of the MID seal. The influence rules of some parameters on adhesive performances were presented, including material parameters, geometric parameters, and parameters of surface morphology. The research findings are proven to be favorable for the design, machining, assemblage and actual service of the MID seal, and can be also used for other elastomeric seals.

Controlling the adhesive pull-off force via the change of contact geometry

A first-order asymptotic analysis of the Griffith energy balance in the Johnson-Kendall-Roberts model of adhesive contact under non-symmetric perturbation of the contact geometry is presented. The pull-off force is evaluated in explicit form. A particular case of adhesive contact between a relatively stiff sphere and an elastic half-space is considered under the assumption that the sphere geometry is changed by the application of an arbitrary lateral normal surface loading. The effect of the sphere Poisson's ratio on controlling the adhesive pull-off force is considered. This article is part of a discussion meeting issue 'A cracking approach to inventing new tough materials: fracture stranger than friction'.

A generalized Maugis-Dugdale solution for adhesion of power-law graded elastic materials

The present paper provides a generalized Maugis-Dugdale analytical model of adhesive contact between an axisymmetric rigid punch of arbitrary shape and a graded elastic half-space with Young's modulus varying with depth according to a power-law. Based on the principle of superposition and an equivalent indentation method to solve an axisymmetric external crack problem, a series of closed-form solutions are derived for power-law punch profiles which reduce to the existing solutions for homogeneous isotropic materials and for paraboloidal geometries as special cases. A complete transition between the JKR and DMT type models is captured through a generalized Tabor parameter. Finite element analysis is performed to validate the obtained analytical solutions and examine the effect of interaction potential form on the pull-off force. Further important findings from this study include: (1) The pull-off force for non-paraboloidal punch profiles is found to be largely dependent on the form of the interaction potential, which is distinct from the familiar paraboloid case; (2) For the so-called Gibson solid, the independence of the pull-off force on the Tabor parameter is valid only for the paraboloidal punch profiles; (3) A three-dimensional adhesion map has been constructed to aid the appropriate choice of different contact models involving power-law graded elastic materials.

On the controlled adhesive contact and electrical performance of vertical contact-separation mode triboelectric nanogenerators with micro-grooved surfaces

The triboelectric nanogenerator (TENG) is a new energy technology to convert mechanical energy into electricity based on contact electrification and electrostatic induction. Altering the alignment state of TENG with two micro-grooved surfaces is a simple way to achieve controlled electrical output. To guide the output regulation of TENG with micro-grooved surfaces and avoid causing interface separation failure, comprehensive analysis based on adhesive contact mechanics is essential. In this paper, the relation between charge density and contact stress is measured. Numerical adhesive contact and electrical models of TENG with grooved surfaces considering the charge density-contact stress relation and material properties are established to quantitatively analyze the effects of alignment angle, applied pressure and loading time on adhesive contact and electrical performance. It is shown that there exists an exponential relation between charge density and contact stress. Alignment angle of grooved surfaces in TENG significantly affects the adhesive contact and electrical performance. Under low and high applied pressures, the variations of output voltage with alignment angle are 630.5% and 301.1%, respectively, while same pull-off force variation of 532.6% is exhibited. Large pull-off force usually comes along with high voltage output, except for alignment angle of 10°, at which relative high voltage and low pull-off force are exhibited. Strategies for the output regulation of TENG with grooved surfaces without causing interface separation difficulty are presented.

Systematic design optimization of grabs considering bulk cargo variability

Ship unloader grabs are usually designed using the manufacturer’s in-house knowledge based on a traditional physical prototyping approach. The grab performance depends greatly on the properties of the bulk material being handled. By considering the bulk cargo variability in the design process, the grab performance can be improved significantly. A multi-objective simulation-based optimization framework is therefore established to include bulk cargo variability in the design process of grabs. The primary objective is to reach a maximized and consistent performance in handling a variety of iron ore cargoes. First, a range of bulk materials is created by varying levels of cohesive forces and plasticity in the elasto-plastic adhesive DEM contact model. The sensitivity analysis of the grabbing process to the bulk variability allowed three classes of iron ore materials to be selected that have significant influence on the product performance. Second, 25 different grab designs are generated using a random sampling method, Latin Hypercube Design, to be assessed as to their handling of the three classes of iron ore materials. Of this range of grab designs, optimal solutions are found using surrogate modelling-based optimization and the NSGA-II genetic algorithm. The optimization outcome is verified by comparing predictions of the optimization algorithm and results of DEM-MBD co-simulation. The established optimization framework offers a straightforward and reliable tool for designing grabs and other similar equipment.

A validated co-simulation of grab and moist iron ore cargo: Replicating the cohesive and stress-history dependent behaviour of bulk solids

The traditional design approach of grabs and other bulk handling equipment consists of manufacturing and testing physical prototypes. A novel design approach is to use a co-simulation of MultiBody Dynamics (MBD) and Discrete Element Method (DEM), in which the virtual prototype of a new concept interacts with bulk solids. Therefore, this study develops and validates a full-scale co-simulation that models the grabbing process of cohesive and stress-history dependent iron ore. First, by executing in-situ measurements during the unloading of a vessel, grab-relevant bulk properties of the cargo, such as penetration resistance, are determined. Second, full-scale grabbing experiments are conducted in the cargo hold, which allows the process to be recorded in realistic operational conditions. Third, full-scale co-simulation is set up using the material model that has been calibrated based on an elasto-plastic adhesive contact model. Fourth, the co-simulation is validated by comparing its predictions to experimental data from various aspects, such as the force in cables and the torque in winches. The validated co-simulation proves that the stress-dependent behaviour of cohesive cargo as it interacts with the grab could be captured successfully. Valuable information such as a grab’s kinematics and dynamics, as well as the porosity distribution of collected bulk solids, can be extracted from the simulation, supporting engineers to enhance the design and operation of equipment.

High Temperature Sealing of Silicon Oxide/Oxide Bonding Interfaces

High temperature (>1000°C) annealing is necessary to completely close a bonding interface. Using interface synchrotron high energy X-ray reflectivity, we have investigated the sealing behavior of silicon thermal oxide/ silicon thermal oxide interfaces, with different surface roughnesses. Interface X-ray reflection is able to measure accurately the width and the depth of the electron density gap across a bonding interface with a sub-nanometer accuracy, whose product is a good marker of the interface closure.The uncomplete closure of the interface is the result of a balance between attractive and repulsive force between the two bonded solids. These two forces depend on the statistics of asperity and on the mechanical behavior of individual asperity [1]. For attraction, at high temperatures, a key factor is the gain of contact surface area which reduces the surface energy due to uncontacted zones. Repulsion is due to compression of contacting asperities. At high temperature, both elastic and plastic behavior can be expected, with some yield of the asperities compressed above the elastic limit.In the elastic solid regime, we have shown that equilibrium distance of rough silicon oxide to rough siliconoxide bonding can be predicted using standard adhesive contact models such as Johnson-Kendall Roberts (JKR) or Deryagin-Müller-Toporov (DMT) with Gaussian roughness statistics[2]. The equilibrium distance is dependent and sensitive to the Fuller Tabor adhesion parameter, θ=E σ3/2 R1/2 / 2γR where E is the Young modulus, σ the mean roughness, R the radius of the asperity, while 2γ is the adhesion energy at contact points. When θ>>1 , the equilibrium distance is large and the contact area is small (unclosed interface) . When θ<<1 the adhesion energy is strong and the system will have a strong tendency to seal.In the plastic regime, both repulsive and attractive curves share the same dependence with distance so that the system is expected to be unstable, between a weakly evolving unsealed interface and a fully sealed interface when the asperity pressure stress is above the elastic limit.The kinetics of the sealing can also be modeled in this case, monitoring the interfacial gap as a function of time (Figure). It is found that the sealing characteristic time scales directly with the silicon oxide viscosity (Figure insert). This dependence can be understood using the plastic asperity contact model, assuming the fluid part flows according to the Stokes equation around asperities.

Deterministic normal contact of rough surfaces with adhesion using a surface integral method.

The fundamental problem of adhesion in the presence of surface roughness and its effect on the prediction of friction has been a hot topic for decades in numerous areas of science and engineering, attracting even more attention in recent years in areas such as geotechnics and tectonics, nanotechnology, high-value manufacturing and biomechanics. In this paper a new model for deterministic calculation of the contact mechanics for rough surfaces in the presence of adhesion is presented. The contact solver is an in-house boundary element method that incorporates fast Fourier transform for numerical efficiency. The adhesive contact model considers full Lennard-Jones potentials and surface integration at the asperity level and is validated against models in the literature. Finally, the effect of surface roughness on the adhesion between surfaces was studied, and it was shown that the root mean square gradient of surface roughness can change the adhesive pressures irrespective of the root mean square surface roughness. We have tested two adhesion parameters based on Johnson's modified criteria and Ciavarella's model. We showed that Civarella's model introduces the most reasonable criteria suggesting that the RMS roughness and large wavelength of surfaces roughness are the important parameters of adhesion between rough surfaces.

Investigations of Adhesion under Different Slider-Lube/Disk Contact States at the Head–Disk Interface

Adhesion is the key factor influencing the failure of the hard disk drive operating under ultra-low flying height. In order to mitigate the negative effects of adhesion at the head–disk interface (HDI) and promote further development of the thermal flying height control (TFC) technology, an adhesive contact model based on the Lifshitz theory accounting for the thermal protrusion (TP) geometry of TFC slider, the layered structures of the head and disk, and the operation states of the slider was proposed to investigate the static contact characteristics at the HDI. The simulation results demonstrated the undesirable unstable regions during the transitions between different operation states and the necessity of applying TFC technology. The reduction in the head–media spacing (HMS) was found to be achieved by properly increasing the TP height, decreasing the thickness of the lubricant layer or the thickness of the diamond–like carbon (DLC) layer during the flying state or the TP–lube contact state. At the TP–DLC contact regime, the attractive interaction was stronger than other states, and the strong repulsive interaction made the HMS difficult to be further reduced through the increase in the TP height or the decrease in the lubricant thickness.

Numerical analysis and simulation of an adhesive contact problem with damage and long memory

<p style='text-indent:20px;'>This paper studies an adhesive contact model which also takes into account the damage and long memory term. The deformable body is composed of a viscoelastic material and the process is taken as quasistatic. The damage of the material caused by the compression or the tension is involved in the constitutive law and the damage function is modelled through a nonlinear parabolic inclusion. Meanwhile, the adhesion process is modelled by a bonding field on the contact surface while the contact is described by a nonmonotone normal compliance condition. The variational formulation of the model is governed by a coupled system which consists of a history-dependent hemivariational inequality for the displacement field, a nonlinear parabolic variational inequality for the damage field and an ordinary differential equation for the adhesion field. We first consider a fully discrete scheme of this system and then focus on deriving error estimates for numerical solutions. Under appropriate solution regularity assumptions, an optimal order error estimate is derived. At the end of this paper, {we report some two-dimensional numerical simulation results} for the contact problem in order to provide numerical evidence of the theoretical results.

Nonlinear ultrasonic nondestructive detection and modelling of kissing defects in high voltage composite insulators

This paper investigates a feasible nondestructive method for micron-sized kissing defects in the composite insulators, which are commonly utilized in ultra-high voltage (UHV) power grid. In this paper, cylindrical adhesive contact model was established to describe the obvious distortion of ultrasound propagation caused by kissing defects on the polymeric interface of composite insulators. The ultrasonic nonlinearity parameter was measured for each of the kissing defective and intact specimens. Moreover, the theoretical analysis has been proved by the test results on the kissing defective samples, which indicates that the 0.7 μm kissing defects existed in the composite insulators could be successfully recognized by the nonlinear ultrasonic methods and the nonlinearity is reduced by interface roughness.

A hybrid particle-geometric scaling approach for elasto-plastic adhesive DEM contact models

The computation time of Discrete Element Method (DEM) simulations increases exponentially when particle size is reduced or the number of particles increased. This critical challenge limits the use of DEM simulation for industrial applications, such as powder flow in silos. Scaling techniques can offer a solution to reduce computation time. In this paper, we have developed a hybrid particle-geometric scaling approach with a focus on Elasto-Plastic Adhesive contact models. It established relationships between particle scaling factors and DEM contact input parameters. The isolated effects of varying particle size and geometric dimensions on bulk properties were also evaluated using uniaxial consolidation, static angle of repose, and ring shear tests. This paper shows how the particle scaling can be applied together with geometric scaling to incorporate two important aspects of bulk materials, their Elasto-Plastic behaviour and their cohesive forces.

An Equivalent Indentation Method for the External Crack with a Dugdale Cohesive Zone

An equivalent indentation method is developed for the external crack problem with a Dugdale cohesive zone in the both axisymmetric and two-dimensional (2D) cases. This is achieved based on the principle of superposition by decomposing the original problem into two simple boundary value problems, with one considering action of a constant traction within the cohesive zone, and the other corresponding to indentation by a rigid concave punch. Closed-form expressions are derived for the distributions of displacement and traction on the crack interface, which are consistent with the classical results in fracture mechanics. Results show that the interfacial traction distributions in the axisymmetric and 2D cases share the similar mathematical forms except for different coordinate parameters. Finite element analysis is performed to validate the obtained analytical solutions. The proposed method relies solely on a few contact solutions on the surface irrespective of a general elasticity solution in the whole body, and it may find applications in the external crack analysis and adhesive contact model involving functionally graded elastic solids or piezoelectric materials.

Phenomenological Model of Surface Forces and Contact of Rough Surfaces

Analytical dependences are obtained for calculating main characteristics of discrete contact of technical surfaces—approach of surfaces (contact deformation) and real contact area—taking into account adhesive forces. Model of discrete adhesive contact of rough bodies is based on phenomenological model of surface forces (by A.V. Kovalev, I.N. Kovaleva and N.K. Myshkin) and well-known model of contact of rough surfaces by N. B. Demkin.