Theoretical Contact Mechanics Research Articles

A Theoretical Contact Mechanics Model of Machine Joint Interfaces Based on Fractal Theory

To obtain more accurate contact mechanics model of joint interfaces theoretically, A theoretical contact mechanics model of joint interfaces based on fractal theory was proposed. An improved 3D WM fractal function was used to characterize the contact surface, contact load and contact area equations of asperities in elastoplastic deformation regime were established, solutions for the relationships of area-displacement and force-displacement in the elastoplastic deformation regime was done based on Hertz contact theory and fractal theory, and the present model was proven to be effective by comparing the present model to other four classical contact models and test data. Furthermore, simulations and numerical calculation results reveal nonlinear relation between the influence factors and the contact area.

A GPU-based discrete element modeling code and its application in die filling

In this study, parallelization of a Discrete Element Method (DEM) code titled Trubal was carried out based on the CPU–GPU heterogeneous architecture where both two- and three-dimensional cases were assessed. In Trubal, the particle–particle/wall interaction rules are governed by the theoretical contact mechanics which enable the direct use of real physical material properties in the calculation. We reconstructed Trubal in two steps: reconstruction of the static storage structure; essential parallelism on the relative newer version code. Numerical simulations were implemented to present the benefits of this research. Firstly, two simulations of die filling with a moving shoe involving 6000 and 60,000 two-dimensional particles were conducted under (i) NVIDIA Tesla C2050 card together with Intel Core-Duo 2.93GHz CPU and (ii) NVIDIA Tesla K40c card along with Intel Xeon 3.00GHz CPU. Average speedups of (i) 4.69 and 12.78 as well as (ii) 6.52 and 18.60 in computational time were obtained, respectively. Then, a simulation of die filling with a stationary shoe containing 20,000 three-dimensional particles was carried out under the same conditions where average speedups of (i) 12.90 and (ii) 19.66 in computational time were obtained, respectively. It is shown that the final version parallel code gave a substantial acceleration on the original Trubal.

Numerical investigation on the role of discrete element method in combined LBM–IBM–DEM modeling

Particle collisions play a very important role in determining the fluid–particle multiphase flow, and thus it is crucial to treat the particle–particle interaction using a felicitous method in numerical simulations. A novel combined lattice Boltzmann method (LBM)–immersed boundary method (IBM)–discrete element method (DEM) scheme is presented in this study with its application to model the sedimentation of 2D circular particles in incompressible Newtonian flows. The hydrodynamic model of the incompressible Newtonian flow is based on the Bhatnagar–Gross–Krook LBM, and a momentum exchange-based IBM is adopted to calculate the fluid–solid interaction force. The kinematics and trajectory of the discrete particles are evaluated by DEM, in which the particle–particle interaction rules are governed by theoretical contact mechanics to enable the direct use of real particle properties. This eliminates the need of artificial parameters and also improves the reliability of the numerical results. By using a more accurate and physical description of particle interaction, a ‘safe zone’ or threshold is also no longer required. Case studies of single particle settling in a cavity, and two particles settling in a channel were carried out, the velocity characteristics of the particle during settling and near the bottom were examined. A numerical example of sedimentation involving 504 particles was finally presented to demonstrate the capability of the combined scheme.

A hybrid DEM/CFD approach for solid-liquid flows

A hybrid scheme coupling the discrete element method (DEM) with the computational fluid dynamics (CFD) is developed to model solid-liquid flows. Instead of solving the pressure Poisson equation, we use the compressible volume-averaged continuity and momentum equations with an isothermal stiff equation of state for the liquid phase in our CFD scheme. The motion of the solid phase is obtained by using the DEM, in which the particle-particle and particle-wall interactions are modelled by using the theoretical contact mechanics. The two phases are coupled through the Newton's third law of motion. To verify the proposed method, the sedimentation of a single spherical particle is simulated in water, and the results are compared with experimental results reported in the literature. In addition, the drafting, kissing, and tumbling (DKT) phenomenon between two particles in a liquid is modelled and reasonable results are obtained. Finally, the numerical simulation of the density-driven segregation of a binary particulate suspension involving 10 000 particles in a closed container is conducted to show that the presented method is potentially powerful to simulate real particulate flows with large number of moving particles.

Numerical Modeling of Liquid-Particle Flows by Combining SPH and DEM

We present a numerical method combining the smoothed particle hydrodynamics (SPH) and discrete element method (DEM) to model liquid flows in the presence of solid particles. This approach does not require a computational mesh, nor does it rely on empirical models to couple the liquid and solid phases. In the present method, the SPH is applied to simulate the liquid flows, and the DEM together with interaction laws based on the theoretical contact mechanics is introduced into the SPH method to calculate the effects of contact between solid particles and between solid particles and walls. The introduced DEM model is verified by experimental analyses for the collapse of six solid cylinder layers. The proposed DEM–SPH model is then applied to simulate the free falling of a rigid sphere in water with a free surface.

Contact mechanics approach to determine contact surface area between bipolar plates and current collector in proton exchange membrane fuel cells

A significant portion of the power loss in a fuel cell stack can be attributed to the contact resistance between the gas diffusion layer/bipolar plate and the current collector/bipolar plate interfaces, especially when an oxide layer is formed on a stainless steel bipolar plate. Researchers have already studied methods to decrease the contact resistance between fuel cell components, but never has a theoretical contact mechanics model been applied to contact resistance problems in fuel cells. Therefore, the purpose of this research is to utilize a theoretical contact mechanics model in order to study real contact area in fuel cell components as a function of surface roughness, material properties, and clamping force. Specifically, the effects of bipolar plate surface roughness, coating thickness of the gold plating on the current collector, and the clamping force on real contact area have been studied. It was found that smoother materials, thicker gold coating, and higher clamping force resulted in a higher real percentage contact area.

Fully-3D DEM simulation of fluidised bed spray granulation using an exploratory surface energy-based spray zone concept

An exploratory spray-zone modelling concept which uses energy of adhesion at the contact of particles has been incorporated in a fully-3D Discrete Element Method (DEM)-based granular dynamics fluidised bed model and applied to fluidised bed spray granulation. In this ‘proof-of-concept’ spray model, simple functional relationships are used to notionally model surface energy pick-up by particles traversing the spray zone and an increase in energy of adhesion at drying interparticle bonds. The formation and breakage of interparticle bonds is thus governed by theoretical contact mechanics. Early-stage granulation simulation results provide qualitatively realistic granule size evolution trends.

Biological and artificial attachment devices: Lessons for materials scientists from flies and geckos

In insects, spiders, and geckos, adhesion to surfaces is mediated by finely structured contact elements. We have studied the structure and function of these elements on the micro and nano level by microscopical and nanomechanical techniques. Local mechanical properties and adhesion forces are measured by novel test methods and compared with predictions based on theoretical contact mechanics. Structure, size and shape of the contact elements are found to play important roles; in particular the principle of “contact splitting” has been identified: finer contact elements (down to sub-micron level) produce larger contact forces in heavier animals. The insight gained in studying biological systems can be transferred to the development of optimized artificial attachment devices. From our findings, the desired mechanical parameters of attachment structures can conveniently be delineated in newly developed adhesion design maps. Based on these investigations, a clearer strategy for producing optimum bio-inspired attachment structures is beginning to emerge. This paper gives an overview of our recent work in theory and experimental measurement of such adhesion phenomena.

Discrete particle-continuum fluid modelling of gas–solid fluidised beds

This paper describes a fluidised bed model developed from the DEM-based Aston granular dynamics code in which the solid–solid interaction rules are based on theoretical contact mechanics thereby enabling particles to be directly specified by material properties such as friction, elasticity, elasto-plasticity and auto-adhesion. For the gas phase which is treated as a continuum, the equations of motion are evaluated with a Navier–Stokes solver originally developed for a two-fluid model. The coupling of the discrete particulate phase and the continuous fluid phase equations is an important attribute of such Eulerian–Lagrangian models and two forms of the coupling terms have been examined: one employing the pressure gradient force (PGF model) and the other a buoyancy force based on the fluid density (FDB model). Uniform fluidisation simulations for a superficial gas velocity of 2.5 m s −1 and bed pressure drop-superficial gas velocity simulations for gas flows from 0.3 to 3.0 m s −1 have been carried out in a pseudo-2D bed of 2400 4 mm -diameter spherical particles using the two model formulations. The two formulations yielded minor differences in qualitative fluidisation behaviour with a gas flow of 2.5 m s −1 . However, there were significant differences in the pressure drop-superficial gas velocity profiles in the fixed bed regime and corresponding significant differences in the prediction of minimum fluidisation velocity. The PGF model showed the best agreement with pressure drop-superficial gas velocity trends and minimum fluidisation velocities predicted by empirical correlations. Two new methods for determining the minimum fluidisation velocity have been introduced and found to give predictions in good agreement with those obtained from pressure drop-superficial gas velocity profiles and empirical correlations.

Towards a micromechanical understanding of biological surface devices

Abstract Interesting analogies exist between the science of materials in small dimensions and the mechanics of biological systems. In both instances, mechanical forces and displacements are measured and the results are interpreted in the light of theoretical expectations. The approaches are, however, fundamentally different: consideration of structural diversity and global performance in biology contrasts with more quantitative evaluation of local properties and emphasis on micromechanical modeling in materials science. We report on two examples of our first efforts to combine these approaches in the investigation of biological attachment systems in insects. Nanoindentation studies were performed on the head–neck joint and the wing-locking mechanism in a beetle, and the foot–substrate adhesion in flies and geckos has been examined from the perspective of theoretical contact mechanics. By aiming at a quantitative understanding of the interrelationship between local materaal properties and complex structure...

An improved contact model for ball mill simulation by the discrete element method

The discrete element method (DEM) has emerged as a powerful tool for simulating discrete particle systems. It has wide application in the field of mining and mineral processing where it can be used as a design and diagnostic tool for industrial size tumbling mills. The simulation of tumbling mills involves the calculation of contact forces between the colliding balls in order to predict the enmasse charge motion, ball wear, power draw, collision spectra, etc. These calculations are made by modeling the contact by a set of spring, dashpot and slider elements. The manner of implementation of these contact elements within the DEM framework and the associated contact parameters determine the accuracy of the simulation. A physically more realistic contact model based on theoretical contact mechanics is described. The model is not implementation specific and requires only material parameters that may be obtained from standard tests. In addition, the model allows for a variation in the coefficient of restitution as a function of impact velocity. The implementation of this contact model is illustrated by simulating a 0.545 m diameter ball mill.

Distinct element simulation of impact breakage of lactose agglomerates

Traditional theoretical and experimental investigations of the mechanical behavior of particulate solids are restricted by the limited quantitative information about what actually happens inside particulate assemblies. This paper presents computer simulation results of the breakage of lactose agglomerates due to impact on a target plate using distinct element analysis. The agglomerates of interest here are generally weak and easy to disintegrate as no binder other than weak surface forces is holding the primary particles together. Particle interaction laws in the simulation code are based on theoretical contact mechanics, where adhesive interface energy determines the bond strength between individual particles of the assembly. Experimental investigations have been conducted to validate the computer simulation results, using a simple air-eductor where particles are accelerated to the required velocity by an air flow and impacted against a rigid target plate. Computer graphics of the simulation results of agglomerate breakdown are compared with the images obtained by high speed video recording of the impact events. A good agreement has been found between the simulation results and experimental measurements. Dynamic features and loading compliance of weak agglomerates are found to be distinctly different from those of high strength agglomerates and solid particles.