Mechanical Contact Forces Research Articles

Micro‐mechanical analysis of one‐dimensional compression of clay with DEM

AbstractIn order to clarify the micro‐mechanics of clay during compression, the behavior of clay subjected to one‐dimensional compression is investigated at the particle scale using discrete element method (DEM). The flaky clay particles in the simulation are approximated by clumps made of spheres. A new contact model is implemented to account for the double‐layer repulsive force, the van der Waals attractive force and the mechanical contact force. The effect of sphere arrangement in the particle clump is discussed. The DEM model is validated against experimental observations in terms of macroscopic compressibility, particle dip angle as well as the over consolidated behavior. The e‐logσv curve shows a concave‐to‐linear shape. The evolution of dip angle of clay particles indicates that particles tend to have an anisotropy with a preferential orientation towards horizontal direction. The increase of preconsolidation pressure decreases the initial compressibility due to the increase the number of mechanical contacts. The evolution of average coordination and sphere‐sphere contact shows that the majority of mechanical contacts are generated before the compressive stress reaches 100 kPa. Evolution of soil fabric is presented and discussed.

A statistical RVE model for effective mechanical properties and contact forces in lithium-ion porous electrodes

A statistical representative volume element for a cathodic NMC active porous materials is developed as a periodic face-centered-cubic structure. Finite element analyses (FEA) are conducted to calculate effective elastic properties and contact force distributions for various numbers of particle–particle and binder-particle contacts. The statistics for the RVE is attained through simulations of stochastic distributions of contact conditions. The variations of effective properties with changing particle–particle and binder-particle contacts are compared to experimental results in the literature. The stiffening behavior at compressive loading is correlated with increasing interparticle connections in the RVE. The average values and the statistical distribution of particle–particle and binder-particle contact forces that result from particle swelling are as well investigated. The significance of contact force distributions, for both particle–particle and binder-particle contacts, on potential particle cracking and binder debonding is addressed. It is noted that particle–particle contact forces appear that are 4–5 times larger than their corresponding average values. Conclusions drawn from differences in average contact forces between particle–particle and particle-binder contacts as well as normalized standard deviations of contact forces are utilized to improve a previously developed analytical model for effective stiffness properties and contact forces. Excellent agreements are found in comparisons to the numerical simulations.

Electromagnetic and Flow Field Analysis of Multi-DOF Motor With Air Bearing Structure

Due to the traditional multi-DOF motor active joint using the traditional joint ball bearing, it often has friction loss and mechanical contact force and cause low output accuracy. In this paper, an air-floating multi-DOF motor (AMM) supports the rotor with the compressed gas. Firstly, the FEM is adopted to conduct electromagnetic analysis on AMM to obtain the magnetic field distribution of the motor. Then, according to the gas lubrication theory, the flow field is numerically analyzed. The finite element software is also used to simulate the air flow in the air film flow field between the stator and the rotor. Finally, the experimental platform of AMM was established, and the vibration curve of the rotor during the movement was obtained. The experimental results prove the validity of special design requirements of the motor.

Mechanical behavior investigation of press-fit connector based on finite element simulation and its reliability evaluation

As the successor of soldering, the press-fit electrical connection is more and more widely used on account of a number of advantages, such as fast assembly. However, in practice, some contact failures were found. Most of the failures are related to the force which the structure bears. Because of complex elastic-plastic deformation characteristic, analyzing the mechanical behavior of the press-fit is challenging. Some studies have been done on the assembly force behavior of the press-fit such as press in force. However, the stress behavior on the assembly is not clearly studied. In this paper, the mechanical behavior of a typical press-fit connector named as eye of needle (EON) was investigated based on finite element (FE) simulation. Firstly, FE models were established and verified. Then, basing on model analysis, the mechanical behaviors (mechanical stress, thermal stress, and contact force) of the EON were obtained, which makes it clear that the relationships between the EON's structural parameters and its work conditions. Finally, the long-term reliability of the EON connector under the stress relaxation and the thermal fatigue was evaluated, respectively. The FE models here are effective on analyzing the press-fit mechanical behavior, which can be adopted to help design and manufacture.

Artificially innervated self-healing foams as synthetic piezo-impedance sensor skins

Human skin is a self-healing mechanosensory system that detects various mechanical contact forces efficiently through three-dimensional innervations. Here, we propose a biomimetic artificially innervated foam by embedding three-dimensional electrodes within a new low-modulus self-healing foam material. The foam material is synthesized from a one-step self-foaming process. By tuning the concentration of conductive metal particles in the foam at near-percolation, we demonstrate that it can operate as a piezo-impedance sensor in both piezoresistive and piezocapacitive sensing modes without the need for an encapsulation layer. The sensor is sensitive to an object’s contact force directions as well as to human proximity. Moreover, the foam material self-heals autonomously with immediate function restoration despite mechanical damage. It further recovers from mechanical bifurcations with gentle heating (70 °C). We anticipate that this material will be useful as damage robust human-machine interfaces.

Spatial pattern and surface-specificity of particle and microorganism deposition and attachment: Modeling, analytic solution and experimental test

Hypothesis: Understanding microparticle and living cell deposition and attachment on surfaces from a flow is a long-standing surface-science problem, pivotal to developing antifouling strategies. Recent studies indicate a complex non-conservative and surface-specific nature of adhesion and mechanical contact forces that determine attachment kinetics. This requires new models and kinetic data, however, observed deposition rates (e.g., in parallel-plate flow chamber, PPFC) represent a superposition of attachment and bulk transport. Here, we propose to deduce attachment rates (as an appropriate rate constant) from spatial deposition profiles along PPFC and develop an analytical solution for the full problem, suitable for deposition data analysis and parameter fitting.Experiments: Analytical solution, validated by numerical simulations, reveals relation between the deposition profile along PPFC and key model parameter B, the ratio of sedimentation and attachment rates. Its use is demonstrated on experimental data obtained in a PPFC for particles and bacteria on various surfaces.Findings: Fitted B values highlight correlation with the particle/substrate nature and consistently explain the observed trends along PPFC, both decreasing and increasing. Thus derived attachment rates will serve as basis for future microscopic modelling that would relate attachment to appropriate surface and contact-mechanical characteristics of particles and substrate and flow conditions.

Shear Thickening of Concentrated Suspensions: Recent Developments and Relation to Other Phenomena

Shear thickening is the increase of the apparent viscosity as shear rate or shear stress increases. This phenomenon is pronounced in concentrated (dense) suspensions of both colloidal-scale and larger particles, with an abrupt form, known as discontinuous shear thickening, observed as the maximum flowable solid fraction is approached. An overview of observed shear thickening behavior is presented, with a discussion of present understanding of the relationship of suspension shear thickening to granular jamming. Mechanistic arguments for the extreme change in rheological properties are outlined, and recent evidence from experiment and simulation for the role of contact forces is presented. Interactions of particles by fluid mechanical lubrication, contact, and steric and electrostatic forces, together with extreme stresses that may lead to solid deformation, require consideration of surface interactions and their tribological consequences in describing shear thickening.

Analysis of the Melt Erosion Patterns at Rail-Armature Contact of Rail Launcher in Current Range of 10–20 kA/mm

Sliding electrical contact between rail and armature in rail launchers is characterized by high speed and large current. Melt erosion is caused by the current concentration on the contact surface of armature. This current melt erosion (CME) has been experimentally studied in the current range of 10–20 kA/mm with a 20-mm caliber lab-scale rail launcher, and payload separated method was used to keep the recovered armatures intact. Thus, the onset behavior and erosion patterns of CME can be successfully observed in such relatively low current densities. In this paper, we will further our study to analyze the patterns of CME with the increasing current densities under the effect of different initial mechanical contact force. Results show that CME all start at the side edge, and then spreading longitudinally and transversely with the increase of current. It is found that the starting position of CME appears near the trailing edge at small mechanical contact force and moves to the leading edge with larger mechanical contact force. At the same current magnitude, erosion amounts are much more under small mechanical contact force. Stationary mechanical contact pressures and current distribution of armature surface are simulated to investigate the effect of initial mechanical contact force on starting position of CME. Current distributes at the edges of actual contact zone and the maximum of current density appears at the side edges of armature. As the initial mechanical contact force increases, actual contact zone and current density move toward to the leading edge from trailing edge. It can be confirmed that the erosion starting position is at the side edge along actual contact zone and will move toward to the leading edge from trailing edge with the increase of initial mechanical contact force. Smaller initial mechanical contact force will cause more erosion at the same current magnitude because of larger current density. Furthermore, an average friction coefficient calculation method is proposed to evaluate the melt lubrication effect of CME. The result shows that the lubrication effect becomes better with larger current densities.

Acoustical boundary hologram for macroscopic rigid-body levitation.

In previous studies, acoustical levitation in the far-field was limited to particles. Here, this paper proposes the "boundary hologram method," a numerical design technique to generate a static and stable levitation field for macroscopic non-spherical rigid bodies larger than the sound wavelength λ. This paper employs boundary element formulation to approximate the acoustic radiation force and torque applied to a rigid body by discretizing the body surface, which is an explicit function of the transducer's phase and amplitude. Then, the drive of the phased array is numerically optimized to yield an appropriate field that stabilizes the body's position and rotation. In experiments, this paper demonstrates the levitation in air of an expanded polystyrene sphere with a diameter of 3.5 λ and a regular octahedron with diagonal length of 5.9 λ, both located 24 λ from the acoustic elements, by a 40 kHz (λ = 8.5 mm) ultrasonic phased array. This method expands the variety of objects that can be levitated in the far-field of an ultrasonic phased array.

Discrete-element model of electrophoretic deposition in systems with small Debye length: effective charge, lubrication force, characteristic scales, and early-stage transport

Electrophoresis was recently proposed as a new method for controlling properties and structure of the interface between cement and steel elements in petroleum and construction industries (Lavrov A et al., 2018). Cement slurries typically contain a wide range of particle sizes (micron to hundred micron), and the particles have small Debye lengths (nanometer). The relative magnitude of different forces acting on particles in such systems was examined. A formulation based on the DLVO theory was used to calculate van der Waals forces and electric repulsion forces between particles. It was shown that the following forces need to be included in a discrete-element model of electrophoretic deposition in this case: viscous drag force, force due to the external electric field, gravity + buoyancy force, lubrication force, van der Waals force, and direct mechanical contact force. The recommended cut-off gaps for van der Waals and lubrication forces are equal to the radius of the larger particle participating in the interaction. An example DEM simulation has revealed that deposition starts with deposing finer particles. Shortly after, larger particles are deposited on the finer substrate. This is due to the larger speed-up time of larger particles. The difference in the speed-up time leads to some size segregation at the early stage of deposition, even though the particle mobility is independent of the particle size. The model enables some insight into the processes that take place during the earlier stages of electrophoretic deposition that are difficult to capture and analyze in laboratory experiments.

Finite Element Analysis on Forces, Currents, and Magnetic Fluxes of Misaligned GIS Disconnector

This paper provides an effective misalignment fault diagnosis method of the gas-insulated switchgear (GIS) disconnector. Relationships between contact forces and the surrounding magnetic field in various mechanical assembly conditions were studied by the 3-D electromagnetic-mechanical coupled finite-element model and physical experiments. First, spring deformation and contact forces on individual contact elements were obtained by mechanical field analysis. Holding springs were modeled by equivalent elastic rings during analysis. Second, the magnetic field around the GIS disconnector was obtained by electromagnetic-field analysis. Contact resistances on contact interfaces were modeled by cylindrical bridges, which use mechanical contact forces as parameter inputs. Physical tests have been carried on the prototype GIS disconnector with various artificial misalignment faults. In different situations of misalignment faults under ac current, surrounding magnetic flux densities were measured by the Hall sensor array. Calculated and measured results have the same trend that the mechanical misalignment fault could change distributions of contact forces then make distortion of currents and the surrounding magnetic field. Magnetic-field behaviors of the GIS disconnector with different mechanical assembly conditions were studied. The results show that the deviation of magnetic flux density becomes larger while reducing holding spring stiffness, decreasing movable conductor insert depth, and increasing the movable conductor docking angle.

Dynamic simulation of powder packing structure for powder bed additive manufacturing

Powder packing structure is a critical parameter of powder bed-based additive manufacturing (AM). Experimental characterization of powder is typically limited to measuring bulk properties, whereas many numerical models of AM powder packing are based on geometrical consideration without accounting for particle-to-particle interactions. In the present paper, the powder packing dynamics is simulated using a discrete element method (DEM)-based model that solves the mechanical contact forces and moments between individual particles. As DEM uses explicit time integration, a main challenge in modeling dynamics of metallic powder packing is the need for extremely fine time increment size (e.g., in the order of 1 ns for a 10-μm-diameter particle). The effect of mass scaling, employed for speeding up the calculation, on the simulation results is examined in a test case of powder particles packed inside a box container. The calculated packing density for two different particle size distributions is validated against independent literature data for laser powder bed AM with AISI 316L stainless steel powder. The sensitivity of key input parameters (e.g., friction coefficient) is further evaluated in this test case. The powder packing model is then applied to a practical situation of binder jet AM involving rolling of multiple layers of IN718 powder particles onto a powder bed, for which the calculated packing density is also validated with independent literature data.

State observer design for a walking in-pipe robot

In this paper, a state observer design for a walking in-pipe robot is studied. The necessity of using a state observer is related to the fact that sensors have limited accuracy and are prone to producing noise. This is especially problematic for in-pipe walking robots, since they use model-based control and require accurate information of their current state. The paper shows that an iterative state observer based on solving Riccati equation provides significant improvements in the behaviour of the control system. It allows to smooth out the spikes in the control actions requested by controller and to minimize tremor of the robot links. In order to study the behaviour of the observer when different sensors are used, a performance function was introduced. It was shown that the observer allows to improve the performance of the control system for a wide range of sensor parameters. Additionally, it was shown that the introduction of the observer allows to choose higher feedback controller gains, enabling more precise control. Simulations on the full robot model, taking into account mechanical constraints and contact forces showed that the linear observer is capable of improving the behaviour of the control system of the walking robot, if measurements of the reaction forces are provided. The effects that the noise and quantization in the reaction forces measurements have on the behaviour of the state observer is studied.

A Flow Perfusion Bioreactor System for Vocal Fold Tissue Engineering Applications.

The human vocal folds (VFs) undergo complex biomechanical stimulation during phonation. The aim of the present study was to develop and validate a phono-mimetic VF flow perfusion bioreactor, which mimics the mechanical microenvironment of the human VFs in vitro. The bioreactor uses airflow-induced self-oscillations, which have been shown to produce mechanical loading and contact forces that are representative of human phonation. The bioreactor consisted of two synthetic VF replicas within a silicone body. A cell-scaffold mixture (CSM) consisting of human VF fibroblasts, hyaluronic acid, gelatin, and a polyethylene glycol cross-linker was injected into cavities within the replicas. Cell culture medium (CCM) was perfused through the scaffold by using a customized secondary flow loop. After the injection, the bioreactor was operated with no stimulation over a 3-day period to allow for cell adaptation. Phonation was subsequently induced by using a variable speed centrifugal blower for 2 h each day over a period of 4 days. A similar bioreactor without biomechanical stimulation was used as the nonphonatory control. The CSM was harvested from both VF replicas 7 days after the injection. The results confirmed that the phono-mimetic bioreactor supports cell viability and extracellular matrix proteins synthesis, as expected. Many scaffold materials were found to degrade because of challenges from phonation-induced biomechanical stimulation as well as due to biochemical reactions with the CCM. The bioreactor concept enables future investigations of the effects of different phonatory characteristics, that is, voice regimes, on the behavior of the human VF cells. It will also help study the long-term functional outcomes of the VF-specific biomaterials before animal and clinical studies.

Analysis of Graphene Coating on Si Anode and Defect Engineering for Lithium Ion Battery

Silicon is the most promising anode material due to its huge capacity about 4200mAh/g and their abundance. However its usage is hindered by large volume expansion, causing pulverization and so on. To reduce the problems, coating on silicon anode is one of the efficiency methods. In this presentation, computational study on the effect of graphene coating to Si anode material is performed by using density functional theory calculations. We construct the atomic model to examine interactions between amorphous silicon and graphene during lithiation. The lithiation of Si anode increases the not only mechanical contact force between outer graphene layer and amorphous silicon but also the shear resistance. To explain the interaction between graphene and silicon, we examine the charge distribution of silicon and graphene considering lithiation insertion. The number of density, electron field distribution and electric potential are also calculated. Charge-non polar interaction between Li-ion and graphene increase the contact energy between graphene-silicon. In addition, we also study the effect of defect on graphene such as mono, di, and stone-wales defect. These defects increase the contact force between defected graphene and lithium silicide.

Balloon catheters with integrated stretchable electronics for electrical stimulation, ablation and blood flow monitoring

Balloon catheter-based techniques that apply mechanical force to blood vessel walls have dramatically advanced the standard of care for patients with vascular disease. But balloons are limited in their therapeutic potential because they rely on mechanics alone and lack electronically active materials. Recent advances in balloon catheters with integrated electrodes have offered additional treatment capabilities through the application of both mechanical contact force and radio frequency electromotive force. However, these smart balloons remain limited in their mechanics and their ability to provide diagnostic information about local vessel hemodynamics and tissue health near treatment zones. Here, we demonstrate a novel instrumented balloon catheter system that contains stretchable electrodes and thermal-based blood flow sensors, providing hemodynamic information, electrical stimulation, and ablation therapy on a single device. This system accommodates high inflation pressure (∼2 atm) and twisting and bending over a broad range of vascular diameters and geometries. Finite element and analytical modeling capture the anisotropic mechanical and thermal properties of the device during cycles of inflation and deflation in a simulated biological environment. Bench-top ablation and in vivo blood flow measurements highlight the utility of this technology for clinical application.

Computational studies on interparticle forces between nanoellipsoids

The adapted continuum models pertinent to ellipsoidal microparticles do not generally hold at the nanoscale due to the approximations apart from the surface effects and the neglect of atomic discrete structure. The governing equation of describing the interactions, non-contact forces in particular, between ellipsoidal nanoparticles is lacking. In this work, the interaction forces including van der Waals (vdW) attraction, Born repulsion and mechanical contact forces between nanoellipsoids are studied by molecular dynamics (MD) simulation and compared with those predicted by the adapted continuum models (Hamaker or Hertz model). The results show that the interaction forces between ellipsoidal nanoparticles are complicated and the ratios of interaction forces obtained from the MD simulations to those from the adapted Hamaker equations are dependent on surface separation, particle size, aspect ratio and configurations. Under different configurations, two formulas have been proposed for vdW attraction and Born repulsion forces. In particular, under parallel configuration, both the vdW attraction and Born repulsion forces between nanoellipsoids show an obvious periodic variation stemming from the step-like atomic structure and can be described by a second-order Fourier expansion and correspondingly another two relatively more accurate formulas are proposed for vdW attraction and Born repulsion forces. Moreover, the mechanical contact force between ellipsoidal nanoparticles at low compression still can be described by the Hertz model. This work can provide quantitative insights into interaction forces between nano-ellipsoids and should be useful in the applications where ellipsoidal particles are involved, such as self-assembly by virtue of inter-particle or external forces.

Interactions between crystalline nanospheres: comparisons between molecular dynamics simulations and continuum models

Although the breakdown of the continuum model at the nanoscale has been predicted, the origin of failure is not well understood. This in turn has affected our better understanding of interparticle forces. In this work, the interaction forces including the van der Waals (vdW) attraction, Born repulsion and mechanical contact forces between crystalline silicon carbide nanospheres are studied using molecular dynamics (MD) simulations. The origin of the deviation from the Hamaker approach is fully analyzed and ascribed to the underestimated number density of atoms apart from the neglect of atomic discrete structure and surface effects. Two equations have been proposed to describe the vdW attraction and Born repulsion forces between silicon carbide nanospheres. Moreover, MD simulated results confirm that the mechanical contact forces between silicon carbide nanospheres at low compression can be largely described by the Hertz model. Also, the definition of contact at the atomic level when a mechanical contact force initiates at equilibrium separation is validated. Finally, the theory of normal impact between two frictionless elastic bodies is evaluated at the atomic level, which shows that the maximum compression achieved is in semi-quantitative agreement with continuum prediction. The origin of existing quantitative differences is critically studied and the continuum equation is adapted to predict the maximum compression at the nanoscale.

Interaction forces between carbon nanospheres: A molecular dynamics simulation study

In this work, the interaction forces between carbon nanospheres are studied using molecular dynamics (MD) simulations. It is shown that the conventional Hamaker approach cannot be directly applied to reliably estimate the van der Waals attraction and Born repulsion forces for nanospheres of different carbon materials. Yet, there are some common features identified, including a finite value of the forces at the surface separation d≈0nm, a maximum ratio between the interaction forces obtained from the MD simulation and Hamaker approach at d≈0.4nm, a turning point of interaction forces at d≈0.15nm. These features can be quantitatively described with a similar mathematical form formulated for silica. Moreover, it is demonstrated that the mechanical contact force between carbon nanospheres at a low compression can be described by the classical Hertz model. Finally, the minimum gap between carbon nanoparticles in collision can be described by the conventional contact mechanics, which is adapted to evaluate the minimum gap.

Interaction forces between a spherical nanoparticle and a flat surface

Due to the breakdown of Derjaguin approximation at the nanoscale level apart from the neglect of the atomic discrete structure, the underestimated number density of atoms, and surface effects, the continuum Hamaker model does not hold to describe interactions between a spherical nanoparticle and a flat surface. In this work, the interaction forces including van der Waals (vdW) attraction, Born repulsion and mechanical contact forces between a spherical nanoparticle and a flat substrate have been studied using molecular dynamic (MD) simulations. The MD simulated results are compared with the Hamaker approach and it is found that the force ratios for one nanosphere interacting with a flat surface are different from those for two interacting nanospheres, both qualitatively and quantitatively. Thus two separate formulas have been proposed to estimate the vdW attraction and Born repulsion forces between a nanosphere and a flat surface. Besides, it is revealed that the mechanical contact forces between a spherical nanoparticle and a flat surface still can be described by the continuum Hertz model.