Contact Geometry Research Articles

Analyzing of continuous and discontinuous contact problems of a functionally graded layer: theory of elasticity and finite element method

Contact mechanics analysis is crucial because such problems often arise in engineering practice. When examining contact mechanics, the material property of the contacting components is a crucially significant aspect. It is more complex to solve the contact mechanics of systems that are composed of materials that do not have a homogenous structure compared to materials that have homogeneous qualities throughout. While many studies on contact problems with homogeneous materials exist, those involving non-homogeneous materials are scarce in the literature. As material technology improves fast, there will be a greater need to solve such problems. In this respect, analytical and finite element method (FEM) solutions of the continuous and discontinuous contact problems of a functionally graded (FG) layer are carried out in this article. The FG layer in the problem rests on a rigid foundation and is pressed with a rigid punch. From the solutions, the contact length, contact stress, initial separation distance, and beginning and ending points of separation were determined, and the results were compared. It has been concluded that the FEM findings are consistent with the analytical results to a satisfactory degree. This study analyzes contact problem using different approaches and accounts for the influence of body force in a contact geometry that has yet to be reported.

Pitting Influence on Electrical Capacitance in EHL Rolling Contacts

This work presents an investigation on the influence of pitting in EHL rolling contact. The pitting geometry comes as an extension on the architecture for multi-physical numerical calculations of rolling element bearing contacts based on OpenFOAM. The model utilized is built according to the state-of-the-art for fluid–solid interaction and electro-quasi-static fields. In this framework, the contact is reduced to be two dimensional in order to reduce the computing costs needed. The changes in the electric properties, which are caused by pittings, are measured using the electric capacitance of the contact and put in perspective with regard to the EHL contacts geometry. The investigation delivers an evaluation on how surface degradation, in both the pitting width w and pitting depth d of the contact, affects the electric properties. It can be seen that the maximum deviations with different reduced radii for the same pitting structures are around 5% and would therefore hardly be distinguishable with corresponding measurements. By comparing the same data for the same ratio of pitting width to reduced radius, on the other hand, differences of up to 50% can be seen.

Multibody system dynamics modelling and simulation of a high-speed train for its suspension optimization

Multibody system (MBS) dynamics modelling and simulation of high-speed train system is essential and meaningful in the context of train dynamics research and product design. In this work, MBS modelling of a CRH high-speed train is carried out, in the modelling process, nonlinear geometry of wheel-rail contact, nonlinear characteristics of the suspensions and nonlinear railroad irregularities are fully considered. With the obtained MBS model, simulation is performed and demonstrates that critical speed of the CRH train is 378 km/h, vertical ride comfort of the CRH train is in “Excellent” level and lateral ride comfort is in “Good” level, in addition, RMS derailment coefficient of the train is less than 0.5 which means curving performance of the train is good. Agreement of simulation result with actual test or running data validates the established MBS model, thus, the MBS model developed in this study is accurate and would be useful in the next step vibration analysis and suspension optimization of the CRH train.

On the vibration control of semi-active friction dampers with piecewise defined contact geometries

Suppressing unwanted vibrations has been a major challenge for more than a century. Semi-active dampers offer a compromise between the high energy costs of active solutions and their increased flexibility. A nonlinear system that utilizes dry friction and piecewise defined contact geometries is used as the basis for a semi-active damper. Furthermore, a slow frequency-based control that does not solely rely on dissipation is considered and compared to the conventional Skyhook Control. Simulations show that the strategy is an effective approach for vibration reduction.

A terramechanics model for high slip angle and skid with prediction of wheel-soil interaction geometry

Previously developed terramechanics models of wheel-soil interaction forces do not cover the full span of possible wheel states, including large slip angles and ratios. This paper synthesizes a model that covers the full range of slip and skid ratios and slip angles by building on classic terramechanics and soil failure models. The need for wheel and soil specific tuning is reduced through use of a closed-form model of soil flow around the wheel to determine the wheel-soil contact geometry. The terramechanics model is validated both with and without the soil flow model on two wheels in sand for slip ratios from −1 to 0.9 and slip angles from 0° to 60°, showing good prediction of tractive forces, sidewall forces, and sinkage over a wide variety of states. The data from these experiments is also presented, as the only open source data set to cover both a high range of slip angles and slip ratios.

Au-MoS2 contacts: Quantum transport simulations using a continuum description

We present a novel method of modeling the contact between a metal and a two-dimensional semiconductor. Using Au on MoS2 as an example, we self-consistently solve the Schrödinger and Poisson equations and obtain the charge density in the contact. We consider open boundary conditions using the quantum transmitting boundary method and model the electron current through the contact region. We then investigate the effect of effective Schottky barrier height, electrostatic doping, and length of the overlap region on the contact resistance in a top contact geometry. By using data from experiments or from ab initio calculations for the fitting of parameters, such as the effective Schottky barrier height, the model can be used to efficiently obtain the contact resistance and, therefore, the quality of the contact. Furthermore, we investigate the effect of sampling of the Brillouin zone in the transverse direction on the numerical calculation of key quantities, such as contact resistance and free charge density. Additionally, we show that the boundary conditions applied to the Poisson equation during the calculation of the free charge density have a significant impact on the calculated contact resistance and that the impact is more pronounced in heterostructures with a larger Schottky barrier. We found that the contact resistance may be significantly underestimated, by up to one order of magnitude, when the height of the simulation domain is not large enough.

Material-geometry interplay in damping of biomimetic scale beams

Biomimetic scale-covered substrates are architected meta-structures exhibiting fascinating emergent nonlinearities via the geometry of collective scales contacts. Despite much progress in understanding their elastic nonlinearity, their dissipative behavior arising from scales sliding is relatively uninvestigated in the dynamic regime. Recently discovered is the phenomena of viscous emergence, where dry Coulomb friction between scales can lead to apparent viscous damping behavior of the overall multi-material substrate. In contrast to this structural dissipation, material dissipation common in many polymers has never been considered, especially synergistically with geometrical factors. This aspect is addressed here, where material viscoelasticity is introduced via a simple Kelvin–Voigt model for brevity and clarity. The results contrast the two damping sources in these architectured systems: material viscoelasticity and geometrical frictional scales contact. It is discovered that although topically similar in effective damping, viscoelastic damping follows a different damping envelope than dry friction, including starkly different effects on damping symmetry and specific damping capacity.

The role of 3D contact geometry in modeling dynamic wheel-rail interaction at short-wave irregularities on rail surface

Modelling wheel-rail interaction at short-wave irregularities is an important issue under the context of vehicle-track system dynamics for analyzing and preventing severe impact. The present paper broadens the applicability of vehicle-track dynamics model to account for 3D short-wave irregularities on rail surface. The key is developing a meshing grid approach combined with the accurate conjugate gradient (CG) method for contact mechanics under arbitrary 3D contact geometry. Therefore, both the global dynamics and local contact solutions can be provided. In this study, short-pitch rail corrugation is employed for investigating the role of 3D contact geometry in modelling dynamic wheel-rail interaction. The traditional vehicle-track coupled dynamics model using Hertzian spring is used for comparison. The results show short-pitch rail corrugation contributes two different aspects, i.e., wheel center trajectory and time-varying contact stiffness. These two aspects are found to be the root causes for explaining the influence of the varying 3D geometry irregularity size on dynamic wheel-rail contact force.

Edge Contacts to Atomically Precise Graphene Nanoribbons.

Bottom-up-synthesized graphene nanoribbons (GNRs) are an emerging class of designer quantum materials that possess superior properties, including atomically controlled uniformity and chemically tunable electronic properties. GNR-based devices are promising candidates for next-generation electronic, spintronic, and thermoelectric applications. However, due to their extremely small size, making electrical contact with GNRs remains a major challenge. Currently, the most commonly used methods are top metallic electrodes and bottom graphene electrodes, but for both, the contact resistance is expected to scale with overlap area. Here, we develop metallic edge contacts to contact nine-atom-wide armchair GNRs (9-AGNRs) after encapsulation in hexagonal boron-nitride (h-BN), resulting in ultrashort contact lengths. We find that charge transport in our devices occurs via two different mechanisms: at low temperatures (9 K), charges flow through single GNRs, resulting in quantum dot (QD) behavior with well-defined Coulomb diamonds (CDs), with addition energies in the range of 16 to 400 meV. For temperatures above 100 K, a combination of temperature-activated hopping and polaron-assisted tunneling takes over, with charges being able to flow through a network of 9-AGNRs across distances significantly exceeding the length of individual GNRs. At room temperature, our short-channel field-effect transistor devices exhibit on/off ratios as high as 3 × 105 with on-state current up to 50 nA at 0.2 V. Moreover, we find that the contact performance of our edge-contact devices is comparable to that of top/bottom contact geometries but with a significantly reduced footprint. Overall, our work demonstrates that 9-AGNRs can be contacted at their ends in ultra-short-channel FET devices while being encapsulated in h-BN.

Computational model and simulations of contact angle and geometry effects on centrifugal microfluidic step-emulsification

Computational model and simulations of contact angle and geometry effects on centrifugal microfluidic step-emulsification

Revealing capacitive and inductive effects in modern industrial c-Si photovoltaic cells through impedance spectroscopy

To achieve a high performance in sub-module power conditioning circuits, it is important that power converters are designed in accordance with the photovoltaic (PV) cell impedance at the input. Taking this one step further, exploiting the impedance of cell strings could even support novel power conditioning approaches in PV modules. In this work, we characterize the impedance of eight single-cell laminates based on different industrial c-Si PV cell architectures. This characterization is carried out by impedance spectroscopy in dark conditions at room temperature, and the capacitive and inductive effects are evaluated through equivalent model fitting. By comparing the results for the different laminates, it is revealed how the cell design affects its impedance. Our experiments show that the PN junction capacitance at maximum power point varies for the different cells between 0.30 and 45.6 μF/cm2. The two main factors contributing to a high PV cell capacitance at maximum power point are (i) a low wafer dopant concentration and (ii) a high maximum power point voltage. In high-efficiency c-Si PV cells that will be fabricated in the coming years, increasing capacitances are expected for operation near the maximum power point. Furthermore, the single-cell laminates exhibit inductances between 63 and 130 nH, and our results indicate that the inductance is mostly affected by the number of busbars and the geometry of the metal contacts.

Thermodynamic Entropy as a Noether Invariant from Contact Geometry.

We use a formulation of Noether's theorem for contact Hamiltonian systems to derive a relation between the thermodynamic entropy and the Noether invariant associated with time-translational symmetry. In the particular case of thermostatted systems at equilibrium, we show that the total entropy of the system plus the reservoir are conserved as a consequence thereof. Our results contribute to understanding thermodynamic entropy from a geometric point of view.

Contact-engineered reconfigurable two-dimensional Schottky junction field-effect transistor with low leakage currents

Two-dimensional (2D) materials have been considered promising candidates for future low power-dissipation and reconfigurable integrated circuit applications. However, 2D transistors with intrinsic ambipolar transport polarity are usually affected by large off-state leakage currents and small on/off ratios. Here, we report the realization of a reconfigurable Schottky junction field-effect transistor (SJFET) in an asymmetric van der Waals contact geometry, showing a balanced and switchable n- and p-unipolarity with the Ids on/off ratio kept >106. Meanwhile, the static leakage power consumption was suppressed to 10−5 nW. The SJFET worked as a reversible Schottky rectifier with an ideality factor of ~1.0 and a tuned rectifying ratio from 3 × 106 to 2.5 × 10−6. This empowered the SJFET with a reconfigurable photovoltaic performance in which the sign of the open-circuit voltage and photo-responsivity were substantially switched. This polarity-reversible SJFET paves an alternative way to develop reconfigurable 2D devices for low-power-consumption photovoltaic logic circuits.

Uncertainty in overturning of precariously balanced rocks due to basal contact

AbstractPrecariously balanced rocks (PBRs) are a type of naturally occurring freestanding structure that provides valuable information to constrain seismic hazard at return periods which are important for critical facilities such as nuclear power plants and nuclear repositories. Exposure ages have been established to be in excess of 10,000–30,000 years, which is why precarious rocks are one of the only available means to validate seismic hazard associated with long return periods. One critical component for constraining seismic hazard in this way is the overturning estimate of a given precarious rock as a function of earthquake intensity. However, current state‐of‐the‐art methods for modeling the seismic response of precarious rocks involve significant sources of uncertainty. One of the main sources of uncertainty stems from the interface of the rock, which is usually occluded during surveying and assumed during modeling. Through extensive shake table testing, this study analyzes the uncertainty in the overturning response of a granite precarious rock specimen incorporating various degrees of interface contact. The results indicate that a small variation in the contact geometry could result in a substantial increase in the stability of the specimen, which is significant given the difficulty of surveying the interface of PBRs in the field. Repeatability tests indicate that the overturning demand can vary up to nearly ±50%. The probabilistic overturning responses are compared across the interface changes to bound uncertainty; and, the effect of modeling parameters, namely the contact normal stiffness, is evaluated through a parametric study and comparison with experimental results.

Geometrothermodynamic Cosmology.

We review the main aspects of geometrothermodynamics, a formalism that uses contact geometry and Riemannian geometry to describe the properties of thermodynamic systems. We show how to handle in a geometric way the invariance of classical thermodynamics with respect to Legendre transformations, which means that the properties of the systems do not depend on the choice of the thermodynamic potential. Moreover, we show that, in geometrothermodynamics, it is possible to apply a variational principle to generate thermodynamic fundamental equations, which can be used in the context of relativistic cosmology to generate cosmological models. As a particular example, we consider a fundamental equation that relates the entropy with the internal energy and the volume of the Universe, and construct cosmological models with arbitrary parameters, which can be fixed to reproduce the main aspects of the inflationary era and the standard cosmological paradigm.

Influence of Pile Cap–Ground Contact Geometry on the Behavior of Piled Foundations

Influence of Pile Cap–Ground Contact Geometry on the Behavior of Piled Foundations

Consideration of the Variable Contact Geometry in Vibratory Roller Compaction

Vibratory rollers are mainly used for the near-surface compaction of granular media for a wide variety of construction tasks. In addition to the pronounced depth effect, vibratory rollers have offered the possibility of work-integrated compaction control (intelligent compaction) for decades. State-of-the-art measurement values for intelligent compaction (ICMVs) only take into account, if at all, a constant geometry of the contact area between the drum and soil. Therefore, this paper introduces a comparatively simple mechanical model, which describes the dynamic interaction between the vibrating drum and the underlying soil during compaction to investigate the influence of the changing geometry of the contact area on the motion behavior of the vibrating drum. The model is tested on realistic soil and machine parameters, and the results of the simulation with varying drum contact geometry are compared to a conventional simulation with a fixed contact geometry. The analysis shows that only a consideration of the varying drum contact geometry can map the dynamic interaction between the vibrating drum and soil sufficiently and provide a motion behavior of the drum that is in good accordance with the field measurements.

Pure edge-contact devices on single-layer-CVD-graphene integrated into a single chip

We present a simple and cost-effective fabrication technique for on-chip integration of pure edge contact two-terminal (2T) and Graphene field effect transistor (GFET) devices with low contact resistance and nonlinear characteristics based on single-layer chemical-vapor-deposited (CVD) graphene. We use a smart print-based mask projection technique with a 10X magnification objective lens for maskless lithography followed by thermal evaporation of the contact material Cr-Pd-Au through three different angles (90° and ± 45°) using a customized inclined-angle sample-holder to control the angle during normal incidence evaporation for edge-contact to graphene. Our fabrication technique, graphene quality, and contact geometry enable pure metal contact to 2D single-layer graphene allowing electron transport through the 1D atomic edge of graphene. Our devices show some signatures of edge contact to graphene in terms of very low contact resistance of 23.5 Ω, the sheet resistance of 11.5 Ω, and sharp nonlinear voltage-current characteristics (VCC) which are highly sensitive to the bias voltage. This study may find application in future graphene-integrated chip-scale passive or active low-power electronic devices.

A force–displacement relation based on the JKR theory for DEM simulations of adhesive particles

Studying the collective behavior of adhesive particles with the discrete element method (DEM) requires well-founded force–displacement relations (force models). While the Johnson–Kendall–Roberts (JKR) theory reliably predicts the dependence of the contact radius a on the force F, it has remained a challenge in this framework to obtain a straightforward force–displacement relation F(δ) with physically meaningful parameters to calculate the force F from the displacement δ. We derive a novel force–displacement relation from the JKR theory as a composition of functions F(δ)=(F∘a∘λ)(δ), with the intermediate functions contact radius a(λ) and effective adhesive contact radius λ(δ). We also analyze contact geometry errors in the Hertz and JKR models, derive the exact contact centroid to accurately calculate contact torques and relative tangential velocities, and propose a smoothed JKR model to avoid discontinuities in force and energy. We find that incorrect torques and relative tangential velocities are obtained when the stiffness quotient is neglected because it affects the exact location of the contact centroid. We also find that contact geometry errors can become non-negligible for nanoparticles due to their large relative contact size. In addition, we show from bouncing ball simulations that the JKR models with a nominal coefficient of restitution derived for the Hertz model result in higher damping and thus reduced actual coefficients of restitution. Our analysis serves as a foundation for contact-mechanics-based force models for DEM simulations of adhesive particles to investigate their collective behavior in the future.

A hybrid analytical-FEM 3D approach including wear effects to simulate fretting fatigue endurance: Application to steel wires in crossed contact

Fretting fatigue causes failures in steel wire ropes. A crossed wires set-up was used to investigate the influence of displacement amplitude on fretting fatigue damage. The analysis confirms previous literature results, showing a beneficial effect of wear in gross slip regime, which extends the contact area and thus decreases the contact stresses. The computation of such combined cracking and wear processes is very expensive using a classical FEM strategy, particularly for 3D contact geometries. To palliate such limitations, a hybrid analytical-numerical strategy was developped. It consists in estimating pressure and shear stress surface fields using analytical formulations, then to transpose these fields to an FEM model. This allows to simulate contact geometry modifications in a fast and efficient way.