This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among ...the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.
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The contact between rough metallic bodies almost always involves plastic flow in the area of real contact. We performed indentation experiments on sandblasted aluminum surfaces to explore the plastic ...deformation of asperities and modeled the contact mechanics using the boundary element method, combined with a simple numerical procedure to take into account the plastic flow. The theory can quantitatively describe the modification of the roughness by the plastic flow. Since the long-wavelength roughness determines the fluid leakage of metallic seals in most cases, we predict that the leakage can be estimated based on the elastoplastic contact mechanics model employed here.
We study the distribution of interfacial separations at the contact region between two elastic solids with randomly rough surfaces. An analytical expression is derived for the distribution of ...interfacial separations using Persson's theory of contact mechanics, and is compared to numerical solutions obtained using (a) a half-space method based on the Boussinesq equation, (b) Green's function molecular dynamics technique and (c) smart-block classical molecular dynamics. Overall, we find good agreement between all the different approaches.
Detailed understanding of wear processes is required to improve the wear resistance and lifetime of machine components. Atomic force microscopy (AFM) is used to measure surface height profiles with ...high precision, before and after a wear experiment. The distribution and depth of wear on steel surfaces is then calculated using a relocation method. A numerical investigation of wear based on Archard׳s equation is conducted on the same measured surfaces. A good correlation was found between the model and experiment for wear larger than a hundred nm. The wear mechanisms considered in the numerical simulation was thus found to be the cause of the majority of the wear. On the scale of tens of nm the correlation was limited, but the measured wear was still analysed in detail.
•Surface topographies are measured before and after wear with AFM.•A relocation method is applied mapping wear with a precision of nanometers.•The same topography is used in a numerical wear simulation.•The wear from numerical simulation is compared with the measured wear.•Simulation and experiment correlates well for wear depths of a few hundred nm.
• Numerical contact pressure and thereof depending wear calculations. • Experimental wear investigations. • A comparison between numerical investigation and experiment. • Results indicating clear ...deviations and similarities between simulation and experiment. • Numerical contact pressure and (developed for the specific conditions) thereof depending wear calculations.
A wear model including a deterministic FFT-accelerated contact mechanical tool to calculate pressure and elastic–plastic deformation, is employed to simulate the time dependent wear in a sphere on flat contact. The results of the wear simulations compared to experimental results from a reciprocating test in a ball on disk tribometer. The conditions of the simulations and the experiments are independently adjusted to match up. Similarities and differences shows upon the usefulness and limitation of wear modelling of this type.
A low degree of freedom (LDOF) approach for expedient prediction of friction in finite EHL line contact is presented. Friction prediction are in good agreement with existing results for all of the ...reference cases tested. With the present LDOF approach, friction predictions can be obtained thousands of times faster than if using a fully coupled FE-based model. It can, therefore be used to simulated the tribological contacts in multi-body dynamic (MBD) systems with millions of degrees of freedoms. The model was utilised to study edge stress concentrations due to roller tilt. The results confirmed that increasing the tilting angle increases the maximum pressure and shifts its location towards the edge of roller. An increase in total friction was also observed.
•The LDOF approach computes EHL friction thousands of times faster than a fully coupled FE-based model.•The accuracy of the friction prediction is high and the LDOF approach can be used to study edge stress effects.•A low degrees of freedom (LDOF) approach for prediction of friction in finite EHL line contacts is presented.
On compressible and piezo-viscous flow in thin porous media Pérez-Ràfols, F.; Wall, P.; Almqvist, A.
Proceedings of the Royal Society. A, Mathematical, physical, and engineering sciences,
01/2018, Letnik:
474, Številka:
2209
Journal Article
Recenzirano
Odprti dostop
In this paper, we study flow through thin porous media as in, e.g. seals or fractures. It is often useful to know the permeability of such systems. In the context of incompressible and iso-viscous ...fluids, the permeability is the constant of proportionality relating the total flow through the media to the pressure drop. In this work, we show that it is also relevant to define a constant permeability when compressible and/or piezo-viscous fluids are considered. More precisely, we show that the corresponding nonlinear equation describing the flow of any compressible and piezo-viscous fluid can be transformed into a single linear equation. Indeed, this linear equation is the same as the one describing the flow of an incompressible and iso-viscous fluid. By this transformation, the total flow can be expressed as the product of the permeability and a nonlinear function of pressure, which represents a generalized pressure drop.
Vertical Scanning Interferometry (VSI) may induce optical artefacts in surface topography measurements. The influence of these optical artefacts on the calculation of Rk surface roughness parameters, ...contact stiffness and flow factors were studied. Two surface measurement techniques were used: Atomic Force Microscopy (AFM) and VSI. Calibration grids were used to make it easier to isolate the causes of these artefacts, while a real engineering surface was used to compare these two techniques in an industrially applied case. It was found that the optical artefacts have a large influence on all the roughness parameters, contact stiffness and flow factors calculated on the calibration grids. However, for the engineering surface the differences between AFM and VSI measurements were much smaller.
► Comparison undertaken of VSI and AFM measurement techniques. ► Silicon ‘calibration’ grids and a ‘real’ surface measured. ► Roughness parameters, contact mechanics and flow factors investigated. ► VSI artefacts significantly influence results for surfaces with extreme geometries. ► However, VSI found to be sufficiently accurate for a typical engineering surface.
Wearing-in of a machine component can increase the conformity between contacting pairs and smoothen the surface topography. A two scale model, combining the wearing-in effects, resulting in changes ...in the surface topography, with the wear that occurs on the component, is presented. The geometry of the components is represented with measured coordinates. Wear leads to changes of the geometry, which has an effect on several tribological conditions, such as contact forces, relative velocities and conformity. Due to the wear on the topography scale, the load sharing is also affected. The model is applied to orbital hydraulic motors. The wear depth predicted with the model is qualitatively in good agreement with the wear depth recorded in experiments.
•A mixed lubrication running-in model is presented.•Changes in the surface topography due to wear affect the load sharing.•Running-in is observed in the model due to the wear on two length scales.•Measured surface profiles are used as input to the model.•Simulation results correlate well qualitatively with experiment.
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