| 1. |
- Almqvist, Andreas
(författare)
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Fundamentals of Physics-Informed Neural Networks Applied to Solve the Reynolds Boundary Value Problem
- 2021
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Ingår i: Lubricants. - : MDPI. - 2075-4442. ; 9:8
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a complete derivation and design of a physics-informed neural network (PINN) applicable to solve initial and boundary value problems described by linear ordinary differential equations. The objective with this technical note is not to develop a numerical solution procedure which is more accurate and efficient than standard finite element- or finite difference-based methods, but to give a fully explicit mathematical description of a PINN and to present an application example in the context of hydrodynamic lubrication. It is, however, worth noticing that the PINN developed herein, contrary to FEM and FDM, is a meshless method and that training does not require big data which is typical in machine learning.
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| 2. |
- Berglund, Kim, 1982-, et al.
(författare)
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A novel reciprocating tribometer for friction and wear measurements with high contact pressure and large area contact configurations
- 2021
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Ingår i: Lubricants. - : MDPI. - 2075-4442. ; 9:12
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Tidskriftsartikel (refereegranskat)abstract
- There are many moving machine assemblies with conformal tribological contacts at very high contact pressures, e.g., sliding bearings, propeller shaft bearings and machine guideways. Furthermore, applications such as trunnion and guide vane bearing in Kaplan turbines have very low sliding speeds and oscillatory types of motion. Although there is a vast selection of tribology test rigs available, there is still a lack of test equipment to perform friction and wear tests under high contact pressure, reciprocatory sliding and large area contact. The aim of this work is thus to develop a novel reciprocating tribometer and test method that enables friction and wear tests under low-speed reciprocatory sliding with contact pressures up to 90 MPa in a flat-on-flat contact configuration. First, a thorough description of the test rig design is given. Secondly, the influence of contact pressure and stroke length on the tribological properties of a stainless steel and polymer composite material combination is studied. The significance of considering creep, friction during the stroke and contact temperature is specifically highlighted. The novel tribometer can be used to screen different bearing and shaft material combinations and to evaluate the friction and wear performance of self-lubricating bearings for the specific operating conditions found in Kaplan turbines.
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| 3. |
- Berglund, Kim, 1982-, et al.
(författare)
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Friction and Wear of Self-Lubricating Materials for Hydropower Applications under Different Lubricating Conditions
- 2017
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Ingår i: Lubricants. - : MDPI. - 2075-4442. ; 5:3
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Tidskriftsartikel (refereegranskat)abstract
- Self-lubricating bearings in hydropower applications are often lubricated with water under boundary lubricating conditions. Polyhydric alcohols replacing water have shown the potential to reduce both friction and wear. The objective of this work is, therefore, to evaluate the effect of a polyhydric alcohol-based environmentally-acceptable lubricant (EAL) on the friction and wear of self-lubricating materials for conformal contacts under boundary lubricating conditions. The lubricating properties of four commercially-available self-lubricating bearing materials were investigated under three different lubricating conditions: dry, water and a new polyhydric alcohol-based EAL. Bearing materials include one metallic composite and three polymer composites. A reciprocating motion test rig was used to evaluate the wear and friction properties. Surface analysis was performed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and optical profilometry. Results show that the wear rate for the polymer composites is increased when water is present compared to dry operating conditions. The new polyhydric alcohol-based EAL substantially improves both friction and anti-wear performance of all four self-lubricating bearing materials compared to both dry and water conditions. Surface analysis indicates that the material transfer to the counter-surface is limited when the polyhydric alcohol-based EAL is used.
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| 4. |
- Dahlberg, Carl F. O., 1980-, et al.
(författare)
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Correlation of Global Quantities at Material Characterization of Pressure-Sensitive Materials Using Sharp Indentation Testing
- 2021
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Ingår i: LUBRICANTS. - : MDPI AG. - 2075-4442. ; 9:3
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Tidskriftsartikel (refereegranskat)abstract
- Correlation of sharp indentation problems is examined theoretically and numerically. The analysis focuses on elastic-plastic pressure-sensitive materials and especially the case when the local plastic zone is so large that elastic effects on the mean contact pressure will be small or negligible as is the case for engineering metals and alloys. The results from the theoretical analysis indicate that the effect from pressure-sensitivity and plastic strain-hardening are separable at correlation of hardness values. This is confirmed using finite element methods and closed-form formulas are presented representing a pressure-sensitive counterpart to the Tabor formula at von Mises plasticity. The situation for the relative contact area is more complicated as also discussed.
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| 5. |
- de Boer, Gregory, et al.
(författare)
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On the Two-Scale Modelling of Elastohydrodynamic Lubrication in Tilted-Pad Bearings
- 2018
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Ingår i: Lubricants. - : MDPI. - 2075-4442. ; 6:3
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Tidskriftsartikel (refereegranskat)abstract
- A two-scale method for modelling the Elastohydrodynamic Lubrication (EHL) of tilted-pad bearings is derived and a range of solutions are presented. The method is developed from previous publications and is based on the Heterogeneous Multiscale Methods (HMM). It facilitates, by means of homogenization, incorporating the effects of surface topography in the analysis of tilted-pad bearings. New to this article is the investigation of three-dimensional bearings, including the effects of both ideal and real surface topographies, micro-cavitation, and the metamodeling procedure used in coupling the problem scales. Solutions for smooth bearing surfaces, and under pure hydrodynamic operating conditions, obtained with the present two-scale EHL model, demonstrate equivalence to those obtained from well-established homogenization methods. Solutions obtained for elastohydrodynamic operating conditions, show a dependency of the solution to the pad thickness and load capacity of the bearing. More precisely, the response for the real surface topography was found to be stiffer in comparison to the ideal. Micro-scale results demonstrate periodicity of the flow and surface topography and this is consistent with the requirements of the HMM. The means of selecting micro-scale simulations based on intermediate macro-scale solutions, in the metamodeling approach, was developed for larger dimensionality and subsequent calibration. An analysis of the present metamodeling approach indicates improved performance in comparison to previous studies.
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| 6. |
- Homayoun, Mohammad-Reza, et al.
(författare)
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Effect of Hygrothermal Ageing on Tribological Behaviour of PTFE-Based Composites
- 2018
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Ingår i: Lubricants. - : MDPI. - 2075-4442. ; 6:4
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Tidskriftsartikel (refereegranskat)abstract
- The present study investigates the influence of hygrothermal ageing on the tribological behaviour of polytetrafluoroethylene (PTFE) polymer composites. Three PTFE composites along with unfilled PTFE were tested in sliding contact against Inconel 625 (a Ni-based alloy) plates in both dry and water-lubricated conditions, utilising a unidirectional pin-on-disc tribometer. The tribo-tests were performed at a constant sliding speed of 0.13 m/s with a normal load of 84N providing an apparent contact pressure of 5 MPa. Hygrothermal conditioning was carried out at two different temperatures, and the water absorption evolution and kinetic parameters were estimated. Various characterisation methods were used to identify the wear mechanisms and influence of hygrothermal ageing on the degradation of the filler/matrix. The different tribological behaviour for different PTFE composites was observed within the ageing timeframe. The wear resistance of the fibre-filled samples was reduced compared to the non-aged ones over the ageing timeframe. However, the friction and wear resistance of the bronze-filled PTFE were enhanced by hygrothermal ageing.
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| 7. |
- Huang, De, et al.
(författare)
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Numerical Simulation of Static Seal Contact Mechanics Including Hydrostatic Load at the Contacting Interface
- 2021
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Ingår i: Lubricants. - Switzerland : MDPI. - 2075-4442. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- A finite element model of a static seal assembled in its housing has been built and is utilized to study how the seal deforms under varying loading conditions. The total contact load on the sealing surface is balanced by the sealed fluid pressure and the friction between the seal and the housing sidewall perpendicular to the sealing surface. The effect of the sealed fluid pressure between the sealing surfaces was investigated and the simulation showed that the surface profile is distorted due to the hydrostatic pressure. We study the distorted contact profile with varying sealed fluid pressure and propose five parameters to describe the corresponding contact pressure profile. One of these parameters, overshoot pressure, a measure of the difference between maximum contact pressure and the sealed fluid pressure, is an indicator of sealing performance. The simulations performed show different behaviors of the overshoot pressure with sealed fluid pressure for cosinusoidal and parabolic surfaces with the same peak to valley (PV) value.
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| 8. |
- Huang, Xianzhu, et al.
(författare)
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Tribological Properties of Porous PEEK Composites Containing Ionic Liquid under Dry Friction Condition
- 2017
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Ingår i: Lubricants. - Basel : MDPI. - 2075-4442. ; 5:2
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Tidskriftsartikel (refereegranskat)abstract
- NaCl particles were added into Polyetheretherketone (PEEK) and its composites to produce porous PEEK-based materials by washing NaCl away after the high-temperature compression molding process. After that, an ionic liquid was added into the porous materials under vacuum condition. Carbon fibers (CF), as reinforcement, and PTFE, as an internal solid lubricant, were employed to prepare PEEK composites. Tribological properties under dry friction condition were studied on a ring-on-disc tribo-meter. The influence of CF and PTFE on tribological properties was carefully investigated. The results indicated that, in comparison with traditional PEEK composites (CF/PTFE/PEEK), the porous PEEK composites containing ionic liquid showed much better tribological properties. It is found that CF can help PEEK form effective pores to suck in the ionic liquid resulting in a better tribological performance. CF reinforced porous PEEK containing ionic liquid (p-CF/PEEK + IL) demonstrated the lowest friction coefficient (27% of CF/PTFE/PEEK) and the lowest wear loss (only 0.9% of CF/PTFE/PEEK). Long time tribological test revealed that the wear mass loss comes from the running-in period, while its wear is negligible after this period. It is also found that the addition of PTFE has a negative influence on the tribological behaviors, especially under high sliding velocity and applied load
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| 9. |
- Hultqvist, Tobias, 1991-, et al.
(författare)
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On Waviness and Two-Sided Surface Features in Thermal Elastohydrodynamically Lubricated Line Contacts
- 2020
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Ingår i: Lubricants. - : MDPI. - 2075-4442 .- 2075-4442. ; 8:6
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Tidskriftsartikel (refereegranskat)abstract
- Machine components are designed to endure increasingly severe operating conditions due to the strive for improved energy efficiency of mechanical systems. Consequently, lubricated non-conformal contacts must rely on thin lubricant films where the influence of surface topography on the lubricating conditions becomes significant. Due to the complexity of the multiphysical problem, approximate assumptions are often employed to facilitate numerical studies of elastohydrodynamically lubricated (EHL) contacts. In this work, the rough, time dependent, thermal EHL problem is solved with focus on two main analyses. The first analysis focuses on the influence of sinusoidal roughness and the difference between a thermal non-Newtonian approach and an isothermal Newtonian approach. The second analysis is focused on the lubricating mechanisms taking place when two-sided surface features overtake within the thermal EHL contact. The results indicate that the film thickness in the outlet of the contact may be significantly overestimated by an isothermal Newtonian approach and that differences in the high-pressure region may also occur due to viscosity variations in the inlet of the contact. Moreover, for the studied two-sided surface features, it became evident that not only the surface feature combination but also the overtaking position influence the film thickness and pressure variations significantly.
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| 10. |
- Jain, Ayush, et al.
(författare)
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Development and Characterisation of Multi-Scale Carbon Reinforced PPS Composites for Tribological Applications
- 2019
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Ingår i: Lubricants. - : MDPI. - 2075-4442. ; 7:4
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Tidskriftsartikel (refereegranskat)abstract
- Polymer-based materials show to be of increasing interest in replacing metal based materials in tribological applications due to their low weight, cost and easy manufacturability. To further reduce the environmental impact of these bearing materials recyclability is becoming more crucial, stimulating the need for high performing thermoplastic materials. In this study, polyphenylene sulfide (PPS) composites were prepared in an effort to enhance its tribological properties. Short carbon fibres (SCFs), graphene oxide (GO) and nano diamonds (NDs) as well as polytetrafluoroethylene (PTFE) were used as micro and nano reinforcements. The addition of SCFs especially decreased the linear coefficient of thermal expansions while enhancing the micro hardness and wettability of the polymer. Under water lubricated conditions, a decrease in friction up to 56% and a reduction of wear rate in the order of 103 was observed by the addition of SCF. The reduction in friction and wear was further enhanced by the addition of NDs, providing a synergistic effect of the reinforcements in micro and nano scale. By testing the individual reinforcements under dry conditions, PTFE and SCFs were especially effective in reducing friction while the release and consequent abrasion of NDs and SCFs increased the wear under a higher contact pressure.
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