Ústav konstruování

Browse

Now showing 1 - 5 of 95

Computational and experimental investigation of thermally auxetic multi-metal lattice structures produced by laser powder bed fusion
(Taylor & Francis Group, 2024-09-11) Červinek, Ondřej; Tucker, Michael Robert; Koutný, Daniel; Bambach, Markus
Communication antennas and optical systems of space-borne satellites require highly accurate relative positioning of components despite large variations in ambient temperature. As a potential solution, additive manufacturing technologies, such as laser powder bed fusion, enable the production of metamaterial structures with complex local geometries that can be designed to achieve the desired thermal and mechanical behaviours. Recent advances enable the processing of multiple materials within a single build to achieve composite structural properties that are infeasible using conventional single materials. This study investigates the potential of tailoring the structural thermal expansion properties of several configurations of a multi-metal re-entrant lattice structure made of stainless steel 316L and the copper alloy CuCr1Zr. Unit cells and lattice structure segments with theoretical coefficients of thermal expansion ranging from 1.64×105 °C1 to 2.51×105 °C1 (16% more than CuCr1Zr) are evaluated by finite element analysis and validated experimentally. Imperfections related to the manufacturing process are shown to have a significant effect on net expansion. The results indicate good agreement despite the imperfections. The study demonstrates the feasibility of designing and fabricating metal lattice structures for a specific thermal expansion within, as well as above and below, the range of thermal expansion of the parent materials.
Understanding frictional behavior in fascia tissues through tribological modeling and material substitution
(Elsevier, 2024-07-01) Streďanská, Alexandra; Nečas, David; Vrbka, Martin; Suchánek, Jakub; Matonohová, Jana; Toropitsyn, Evgeniy; Hartl, Martin; Křupka, Ivan; Nešporová, Kristina
The objective of this study is to develop a reliable tribological model to enable a more thorough investigation of the frictional behavior of fascia tissues connected to non-specific lower back pain. Several models were designed and evaluated based on their coefficient of friction, using a low-frequency, low-load reciprocating motion. The study found that two technical elastomers, layered on PDMS to simulate the fascia and underlying muscle, are suitable substitutes for biological tissue in the model. The influence of tribopair geometry was also examined, and the results showed that greater conformity of contact leads to a lower COF, regardless of the material combination used. Finally, the friction properties of HA of various molecular weights and concentrations were tested.
Thickener Behaviour in Rolling Elastohydrodynamic Lubrication Contacts
(SPRINGER/PLENUM PUBLISHERS, 2024-06-04) Okál, Michal; Košťál, David; Sakai, Kazumi; Křupka, Ivan; Hartl, Martin
This paper focuses on the study of thickener behaviour in elastohydrodynamic contacts using fluorescence microscopy and optical interferometry. A standard Ball-on-disc configuration tribometer was used as well as a newly developed Ball-on-ring configuration tribometer mimicking the conditions in a 6314 deep groove ball bearing. Three types of custom-made greases were used with the same base oil and Coumarin 6 fluorescent dye on the thickener. These greases contained no additives and were of the lithium complex and alicyclic di-urea type. It was confirmed that the behaviour of the grease was directly influenced by both the structure and type of thickener, and the differences were successfully observed by fluorescence microscopy. The concentration of thickener was found to change due to the formation or breakdown of the deposited thickener layer on the track. At low speeds, the alicyclic di-urea forms a thickener layer on the track that is more pronounced than the lubricating film formed by the base oil alone. The thickener layer formation also occurs at higher speeds when fully flooded conditions are beneficial in building the layer. Experiments with a Ball-on-ring tribometer show that the natural replenishment of the real contact geometry is more effective than on a Ball-on-disc tribometer, but the formation of the thickener layer on the track is more limited than in a Ball-on-disc tribometer with artificial replenishment. At higher velocities, a smaller but stable layer is formed where the layer does not change much from a certain velocity.
Grasping the behavior of magnetorheological fluids in gradient pinch mode via microscopic imaging
(AIP Publishing, 2024-04-03) Kubík, Michal; Žáček, Jiří; Goldasz, Janusz; Nečas, David; Sedlačík, Michal; Blahuta, Jiří; Bańkosz, Wojciech; Sapinski, Bogdan
Magnetorheological (MR) fluids are suspensions of micrometer-sized ferromagnetic particles in a carrier fluid, which react to magnetic fields. The fluids can be operated in several fundamental modes. Contrary to the other modes, the rheology and microstructure formation of the MR fluid in the gradient pinch mode have been studied to a far lesser extent. The magnetic field distribution in the flow channel is intentionally made non-uniform. It is hypothesized that the Venturi-like contraction is achieved via fluid property changes, leading to a unique behavior and the presence of a pseudo-orifice. The main goal is to investigate the presence of the Venturi-like contraction effect in the fluid by means of optical imaging and hydraulic measurements. To accomplish the goal, a unique test rig has been developed including a fluorescence microscope and MR valve prototype. The Venturi-like contraction hypothesis was confirmed. The results indicate that the effective flow channel size decreases by 92% at the maximum magnetic flux applied. This has a direct impact on the flow characteristics of the MR valve. The variation of the pressure–flow rate curve slope with magnetic field was demonstrated. The results provide valuable information for understanding the rheology and microstructure formation mechanism in MR fluids in the pinch mode.
Testing and modelling of transient adhesion phenomena in rolling-sliding contacts
(Tsinghua University, 2023-12-15) Kvarda, Daniel; Meierhofer, Alexander; Six, Klaus
Transient adhesion effects in rolling-sliding contacts influence all aspects of train-track interaction. This is of high importance specifically when these effects result in critically low adhesion, which poses a risk to traction and braking of railway vehicles. This study presents a model that can replicate the transient changes of the coefficient of adhesion with tested water and solid particle mix. The experimental data for the model are measured using a commercial ball-on-disc tribometer. The experimental results showed a liquid reservoir in front of the contact area that slowly reduces in size. This observation was used in the modelling approach to divide the calculation into two stages where the reservoir is present and when it disappears. The model was able to reproduce the occurrence of low adhesion region seen in experimental results with different particle concentrations.

Browse

Recent Submissions