Středoevropský technologický institut VUT
Browse
Recent Submissions
Now showing 1 - 5 of 21
- ItemCrack Propagation Analysis of Compression Loaded Rolling Elements(MDPI, 2021-05-01) Dlhý, Pavol; Poduška, Jan; Berer, Michael; Gosch, Anja; Slávik, Ondrej; Náhlík, Luboš; Hutař, PavelThe problem of crack propagation from internal defects in thermoplastic cylindrical bearing elements is addressed in this paper. The crack propagation in these elements takes place under mixed-mode conditions-i.e., all three possible loading modes (tensile opening mode I and shear opening modes II and III) of the crack are combined together. Moreover, their mutual relation changes during the rotation of the element. The dependency of the stress intensity factors on the crack length was described by general parametric equations. The model was then modified by adding a void to simulate the presence of a manufacturing defect. It was found that the influence of the void on the stress intensity factor values is quite high, but it fades with crack propagating further from the void. The effect of the friction between the crack faces was find negligible on stress intensity factor values. The results presented in this paper can be directly used for the calculation of bearing elements lifetime without complicated finite element simulations.
- ItemSide-groove effect on fracture mechanical fatigue testing of PLA material(Elsevier, 2023-01-03) Milovanović, Aleksa; Poduška, Jan; Trávníček, Lukáš; Náhlík, Luboš; Sedmak, Aleksandar; Milošević, Miloš; Berto, FilippoPLA polymer is probably the most used thermoplastic material in FDM technology nowadays. Besides prototyping purposes, FDM materials are also considered for functional application use, thanks to the fast fabrication of components, relatively simple workflow, and the absence of material waste that this technology offers. Except for static material properties, data concerning the expected lifetime and reliability of AM parts under cyclic loading are also necessary for functional purposes. Due to the exceptional structural complexity of FDM parts, minor modifications in material testing must be used sometimes. The subject of this paper is the fracture mechanics-based fatigue testing of CT specimens that results in crack kinetics description like the Paris’ law. One of the objectives is also to describe the effect of SGs placed on the surface of CT specimens, and the effect of different layer heights. The main intention of the SGs is to secure a straight crack propagation path, thus imposing the crack to follow the Mode I condition, predominantly. A set of regular CT specimens and specimens with SGs was prepared with a full infill density interior and with different layer heights (0.1 mm and 0.3 mm). Crack kinetics measured on these specimens are presented in this paper and discussed.
- ItemInvestigation into the Effect of Molds in Grasses on Their Content of Low Molecular Mass Thiols(MDPI, 2012-10-24) Skládanka, Jiří; Adam, Vojtěch; Zítka, Ondřej; Kryštofová, Olga; Beklová, Miroslava; Kizek, René; Havlíček, Zdeněk; Sláma, Petr; Nawrath, adamThe aim of this study was to investigate the effect of molds on levels of low molecular mass thiols in grasses. For this purpose, the three grass species Lolium perenne, Festulolium pabulare and Festulolium braunii were cultivated and sampled during four months, from June to September. The same species were also grown under controlled conditions. High-performance liquid chromatography with electrochemical detection was used for quantification of cysteine, reduced (GSH) and oxidized (GSSG) glutathione, and phytochelatins (PC2, PC3, PC4 and PC5). Data were statistically processed and analyzed. Thiols were present in all examined grass species. The effect of fungicide treatments applied under field conditions on the content of the evaluated thiols was shown to be insignificant. Species influenced (p < 0.05) PC3 and GSSG content. F. pabulare, an intergeneric hybrid of drought- and fungi-resistant Festuca arundinacea, was comparable in PC3 content with L. perenne and F. braunii under field conditions. Under controlled conditions, however, F. pabulare had higher (p < 0.05) PC3 content than did L. perenne and F. braunii. Under field conditions, differences between the evaluated species were recorded only in GSSG content, but only sampling in June was significant. F. pabulare had higher (p < 0.05) GSSG content in June than did L. perenne and F. braunii.
- ItemQuo vadis progress in polymer stabilization?(Crinson Publishers, 2021-07-30) Tocháček, JiříSynthetic polymers processed and applied in practice undergo degradation changes that result in the potential loss of physico-chemical properties. Polymer stabilizers are used to prevent these changes and help the polymer utility properties get retained. Despite the visible progress in other fields of human activity, polymer stabilization chemistry has exhibited only minimum changes. No new chemistry of polymer stabilization has been introduced within the several past decades. All the “new” stabilizers promoted by different suppliers were mostly the modifications of the existing ones. Any new and commercially really successful stabilizer capable of competing the performance of old existing structures has not appeared on the market for many years.
- ItemPorous pseudo-substrates for InGaN quantum well growth: Morphology, structure, and strain relaxation(AIP Publishing, 2023-10-10) Ji, Yihong; Frentrup, Martin; Zhang, Xiaotian; Pongrácz, Jakub; Fairclough, Simon M.; Liu, Yingjun; Zhu, Tongtong; Oliver, Rachel A.Strain-related piezoelectric polarization is detrimental to the radiative recombination efficiency for InGaN-based long wavelength micro-LEDs. In this paper, partial strain relaxation of InGaN multiple quantum wells (MQWs) on the wafer scale has been demonstrated by adopting a partially relaxed InGaN superlattice (SL) as the pseudo-substrate. Such a pseudo-substrate was obtained through an electro-chemical etching method, in which a sub-surface InGaN/InGaN superlattice was etched via threading dislocations acting as etching channels. The degree of strain relaxation in MQWs was studied by x-ray reciprocal space mapping, which shows an increase of the in-plane lattice constant with the increase of etching voltage used in fabricating the pseudo-substrate. The reduced strain in the InGaN SL pseudo-substrate was demonstrated to be transferable to InGaN MQWs grown on top of it, and the engineering of the degree of strain relaxation via porosification was achieved. The highest relaxation degree of 44.7% was achieved in the sample with the porous InGaN SL template etched under the highest etching voltage. Morphological and structural properties of partially relaxed InGaN MQWs samples were investigated with the combination of atomic force and transmission electron microscopy. The increased porosity of the InGaN SL template and the newly formed small V-pits during QW growth are suggested as possible origins for the increased strain relaxation of InGaN MQWs.(c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)