Ústav mechaniky těles, mechatroniky a biomechaniky

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Critical length parameter of HDPE and its use in fatigue lifetime predictions
(GRUPPO ITALIANO FRATTURA, 2024-12-12) Kozáková, Kamila; Trávníček, Lukáš; Poduška, Jan; Klusák, Jan
The contribution describes the fatigue lifetime predictions of polyethylene notched specimens based on the theory of critical distance. The approach uses the line method, which averages the axial stress over the critical distance. The critical distance is determined from experimental data of Cracked Round Bar (CRB) specimens and specimens with a model notch. From these two sets of experimental fatigue data and corresponding axial stress distributions, the critical distance is determined. The critical distance depends on the number of cycles to failure and notch radius. For this reason, the critical distance is modified by the ratio of stress concentration factors and the modified distance is used for fatigue lifetime predictions of notched specimens with various notch radii. Using this approach significant savings in testing time can be achieved. CRB fatigue tests are commonly used tests for ranking PE pipe grades and are easily available. Adding the fatigue tests of notched specimens with a model notch, the critical parameter can be found, and fatigue lifetime predictions of various notches can be calculated.
Mixed mode I/III fatigue behaviour of Polyetheretherketone
(Elsevier, 2025-05-13) Arbeiter, Florian J.; Gosch, Anja; Vojtek, Tomáš; Pinter, Gerald; Hutař, Pavel; Berer, Michael
In this work, the fatigue behaviour of Polyetheretherketone (PEEK) under tensile (mode I) and out of plane shear (mode III) loading was examined by using razor blade notched cylindrical bars. By varying the applied mode I and mode III loadings, fatigue fracture curves of pure mode I and mixed mode I/III were established. The aim was to find a suitable method to describe the behaviour of both pure mode I and mixed mode I/III loading via a singular equivalent stress intensity factor. This was found possible by increasing the coefficient of the mode III contribution by more than 500 % compared to previous work on other thermoplastic materials. This significant change of the mode III contribution was mainly attributed to the different thermo-mechanical state of PEEK at the testing temperature of 23 degrees C, as well as the good friction properties of PEEK which leads to a transferal of mode III contributions to mode I contributions due to crack flank sliding of the formed factory roof formations on the fracture surfaces.
Dynamic performance and wear of ceramic aerodynamic tilting-pad journal bearings: Tested and simulated under excessive vibrations
(Elsevier, 2025-02-06) Pokorný, Jan; Návrat, Tomáš; Vajdák, Michal; Cabaj, Gabriel; Sliwková, Petra
Ceramic aerodynamic tilting-pad journal bearings are emerging as a crucial component in hydrogen-electric mobility, specifically within electric compressors that deliver compressed air to hydrogen fuel cells. These bearings provide an environmentally friendly solution by eliminating the need for oil lubrication, thus preventing contamination of the fuel cells. This study focuses on the wear performance of these bearings under extreme conditions, operated near critical bending speed with elevated vibrations surpassing assembly clearance levels. To assess performance, a custom high-speed test rig was developed, designed without coupling to ensure precise measurements and to prevent the transfer of vibrations from the electric drive to the bearing system. Detailed vibration measurements were conducted under extreme conditions across various rotational speeds, reaching up to 70000 rpm. Alongside this, a dynamic computational model of the aerodynamic bearing was created, incorporating an analysis of rotor vibrations. Experimental results were systematically compared with simulation data to validate the model’s accuracy. Key to this study is the wear assessment of the bearing pads under these demanding conditions. The findings reveal that the proposed bearings exhibit reliable performance even in highly demanding scenarios, demonstrating their robustness and potential applicability in other critical and high-stress environments.
Modeling Mechanical Properties of Titanium Scaffolds with Variable Microporosity
(WILEY-V C H VERLAG GMBH, 2024-10-01) Slámečka, Karel; Skalka, Petr; Pokluda, Jaroslav
The article introduces a two-level finite element model for metallic scaffolds with porosity at both design and material levels. Despite several additive manufacturing methods producing structures with controlled hierarchical porosity, their functional properties remain largely unknown, hindering industrial utilization. This article examines how material microporosity affects the mechanical properties of a scaffold prepared by direct ink writing from pure titanium with dimensions typical for orthopedic implants. The study focuses on the compressive response of scaffolds with microporosity ranging from 0.05 to 0.65. The article demonstrates the practical application of the model by estimating the effective Young's modulus and the relative length of the fatigue crack initiation stage. Tensile plastic strains at critical sites exhibit a delocalization from around micropores followed by relocalization into thinning interpore walls with increasing microporosity, resulting in the highest fracture strain predicted for microporosities between 0.2 and 0.3. These strains enable the estimation of the length of the fatigue crack initiation stage, which proves to be very short for all microporosities. This emphasizes the crucial role of the crack growth stage in scaffold fatigue life and confirms the potential for additional experiments on scaffolds with microporosities exceeding 0.15 to enhance their fatigue resistance. The article presents a finite element model for metallic scaffolds, investigating the impact of material microporosity on mechanical behavior. Specifically, the study investigates regular microporous titanium scaffolds prepared by direct ink writing, highlighting the effects of strain delocalization and relocalization on fracture strain. Moreover, the model estimates the relative length of the fatigue crack initiation stage in these materials.image (c) 2024 WILEY-VCH GmbH
Long-term zinc treatment alters the mechanical properties and metabolism of prostate cancer cells
(BMC, 2024-10-11) Navrátil, Jiří; Kratochvílová, Monika; Raudenská, Martina; Balvan, Jan; Vičar, Tomáš; Petrláková, Kateřina; Suzuki, Kanako; Jadrná, Lucie; Burša, Jiří; Kräter, Martin; Kyoohyun, Kim; Masařík, Michal; Gumulec, Jaromír
The failure of intracellular zinc accumulation is a key process in prostate carcinogenesis. Although prostate cancer cells can accumulate zinc after long-term exposure, chronic zinc oversupply may accelerate prostate carcinogenesis or chemoresistance. Because cancer progression is associated with energetically demanding cytoskeletal rearrangements, we investigated the effect of long-term zinc presence on biophysical parameters, ATP production, and EMT characteristics of two prostate cancer cell lines (PC-3, 22Rv1). Prolonged exposure to zinc increased ATP production, spare respiratory capacity, and induced a response in PC-3 cells, characterized by remodeling of vimentin and a shift of cell dry mass density and caveolin-1 to the perinuclear region. This zinc-induced remodeling correlated with a greater tendency to maintain actin architecture despite inhibition of actin polymerization by cytochalasin. Zinc partially restored epithelial characteristics in PC-3 cells by decreasing vimentin expression and increasing E-cadherin. Nevertheless, the expression of E-cadherin remained lower than that observed in predominantly oxidative, low-invasive 22Rv1 cells. Following long-term zinc exposure, we observed an increase in cell stiffness associated with an increased refractive index in the perinuclear region and an increased mitochondrial content. The findings of the computational simulations indicate that the mechanical response cannot be attributed exclusively to alterations in cytoskeletal composition. This observation suggests the potential involvement of an additional, as yet unidentified, mechanical contributor. These findings indicate that long-term zinc exposure alters a group of cellular parameters towards an invasive phenotype, including an increase in mitochondrial number, ATP production, and cytochalasin resistance. Ultimately, these alterations are manifested in the biomechanical properties of the cells.

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