Ústav mechaniky těles, mechatroniky a biomechaniky

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Analysing plate fixation of a comminuted fracture of the proximal ulna in relation to the elbow joint: a finite element study
(BMC, 2025-07-28) Šafran, Jindřich; Pavlacký, Tomáš; Marcián, Petr; Herůfek, Radim; Veselý, Radek
This study investigated the biomechanical behavior of four different screw configurations used to fix comminuted proximal ulna fractures with a locking compression plate (LCP), via a detailed finite element model based on realistic anatomical geometry. The model incorporated realistic anatomical geometry including both cortical and cancellous bone, soft tissue constraints, and loading conditions representing the physiological self-weight of the forearm, with the humerus fixed at its proximal end. The stress distribution on the plate, strain intensity within the bone tissue, and interfragmentary motion (IFM) between fracture fragments were evaluated for each configuration. The results indicate that all the tested configurations provide adequate stability under normal loading conditions, with no risk of material failure. However, excessive stress concentrations were observed in specific screw regions depending on the configuration, particularly when proximal screws anchoring the olecranon (e.g. screws 2 and 3 in Variant 3) were omitted. Strain analysis revealed moderate physiological bone loading across variants, whereas IFM assessment highlighted the importance of securing the coronoid and apical fragments to prevent compromised healing. These findings suggest that a specific reductions in osteosynthetic material, such as omitting certain diaphyseal screws while maintaining crucial olecranon and coronoid fixation, may provide sufficient fracture stabilisation under the modelled conditions, potentially minimising implant-related complications. This modelling approach offers a valuable tool for preclinical assessment of osteosynthesis strategies and supports future comparative research on fixation methods with varying biomechanical properties.
Backstress shift modelling concept for improving uniaxial ratcheting predictions for wrought 304 stainless steel and additively manufactured Inconel 718
(Elsevier, 2025-11-01) Adamec, Tomáš; Hassan, Tasnim; Zapletal, Josef; Kondepati, Sudhir Kumar; Šebek, František
The Chaboche model is one of the widely used models, but it still shows limitations in predicting various complex responses. For example, issues in predicting ratcheting responses of metals and alloys under stress-controlled loading, especially under uniaxial cyclic loading, have been demonstrated. Therefore, this study evaluates the performance of the Chaboche model under uniaxial cyclic loading with an emphasis given to the simulation of uniaxial ratcheting responses. A modification to the model is proposed to enhance its prediction of the uniaxial ratcheting response for a wide range of ratcheting rates. The modification technique is called the backstress shift model, developed on the basis of experimental observations of the similarity between the strain- and stress-controlled hysteresis loops. A backstress memory surface is introduced and its material parameters are calibrated using responses of stainless steel 304 and Inconel 718 superalloy. For this study, data for steel are collected from the literature and experiments are performed on superalloy to acquire a set of data for development and validation of the proposed model. The modified model demonstrates better predictability of the uniaxial ratcheting responses compared to the Chaboche model with the threshold, especially for the additively manufactured nickel-based superalloy. The modified model also works well for the wrought 304 stainless steel.
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.

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