NCK MESTEC - sekce mechaniky těles a mechatroniky

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    Eulerian Description of Rail Straightening Process
    (Trans Tech Publications, 2014-08-18) Návrat, Tomáš; Petruška, Jindřich
    The paper deals with numerical analysis of the process of roller straightening of rails. The problem of repeated elasto-plastic bending is solved by a program in MATLAB based on FEM algorithm with Eulerian description of material flow through the straightening machine. Beam element formulation with a shear deformation effect is used for the rail discretization. The results are compared with literature and standard FE analysis with Lagrangian description of material flow. Effectiveness of presented formulation is discussed and its applicability for fast iterative optimization of the straightening process is illustrated.
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    Biomechanical Testing of Spinal Segment Fixed by Arcofix System on the Swine Spine
    (Korean Society of Spine Surgery, 2015-07-28) Kelbl, Martin; Kočiš, Ján; Veselý, Radek; Florian, Zdeněk; Návrat, Tomáš; Vosynek, Petr
    To evaluate the mechanical properties of the spinal segment in the intact, injured, and stabilized state after fixation by an Arcofix implant. Several types of thoracolumbar spine injury necessitates anterior instrumentation. The Arcofix plate represents the latest generation of angular stablity systems. The biomechanical properties of these implants have not been sufficiently studied yet. A total of ten porcine specimens (levels Th12-L3) were prepared. The tests were performed for intact, injured, and implanted specimens. In each state, the specimen was subjected to a tension load of a prescribed force, and subsequently, twisted by a given angle. The force load was 200 N. The torsion load had a deformation character, i.e., the control variable was the twisting angle and the measured variable was the moment of a couple. The amplitude of the load alternating cycle was 3°. Another parameter that was evaluated was the area of the hysteresis loop. The area corresponds to the deformation energy which is dissipated during the cycle. A statistically significant difference was found between the intact and injured states as well as between the injured and implanted specimens. The statistical evaluation also showed a statistically different value of the hysteresis loop area. In the case of instability, the area decreased to 33% of the physiological value. For the implanted sample, the area increased to 170% of the physiological value. The Arcofix implant with its parameters appears to be suitable and sufficiently stable for the treatment of the anterior column of the spine.
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    Characterization of Polyvinylidene Fluoride (PVDF) Electrospun Fibers Doped by Carbon Flakes
    (MDPI, 2020-11-24) Kaspar, Pavel; Sobola, Dinara; Částková, Klára; Knápek, Alexandr; Burda, Daniel; Orudzhev, Farid; Dallaev, Rashid; Tofel, Pavel; Trčka, Tomáš; Grmela, Lubomír; Hadaš, Zdeněk
    Polyvinylidene fluoride (PVDF) is a modern polymer material used in a wide variety of ways. Thanks to its excellent resistance to chemical or thermal degradation and low reactivity, it finds use in biology, chemistry, and electronics as well. By enriching the polymer with an easily accessible and cheap variant of graphite, it is possible to affect the ratio of crystalline phases. A correlation between the ratios of crystalline phases and different properties, like dielectric constant as well as piezo- and triboelectric properties, has been found, but the relationship between them is highly complex. These changes have been observed by a number of methods from structural, chemical and electrical points of view. Results of these methods have been documented to create a basis for further research and experimentation on the usability of this combined material in more complex structures and devices.
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    Automated calibration of advanced cyclic plasticity model parameters with sensitivity analysis for aluminium alloy 2024-T351
    (SAGE Publications, 2019-03-18) Peč, Michal; Šebek, František; Zapletal, Josef; Petruška, Jindřich; Hassan, Tasnim
    The plasticity models in finite element codes are often not able to describe the cyclic plasticity phenomena satisfactorily. Developing a user-defined material model is a demanding process, challenging especially for industry. Open-source Code_Aster is a rapidly expanding and evolving software, capable of overcoming the above-mentioned problem with material model implementation. In this article, Chaboche-type material model with kinematic hardening evolution rules and non-proportional as well as strain memory effects was studied through the calibration of the aluminium alloy 2024-T351. The sensitivity analysis was performed prior to the model calibration to find out whether all the material model parameters were important. The utilization of built-in routines allows the calibration of material constants without the necessity to write the optimization scripts, which is time consuming. Obtaining the parameters using the built-in routines is therefore easier and allows using the advanced modelling for practical use. Three sets of material model parameters were obtained using the built-in routines and results were compared to experiments. Quality of the calibration was highlighted and drawbacks were described. Usage of material model implemented in Code_Aster provided good simulations in a relatively simple way through the use of an advanced cyclic plasticity model via built-in auxiliary functions.
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    Implementation of American weld connection standards into finite element computations
    (IOP Publishing, 2017-03-01) Peč, Michal; Vosynek, Petr; Šebek, František; Návrat, Tomáš
    Presented paper summarizes the weld assessment on the basis of national standards. The approach concerning the static loading has been still not adequately developed in standards. The standard of American Institute of Steel Construction was considered as the best candidate for implementation into the finite element method using post-processing of a particular weld. A comparative analysis of maximum allowable loadings of weld connection models loaded in-plane and out-of-plane has been performed. Results obtained by the finite element method and American and Czech standards were compared and the conclusions of applicability and reliability were made.