Lidé na fakultě
Ing. Nela Krčmářová

2024, Vol. 48 (2024): 19th Youth Symposium on Experimental Solid Mechanics, Praha, České vysoké učení technické v Praze), p. 15-21), ISBN 978-80-01-07358-2

The paper deals with the examination of the ageing effects on the mechanical properties stability of 3D printed material via stereolithography under compression when subjected to various conditions, including UV radiation, X-rays, and the effects of time, from the opening of the bottle with the material to the 3D-printing process. The sets of samples under investigation were subjected to quasi-static and dynamic compression loading using an Split Hopkinson Pressure Bar. The aim of this paper is to investigate the long-term stability of the samples in terms of their mechanical properties and material behaviour and their degradation pattern. Despite the manufacturer’s information, it was found that the mechanical behaviour of the printed samples was significantly affected by the ageing process.

Stať ve sborníku z prestižní konf. (Scopus)

2024, Transforming Construction: Advances in Fiber Reinforced Concrete, Springer Nature), p. 573-580), ISBN 978-3-031-70144-3, ISSN 2211-0844

he Ultra high-performance steel fibres reinforced concrete (UHPFRC) investigated in this paper is a fine-grained cement-based composite material with outstanding mechanical properties. Its key attributes include an ultra-high compressive strength in excess 150 MPa and a permanent post-cracking strength in excess 5 MPa. To increase its structural integrity, steel fibres 13 mm long and 0.2 mm in diameter are added to the matrix to reinforce it. In order to assess the properties of the UHPFRC under varying loading conditions, the prism-shaped specimens are subjected to three-point bending tests over a range of loading rates from quasi-static regime to dynamic impacts at intermediate strain rates. The experiments are performed using an in-house developed testing machine based on linear motors and are conducted at 4 different loading velocities with at least 5 specimens tested at each strain rate. The tests are observed using a high-speed camera. For a better understanding of the material behaviour, the testing equipment is combined with a laboratory high power X-ray imaging set-up that allows internal inspection of the samples to analyze the effect of imperfections, inhomogeneities, voids and dominant fibre orientation. X-ray imaging is performed before and after mechanical testing and also in-situ during the loading using a high-speed X-ray imaging camera. A significant dynamic increase factor is observed between the individual strain rates, while the dominant fibre orientation is identified as a crucial aspect causing the differences between the specimens. This innovative experimental approach provides invaluable insights into the material response to dynamic loading conditions and offers a comprehensive understanding that is crucial for optimizing its performance in a variety of real-world applications.

Stať ve sborníku z prestižní konf.

2024, Materials Letters, 2024 (354), p. 1-4), ISSN 0167-577X

Processes of internal damage development during localized dynamic penetration represent a crucial mechanism important for relevant analysis of deformation and failure of plates and sandwich panels under high strain rate conditions. Soft cellular materials are of special importance as the internal damage defines mode of collapse and energy absorption capabilities. In this paper, a fast X-ray radiography is employed for in-situ analysis of the internal damage development in soft closed-cell aluminum foam subjected to a localized high strain rate penetration using an instrumented projectile in a direct impact Hopkinson bar apparatus. The process with a typical duration of a few milliseconds is visualized using four X-ray projections acquired using a flash X-ray system and a high-speed camera. Internal damage such as cracking, shear failure in the vicinity of the projectile, and compaction of the material is successfully identified. This unique method utilizing a laboratory based X-ray source allows for characterization of the penetration mechanism that has been usually analyzed only in post-mortem state.

Článek v periodiku excerpovaném SCI Expanded

2024, Emergent Materials, 2024, ISSN 2522-5731

Excellent mechanical properties of ultra high performance concrete make it suitable for use in special applications, where the material is subjected to dynamic phenomena such as impacts, explosions, or earthquakes. This paper presents a novel experimental approach that integrates a Split Hopkinson Pressure Bar with a flash X-ray system and high-speed optical imaging to investigate the dynamic behavior of steel fiber reinforced UHPC under high strain rate uni-axial compression. In-situ Flash X-ray radiography emerges as a particularly effective tool, providing clear visualization of deformation response and overcoming challenges associated with flying debris encountered in optical inspection. Moreover, computed tomography and scanning electron microscopy appear as a vital technique for analyzing micro-structure and fiber distribution and orientation. The combined approach offers a promising method to study the dynamic behavior of steel fiber reinforced ultra high performance concrete and also holds promise for analyzing more complex modes of deformation and material interactions, providing valuable insights for enhancing the design and performance of critical infrastructure subjected to dynamic loading events.

Článek v periodiku excerpovaném databází Scopus

2023, Young Transportation Engineers Conference 2022, Praha, České vysoké učení technické v Praze), p. 89-97), ISBN 978-80-01-07224-0, ISSN 2336-5382

The article presents a study of the mechanical processes occurring during the aluminothermic reaction using experimental methods (strain gauges, digital image correlation, thermography, scanning electron microscopy, profilometry). The aluminothermic reaction is a highly efficient welding method due to its exothermic behaviour, however, it places considerable demands not only on the welding technique, but also on the capabilities of the experimental methods used; these limitations are also discussed in the article. The aluminothermic reaction is associated with the formation of a localised heat source with a time evolution dictated by the technological procedure, which manifests itself in heat propagation to the surrounding weld material. The unequal evolution of the temperature field is the fundamental cause of the appearance of the heat affected zone or local deformations or surface curvature, which was the focus of the experimental methods deployed above and the results of which are shown in the article.

Stať ve sborníku z prestižní konf. (Scopus)

2023, Vol. 42 (2023): 18th Youth Symposium on Experimental Solid Mechanics, Praha, České vysoké učení technické v Praze), p. 17-21), ISBN 978-80-01-07237-0, ISSN 2336-5382

This article focuses on the mechanical properties of basalt in compressive loading at different strain-rates. The study employs advanced instrumentation for the evaluation of the results in dynamic conditions, while standard uni-axial loading device is used for evaluation in quasi-static conditions. Basalt specimens were subjected to four different loading-rates from 200-600 s−1 on which the stress-strain dependence was evaluated together with DIC analysis of crack initiation and disintegration process. Understanding the mechanical properties of basalt can provide insights for engineers and designers in creating structures that are durable and able to withstand different loading conditions. The findings of this study can have implications for a wide range of industries, including aerospace, automotive, and construction, among others.

Stať ve sborníku z prestižní konf. (Scopus)

2023, Vol. 42 (2023): 18th Youth Symposium on Experimental Solid Mechanics, Praha, České vysoké učení technické v Praze), p. 51-54), ISBN 978-80-01-07237-0, ISSN 2336-5382

Ultra high performance concrete is a modern cementitious material which exhibits excellent mechanical properties such as damage tolerance, fracture toughness and durability. These features make this materials suitable for wide range of applications where is the material subjected to different modes of loading and different loading rates. This paper deals with measurement of the Ultra high performance concrete reinforced with steel fibres in quasi-static compression mode of deformation and two elevated strain rates using split Hopkinson pressure bar. The results of the measurement show high increase of the mechanical properties with elevated strain rate.

Stať ve sborníku z prestižní konf.

2017

Zkoušky určování povrchové tvrdosti představují jedny z nejpoužívanějších zkoušek materiálů. Výhodou tohoto typu testu je jeho minimální destruktivnost, nízké náklady na jeho provádění a relativně jednoduché vyhodnocení. Tato diplomová práce se zabývá vytvořením automatizované procedury měření povrchové tvrdosti porézních materiálů. Tyto poznatky jsou využity pro napsání programu pro určování nejvhodnějších lokací pro indentaci. Vstupy pro vytvořený program jsou fotografie porézního materiálů. Složením těchto snímků je vytvořen pracovní obraz na němž jsou následně prováděny procedury sloužící k identifikaci nejvhodnějších lokací pro indent. Výstupem skriptu je G-kód pro CNC proceduru. Vytvořený G-kód je použit pro experimentální měření tvrdosti. Vytvořené indenty jsou nafoceny, poté jsou změřeny velikosti jejich úhlopříček, které poslouží jako základ pro výpočet tvrdosti podle Vickerse.

Diplomová práce

2017, ExNum 2016, Praha, CESKE VYSOKE UCENI TECHNICKE V PRAZE), p. 29-32), ISBN 978-80-01-06070-4, ISSN 2336-5382

The paper deals with investigation of deformation behaviour of gellan gum (GG) based structures prepared for regenerative medicine purposes. Investigated material was synthesized as porous spongy-like scaffold reinforced by bioactive glass (BAG) nano-particles in different concentrations. Deformation behavior was obtained employing custom designed experimental setup. This device equipped with bioreactor chamber allows to test the delivered samples under simulated physiological conditions with controlled flow and temperature. Cylindrical samples were subjected to uniaxial quasistatic loading in tension and compression. Material properties of plain GG scaffold and reinforced scaffold buffered by 50wt% and 70wt% BAG were derived from a set of tensile and compression tests. The results are represented in form of stress-strain curves calculated from the acquired force and displacement data.

Stať ve sborníku z prestižní konf.

2017, Acta Polytechnica, 57 (1), p. 14-21), ISSN 1210-2709

This study is focuses on an investigation of the reinforcement effect of the bioactive glass nano-particles in the gellan gum (GG) scaffolds used in bone tissue engineering. The investigated material was synthesized as the porous spongy-like structure improved by the bioactive glass (BAG) nano-particles. Cylindrical samples were subjected to a uniaxial quasi-static loading in tension and compression. Very soft nature of the material, which makes the sample susceptible to damage, required employment of a custom designed experimental device for the mechanical testing. Moreover, as the mechanical properties are significantly influenced by testing conditions the experiment was performed using dry samples and also using samples immersed in the simulated body fluid. Material properties of the pure GG scaffold and the GG-BAG reinforced scaffold were derived from a set of tensile and compression tests under dry and simulated physiological conditions. The results are represented in the form of stress-strain curves calculated from the acquired force and displacement data.

Článek v periodiku excerpovaném databází Scopus cizojaz.

2017, ExNum 2016, Praha, CESKE VYSOKE UCENI TECHNICKE V PRAZE), p. 72-75), ISBN 978-80-01-06070-4, ISSN 2336-5382

Metal foams are innovative porous material used for wide range of application such as deformation energy or sound absorption, filter material, or microbiological incubation carrier. To predict mechanical properties of the metal foam is necessary to precisely describe elasto–plastic properties of the foam on cell–wall level. Indentation with low load is suitable tool for this purpose. In this paper custom designed instrumented microindentation device was used for measurement of cell-wall characteristics of two different aluminium foams (ALPORAS and ALCORAS). To demonstrate the possibility of automated statistical estimation of measured characteristics the device had been enhanced by semi-automatic indent positioning and evaluation procedures based on user-defined grid. Vickers hardness was measured on two samples made from ALPORAS aluminium foam and one sample from ALCORAS aluminium foam. Average Vickers hardness of ALPORAS foam was 24.465HV1.019 and average Vickers hardness of ALCORAS was 36.585HV1.019.

Stať ve sborníku z prestižní konf.

2017, Materials and Technology, 51 (3), p. 397-402), ISSN 1580-2949

The presented work is aimed at a demonstration of modern radiological methods for an investigation of the deformation behaviour of bone scaffolds. Bone scaffold is an artificial structure used for the repairs of trabecular bones damaged by injuries or degenerative diseases. In bone-tissue engineering a proper description of its deformation behaviour is one of the most important characteristics for an assessment of the biocompatibility and bone-integration characteristics of the proposed structure intended to be used as a bone scaffold. According to recent studies bioactive-glass-reinforced gellan-gum (GG-BAG) is a promising material for bone-scaffold production. However, its low specific stiffness and simultaneous low attenuation to X-rays makes both the mechanical and imaging parts of the deformation experiments difficult. As a result a state-of-the-art experimental setup composed of high-precision micro-loading apparatus designed for the X-ray observation of deformation processes and an advanced radiographical device is required for such experiments. High-resolution time-lapse micro-focus X-ray computed tomography (micro CT) under loading in three different imaging modes was performed to obtain a precise structural and mechanical description of the observed deforming GG-BAG scaffolds.

Článek v periodiku excerpovaném SCI Expanded

2017, 25th INTERNATIONAL CONFERENCE ON MATERIALS AND TECHNOLOGY - PROGRAM AND BOOK OF ABSTRACTS, Ljubljana, Inštitut za kovinske materiale in tehnologije), ISBN 978-961-94088-1-0

The microtomography inspection was performed using the patented (European patent no. EP2835631) in-house designed modular radiographical imaging device equipped with scintillators, large single photon counting and spectroscopic detectors. From reconstructed volumetric data internal microarchitecture, porosity, cell-wall thickness and BAG distribution was derivated. The results were compared with the results of the planar analysis of thin scaffold layers prepared by cryosection.

Abstrakt ve sborníku z mezinár. konf.

2016, Engineering Mechanics 2015, Zurich, TRANS TECH PUBLICATIONS LTD), p. 665-670), ISSN 1662-7482

This work presents deformation behaviour of gellan gum and gellan gum - bioactive glass composites as novel hydrophilic materials for production of scaffolds in the field of bone-tissue engineering. According to recent studies such materials are attractive for personalized design of implants thanks to their biocompatibility and wide range of available fabrication methods. Batch of samples was subjected to uni-axial compression loading in a custom designed loading device to obtain their elastic and plastic characteristics. However the testing procedure was challenging because of very low stiffness of the material acquired results show a significant reinforcement effect of bioactive glass and its influence to the elastic modulus.

Stať ve sborníku z mezinár. konf.

2016, XIVth Youth Symposium on Experimental Solid Mechanics, Praha, ČVUT v Praze, Česká technika - nakladatelství ČVUT), p. 71-74), ISBN 978-80-01-05885-5, ISSN 2336-5382

This work presents deformation behaviour of cost effective chopped fiber composites. Use of chopped fibre is advantageous for manufacturing however complex shape parts production technology could be challenging. Batches of samples with different fibres composition were subjected to uni-axial tensile loading to obtain overall materials properties and inspection of proper manufacturing based on local deformation inhomogeneities. Both crossbeam displacement and optical strain measurement were used for elastic characteristics evaluation. Deformation response was derived from full-field optical strain measurements based on digital image correlation method. Relatively large variation of mechanical properties testing of samples was found.

Stať ve sborníku z prestižní konf.

2016, Book of Abstracts of International Symposium on Experimental Methods and Numerical Simulation in Engineering Sciences 2016, Praha, katedra mechaniky a materiálů), ISBN 978-80-01-06009-4

This study is focused on investigation of the reinforcement effect of the nano-particles in gellan gum (GG) scaffolds used in bone tissue engineering. This hydrophilic polysaccharide-based material is attractive for personalized design of implants thanks to its biocompatibility and wide range of available fabrication methods. Material properties of pure GG scaffold and GG-BAG reinforced scaffold were derived from set of tensile and compression tests under dry and simulated physiological conditions. The results are represented in form of stress-strain curves calculated from acquired force and displacement data. It can be concluded that significant reinforcement effect of BAG was observed only during the compressive loading.

Abstrakt ve sborníku z mezinár. konf. cizojazyčně

2016, Book of Abstracts of International Symposium on Experimental Methods and Numerical Simulation in Engineering Sciences 2016, Praha, katedra mechaniky a materiálů), ISBN 978-80-01-06009-4

This work presents deformation behaviour of cost effective chopped fiber composites. Use of chopped fibre is advantageous for manufacturing however complex shape parts production technology could be challenging. Batches of samples with different fibres composition were subjected to uni-axial tensile loading to obtain overall materials properties and inspection of proper manufacturing based on local deformation inhomogeneities. Deformation behaviour of chopped fibre composites represented by stress-strain curves and strain maps was described based on optical measurement of standard tensile test. Local strain concentrations predicting failure area was identified.

Abstrakt ve sborníku z mezinár. konf. cizojazyčně

2016, XIVth Youth Symposium on Experimental Solid Mechanics, Praha, ČVUT v Praze, Česká technika - nakladatelství ČVUT), p. 35-38), ISBN 978-80-01-05885-5, ISSN 2336-5382

In this study high resolution thermography is used for identification of damaged zones in Carbon fiber/polyphenylene sulfide (C/PPS) long fiber composite specimens with induced impact damage subjected to tensile fatigue loading. Image processing techniques were applied on thermographs from all loading cases to obtain segmented images of the damaged location that were then used for calculation of the heated area. Results show that the considered method can be used to identify heated area in the vicinity of damage with high confidence at low number of cycles where no significant fatigue effect is present in the material.

Stať ve sborníku z prestižní konf.

2016, Program and book of abstracts, Ljubljana, IMT Ljubljana), ISBN 978-961-94088-0-3

Regeneration of human bone tissue damaged by injuries or by degenerative diseases using biodegradable bone scaffold is one of the promising alternatives to the established treatment methods. This paper is focused on monitoring of BAG particles distribution and material properties mapping employing scanning electron microscopy (SEM) observation and nanoindentation measurement.

Abstrakt ve sborníku z mezinár. konf.

2016

SEM inspection, EDX analysis, hardness testing and grain structure analysis of delivered rectangular hollow structural section performed using in-house developad evaluation procedures.

Výzkumná zpráva cizojazyčně

2015, Proceedings of the 11th Conference on Local Mechanical Properties, Uetikon-Zurich, Trans Tech Publications), p. 129-133), ISBN 978-3-03835-555-7, ISSN 1013-9826

This paper deals with evaluation of mechanical properties of human trabeculae in the interconnection area. Local changes in the trabecular connections were evaluated using both quasi-static nanoindenation and modulus mapping technique. Connecting point of two trabeculae was revealed by precise grinding and polishing. A rectangular region in the interconnection was selected and inspected by modulus mapping procedure. Moreover several quasi-static indentation measurements using cube-corner indenter were performed along distinct lamellae. The obtained elastic properties were then compared with the values of the rod-like trabeculae. The comparison does not indicate significant differences in elastic properties between the trabecular rods and interconnections.

Stať ve sborníku z prestižní konf. (Scopus)

2015, Proceedings of the 11th Conference on Local Mechanical Properties, Uetikon-Zurich, Trans Tech Publications), p. 138-141), ISBN 978-3-03835-555-7, ISSN 1013-9826

Bone implants in form of artificial scaffolds manufactured from poly-lactic acid (PLA) represent an attractive alternative to traditional surgical treatments of defective bones (i.e. autografts and allografts). In this work factors influencing biocompatibility and primary stability of implants manufactured from PLA using direct 3D printing were assessed using nanoindentation. For this reason bulk sample of the PLA material and a printed object were subjected to nanomechanical measurement. Quasi-static nanoindentation was employed to identify elastic modulus and hardness distribution on surface and within volume of the samples. Moreover mechanical properties along scanning direction and interlayer characteristics were also assessed. Gradients in mechanical properties have been identified within volume of the material, within the printing layers and at contact between individual layers.

Stať ve sborníku z prestižní konf. (Scopus)

2015

Předložená práce se zabývá návrhem, výrobou a zprovozněním indentačního zařízení schopného indentace při nízkém zatížení metodou Vickers. Za tímto účelem byly navrženy dva způsoby zatěžování, první mód pro zatěžování konstantní silou při osazení indentoru závažím a druhý mód pro zatěžování řízené silou. Pro řízení indentačního zařízení technologií CNC byl vybrán modul LinuxCNC verze 2.6.4. Pro oba způsoby zatěžování vzniklo vlastní uživatelské rozhraní. Pro kontrolu správné funkce indentoru a vyhodnocovacích procedur bylo provedeno měření na referenční tvrdoměrné destičce. Toto měření dokázalo plnou funkčnost indentačního zařízení při nízkých zatěžovacích silách. Chyba měření byla mnohem menší, než chyba předepsaná normou.

Diplomová práce

2015

Předmětem díla je porovnání neoriginálního náhradního dílu s originálním dílem kapoty Škoda Fabia II z pohledu bezpečnosti dopravy. Dále je předmětem díla zjištění zda neoriginální náhradní díly jednoho výrobce mají stejné mechanické vlastnosti.

Výzkumná zpráva v češtině

2014, 9th International Conference on Local Mechanical Properties, Zürich, Transtech Publications), p. 257-260), ISBN 978-3-03785-876-9, ISSN 1013-9826

The paper presents application on digital image correlation (DIC) and microindentation for investigation of plastic flow under Brinell ball indenter applied on steel specimen made of two screwed together parts. Specimens in two different material state (a) as delivered (b) annealed were investigated. This approach enables internal surface to act as the external one and to be examined by mentioned methods. Results obtained by application of DIC on scanning electron microscope (SEM) images are compared to microhardness maps and agreement is demonstrated.

Stať ve sborníku z prestižní konf.

2013, Proceedings of 12th Youth Symposium on Experimental Solid Mechanics, Bari, Politecnico di Bari)

Presented article is focused on plastic strain distribution assessed by microindentation technique. If the nature of the specimen is unsuitable for radiological inspection destructive sample preparation is inevitable to describe its inner state. Two mutually perpendicular planes in deformed zone were manufactured by electrical discharge machining and polishing. One of the planes was created prior the loading procedure and during the loading the parts were connected by screws. In the perpendicular plane the specimen was cut after the loading. Specimens prepared using this procedure were then subjected to microindentation testing. Each set of indents consisted of approximately 150 Vickers microindents covering area of 0.5 mm2. Images of imprint arrays on the specimens surface were acquired by scanning electron microscope. Diagonals of indents were determined and then Vickers hardness values were calculated. Map of hardness distribution was created using cubic interpolation method.

Stať ve sborníku z mezinár. konf.

2012, Experimental Methods and Numerical Simulation in Engineering Sciences, Proceedings of XIIIth Bilateral Czech/German Symposium, Praha, České vysoké učení technické v Praze, Fakulta dopravní), p. 31-34), ISBN 978-80-01-05062-0

This paper deals with evaluation of hardness fields using Vickers indentation device and a semi-automatic software tool for local hardness distribution mapping. Specimens from low carbon pipeline steels were investigated. Dense grids of Vickers imprints were created on the specimens' surface. Image data of the surface were acquired by a high resolution flatbed scanner with resolution 6400dpi. The imprints were measured using an custom-developed image processing tool, and from the obtained geometrical characteristics local hardness values were derived. From the local hardness values and their coordinates the hardness field in the whole specimen was interpolated.

Stať ve sborníku z mezinár. konf. cizojazyčně