Lidé na fakultě
Ing. Tomáš Doktor, Ph.D.

2024, Composite Structures, 333, p. 1-10), ISSN 0263-8223

The study presents an extensive mechanical and computational characterisation of novel cellular metamaterial with axisymmetric chiral structure (ACS) at different strain rates. The Direct Impact Hopkinson Bar (DIHB) testing device was used for impact testing up to 21 m/s striker speed, which was insufficient to reach the shock deformation regime. Thus, using computational simulations to estimate the structure behaviour at high strain rates was necessary. Experimental and computational results showed that all ACS structures exhibit a nominal stress–strain relationship typical for cellular materials. As the loading conditions shifted to a dynamic regime, the micro–inertia effect became increasingly pronounced, leading to a corresponding rise in structure stiffness. The Poisson's ratio in all ACS increases gradually, making them superior to traditional cellular materials, which experience a sudden increase in Poisson's ratio during loading. Additionally, the study found that the structures exhibited a rise in the auxetic effect with an increase in strain rate, highlighting the benefits of axisymmetric structures in high-loading regimes. Overall, the obtained results provide valuable insights into the mechanical properties of ACS under different loading regimes and will contribute to further design improvements and the fabrication of novel ACS metamaterials.

Článek v periodiku excerpovaném SCI Expanded

2024, ISBN 978-80-01-07358-2, ISSN 2336-5382

Sborník z mezinár. konf. cizojazyčně

2023, ISBN 978-80-01-07237-0, ISSN 2336-5382

Sborník z mezinárodní konf. v češtině

2022, Materials Science and Engineering A - Structural Materials: Properties, Microstructure and Processing, 852 (5), ISSN 0921-5093

One of the most promising options for future crashworthiness applications is thin-walled tubes filled with various cellular materials (e.g. metal foam). Of higher interest are the shell-based lattices, which have lately gained popularity due to their superior qualities over strut-based lattices. In this work, we investigate the mechanical response of foam-filled tubes where the tube's core was represented by Triply Periodic Minimal Surface (TPMS) diamond lattices. Samples made of stainless steel 316L comprising the diamond lattice core, empty tubes, and in-situ TPMS-filled tubes were additively manufactured and mechanically tested under compressive loading. As-fabricated welded tubes and ex-situ TPMS-filled tubes were also analysed and compared. Under the axial loading, the ex-situ and in-situ TPMS-filled tubes showed very similar behaviour. Enhanced energy absorption up to 21% and 44% compared to the sum of empty tubes and the core responses was noted. The energy absorption enhancement of 12% in the case of transversal loading is limited to in-situ TPMS-filled tubes, where the connection between the tube and core prevents the tube's walls from buckling. Computational models with homogenised core were developed and validated based on the experimental data. These straightforward, fast, and accurate computational models can be efficiently used for large-scale real-life applications, e.g. crash and impact.

Článek v periodiku excerpovaném SCI Expanded

2022, ENGINEERING MECHANICS 2022, Prague, Institute of Theoretical and Applied Mechanics, AS CR), p. 85-88), ISBN 978-80-86246-51-2, ISSN 1805-8256

In this study, the relation between the presence of the filler in different types of open auxetic lattices and their Poisson’s functions was investigated using optical strain measurement technique and Digital Image Correlation (DIC) algorithms. Three different types of auxetics were manufactured using Selective Laser Sintering (SLS) technique from 316L–040 stainless steel alloy: (i) 2D re-entrant, (ii) 3D re-entrant and (iii) 2D missing rib structure. All types of SLS printed auxetics were then divided into three different groups according to the presence of the filler: (a) unfilled and filled with (b) porous polyurethane foam and (c) ordnance gelatin. All groups of sam- ples were tested in uniaxial compression mode under both quasi-static and high strain rates in the range of thousands strains per second using the Split Hopkinson pressure bar. During the loading tests, the deforming structure was observed optically and from the captured image data, the in-plane displacements were calculated using DIC. Based on these displacements, Poisson’s functions among the tested groups were compared. The results show that in the case of both types of polymeric fillers, the auxetic behaviour is suppressed with increasing values of longitudinal strain.

Stať ve sborníku z mezinár. konf. cizojazyčně

2021, Materials Science and Engineering A - Structural Materials: Properties, Microstructure and Processing, 800, ISSN 0921-5093

Light-weight cellular solids, such as aluminium foams, are promising materials for use in ballistic impact mitigation applications for their high specific deformation energy absorption capabilities. In this study, three different types of aluminium alloy based in-house fabricated cellular materials were subjected to dynamic penetration using the in-house experimental setup to evaluate their deformation and microstructural response. Two-sided direct impact Hopkinson bar apparatus instrumented with two high-speed cameras observing the impact area and the penetrated surface of the specimens was used. Advanced wave separation technique was employed to process strain-gauge signals recorded during penetration. Images captured by one of the cameras were processed using an in-house Digital Image Correlation method with sub-pixel precision, that enabled validation of the wave separation results of the strain-gauge signals. The second camera was used to observe the penetration into the tested specimens for correct interpretation of the measured signals with respect to derived mechanical and microstructural properties at different impact velocities. Differential X-ray computed tomography of selected specimens was performed, which allowed for an advanced pre- and post-impact volumetric analysis. Results of performed experiments and elaborate analysis of the measured experimental data are shown in this study.

Článek v periodiku excerpovaném SCI Expanded

2020

The thesis deals with the investigation of the mechanical behaviour of cellular materials at higher strain-rates and enhancement of their energy absorption capabilities. Three types of cellular solids were tested: closed- and open-cell aluminium foam and SLS printed auxetic lattices. To induce a strain-rate sensitive response in open-cell structures, different types of polymeric fillers were tested (polyurethane putty, polyurethane foam and ordnance gelatin) to form Interpenetrating- phase composites (IPCs). At the micro level, the tests were performed using an in-house apparatus in conjunction with the optical strain measurement using Digital Image Correlation (DIC). At the macro-level, for the investigation of the cellular structure a time-lapse X-ray tomography of the compression test was performed. Two types of impact tests were used to cover a broader range of the strain-rates, drop tower and Hopkinson bar (SHPB). A strain-rate sensitive response was observed at moderate strain-rates in the ordnance gelatin and IPC, while the response of unfilled aluminium foam remained unchanged. The SHPB tests showed a strain-rate sensitivity in the energy absorption for all the tested materials. The DIC strain measurement of the SHPB impact tests of the polymer-filled auxetics showed a reduction in the auxetic nature with the filling.

Disertační práce (PhD)

2019, ISBN 978-80-86246-45-1

Sborník z mezinár. konf. cizojazyčně

2019, 17th YOUTH SYMPOSIUM ON EXPERIMENTAL SOLID MECHANICS, Praha, Česká technika - nakladatelství ČVUT), p. 17-20), ISBN 978-80-01-06670-6, ISSN 2336-5382

Presented paper deals with experimental study on compressive properties of auxetics with controlled stiffness of strut joints. The variable strut joints properties were simulated by adding extra amount of material in the struts’ intersection regions. Four groups of inverted honeycomb structures were prepared by multi-jet 3D printing and tested in quasi-static compression. The structure collapsed gradually, however after the first collapse, failure in entire cross-section occurred due to the brittle nature of the base material. The behavior up to the first collapse was consistent among the specimens within each group, while differed slightly subsequently. With higher reinforcement in the joints, results showed increasing stress at the first collapse (ultimate compressive stress) while the strain at the first collapse remained unchanged. The auxetic behaviour became less significant with increasing joints’ reinforcement.

Stať ve sborníku z prestižní konf.

2019, 17th YOUTH SYMPOSIUM ON EXPERIMENTAL SOLID MECHANICS, Praha, Česká technika - nakladatelství ČVUT), p. 21-24), ISBN 978-80-01-06670-6, ISSN 2336-5382

In this study behavior of the selected types of filling material for the inter-penetrating phase composites was tested in compressive loading mode at low and high strain-rates. Three types of the filling material were tested, (i) ordnance gelatin, (ii) low expansion polyurethane foam, and (iii) polyurethane putty. To evaluate their impact energy absorption bulk samples of the selected materials were tested in compression loading mode at strain-rates 1000 s−1 to 4000 s−1 . The high strain-rate compressive loading was provided by Split Hopkinson Pressure Bar (SHPB) which was equipped with PMMA bars to enable testing of cellular materials with low mechanical impedance. Based on the comparative measurement response to compression at both low and high strain-rates was analysed. The results show a significant strain-rate sensitivity of the ordnance gelatin and of the polyurethane putty, while strain-rate effect in the polyurethane foam was not observed.

Stať ve sborníku z prestižní konf.

2019, ISBN 978-80-01-06670-6, ISSN 2336-5382

Sborník z mezinár. konf. cizojazyčně

2018, 16th Youth Symposium On Experimental Solid Mechanics, Praha, Česká technika - nakladatelství ČVUT, ČVUT v Praze), p. 44-47), ISBN 978-80-01-06474-0, ISSN 2336-5382

An experimental study on energy absorption capabilities and strain rate sensitivity of ordnance gelatine was performed. Strain energy density under quasi static compression and moderate strain rate impact tests was compared. In the study two types of material were tested, bulk ordnance gelatine and polymeric open-cell meshwork filled with ordnance gelatine. From the results a significant strain-rate effect was observed in terms of ultimate compressive strength and strain energy density. In comparison of the deformation behaviour under quasi static conditions and drop weight test the difference was very significant, however slight increase in both strength and strain energy density was observed even between different impact energies and velocities during the impact testing. The peak acceleration was significantly reduced in polymer meshwork filled by gelatine in comparison to the bulk gelatine.

Stať ve sborníku z prestižní konf.

2018, EPJ Web of Conferences - Volume 183 (2018) - DYMAT 2018 - 12th International Conference on the Mechanical and Physical Behaviour of Materials under Dynamic Loading, Les Ulis Cedex A, EDP Sciences - Web of Conferences), p. 1-6), ISBN 978-2-7598-9053-8, ISSN 2100-014X

In this paper, a split Hopkinson pressure bar (SHPB) was used for impact loading of an auxetic lattice (structure with negative Poisson’s ratio) at a given strain-rate. High strength aluminum and polymethyl methacrylate bars instrumented with foil strain-gauges were used for compression of an additively manufactured missing-rib auxetic lattice. All experiments were observed using a high-speed camera with frame-rate set to approx. 135.000 fps. High-speed images were synchronized with the strain-gauge records. Dynamic equilibrium in the specimen was analyzed and optimized pulse-shaping was introduced in the selected experiments. Longitudinal and lateral in-plane displacements and strains were evaluated using digital image correlation (DIC) technique. DIC results were compared with results obtained from strain-gauges and were found to be in good agreement. Using DIC, it was possible to analyze in-plane strain distribution in the specimens and to evaluate strain dependent Poisson’s ratio of the auxetic structure.

Stať ve sborníku z prestižní konf.

2018, ISBN 978-80-01-06474-0, ISSN 2336-5382

Sborník z mezinár. konf. cizojazyčně

2017, Advanced Engineering Materials, 19 (10), ISSN 1438-1656

In this paper, impact testing of auxetic structures filled with strain rate sensitive material is presented. Two dimensional missing rib, 2D re-entrant honeycomb, and 3D re-entrant honeycomb lattices are investigated. Structures are divided into three groups according to type of filling: no filling, low expansion polyurethane foam, and ordnance gelatine. Samples from each group are tested under quasi-static loading and dynamic compression using Split Hopkinson Pressure Bar. Digital image correlation is used for assessment of in-plane displacement and strain fields. Ratios between quasi-static and dynamic results for plateau stresses and specific energy absorption in the plateau are calculated. It is found out that not only the manufactured structures, but also the wrought material exhibit strain rate dependent properties. Evaluation of influence of filling on mechanical properties shows that polyurethane increases specific absorbed energy by a factor of 1.05–1.4, whereas the effect of gelatine leads to increase of only 5–10%. Analysis of the Poisson's function reveals influence of filling on achievable (negative) values of Poisson's ratio, when compared to unfilled specimens. The results for the Poisson's function yielded apparently different values as the assessed minima of quasi-static Poisson's ratio in small deformations are constrained by a factor of 15.

Článek v periodiku excerpovaném SCI Expanded

2017

Tato práce řeší problematiku instrumentace dělené Hopkinsonovy tyče modifikované pro dynamická měření materiálů s nízkou mechanickou impedancí. Zaměřuje se na návrh a realizaci součástí souvisejících se samotným záznamem deformačních pulzů z tenzometrických snímačů (jednotka pro tenzometrická měření, napájecí jednotka, filtrace) až po součásti sloužící k řízení a synchronizaci experimentu (modul optických bran a trigger vysokorychlostní kamery). Součástí práce je také návrh softwarového řešení, s grafickým uživatelským rozhraním, umožňujícím nastavení parametrů záznamu měření, samotné zaznamenání měřených veličin a jejich uložení pro potřeby vyhodnocení experimentu. Použitelnost a spolehlivost implementovaného řešení pro rutinní experimenty instrumentovaného SHPB je potvrzena provedenými experimenty se vzorky auxetických struktur. Tyto experimenty se následně podařilo spolehlivě vyhodnotit. Výsledkem této práce je soubor technických řešení (hardwarových i softwarových) zlepšujících kvalitu a spolehlivost měřených dat a také zvyšující celkový komfort obsluhy instrumentovaného zařízení SHPB.

Diplomová práce

2017, 25th INTERNATIONAL CONFERENCE ON MATERIALS AND TECHNOLOGY - PROGRAM AND BOOK OF ABSTRACTS, Ljubljana, Inštitut za kovinske materiale in tehnologije), p. 52-52), ISBN 978-961-94088-1-0

In this study behavior of selected types of filling material were tested in compressive loading mode at high strain rates. Four types of filling material were tested, (i) ordnance gelatin, (ii) low expan sion polyurethane foam, (iii) thixotropic polyurethane putty and (iv) silicon putty. To evaluate their contribution to the impact energy absorption in IPC bulk samples of selected materials were subjected to high strain rate compression. The high strain ra te compressive loading was provided by Split Hopkinson Pressure Bar (SHPB) which was adjusted to be able to test cellular and soft materials. From the tests stress - strain diagrams of investigated materials were obtained, which provided relevant mechanical properties (plateau stress and strain, strain energy density).

Abstrakt ve sborníku z mezinár. konf.

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, 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.