People
Ing. Petr Koudelka, Ph.D.

2025, Measurement: Sensors, ISSN 2665-9174

Hopkinson pressure bar is a commonly used technique to test the response of a specimen to shock load under constant strain rate to determine its material parameters. In addition to that, we propose that the Hopkinson bar apparatus can be employed to test the response of the specimen in the sense of frequency analysis; that is, specimens made of a complex metamaterial could behave as a filter of specific frequencies. Here, several difficulties arise. The structure of the metamaterial affects only those waves that have a wave length comparable to the specific length in the metamaterial of the specimen. However, the bar geometry of the apparatus itself behaves as a low-pass filter, so the high frequencies are attenuated with distance traveled. Hence, here we have the situation that the longer the specimen is, we lose the ability to investigate high frequencies, and, at the same time, the shorter the specimen is, the higher the lowest affected frequency is. Our contribution is to find a compromise for the length of the sample and to design a high-frequency testing method for such an investigation of metamaterials.

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.

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.

2023, Advanced Engineering Materials, 25 (24), ISSN 1527-2648

The paper deals with the dynamic penetration of 3D printed panels with auxetic and conventional honeycomb unit cell-based cores. The geometry of the unit cells and their periodic assembly in the resulting lattices were selected to ensure the same relative density and overall weight of the individual sample types. Such a similarity of both specimen types allowed the evaluation of differences between conventional and auxetic lattices in terms of penetration characteristics and deformation energy mitigation properties. Dynamic penetration of the samples was performed using a fully strain-gauge instrumented Open Hopkinson Pressure Bar (OHPB) at three impact velocities resulting in three loading scenarios. All performed experiments were captured by two optical cameras for detailed observation and for tracking of an impactor movement using Digital Image Correlation (DIC). The force-penetration depth relation was used to evaluate the elastic and post-yield compression characteristics of the lattices together with their deformation energy mitigation capabilities. The results show that the main differences in the deformation response of lattices consist of lower overall stiffness and effective yielding of the auxetic lattices at higher penetration depth. Numerical simulation using an explicit solver was performed to analyze the deformation mechanism of the individual core types.

2022, International Conference on Nonlinear Solid Mechanics, abstract book, International Research Center on Mathematics and Mechanics of Complex Systems), p. 107-107)

Additively manufactured materials represent an advanced type of engineering material allowing for rapid building of parts with complex design. Additively manufactured metallic materials are particularly promising for application in high-tech industry, requiring optimized parts with complex shape and high performance mechanical properties. In this contribution, the cylindrical specimens manufactured from 316L powdered stainless steel built in different orientations to the printing platform are subjected to compression at high strain rates using split Hopkinson pressure bar (SHPB). The specimens are subjected to quasi- static and dynamic compression at strain rates ranging from 1500/s to 5000/s. Changes in damage development and failure mode are investigated through combination of high speed optical imaging with data of the SHPB instrumentation. For the testing, the SHPB with high strength aluminum alloy bars, soft copper pulse shapers and two sizes of the striker bar is used. The bars are instrumented with a set of foil strain-gauges. The experiments are observed by stateof-the-art high speed camera with frame rate of approximately 250kfps. The camera is time synchronized with the data acquisition system. Strain localization and changes in failure mode related to the printing orientation and strain rate, particularly occurrence of the fatal macroscopic crack and identification of the corresponding failure strain, are investigated using digital image correlation (DIC). It is found out that the failure mode changes dramatically with the increasing strain rate resulting in sudden and complete failure of the specimen during high strain rate compression. The failure is dependent on both the printing orientation and the strain rate.

2022, International Conference on Nonlinear Solid Mechanics, abstract book, International Research Center on Mathematics and Mechanics of Complex Systems), p. 104-104)

The mechanical response of sandwich panels tailored to specific applications investigated becomes an extensively topic for research teams. Sandwich panels typically consist of a lightweight core (porous materials, meta-materials structures) and covering shell (solid or composite layer). These materials can be used as, e.g., the main component of crumple zones in vehicles or low-velocity protection in many applications, due to their high specific energy absorption and low density. An unique loading mode in dynamic mechanical testing is a dynamic indentation where combining multi-directional stress distribution in sandwich panels is not an easy task for description. The main aim of this work is to compare sandwich panels with two different types of core (3D inverted honeycomb and conventional honeycomb structures with similar specific densities). Based material of the aforementioned cores is photopolymer resin which allows the manufacturing of complex shapes of cores by stereolithography technology. All specimens, equipped with a spreading thin layer of polyethylene shell, are subjected to dynamic penetration to evaluate the mechanical behavior, penetration resistance, and energy-absorbing capability at different impact velocities. An in-house developed direct impact Hopkinson bar is used for dynamic indentation experiments. The loading apparatus is equipped with strain gauges and the measured signals are used for the calculation of an applied force and impact velocity. A pair of highspeed cameras are used for optical inspection of the experiments. A targeted camera is used for evaluating the velocity of the projectile using the digital image correlation method (DIC) for comparison with strain-gauge measurement, and an overview camera is used for capturing the surroundings of the impact plane.

2022, ENGINEERING MECHANICS 2022, Prague, Institute of Theoretical and Applied Mechanics, AS CR), p. 285-288), ISBN 978-80-86246-51-2, ISSN 1805-8256

Research into the mechanical behaviour of lattice structures and metal foams at high strain rates using experiments based on a direct impact Hopkinson bar (DIHB) method has been recently proposed to overcome several limitations of the conventional split Hopkinson pressure bar (SHPB). Especially, the socalled open Hopkinson pressure bar (OHPB), a modification of DIHB with strain measurement points on both bars, has been proved to be a suitable experimental technique for testing of materials with low mechanical impedance. However, experimental testing is usually limited in terms of resources and, hence, it is convenient to employ numerical methods to predict the results of experiments and, if necessary, adjust the parameters of the experimental procedure based on the preceding numerical analysis of the problem. Developing a numerical model of the whole experimental set-up is, thus, a key method to achieve a reliable analysis. In this paper, we present a numerical model of an OHPB apparatus and demonstrate its suitability for inverse numerical simulations of the closed-cell aluminium foam.

2022, Dynamic Behaviour of Additively Manufactured Structures & Materials, Freiburg im Breisgau, Albert-Ludwigs-Universität Freiburg), p. 187-192)

We demonstrate numerical modelling of the mechanical response of auxetic structures sub- jected to dynamic uniaxial compressive load- ing in split Hopkinson pressure bar (SHPB) at the strain rates of 1500 s−1 and 3000 s−1. The stress-strain characteristics as well as com- pressive strain dependent Poisson’s ratio of re-entrant honeycomb and missing-rib aux- etic lattices are assessed in LS-DYNA simula- tions with explicit time integration. Numer- ical results are supported by SHPB experi- ments utilized for both calibration of finite el- ement modeling and verification of the sim- ulations. The studied lattices were additively manufactured by laser powder bed fusion from 316L stainless steel. The numerical aspects of the simulations together with the influence of the 3D printing quality on the reliability of the results are discussed.

2022, Dynamic Behaviour of Additively Manufactured Structures & Materials, Freiburg im Breisgau, Albert-Ludwigs-Universität Freiburg), p. 103-110)

Open Hopkinson Pressure Bar (OHPB) appa- ratus is used, together with conventional split Hopkinson pressure bar (SHPB), for dynamic testing of additively manufactured cellular me- tamaterials at intermediate and high strain rates. Benefits of the OHPB testing method over standard established methods are dis- cussed. The investigated metamaterials in- clude various types of auxetic lattices manu- factured from powdered austenitic steel by powder bed fusion technology. It is found out that the investigated type of metamate- rials exhibits significant strain rate sensitivity of the stress-strain curves as well as of the apparent auxeticity. Moreover, its deforma- tion mechanism changes with the increasing impact velocity as the buckling of the individ- ual struts is reduced by the inertia effects.

2022, Materials, 15 (3), ISSN 1996-1944

The main aim of the study was to analyse the strain rate sensitivity of the compressive deformation response in bulk 3D-printed samples from 316L stainless steel according to the printing orientation. The laser powder bed fusion (LPBF) method of metal additive manufacturing was utilised for the production of the samples with three different printing orientations: 0◦, 45◦, and 90◦. The specimens were experimentally investigated during uni-axial quasi-static and dynamic loading. A split Hopkinson pressure bar (SHPB) apparatus was used for the dynamic experiments. The experiments were observed using a high-resolution (quasi-static loading) or a high-speed visible-light camera and a high-speed thermographic camera (dynamic loading) to allow for the quantitative and qualitative analysis of the deformation processes. Digital image correlation (DIC) software was used for the evaluation of displacement fields. To assess the deformation behaviour of the 3D-printed bulk samples and strain rate related properties, an analysis of the true stress–true strain diagrams from quasi-static and dynamic experiments as well as the thermograms captured during the dynamic loading was performed. The results revealed a strong strain rate effect on the mechanical response of the investigated material. Furthermore, a dependency of the strain-rate sensitivity on the printing orientation was identified.

2021, Metals — Open Access Metallurgy Journal, 11 (8), ISSN 2075-4701

Compressive deformation behaviour of additively manufactured lattice structures based on re-entrant tetrakaidecahedral unit-cell geometry were experimentally investigated under quasi-static and dynamic loading conditions. Specimens of four different structures formed by three-dimensional periodical assembly of selected unit-cells were produced by a laser powder bed fusion technique from a powdered austenitic stainless steel SS316L. Quasi-static compression as well as dynamic tests using split Hopkinson pressure bar (SHPB) apparatus at two strain-rates were conducted to evaluate the expected strain-rate sensitivity of the fundamental mechanical response of the structures. To evaluate the experiments, particularly the displacement fields of the deforming lattices, optical observation of the specimens using a high-resolution camera (quasi-static loading) and two synchronised high-speed cameras (SHPB experiments) was employed. An in-house digital image correlation algorithm was used in order to evaluate the anticipated auxetic nature of the investigated lattices. It was found that neither of the investigated structures exhibited auxetic behaviour although strain-rate sensitivity of the stress–strain characteristics was clearly identified for the majority of structures.

2021, Metals — Open Access Metallurgy Journal, 11 (1), ISSN 2075-4701

The mechanical behaviour of three different auxetic cellular structures, hexa-chiral 2D, tetra-chiral 2D and tetra-chiral 3D, was experimentally investigated in this study. The structures were produced with the powder bed fusion method (PBF) from an austenitic stainless steel alloy. The fundamental material mechanical properties of the sample structures were determined with classic quasi-static compressive tests, where the deformation process was captured by a high-resolution digital camera. The Split Hopkinson Pressure Bar (SHPB) apparatus was used for dynamic impact testing at two impact velocities to study the strain-rate dependency of the structures. Two synchronised high-speed cameras were used to observe the impact tests. The captured images from both quasi-static and dynamic experiments were processed using a custom digital image correlation (DIC) algorithm to evaluate the displacement/strain fields and the Poisson’s ratio. Predominant auxetic behaviour was observed in all three structures throughout most of the deformation process both under quasi-static and impact loading regimes. The tetra-chiral 2D structure showed the most significant auxetic behaviour. Significant stress enhancement in all tested structures was observed in dynamic testing. The Poisson’s ratio strain-rate dependency was confirmed for all three auxetic structures.

2021, International Journal of Impact Engineering, 148, ISSN 0734-743X

Direct impact testing with a Hopkinson bar is, nowadays, a very popular experimental technique for investigating the behavior of cellular materials, e.g., lattice metamaterials, at high strain-rates as it overcomes several limitations of the conventional Split Hopkinson Pressure Bar (SHPB). However, standard direct impact Hopkinson bars (DIHB) have only single-sided instrumentation complicating the analysis. In this paper, a DIHB apparatus instrumented with conventional strain-gauges on both bars (a so called Open Hopkinson Pressure Bar - OHPB) is used for dynamic impact experiments of cellular materials. Digital image correlation (DIC) is used as a tool for investigating the displacements and velocities at the faces of the bars. A straight-forward wave separation technique combining the data from the strain-gauges with the DIC is adopted to increase the experiment time window multiple times. The experimental method is successfully tested at impact velocities in a range of 5-30 m/s with both linear elastic and visco-elastic bars of a medium diameter. It is shown that, under certain circumstances, a simple linear elastic model is sufficient for the evaluation of the measurements with the visco-elastic bars, while no additional attenuation and phase-shift corrections are necessary. The applicability of the experimental method is demonstrated on various experiments with conventional metal foams, hybrid foams, and additively manufactured auxetic lattices subjected to dynamic compression.

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.

2021, Advanced Engineering Materials, 23 (5), p. 1-15), ISSN 1438-1656

With their increased energy absorption capacity, auxetic materials are perfectly fit to develop new, enhanced lightweight crash absorbers for cars. Herein, the mass distribution along the struts is optimized via finite element analysis with a parameterized optimization. Four different auxetic unit cells are taken from the literature and their struts parameterize, the models simulate, and the mass specific energy absorption capacity optimizes. The two models with the highest energy absorption capacity are then selected for experimental investigation and produced by additive manufacturing from a polymer. To further enhance the mechanical properties, the specimens are electrochemically coated with nickel and the polymer molten out by pyrolysis. Those Ni/polymer hybrids are subjected to quasistatic and dynamic impact experiments. Only a small strain rate sensitivity can be detected under dynamic loading, namely, a higher plastic collapse and higher plateau stress. The hollow struts are folding instead of bending, which render them much weaker than predicted by the simulation. In conclusion, it is possible to improve existing crash absorber elements with tailored auxetic hybrid structures. They absorb higher amounts of energy without changing their stiffness under dynamic loading while saving mass and cost.

2021, Advanced Engineering Materials, 23 (1), ISSN 1438-1656

Metamaterials produced using additive manufacturing represent advanced structures with tunable properties and deformation characteristics. However, the manufacturing process, imperfections in geometry, properties of the base material as well as the ambient and operating conditions often result in complex multiparametric dependence of the mechanical response. As the lattice structures are metamaterials that can be tailored for energy absorption applications and impact protection, the investigation of the coupled thermomechanical response and ambient temperature‐dependent properties is particularly important. Herein, the 2D re‐entrant honeycomb auxetic lattice structures additively manufactured from powdered stainless steel are subjected to high strain rate uniaxial compression using split Hopkinson pressure bar (SHPB) at two different strain rates and three different temperatures. An in‐house developed cooling and heating stages are used to control the temperature of the specimen subjected to high strain rate impact loading. Thermal imaging and high‐speed cameras are used to inspect the specimens during the impact. It is shown that the stress–strain response as well as the crushing behavior of the investigated lattice structures are strongly dependent on both initial temperature and strain rate.

2020

The dissertation thesis is focused on the numerical modelling of the mechanical response of auxetic structures to uniaxial compressive loading. According to the primary application of auxetics in terms of deformation energy mitigation, where their unique characteristics arising from the negative Poisson’s ratio of the structure, high strain rate response is emphasised. Additionally, quasi-static characteristics are assessed to obtain reference data for the evaluation of the strain-rate dependency induced particularly by micro-inertia effects. Mechanical properties are studied using stress-strain characteristics, whereas the microstructural response is evaluated based on the function of Poisson’s ratio. In the thesis, three auxetic unit-cells having uni- or bi-axial auxetic characteristics are considered. The structures are developed by a periodic assembly of unit-cells in the respective spatial directions. Due to the complex deformation response of the auxetic structures, the reference data for the development of numerical simulations are obtained from the experiments with the samples of structures manufactured using 3D printing. Dynamic loading is performed using a Split Hopkinson Pressure Bar (SHPB) apparatus, while an approach to the numerical simulations consisting of the development of a full-scale virtual SHPB for an explicit time integration scheme in LS-DYNA was selected. In the dynamic simulations, geometrical models of the lattices precisely corresponding to the geometry of the structures for the 3D printing are used. The numerical aspects of the simulations together with the influence of the 3D printing quality on the reliability of the results are discussed. The ability of the numerical simulations to describe the deformation response of the investigated auxetic lattices is assessed based on the numerical stress-strain curves and the graphs of the strain-dependent Poisson’s ratio.

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.

2019, International Conference on Nonlinear Solid Mechanics - ICoNSoM2019, Palazzo Argiletum, Roma, Italy), p. 131-131)

Cellular solids, such as metal foams, hybrid foams, 3D printed lattices or additively manufactured auxetic structures are complex lightweight cellular materials with high energy absorption capabilities and possible functionally graded material properties. Engineering applications of such materials require optimization of their design, and thus their mechanical behavior under the representative loading conditions (i. e., dynamic impact, blast). The design and optimization procedures require a relevant material model based on the experimental investigation of the constructs. In this study, the application of the Digital Image Correlation (DIC) technique on the cellular solids in quasi-static and dynamic compression is discussed and the representative results of the method in this application are presented. Here, digital image correlation is used as an advanced method for the complex experimental analysis of the displacement and strain fields of several cellular solids under quasi-static compression and high strain-rate loading using the Split Hopkinson Pressure Bar (SHPB) apparatus. The data from the experiments with the specimens of the selective laser sintered auxetic lattices, made of powdered austenitic steel, and with hybrid nickel-polyurethane aluminum foam were processed using a custom digital image correlation tool. Results covering the evaluation of the displacement and strain fields, different methods for evaluation of Poisson’s ratio, and the analysis of the digital image correlation reliability are presented in the study. The study is focused particularly on the application of the digital image correlation on the data captured by a high-speed camera during high strain-rate experiments and the analysis of the cellular solids during dynamic impact. Comparison of the digital image correlation results with the other methods, its limitations and the actual challenges in this field are also discussed in the study.

2019, Temperature dependence of material behaviour at high strain-rate, Politecnico di Torino), ISBN 978-88-85745-27-8

Two types of the hybrid polymer-nickel auxetic specimens were subjected to the quasi-static compression and compressive impact loading using Open Hopkinson Pressure Bar. Two variants of the 3D re-entrant auxetic lattice were used: i) structures with rectangular struts and ii) structure with rounded struts. The specimens were numerically optimized, prepared using computer aided design, and the base constructs were 3D printed from VisiJet EX200 polymer. The constructs were then coated using the electrodeposition of the nanocrystalline nickel in two nominal thicknesses of the coating (60 μm and 120 μm). After the coating process, the core part of the constructs was removed by the burning-out of the polymer at elevated temperature. The structures were subjected to the quasi-static compression and simultaneously inspected using an CCD camera, while Hopkinson bar was used for the impact loading of the specimens at two different impact velocities (ca. 5 m/s and 26 m/s). Dynamic experiments were observed with a pair of high-speed cameras and an infrared camera. The high-speed camera images were processed using a custom digital image correlation algorithm. Mechanical as well as thermal behavior of the hybrid auxetic structures subjected to the different loading conditions was analyzed and summarized in this paper.

2019, 10th International Conference Auxetics and other materials and models with ”negative” characteristics - abstract book, Poznań, Institute of Molecular Physics), ISBN 978-83-933663-8-5

2019, 17th YOUTH SYMPOSIUM ON EXPERIMENTAL SOLID MECHANICS, Praha, Česká technika - nakladatelství ČVUT), p. 25-31), ISBN 978-80-01-06670-6, ISSN 2336-5382

The paper aims at the numerical simulation of the wave propagation in compressive Split Hopkinson Pressure Bar (SHPB) experiment. The paper deals with principles of SHPB measurement, optimisation of a numerical model and techniques of pulse shaping. The parametric model of the typical SHPB configuration developed for LS-DYNA environment is introduced and optimised (in terms of element size and distribution) using the sensitivity study. Then, a parametric analysis of a geometric properties of the pulse shaper is carried out to reveal their influence on a shape of the incident pulse. The analysis is algorithmized including the pre- and post-processing routines to enable automated processing of numerical results and comparison with the experimental data. Results of the parametric analysis and the influence of geometric properties of the pulse shaper (diameter, length) on the incident wave are demonstrated.

2019, Engineering Mechanics 2019: Book of full texts, Prague, Institute of Thermomechanics, AS CR, v.v.i.), p. 29-33), ISBN 978-80-87012-71-0, ISSN 1805-8248

In this paper, digital image correlation method (DIC) is introduced as a tool for evaluation of high strain-rate experiments perfomed using Hopkinson Bar apparatus. Samples of advanced cellular materials with predefined periodic structure and negative Poisson’s ratio (auxetic structures) were investigated in this study. In-house Hopkinson Pressure Bar apparatus was used to perform the impact experiments and the experimental setup was observed using a pair of high-speed cameras. Custom DIC software tool was used to evaluate highspeed cameras records. Selected representative results of DIC applications on Hopkinson Bar experiments are provided in this paper.

2019, Advanced Engineering Materials, 21 (8), ISSN 1438-1656

This paper deals with experimental investigation into a strain‐rate dependent function of Poisson's ratio of three auxetic structures subjected to compressive loading. The missing rib, the 2D re‐entrant honeycomb, and the 3D re‐entrant honeycomb lattices printed using selective laser sintering from powdered SS316L austenitic steel are investigated. The samples are subjected to uni‐axial compression under quasi‐static conditions and dynamic conditions using the Split Hopkinson Pressure Bar (SHPB). The deforming specimens are optically observed in order to apply a digital image correlation for evaluation of the in‐plane displacement and strain fields. From the calculated strain fields, the function of Poisson's ratio is calculated for each experiment using different methods taking specific regions of interest of the specimen microstructures into account. The obtained functions of Poisson's ratio are plotted for each microstructure and strain‐rate. The analysis of the results shows that the strain‐rate has a significant influence on the deformation characteristics of all the investigated microstructures yielding differences in the magnitude of the minima of Poisson's ratio and the differences in the maximum overall compressive strain, where the lattices are still auxetic.

2019, 17th YOUTH SYMPOSIUM ON EXPERIMENTAL SOLID MECHANICS, Praha, Česká technika - nakladatelství ČVUT), p. 68-72), ISBN 978-80-01-06670-6, ISSN 2336-5382

The paper is focused on evaluation of the relation between mechanical properties of 3D printed stainless steel 316L-0407 and printing direction (i.e. the orientation of the part which is being printed in the manufacturing device) subjected to compressive loading at different strain-rates. In order to evaluate the strain rate dependency of the 3D printed material’s compressive characteristics, dynamic and quasi-static experiments were performed. Three sets of bulk specimens were produced, each having a different printing orientation with respect to the powder bed plane (vertical, horizontal and tilted). To assess the deformation behaviour of the 3D printed material, compressive stress-strain diagrams and compressive yield strength and tangent modulus were evaluated.

2019, 10th International Conference Auxetics and other materials and models with ”negative” characteristics - abstract book, Poznań, Institute of Molecular Physics), p. 37-39), ISBN 978-83-933663-8-5

n this work, selective laser sintered (SLS) auxetic lattices printed from the powdered 316L–0407 austenitic steel were subjected to compressive loading at several strain-rates. Three types of the auxetic lattices were tested: i) 2D re-entrant honeycomb, ii) 2D missing rib, and iii) 3D re-entrant honeycomb. The structures were subjected to the quasi-static uni-axial compression using a standard electromechanical loading device. The experiments were observed using a CCD camera. In dynamic experiments, the specimens were compressed at four different strain-rates using two Hopkinson bar techniques. Data recorded by the strain-gauges mounted on the measurement bars were used for evaluation of the mechanical behavior of the specimen (e. g., stress-strain and strain-rate-strain diagrams). A custom digital image correlation (DIC) tool based on Lucas-Kanade tracking algorithm was used for the advanced analysis of the displacement and strain fields in the specimens of both quasi-static and dynamic experiments. At least 5 specimens of the each structure were tested per one strain-rate to ensure a sufficient statistics and relevancy of the results.

2019, Temperature dependence of material behaviour at high strain-rate, Politecnico di Torino), ISBN 978-88-85745-27-8

Specimens based on re-entrant honeycomb auxetic lattice were printed from powdered austenitic steel using selective laser sintering and subjected to dynamic compression using Split Hopkinson Pressure Bar (SHPB). To study the influence of strain-rate and temperature on mechanical properties of the lattices, heating and cooling devices integrated into the SHPB apparatus were developed and the experiments were performed at two different strain rates given by different striker impact velocities. As a result, the dynamic compression was performed at two strain rates and three temperature levels (reduced, room, and elevated temperature) with 5 specimens for each combination. The specimen were observed by a pair of high-speed CMOS optical cameras and a high-speed thermal imaging camera. Optical cameras were used for evaluation of strain fields of the compressed samples using digital image correlation and for inspection of experiment validity. Thermograms were used for qualitative evaluation of heat distribution within the sample microstructure during its deformation. It has been found out that increase of strain-rate results in increase of plateau stress together with decrease of densification strain. The difference in specimen temperature led to changes in the mechanical properties, the absolute temperature of the fully compressed sample and increase of maximum measured temperature during the experiment.

2018, 16th Youth Symposium On Experimental Solid Mechanics, Praha, Česká technika - nakladatelství ČVUT, ČVUT v Praze), p. 15-19), ISBN 978-80-01-06474-0, ISSN 2336-5382

In recent years, open-source applications have replaced proprietary software in many fields. Especially open-source software tools based on Linux operating system have wide range of utilization. In terms of CNC solutions, an open-source system LinuxCNC can be used. However, the LinuxCNC control software and the graphical user interface (GUI) could be developed only on top of Hardware Abstraction Layer. Nevertheless, the LinuxCNC community provided Python Interface, which allows for controlling CNC machine using Python programming language, therefore whole control software can be developed in Python. The paper focuses on a development of a multi-process control software mainly for in-house developed loading devices operated at our institute. The software tool is based on the LinuxCNC Python Interface and Qt framework, which gives the software an ability to be modular and effectively adapted for various devices.

2018, 16th Youth Symposium On Experimental Solid Mechanics, Praha, Česká technika - nakladatelství ČVUT, ČVUT v Praze), p. 38-43), ISBN 978-80-01-06474-0, ISSN 2336-5382

The paper is focused on numerical analysis of mechanical behaviour of auxetic structures with re-entrant tetrakaidecahedral unit cell subjected to uni-axial quasi-static compression. The mechanical behaviour was evaluated inversely with respect to selected geometrical parameters of the unit cell and two different loading modes. Finite element method was used for the numerical analysis of the problem. A set of fully parametric tools has been developed, which enabled automated execution and evaluation of virtual experiments. From results of the simulations, Young’s modulus, the characteristics of the Poisson’s ratio function, and the deformation energy density were estimated. The relation between these characteristics and geometry of the unit cell, particularly the re-entrant angle and the relative density, was evaluated. Results of the numerical simulations for the unit cell and representative volume element of its three-dimensional periodic assembly are presented.

2018

Předmětem této bakalářské práce je shrnutí poznatků a faktů o šíření vlnění jednorozměrným kontinuem, zejména v dlouhých tenkých tyčích, o principech měření metodou SHPB, o technikách tvarování pulzu a okrajově i o metodách numerické mechaniky. Z teoretického úvodu vychází část praktická, kde je vytvořen zdrojový kód pro tvorbu parametrického modelu typické sestavy SHPB pro prostředí LS-DYNA. Redukovaná sestava je pak metodou konečných prvků podrobena citlivostní studii. Nakonec je provedena parametrická studie tvarovače pulzu, která sleduje vliv geometrických vlastností tvarovače na tvar napěťového pulzu a která je včetně s procesů s ní spjatých algoritmizována, čímž je umožněno snadné zpracování výsledků a případné srovnání s experimentálními daty. Primárním výstupem práce jsou výsledky parametrické studie v podobě kvantifikované míry vlivu geometrie tvarovače na změnu tvaru pulzu a automatizační algoritmy pro tvorbu modelů, provedení simulací a vyhodnocení výsledků.

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.

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.

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.

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.

2017

The aim of the dissertation is investigation of mechanical behavior of auxetic materials using numerical modeling techniques. The studied auxetic constructs are intended to be used as a matrix in modular panels for ballistic protection and mitigation of deformation energy of blasts. Thus, the analyses will be primarily focused on development of modeling schemes for reliable prediction of the materials’ mechanical properties and their optimization in the given design domain according to required characteristics. Particular attention will be paid in the numerical studies to evaluation of influence of structural characteristics on overall mechanical characteristics including the Poisson’s function and specific absorbed deformation energy. According to analogy of the metal foams and beam-like discretization schemes of their microstructure in terms of mechanical properties, the same approach can be used for numerical modeling of auxetic structures, both produced by modification of an existing foam and auxetic constructs with directly controlled geometry. Hence, the methods developed for inverse estimation of the effective mechanical properties of metal foams will be used as a basis for development of prediction and optimization schemes for the auxetic structures. The analyses will be at first performed with structures exhibiting in-plane negative Poisson’s ratio and, only after verification of the used methods, fully 3D structures with volumetric negative Poisson’s ratio characteristics will be simulated. After verification of the quasi-static results, explicit dynamics simulations will be carried out in large deformation field and as a self-contact problem. Numerical simulations will be complemented by experimental tests at micro- and macroscale.

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).

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.