People
doc. Ing. Tomáš Fíla, Ph.D.

2026, Measurement, 2026 (258), ISSN 1873-412X

In this paper, performance of a novel contactless method for direct impact testing that replaces strain-gauges and optic instrumentation with a wireless high-speed linear magnetic encoders is demonstrated together with in-situ flash X-ray (FXR) radiography. Conventional direct impact testing with the Hopkinson bar is generally conducted with one-sided instrumentation, potentially complicating material response analysis. A single wireless encoder sensor on the impact side with wave-separation scheme, grounded in the free-end boundary condition, is used to decompose forward and backward strain-waves and reconstructs interface force and displacement at the specimen. We characterize bandwidth and noise via void tests and calibration on cellular foam, showing that the encoder frequency bandwidth up to 25kHz with only 3dB signal loss is adequate for wide range of dynamic tests. Force reconstructions achieve SNR ≥ 20 dB above 1.4 kN on the projectile side, and void tests confirm force equilibrium and consistent velocity histories between input and output bars. Penetration experiments on 3D re-entrant auxetic and 3D honeycomb sandwich panels at 27 m/s demonstrate that projectile-side encoder traces provide a clearer mechanical record than commonly used transmission-side strain-gauges (which are affected by propagation through the tested material). Compared with digital image correlation or Doppler velocimetry, the encoder approach is low-cost, compact, and line-of-sight independent, improving robustness to debris, scene quality and space requirements while retaining velocity record capability although with lower frequency bandwidth. These attributes make encoder-based instrumentation a practical alternative or improvement to existing dynamic devices using other instrumentation approaches.

2025, Results in Engineering, 28, ISSN 2590-1230

The Split Hopkinson bar is a well-established instrument for testing material properties at high strain rates. Despite its popularity, the method has limitations due to its measurement principle, which involves the propagation of the strain wave in elastic slender bars. A key limitation is the superposition of strain waves, which primarily limits the maximum duration of experiments. To address this, wave separation (or wave deconvolution) techniques have been developed to separate overlapping strain waves. However, existing methods often involve complex algorithms, wave dispersion issues requiring an analytical model of the bar's material, or expensive experimental equipment. This paper introduces a simple wave separation technique using linear magnetic encoders as velocity sensors in a Split Hopkinson bar. The approach relies on solving wave propagation equations at a single point, using velocity signals from the linear encoder and strain data from a conventional strain-gauge. The method offers several advantages, including simplicity in instrumentation and calculation, suppression of wave dispersion effects, easy implementation, and cost-effective sensors. We validate the method through numerical simulations with a custom finite element code, analyzing error sources and their impact. Experimentally, we demonstrate the principle using void tests (experiments without a specimen) and compare the results with the conventional strain-gauge method. The technique is further applied to various Split Hopkinson bar systems and materials, including compression, tension, and cellular materials. The results are promising, with good performance within the application range of the method. The paper concludes with a discussion of its advantages and limitations.

2025, Young Transportation Engineers Conference 2024, Praha, České vysoké učení technické v Praze), p. 43-49), ISBN 978-80-01-07445-9, ISSN 2336-5382

Split Hopkinson Bar is an experimental apparatus designed for acquiring material properties in dynamic regime. The apparatus is usually constructed as a single purpose experimental device for specific type of loading, most commonly either compression (Split Hopkinson Pressure Bar) or tension (Split Hopkinson Tensile Bar). In this paper, a simple and highly modular design capable of testing for multiple types of loading is introduced, which can be a great advantage to obtain material properties under different loading conditions. The device was successfully assembled, including its electrical installation and implementation of a pneumatic launching system and was fitted with multiple sensors. Series of calibration procedures was conducted successfuly to analyze correct behaviour of the system. After calibration, the apparatus was subjected to multiple pilot tests with samples manufactured using 3D printing. The results were compared to those obtained using the quasi-static method.

2025, International Journal of Impact Engineering, 206, ISSN 1879-3509

Along with metal foams and lattices based on various unit-cell architectures, sandwich panels have become a prospective solution to problems involving deformation energy mitigation. The combination of a face sheet and a porous core in the sandwich panel exhibits synergistic effects in its effective properties, which has attracted attention in the field of dynamic impact and blast perforation. Further advances can be inspired by the performance of interpenetrating phase composites (IPPCs). These novel meta-materials consist of two or more topologically continuous and three-dimensionally interconnected phases, where the matrix is reinforced by the microstructure of a foam or lattice. Using a combination of IPPC with a sandwich panel architecture based on a suitable filling material, it is possible to further enhance the specific deformation mitigation characteristics of the panel with respect to the strain rate dependence. Here, shear thickening non-Newtonian fluids (STFs) are a type of filling with the potential to greatly enhance the performance of sandwich panels in comparison to Newtonian fluid filling materials. In this paper, we investigate STF (polyethylene glycol with hydrophilic fumed silica) filled sandwich panels with an additively manufactured periodic core subjected to dynamic penetration at an intermediate and a high strain rate. Intermediate strain rate loading (maximum impact velocity of 2 m/s) is induced by a loading apparatus based on linear motors. The high strain rate loading is carried out using a direct impact Hopkinson bar (DIHB) apparatus at impact velocities of 10 m/s and 20 m/s. Both types of experiments were amply instrumented by high-speed cameras and in-situ X-ray radiographical imaging was used to reveal deformation processes within the microstructure of the panels including flash X-ray radiography of the DIHB experiments. The DIHB apparatus was equipped with a novel wireless instrumented striker recording its velocity using contactless linear encoders. To explore the potential and characteristics of such a bar velocity measurement, finite element simulations of void tests (i.e., impact experiments without a specimen) were performed in LS-DYNA and evaluated using the same methods and algorithms used to process the experimental data. A strong strain rate dependence was revealed in the impact behavior of the sandwich panels, while the contribution of the STF filling was clearly identified in both the mechanical and image data acquired during the experiments.

2025, Engineering Mechanics 2025: Book of full texts, Prague, Institute of Thermomechanics, AS CR, v.v.i.), ISBN 978-80-86246-99-4, ISSN 1805-8256

This paper presents two advanced in-situ X-ray imaging facilities developed for studying material behavior under dynamic loading conditions at both intermediate and high strain rates. The first facility, optimized for intermediate strain rate testing using an apparatus based on linear motors, utilizes continuous axial X-ray radiography to capture internal material processes during impact testing, with frame rates ranging from 100 fps to 500 fps using a laboratory camera, and up to 2000 fps using a high-speed camera. The second facility, designed for high strain rate testing, integrates a flash X-ray system capable of capturing ultra-high-speed events with frame rates in the tens of thousands of frames per second. This imaging system is coupled with modular Split Hopkinson Bar (SHPB), Direct Impact Hopkinson Bar (DIHB), and Open Hopkinson Pressure Bar (OHPB) devices for dynamic material testing, enabling precise measurements of force, velocity, and displacement. The facilities have enabled successful visualization and analysis of complex deformation and failure mechanisms in various materials, providing significant insights into deformation response under dynamicconditions. Despite challenges related to resolution, frame rates, and contrast, these systems offer valuableinsights with the potential for further optimization and comparison with particle-accelerator based results.

2025, Proceedings : ECCOMAS MSF 2025, Sarajevo, Association of Computational Mechanics in Bosnia and Herzegovina)

The study examines the impact of nanoparticle concentration and distribution on the mechanical behavior of shear-thickening fluids (STFs) integrated with additively manufactured structures under intermediate and high strain rates. Two STFs were prepared using hydrophilic fumed silica and styrene-acrylate copolymer nanoparticles in polyethylene glycol, with concentrations of 10–30 wt%. Scanning electron microscopy revealed nanoparticle agglomeration, influencing STF uniformity. At intermediate strain rates, STFs demonstrated effective energy absorption through prismatic collapse. At higher strain rates, inhomogeneous propagation allowed partial impactor penetration, limiting energy dissipation. However, the consistent thickening behavior highlights STFs’ potential for adaptive energy absorption. Ongoing research will focus on optimizing STF composition and integration for advanced engineering applications.

2025, IMEKO XXIV World Congress Proceedings, Berlin, Elsevier), ISSN 2665-9174

In this contribution, we introduce a control system and instrumentation of the in-house developed dynamic testing device with linear motors suitable for experiments ranging from quasi-static regime up to intermediate strain rates with impact velocities of up to 8 m/s. In the contribution, we demonstrate the temporal resolution of the whole system to perform dynamic experiments with closed-loop control of displacement, velocity, or force within a period of a few milliseconds. Frequency bandwidth and testing capabilities of standard membrane pancake load-cells as well as quartz-based piezoelectric load-cells for impact testing are analysed. The system is combined with high-resolution optical inspection, high-speed photography, or even X-ray imaging. The advantages of the device and instrumentation are demonstrated in a case study revealing its potential.

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.

2024, Advanced Engineering Materials, 26 (24), ISSN 1527-2648

Metal foams are an interesting class of bio-inspired materials for lightweight construction and energy absorption. Commonly, aluminum (Al) foams are used. However, the mechanical properties have been improved by coating pure Al foams with a nanocrystalline nickel (Ni) coating resulting in Ni/Al hybrid foams. Herein, the meso-mechanical deformation mechanisms in Al foams and the changes in the mechanisms in Ni/Al hybrid foams are studied using time-lapse micro-computed tomography measurements in comparison between numerical modeling using voxel finite element models and evaluation of displacement fields using 3D optical flow. This gives never-seen insights into the highly localized 3D deformation mechanisms within the entire volume of the foams and not only on the surfaces as given by conventional digital image correlation methods. Displacements calculated by the 3D optical flow algorithm demonstrate its possibility to reveal a significant concentration of plastic deformation, particularly evident when deformation occurs within a distinct, slightly inclined band in the central region. Common numerical simulations using standard plasticity models do not accurately capture this localized deformation, underscoring the need to integrate damage and softening models into the simulation.

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.

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, Vol. 42 (2023): 18th Youth Symposium on Experimental Solid Mechanics, Praha, České vysoké učení technické v Praze), p. 1-5), ISBN 978-80-01-07237-0, ISSN 2336-5382

This paper focuses on stereolithography (an additive manufacturing technology working on the principle of curing liquid resins layer by layer using ultraviolet radiation) and the effect of aging on the mechanical properties of the material and printed samples. The aging of the material could be a problem for its subsequent use as the stability of the mechanical properties would not be maintained and unwanted deterioration of the material could occur. As part of the research, sets of samples were printed and subjected to different aging methods and subsequently subjected to quasi-static and dynamic uni-axial load tests. From the data obtained, the basic mechanical properties of the material were calculated and compared with each other. The aim of this paper was to investigate whether aging process causes significant changes in the mechanical properties of the materials used, which could have a consequential impact on their use in different industries.

2023, Computer Methods in Applied Mechanics and Engineering, 413, p. 1-26), ISSN 1879-2138

The derivation and implementation of an asynchronous direct time integration scheme for domain-decomposed finite element models is presented. To maximize clarity in the description of the proposed asynchronous integration, the scheme is restricted to the linear-elastic stress wave propagation case. The proposed method allows the integration of individual subdomains with independent time steps. There is no requirement for an integer time steps ratio of the interacting domains while maintaining zero interface energy. The subdomains are connected by the condition of the continuity of the acceleration field at the interface. In addition, the a posteriori technique is applied to satisfy the continuity of the displacement and velocity fields. Another important contribution of this paper lies in the description of the implementation — we offer the reader a general description of the implementation of the case of any number of subdomains with any number of constraints between them, while the basics of the algorithm are explained on a single domain pair. The functionality of the asynchronous integrator is verified by solving selected problems and comparing with analytical solutions and experimental measurements obtained using a Split Hopkinson pressure bar setup. © 2023 Elsevier B.V.

2023, Wave Motion, 121, ISSN 0165-2125

This is a presentation of robust and accurate explicit time-stepping strategy for finite element modeling of elastic discontinuous wave propagation in strongly heterogeneous, multi-material and graded one-dimensional media. One of the major issues in FEM modeling is the existence of spurious numerical stress oscillations close to theoretical wave fronts due to temporal-spatial dispersion behavior of FE discretization. The numerical strategy presented for modeling of 1D discontinuous elastic waves is based on (a) pushforward-pullback local stepping — ensuring the elimination of dispersion due to different critical time step sizes of finite elements, (b) domain decomposition via localized Lagrange multipliers — to satisfy coupling kinematics and dynamic equations , (c) asynchronous time scheme — ensuring the correct information transfer of quantities for the case of integer ratios of time step size for all domain pairs. Dispersion behaviors, existence of spurious stress oscillations, and sensitivity of the dispersion to time step size are then suppressed. The proposed method is numerically tested with regard to the rectangular step pulse elastic propagation problem considering in-space varying Young’s modulus. To prove robustness and accuracy, a comparison with results from commercial software, an analytical solution, and experimental data from partial assembly of a split Hopkinson pressure bar (SHPB) setup is provided.

2023, Vol. 42 (2023): 18th Youth Symposium on Experimental Solid Mechanics, Praha, České vysoké učení technické v Praze), p. 77-82), ISBN 978-80-01-07237-0, ISSN 2336-5382

X-ray radiography and computed tomography have become well-established methods for investigation of internal structure of objects and for defectoscopy. Recently, the methods have even been used for in-situ analysis of materials under mechanical loading. Although the techniques would be very suitable for analysis during dynamic events, their application is constrained by typical achievable frame rates. Therefore, fast imaging is usually limited to facilities providing sufficient flux like particle accelerators. In this paper, we test imaging performance of a laboratory-based setup with a high-power X-ray tube, a scintillation panel, and an optical camera. Fast-rotating object and typical specimens for impact testing are irradiated with different power settings and quality of captured images is evaluated and analyzed. It is found out that the system can be successfully used for imaging at several hundred frames per second allowing for inspection of slow impact dynamics experiments.

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.

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.

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.

2022, ENGINEERING MECHANICS 2022, Prague, Institute of Theoretical and Applied Mechanics, AS CR), p. 109-112), ISBN 978-80-86246-51-2, ISSN 1805-8256

Stress uniformity in a specimen represents a crucial parameter for validity of impact experiment conducted in a split Hopkinson bar. Principle of the experimental method is to measure elastic stress waves that propagate in the bars of an experimental setup and to calculate stress to strain relations at the interfaces with the specimen. However, standard procedure to evaluate the experiment can only be valid when stress is distributed approximately uniformly along the specimen. As the stress wave has to propagate through the specimen during the experiment, a time period of significantly non-uniform stress distribution is present and a certain number of wave transits is required to achieve approximately uniform stress distribution. In this paper, a simple analytical model assuming one-dimensional wave propagation theory, non-equal mechanical impedance of the bars and linear elastic material model of the specimen is introduced to determine a number of transits required to achieve uniform stress distribution in the specimen. Potential of the calculated results is discussed in an experiment using a direct impact Hopkinson bar and a material with significant plateau in its stress-strain response.

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, 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.

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

The problem of the linear elastodynamics including domain decomposition via localized Lagrange multipliers method [2] is solved using finite element method and direct time integration. Time integration of domains is performed separately with different time steps with integer ratio. The known asynchronous integrator scheme [1] is generalized for multiple domains problem and enhanced by the use of a local variant of the pushforward-pullback method, which effectively avoids spurious oscillation in steep stress pulses response. The proposed method is applied to the rectangular step pulse propagation problem considering the linearly varying Young modulus in space as well as the bi-material interface problem. To prove the robustness and the accuracy, the comparison with analytical solution and commercial software outputs is provided.

2022, International Conference on Nonlinear Solid Mechanics, abstract book, International Research Center on Mathematics and Mechanics of Complex Systems), p. 35-35)

The problem of the linear elastodynamics including domain decomposition via localized La- grange multipliers method [2] is solved using finite element method and direct time integration. Time integration of domains is performed separately with different time steps with integer ra- tio. The known asynchronous integrator scheme [1] is generalized for multiple domains prob- lem and enhanced by the use of a local variant of the pushforward-pullback method, which effectively avoids spurious oscillation in steep stress pulses response. The proposed method is applied to the rectangular step pulse propagation problem considering the linearly varying Young modulus in space as well as the bi-material interface problem. To prove the robustness and the accuracy, the comparison with analytical solution and commercial software outputs is provided.

2022, Advanced Engineering Materials, 24 (3), ISSN 1438-1656

Open-cell metal foams are a versatile class of porous lightweight materials, which are predominantly used as kinetic energy absorbers in a wide scope of applications. Based on their bio-inspired inhomogeneous 3D porous structure, they are capable to significantly reduce the mass of structural designs. Starting with a polyurethane (PU) template foam, the specimens in the present contribution are manufactured by an electrochemical nickel (Ni) deposition. This manufacturing process is beneficial regarding both the specimen design and the adjustment of mechanical properties correlated with the Ni-coating thickness. Herein, the strain-rate sensitivity of open-cell Ni/PU hybrid metal foams is investigated by quasistatic compression tests and high-velocity impact tests conducted with a conventional split-Hopkinson pressure bar device.

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

This doctoral thesis is focused on the experimental analysis of cellular meta-materials subjected to a dynamic impact with a high strain-rate. In particular, additively manufactured auxetic lattices (structures with a negative Poisson’s ratio) are investigated. Two in-house Hopkinson bar experimental setups are developed for the testing of the structures: i) a conventional Split Hopkinson Pressure Bar (SHPB, Kolsky bar), and ii) a novel direct impact Open Hopkinson Pressure Bar (OHPB). Both setups are tailored for the application on the low impedance materials and are used for the experiments subjecting the cellular meta-materials to a high strain-rate uni-axial compression. In the thesis, the developed apparatuses, the instrumentation, evaluation methods and the experimental program are described in detail. The experiments are optically inspected using several high-speed cameras and a digital image correlation technique is employed for the advanced analysis of the deformation behavior of the meta-materials. Using the data from several experimental campaigns, the strain-rate sensitivity of the selected auxetic lattices and their Poisson’s ratio is investigated in detail. It is found out that the auxetic structures are, in general, strain-rate sensitive and their Poisson’s ratio is both strain-rate and strain dependent. Other representative results exploiting the deformation behavior of cellular materials, e.g., hybrid open-cell foams and hybrid hollow strut auxetic lattices are also presented in the study.

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, 17th YOUTH SYMPOSIUM ON EXPERIMENTAL SOLID MECHANICS, Praha, Česká technika - nakladatelství ČVUT), p. 32-35), ISBN 978-80-01-06670-6, ISSN 2336-5382

The presented paper is focused on embedding of the serially manufactured piezo-electric impact load-cell into Split Hopkinson Pressure Bar (SHPB) for a direct force measurement during dynamic loading. Conventionally, during the SHPB test dynamic force equilibrium is investigated by a comparison of the transmitted signal wave and the difference between the incident and reflected signals waves to the incident bar, measured by strain gauges \cite{bib1}. However, in the experiments with specimens with low \linebreak mechanical impedance, a major portion of the incident wave is reflected back on the boundary between the bar and the specimen. Comparison between two-large amplitude incident and reflected pulse and \linebreak a small-amplitude transmitted pulse can be influenced by large error and resulting force equilibrium can be inaccurate. Therefore, a piezo-electric quartz impact force transducer was used to directly measure the axial forces in the vicinity of the specimen end surfaces, allowing to analyze the force equilibrium which is an essential characteristic for reliable measurement. Measured values from strain gauges were compared with values obtained from force transducer, to verify the validity of the acquired signals which will increase the reliability of the measured data. The presented solution will help to determine the mechanical properties of advanced materials which is necessary for investigation of complex modern material structures behaviour.

2019, 17th YOUTH SYMPOSIUM ON EXPERIMENTAL SOLID MECHANICS, Praha, Česká technika - nakladatelství ČVUT), p. 1-5), ISBN 978-80-01-06670-6, ISSN 2336-5382

Three different tools for Digital Image Correlation (DIC) were used for evaluation of dynamic experiments performed using custom Open Hopkinson Pressure Bar (OHPB) apparatus. High strain-rate measurements were performed on specimens of advanced cellular materials with predefined structure and negative Poisson's ratio. Low impedance polymethyl methacrylate (PMMA) bars instrumented with foil strain-gauges were used for dynamic loading of the specimens. Experiments were observed using a pair of high-speed cameras for imaging of loading process in sufficient quality. Custom developed evaluation DIC tool implemented in Matlab, open-source Matlab tool (NCorr) and commercial DIC software (ISTRA 4D) were all used for evaluation of image sequences recorded by high-speed cameras. Comparison of results obtained using all three different DIC tools and results of complementary strain-gauge measurement are shown in this paper. Verification of reliability of custom made DIC software tool is presented.

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.

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, 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. 97-101), ISBN 978-80-87012-71-0, ISSN 1805-8248

In this paper, an implementation of Split Hopkinson Pressure Bar (SHPB) and its modification Open Hopkinson Pressure Bar (OHPB) for testing of cellular structures is presented. Dynamic testing of materials with low mechanical impedance is very demanding in terms of achieving the requested experimental device performance. Key elements of the Hopkinson bar instrumentation that were successfully employed in dynamic testing of cellular materials are presented in this paper. Detailed overview of the strain-gauges instrumentation including our best practices for installation and noise reduction is provided. Information about the instrumentation of Hopkinson bar with high-speed cameras are also given. To demonstrate the performance of the proposed instrumentation some examples of a typical results are summarized in the text.

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, The 2nd International Conference of Wave Propagation in Solids - Book of abstracts, Praha, Ústav termomechaniky AV ČR, v. v. i.), p. 43-44), ISBN 978-80-87012-67-3

In this contribution, an Open Hopkinson Pressure Bar (OHPB) apparatus was used for high strain-rate compression of the additively manufactured auxetic lattices and selected cellular metals. The principle of OHPB is based on Direct Impact Hopkinson Bar (DIHB). Instead of striker bar, the incident bar is accelerated in the gas-gun and it hits directly the specimen mounted on the face of the transmission bar. The incident bar is instrumented using strain-gauges and so, in contrast with Taylor anvil test, strain histories corresponding to the both contact faces of the specimen are measured and thus are known. Unlike conventional SHPB test, OHPB allows for high maximum strain in the specimen at constant strain-rate with good conditions of dynamic equilibrium as the strain waves propagate from the specimen boundaries.

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.

2018, 16th Youth Symposium On Experimental Solid Mechanics, Praha, Česká technika - nakladatelství ČVUT, ČVUT v Praze), p. 10-14), ISBN 978-80-01-06474-0, ISSN 2336-5382

This paper presents an overview of the custom design instrumentation of a Split Hopkinson Pressure Bar modified for dynamic testing of materials with low mechanical impedance, particularly for cellular metallic materials (e. g. metal foams, laser sintered structures). Design and implementation of the components related to the strain wave measurement based on strain gauges (i.e. strain-gauge measurement unit, power supply unit, filtration) and the components used for the control and synchronization of the experiment, such as module of laser trough-beam photoelectric sensor are summarized in the paper. Aside from the design of the hardware components, the contribution deals also with development of a control software with graphical user interference using LabView (National Instruments, USA) programming environment, that allows selection of parameters of the dynamic tests and their storage for the evaluation of experiments.

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. 77-81), ISBN 978-80-01-06474-0, ISSN 2336-5382

This work presents a data preprocessing procedure for signal acquired during high strain-rate loading using a custom Split Hopkinson Pressure Bar (SHPB). Before the evaluation of the experimental data, preprocessing of the measured signals including application of suitable digital or analog filter needs to be performed. Our department mainly focuses on measurements performed on advanced materials (e.g. materials with predefined structures or hybrid foams). For such measurements, it is essential to perform data preprocessing and apply suitable filter, to be able to appropriately determine deformation pulses on the measuring bars. This paper focuses foremost on spectral analysis of the measured signals, and design of optimal method of data filtering. Data from several different SHPB experiments were processed and results of different filtering methods are shown in this paper. Parameters of the best performing filter were optimized and shown to be universal for wide range of SHPB measurements.

2018, Engineering Mechanics 2018: Book of Full Texts, Praha, Ústav teoretické a aplikované mechaniky AV ČR, v. v. i.), p. 213-216), ISBN 978-80-86246-88-8, ISSN 1805-8248

In this paper, a simple analytic model of Split Hopkinson Pressure Bar (SHPB) propelled by gas-gun is introduced. The model allows for prediction of the output of SHPB experiment or can be used inversely as a design tool for gas-gun propelled SHPB. The model is based on existing models of light gas-gun working according to adiabatic process and on one-dimensional wave propagation theory in linear elastic cylindrical slender bar. The model allows for calculation with elementary drag effects. Model functionality was evaluated and compared with the experimental results of a SHPB setup equipped with linear elastic (aluminium) bars and visco-elastic (polymethyl metacrylate) bars at two different impact velocities.

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.

2018, 16th Youth Symposium On Experimental Solid Mechanics, Praha, Česká technika - nakladatelství ČVUT, ČVUT v Praze), p. 72-76), ISBN 978-80-01-06474-0, ISSN 2336-5382

In this paper Split Hopkinson pressure bar (SHPB) was used for dynamic testing of nickel coated polyurethane hybrid foams. The foams were manufactured by electrodeposition of a nickel coating on the standard open-cell polyurethane foam. High strength aluminium alloy bars instrumented with foil strain-gauges were used for dynamic loading of the specimens. Experiments were observed using a high-speed camera with frame-rate set to approx. 100.000 – 150.000 fps. Precise synchronisation of the high-speed camera and the strain-gauge record was achieved using a through-beam photoelectric sensor. Dynamic equilibrium in the specimen was achieved in all measurements. Digital image correlation technique (DIC) was used to evaluate in-plane displacements and deformations of the samples. Specimens of two different dimensions were tested to investigate the collapse of the foam structure under high-speed loading at the specific strain-rate and strain.

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, 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

In this paper, a Split Hopkinson Pressure Bar (SHPB) apparatus is used for impact loading of the selected cellular metallic materials. The experimental setup is arranged as a modified Kolsky setup. The experiment is observed using a high-speed camera for assessment of in-plane displacement and strain fields in the sample using digital image correlation (DIC). The functionality of the system is demonstrated on experiments where additively manufactured metallic auxetic lattices and nickel-coated open-cell polyurethane foam were used as specimens.

2017

Předložená práce se zabývá vyhodnocováním dynamických experimentů prováděných na instrumentovaném zařízení SHPB modifikovaném pro měření materiálů s nízkou mechanickou impedancí. V rámci práce je navržena a implementována metodika předzpracování zaznamenaných experimentálních dat, vytvořena sada softwarových nástrojů umožňující konzistentní a přesné vyhodnocení. Celé řešení je implementováno v modulárním uživatelském rozhraní, které umožňuje automatizované, rychlé a spolehlivé vyhodnocení zaznamenaných experimentálních dat. Navržené řešení je ověřeno v rámci vyhodnocení experimentů provedených na auxetické struktuře a na vzorku porézní polymerní pěny. Výsledky získané s použitím navrženého řešení jsou konzistentní a odpovídají již dříve publikovaným hodnotám. Výsledkem práce je soubor softwarových řešení implementovaný v uživatelském prostředí, který značně usnadňuje a zrychluje proces vyhodnocení experimentů provedených na SHPB.

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

In this study, 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 gelatin. 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 gelatin 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, 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.