Lidé na fakultě
Ing. Jaromír Kylar

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.

Článek v periodiku excerpovaném SCI Expanded

2025, Institute of Theoretical and Applied Mechanics of the Czech Academy of Sciences, 31, p. 1-228), ISSN 1805-8248

Wave propagation in 3D printed metals is an area of research that focuses on the behavior of mechanical waves in metallic materials produced by additive technologies, specifically 3D printing. This process is increasingly being used to produce complex geometric structures that can exhibit different properties compared to traditionally manufactured materials. In this context, the focus is on how the structure of the printing material, the microscopic arrangement of particles, porosity, and anisotropy affect the wave behavior. The aim of the research is to understand how different printing parameters (e.g., layer orientation, material composition) affect wave propagation and how this knowledge can be applied for e.g., defect detection, sound insulation, or structures with optimized mechanical properties. This work focuses on the instrumentation of 3D printed samples with actuators and measurement labels, the creation of an experimental setup and the measurement of initial high frequency mechanical wave propagation experiments.

Článek v odborném recenzovaném periodiku cizojazyčně

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.

Stať ve sborníku z mezinár. konf. cizojazyčně

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.

Abstrakt ve sborníku z lokální konf.

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.

Stať ve sborníku z prestižní konf. (Scopus)

2025, Measurement: Sensors, 38, 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.

Článek v odborném recenzovaném periodiku