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Vavrik, D., Benes, P., Fila, T., Koudelka, P., Kumpova, I., Kytyr, D., Vopalensky, M., Vavro, M., Vavro, L. (2021) International Journal of Rock Mechanics and Mining Sciences, 138, art. no. 104578, DOI: 10.1016/j.ijrmms.2020.104578
It is well known that the measured values of the fracture toughness of rocks are influenced by material heterogeneity, dimensions, boundary conditions, and asymmetric mechanical behavior. Consequently, the results obtained by standard testing methods developed primarily for homogenous materials with symmetric mechanical behavior, can significantly differ. The standard methods take global approach. Thus, they suppose that the material tested will follow a specific physical model and that one can consider the selected testing method as a black box in which some simple characteristics are measured and the required values can be evaluated. If the material behavior is too different from the theoretically expected one, this global approach will fail. The authors present a method called Local Fracture Toughness Testing (LFTT) to overcome these obstacles. LFFT is calculated independently of the boundary conditions and the crack length. LFTT is based on a complex methodology using a series of tomographic reconstructions, for which data are recorded during specimen loading. Subsequent extended data processing using digital image correlation serves for calculating the evolution of the displacement/strain fields and for identifying the crack which develops during increased loading. Later on, the crack tip opening displacement and the local fracture toughness KIC are calculated at arbitrarily selected positions independent of the geometry and boundary conditions. The LFTT methodology was tested on a sandstone specimen, since such material is usually considered to be brittle. In this work, the authors demonstrate that even a stable crack extension can be identified after maximal loading. Using a loading machine developed in-house, the experimental data allowed for the measurement of fracture toughness at five loading levels/crack lengths. In addition, fracture toughness was measured in nine planes crossing the crack tip for each loading level.

Šleichrt, J., Fíla, T., Koudelka, P., Adorna, M., Falta, J., Zlámal, P., Glinz, J., Neuhäuserová, M., Doktor, T., Mauko, A., Kytýř, D., Vesenjak, M., Duarte, I., Ren, Z., Jiroušek, O. (2021) Materials Science and Engineering A, 800, art. no. 140096, DOI: 10.1016/j.msea.2020.140096
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 testing using an in-house experimental setup to evaluate their deformation and microstructural response. A 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. An advanced wave separation technique was employed to process the strain-gauge signals recorded during the penetration. The images captured by one of the cameras were processed using an in-house Digital Image Correlation method with sub-pixel precision, that enabled the 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 the correct interpretation of the measured signals with respect to the derived mechanical and the microstructural properties at the different impact velocities. A differential X-ray computed tomography of the selected specimens was performed, which allowed for an advanced pre- and post-impact volumetric analysis. The results of the performed experiments and elaborate analysis of the measured experimental data are shown in this study.

Top 5 results

Kytýř, D., Zlámal, P., Koudelka, P., Fíla, T., Krčmářová, N., Kumpová, I., Vavřík, D., Gantar, A., Novak, S. (2017) Materials and Design, 134, pp. 400-417. DOI: 10.1016/j.matdes.2017.08.036
Porous hydrogel-based structures reinforced by bioactive nano-particles allows one to design scaffolds with controlled stiffness, strength, and permeability for bone-tissue engineering applications. To be able to reliably assess the mechanical properties, it is necessary to study the material's deformation response on a volumetric basis and in high detail. In this paper, we present an investigation on the compressive characteristics of highly-relaxing gellan-gum bioactive-glass scaffold subjected to continuous uniaxial quasi-static compression. The sample was compressed with a loading rate of 0.4 μm⋅s−1 and simultaneously irradiated by X-rays during several micro-tomographical scans to obtain data for the evaluation of the deformation and strain fields using digital volume correlation (DVC). Such DVC evaluated on-the-fly micro-tomography was very challenging due to the low thickness of cell-walls and the material's intrinsic low attenuation of X-rays. Thus, we employed loading and tomographical devices equipped with a single-photon counting detector coupled with a DVC procedure, all developed in-house. From the acquired 34 tomographical scans, high-resolution voxel models with a resolution of 29.77 μm were developed and subjected to DVC to obtain detailed deformation and strain fields of the material. It is shown that the presented method is suitable for the precise determination of the deformation response of the predominantly organic material developed as a biocompatible, bioresorbable bone scaffold.

Koudelka, P., Fila, T., Rada, V., Zlamal, P., Sleichrt, J., Vopalensky, M., Kumpova, I., Benes, P., Vavrik, D., Vavro, L., Vavro, M., Drdacky, M., Kytyr, D. (2020) Materials, 13 (6), art. no. 1405, DOI: 10.3390/ma13061405
Several methods, including X-ray radiography, have been developed for the investigation of the characteristics of water-saturated quasi-brittle materials. Here, the water content is one of the most important factors influencing their strength and fracture properties, in particular, as regards to porous building materials. However, the research concentrated on the three-dimensional fracture propagation characteristics is still significantly limited due to the problems encountered with the instrumentation requirements and the size effect. In this paper, we study the influence of the water content in a natural quasi-brittle material on its mechanical characteristics and fracture development during in-situ four-point bending by employing high-resolution X-ray differential micro-tomography. The cylindrical samples with a chevron notch were loaded using an in-house designed four-point bending loading device with the vertical orientation of the sample. The in-house designed modular micro-CT scanner was used for the visualisation of the specimen's behaviour during the loading experiments. Several tomographic scans were performed throughout the force-displacement diagrams of the samples. The reconstructed 3D images were processed using an in-house developed differential tomography and digital volume correlation algorithms. The apparent reduction in the ultimate strength was observed due to the moisture content. The crack growth process in the water-saturated specimens was identified to be different in comparison with the dry specimens.

Fíla, T., Šleichrt, J., Kytýř, D., Kumpová, I., Vopálenský, M., Zlámal, P., Rada, V., Vavřík, D., Koudelka, P., Senck, S. (2018) Journal of Instrumentation, 13 (11), art. no. C11021, DOI: 10.1088/1748-0221/13/11/C11021
In this work, an in-house designed table top loading device equipped with a bioreactor is used for the in-situ compression of a spongious sample in simulated physiological conditions. On-the-fly 4D computed tomography is used as a tool for the advanced volumetric analysis of the deforming microstructure of the specimen. The loading device with the bioreactor was placed directly onto the rotational stage of a modular X-ray scanner. As the loading device is equipped with a slip-ring cable system, it can perform an unlimited number of revolutions during the on-the-fly scanning procedure. A complementary metal-oxide-semiconductor flat panel detector with a fast readout was used for the acquisition of the X-ray images. The specimen was compressed with a low loading velocity. A set of the volumetric data capturing the deformation of the specimen during the experiment was prepared from the images acquired by the detector. A digital volume correlation algorithm was used for the evaluation of the volumetric strain fields in the specimen.

Kumpová, I., Vavřík, D., Fíla, T., Koudelka, P., Jandejsek, I., Jakůbek, J., Kytýř, D., Zlámal, P., Vopálenský, M., Gantar, A. (2016) Journal of Instrumentation, 11 (2), art. no. C02003, DOI: 10.1088/1748-0221/11/02/C02003
To overcome certain limitations of contemporary materials used for bone tissue engineering, such as inflammatory response after implantation, a whole new class of materials based on polysaccharide compounds is being developed. Here, nanoparticulate bioactive glass reinforced gelan-gum (GG-BAG) has recently been proposed for the production of bone scaffolds. This material offers promising biocompatibility properties, including bioactivity and biodegradability, with the possibility of producing scaffolds with directly controlled microgeometry. However, to utilize such a scaffold with application-optimized properties, large sets of complex numerical simulations using the real microgeometry of the material have to be carried out during the development process. Because the GG-BAG is a material with intrinsically very low attenuation to X-rays, its radiographical imaging, including tomographical scanning and reconstructions, with resolution required by numerical simulations might be a very challenging task. In this paper, we present a study on X-ray imaging of GG-BAG samples. High-resolution volumetric images of investigated specimens were generated on the basis of micro-CT measurements using a large area flat-panel detector and a large area photon-counting detector. The photon-counting detector was composed of a 10 × 10 matrix of Timepix edgeless silicon pixelated detectors with tiling based on overlaying rows (i.e. assembled so that no gap is present between individual rows of detectors). We compare the results from both detectors with the scanning electron microscopy on selected slices in transversal plane. It has been shown that the photon counting detector can provide approx. 3× better resolution of the details in low-attenuating materials than the integrating flat panel detectors. We demonstrate that employment of a large area photon counting detector is a good choice for imaging of low attenuating materials with the resolution sufficient for numerical simulations.

Fíla, T., Kumpová, I., Koudelka, P., Zlámal, P., Vavřík, D., Jiroušek, O., Jung, A. (2016) Journal of Instrumentation, 11 (1), art. no. C01005, DOI: 10.1088/1748-0221/11/01/C01005
In this paper, we employ dual-energy X-ray microfocus tomography (DECT) measurement to develop high-resolution finite element (FE) models that can be used for the numerical assessment of the deformation behaviour of hybrid Ni/Al foam subjected to both quasi-static and dynamic compressive loading. Cubic samples of hybrid Ni/Al open-cell foam with an edge length of 15 mm were investigated by the DECT measurement. The material was prepared using AlSi7Mg0.3 aluminium foam with a mean pore size of 0.85 mm, coated with nanocrystalline nickel (crystallite size of approx. 50 nm) to form a surface layer with a theoretical thickness of 0.075 mm. CT imaging was carried out using state-of-the-art DSCT/DECT X-ray scanner developed at Centre of Excellence Telč. The device consists of a modular orthogonal assembly of two tube-detector imaging pairs, with an independent geometry setting and shared rotational stage mounted on a complex 16-axis CNC positioning system to enable unprecedented measurement variability for highly-detailed tomographical measurements. A sample of the metal foam was simultaneously irradiated using an XWT-240-SE reflection type X-ray tube and an XWT-160-TCHR transmission type X-ray tube. An enhanced dual-source sampling strategy was used for data acquisition. X-ray images were taken using XRD1622 large area GOS scintillator flat panel detectors with an active area of 410 x 410 mm and resolution 2048 x 2048 pixels. Tomographic scanning was performed in 1,200 projections with a 0.3 degree angular step to improve the accuracy of the generated models due to the very complex microstructure and high attenuation of the investigated material. Reconstructed data was processed using a dual-energy algorithm, and was used for the development of a 3D model and voxel model of the foam. The selected parameters of the models were compared with nominal parameters of the actual foam and showed good correlation.