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Emil Svoboda between Bratislava and Prague - Flying Dutchmen and Anquish of a Prague Cage Bird?
Kober, Jan
Following the nearly forgotten story of Slovak porfessorship of Emil Svoboda, the article analyses the historical process of obtaining a professorship in Czech legal academia of the early 1920s, in the time of the university boom. Emil Svoboda was working for his professorship in Bratislava. His attempt to enforce the appointment of a professor in Prague for the promise of several years of 'pioneering work' in Slovakia failed, as did the less successful attempt to obtain at least the promise of a future Prague professorship as a counterbalance to leaving for Slovakia. Still, he elft for Slovakia, believing that the position at the law faculty would increase his chances of future success. However, the knowledge that he will lose his academic positions in Prague led him to resign quickly from the Bratislava university chair and to be satisfied with the certainty of the Prague Technical University professorship and with the associate professor position at the Faculty of Law of Charles University. In the end, he proved to be right and he would be an imaginary winner in this situation - a brief presence in Bratislava preovided him with the possibility of external lecgturing at this university for several years without losing his two teaching positions in Prague.
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Nonlinear ultrasonic porosimetry of 3D printed metallic parts
Převorovský, Zdeněk ; Krofta, Josef ; Kober, Jan ; Kirschner, A.
Additive manufacturing process optimization requires a feedback from nondestructive testing (NDT) and evaluation. In this contribution are discussed NDT results obtained by nonlinear elastic wave spectroscopy methods to classify presence of defects in Ti-6Al-4V prismatic samples fabricated by electron beam printer. Quantitative calibration of porosity was realized by metallography. The best porosity classification results were obtained by nonlinear wave modulation spectroscopy with chirp excitation and by ESAM (Excitation Symmetry Analysis).
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Computational time reversal method based on finite element method: influence of temperature
Mračko, Michal ; Kolman, Radek ; Kober, Jan ; Převorovský, Zdeněk ; Plešek, Jiří
Time reversal method is used to focus elastic waves to the location of the original source and reconstruct its source time function. The procedure consists of two steps: Frontal task and Reversal task. In the Frontal task, the medium is excited by an arbitrary source, elastic waves propagate through a body of interest and the dynamic response at few points on boundary is recorded. In the second step (say the Reversal task) the response signal is reversed in time and transmitted back into the medium resulting in focusing in the original source location. It is of practical importance to investigate a case when the medium changes its properties between the frontal and reversal wave propagation steps. An example is a problem of transferring experimentally recorded data to a computational model, where discrepancies in geometry, elastic properties and boundary conditions are expected. Our motivation is to develop a methodology for computation of time reversal problems in commercial finite element software. The results prove that this method is extremely sensitive to the change of temperature and one have to pay special attention to tuning of elastic parameters relevant to the\nexperiment.
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Localization of continuous acoustic emission sources using time reversal procedure
Převorovský, Zdeněk ; Krofta, Josef ; Kober, Jan ; Chlada, Milan
A new solution with the help of Time Reversal (TR) signal processing approach is suggested in this paper. It allows planar location of burst AE sources under high background noise using only one transducer, and continuous AE sources with two transducers, both under high background noise from other sources. TR procedure was in this approach applied to long random noise signals for the first time. Suggested method was verified by means of experimentally simulated AE sources on a steel plate and tube. Source location results certified high robustness of suggested approach. Certain disadvantage of the new method is requirement of detailed scanning of region around a presumed source. Scanning may be eliminated if we use numerical simulations with TR signals put into a computer model of the structure. Localization accuracy of suggested procedure (approx. 1 mm) is better than the wavelength and transducers aperture.
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NDT in additive manufacturing of metals
Převorovský, Zdeněk ; Krofta, Josef ; Kober, Jan
In this study we tested possibilities of nonlinear elastic wave spectroscopy (NEWS) methods to classify two types of artificially created defects in Ti-6Al-4V prismatic samples fabricated by Electron Beam Melting (EBM) system. Two series of samples with variable internal defects were tested: samples with central circular defects (missing material gaps), and samples with gradually growing stochastic porosity. Each group of samples contained also fully compact sample for a comparison. Five ultrasonic NEWS procedures were used to classify defects variability. Relatively simple experimental facility with two piezoelectric transducers allows interrogate samples and record their response. The best results, documented in this paper, were obtained by two modifications of NWMS procedure. Results of NEWS tests indicate that selected NEWS procedures can be used also for online AM monitoring.
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On finite element modelling in time reversal problems
Mračko, Michal ; Kober, Jan ; Kolman, Radek ; Převorovský, Zdeněk ; Plešek, Jiří ; Masák, Jan ; Kruisová, Alena
In this paper we analyse suitability and accuracy of computational techniques in time reversal applications based on finite element method (FEM) for detection and localization of defects, cracks or other acoustic emission sources in bodies and structures. As it is known, a classical explicit integration scheme - central difference is reversible. The central difference scheme as a time integrator is widely used for linear and nonlinear finite element analyses and it is also implemented in commercial and open-source finite element software. In the paper properties of the explicit FEM in time reversal problems are studied and analysed. We use the standard Galerkin FEM formulation with linear shape functions, one-point Gauss integration and lumped mass matrix. Loading by the Ricker pulse was applied for modelling of the acoustic source in an elastic square domain. A special attention is paid to the choice of boundary conditions in reverse problem which keep the reversibility of problems of interest. Finally, we show the quality of refocusing of the original acoustic source.
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Between Place, Representation and Possibilities: Searching of Urban Location and Architecture of the Czechoslovak Parliament Building
Kober, Jan ; Švácha, Rostislav (advisor) ; Svobodová, Markéta (referee)
Keywords Representation - Building of Parliament - competitions - public debate - Centre of Prague - Czechoslovakia - Central Europe Resume Jan Kober: Between Place, Representation and Possibilities: Searching of Urban Location and Architecture of the Czechoslovak Parliament Building. Thesis, Charles University in Prague, 242 pages. The thesis researches the nearly forgotten story of the efforts to build the new parliament building. At the beginning of the 19th century, the Parliament of the Kingdom of Bohemia left its age-old seat at the Prague Castle for the newly-rebuild seat in the palace of the Prague Lesser Town. At the beginning of the 20th century, Prague became a quickly developing city, but, unlike the European capitals of that time, with few exceptions lacking the new monumental architecture of various public buildings. This led to the idea of the new larger and incomparably more representative parliamentary design (Antonín Balšánek), influenced by the pattern of Paris. The renewal of the independent state in 1918 as well as the rise of the number of deputies immediately led to the requests for the new building of the Parliament and to the emergence of the new projects of Václav Roštlapil, Max Urban, Antonín Balšánek and Bohuslav Fuchs. The political changes enabled also the old vision of the...
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Verification of diagnostic device using NEWS methods
Chlada, Milan ; Krofta, Josef ; Kober, Jan
Within the framework of the project „Verification of diagnostic device using NEWS methods“ supported by SIC, several calibration tests of newly developed apparatus for non-destructive testing of materials by NEWS (Non-linear Elastic Wave Spectroscopy) methods were realized. The report summarizes the results of characteristics measuring for case of particular components of the device and the demonstration tests of defect detection in selected structure parts.
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The System for Automatic Location of Impacts
Chlada, Milan ; Kober, Jan ; Převorovský, Zdeněk ; Krofta, Josef
The report deals with the automatic target system for on-line evaluation of shooting precision based on acoustic emission (AE) method, known from the field of non-destructive testing as an approach for location of material defects. Since the bullet stroke on a steel plate can be considered as an AE source, the method can be included in a target system consisting of impact plate, AE sensors and AE measuring system with implemented advanced location algorithms. Whole system can be sized and matched to specific customer needs. At the end of the report, the real demonstration of the system is shown.
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