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1.
Behaviour of cellular metamaterials under impact loading conditions : doctoral disertation
Yunus Emre Yilmaz, 2024, doctoral dissertation

Abstract: This doctoral thesis investigates cell-size-graded metallic and non-metallic Triply Periodic Minimal Surface (TPMS) structures' behavior under varying loading rates. Using experimental tests, analytical calculations, and advanced computer simulations, the research explores the interplay between material properties, cell size grading, and deformation mechanisms under different strain rates. The study focuses on enhancing the Direct Impact Hopkinson Bar (DIHB) setup for accurate force and displacement measurements and pioneering a method for quantifying inertial forces, critical at high strain rates. Key findings show that cell-size grading significantly affects deformation patterns, with initial deformation occurring in regions with smaller and lower stiffness cells across different loading rates and TPMS geometries. The research also highlights topology's influence on mechanical response, with photopolymer-based diamond structures showing superior energy absorption and gas-atomized steel structures favoring gyroid configurations. This underscores the importance of considering both topology and base material selection during TPMS design. The study demonstrates the increasing prominence of inertial forces as deformation rates rise, impacting structural response and failure likelihood in TPMS structures. These insights inform the design of optimized cellular metamaterials for high-performance applications requiring superior energy absorption and structural integrity under high loading rates. The research advances material characterization techniques and computational modelling capabilities, contributing to the development of next-generation cellular metamaterials for broader engineering applications.
Keywords: Triply Periodic Minimal Surfaces, TPMS, Cell-size-grading, Impact, High-strainrate, Digital Image Correlation
Published in DKUM: 17.10.2024; Views: 0; Downloads: 24
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Complexity of the COVID‑19 pandemic in Maringá
Andre S. Sunahara, Arthur A. B. Pessa, Matjaž Perc, Haroldo V. Ribeiro, 2023, original scientific article

Abstract: While extensive literature exists on the COVID-19 pandemic at regional and national levels, understanding its dynamics and consequences at the city level remains limited. This study investigates the pandemic in Maringá, a medium-sized city in Brazil’s South Region, using data obtained by actively monitoring the disease from March 2020 to June 2022. Despite prompt and robust interventions, COVID-19 cases increased exponentially during the early spread of COVID-19, with a reproduction number lower than that observed during the initial outbreak in Wuhan. Our research demonstrates the remarkable impact of non-pharmaceutical interventions on both mobility and pandemic indicators, particularly during the onset and the most severe phases of the emergency. However, our results suggest that the city’s measures were primarily reactive rather than proactive. Maringá faced six waves of cases, with the third and fourth waves being the deadliest, responsible for over two-thirds of all deaths and overwhelming the local healthcare system. Excess mortality during this period exceeded deaths attributed to COVID-19, indicating that the burdened healthcare system may have contributed to increased mortality from other causes. By the end of the fourth wave, nearly three-quarters of the city’s population had received two vaccine doses, signifcantly decreasing deaths despite the surge caused by the Omicron variant. Finally, we compare these fndings with the national context and other similarly sized cities, highlighting substantial heterogeneities in the spread and impact of the disease.
Keywords: complex system, correlation, epidemics, COVID-19
Published in DKUM: 17.07.2024; Views: 117; Downloads: 21
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4.
High strain-rate deformation analysis of open-cell aluminium foam
Anja Mauko, Mustafa Sarıkaya, Mustafa Güden, Isabel Duarte, Matej Borovinšek, Matej Vesenjak, Zoran Ren, 2023, original scientific article

Abstract: This study investigated the high-strain rate mechanical properties of open-cell aluminium foam M-pore®. While previous research has examined the response of this type of foam under quasi-static and transitional dynamic loading conditions, there is a lack of knowledge about its behaviour under higher strain rates (transitional and shock loading regimes). To address this gap in understanding, cylindrical open-cell foam specimens were tested using a modified Direct Impact Hopkinson Bar (DIHB) apparatus over a wide range of strain rates, up to 93 m/s. The results showed a strong dependency of the foam's behaviour on the loading rate, with increased plateau stress and changes in deformation front formation and propagation at higher strain rates. The internal structure of the specimens was examined using X-ray micro-computed tomography (mCT). The mCT images were used to build simplified 3D numerical models of analysed aluminium foam specimens that were used in computational simulations of their behaviour under all experimentally tested loading regimes using LS-DYNA software. The overall agreement between the experimental and computational results was good enough to validate the built numerical models capable of correctly simulating the mechanical response of analysed aluminium foam at different loading rates.
Keywords: Open-cell aluminium foam, Micro-computed tomography, High-strain rate, Direct impact hopkinson bar, Digital image correlation, Computer simulation
Published in DKUM: 06.12.2023; Views: 428; Downloads: 42
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5.
Radon anomalies in soil gas caused by seismic activity
Boris Zmazek, Mladen Živčić, Ljupčo Todorovski, Sašo Džeroski, Janja Vaupotič, Ivan Kobal, 2004, original scientific article

Abstract: At the Orlica fault in the Krško basin, combined barasol detectors were buried in six boreholes, two along the fault itself and four on either side of it, to measure and record radon activity, temperature and pressure in soil gas every 60 minutes for four years. Data collected have been analysed in a manner aimed at distinguishing radon anomalies resulting from environmental parameters (air and soil temperature, barometric pressure, rainfall) from those caused solely by seismic events. The following approaches have been used to identify anomalies: (i) ± 2σ deviation of radon concentration from the seasonal average, (ii) correlation between time gradients of radon concentration and barometric pressure, and (iii) prediction with regression trees within a machine learning program. In this paper results obtained with regression trees are presented. A model has been built in which the program was taught to predict radon concentration from the data collected during the seismically inactive periods when radon is presumably influenced only by environmental parameters. A correlation coefficient of 0.83 between measured and predicted values was obtained. Then, the whole data time series was included and a significantly lowered correlation was observed during the seismically active periods. This reduced correlation is thus an indicator of seismic effect.
Keywords: radon in soil gas, environmental parameters, earthquakes, correlation, regression trees, forecasting
Published in DKUM: 15.05.2018; Views: 1784; Downloads: 89
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6.
Productivity and economic growth in the European Union : impact of investment in research and development
Andreja Nekrep, Sebastjan Strašek, Darja Boršič, 2018, original scientific article

Abstract: This paper focuses on investment in research and development as a factor of labour productivity and economic growth. Our analysis confirms the link between expenditure for research and development (expressed in % of GDP) and labour productivity (expressed in the number of hours worked) based on selected data for EU Member States in the period 1995-2013. A causal link between variables of the concave parabola was confirmed, and the value of expenditure for research and development (2.85% of EU GDP) maximising productivity (per hour of work) was determined based on the examined data. In accordance with these findings, EU’s target of reaching 3% of GDP spent on research and development to be achieved by 2020 seems in support of reaching maximum productivity in the EU.
Keywords: investment in research and development, productivity, economic growth, correlation, panel analysis
Published in DKUM: 03.05.2018; Views: 1991; Downloads: 238
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7.
The influence of broadband regulation in EU on the development of the regulated technology
Andrej Mihevc, 2011, original scientific article

Abstract: The aim of the article is to answer the question if the level of intensity of `ex ante` regulation (also `regulation`) imposed by NRA (national regulatory Authority or regulators) influence on the development of incumbent DSL technology against other Access technologies. There are several approaches which support the basic idea of regulation, that Ćex anteĆ regulation promotes the competition. The approaches must be also based on the size of themarket. In the internet world there are different access technologies. OpenAccess is crucial for the development of competition. Regulators have to ensure, that also new entrants can reach end users through the facilities of the incumbent operator. Based on the approach of three criteria, European Commission defines two wholesale relevant access markets both based on DSL technology (`bitstream` access and unbundled local loop) susceptible to sector specific `ex ante` regulation. In the last stage also optical access isincluded on the relevant markets. Other technologies are still not equivalent to incumbent DSL technology according to the opinion of the commission. The intensity of regulation influences on the competition conditions. It influences on the level where and at which point of the investment ladder entrants will enter the market through wholesale inputs or through building its own infrastructure.
Keywords: regulation, market analyses, broadband, correlation, DSL technology
Published in DKUM: 22.01.2018; Views: 1281; Downloads: 400
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8.
Asymmetric correlation of sovereign bond yield dynamics in the Eurozone
Silvo Dajčman, 2013, original scientific article

Abstract: This paper examines the symmetry of correlation of sovereign bond yield dynamics between eight Eurozone countries (Austria, Belgium, France, Germany, Ireland, Italy, Portugal, and Spain) in the period from January 3, 2000 to August 31, 2011. Asymmetry of correlation is investigated pair-wise by applying the test of Yongmiao Hong, Jun Tu, and Guofu Zhou (2007). Whereas the test of Hong, Tu, and Zhou (2007) is static, the present paper provides also a dynamic version of the test and identifies time periods when the correlation of Eurozone sovereign bond yield dynamics became asymmetric. We identified seven pairs of sovereign bond markets for which the null hypothesis of symmetry in correlation of sovereign bond yield dynamics can be rejected. Calculating rolling-window exceedance correlation, we found that the time-varying upper- (i.e. for positive yield changes) and lower-tail correlations (i.e. for negative yield changes) for pair-wise observed sovereign bond markets normally follow each other closely, yet during some time periods (for most pair-wise observed countries, these periods are around the September 11 attack on the New York City WTC and around the start of the Greek debt crisis) the difference in correlation does increase. The results show that the upper- and lower-tail correlation was symmetric before the Eurozone debt crisis for most of the pair-wise observed sovereign bond markets but has become much less symmetric since then.
Keywords: asymmetric correlation, sovereign bonds, Eurozone, financial crisis
Published in DKUM: 03.08.2017; Views: 1209; Downloads: 369
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9.
Phase equilibrium measurements and data correlation for the ternary system oleic acid + 1-octanol + carbon dioxide
Chiara Giulia Laudani, Mateja Primožič, Željko Knez, Maja Leitgeb, 2009, original scientific article

Abstract: A study of the high-pressure phase equilibria in the ternary system oleic acid/1- octanol/supercritical CO2 was performed to obtain information for optimization of enzymatic synthesis of n-octyl oleate. Equilibrium data were measured at temperatures of 308.15, 323.15 and 343.15 K over a pressure range from 1 to 25 MPa. Two different apparatuses: a Variable Volume View Cell and a Batch Stirred Tank were used employing synthetic and static-analytic measurement methods, respectively. The measured solubility of dense CO2 in the binary liquid mixture oleic acid/1-octanol covered the range from 0.0781 to 0.7686 of CO2 molar fraction. CO2 solubility increased with increasing pressure up to 10 MPa. At higher pressures, no significant increase was observed. The Soave-Redlich-Kwong equation-of-state (SRK-EoS) with quadratic mixing rules was successfully used for data correlation in the whole P-T-x region studied.
Keywords: data correlation, 1-octanol, oleic acid, supercritical carbon dioxide, Soave-Redlich-Kwong equation-of-state, vapor-liquid equilibria
Published in DKUM: 05.07.2017; Views: 1558; Downloads: 120
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10.
Characterization of heterogeneous arc welds through miniature tensile testing and Vickers-hardness mapping
Stijn Hertelé, Jonas Bally, Nenad Gubeljak, Primož Štefane, Patricia Verleysen, Wim De Waele, 2016, original scientific article

Abstract: The heterogeneity of arc-welded connections is often ignored in structural assessments, giving rise to inaccuracies. Improved assessments taking into account heterogeneity require the characterization of local constitutive properties. We have compared two methods to do this: Vickers-hardness mapping and miniature tensile testing. Whereas the former is more straightforward to apply, the latter provides full-range stress-strain data. This paper discusses an experimental comparison of both methods on a heterogeneous arc weld. Miniature tensile tests were performed, using digital image correlation to measure the strain. The specimens were indented to compare their stress-strain response with Vickers hardness. Notwithstanding that small natural flaws invalidated some tests, reliable stress-strain curves were obtained. Vickers hardness testing is a convenient alternative if the yield and ultimate tensile strength are the only points of interest and the corresponding conversion inaccuracy is acceptable.
Keywords: arc weld, heterogeneity, hardness, miniature tensile testing, digital image correlation
Published in DKUM: 14.03.2017; Views: 1364; Downloads: 440
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