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On the application of the multigrid method to topology optimization with orthotropic material with varying orientation
No full textMultigrid methods are known to show good performance for most topology optimization problems, but suffer from anisotropies. The main problems are caused by poor performance of the smoothing algorithms. This paper analyzes different standard and advanced smoothing algorithms using the local Fourier analysis with regard to orthotropic materials in the linear elasticity problem. The results are generalized to real problems and contrast-rich topology designs using numeric experiments. Based on the analyses, recommendations are given and a new spatial Jacobi smoothing algorithm is presented. The results are finally approved by a topology optimization example and a material orientation and topology optimization example
Completion of the Central Italy daily precipitation instrumental data series from 1951 to 2019
No full textPrecipitation is a critical part of the global hydrological cycle that determines the distribution of water resources. It is also an essential meteorological variable used as input for hydroclimatic models and projections. However, precipitation data frequently lack complete series, especially at daily and sub-daily precipitation stations, which are usually large, bulky, and complex. To address this, gap filling is commonly used to produce complete hydrometeorological data series without missing values. Several gap-filling methods have been developed and improved. This study seeks to fill the gaps of 201 daily precipitation time series in Central Italy by localizing the approach used to generate the Serially Complete dataset for the Planet Earth (SC-Earth). This method combines the outcome of 15 strategies based on four various gap-filling techniques (quantile mapping, spatial interpolation, machine learning, and multi-strategy merging). These strategies employ the daily dataset of the neighbouring stations and the matched ERA5 data to estimate missing values at the target stations. Both raw data and the final serially complete station datasets (SCDs) underwent comprehensive quality control. Many accuracy indicators have been utilized to evaluate the performance of the strategies' estimations and the final SCD, such as Correlation Coefficient (CC), Root mean square error (RMSE), Relative bias (Bias %), and Kling-Gupta efficiency (KGE″). Multi-strategy merging strategy based on the Modified Kling-Gupta efficiency (MS1) shows the highest performance as an individual precipitation gap-filling strategy. However, the machine learning strategy using random forest (ML3) has the most outstanding share in the final estimates among all other strategies. In the end, the temporal–spatial performance of the final SCD is promising and depends on the pattern of the missing values (MV%). The mean values of KGE″, CC, variability (α), and bias term (β) are 0.9, 0.93, 1.064, and 4.98 × 10−7, respectively
Molecular mechanisms of macroscopic ductility within brittle epoxy resins
No full textVarious studies show that the mechanical behavior of epoxy resins depends not only on the test temperature, the load case, and the test speed, but also on the test volume. This so-called size effect is evident in epoxy resins with an expected increase in strength. At the same time, there is also a significant increase in ductility as the test volume decreases. This means that the deformation behavior of microscale epoxy resins, such as the matrix areas between the fibers in fiberreinforced polymers (FRPs), differs significantly from the classic brittle behavior of standard bulk samples with comparatively large test volumes. However, to date, there is no comprehensive physical, mechanical-chemical, or molecular explanation for this increased ductile deformation capacity of microscale epoxy samples. For this reason, as part of this thesis, the mechanical behavior and underlying molecular mechanisms of epoxy resin films under tensile load were investigated with a greatly reduced test volume compared to standard samples. For this purpose, a suitable process was first developed and optimized to manufacture films with a thickness between 15 and 100 μm. The films produced were first examined using differential scanning calorimetry (DSC) and near-Infrared spectroscopy (NIR) with respect to their crosslinking behavior. No evidence of incomplete cross-linking was found. The samples were then shaped by punching and laser cutting the films. To clarify the relationships between the pronounced deformation capability and microstructural processes, the epoxy film samples were analyzed mechanically and spectroscopically in tensile, creep, fatigue, and relaxation tests. The loaded samples showed considerable necking and shear bands with a reduced thickness compared to the initial thickness. Shear bands were detected by photoelastic imaging. Ductility increased significantly with decreasing specimen thickness or reduced test volume. Film samples with a test volume of 0.06mm3 achieved elongations at break of up to 80% in tensile tests, which is significantly higher compared to standard bulk type 1BA samples with a test volume of 500mm3 (5-12% elongation at break) investigated in this study. In addition, the stress-strain curves of the film samples showed strain softening and hardening mechanisms during the deformation process. Spectroscopic methods were employed to analyze the molecular processes of macroscopic deformation. Polarized Raman and Infrared (IR) measurements revealed a molecular orientation of the main chains in the load direction. This explains the decrease and increase in stress after reaching the yield stress in the tensile test, as the chains “entangle” and align themselves in the load direction. After the stress has been partially relaxed by the molecular movements and macroscopically visible shear bands have formed, the stress increases again due to the strengthening effect of the molecular chains aligned in the tensile direction within the deformed sample. High-resolution microscopic IR investigations have also shown that within the deformed sample areas in the shear bands, the carbon bonds of the aromatics in the main chains are stretched, which is reflected in a peak shift towards lower wavenumbers. Following the orientation, there is therefore also a measurable elongation of the main molecular chains in the tensile direction. Digital image correlation (DIC) studies confirmed that high local strains and deformations occur particularly in the forming shear bands. In situ IR measurements
also enabled a direct correlation of the decrease in the aromatic and stress-sensitive peak wavenumber with increasing macroscopic strain during mechanical tests. This allowed investigation of the stress states present in the epoxy under different external mechanical loads.
For validation, the mechanically loaded and deformed samples were stored in an oven above the glass transition temperature Tg. This allowed the constricted samples to return to their original shape and the shear bands to be eliminated. After thermal annealing, the molecular main chains returned to an amorphous, undeformed state without long-range order. The shear bands and constrictions reappeared in the same way when the material was mechanically loaded again after thermal annealing. The high deformation ability of fully cross-linked epoxy can therefore be explained by reversible molecular structural changes in the form of alignment and stretching of the molecular chains, particularly in the area of the aromatic structures. The present doctoral thesis thus offers valuable insight into the molecular processes in microscopic test volumes that are favored by plane stress states and lead to an increased deformation capacity of epoxies. For even more accurate FRP modeling and design in microscale areas, the parameters of microscopic epoxy samples should be used instead of the mechanical properties of bulk epoxy samples to exploit the full potential of epoxies. In addition, the epoxy volume could be specifically reduced in critical areas of FRP, increasing the deformability locally.Verschiedene Studien zeigen, dass das mechanische Verhalten von Epoxidharzen neben der Testtemperatur, des Belastungsfalls und der Prüfgeschwindigkeit auch vom Volumen abhängt. Dieser sogenannte Größeneffekt (engl. size effect) zeigt sich bei Epoxidharzen mit einer erwarteten Zunahme der Festigkeit bei Abnahme des Volumens. Gleichzeitig kommt es auch zu einer signifikanten Erhöhung der Duktilität mit abnehmendem Prüfvolumen. Damit unterscheidet sich das Verformungsverhalten von mikroskaligen Epoxidharzen, wie sie beispielsweise als Matrixbereiche zwischen den Fasern in Faser-Kunststoff-Verbunden (FKV) vorliegen können, signifikant von dem klassisch spröden Verhalten von Standard Proben mit vergleichsweise großen Prüfvolumina. Bisher gibt es jedoch keine umfassende physikalische, mechanisch-chemische oder molekulare Erklärung für dieses erhöhte duktile Verformungsvermögen von mikroskaligen Epoxidharzbereichen. Aus diesem Grund wurde ihm Rahmen der vorliegenden Promotion das mechanische Verhalten und die zugrundeliegenden molekularen Mechanismen von Epoxidharzfolien unter Zuglast mit im Vergleich zu Standardproben stark reduziertem Prüfvolumina untersucht. Dafür wurde zunächst ein geeignetes Verfahren zur Herstellung von Folien mit einer Dicke zwischen 15 bis 100 μm entwickelt und optimiert. Die hergestellten Folien wurden zunächst mittels Dynamische Differenzkalorimetrie (DSC) und Nahinfrarot- Spektroskopie (NIR) untersucht, um eine nahezu vollständige Vernetzung sicherzustellen. Dabei konnten keine Hinweise auf eine unvollständige Vernetzung gefunden werden. Durch anschließendes Stanzen und Laserschneiden der Folien erfolgte die Probenformgebung. Zur Aufklärung der Zusammenhänge zwischen dem ausgeprägten Verformungsvermögen und der mikrostrukturellen Vorgänge wurden die so erzeugten Proben in Zug-, Kriech, Ermüdungs- und Relaxationsversuchen mechanisch und spektroskopisch analysiert. Die belasteten Proben zeigten erhebliche Einschnürungen und es bilden sich Scherbänder mit im Vergleich zur Ausgangsdicke reduzierter Dicke aus, welche mittels spannungsoptischer Mikroskopaufnahmen nachgewiesen werden. Es konnte gezeigt werden, dass die Duktilität mit abnehmender Probendicke bzw. verringertem Prüfvolumen signifikant zunimmt. Folienproben mit einem Prüfvolumen von 0.06mm3 erreichten im Zugversuch Bruchdehnungen von bis zu 80 %, was signifikant erhöht ist im Vergleich zu den im Rahmen der Arbeit untersuchten Standard Proben vom Typ 1BA mit einem Prüfvolumen von 500mm3 (5-12% Bruchdehnung). Außerdem zeigten sich in den Spannungs-Dehnungs-Kurven der Folien Proben Dehnungserweichungs- und Kaltverfestigungsmechanismen während des Verformungsvorgangs. Mit spektroskopischen Methoden wurden parallel zu den makroskopischen die molekularen Vorgänge analysiert. Durch polarisierte Raman und Infrarot (IR) Messungen konnte eine molekulare Orientierung der Hauptketten in Lastrichtung nachgewiesen werden. Dies erklärt die Spannungsab- und -zunahme nach dem Erreichen der Fließspannung im Zugversuch, da die Ketten sich „entschlaufen“ und in Lastrichtung ausrichten. Nachdem die Spannung durch die Molekülbewegungen teilweise abgebaut wurde und sich in einigen Probenbereichen makroskopisch sichtbare Scherbänder gebildet haben, steigt die Spannung aufgrund der Verfestigungswirkung der in Zugrichtung ausgerichteten Molekülketten innerhalb der deformierenden Probe wieder an. Hochauflösende mikroskopische IR Untersuchungen haben außerdem gezeigt, dass innerhalb der verformten Probenbereichen in den Scherbändern die aromatische Kohlenstoffverbindungen in den Hauptketten verstreckt vorliegen, was sich in einer Peak Verschiebung (engl. Peak Shift) zu geringeren Wellenzahlen zeigt. Es kommt demnach im Anschluss an die Orientierung auch zu einer messbaren Dehnung der Molekülketten in Zugrichtung. Mittels Digitalen Bildkorrelations-Untersuchungen (DIC) konnte bestätigt werden, dass es insbesondere in den Scherbändern zu hohen lokalen Dehnungen und Verformungen kommt. IR Messungen ermöglichten zudem eine direkte Korrelation der aromatischen und spannungs-sensitiven Peak Wellenzahl und makroskopischer Dehnung. Dies ermöglichte die Untersuchung der im Epoxidharz vorliegenden Spannungszustände bei verschiedenen externen mechanischen Belastungen. Zur Validierung wurden die mechanisch belasteten und verformten Proben oberhalb der Glasübergangstemperatur ausgelagert. Dabei konnten die eingeschnürten Proben wieder in ihre Ausgangsform zurückgeführt werden und die Scherbänder vollständig eliminiert werden. Nach der thermischen Auslagerung, liegen die molekularen Hauptketten wieder in einem amorphen, unverstreckten Zustand ohne Fernordnung vor. Bei einer erneuten Belastung nach thermischer Rückführung, entstanden die Scherbänder und Einschnürungen wieder in gleicher Art und Weise. Die hohe Duktilität von vollständig vernetzten Epoxidharzen kann somit durch reversible molekulare Strukturänderungen in Form von Ausrichtung und Strecken der Molekülketten insbesondere im Bereich der aromatischen Strukturen erklärt werden. Die vorliegende Promotion bietet damit wertvolle Erkenntnisse in die durch ebene Spannungszustände begünstigten molekularen Vorgänge in mikroskopischen Prüfvolumina, welche zu einem erhöhten Verformungsvermögen von Epoxidharzen führen. Demnach sollten zukünftig für noch genauere FKV Modellierungen und Auslegungen in mikroskalaren Bereichen nicht die mechanischen Kennwerte von Standard- bzw. Norm-Probenkörpern verwendet werden, sondern die Kennwerter von mikroskopischen Proben, um das Materialverhalten von Epoxidharzen richtig zu berücksichtigen. Zudem könnte in kritischen FKV Bereichen gezielt das Harzvolumen reduziert und damit die Verformbarkeit lokal erhöht werden.Deutsche Forschungsgemeinschaft (DFG
Optical properties of nanoporous gold and their change upon electrochemical oxidation
No full textNanoporous gold shows a strong change of color upon electrochemical oxidation. This can result from a reduced volume of plasmonic gold, dielectric coating on the gold surface or an increase of electron collision frequency. Using brute-force simulations we identify the contribution of these effects
Cooperative distributed model predictive control for embedded systems: experiments with hovercraft formations
No full textThis paper presents experiments for embedded cooperative distributed model predictive control applied to a team of hovercraft floating on an air hockey table. The hovercraft collectively solve a centralized optimal control problem in each sampling step via a stabilizing decentralized real-time iteration scheme using the alternating direction method of multipliers. The efficient implementation does not require a central coordinator, executes onboard the hovercraft, and facilitates sampling intervals in the millisecond range. The formation control experiments showcase the flexibility of the approach on scenarios with point-to-point transitions, trajectory tracking, collision avoidance, and moving obstacles
N-heterocyclic carbene as robust and conductive linkages to couple gold nanoparticles and conductive polymers
No full textN-heterocyclic carbene (NHC) anchors enable electron delocalization over the gold/conductive polymer (CP) interface, resulting in an improved conductivity. Here we developed a new class of real hybrid nanocomposites using NHC to coupling AuNPs and CPs, including small Au NP@NHC-CP by bottom-up methods and large Au NP@NHC-CP by top-down methods
Solvation free energies of anions: from curated reference data to predictive models
No full textPredicting the physicochemical properties of ionizable solutes, including solubility and lipophilicity, is of broad significance. Such predictions rely on the accurate determination of solvation free energies for ions. However, the limited availability of high-quality reference data poses a challenge in developing accurate, inexpensive computational prediction methods. In this study, we address both issues of data quality and availability. We present three databases and models related to ionic phenomena: (1) 8,241 pKa data points across 8 solvents, (2) 5,536 gas-phase acidities from DLPNO-CCSD(T) QM calculations, and (3) 6,090 solvation free energies of anions across 8 solvents obtained from a thermodynamic cycle. We also report 6,088 solvation free energies of neutral conjugate solutes computed using the COSMO-RS method. The pKa data were obtained from the iBonD database, cleaned, and combined with a separate compilation of trustworthy reference pKa data. Gas-phase acidities were computed for most of the acids present in the pKa corpus. Leveraging these data, we compiled values for solvation free energies of anions. We then trained several graph neural network models, which can be used as an alternative to QM approaches to quickly estimate these properties. The pKa and gas-phase acidity models accept reaction SMILES strings of the acid dissociation as inputs, whereas the solvation energy model accepts the SMILES string of the anion. Our microscopic pKa model achieves good accuracy, with an overall test mean average error of 0.58 units on unseen solutes and 0.59 on the SAMPL7 challenge (the lowest error so far among multisolvent models). Our gas-phase acidity model had mean absolute errors slightly above 2 kcal mol-1 when evaluated against experimental data. The anionic solvation free energy model had mean absolute errors of less than 3 kcal mol-1 in several test evaluations, comparable to (though less reliable than) several widely used QM-based solvation models
Recent progress in Selenium remediation from aqueous systems: state-of-the-art technologies, challenges, and prospects
No full textThe contamination of drinking water sources with selenium (Se) oxyanions, including
selenite (Se(IV)) and selenate (Se(VI)), contains serious health hazards with an oral intake
exceeding 400 μg/day and therefore requires urgent attention. Various natural and anthropogenic
sources are responsible for high Se concentrations in aquatic environments. In
addition, the chemical behavior and speciation of selenium can vary noticeably depending
on the origin of the source water. The Se(VI) oxyanion is more soluble and therefore more
abundant in surface water. Se levels in contaminated waters often exceed 50 μg/L and may
reach several hundred μg/L, well above drinking water limits set by the World Health
Organization (40 μg/L) and Germany (10 μg/L), as well as typical industrial discharge
limits (5–10 μg/L). Overall, Se is difficult to remove using conventionally available physical,
chemical, and biological treatment technologies. The recent literature has therefore
highlighted promising advancements in Se removal using emerging technologies. These
include advanced physical separation methods such as membrane-based treatment systems
and engineered nanomaterials for selective Se decontamination. Additionally, other
integrated approaches incorporating photocatalysis coupled adsorption processes, and bioelectrochemical
systems have also demonstrated high efficiency in redox transformation
and capturing of Se from contaminated water bodies. These innovative strategies may offer
enhanced selectivity, removal, and recovery potential for Se-containing species. Here, a
current review outlines the sources, distribution, and chemical behavior of Se in natural
waters, along with its toxicity and associated health risks. It also provides a broad and
multi-perspective assessment of conventional as well as emerging physical, chemical, and
biological approaches for Se removal and/or recovery with further prospects for integrated
and sustainable strategies
A framework for demonstrating and mitigating CAN injection attacks in vector CANoe: a case study using ABS
No full textModern vehicles rely heavily on the Controller Area Network (CAN) for communication between electronic control units, yet CAN lacks inherent security features like authentication, making it susceptible to attacks, such as message injection. In this paper, we develop a means of simulating and visualizing the effect of an attack on the communication traffic and the actual system and the effect of a countermeasure. Our tool is based on Vector CANoe simulation and demonstrates a CAN message injection attack and a simple rule-based Intrusion Detection System (IDS). The setup includes a custom Graphical User Interface (GUI) for interactive demonstration. This framework serves as an educational tool and a foundation for future research into more advanced attack and defense scenarios. This work takes a step towards developing simulation platforms or frameworks for security of in-vehicle networks that are flexible and based on industry suitable toolchains
Solar-based timekeeping for batteryless devices
No full textThe Internet of Things (IoT) is steadily gaining traction, but its reliance on battery power raises significant challenges. To address this, researchers are developing energyharvesting IoT devices that operate on intermittent power, eliminating the need for batteries but complicating accurate timekeeping due to unreliable energy supply. Traditional Real-Time Clocks (RTCs) and synchronization methods are ineffective under such conditions, hindering tasks that require precise timing. We propose timekeeping using TinyML to predict sunrise and sunset based on power usage patterns, enabling autonomous time inference without continuous power or external synchronization. We trained and evaluated multiple lightweight models through simulation and with real hardware, and we demonstrate their potential and shortcomings compared to conventional methods