Open-Access-Journals @ Otto-von-Guericke-Universität Magdeburg
Not a member yet
2156 research outputs found
Sort by
An Elastohydrodynamic Lubrication Framework for Hip Prostheses based on Isogeometric Analysis
No full textThe hip prosthesis modeling involves complex fluid-structure interaction between lubricant flow and solid deformation, which is a typical elastohydrodynamic lubrication regime. This study introduces a novel numerical framework for addressing such challenges through Non-Uniform Rational B-Splines (NURBS)-based isogeometric analysis (IGA). A new form of the Reynolds equation, derived in the parametric space of curved surfaces rather than spherical coordinates, is introduced to streamline discretization within the IGA framework and integration with solid mechanics. Additionally, the momentum balance equation is utilized for calculating solid deformations, providing a more accurate description than the conventional Boussinesq approximation. Furthermore, a developed mass-conserving cavitation formulation is incorporated to depict the formation and volumetric distribution of cavitation bubbles in hip prostheses. These equations are discretized utilizing NURBS basis functions and solved simultaneously via the Newton-Raphson method, enabling efficient strong coupling and rapid convergence. This IGA-based framework also facilitates future integration of nonlinear solid deformation and frictional contact
Influence of cyclic stress wave form on the fatigue behavior of bolts
No full textBolted joints offer the great advantage of a non-destructive disassembly and can transfer higher loads than otherdetachable connections during operation. Therefore, bolted joints are one of the most commonly used joining methods inmechanical engineering and steel construction. In current guidelines the wave form and frequency of the load are neglected whenestimating the fatigue strength. Even in rainflow counting the influence of load frequency and load wave form is also neglected.Although it is known from unnotched material samples that the wave form and frequency of the load have a measurable influence.In this study, bolts of size M10 are examined at different test frequencies and wave forms in the finite life range. The mean load iskept constant for all different wave forms during the tests. Each test is performed at a constant amplitude. For this purpose, a testsetup was designed to perform fatigue tests with fluctuating axial load. It is shown that a long application time of the load shortensthe fatigue life. In addition, the effective value of the load has an influence on the achievable service life. The greater the effectivevalue of the load, the shorter the fatigue life
Development of a multi-lane cargo bike with a view to driving behaviour
No full textCargo bikes are a suitable alternative to cars for transporting food or loads for private individuals or delivery services,especially in city centers. Major advantages are that the use of cargo bikes is emission-free and causes no noise. In comparison,the intensive use of private cars in inner-city traffic is very harmful to the traffic system and the environment. In order to solvethis problems, this project deals with the product development of a multi-lane cargo bike, which is designed for the transport ofloads over short and medium distances. The funded SteigtUM project therefore developed an urban mobility concept based onmicromobiles. One part of these micromobiles are the cargo bikes mentioned above, which are used with an app-based hire system.The aim for the newly developed cargo bike with two wheels in the front is to achieve a driving behavior that is comparable to thedriving behavior of conventional single-lane bicycle. The reasons for the multi-lane design are an autonomous parking function,which is one of the overall objectives of the project and a safe standing even with loads. In order to gain a comparable drivingbehavior like single-lane bicycles, the strong dependence of the geometric design on the steering head angle and the caster mustbe given special consideration. In addition, the torsional stiffness and the track adjustment of the multi-lane axle are of greatimportance. The developments in the overall project take particular account of suitability for everyday use. Furthermore, they arebased on various user studies on driving behavior and thus ensure a high level of acceptance and usability. Cargo bikes thereforeoffer a great opportunity to further strengthen sustainable inner-city mobility and reduce the use of cars as a means of transport
On the characteristic electrical and thermal properties of beta-phase gallium oxide (β–Ga2O3) Schottky diodes in view of their potential application in power electronics
No full textDer Halbleiter beta-Galliumoxid (β–Ga2O3) vereint eine 4.6–4.9 eV breite Bandlücke mit der Verfügbarkeit schmelzgewachsener Wafer und könnte dazu beitragen, den steigenden Bedarf an hocheffizienter und kostengünstiger Leistungselektronik zu decken. Jedoch konzentriert sich die Forschung derzeit noch hauptsächlich auf die grundlegenden Strukturen der Bauelemente. Außerdem weckt die geringe thermische Leitfähigkeit von Ga2O3 Bedenken hinsichtlich möglicher Probleme bei der Wärmeabfuhr. Ziel dieser Arbeit ist, einen Übergang von der bisherigen Forschung auf Bauelementestruktur-Ebene hin zur Anwendung in der Leistungselektronik zu schaffen, indem elektrische und thermische Eigenschaften neuartiger β–Ga2O3 Dioden aus einer geplanten Fertigungslinie experimentell und simulativ untersucht werden. Bei mehreren β–Ga2O3 Dioden ist eine Änderung des Leitmechanismus zu beobachten, die zu einer anfänglichen Abnahme und dann zu einem Anstieg der Leitungsverluste mit steigender Temperatur führt. Dies scheint jedoch auf die Herstellung der Chips zurückzuführen zu sein und nicht direkt auf die intrinsischen Eigenschaften von β–Ga2O3. Trotz einer starken Variation der Materialeigenschaften zwischen Dioden desselben Typs wird ein geringerer Anstieg des differentiellen Durchlasswiderstands mit steigender Temperatur im Vergleich zu Siliziumkarbid (SiC) Dioden beobachtet, und Messungen der temperaturabhängigen Idealitätsfaktoren und Schottky-Barrierenhöhen weisen auf stabile Sperrschicht- bzw. Grenzflächeneigenschaften hin. Die Entwärmung von Ga2O3 Dioden kann durch Abdünnen der derzeit 600 μm dicken Standardbauteile auf Dicken von 200 μm signifikant verbessert werden. Im Gegensatz zu SiC erweist sich jedoch die sperrschichtseitige Kühlung trotz der kleineren Kühlfläche als wesentlich effektiver bei der Senkung der Bauteiltemperatur, wenn die gesamte Anodenfläche mit Lot- bzw. Sinterpaste bedeckt ist. In Verbindung mit den potenziell geringen Leitungsverlusten scheint es realistisch, dass künftige Ga2O3 Dioden bei gleichem Durchlassstrom ähnliche Sperrschichttemperaturen erreichen wie moderne kommerzielle SiC Dioden. Selbst 600 μm dicke Dioden werden erfolgreich in einem 400 V Abwärtswandler eingesetzt, der bei Schaltfrequenzen von bis zu 350 kHz betrieben wird. Es werden maximale Anstiegsgeschwindigkeiten der Spannung von über 100 V/ns erreicht, aber im Dauerbetrieb weisen die Ga2O3 Dioden einen höheren Temperaturanstieg auf als die SiC Vergleichsbauteile. Dennoch können mit den Ga2O3 Freilaufdioden trotz des aktuell höheren Durchlasswiderstandes aufgrund der fehlenden Rekombinationsverluste die Effizienzen mit einer modernen Silizium-Diode ähnlicher Größe übertroffen werden.The semiconductor beta gallium oxide (β–Ga2O3) combines a 4.6–4.9 eV wide band gap with the availability of melt-grown wafers and could help meet the growing demand for high-efficiency and low-cost power electronics. However, research still mostly focuses on basic device structures, and the low thermal conductivity raises concerns about potential thermal management problems. The objective of this work is to transition from fundamental device structure research to power electronics applications by experimentally and through simulation investigating the electrical and thermal characteristics of novel β–Ga2O3 diodes from a planned production line. A change of the conduction mechanism leading to an initial decrease and then increase of the conduction losses with rising temperature is observed for multiple but not all diodes. This seems to originate from the device processing rather than the intrinsic properties of β–Ga2O3. Despite a strong variation of the material properties between diodes of the same type, a lower increase in differential on-resistance with rising temperature is observed compared to silicon carbide (SiC) diodes, and measurements of the temperature-dependent ideality factors and Schottky barrier heights indicate stable junction properties. The heat dissipation in Ga2O3 diodes can be improved by thinning the currently 600 μm thick standard devices to thicknesses of 200 μm. In contrast to SiC, however, cooling the devices from the junction side is found to be significantly more effective in reducing the junction temperature despite the smaller cooling area, if the entire anode area is covered with die-attach material. Combined with the potentially low conduction losses, it seems realistic for future Ga2O3 diodes to achieve similar junction temperatures as modern commercial SiC diodes at the same forward current. Even 600 μm thick diodes are successfully implemented in a 400 V buck converter operated at frequencies up to 350 kHz. Peak voltage slew rates exceeding 100 V/ns are achieved, but in continuous operation the diodes exhibit a higher temperature rise than their SiC counterparts. Nevertheless, the efficiencies with a state-of-the-art silicon diode of similar size can be surpassed with the Ga2O3 diodes despite the still higher on-resistance, owing to the absence of recombination losses
Numerical evaluation of mechanical behaviour of lattice structures for rotating blades
No full textLattice structures have significant potential in engineering, allowing for material and weight reduction while maintainingdesired mechanical properties. This versatility is crucial in applications ranging from aerospace to medical implants, wherecustomisability and efficiency are essential. This study investigates the mechanical performance of four lattice structures withmultiple and single-cell model approaches. The aim is to elucidate the impact of lattice design parameters on the structural integrity and performance of components subjected to dynamic loads typical of rotating blades in aerospace applications. Utilising Finite Element Analysis (FEA), this research is conducted to characterise the overall mechanical behaviour and to simulate the behaviour of these structures under conditions that represent real-world operational conditions for rotating blades. The loading conditions considered are tension, compression, shear and periodic boundary conditions are applied. By comparing the mechanical behaviour of these lattice structures against each other, this research aims to identify optimised lattice designs that enhance the performance and durability of rotating blades. This study is expected to contribute to the broader field of materials science and engineering by providing guidelines for designing more efficient, lightweight, high-performance components in various industrial applications
Klimaschutz auf regionaler Ebene im Land Sachsen-Anhalt : Eine vergleichende Analyse der 11 Landkreise
No full textDer Beitrag untersucht das klimapolitische Handeln von Landkreisen in Sachsen-Anhalt anhand der drei Bereiche "Konzepte und Strategien", Personalstellen und Fördermittel" sowie "Außendarstellung & Information". Er adressiert damit eine zentrale Forschungslücke, indem erstmals eine umfassende und differenzierte Bestandsanalyse auf kommunaler Ebene für ein gesamtes Bundesland vorgelegt wird. Im Ergebnis bestehen zwei Gruppen an Landkreisen, wobei die Nachzügler-Gruppe keinen/kaum Klimaschutz betreibt, während die Vorreiter-Gruppe substanzielles Handeln zeigt. Unterschiede bestehen allerdings auch innerhalb der Vorreiter-Gruppe, sowohl bei Umfang als auch zeitlicher Verortung der Aktivitäten. Ein Desiderat künftiger Forschung ist die Analyse abgebrochener Pfade, d. h. Hindernissen der Versteigerung von Klimaschutzaktivitäten
Dynamic behaviour of cargo bikes: An approach for quantitative evaluation
No full textPeople’s desire for individuality and flexibility in inner-city traffic often leads to high emissions and a critical parking situation. The extensive use of private cars puts a strain on the entire transportation system. In order to counteract, this paper deals with quantifying driving behaviour in order to influence the product development of cargo bikes, which are designed to transportloads for short distances. The overall bicycle developments are based on user studies on riding behaviour and thus focus on higher acceptance. The aim is that the newly developed multi-lane cargo bikes will achieve the same driving behaviour as a conventional bicycle and increase public acceptance of sharing systems and usability of cargo bikes. A single-lane cargo bike is presented, which is used for the active recording of measured values. This enables the assessment of forces such as saddle forces, pedal forces and steering forces, as well as steering movement and velocity. Statistical methods can be used to draw conclusions about the driving behaviour of cargo bikes, which is significantly influenced by the load distribution. Cargo bikes present a valuable opportunity to reinforce sustainable mobility and supplant the automobile as a mode of transportation. In particular, if the transportation of heavy loads remains within the rider’s control, a positive riding experience can be achieved, as is the case with conventional bicycles
Numerical and Experimental Considerations of Non-linearities for a Trifilar Pendulum
No full textMoments of inertia of machine-components or whole machines are essential for numerical simulations, especially for Mutli-Body-Simulations. Various measuring techniques exists, which can be rougly divided in techniques using oscillations or accelerations. One possibility to measure indirectly the inertia properties are multifilar pendulums, for example the trifilar pendulum using a platform and three ropes or filars. A lot of literature dealing with the trifilar pendulums exists. We will give a brief overview of the literature with emphasis to non-linearities. Furthermore, we will investigate nonlinear effects and compare them with experimental and numerical results
Dynamic Optimization in Peer-To-Peer Transportation with Acceptance Probability Approximation
No full textCrowdsourced transportation by independent suppliers (or drivers) is central to urban delivery and mobility platforms. While utilizing crowdsourced resources has several advantages, it comes with the challenge that suppliers are not bound to assignments made by the platforms. In practice, suppliers often decline offered service requests, e.g., due to the required travel detour, the expected tip, or the area a request is located. This leads to inconveniences for the platform (ineffective assignments), the corresponding customer (delayed service), and also the suppliers themselves (non-fitting assignment, less revenue). In this work, we show how approximating suppliers’ acceptance behavior by analyzing their past decision making can alleviate these inconveniences. To this end, we propose a dynamic matching problem where suppliers’ acceptances or rejections of offers are uncertain and depend on a variety of request attributes. Suppliers who accept an offered request from the platform are assigned and reenter the system after service looking for another offer. Suppliers declining an offer stay idle to wait for another offer, but leave after a limited time if no acceptable offer is made. Every supplier decision reveals partial information about the suppliers’ acceptance behavior, and in this paper, we present a corresponding mathematical model and a solution approach that translates supplier responses into the probability of a specific supplier to accept a specific future offer and uses this information to optimize subsequent offering decisions. We show that our approach leads to overall more successful assignments, more revenue for the platform and most of the suppliers, and less waiting for the customers to be served. We also show that considering individual supplier behavior can lead to unfair treatment of more agreeable suppliers