303 research outputs found
New approaches to solar tracking and concentration through numerical optimization of lens arrays
No full textSolar concentrators are essential for enabling several solar energy applications, including high-efficiency photovoltaic conversion and high-temperature solar thermal energy. These concentrators require accurate solar tracking, commonly performed by rotating them towards the sun, which adds to the bulk and complexity of the system.
In this thesis, we investigate optically tracking the sun using millimeter-scale translation instead of rotating the complete concentrator — a concept known as tracking integration. We show how the performance of these systems can be pushed beyond the current state of the art through a broad exploration of the design space using a custom sequential ray-tracer in combination with memetic multi-objective optimization algorithms. We explore two classes of tracking integration: beam-steering lens arrays that consist of an afocal stack of lens arrays, and microtracking concentrators that concentrate sunlight to an array of discrete focal spots where micro-PV cells can convert the solar energy to electricity. Further, we propose possible étendue-squeezing solar concentrator designs that may benefit from tracking integration.
First, we perform a broad exploration of beam-steering lens array configurations for full-day stationary solar tracking. We identify several promising configurations, including one capable of redirecting sunlight into a 2000x concentration ratio at a two-axis ±60◦tracking range. Finally, we demonstrate a line-focus concentrator with a simulated effective concentration ratio of 218x at a ±1◦ acceptance angle that employs étendue squeezing to go beyond the conventional two-dimensional concentration limit. To the best of our knowledge, this is the first demonstration of how a line-focus concentrator can be directly designed as a three-dimensional concentrator to operate beyond the 2D limit (which is 57x at the ±1◦ acceptance angle). This concentrator, combined with beam-steering lens arrays, may enable the development of a new class of highconcentration trough-like solar concentrators
Relating stress corrosion cracking behavior to microstructural and surface properties of biocompatible AZ31 alloy
No full textMagnesium (Mg) and its alloys have attracted significant attention as temporary implant materials due to their excellent biocompatibility with human physiology. In fact, Mg is essential to the human metabolism as a cofactor for many enzymes and Mg ions are well-known to facilitate tissue-healing. In addition, the mechanical properties (density, elastic modulus, yield strength and ultimate tensile strength) of Mg and its alloys resemble those of natural bone reducing the risk of the stress-shielding-related problems observed with other metallic implant materials such as stainless steel, titanium and Co-Cr alloys. However, despite their high potential, Mg and its alloys are not yet utilized in biomedical applications. This is due to the (1) rapid corrosion and degradation in the human body that leads to a loss of mechanical integrity before tissues have sufficient time to heal, (2) the evolution of hydrogen as corrosion product accompanied by hydrogen pocket formation that hampers healing or even cause the death of patients through the blockage of the blood stream and (3) the sudden fracture of implants due to the simultaneous action of the corrosive human-body-fluid and mechanical loads through corrosion-assisted cracking phenomena (stress corrosion cracking (SCC) and corrosion fatigue (CF)).
In the past years, several approaches have been developed to improve the corrosion resistance of Mg and its alloys. These approaches can be divided into two main groups, one characterized by the modification of the bulk and the other by the modification of the surface. Among the former, Severe Plastic Deformation (SPD) techniques, such as Equal Channel Angular Pressing (ECAP), have attracted attention as possibility for inducing a very fine and homogeneous microstructure throughout all the samples. The latter group relies on surface modifications obtained by mechanical processing (e.g. cryogenic machining) or by the protection through coatings deposited by various techniques (e.g. sputter and Atomic Layer Deposition (ALD)). However, the assessment of the effectiveness of the different approaches in improving the resistance of Mg and its alloys to corrosion-assisted cracking phenomena is still underexplored.
In an attempt to understand the fundamental mechanisms linking the microstructural and surface properties to the SCC susceptibility, this thesis investigates how selected procedures initially intended for improving the corrosion resistance of Mg and its alloys impact the SCC susceptibility of AZ31 alloys in Simulated Body Fluid (SBF) at 37 °C. The procedures selected from an extensive literature review investigating the different procedures used to improve the corrosion behavior and the mechanisms regulating the SCC phenomenon were ECAP, cryogenic machining and coatings obtained by means of ALD.
1, 2 and 4 passes of ECAP were carried out on an AZ31 alloy and samples subjected to one pass of ECAP have been shown to be less susceptible to SCC compared to the material in the as-received condition (the elongation to failure was increased by 150%) due to the improved corrosion resistance as a consequence of a reduced grain size. The reduced SCC susceptibility after one pass of ECAP was also confirmed by the morphology of the fracture surfaces that reveals an increased ductility compared to the as-received material. However further ECAP processing (2 and 4 passes) are reported to worsen the SCC susceptibility due to an increased brittleness of the material as a consequence of an increased amount of hydrogen evolved. This is due to the unfavorable texture evolution, as confirmed by the mechanical characterization (tensile tests and hardness measurements).
AZ31 samples were machined under cryogenic cooling and afterwards subjected to Slow Strain Rate Tests (SSRTs) at a strain rate of 3.5·10-6 s-1 to evaluate the SCC susceptibility. Cryogenic machined samples were characterized by lower SCC susceptibility than dry cut samples (the elongation to failure was increased by 28%) as a consequence of their improved corrosion performances due to the presence of a wider nanocrystalline layer, resulting in a faster formation of passivating surface oxides, and to the presence of compressive residual stresses instead of tensile.
Being ALD a recently developed technique still underexplored in terms of corrosion and biological properties, it was compared to sputter technique in terms of corrosion protectiveness and the induced biocompatibility of three different coatings were evaluated. The ALD technique has been shown to provide the better corrosion protection (assessed by means of potentiodynamic polarization curves and hydrogen evolution experiments) both in case of smooth and rough surfaces due to an increased surface integrity (observed by SEM and XPS analyses). In addition, in the case of 3D porous structures, the improvements provided by the ALD technique were even higher as a consequence of the line-of-sight limitation of sputtering (confirmed by means of SEM analyses). In addition, the biocompatibility of TiO2, ZrO2 and HfO2 coatings obtained by means of ALD have been investigated by means of MTS assay on L929 cells and the HfO2 coatings were shown to provide the best biocompatibility due to the highest corrosion resistance. This can be reasoned by their lower wettability and their higher electrochemical stability and surface integrity (in terms of cracks and pores). TiO2, though generally considered a biocompatible coating, was found to provide the lowest improvements in terms of corrosion resistance and cell viability. Interestingly, TiO2 coatings are characterized by grade 3 cytotoxicity after 5 days of culture due to their high corrosion rate, which does not meet the demands for cellular applications. These results indicate the strong link between biocompatibility and corrosion protection and signify the need of considering the latter when choosing a biocompatible coating to protect temporary Mg based alloys before implantation.
Finally, the SCC susceptibility of TiO2 and ZrO2 ALDed coated AZ31 alloys have been evaluated and the ZrO2 coated samples were reported to have the lowest SCC susceptibility. In fact, the elongation to failure of the TiO2 coated samples were increased by 125% and that of ZrO2 coated samples by 220%. The different SCC susceptibility was attributed to the improved corrosion of the ZrO2 coated samples compared to the TiO2 coated samples as a consequence of four main aspects, i.e. different cohesive energies, different wettability, different defect densities and sizes and different mechanical properties
The genetic and environmental architecture of substance use development from early adolescence into young adulthood: a longitudinal twin study of comorbidity of alcohol, tobacco and illicit drug use
No full textThis is the peer reviewed version of the following article: Waaktaar, T., Kan, K.-J., and Torgersen, S. (2017) The genetic and environmental architecture of substance use development from early adolescence into young adulthood: a longitudinal twin study of comorbidity of alcohol, tobacco and illicit drug use. Addiction, which has been published in final form at http://dx.doi.org/10.1111/add.14076. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.
Polarization-driven reversible actuation in a photo-responsive polymer composite
No full textLight-responsive polymers and especially amorphous azopolymers with intrinsic anisotropic and polarization-dependent deformation photo-response hold great promises for remotely controlled, tunable devices. However, dynamic control requires reversibility characteristics far beyond what is currently obtainable via plastic deformation of such polymers. Here, we embed azopolymer microparticles in a rubbery elastic matrix at high density. In the resulting composite, cumulative deformations are replaced by reversible shape switching – with two reversible degrees of freedom defined uniquely by the writing beam polarization. We quantify the locally induced strains, including small creeping losses, directly by means of a deformation tracking algorithm acting on microscope images of planar substrates. Further, we introduce free-standing 3D actuators able to smoothly undergo multiple configurational changes, including twisting, roll-in, grabbing-like actuation, and even continuous, pivot-less shape rotation, all dictated by a single wavelength laser beam with controlled polarization
Fracture behaviour of notched as-built EBM parts: Characterization and interplay between defects and notch strengthening behaviour
No full textAdditive manufacturing (AM) offers the potential to economically produce customized components with complex geometries. However, the introduction of complex geometry goes hand in hand with the presence of notches. Thus, the basic understanding of the tensile behaviour of AM fabricated notched components must be substantially improved so that the unique features of this rapidly developing technology can be utilized in critical load bearing applications. This work aims to assess the tensile behaviour of different notched specimens manufactured by means of the Electron Beam Melting (EBM) technology and tested in their as-built conditions in order to reveal the interplay between notch geometry and AM specific processing. Scanning Electron Microscopy (SEM) was used to investigate the fracture surface of the broken samples, revealing the presence of process-induced defects, harmfully affecting tensile strength with a reduction of 10% and elongation to failure with 40% with respect to values reported in literature for heat treated AM parts, respectively. Interestingly, the authors have discovered an increase in the tensile strength with the severity of the stress concentrators, quite contrary to what is commonly observed for wrought Ti-6Al-4V. This notch strengthening behaviour particular to AM specimens is related to the influence of defects on the failure driving force. The authors provide a qualitative explanation for this phenomenon using 3D FE analyses together with a theoretical description via the ellipse criterion
Die neutrale Normativität der Technikfolgenabschätzung : Konzeptionelle Auseinandersetzung und praktischer Umgang
No full textTechnikfolgenabschätzung (TA) ist unparteilicher Expertise ebenso wie demokratischen Grundwerten verpflichtet. Und darüber hinaus? In welchem normativen Rahmen bewegt sie sich, ist dieser überall gleich oder unterscheidet er sich je nach Thema, gesellschaftlicher Aufgabe oder Land und politischer Kultur? Wie soll TA mit normativen Ansprüchen umgehen, die von außen an sie herangetragen werden, und wie mit solchen, die von innen, aus der TA selbst, kommen? Welche Möglichkeiten der Identifikation und der Verarbeitung normativer Ansprüche hat TA, und wie kann und soll sie sich im Konzert widerstreitender politischer Interessen und divergierender Weltbilder positionieren? Ist „neutrale“ Expertise dabei ein Hemmschuh oder eine Hilfe; kann es sie überhaupt (noch) geben? Auf derartige Fragen versuchen die Autoren und Autorinnen des Bandes Antworten zu geben oder sich zumindest der Problematik anzunehmen, vor der immer vielfältigere TA-Ansätze in Zeiten schärferer politischer und ökonomischer Gegensätze und beschleunigter technischer Entwicklung stehen.
Mit Beiträgen von
Armin Grunwald, Niklas Gudowsky-Blatakes | Christoph Kehl | Helge Torgersen, Julia Hahn, Jan-Hendrik Kamlage | Julia Reinermann, Marcel Krüger | Philipp Frey, Linda Nierling | Maria Udén, Poonam Pandey | Aviram Sharma, Diana Schneider, Stefan Strau
Fatigue strength of blunt V-notched specimens produced by selective laser melting of Ti-6Al-4V
No full textSelective Laser Melting (SLM) process is an Additive Manufacturing (AM) technique that allows producing metallic parts of any kind of geometry with densities greater than 99.5%. Complex shapes lead however to notches with different radii of curvature that may reduce load bearing capacities. This work is aimed to assess the fatigue strength of Ti-6Al-4V blunt V-notched samples produced by SLM. Results were compared with those of the corresponding smooth samples and Environmental Scanning Electron Microscopy (ESEM) have been used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Finally, the strain energy density approach was used to evaluate the critical radius value. Despite the observed fatigue strength reduction induced by the notch, samples showed a sufficient low notch sensitivity that it was not possible to define a critical radius for the material analysed
3D Printing and Carbonization of High Performing Polyimide Formulations
No full textStereolitografisk 3D-printing (SLA) er en moden form for additiv tilvirkning, og blir i økende grad brukt til produksjon av deler som inngår i et ferdig produkt. I denne oppgaven utforskes muligheten for å bruke SLA til å produsere gassdiffusjonslagene som inngår i proton-utvekslende brensel-celler. En studie av eksisterende litteratur presenteres,og denne studien indikerer at oppløsningen som er oppnåelig med SLA er kompatibel med designkravene for gassdiffusjonslag. Teknikker for effektiv generering av gitterkonstruksjoner med slik oppløsing har blitt utviklet. Dette er gjort ved å hoppe over 3D modellering som et steg i prosessen og isteden produsere 2D tverrsnitt med et python script. Sammenligninger som er utført viser at denne teknikken er størrelsesordener raskere enn vanlig 3D-modellering. Videre tillater teknikken nøyaktig innstilling av energidoseringen gjennom pikselbasert intensitetskontroll. Dette muliggjorde 3d-printing av gitterkonstruksjoner med bjelker bare en piksel brede (50uM), adskilt av mellomrom kun tre piksler (150uM) brede på en kommersielt tilgjengelig DLP-SLA 3D-printer. Denne masteroppgaven undersøker også to polyimidbaserte materialer som kandidatmaterialer for de 3D-printede gassdiffusjonslagene. Resultatene indikerer at det er mulig å karbonisere UV-herdbar polyimid, men også at mer arbeid er nødvendig for å avgjøre om materialet er egnetStereolithographic 3D printing (SLA) is a mature additive manufacturing technology and is gaining traction for the production of certain end-use parts. In this thesis, the possibility of using SLA to produce the gas diffusion layers found in proton exchange membrane fuel cells is explored. A study of the existing literature is presented, which indicates that the resolution of SLA is compatible with the design requirements for gas diffusion layers. A technique for efficiently designing lattice structures with sufficient resolution and a large number of features have been developed. This has been done by avoiding 3D modeling and instead directly producing 2D cross sections with a Python script. Comparisons show that this technique could be orders of magnitudes faster than conventional 3D modeling. Further, the technique allows for precise tuning of the energy dosage through pixel-based intensity control, this enables 3D printing of lattice structures with beams only one pixel wide 50 μm, separated by only three pixels 150 μm gaps on a commercially available a DLP-SLA printer. The thesis also investigates two polyimide-based materials as candidate materials for the 3D printed gas diffusion layers. The results indicate that it is possible to carbonize UV-curable polyimide, but also that more work is needed to determine if the material is suitable
Topology Optimization in Mechanical Engineering: Implementation and practical aspects
No full textOne of the most applied optimization methods in mechanical engineering is topology optimization (TO). The benefits of its integration in the product development process are several, such as reducing material usage in manufacturing, shortening the design cycle, and enhancing product quality. However, the implementation of TO is characterized by the following bottlenecks: the geometrical complexity of its optimized designs, the long optimization times, the sensitivity of its results to the given parameters, and the need for numerous inputs during its workflow. All these issues make TO a complex and time-demanding procedure dependent on designers’ starting guesses and choices during its implementation. It is clear that there is a need for a more automatic and effective optimization procedure.
In this thesis, the author uses TO for the weight reduction of structures in mechanical engineering. First, he explores the workflow of the TO and identifies interesting practical aspects in its implementation. The most popular TO-methods, such as SIMP and BESO, are described, categorized, and compared. In addition, the three following TO-practices are developed with respect to the size of the available design space for optimization: TO with limited design space, TO with maximum possible design space, and combined size/shape/topology optimization. The author states that the designer’s choices (inputs) affect the TO-results and categorizes them into five clusters of parameters: design constraints, supports and connections, loads, geometric restrictions due to manufacturing constraints, and software constraints. The sensitivity of the TO-results to the variations of these parameters is explored.
Furthermore, different multi-objective, multi-level, and multi-scale optimization workflows are used in the pursuit of the lightest design solutions. To identify the software constraints, a literature review is conducted among the most applied TO-software platforms. As a result, an online library of 70 commercial and open source TO software is developed in the form of a table. This table encompasses the name, company, optimization types, and methods that software uses, as well as its available objective functions and constraints in TO. Moreover, relative research works and representative literature for each software are included. Different 3D models are designed, optimized, and used as case studies to support the theory and tie the academic text to real-world applications of TO. Finally, the educational perspective of TO is checked. The author developed an educational framework of a topology optimization-based learning (TOBL) combining the CDIO-approach and TO. The implementation of the developed TOBL-framework in any study program in CAD-engineering can educate modern CAD-designers to conceive, design, implement, and operate optimized products.
The current research work is addressed to practitioners, researchers, teachers, and other engineers looking for new lightweight design concepts. Hence, the aim of this thesis is to provide them, through valuable insights, with a better understanding of TO, as well as to advise them with guidelines and recommendations to avoid common pitfalls
Development of Hybrid Aluminum Carbon Fiber Composite Wheels for a Formula Style Race Car
No full textModern short product life cycles and the necessity to rapidly manufacture components with minimal production needs poses stringent requirements on both time and sophistication of modern product design. Light weight components are more and more in demand, which drives industry to utilize advanced materials like Carbon Fiber Reinforced Polymer (CFRP). The ever-increasing demands of conventional design methods often fail to fulfill the necessary requirements for complex, high performing components. For example, conventional design methods for composites are done manually and often based on experience which is time consuming and can result in non-optimal designs. Using simulation based design (SBD) approaches, this thesis presents FE optimization models that can lead to next generation CFRP designs. A two module wheel is presented consisting of a topology optimized aluminum center for easy machinability and a CFRP rim, for which the layup was optimized in a self-developed evolution based material optimization algorithm. The design considerations and developed algorithm are presented in this thesis as well as the results on optimization with the weight target of 700 g and maximization of the total global stiffness which yielded a deflection of maximum 2.27 mm in cornering at 110 km/h. The optimization was based on two quasi static load scenarios gathered from vehicle dynamic simulations performed by collaborators. The optimized rim shell has an increased specific stiffness of around 45 % and a decreased rotational inertia of nearly 70 %, compared to an aluminum rim shell. The combination of the optimized design and a high quality production resulted in high performing rims that worked well throughout the competitions and during tuning of the car. Mechanical tests on the wheel showed the perfect agreement between the simulation and the experimental results. The largest discrepancy was found in large static loads up to 200 kg, where the discrepancy of the total deformation was 15.6 %.
To the best of the author's knowledge, this work presents the first complete and successful approach towards the parametric material optimization of a CFRP component linked to an entire product development process from the initial design consideration all the way to mechanical testing and application. It is likely helpful to many researchers and developers of next-generation composite designs
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