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Hou, An. "Strength of composite lattice structures." Diss., Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/12475.
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Obiedat, Mohammad. "Incrementally Sorted Lattice Data Structures." Thesis, The George Washington University, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3732474.
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Data structures are vital entities that strongly impact the efficiency of several software applications. Compactness, predictable memory access patterns, and good temporal and spacial locality of the structure's operations are increasingly becoming essential factors in the selection of a data structure for a specific application. In general, the less data we store and move the better for efficiency and power consumption, especially in infrastructure software and applications for hand-held devices like smartphones. In this dissertation, we extensively study a data structure named lattice data structure (LDS) that is as compact and suitable for memory hierarchies as the array, yet with a rich structure that enables devising procedures with better time bounds.
To achieve performance similar to the performance of the optimal O(log(N)) time complexity of the searching operations of other structures, we provide a hybrid searching algorithm that can be implemented by searching the lattice using the basic searching algorithm when the degree of the sortedness of the lattice is less than or equal to 0.9h, and the jump searching algorithm when the degree of the sortedness of the lattice is greater than 0.9h. A sorting procedure that can be used, during the system idle time, to incrementally increase the degree of sortedness of the lattice is given. We also provide randomized and parallel searching algorithms that can be used instead of the usual jump-and-walk searching algorithms.
A lattice can be represented by a one-dimensional array, where each cell is represented by one array element. The worst case time complexity of the basic LDS operations and the average time complexity of some of the order-statistic operations are better than the corresponding time complexities of most of other data structures operations. This makes the LDS a good choice for memory-constrained systems, for systems where power consumption is a critical issue, and for real-time systems. A potential application of the LDS is to use it as an index structure for in-memory databases.
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Kouach, Mona. "Methods for modelling lattice structures." Thesis, KTH, Hållfasthetslära (Avd.), 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-260498.
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The application of lattice structures have become increasingly popular as additive manufacturing (AM) opens up the possibility to manufacture complex configurations. However, modelling such structures can be computationally expensive. The following thesis has been conducted in order for the department of Structural Analysis, at SAAB in Järfälla, to converge with the future use of AM and lattice structures. An approach to model lattice structures using homogenization is presented where three similar methods involving representative volume element (RVE) have been developed and evaluated. The stiffness matrices, of the RVEs, for different sizes of lattice structures, comprising of BCC strut-based units, have been obtained. The stiffness matrices were compared and analysed on a larger solid structure in order to see the deformational predictability of a lattice-based structure of the same size. The results showed that all methods were good approximations with slight differences in terms of boundary conditions (BCs) at the outer edge. The comparative analyses showed that two of the three methods matches the deformational predictability. The BCs in all methods have different influences which makes it pivotal to establish the BCs of the structure before using the approach presented in this thesis.Ökad implementering av gitterstrukturer i komponenter är ett resultat av utvecklingen inom additiv tillverkning. Metoden öppnar upp för tillverkning av komplexa strukturer med färre delmoment. Dock så uppkommer det svårigheter vid simulering av dessa komplexa strukturer då beräkningar snabbt tyngs ner med ökad komplexitet. Följande examensarbete har utförts hos avdelningen Strukturanalys, på SAAB i Järfälla, för att de ska kunna möta upp det framtida behovet av beräkningar på additivt tillverkade gitterstrukturer. I det här arbetet presenteras ett tillvägagångsätt för modellering av gitterstrukturer med hjälp av represantiva volymselement. Styvhetsmatriser har räknats fram, för en vald gitterkonfiguration, som sedan viktats mot tre snarlika representativa volymselement. En jämförelseanalys mellan de olika styvhetsmatriserna har sedan gjorts på en större och solid modell för att se hur väl metoderna förutsett deformationen av en gitterstruktur i samma storlek. Resultaten har visat att samtliga metoder är bra approximationer med tämligen små skillnader från randeffekterna. Vid jämförelseanalysen simulerades gitterstrukturen bäst med två av de tre metoder. En av slutsatserna är att det är viktigt att förstå inverkan av randvillkoren hos gitterstrukturer innan implementering görs med det tillvägagångssätt som presenterats i det här examensarbetet.
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Pugh, David John Rhydwyn. "Topological structures in lattice gauge theory." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.279896.
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Papachristou, Petros G. "Probabilistic relaxation for square lattice structures." Thesis, University of Surrey, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.241395.
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AGUILERA, JEAN RODRIGO FERREIRA. "LIGHT LATTICE STRUCTURES UNDER WIND ACTION." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2007. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=10538@1.
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CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICONo projeto de estruturas treliçadas esbeltas, um ponto relativamente em aberto é quanto à resposta dinâmica dessas torres sob ação do vento. Já foram observadas várias ocorrências de queda de torres por tais efeitos no Brasil e ultimamente, essas incidências vêm aumentando, trazendo diversos transtornos e prejuízos significativos à sociedade e às empresas concessionárias. Nesse contexto, brotam dois aspectos centrais: a modelagem da estrutura e a discretização da ação do vento. A montagem desses dois cenários é feita com base em um modelo numérico, no SAP2000, de uma torre de 73,75 m de altura da linha de transmissão LT - 103, na Amazônia, e uma torre de TV com 192 m de altura, localizada em Brasília-DF, ambas no Brasil. Para avaliação da excitação do vento, é utilizada a norma brasileira NBR 6123. Em estudo preliminar, propõe-se uma forma de representação simplificada das forças do vento sobre a torre, de modo a serem utilizadas resultantes por módulos, convenientemente distribuídas por seus nós principais, visando-se quer a resposta estática, quer a dinâmica. A torre de TV é ensaiada sob a ação de pulsos isolados do vento de projeto e por sucessões diversas desses pulsos com o intuito de simular rajadas de vento. Investiga-se também, para a ação de ventos normalizados extremos, a resposta linear e não-linear P-Delta do sistema. Em consequência, identificam-se pontos de insuficiência estrutural e, para as ações extremas, ensaiam-se recursos mecânicos para controle dos deslocamentos e esforços máximos produzidos pela ação estática e dinâmica do vento.
The dynamic response of slender latticed tower structures under wind excitation is still an open point in the design of such systems. In Brazil, an expressive number of accidents have been registered, in the last few decades, and a large number of material and financial losses have been equally reported, for both people and industrial plant owners. In the structural analysis scenario, two central points dominate the structural engineer concerns: the modeling of the structure and of the wind action. Two tower models are used, a 73,75m high transmission line trussed structure, LT 103, settled in the Brazilian Amazon Basin and a 193m tall trussed TV tower, built in Brasilia-DF. The wind action on the tower members is computed according to the brazilian recommendation, NBR- 6123. In a preliminary study, a simplified procedure is proposed to evaluate the wind forces on the LT-103 tower sections and to conveniently distribute them on the main tower model nodes (joints). The TV Tower is evaluated under the static and dynamic action of the wind forces, either by isolated pulses or by a train of them to model a wind gust. The tower response is computed under a linear and non-linear P-Delta behavior; some faulting spots are identified in the response reports and a combined vibration control solution is proposed incorporating steel tendons conjugated with multiple tuned mass absorbers. A comparison is also made with the NBR 15307 recommendation and a couple of comments are addressed to those who may intend to apply this regulation to investigate the behavior of slender trussed tower structures.
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Leung, Anthony Chi Hin. "Actuation properties of kagome lattice structures." Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613328.
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Hammetter, Christopher Ian. "Designing pyramidal lattice structures for energy absorption." Thesis, University of California, Santa Barbara, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3602080.
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Applications for energy absorption materials range from athletic equipment, to vehicle crumple zones, to blast protection for military vehicles and personnel. Many energy absorption structures employ stochastic foams because of their plateau-like stress-strain response that allows for the absorption of large amounts of energy at relatively low stresses over large compressive strains. Periodic lattice structures, when properly designed, provide the same capabilities as stochastic systems, but with a more tailorable response that provides potential for improved specific strength and energy absorption. The present dissertation provides an in-depth study of the pyramidal lattice: one particular periodic structure that strikes a good compromise between performance and manufacturability. Through finite element and analytical modeling, this study identifies key parameters of the geometry, boundary conditions, and parent material properties that determine the compressive stress-strain response of the structure. In conjunction with experimental investigations, these models are used to understand and determine the potential for improving the response of the as-manufactured polymeric pyramidal lattice structures through additional heat treatment and filling the lattice void-space with stochastic foam. Finally, additional models are developed to understand and predict the structural rate effects that arise from inertial stabilization of strut buckling during dynamic loading. Particular emphasis is given to the effects of yield strain and density of the parent material on failure modes and dynamic response. In addition to providing a strong basis for the design of pyramidal lattice materials, this work provides useful insight into the design of energy absorption materials in general.
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Novak, Jurica. "Simulated mesoscopic structures in a ferroelastic lattice." Thesis, University of Cambridge, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.621535.
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Craig, Adam Patrick. "Novel structures for lattice-mismatched infrared photodetectors." Thesis, Lancaster University, 2016. http://eprints.lancs.ac.uk/82854/.
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Using the interfacial misfit (IMF) array growth mode, GaSb p-i-n diodes were grown on Si and GaAs lattice-mismatched substrates by molecular beam epitaxy (MBE) under optimised growth conditions. For the sample grown on Si, an AlSb nucleation layer was used to reduce the occurrence of twinning defects. In addition to the samples grown on mismatched substrates, an equivalent structure was further grown on a native GaSb substrate, for comparison. X-ray diffraction (XRD) was used to demonstrate that the layers were fully relaxed, and transmission electron microscopy (TEM) imaging showed arrays of 90° misfit dislocations with measured periodicities in agreement with atomistic modelling. However, after processing, device dark current densities of 0.9 Acm^-2 and 0.18 Acm^-2 were recorded for the sample grown on Si and the sample grown on GaAs, respectively, at -1.0 V and 300 K. These were compared to the sample grown on native GaSb, which had a dark current density of 0.01 Acm^-2 under the same conditions. Furthermore, TEM analysis revealed relatively high threading dislocation densities (TDDs) of ~10^8 cm^-2. It was proposed that not all the interfacial strain could be accommodated by the IMF arrays, since the array periods (9:8 for AlSb/Si and 13:14 for GaSb/GaAs) were not in exact agreement with ratio of the lattice constants (of AlSb to Si and GaSb to GaAs), i.e. a population of 60° misfit dislocations was still formed. It was therefore decided to investigate the use of nBn detector structures as lattice mismatched photodetectors. Using a design based on an InAsSb bulk-material absorber, a comparison was again drawn between two samples, one grown on mismatched GaAs and a second grown on native GaSb. This time, device dark current densities were found to be relatively similar when comparing the two samples (1.6×10^-5 Acm^-2 vs 3×10^-6 Acm^-2 at 200 K). D^* performance figures were also found to be within one order of magnitude (1.5×10^10 cmHz^1/2 W^-1 vs 9.8×10^10 cmHz^1/2 W^-1 at 200 K). Furthermore, diffusion limited performance was exhibited at all temperatures tested, so that the effects of Shockley Read Hall (SRH) generation were established to be absent (or at least much less significant). It was also found that absorption layer doping of around ~4×10^17 cm^-3 was necessary to ensure diffusion limited performance for the sample grown on GaAs and that, with this modification, diffusion limited performance was achieved even for a sample with a highly lattice-mismatched absorption layer (with higher Sb content and longer cut-off wavelength).While nBn detector structures offer very low dark currents, it will sometimes be necessary to have a detector which is sensitive to very weak signals. In telecoms applications, avalanche photodiode (APD) structures are often used as receivers for long-haul fibre optic systems. However, relatively few avalanche photodiode designs exist for wavelengths beyond 1.55 μm. Two novel separate-absorption-and-multiplication (SAM) APD structures were therefore demonstrated based on the IMF growth mode. In particular, by transitioning the lattice from 5.65 Å to 6.09 Å, it was possible to combine GaSb absorption layers with GaAs and (for improved noise performance) Al0.8Ga0.2As multiplication layers. Multiplication profiles were established using capacitance voltage modelling (together with ionisation coefficients from the literature) and excess noise measurements were then carried out. Through the presence of 1.55 μm photocurrent, it was confirmed that absorption took place in the GaSb regions, with transport to the p-n junction (in the multiplication region) taking place by diffusion. Through measurements showing 0.2
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Melpal, Gopalakrishna Ranjan. "Conformal Lattice Structures in Additive Manufacturing (AM)." University of Cincinnati / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1535382325233769.
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Tornatore, Dario. "Damping Capability of Lattice Structures: a Numerical Study." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2019.
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Lattices are recognized as ultra-lightweight materials with high specific stiffness and high specific strength. The applications of this architectural material range from the aerospace and automotive industry up to the biomedical one.
In the literature, most of the studies address the mechanical responses of lattice structures under static, dynamic (impact) and fatigue loading conditions while only few works deal with the damping capabilities of such structures.
This study focuses on the damping capability of classical lattice configurations whose architecture is made of struts (i.e. CC, CBCC, ACC, Octet, Rhombic dodecahedron). The influence of three aspects has been investigated: the geometrical parameters defining the structure of the lattice cell, the introduction of a compressive pre-stress field within the cell and the plastic constitutive behaviour of the material used for the struts. A broad sensitivity campaign has been performed in order to evaluate the amount of dissipated energy for the different cell architectures according to the variation of the strut diameter and to the presence of local instabilities (post-buckling behaviour of the struts).
The numerical results highlight how the damping capability of the considered cell, for the same loading condition, is strongly related to the topology of the cell and to its relative density. By smartly tailoring these parameters, the damping capability without pre-stress can be increased up to 23% and, if the pre-stress is introduced within the cell, the damping effects can be ulteriorly intensified .
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Haq, Sirajul. "Mathematical models of defects in discrete lattice structures." Thesis, University of Liverpool, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.408552.
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Paupitz, Goncalves Paulo José. "Dynamic analysis and active control of lattice structures." Thesis, University of Southampton, 2007. https://eprints.soton.ac.uk/69737/.
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This thesis presents an investigation of the factors controlling the performance of two forms of active vibration control applied to lattice structures, such as those used for space applications. The structure considered is based on a lattice structure assembled by NASA in 1984. It consists of a satellite boom with 93 aluminium members connected rigidly through 33 spherical joints. The structure has two distinct forms of motion which are categorized in terms of short and long wavelength modes. The short wavelength modes occurs when the length of the individual members is a multiple of half wavelength of bending waves. The second category, named long wavelength modes occur when the length of the whole structure is a multiple of half wavelength of waves propagating by longitudinal motion in the structure. Simple expressions are derived to identify the factors that control the frequency bands where short and long wavelength modes occur. It is possible to alter the dynamic behaviour of the system by changing some of the factors in these expressions and thus study the active and passive control of vibration in a variety of such structures. The two strategies of active control considered in the thesis are feedforward control and integral force feedback control. Feedforward control usually requires deterministic forms of disturbance sources while feedback control can be applied to random disturbances. It has been found that short wavelength modes can reduce the performance in the feedback control strategy, while the results of feedforward control are not affected so much. To support this analysis, the energy dissipation and power flow mechanisms in the structure are studied. The results in this thesis are based on numerical simulations and experimental tests which have been used to validate the mathematical model of the structure.
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Shalchy, Faezeh, Enrique Cuan-Urquizo, Kevin Jose, Neil Ferguson, Claus Ibsen, and Atul Bhaskar. "Mechanics and manufacture of lattice structures & materials." Thesis, Київський національний університет технологій та дизайну, 2021. https://er.knutd.edu.ua/handle/123456789/19222.
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Folinus, Charlotte Méry. "Stiffness prediction methods for additively manufactured lattice structures." Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/127918.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, May, 2020Cataloged from the official PDF of thesis.
Includes bibliographical references (pages 87-91).
Since the initial 300 pair release of the Futurecraft 4D in April 2017, adidas has scaled its 4D program to mass produce additively manufactured shoe midsoles. The 4D midsoles are constructed from lattice structures, and if there is variation in the manufacturing process, the structure's material and/or geometric properties may be altered. This means midsoles may have the same geometry but different material properties and thus different stiffnesses, and they may also have the same material properties but different overall stiffness due to geometric changes. The current quality control test is slow, expensive, and does not scale well. This thesis explores two potential techniques: using ultrasonic waves to determine the lattices' acoustic properties, and weighing them to determine their mass. Pulse-echo testing data for n = 8 samples shows a statistically significant (p = 0.0398 < 0.05) increase in response time due to sample stiffness. Stiffness scaled linearly with lattice mass for both physical and simulated lattices, and mass predicted lattice stiffness with a minimum accuracy of 90% across a range of simulated manufacturing conditions. An analytical framework parameterized around a bivariate normal distribution can determine accuracy of new test methods or from additional mass-stiffness data. Lastly, cost minimization is presented for a hybrid test protocol which combines mass testing with secondary testing for rejected samples. At specification limits of ±1[sigma], the hybrid test achieves 99% accuracy at 69.8% of the cost for the current test. Increasing the specification limit to ±2[sigma] reduces cost further, achieving 99% accuracy at 16.4% of the current cost.
by Charlotte Méry Folinus.
S.B.
S.B. Massachusetts Institute of Technology, Department of Mechanical Engineering
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Wilmoth, Nathan G. "Determining the Mechanical Properties of Lattice Block Structures." Cleveland State University / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=csu1366275566.
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Bailey, S. J. "Cathodoluminescence of quantum well structures." Thesis, University of Bristol, 1987. http://hdl.handle.net/1983/b02b03a2-c0d8-401d-a94e-8bd26b52b953.
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Ayers, James T. "Hydrodynamic Drag and Flow Visualization of IsoTruss Lattice Structures." Diss., CLICK HERE for online access, 2005. http://contentdm.lib.byu.edu/ETD/image/etd782.pdf.
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Ramsden, M. "Dynamic effects in the progressive failure of lattice structures." Thesis, University of Reading, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.376665.
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Brooks, Wesley Keith. "The creation of lattice structures using selective laser melting." Thesis, University of Liverpool, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.569197.
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This research focuses on reducing the limitations imposed on the repeating topologies in lattice structures that restrict what can be created using the RealiZer Selective Laser Melting (SLM) machine. The creation of regular, randomly perturbed, polar mapped, and random metallic lattice structures using SLM apparatus is reported and discussed in this thesis. It was observed that a new technique was required to generate the slice data files used to control the SLM equipment in order to create structures that measured significantly more than 10 cells in each axis. The research details the motivations behind the development of the computational methods utilised to develop lattice parts and how the iterations of these methods enabled different areas of research to progress. The limits of the angles from the horizontal that elements could be built are reviewed and scanning techniques are developed that create elements below these values. In order to create horizontal links significant proportions of the machine control software were replaced with software developed during the course of the research. This is discussed at length along with how the limitations on the number of processing parameters available could be removed and how pauses which let sections of the melt on horizontal links freeze before processing the next section could be used. It is suggested that systems or experimental set ups are developed that allow greater control over the duration of these pauses. This would enable further research into the processing of horizontal links, developing them to the point where they are mechanically consistent and comparable to other links in the structures
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Goel, Archak. "Design of Functionally Graded BCC Type Lattice Structures Using B-spline Surfaces for Additive Manufacturing." University of Cincinnati / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1552398559313737.
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Ashok, Ramya. "A database system to store and retrieve concept lattice structures." Cincinnati, Ohio : University of Cincinnati, 2005. http://www.ohiolink.edu/etd/view.cgi?acc%5Fnum=ucin1130552767.
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Masoumi, Khalil Abad Ehsan. "Fatigue design of lattice materials: application to stent-like structures." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=114478.
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A lattice material is a cellular structure with a periodic arrangement of cells in either two or three dimensions. Lattice materials are attractive candidates for potential use in a broad range of applications, including battery electrodes, vibration insulators, ultra lightweight sandwich panels, and biomedical implants. This thesis focuses on the design of planar lattices for micro-architectured materials and medical devices. Strength of a lattice material degrades under cyclic loading conditions. In this thesis a computational method based on finite element analysis (FEM) is proposed to analyze and design lattice materials and structures for fatigue failure. A comparison with available experimental data contributes to the validity of the method. The effect of the unit cell's architecture on the fatigue resistance of lattice materials is investigated by considering square and hexagonal shapes of unit cells. A shape optimization methodology based on removing the stress concentration caused by the presence of geometrical discontinuities at the inner boundaries of the lattice cell walls is proposed to improve the fatigue resistance of planar lattice materials. The shape optimization method adapted for the fatigue design of a lattice is applied to design intravascular self-expandable characterized by a periodic arrangement of cells, against fatigue failure. In particular, the aim is to improve the fatigue resistance of Nitinol stent grafts with closed-cell, and to design a stent-like device functioning as a protection for an endovascular oxygenator. A parametric study was carried out to assess the effect of different geometrical parameters on the fatigue resistance and radial stiffness of the generated Nitinol stent lattices. Novel stent-like concepts are proposed to protect and guide the state-of-the art intravenous oxygenator developed by ALung Technologies Inc. (Pittsburgh, PA) in partnership with the University of Pittsburgh. The validity of the proposed concepts in protecting the oxygenator was tested in vitro. The structural behavior of the proposed conceptual designs was studied by using FEM, and the level of blood damage caused by catheter rotation is investigated through CFD analysis. Preliminary numerical and experimental observations suggest that the proposed design can put the oxygenator one step closer to the market.Un matériau en treillis est une structure cellulaire avec une disposition périodique de cellules en deux ou en trois dimensions. Ces structures sont utilisées dans plusieurs applications, y compris les électrodes de la batterie, isolateurs de vibration, panneaux ultra légers en sandwich et implants biomédicaux. Cette thèse met l'accent sur la conception de réseaux plans pour des matériaux ayant une microarchitecture et pour les dispositifs médicaux. Dans plusieurs applications, la résistance d'un matériau en treillis se dégrade dans les conditions de chargement cycliques. Dans cette thèse une méthode numérique basée sur la mécanique de calcul est proposé afin d'analyser et de concevoir des matériaux et des structures en treillis pour prévenir toute rupture causée par fatigue. Une comparaison avec des données expérimentales contribue à la validité de la méthode. L'effet de l'architecture d'une cellule de cette unité sur la tenue en fatigue des matériaux en treillis est étudiée en tenant compte des formes carrées et hexagonales de cellules unitaires. En outre, une méthodologie d'optimisation de forme fondé sur l'élimination de la concentration du stress causé par la présence de discontinuités géométriques aux frontières intérieures des parois cellulaires en treillis est proposé pour améliorer la résistance à la fatigue des matériaux en treillis planaires. Plusieurs topologies de cellules augmentant la résistance à la fatigue sont proposées pour l'amélioration des matériaux et des structures caractérisées par un arrangement périodique de cellules. Cette méthode d'optimisation de forme adaptée pour la conception de fatigue d'un réseau de cellule est appliquée à la conception intravasculaire d'endoprothèses auto-expansibles et aussi à la conception d'un dispositif fonctionnant comme stent offrant une protection pour un oxygénateur endovasculaire.Une géométrie de la cellule avec une meilleure résistance à la fatigue est proposée pour un réseau planaire pour stent. Une étude paramétrique a été réalisée pour évaluer l'effet des différents paramètres géométriques sur la résistance à la fatigue et la raideur radiale des réseaux générés de stent. Plusieurs concepts nouveau empruntent du stent sont proposées pour protéger et guider un oxygénateur intraveineux mis au point par Technologies Inc. Alung (Pittsburgh, PA), en partenariat avec l'Université de Pittsburgh. La validité des concepts proposés assurant une protection de l'oxygénateur a été testée in vitro. Le comportement de la structure des conceptions proposées conceptuels a été étudié en utilisant la méthode des éléments finis tandis que et le niveau de dommages de sang causé par la rotation du cathéter a était évaluer à travers une modélisation numérique et dynamique des fluides. Les observations numériques et expérimentales suggèrent que la conception proposée mettrait l'oxygénateur un pas de plus vers le marché.
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Cochran, Christopher S. "Even-number spin correlations on two-dimensional Ising lattice structures." Virtual Press, 2002. http://liblink.bsu.edu/uhtbin/catkey/1237760.
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Many physical systems can be represented by a regular arrangement of molecules in a lattice structure. Knowing how neighboring molecules in the lattice interact with one another can give great insight into a material's macroscopic behavior. A very popular and effective means of investigating these microscopic interactions is the Ising Model. This model, suggested first by Wilhelm Lenz in 1920 and later expanded by Ernst Ising in 1925, is based on the assumptions that each molecule in a lattice structure can be represented by its spin value (+l or -1) and that only nearest neighbors contribute to the total interaction energy. The Ising Model, which was initially used in the study of ferromagnetic systems, can now be used to study a variety of physical systems. Some of these include antiferromagnetic crystals, binary alloys, DNA, and lattice gasses.Department of Physics and Astronomy
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Adam, James G. "Designing emergence : automatic extraction of stigmergic algorithms from lattice structures." Thesis, University of Essex, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.428979.
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Liu, Biheng. "Reduced order models for the analysis of offshore lattice structures." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2022.
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The lattice is a common frame applied in offshore structures benefiting from their economic construction and easily accessible, for example, the jacket platform of the offshore wind turbine. In order to accelerate the design process of a model, this thesis proposed a method to reduce the order of the offshore lattice structure models. The full order model is constructed based on the FEM. Applying the MDOF mass discrete concept lumps the structure mass on each story center. Meanwhile, apply the DSM to determine the displacement and the forces implied on the mass center for accomplishing the unitary displacement of each DOF of the mass center node. Finally, a reduced order model for the lattice structure will be obtained. That model composed of the stiffness matrix of the mass center node has the same fundamental flexural frequency as the original model. The model can be expressed by symbolic relationship as well as possible to package as a function for further quotation. All computation processes of this thesis were executed on the Python platform.
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Chen, Li. "A quasicontinuum approach towards mechanical simulations of periodic lattice structures." Doctoral thesis, Universite Libre de Bruxelles, 2020. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/314314.
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Thanks to the advancement of additive manufacturing, periodic metallic lattice structures are gaining more and more attention. A major attraction of them is that their design can be tailored to specific applications by changing the basic repetitive pattern of the lattice, called the unit cell. This may involve the selection of optimal strut diameters and orientations, as well as the connectivity and strut lengths. Numerical simulation plays a vital role in understanding the mechanical behavior of metallic lattices and it enables the optimization of design parameters. However, conventional numerical modeling strategies in which each strut is represented by one or more beam finite elements yield prohibitively time consuming simulations for metallic lattices in engineering scale applications. The reasons are that millions of struts are involved, as well as that geometrical and material nonlinearities at the strut level need to be incorporated. The aim of this thesis is the development of multiscale quasicontinuum (QC) frameworks to substantially reduce the simulation time of nonlinear mechanical models of metallic lattices. For this purpose, this thesis generalizes the QC method by a multi-field interpolation enabling amongst others the representation of varying diameters in the struts’ axial directions (as a consequence of the manufacturing process). The efficiency is further increased by a new adaptive scheme that automatically adjusts the model reduction whilst controlling the (elastic or elastoplastic) model’s accuracy. The capabilities of the proposed methodology are demonstrated using numerical examples, such as indentation tests and scratch tests, in which the lattice is modeled using geometrically nonlinear elastic and elastoplastic beam finite elements. They show that the multiscale framework combines a high accuracy with substantial model reduction that are out of reach of direct numerical simulations.Doctorat en Sciences de l'ingénieur et technologie
info:eu-repo/semantics/nonPublished
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Kuper, Lindsey. "Lattice-based data structures for deterministic parallel and distributed programming." Thesis, Indiana University, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3726443.
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Deterministic-by-construction parallel programming models guarantee that programs have the same observable behavior on every run, promising freedom from bugs caused by schedule nondeterminism. To make that guarantee, though, they must sharply restrict sharing of state between parallel tasks, usually either by disallowing sharing entirely or by restricting it to one type of data structure, such as single-assignment locations.
I show that lattice-based data structures, or LVars, are the foundation for a guaranteed-deterministic parallel programming model that allows a more general form of sharing. LVars allow multiple assignments that are inflationary with respect to a given lattice. They ensure determinism by allowing only inflationary writes and "threshold" reads that block until a lower bound is reached. After presenting the basic LVars model, I extend it to support event handlers, which enable an event-driven programming style, and non-blocking "freezing" reads, resulting in a quasi-deterministic model in which programs behave deterministically modulo exceptions.
I demonstrate the viability of the LVars model with LVish, a Haskell library that provides a collection of lattice-based data structures, a work-stealing scheduler, and a monad in which LVar computations run. LVish leverages Haskell's type system to index such computations with effect levels to ensure that only certain LVar effects can occur, hence statically enforcing determinism or quasi-determinism. I present two case studies of parallelizing existing programs using LVish: a k-CFA control flow analysis, and a bioinformatics application for comparing phylogenetic trees.
Finally, I show how LVar-style threshold reads apply to the setting of convergent replicated data types (CvRDTs), which specify the behavior of eventually consistent replicated objects in a distributed system. I extend the CvRDT model to support deterministic, strongly consistent threshold queries. The technique generalizes to any lattice, and hence any CvRDT, and allows deterministic observations to be made of replicated objects before the replicas' states converge.
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Della, Corte Alessandro. "Lattice structures with pivoted beams : Homogenization and nonlinear elasticity results." Thesis, Toulon, 2017. http://www.theses.fr/2017TOUL0019/document.
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Cette thèse est consacrée à la modélisation des structures fibreuses avec des milieuxcontinus généralisés. Dans l’Introduction, l'état de l'art concernant les milieuxcontinus généralisée et applications aux structures fibreuses sont décrits et lesproblèmes ouverts pertinents sont mis en évidence. Dans le Chapitre 1 et 2, uneprocédure d'homogénéisation rigoureuse basée sur des arguments de Gammaconvergenceest appliquée à une structure en treillis et à un model de poutrediscrétisé. Dans le Chapitre 3, un traitement variationnel est utilisé pour formuler unapproche favorable du point de vue numérique. Dans le Chapitre 4 sont discutées lesrésultats expérimentaux concernant le comportement de la structure dans différentstypes de déformation. Cela à motivé les études effectuées dans le Chapitre 5, ou lesMéthodes directes de calcul des variations sont appliquées à poutres d’Euler engrandes déformationsThis thesis focuses on the mathematical modeling of fibrous structures having somepeculiar properties (high strength-to-weight ratio and very good toughness infracture), whose mechanical behavior escapes from standard Cauchy elasticity. Inparticular, it addresses cases in which the presence of a microstructure, consisting ofregularly spaced pivoted beams, entails effects that are well described by generalizedcontinuum models, i.e. models in which the deformation energy density depends notonly on the gradient of the placement but also on the second (and possibly higher)gradients of it. In the Introduction, the state of the art concerning generalizedcontinua and their applications for the description of fibrous structures is describedand some relevant open problems are highlighted. In Chapter 1 and 2 a rigoroushomogenization procedure based on Gamma-convergence arguments is performedfor a lattice (truss-like) structure and for a discrete 1D system (Hencky-type beammodel). In Chapter 3, a variational treatment is employed to formulate acomputationally convenient approach. In Chapter 4 some experimental resultsconcerning the behavior of the structure in various kinds of deformation arediscussed. This motivated the investigation performed in Chapter 5, in which DirectMethods of Calculus of Variations are applied to Euler beams in large deformationsunder distributed load
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Della, Corte Alessandro. "Lattice structures with pivoted beams : Homogenization and nonlinear elasticity results." Electronic Thesis or Diss., Toulon, 2017. http://www.theses.fr/2017TOUL0019.
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Cette thèse est consacrée à la modélisation des structures fibreuses avec des milieuxcontinus généralisés. Dans l’Introduction, l'état de l'art concernant les milieuxcontinus généralisée et applications aux structures fibreuses sont décrits et lesproblèmes ouverts pertinents sont mis en évidence. Dans le Chapitre 1 et 2, uneprocédure d'homogénéisation rigoureuse basée sur des arguments de Gammaconvergenceest appliquée à une structure en treillis et à un model de poutrediscrétisé. Dans le Chapitre 3, un traitement variationnel est utilisé pour formuler unapproche favorable du point de vue numérique. Dans le Chapitre 4 sont discutées lesrésultats expérimentaux concernant le comportement de la structure dans différentstypes de déformation. Cela à motivé les études effectuées dans le Chapitre 5, ou lesMéthodes directes de calcul des variations sont appliquées à poutres d’Euler engrandes déformationsThis thesis focuses on the mathematical modeling of fibrous structures having somepeculiar properties (high strength-to-weight ratio and very good toughness infracture), whose mechanical behavior escapes from standard Cauchy elasticity. Inparticular, it addresses cases in which the presence of a microstructure, consisting ofregularly spaced pivoted beams, entails effects that are well described by generalizedcontinuum models, i.e. models in which the deformation energy density depends notonly on the gradient of the placement but also on the second (and possibly higher)gradients of it. In the Introduction, the state of the art concerning generalizedcontinua and their applications for the description of fibrous structures is describedand some relevant open problems are highlighted. In Chapter 1 and 2 a rigoroushomogenization procedure based on Gamma-convergence arguments is performedfor a lattice (truss-like) structure and for a discrete 1D system (Hencky-type beammodel). In Chapter 3, a variational treatment is employed to formulate acomputationally convenient approach. In Chapter 4 some experimental resultsconcerning the behavior of the structure in various kinds of deformation arediscussed. This motivated the investigation performed in Chapter 5, in which DirectMethods of Calculus of Variations are applied to Euler beams in large deformationsunder distributed load
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Vannutelli, Rafaela S. "Mechanical Behavior of 3D Printed Lattice-Structured Materials." Youngstown State University / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=ysu1516313745997898.
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Farley, Thomas William Dashwood. "Neutron scattering studies of antifluorite compounds at high temperature." Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.257738.
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Sadeghi, Arjang. "Equivalent earthquake loads for some families of barrel vaults." Thesis, University of Surrey, 2002. http://epubs.surrey.ac.uk/1009/.
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Flores, Paul. "CATEGORICAL PROPERTIES OF LATTICE-VALUED CONVERGENCE SPACES." Doctoral diss., University of Central Florida, 2007. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2686.
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This work can be roughly divided into two parts. Initially, it may be considered a continuation of the very interesting research on the topic of Lattice-Valued Convergence Spaces given by Jäger [2001, 2005]. The alternate axioms presented here seem to lead to theorems having proofs more closely related to standard arguments used in Convergence Space theory when the Lattice is L=.Various Subcategories are investigated. One such subconstruct is shown to be isomorphic to the category of Lattice Valued Fuzzy Convergence Spaces defined and studied by Jäger . Our principal category is shown to be a topological universe and contains a subconstruct isomorphic to the category of probabilistic convergence spaces discussed in Kent and Richardson when L=. Fundamental work in lattice-valued convergence from the more general perspective of monads can be found in Gähler . Secondly, diagonal axioms are defined in the category whose objects consist of all the lattice valued convergence spaces. When the latter lattice is linearly ordered, a diagonal condition is given which characterizes those objects in the category that are determined by probabilistic convergence spaces which are topological. Certain background information regarding filters, convergence spaces, and diagonal axioms with its dual are given in Chapter 1. Chapter 2 describes Probabilistic Convergence and associated Diagonal axioms. Chapter 3 defines Jäger convergence and proves that Jäger's construct is isomorphic to a bireflective subconstruct of SL-CS. Furthermore, connections between the diagonal axioms discussed and those given by Gähler are explored. In Chapter 4, further categorical properties of SL-CS are discussed and in particular, it is shown that SL-CS is topological, cartesian closed, and extensional. Chapter 5 explores connections between diagonal axioms for objects in the sub construct δ(PCS) and SL-CS. Finally, recommendations for further research are provided.Ph.D.
Department of Mathematics
Sciences
Mathematics PhD
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Graf, Gregory C. "Development of specialized base primitives for meso-scale conforming truss structures." Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/28238.
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O'Connor, Joseph. "Fluid-structure interactions of wall-mounted flexible slender structures." Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/fluidstructure-interactions-of-wallmounted-flexible-slender-structures(1dab2986-b78f-4ff9-9b2e-5d2181cfa009).html.
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The fluid-structure interactions of wall-mounted slender structures, such as cilia, filaments, flaps, and flags, play an important role in a broad range of physical processes: from the coherent waving motion of vegetation, to the passive flow control capability of hair-like surface coatings. While these systems are ubiquitous, their coupled nonlinear response exhibits a wide variety of behaviours that is yet to be fully understood, especially when multiple structures are considered. The purpose of this work is to investigate, via numerical simulation, the fluid-structure interactions of arrays of slender structures over a range of input conditions. A direct modelling approach, whereby the individual structures and their dynamics are fully resolved, is realised via a lattice Boltzmann-immersed boundary model, which is coupled to two different structural solvers: an Euler-Bernoulli beam model, and a finite element model. Results are presented for three selected test cases - which build in scale from a single flap in a periodic array, to a small finite array of flaps, and finally to a large finite array - and the key behaviour modes are characterised and quantified. Results show a broad range of behaviours, which depend on the flow conditions and structural properties. In particular, the emergence of coherent waving motions are shown to be closely related to the natural frequency of the array. Furthermore, this behaviour is associated with a lock-in between the natural frequency of the array and the predicted frequency of the fluid instabilities. The original contributions of this work are: the development and application of a numerical tool for direct modelling of large arrays of slender structures; the characterisation of the behaviour of slender structures over a range of input conditions; and the exposition of key behaviour modes of slender structures and their relation to input conditions.
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Alzahrani, Mahmoud Ali. "Design of truss-like cellular structures using density information from topology optimization." Thesis, Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52275.
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The advances in additive manufacturing removed most of the limitations that were once stopping designers when it comes to the manufacturability of the design. It allowed designers to produce parts with high geometric complexity such as cellular structures. These structures are known for their high strength relative to their low mass, good energy absorption, and high thermal and acoustic insulation compared to their relative solid counter-parts. Lattice structures, a type of cellular structures, have received considerable attention due to their properties when producing light-weight with high strength parts. The design of these structures can pose a challenge to designers due to the sheer number of variables that are present. Traditional optimization approaches become an infeasible approach for designing them, which motivated researchers to search for other alternative approaches.
In this research, a new method is proposed by utilizing the material density information obtained from the topology optimization of continuum structures. The efficacy of the developed method will be compared to existing methods, such as the Size Matching and Scaling (SMS) method that combines solid-body analysis and a predefined unit-cell library. The proposed method shows good potential in structures that are subjected to multiple loading conditions compared to SMS, which would be advantageous in creating reliable structures. In order to demonstrate the applicability of the proposed method to practical engineering applications, the design problem of a commercial elevator sling will be considered.
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St-Pierre, Luc. "The quasi-static and dynamic responses of metallic sandwich structures." Thesis, University of Cambridge, 2012. https://www.repository.cam.ac.uk/handle/1810/243443.
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Lattice materials are used as the core of sandwich panels to construct light and strong structures. This thesis focuses on metallic sandwich structures and has two main objectives: (i) explore how a surface treatment can improve the strength of a lattice material and (ii) investigate the collapse response of two competing prismatic sandwich cores employed in ship hulls. First, the finite element method is used to examine the effect of carburisation and strain hardening upon the compressive response of a pyramidal lattice made from hollow tubes or solid struts. The carburisation surface treatment increases the yield strength of the material, but its effects on pyramidal lattices are not known. Here, it is demonstrated that carburisation increases the plastic buckling strength of the lattice and reduces the slenderness ratio at which the transition from plastic to elastic buckling occurs. The predictions also showed that strain hardening increases the compressive strength of stocky lattices with a slenderness ratio inferior to ten, but without affecting the collapse mode of the lattice. Second, the quasi-static three-point bending responses of simply supported and clamped sandwich beams with a corrugated core or a Y-frame core are compared via experiments and finite element simulations. The role of the face-sheets is assessed by considering beams with (i) front-and-back faces present and (ii) front face present, but back face absent. These two beam designs are used to represent single hull and double hull ship structures, and they are compared on an equal mass basis by doubling the thickness of the front face when the back face is absent. Beams with a corrugated core are found to be slightly stronger than those with a Y-frame core, and two collapse mechanisms are identified depending upon beam span. Short beams collapse by indentation and for this collapse mechanism, beams without a back face outperform those with front-and back faces present. In contrast, longbeams fail by Brazier plastic buckling and for this collapse mechanism, the presence of a back face strengthens the beam. Third, drop weight tests with an impact velocity of 5 m/s are performed on simply supported and clamped sandwich beams with a corrugated core or a Y-frame core. These tests are conducted to mimic the response of a sandwich hull in a ship collision. The responses measured at 5 m/s are found to be slightly stronger than those measured quasi-statically. The measurements are in reasonable agreement with finite element predictions. In addition, the finite element method is used to investigate whether the collapse mechanism at 5 m/s is different from the one obtained quasi-statically. The predictions indicate that sandwich beams that collapse quasi-statically by indentation also fail by indentation at 5 m/s. In contrast, the simulations for beams that fail quasi-statically by Brazier plastic buckling show that they collapse by indentation at 5 m/s. Finally, the dynamic indentation response of sandwich panels with a corrugated core or a Y-frame core is simulated using the finite element method. The panels are indented at a constant velocity ranging from quasi-static loading to 100 m/s, and two indenters are considered: a flat-bottomed indenter and a cylindrical roller. For indentation velocities representative of a ship collision, i.e. below 10 m/s, the predictions indicate that the force applied to the front face of the panel is approximately equal to the force transmitted to the back face. Even at such low indentation velocities, inertia stabilisation effects increase the dynamic initial peak load above its quasi-static value. This strengthening effect is more important for the corrugated core than for the Y-frame core. For velocities greater than 10 m/s, the force applied to the front face exceeds the force transmitted to the back face due to wave propagation effects. The results are also found to be very sensitive to the size of the flat-bottomed indenter; increasing its width enhances both inertia stabilisation and wave propagation effects. In contrast, increasing the roller diameter has a smaller effect on the dynamic indentation response. Lastly, it is demonstrated that material strain-rate sensitivity has a small effect on the dynamic indentation response of both corrugated and Y-frame sandwich panels.
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Stragiotti, Enrico. "Conception et optimisation de structures lattice modulaires pour des applications aérospatiales." Electronic Thesis or Diss., Toulouse, ISAE, 2024. http://www.theses.fr/2024ESAE0017.
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Dans l'industrie aérospatiale, il existe une demande continue pour des aérostructures plus légères, motivée par la nécessité d'améliorer l'efficacité énergétique et les performances globales de l'avion. Par conséquent, le secteur aérospatial connaît actuellement deux changements majeurs : l'adoption d'avions à hydrogène et électriques, visant à développer des technologies aéronautiques plus propres et durables. Ces évolutions offrent des opportunités pour explorer des concepts innovants tels que l'aile volante ou les ailes haubanées transsoniques, s'éloignant de la configuration traditionnelle tube-et-aile. Une approche prometteuse pour répondre à ces demandes est l'utilisation de structures modulaires en treillis, reconnues pour leurs propriétés ultralégères et leur modularité. La conception modulaire offre divers avantages, notamment l'assemblage de grandes structures à partir de modules répétitifs plus petits, faciles à fabriquer, la réparabilité sur le terrain et l'assemblage rapide pour des structures temporaires.L'objectif de cette thèse est de développer une méthodologie de conception et d'optimisation pour des aérostructures ultralégères et modulaires. Initialement, nous avons réalisé une revue de la littérature existante pour identifier la base algorithmique la plus adaptée à l'optimisation de structures monolithiques (non modulaires). Après une comparaison approfondie, nous avons sélectionné l'approche nommé Truss Topology Optimization (TTO), qui utilise des barres comme élément de discrétisation de la structure. Cependant, la formulation classique du TTO présente des limitations, telles que l'incapacité à traiter les contraintes de flambage, à prendre en compte plusieurs cas de charge, à limiter l'élancement minimum et à assurer la compatibilité mécanique. Pour surmonter ces défis, nous avons formulé une approche globale et développé un algorithme innovant d'optimisation en deux étapes. Cela implique d'utiliser un problème simplifié pour générer une solution initiale, qui sert de point de départ pour l'optimisation à l'aide d'une formulation complète.La deuxième partie de la thèse se concentre sur l'adaptation de la formulation monolithique proposée pour modéliser des structures modulaires. Initialement, nous nous concentrons sur l'optimisation de la topologie d'une structure entièrement modulaire, où un seul module est répété tout au long du design. Nous examinons comment les hyperparamètres, tels que le nombre de sous-domaines et la complexité des modules, affectent les performances mécaniques de la structure. Ensuite, nous explorons un scénario plus complexe en optimisant plusieurs topologies de modules et leur disposition dans la structure. Cela est réalisé grâce à une nouvelle stratégie de résolution basée sur une approche Discrete Material Optimization (DMO), utilisant un optimiseur à descente de gradient.En abordant les défis de la conception légère et de la modularité dans les aérostructures, cette recherche vise à contribuer à l'évolution continue des technologies aérospatiales et à améliorer l'efficacité et les performances des futurs avionsIn the aerospace industry, there is a continuous demand for lighter aerostructures driven by the need to improve fuel efficiency and overall performance. Consequently, the aerospace sector is undergoing two significant shifts: the adoption of hydrogen-powered and electric planes, aimed at developing cleaner and more sustainable aviation technologies. These changes present opportunities to explore innovative concepts such as the flying wing or transonic dry truss-braced wings, deviating from the traditional tube-and-wing configuration. One promising approach to meet these demands is the utilization of modular lattice structures, known for their ultralight properties and modularity. Modular designs offer various advantages, including the assembly of large structures from smaller, easily manufacturable repeating modules, on-field repairability, and rapid assembly for temporary structures.The objective of this thesis is to develop a design and optimization methodology for ultralight and modular aerostructures. Initially, we conducted a review of existing literature to identify the most suitable algorithm basis for optimizing monolithic (non-modular) structures. After a comprehensive comparison, we selected the Truss Topology Optimization (TTO) approach, which utilizes bars as the discretizing element of the structure. However, the classic TTO formulation has limitations, such as the inability to address buckling constraints, consider multiple load cases, limit the minimum slenderness, and ensure mechanical compatibility. To overcome these challenges, we formulated a comprehensive approach and developed an innovative two-step optimization algorithm. This involves using a relaxed problem to generate an initial solution, which serves as the starting point for optimization using a complete formulation.The second part of the thesis focuses on adapting the proposed monolithic formulation to model modular structures. Initially, we concentrate on optimizing the topology of a fully modular structure, where a single module is repeated throughout the design. We investigate how hyperparameters, such as the number of subdomains and module complexity, affect the mechanical performance of the structure. Subsequently, we explore a more complex scenario by optimizing multiple module topologies and their layout within the structure. This is achieved through a newly proposed solving strategy based on a modified Discrete Material Optimization (DMO) approach, employing a gradient-based optimizer.By addressing the challenges of lightweight design and modularity in aerostructures, this research aims to contribute to the ongoing evolution of aerospace technologies and advance the efficiency and performance of future aircraft
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Lübbert, Daniel. "Strain and lattice distortion in semiconductor structures a synchrotron radiation study /." [S.l. : s.n.], 1999. http://pub.ub.uni-potsdam.de/2000/0003/luebbert.pdf.
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Lübbert, Daniel. "Strain and lattice distortion in semiconductor structures : a synchrotron radiation study." Phd thesis, Universität Potsdam, 1999. http://opus.kobv.de/ubp/volltexte/2005/2/.
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Die Arbeit stellt neu entwickelte Röntgenbeugungsmethoden vor, mit deren Hilfe der Verzerrungszustand des Kristallgitters von Halbleiter-Wafern und -Bauteilen im Detail charakterisiert werden kann. Hierzu werden die aussergewöhnlichen Eigenschaften der an modernen Synchrotrons wie der ESRF (Grenoble) verfügbaren Röntgenstrahlung genutzt. Im ersten Teil der Arbeit werden Röntgen-Diffraktometrie und -Topographie zu einer Untersuchungsmethode kombiniert, mit der die makroskopische Krümmung von Halbleiter-Wafern ebenso wie ihre mikroskopische Defektstruktur abgebildet werden kann. Der zweite Teil ist der Untersuchung von epitaktisch gewachsenen und geätzten Oberflächengittern mit Abmessungen im Submikrometer-Bereich gewidmet. Die unterschiedlichen Gitterkonstanten der beteiligten Halbleitermaterialien führen zu einem inhomogenen Verzerrungsfeld in der Probe, das sich im Röntgenbild durch eine charakteristische Verformung des Beugungsmusters in der Umgebung der Bragg-Reflexe äussert. Die Analyse der experimentell gemessenen Beugungsmuster geschieht mit Hilfe eines neu entwickelten Simulationsverfahrens, das Elastizitätstheorie und eine semi-kinematische Röntgenbeugungstheorie miteinander verbindet. Durch quantitativen Vergleich der Simulationsergebnisse mit den Messdaten kann auf den genauen Verlauf des Verzerrungsfeldes in den Proben zurückgeschlossen werden. Dieses Verfahren wird erfolgreich auf verschiedene Halbleiter-Probensysteme angewendet, und schliesslich auch auf die Untersuchung von akustischen Oberflächenwellen in Halbleiterkristallen übertragen.
This thesis presents newly developed X-ray methods which can be used to characterize in detail the state of distortion of the crystal lattice in semiconductor wafers, devices and nanostructures. The methods use the extraordinary properties of the X-rays available from modern synchrotron sources such as the ESRF (Grenoble).
In the first part of the thesis, X-ray diffractometry and X-ray topography are combined into a new method, called X-ray rocking curve imaging, which allows to image the macroscopic curvature of semiconductor wafers as well as the underlying microscopic defect structure. The second part of the thesis deals with the investigation of epitaxially grown and subsequently etched semiconductor gratings with lateral periods below the micrometer. The lattice mismatch between the different materials used in heteroepitaxy leads to a non-uniform strain field in the sample, which is reflected in a characteristic distortion of the X-ray diffraction pattern around each Bragg peak. The experimental data are evaluated with the help of a newly developed simulation procedure which combines elasticity theory with a semi-kinematical theory of X-ray diffraction. From a quantitative comparison of measured and simulated data the detailed shape of the strain field in the samples can be deduced. This procedure is used successfully for the structural characterization of different types of semiconductor gratings, and is finally applied also to the investigation of surface acoustic waves in crystals.
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Molde, Håvard. "Simulation-Based Optimization of Lattice Support Structures for Offshore Wind Energy." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for bygg, anlegg og transport, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18712.
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Today, design of wind energy support structures is to a large extent a manual process. It requires a lot of experience, and the design tools are often based on simplified methods. As larger structures are being developed and installations move to larger water-depths, the need for efficient and accurate design tools increases. Simulation-based design is a promising technique that can help automate this process. In this study, Spall’s simultaneous perturbation stochastic approximation (SPSA) method was implemented to automatically optimize thickness and diameter of the members in offshore lattice tower support structures. The method utilizes a pseudo-gradient based on only two function evaluations per iteration, which allows for a computationally efficient process. Each evaluation of the design consists of time-domain simulations of the complete wind turbine in FEDEM Windpower, subsequent rainflow counting and calculation of joint lifetimes with stress concentration factors. The utilization of both ultimate and fatigue limit states is reported for each joint. Tower weight was chosen as an indicator of cost, and an objective function comprising variables for weight and joint lifetimes was defined. Joint lifetime was ignored whenever its value was above the design lifetime of the tower, allowing the algorithm to search solely for the lightest design, as long as the design lifetime constraint was sustained. The method has shown promising results, and is able to successfully find viable designs, even when starting from highly unacceptable starting points. Some of the major challenges when using SPSA for lattice support structures are to find a good objective function, as well as appropriate values for the parameters controlling perturbation and step size. Existing guidelines were followed when doing this calibration, but for an efficient search the parameters had to be adapted. Results for both appropriate parameters and the optimization itself are reported for the 10MW NOWITECH reference turbine on a full-height lattice tower. These results show that superior results can be achieved, but at a high cost in terms of computational time. Recommendation is given to use alternative methods to come up with a partially optimized staring point, from which the SPSA method can optimize further.
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Reid, Robert. "Propagation and period-doubling of coherent structures in coupled lattice maps." Thesis, University of Warwick, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.369417.
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Behnejad, S. Alireza. "Geometrical data for lattice spatial structures : regularity, historical background and education." Thesis, University of Surrey, 2018. http://epubs.surrey.ac.uk/849532/.
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Dealing with geometrical information has been an important aspect of the knowledge required for construction of a structure. In particular, data generation techniques appropriate for complex geometries are crucial for the design and construction of spatial structures. This may be referred to as ‘Configuration Processing’ and has been the centre of attention for some researchers in the past few decades. A main focus of this thesis is the ‘regularity’ in structural forms and the present research shows that the ‘metric properties’ of structural forms, suggested by the Author, are fundamental for the study of regularity. Metric properties refer to the geometrical information necessary for design, and in particular, construction of lattice spatial structures. To elaborate, the research addresses the following questions: • What are the metric properties for a lattice structure and how can these be evaluated? • What is the definition of regularity for lattice structures and how can this be quantified? • How could the regularity of a lattice structure be improved? The Author is an architect and structural engineer who has been involved in the design and construction of lattice spatial structures for 20 years. The experience of the actual construction over the years has shown that there are advantages in keeping the number of different types of structural components small. In another front, the study of regularity of forms for lattice structures may involve the ‘visual aspects’, ‘arrangements of elements’ or ‘structural components’. The first two aspects are subjective matters and the latter one, that is the focus of the present work, is an objective matter. The present research shows that the metric properties of structural forms are fundamental for the study of component regularity. There are considerable benefits in terms of the construction of structures which have a high degree of regular components. The benefits include savings in time and cost of construction, as well as a reduction in probability of having a wrong arrangement during assembly. In this sense, the present work could be considered as a research of fundamental importance which provides a basis for the knowledge in this field. Most of the examples in the Thesis are single layer lattice structures with straight elements and further research on other types of lattice structures is recommended. This thesis consists of six chapters, the first of which entitled ‘Introduction’ provides background information about the research and discusses the research aims. Chapter 2 on the ‘Literature Review’ concerns the few available publications relevant to the research. The third chapter entitled ‘Metric Properties’ defines a number of geometrical parameters which are being used to generate the geometrical information. Also, the mathematics involved for the necessary calculations are discussed. This chapter is a major contribution of the thesis and to the available knowledge in terms of introduction a set of well defined geometrical parameters for design and construction of lattice spatial structures. Chapter 4 is dedicated to discussion of different aspects of ‘Regularity’ of lattice structures. To begin with, the idea of regularity is elaborated upon and then the concept of ‘regularity indicators’ are discussed. These indicators help to quantify regularity of components. Here again, this chapter presents a novel idea in the field of lattice spatial structures. Another major contribution of this thesis to the general knowledge is Chapter 5 entitled ‘Sphere Packing’. This is a particular technique for configuration processing developed by the Author to improve the member length regularity of lattice structures. An example of the application of the technique for configuration processing of spherical domes is also discussed in details. Moreover, a comparison on the variation of the member lengths of different dome configurations is discussed which shows that around 50% of the members of a dome created by sphere packing technique are with the same length. This proportion of equal length members is considerably higher than that of the other dome configurations (10%-33%). Finally, Chapter 6 provides the conclusions and some important suggestions for the continuation of the research. In addition to the main body of this thesis, copy of the relevant publications by the Author are provided as Annexes in the following three categories: i. Geometrical data generation for lattice spatial structures is the core of the Annexes A to E, then, ii. Annexes F and G are focusing on the education of spatial structures, and finally, iii. Historical background of spatial structures is discussed in the Annexes H and I.
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Gorbushin, Nikolai. "Analysis of admissible steady-state fracture processes in discrete lattice structures." Thesis, Aberystwyth University, 2017. http://hdl.handle.net/2160/352f6a78-6e2f-4936-9389-a439e02fdfc0.
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The purpose of this work is to study physically possible crack propagation at constant velocity inside a discrete solid by means of theoretical analysis supported by numerical simulations. Analytical solutions are delivered for fracture problems in one dimensional chains, a double chain and square lattices. Evaluation of obtained solutions required implementation of numerical algorithms for computation of integral transforms. Consideration of one-dimensional cases, namely a simple chain of oscillators and a chain of masses with non-local interactions, allowed to examine the validity of derived formulae by a complementary computer simulation of a corresponding dynamic system. Starting from simple models, the analysis of physically admissible and forbidden fracture regimes has been performed. The analytical predictions of possible steady states found a good agreement with a purely numerical scheme. The work discusses the advantages of different approaches to study steady-state failure processes: either with energetic or load characteristics. These attributes of fracture mechanics are shown to be effient for quantifying global predictions, e.g. a choice a particular loading condition for achieving a certain value of a crack speed. However, it was demonstrated that derivation of these characteristics is not enough and consideration of the displacement or stress fields should be performed. The results on chains with non-local interactions between the oscillators illustrated the features of failure at micro-level. Namely, different combinations of microscopic parameters, that result in the same bulk quantities, reflect different patterns of crack propagation in discrete solids. A problem of a separation a double chain compounded by two chains with different properties shows the peculiarities of parameters mismatch. Particularly, it was established that, contrary to quasi-static problems, a steady-state separation is necessarily caused by forces, applied to each chain, of different values. Furthermore, distinct material parameters of chains give a chance for the observation of the supersonic fracture of the structure. Increasing the problem dimension from chains to lattices, several new features emerged. For instance, the behaviour of displacements along a crack path changes. Moreover, the admissibility analysis is expanded to the consideration of possible fracture behind a crack tip. The outcomes predict crack propagation regimes with high energy release rates be accompanied by snapping of the springs on the faces of the original moving crack. The evaluation of displacement eld in the direction orthogonal to a crack path is also presented. The contrast in material properties in anisotropic lattices and mismatch of material properties in dissimilar lattices unveiled different scenarios of admissible regimes. Furthermore, the question of the choice of a particular fracture criterion is addressed. Two history-dependent criteria are compared to the classical one of threshold elongation for linear bonds. The results show that steadystate regimes can be reached in the low subsonic crack speed range which can not be according to the classical criterion. Repercussions in terms of load and crack opening versus velocity are explained in details. Once known the steady-state regimes of fracture propagation, a procedure for applying history-dependent criteria emerges as not restricted to the two examined ones and opens the way to dierent and more complex problems.
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Suard, Mathieu. "Characterization and optimization of lattice structures made by Electron Beam Melting." Thesis, Université Grenoble Alpes (ComUE), 2015. http://www.theses.fr/2015GREAI055/document.
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Le récent développement de la Fabrication Additive de pièces métalliques permet d'élaborer directement des structures à partir de modèles 3D. En particulier, la technologie "Electron Beam Melting" (EBM) permet la fusion sélective, couche par couche, de poudres métalliques. Elle autorise la réalisation de géométries très complexes mais apporte de nouvelles contraintes de fabrication.Ce travail se concentre sur la caractérisation géométrique et mécanique de structures treillis produites par cette méthode. Les pièces fabriquées sont comparées au design initial à travers des caractérisations par tomographie aux rayons X. Les propriétés mécaniques sont testées en compression uni-axiale. Pour les poutres de faibles épaisseur, la différence entre la structure numérique et celle fabriquée devient significative. Les écarts au design initial se traduisent pour chaque poutre par un concept de matière mécaniquement efficace. D'un point de vue modélisation, ce concept est pris en compte en remplaçant la poutre fabriquée par un cylindre avec un diamètre mécaniquement équivalent. Ce diamètre équivalent est utilisé dans des simulations et optimisations "réalistes" intégrant ainsi les contraintes de fabrication de la technologie EBM.Différentes stratégies sont aussi proposées pour réduire la proportion de volume "inefficace" et améliorer le contrôle de la taille des poutres, soit en jouant sur les paramètres procédé et les stratégies de fusion, soit en effectuant des post-traitementsThe recent development of Additive Manufacturing for the fabrication of metallic parts allows structures to be directly manufactured from 3D models. In particular, the "Electron Beam Melting" (EBM) technology is a suitable process which selectively melts a powder bed layer by layer. It can build very complex geometries but brings new limitations that have to be quantified.This work focuses on the structural and mechanical characterization of lattice structures produced by such technology. The structural characterization mainly rely on X-ray tomography whereas mechanical properties are assessed by uni-axial compression. The geometry and related properties of the fabricated structures are compared with the designed ones. For small strut size, the difference between the designed structure and the produced one is large enough to impact the desired mechanical properties. The concept of mechanical efficient volume is introduced. For the purpose of simulation, this concept is taken into account by replacing the struts by a cylinder with a textit{mechanical equivalent diameter}. After validation, it has been used into "realistic" simulation and optimization procedures, thus taking into account the process constraints.Post-treatments (Chemical Etching and Electro-Chemical Polishing) were applied on lattice structures to get rid of the inefficient matter by decreasing the surface roughness. The control of the size of the fabricated struts was improved by tuning the process strategies and parameters
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Hinds, Kirsten Bramall. "Shear-Dominated Bending Behavior of Carbon/Epoxy Composite Lattice IsoBeam Structures." BYU ScholarsArchive, 2014. https://scholarsarchive.byu.edu/etd/5709.
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Composite lattice structures known as the IsoBeam™ made with unidirectional carbon/epoxy were manufactured and tested in shear-dominated bending. The manufacturing process consisted of placing tows of carbon fiber pre-impregnated with epoxy resin onto a pin-type mandrel to create members with interwoven joints. The members were consolidated with a half spiral aramid sleeve. The IsoBeam structure consists of two main types of members: longitudinal and diagonal members measuring nominally 0.4 in. (10.2 mm) and 0.2 in. (5.1 mm) in diameter, respectively. The hand-manufactured specimens measured nominally 6 in. (152.4 mm) high by 3 in. (76.2 mm) wide by 2 ft (0.61 m) long with 4 bays, each 6 in. (152.4 mm) long. The beams weighed between 1.82-1.86 lbs (8.09-8.27 N). A finite element analysis of the IsoBeam was compared to the experimental results. The IsoBeam specimens were tested in four-point or three-point bending but were dominated by shear due to short-beam bending because of the low length/height aspect ratio. After testing to failure, individual members that were lightly loaded and appeared to be undamaged were removed and tested in axial compression. The void percentage and fiber volume fraction were also measured. The average maximum strength of the IsoBeam structure was 4.11 kips (18.3 kN), yielding an equivalent shear of 2.06 kips (9.15 kN) and bending moment of 20.2 kip-in (2.29 kN-m). This strength was lower than expected and is attributed primarily to low material quality, insufficient consolidation of members, and inadequate tension on the tows during manufacturing. The structure exhibited ductile behavior absorbing considerable energy after initial failure, as well as exhibiting damage tolerance due to the inherent structural redundancy. The inner diagonal members which are inherently stiffer exhibited higher strains than the side outer diagonal members after initial failure. The members removed and tested exhibited an average compression strength of 86.9 ksi (599 MPa) and compression modulus of 17.8 Msi (122 GPa) which are both lower than observed in members tested in past research. The diagonal members had a higher strength of 111 ksi (767 MPa) than the longitudinal member's compression strength of 62.5 ksi (431 MPa). Most members were seen to have a high percentage of voids with an average of 4.3% for diagonal members and 6.4% for longitudinal members. The average fiber volume fraction content of members was very low at 38%. The linear finite element analysis of the IsoBeam structure predicted failure at a load of 34 kips (151 kN). Without considering buckling, the first member predicted to fail was a vertical outer diagonal. This research demonstrates that increasing the manufacturing quality should yield an IsoBeam structure that is strong, ductile and damage tolerant.
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McConaha, Matthew. "Graded Lattice Structure Density Optimization for Additive Manufacturing." University of Cincinnati / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1523634949822303.
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Nascimento, Barreto Maria do. "Modulated structures." Thesis, University of Oxford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.670400.
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