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1
Conca, Luca. "Mechanical properties of polymer glasses : Mechanical properties of polymer glasses". Thesis, Lyon, 2016. http://www.theses.fr/2016LYSE1050/document.
Testo completoAbstract (sommario):
Ce manuscrit présente des récentes extensions au modèle PFVD, basé sur l'hétérogénéité de la dynamique des polymères vitreux à l'échelle de quelques nanomètres et résolu par simulation en 3D, afin de fournir une description physique unifiée des propriétés mécaniques et dynamiques des polymères vitreux soumis à déformation plastique. Trois sujets principaux sont traités : La plastification. Sous déformation, les polymères atteignent le seuil de plasticité (yield) à quelques pourcents de déformation et quelques dizaines de MPa. Nous proposons que l'énergie élastique absorbée à l'échelle des hétérogénéités dynamiques accélère la dynamique locale. On observe contraintes ultimes de quelques dizaines de MPa à quelques pourcents de déformation et que la plastification est due à un nombre relativement petit d'événements locaux. Il a été observé que la dynamique devient plus rapide et homogène dans le régime plastique et que la mobilité moyenne atteint une valeur stationnaire, linéaire avec le taux de déformation. Nous proposons que la contrainte locale stimule la diffusion de monomères des domaines lents à ceux rapides (mécanisme de facilitation) et accélère dynamique locale. Ceci permets d'observer l'homogénéisation de la dynamique, avec des caractéristiques proches de l'expérience. L'écrouissage, dans les polymères enchevêtrés ou réticulés. A grande déformation, la contrainte augmente avec une pente caractéristique d'ordre 10 – 100 MPa au-dessous de la transition vitreuse. De manière analogue à une théorie récente, nous proposons que la déformation locale oriente les monomères dans la direction d'étirage et ralentie la dynamique, suite à l'intensification des interactions locales. Les modules d'écrouissage mesurés, les effets de la réticulation et du taux de déformation sont comparables aux données expérimentales. En outre, on trouve que l'écrouissage a un effet stabilisateur sur les phénomènes de localisation et sur les bandes de cisaillementThis manuscript presents recent extensions to the PFVD model, based on the heterogeneity of theh dynamics of glassy polymers at the scale of a few nanometers et solved by 3D numerical simulation, which aim at providing a unified physical description of the mechanical and dynamical properties of glassy polymers during plastic deformation. Three main topics are treated: Plasticization. Under applied deformation, polymers undergo yield at strains of a few percent and stresses of some 10 MPa.We propose that the elastic energy stored at the scale of dynamical heterogeneities accelerates local dynamics. We observe yield stresses of a few 10 MPa are obtained at a few percent of deformation and that plastification is due to a relatively small amount of local yields. It has been observed that dynamics becomes faster and more homogeneous close to yield and that the average mobility attains a stationary value, linear with the strain rate. We propose that stress-induced acceleration of the dynamics enhances the diffusion of monomers from slow domains to fast ones (facilitation mechanism), accelerating local dynamics. This allows for obtaining the homogeneisation of the dynamics, with the same features observed during experiments. Strain-hardening, in highly entangled and cross-linked polymers. At large strain, stress increases with increasing strain, with a characteristic slope (hardening modulus) of order 10 – 100 MPa well below the glass transition. Analogously to a recent theory, we propose that local deformation orients monomers in the drawing direction and slows dows the dynamics, as a consequence of the intensification of local interactions. The hardening moduli mesured, the effect of reticulation and of strain rate are comparable with experimental data. In addition, strain-hardening is found to have a stabilizing effect over strain localization and shear banding
2
Guillou, Lionel. "Cell Mechanics : Mechanical Properties and Membrane Rupture Criteria". Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLX041/document.
Testo completoAbstract (sommario):
L’athérosclérose est une maladie artérielle chronique qui est une des causes majeures d’accidents vasculaires cérébraux et de crises cardiaques. Cette thèse a pour objectif de mieux comprendre certains facteurs spécifiques impliqués dans le dévelopement de cette maladie en abordant cette problématique sous l’angle de la mécanique.Deux types de cellules qui jouent un rôle important dans le dévelopement et la progression de l’athérosclérose sont les cellules endothéliales adhérentes et les leucocytes non-adhérents (les globules blancs). Nous avons développé deux systèmes capables de mesurer les propriétés mécaniques de ces deux grands types cellulaires. Le premier, appelé “indentation de profil”, utilise des micropipettes et des microindenteurs pour indenter la cellule, tandis que le second utilise la microfluidique pour soumettre les cellules à une contrainte d’élongation.De plus, nous nous sommes demandé si la mécanique pouvait nous aider à comprendre quand les déformations des cellules, ou les contraintes exercées sur elles, pouvaient les endommager.En effet, lorsque les plaques d’athérosclérose obstruent une partie trop grande du flux sanguin, le traitement le plus courant consiste à rouvrir le vaisseau avec un ballon et à le maintenir ouvert au moyen d’une endoprothèse artérielle, qui est un petit dispositif maillé et tubulaire. Cette procédure exerce des contraintes de compression considérables sur l’endothélium et l’endommage. Nous avons donc cherché à trouver un critère physique prédictif de la rupture de la membrane des cellules endothéliales en compression, puis avons comparé cela aux contraintes exercées sur l’endothélium durant la pose d’une endoprothèse artérielle, afin de voir si les dommages faits à l’endothélium pouvaient potentiellement être évités.De façon similaire, nous avons cherché à obtenir un critère physique prédictif de la rupture de la membrane des leucocytes. Nous avons ensuite comparé les déformations maximales possibles des leucocytes selon que ces déformations soient passives (comme lors du passage dans la microvasculature) ou actives (comme lors de la traversée de l’endothélium par les leucocytes)Atherosclerosis is a chronic disease of the arteries that is a major cause of heart attacks and strokes. This thesis aims to provide novel insight into this disease by looking at specific factors involved in its development from a mechanical standpoint.Two important cell types involved in the development and progression of atherosclerosis are adherent endothelial cells and non-adherent leukocytes (white blood cells). We developed two devices that are able to measure the mechanical properties of both of these cell types. The first one, termed “profile microindentation”, uses micropipettes and microindenters to indent the cell, while the second one uses microfluidics to submit cells to an extensional stress.Further, we wondered if mechanics could help us understand when deformations undergone by cells, or stresses exerted on them, could become harmful.As a matter of fact, when atherosclerotic plaques occlude too much of the blood flow, the most common treatment consists of reopening the vessel with a balloon and keeping it open with a tubular wired mesh called a stent. This procedure exerts considerable compressive stress on the endothelium and is known to be associated with extensive endothelial damage. Hence, we seek to find a physical criterion that is predictive of endothelial cell membrane rupture under compression and to compare this to the stress exerted on the endothelium during the stenting procedure, to see if endothelial damage could potentially be avoided.Similarly, we seek to obtain a physical criterion that is predictive of leukocyte membrane rupture. We then compare and contrast the maximum possible deformations of leukocytes depending on whether those deformations are passive (such as when going through the microvasculature) or active (such as when leukocytes traverse the endothelial barrier)
3
Miao, Yuyang. "Mechanics of textile composites : from geometry to mechanical properties /". Search for this dissertation online, 2005. http://wwwlib.umi.com/cr/ksu/main.
Testo completo
4
Loveless, Thomas A. "Mechanical Properties of Kenaf Composites Using Dynamic Mechanical Analysis". DigitalCommons@USU, 2015. https://digitalcommons.usu.edu/etd/4310.
Testo completoAbstract (sommario):
Natural fibers show potential to replace glass fibers in thermoset and thermoplastic composites. Kenaf is a bast-type fiber with high specific strength and great potential to compete with glass fibers.
In this research kenaf/epoxy composites were analyzed using Dynamic Mechanical Analysis (DMA). A three-point bend apparatus was used in the DMA testing. The samples were tested at 1 hertz, at a displacement of 10 μm, and at room temperature.
The fiber volume content of the kenaf was varied from 20%-40% in 5% increments. Ten samples of each fiber volume fraction were manufactured and tested. The flexural storage modulus, the flexural loss modulus, and the loss factor were reported. Generally as the fiber volume fraction of kenaf increased, the flexural storage and flexural loss modulus increased. The loss factor remained relatively constant with increasing fiber volume fraction.
Woven and chopped fiberglass/epoxy composites were manufactured and tested to be compared with the kanaf/epoxy composites were manufactured and tested to be compared with the kenaf/epoxy composites. Both of the fiberglass/epoxy composites reported higher flexural storage and flexural loss modulus values. The kenaf/epoxy composites reported higher loss factor values. The specific flexural storage and specific flexural loss modulus were calculated for both the fiberglass and kenaf fiber composites. Even though the kenaf composites reported a lower density, the fiberglass composites reported higher specific mechanical properties.
5
Ozdemir, Gokhan. "Mechanical Properties Of Cfrp Anchorages". Master's thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12605890/index.pdf.
Testo completoAbstract (sommario):
Due to inadequate lateral stiffness, many reinforced concrete buildings are highly damaged or collapsed in Turkey after the major earthquake. To improve the behavior of such buildings and to prevent them from collapse, repair and/or strengthening of some reinforced concrete elements is required. One of the strengthening techniques is the use of CFRP sheets on the existing hollow brick masonry infill. While using the CFRP sheets their attachment to both structural and non-structural members are provided by CFRP anchor dowels. In this study, by means of the prepared test setup, the pull-out strength capacities of CFRP anchor dowels are measured. The effects of concrete compressive strength, anchorage depth, anchorage diameter, and number of fibers on the tensile strength capacity of CFRP anchor dowel are studied.
6
Dimitriu, Radu. "Complex mechanical properties of steel". Thesis, University of Cambridge, 2009. https://www.repository.cam.ac.uk/handle/1810/218319.
Testo completoAbstract (sommario):
Whereas considerable progress has been reported on the quantitative estimation of the microstructure of steels as a function of most of the important determining variables, it remains the case that it is impossible to calculate all but the simplest of mechanical properties given a comprehensive description of the structure at all conceivable scales. Properties which are important but fall into this category are impact toughness, fatigue, creep and combinations of these phenomena. The work presented in this thesis is an attempt to progress in this area of complex mechanical properties in the context of steels, although the outcomes may be more widely applied. The approach used relies on the creation of physically meaningful models based on the neural network and genetic programming techniques. It appears that the hot-strength, of ferritic steels used in the powerplant industry, diminishes in concert with the dependence of solid solution strengthening on temperature, until a critical temperature is reached where it is believed that climb processes begin to contribute. It is demonstrated that in this latter regime, the slope of the hot-strength versus temperature plot is identical to that of creep rupture-strength versus temperature. This significant outcome can help dramatically reduce the requirement for expensive creep testing. Similarly, a model created to estimate the fatigue crack growth rates for a wide range of ferritic and austenitic steels on the basis of static mechanical data has the remarkable outcome that it applies without modification to nickel based superalloys and titanium alloys. It has therefore been possible to estimate blindly the fatigue performance of alloys whose chemical composition is not known. Residual stress is a very complex phenomenon especially in bearings due to the Hertzian contact which takes place. A model has been developed that is able to quantify the residual stress distribution, under the raceway of martensitic ball bearings, using the running conditions. It is evident that a well-formulated neural network model can not only be extrapolated even beyond material type, but can reveal physical relationships which are found to be informative and useful in practice.
7
Drodge, Daniel Ryan. "Mechanical properties of energetic composites". Thesis, University of Cambridge, 2010. https://www.repository.cam.ac.uk/handle/1810/265501.
Testo completoAbstract (sommario):
This thesis presents research into the mechanical response of particulate polymer composites, both energetic and inert, that contributes towards the wider understanding of deformation and damage mechanisms in Polymer Bonded Explosive (PBXs). Specifically, high and low strain-rate compression experiments were performed on several composites, with a view to measuring their elastic properties. A brief review of PBXs, polymers and particulate composites forms chapter 1. A key piece of mechanical testing apparatus, the Split Hopkinson Pressure Bar (SHPB), is critically assessed in chapters 2 and 3. The gauge calibration procedure was critically evaluated; the necessity of dispersion correction was investigated; and a method for allowing for the finite specimen transit time was introduced. Chapter 4 presents a comparison of methods of estimating a high strain-rate elastic modulus, including ultrasonic and pulse-shaped SHPB measurements. All methods returned moduli within the expected range and in broad agreement with each other. Chapter 5 describes SHPB and ultrasonic transducer experiments performed on a UK PBX and binder at temperatures ranging from -100�C to 30�C. Results build upon and agree with published findings, demonstrating a lower glass transition temperature in the binder than in the PBX, implying that the binder in the PBX experiences a higher strain-rate. Chapter 6 reports experiments performed on three cast RDX-HTPB composites, where quantifiable damage was introduced at high strain-rate using a Direct Impact Hopkinson Bar, and the resulting composite modulus was measured quasi-statically. The most abrupt decrease in modulus due to damage was measured for the composite containing bimodally distributed filler particles. Finally, in chapter 7, two sets of sugar-HTPB composites were produced: one with fixed particle size distribution with varying particle separation, and the other vice-versa. Microstructural properties, including the distribution of intergranular separations, were measured using X-ray microtomography. Quasi-static and SHPB compression experiments were performed. Particle size and separation were found to be secondary to fill-fraction in governing material properties. A Porter-Gould modulus decay function was fitted to the stress-strain curves. The binder elastic modulus and crystal-binder adhesion energy were estimated at high and low strain-rates.
8
Rains, Jeffrey K. "Mechanical properties of tracheal cartilage". Thesis, University of British Columbia, 1989. http://hdl.handle.net/2429/27994.
Testo completoAbstract (sommario):
Large airways collapse has been implicated as one of the causes of maximal expiratory flow limitation. Since cartilage plays an important role in maintaining the form of these airways, an understanding of the mechanical properties of the cartilage is necessary for a better understanding of the mechanisms which limit maximal expiratory flow. This work establishes a technique whereby the tensile stiffness of human tracheal cartilage can be determined using uniaxial equilibrium tensile tests. A technique was developed in which standard shaped specimens were cut from tracheal cartilage rings and tested in a specially designed tensile tester in order to determine the stress-strain relationship of the specimen. The stress-strain relationship of the cartilage test specimens was found to be linear up to approximately 10 % strain. However, irreversible disruption of the cartilage matrix occurred at strains greater than 10 %. The tensile stiffness of the tracheal cartilage fell in the range 1-20 MPa and was found to decrease with increasing depth from the outer surface of the tissue. This layer-wise variation in tensile stiffness reflected the orientation of the collagen fibrils in the tissue. An age-related increase in the tensile stiffness of tracheal cartilage was found. This age-related change in tensile stiffness may reflect an increase in collagen cross-linking in specimens from older individuals. A possible bias of the test method toward the measurement of the mechanical properties of the collagen fibrils, as opposed the combined effects of the collagen and proteoglycans, was suspected. However, to the extent that equilibrium tensile testing reflects the ability of tracheal cartilage to bend in response to alterations in transmural pressure, these results suggest that age-related changes in large airway cartilage stiffness are not the cause of the age-related decrease in maximal expiratory flow.Applied Science, Faculty of
Chemical and Biological Engineering, Department of
Graduate
9
Lintzén, Nina. "Mechanical properties of artificial snow". Licentiate thesis, Luleå tekniska universitet, Geoteknologi, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-16798.
Testo completoAbstract (sommario):
Mechanical properties of snow have been a subject of research since the mid-20th century. Theresearch done is based on natural snow. During the last decades the winter business industryhas been growing and also the interest for constructing buildings and artwork of snow. Suchconstructions are generally built using artificial snow, i.e. snow produced by snow guns. Up tothe present constructions of snow are designed based on knowledge by experience. Only minorscientific studies on artificial snow and its properties has been published. Hence it is ofimportance to investigate material properties for artificial snow.A survey of current state of the art knowledge of properties for natural snow was done andbasic material properties for different qualities of artificial snow were investigated. Strengthand deformation properties for artificial snow were evaluated through uniaxial compressivetests where cylindrical test specimens were subjected to different constant deformation rates.The results show that artificial snow at low deformation rates will have a plastic deformationbehavior where the initial deformation will cause a hardening of the snow structure. At higherdeformation rates brittle failure may occur. For artificial snow with a homogeneous and finegrained structure the deformation behavior was found to change from plasticity to brittleness ata certain critical deformation rate. Artificial snow with coarse grained structure was found to bebrittle giving unstructured results independent of the load level.Four point loading was applied on beams of artificial snow to study creep deformation, bendingstrength and to determine the ultimate load for the different snow qualities. The results showedcoarse grained artificial snow underwent relatively small creep deformations. Both the creepbehavior and the ultimate strength varied randomly at the same applied load. Large plasticdeformations were observed with the fine grained artificial without any failure of the beams.The ultimate load was relatively high and repeatable results were achieved for all test.Previous presumptions that coarse grained artificial snow with high density would have highstrength and were not confirmed by the experiments performed on different qualities ofartificial snow. The performed tests indicate that fine grained artificial snow of lower densityhave more predictable strength properties of equally high or higher magnitude as for coarsegrained artificial snow. The plastic deformations were however higher for the fine grainedartificial snow. High deformations are not favorable for structures which should maintain theshape during the winter season. When designing constructions of snow both strength anddeformation properties should be taken into account.Godkänd; 2013; 20131002 (ninlin); Tillkännagivande licentiatseminarium 2013-10-23 Nedanstående person kommer att hålla licentiatseminarium för avläggande av teknologie licentiatexamen. Namn: Nina Lintzén Ämne: Geoteknik/Soil Mechanics and Foundation Engineering Uppsats: Mechanical Properties of Artificial Snow Examinator: Professor Sven Knutsson, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Diskutant: Tekn. lic. Lars Vikström, LKAB, Luleå Tid: Fredag den 15 november 2013 kl 10.00 Plats: F1031, Luleå tekniska universitet
10
Root, Samuel E. "Mechanical Properties of Semiconducting Polymers". Thesis, University of California, San Diego, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10745535.
Testo completoAbstract (sommario):
Mechanical softness and deformability underpin most of the advantages offered by semiconducting polymers. A detailed understanding of the mechanical properties of these materials is crucial for the design and manufacturing of robust, thin-film devices such as solar cells, displays, and sensors. The mechanical behavior of polymers is a complex function of many interrelated factors that span multiple scales, ranging from molecular structure, to microstructural morphology, and device geometry. This thesis builds a comprehensive understanding of the thermomechanical properties of polymeric semiconductors through the development and experimental-validation of computational methods for mechanical simulation. A predictive computational methodology is designed and encapsulated into open-sourced software for automating molecular dynamics simulations on modern supercomputing hardware. These simulations are used to explore the role of molecular structure/weight and processing conditions on solid-state morphology and thermomechanical behavior. Experimental characterization is employed to test these predictions—including the development of simple, new techniques for rigorously characterizing thermal transitions and fracture mechanics of thin films.
11
Virues, Delgadillo Jorge Octavio. "Mechanical properties of arterial wall". Thesis, University of British Columbia, 2008. http://hdl.handle.net/2429/923.
Testo completoAbstract (sommario):
The incidence of restenosis has been shown to be correlated with the overstretching of the arterial wall during an angioplasty procedure. It has been proposed that slow balloon inflation results in lower intramural stresses, therefore minimizing vascular injury and restenosis rate. The analysis of the biomechanics of the arterial tissue might contribute to understand which factors trigger restenosis. However, few mechanical data are available on human arteries because of the difficulty of testing artery samples often obtained from autopsy while arteries are still considered "fresh". Various solutions mimicking the physiological environment have been used to preserve artery samples from harvesting to testing. In vitro mechanical testing is usually preferred since it is difficult to test arteries in vivo. Uniaxial and biaxial testing has been used to characterize anisotropic materials such as arteries, although methodological aspects are still debated.
Several objectives were formulated and analyzed during the making of this thesis. In one study, the effect of deformation rate on the mechanical behavior of arterial tissue was investigated. The effect of several preservation methods, including cryopreservation, on the mechanical properties of porcine thoracic aortas was also analyzed. Finally, the differences in the mechanical behavior between three different types of sample geometry and boundary conditions were compared under uniaxial and equi-biaxial testing.
Thoracic aortas were harvested within the day of death of pigs from a local slaughterhouse. Upon arrival, connective tissue was removed from the external wall of the artery. Then the artery was cut open along its length and cut out in rectangular samples for uniaxial testing, and square and cruciform samples for biaxial testing. Samples belonging to the freezing effect study were preserved for two months at -20°C and -80°C in isotonic saline solution, Krebs-Henseleit solution with 1.8 M dimethylsulfoxide, and dipped in liquid nitrogen. Samples belonging to the deformation rate effect study were tested uniaxially and equi-biaxially at deformation rates from 10 to 200 %/s.
The uniaxial and biaxial experiments were simulated with the help of an inverse finite element software. The use of inverse modeling to fit the material properties by taking into account the non-uniform stress distribution was demonstrated. A rate-dependent isotropic hyperelastic constitutive equation, derived from the Mooney-Rivlin model, was fitted to the experimental results (i.e. deformation rate study). In the proposed model, one of the material parameters is a linear function of the deformation rate. Overall, inverse finite element simulations using the proposed constitutive relation accurately predict the mechanical properties of the arterial wall.
In this thesis, it was found that easier attachment of samples (rectangular and cruciform) is accomplished using clamps rather than hooks. It was also found that the elastic behavior of arteries is nonlinear and non-isotropic when subjected to large deformations. Characterization of the arterial behavior at large deformations over a higherdeformation range was achieved using cruciform samples. The mechanical properties of arteries did not significantly change after preservation of arteries for two months. Under uniaxial and biaxial testing, loading forces were reduced up to 20% when the deformation rate was increased from 10 to 200 %/s, which is the opposite to the behaviour seen in other biological tissues.
The differences observed in the mechanical behavior of fresh and thawed samples were not significant, independently of the storing medium or freezing temperature used. The lack of significant differences observed in the freezing study was likely due to the small number of samples tested per storing group. Further studies are required to clarify the impact of cryopreservation on extracellular matrix architecture to help tailor an optimized approach to preserve the mechanical properties of arteries. From the results obtained in the deformation rate study, it is concluded that the stiffness of arteries decreases with an increase in the deformation rate. In addition, the effect of deformation rate was observed to be higher than the effect of anisotropy. The inverse relationship between stiffness and deformation rate raises doubts on the hypothesized relationship between intramural stress, arterial injury, and restenosis.
12
Choi, Hwa-Soon. "Mechanical properties of canine pericardium". Diss., Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/15492.
Testo completo
13
Puaud, Max. "Mechanical properties of biopolymer films". Thesis, University of Nottingham, 2000. http://eprints.nottingham.ac.uk/11624/.
Testo completoAbstract (sommario):
Hard gelatin capsules have been used for drug delivery for a long time. The current production process takes advantage of the very unusual properties of gelatin: gelation, very low viscosity, film mechanical properties and film solubility. Although the hard gelatin capsules present many advantages compared to other drug delivery systems, their uses are restricted because of the animal origin of the gelatin. A HPMC gelling agent system is currently used for producing animal product free hard capsules. This work examines the possibility of using a different system in a similar production process. The gelling conditions of the mixed system, the potential of various film formers and the mechanical properties of some films are considered. Gelling agent filler mixed systems were prepared, and the limit concentration of filler that allowed gelation was noted. It was shown that none of the gelling agents would always gel and gelation was never prevented by the maltodextrin (up to a concentration of 14%). The gelation inhibition obtained is likely to be due to phase separation. The charge densities of the various products were also measured. It showed that when there is little charge density difference, gelation is inhibited. Polymer compatibility is increased by increasing the charge density differences. However, an asymmetry is observed. This is explained by the necessary shift of the binodal that would predict prevention of incompatibility. Many films were cast from various biopolymers. The films were screened via sensory analysis. The process allowed to define terms that discriminate the films. The results showed that cellulose derivatives, alginate and alginate derivative films had sensory analysis scores similar to gelatin. Although none of the starch derivatives had such good scores, some presented some promising results. Alginate and caseinate films were selected for further analysis. The mechanical properties of gelatin and HPMC films were compared by puncture tests. The results at a relative humidity of 44% are similar. However, the effect of the moisture content on both films' mechanical properties showed differences. The fracture patterns and polarised microscopy observation were also very different. Alginate films' mechanical properties were similar to gelatin. However, alginate films are not soluble in acidic environments. The effects of molecular weight on the mechanical properties of cellulose derivatives and alginates films were different. Increasing the calcium content of the alginate sample gave similar results to those obtained by increasing the molecular weight. It is proposed that ultimate deformation occurs through different processes in various films. Alginate/gelatin films are thought to deform through crazing, and the fracture process generates many surfaces (lines). Molecular weight and crosslinking would stabilise the crazes. On the other hand, cellulose derivative would deform through slippage and the energy is dissipated during deformation. This is consistent with the orientation observed after fracture, the lack of new surfaces and the high hydrophobicity of these polymers. Caseinate films of sodium, potassium, calcium and magnesium were studied. Sodium caseinate presented the best mechanical properties. Glycerol proved to be the best plasticiser. Glyoxal crosslinking or increase in pH did not improve the mechanical properties of these films. Caseinate films are poorer than alginate, HPMC or gelatin films. Caseinate deformation processes might occur through both slippage and crazing owing to the low molecular weight and high hydrogen bonding ability. Overall, different deformation processes can lead to similar mechanical behaviour. None of the films studied is likely to replace gelatin or HPMC. More complex systems are proposed for further study.
14
Salisbury, S. T. Samuel. "The mechanical properties of tendon". Thesis, University of Oxford, 2008. http://ora.ox.ac.uk/objects/uuid:97b73cf6-53bc-4606-b974-a1cdc662e9e8.
Testo completoAbstract (sommario):
Although the tensile mechanical properties of tendon have been well characterised, the viscoelastic and anisotropic properties remain uncertain. This thesis addresses the anisotropic and viscoelastic material properties of tendon. A method to characterise the three-dimensional shape of tendon is reported and experiments to characterise the fibre-aligned and fibre-transverse viscoelastic properties of tendon are presented. The cross-sectional profiles of bovine digital extensor tendons were determined by a laser-slice method. Linear dimensions were measured within 0.15 mm and cross-sectional areas within 1.7 mm². Tendons were compressed between two glass plates in creep loading at multiple loads. Compression was then modelled in a finite element environment. Tendon was found to be nearly incompressible and reproduction of its isochronal load-displacement curve was achieved with a neo-Hookean material model (E ≃ 0.3 MPa). The fibre-aligned tensile mechanical properties were described using a Quasi-Linear Viscoelastic model. The model was effective at reproducing cyclic loading; however, it was ineffective at predicting stress relaxation outside the scope of data used to fit the model. When all experimental results are considered together, two significant conclusions are made: (1) tendon is much stiffer in fibre-aligned tension than in fibre-transverse compression and (2) the fibre-aligned tensile response is strain dependant, while the transverse response is not.
15
McCullough, Kieran. "Mechanical properties of metallic foams". Thesis, University of Cambridge, 1999. https://www.repository.cam.ac.uk/handle/1810/272153.
Testo completo
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King, Raymond John. "Dynamic Mechanical Properties of Resilin". Thesis, Virginia Tech, 2010. http://hdl.handle.net/10919/33677.
Testo completoAbstract (sommario):
Resilin is an almost perfect elastic protein found in many insects. It can be stretched up to 300% of its resting length and is not affected by creep or stress relaxation. While much is known about the static mechanical properties of resilin, it is most often used dynamically by insects. Unfortunately, the dynamic mechanical properties of resilin over the biologically relevant frequency range are unknown. Here, nearly pure samples of resilin were obtained from the dragonfly, Libellua luctuosa, and dynamic mechanical analysis was performed with a combination of time-temperature and time-concentration superposition to push resilin through its glass transition. The tensile properties for resilin were found over five different ethanol concentrations (65, 70, 82, 86 and 90% by volume in water) between temperatures of -5°C and 60°C, allowing for the quantification of resilinâ s dynamic mechanical properties over the entire master curve. The glass transition frequency of resilin in water at 22°C was found to be 106.3 Hz. The rubber storage modulus was 1.6 MPa, increasing to 30 MPa in the glassy state. At 50 Hz and 35% strain over 98% of the elastic strain energy can returned each cycle, decreasing to 81% at the highest frequencies used by insects (13 kHz). However, despite its remarkable ability to store and return energy, the resilin tendon in dragonflies does not act to improve the energetic efficiency of flight or as a power amplifying spring. Rather, it likely functions to passively control and stabilize the trailing edge of each wing during flight.Master of Science
17
Kappiyoor, Ravi. "Mechanical Properties of Elastomeric Proteins". Diss., Virginia Tech, 2014. http://hdl.handle.net/10919/54563.
Testo completoAbstract (sommario):
When we stretch and contract a rubber band a hundred times, we expect the rubber band to fail. Yet our heart stretches and contracts the same amount every two minutes, and does not fail. Why is that? What causes the significantly higher elasticity of certain molecules and the rigidity of others? Equally importantly, can we use this information to design materials for precise mechanical tasks? It is the aim of this dissertation to illuminate key aspects of the answer to these questions, while detailing the work that remains to be done.
In this dissertation, particular emphasis is placed on the nanoscale properties of elastomeric proteins. By better understanding the fundamental characteristics of these proteins at the nanoscale, we can better design synthetic rubbers to provide the same desired mechanical properties.Ph. D.
18
Bidasaria, Sanjay K. "Electronic and mechanical properties of". Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/28101.
Testo completoAbstract (sommario):
Thesis (M. S.)--Physics, Georgia Institute of Technology, 2009.Committee Chair: Marchenkov, Alexei; Committee Member: Callen, William Russell; Committee Member: First, Phillip; Committee Member: Kindermann, Marcus; Committee Member: Riedo, Elisa.
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Paternoster, Carlo. "Mechanical properties of nanostructured coatings". Doctoral thesis, Università Politecnica delle Marche, 2009. http://hdl.handle.net/11566/242312.
Testo completo
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Mace, Tamara Lee. "Phase segregation study of thermoplastic polyurethanes". Diss., Available online, Georgia Institute of Technology, 2004:, 2003. http://etd.gatech.edu/theses/available/etd-04072004-180051/unrestricted/mace%5ftamara%5fl%5f200312%5fms.pdf.
Testo completo
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MacLean, Sean. "Brain tissue analysis of mechanical properties /". Connect to resource, 2010. http://hdl.handle.net/1811/44968.
Testo completo
22
Tharmann, Rainer. "Mechanical properties of complex cytoskeleton networks". [S.l.] : [s.n.], 2006. http://deposit.ddb.de/cgi-bin/dokserv?idn=97998002X.
Testo completo
23
Yang, Ting. "Mechanical and swelling properties of hydrogels". Doctoral thesis, KTH, Ytbehandlingsteknik, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-105539.
Testo completoAbstract (sommario):
Hydrogels have been used as one of the novel soft materials in many biomedical applications such as drug delivery and tissue engineering for recent decades. In the main part of this work, bi-functional poly(ethylene glycol) (PEG) precursors with either thiols (PEG-SH) or allyls (PEG-Al) , covering molecular weights from 3 kDa to 8 kDa were synthesized and thoroughly characterized by 1H NMR, 13C NMR, FT-Raman and MALDI-TOF techniques. By combining PEG precursors with complementary trifunctional crosslinkers, a library of well-defined single-network hydrogels was efficiently constructed via the robust UV-initiated thiol-ene coupling (TEC) chemistry. Novel sequential interpenetrating network (seqIPN) hydrogels based on PEG were fabricated by diffusing and afterwards crosslinking secondary-network precursors within dense (2 kDa) to loose (8 kDa) primary networks. The impacts of polymer chain length and diffusion time on the swelling and mechanical properties were assessed for the seqIPN hydrogels. Additionally, disperse red 13 decorated PEG 2 kDa and 8 kDa were synthesized and used as probes to monitor the secondary-network precursor diffusion rate by UV/Vis spectroscopy. FT-Raman and leaching tests were conducted to evaluate the efficiency of the TEC reaction for the development of PEG networks and their gel fractions. All gels were fully crosslinked within 5 minutes and with the gel fraction above 84%. The chain length of PEG, location of functional groups of PEGs, solvents, solid content were found to have directly influence on the mechanical and swelling properties of PEG single-network hydrogels. The utilization of the diffusion time dependent seqIPN strategy enabled further freedom to control the swelling and mechanical properties of PEG hydrogels, with the degree of water swelling ranged from 280 – 870% and the tensile modulus ranging from 1135 kPa to 175 kPa. Furthermore, the seqIPN strategy was utilized for fiber reinforced free radical polymerized hydrogels. N, N-dimethylacrylamide (DMA) with crosslinker poly(ethylene glycol) diacrylate were diffused in bacterial cellulose (BC) aerogel thereafter UV crosslinked to form BC-DMA hydrogels. FT-Raman and leaching tests were conducted to evaluate the efficiency of the free radical polymerization and the BC-DMA gel fractions. After UV cure for 10 minutes, robust DMA networks were formed within BC aerogels with over 94% gel fraction. The high porosity and robust interpenetrating DMA network within BC fibers were further analysed with FE-SEM. Compression tests showed that fiber reinforced DMA hydrogels have higher compression modulus than DMA hydrogels, ranging from 4.4 to 8.3 MPa with water content from 78 to 70%.QC 2012
24
Savage, Gary. "Mechanical properties of carbon/graphite composites". Thesis, Imperial College London, 1986. http://hdl.handle.net/10044/1/38153.
Testo completo
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Yttergren, Rose-Marie. "Mechanical properties of laminated ceramic composites /". Stockholm : Tekniska högsk, 1999. http://www.lib.kth.se/abs99/ytte0910.pdf.
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Chopra, Prateek. "Effective mechanical properties of lattice materials". Thesis, University of British Columbia, 2011. http://hdl.handle.net/2429/39436.
Testo completoAbstract (sommario):
Lattice materials possess a spatially repeating porous microstructure or unit cell. Their usefulness lies in their multi-functionality in terms of providing high specific stiffness, thermal conductivity, energy absorption and vibration control by attenuating forcing frequencies falling within the band gap region. Analytical expressions
have been proposed in the past to predict cell geometry dependent effective material properties by considering a lattice as a network of beams in the high porosity limit. Applying these analytical techniques to complex cell geometries is cumbersome. This precludes the use of analytical methods in conducting a comparative study involving complex lattice topologies. A numerical method based on the method of long wavelengths and Bloch theory is developed here and applied to a chosen set of lattice geometries in order to compare effective material properties of infinite lattices. The proposed method requires implementation of Floquet-bloch transformation in conjunction with a Finite Element (FE) scheme. Elastic boundary layers emerge from surfaces and interfaces in a finite lattice, or an infinite lattice with defects such as cracks. Boundary layers can degrade effective
material properties. A semi-analytical formulation is developed and applied to a chosen set of topologies and the topologies with deep boundary layers are identified. The methods developed in this dissertation facilitate rapid design calculation and selection of appropriate core topologies in multifunctional design of sandwich
structures employing a lattice core.
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Pope, Stephen Gerard. "Mechanical properties of ion implanted alumina". Diss., Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/19544.
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Kelly, Suzanne Marie. "Cell mechanical properties and volume control". Thesis, McGill University, 1992. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=39551.
Testo completoAbstract (sommario):
We examined the mechanical properties of oocytes and eggs from Xenopus laevis frogs, in relation to the cells' ability to maintain volume in dilute media. Elastic properties were assessed by measuring intracellular pressure (Pic) and cell volume (Vc) during osmotic swelling. Cytoplasmic rheology was determined by magnetometry after microinjection of magnetic iron microparticles. Both oocytes and eggs had a small but measurable Pic. Oocytes markedly increased both Pic and Vc during osmotic swelling. The relationship between Pic and Vc was not linear and showed hysteresis indicating that oocytes are viscoelastic. After removal of the vitelline envelope, the Pic-Vc curve remained qualitatively similar but the Pic rise was reduced by 70%. During osmotic swelling, eggs increased Vc to a lesser extent than did oocytes and Pic did not change. Neither oocytes nor eggs displayed a regulatory volume decrease in response to hypotonic shock and osmolyte loss did not appear to limit swelling. Since Pic was low in eggs and did not increase with swelling, increased stiffness of the vitelline envelope was ruled out as the explanation for their reduced water uptake. Rheologic data indicated that cytoplasmic apparent viscosity and elastic shear modulus were lower in eggs than in oocytes, an observation not compatible with our hypothesis that eggs took up less water because their cytoplasm was a gel. We now feel that Vc of eggs may be controlled by the cortical actin cytoskeleton.
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Morsi, Khaled M. B. E. "Mechanical properties of particle reinforced alumina". Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320644.
Testo completo
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Blewett, Jennifer M. "Micromanipulation of plant cell mechanical properties". Thesis, University of Birmingham, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.520730.
Testo completo
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Horrigan, Emma. "Disordered microstructures and anomalous mechanical properties". Thesis, University of Exeter, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.496775.
Testo completoAbstract (sommario):
This study concerns the effects of structural heterogeneity upon mechanical properties, particularly on negative Poisson's ratio. Two separate methods were used to generate two-dimensional honeycombs optimised for a series of material properties, in particular large negative Poisson's ratio and high stiffness. The properties and heterogeneity of crumpled and recycled materials were compared.
32
Davies, Melissa Lynne F. "Mechanical properties of fish myotomal muscle". Thesis, University of St Andrews, 1995. http://hdl.handle.net/10023/6501.
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Chan, Yan Na-Xin. "The mechanical properties of auxetic foams". Thesis, University of Liverpool, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.528661.
Testo completoAbstract (sommario):
Auxetic materials have a negative Poisson's ratio, that is, they expand laterally when stretched longitudinally. Negative Poisson's ratio is an unusual property that affects many of the mechanical properties of the material, such as indentation resistance, compression, tension and shear stiffness, and certain aspects of the dynamic performance. Auxetic foam was fabricated from a conventional polymeric foam. The unusual mechanical properties of auxetic foams are attributed to the deformation characteristics of re-entrant microstructures. There are four main aspects to the project. Firstly, the fabrication method for auxetic foams has been examined and developed further. Secondly, the microstructure of the foams has been characterised using electron and optical microscopy. Thirdly, the foams have been tested for their static and dynamic mechanical properties. Comparative tests have also been performed on conventional, non-auxetic foams. Fourthly, theoretical models have been developed to relate the observed microstructures to their properties.
34
Khan, A. A. "Mechanical properties of fruit and vegetables". Thesis, University of Reading, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.234479.
Testo completo
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Arce, GarciÌa Isabel. "Mechanical properties of fullerene-like CN_x". Thesis, University of Newcastle Upon Tyne, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.421175.
Testo completo
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Gardham, Louise Marie. "Dynamic mechanical properties of polymer composites". Thesis, University of Leeds, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.395322.
Testo completo
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Gal, Julianna Mary. "Mechanical properties of mammalian intervertebral joints". Thesis, University of Leeds, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305848.
Testo completo
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Chien, H. H. "The mechanical properties of aluminide coatings". Thesis, Cranfield University, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.352970.
Testo completo
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Gandhi, Sunil Kumar. "Quantum mechanical properties of the supermembrane". Thesis, Imperial College London, 1989. http://hdl.handle.net/10044/1/47443.
Testo completo
40
Lee, Hyungsuk. "Mechanical properties of F-actin network". Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/50588.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Includes bibliographical references.
Cells sense, generate and respond to forces in their surroundings through cytoskeletal dynamics. Actin, the most abundant protein found in eukaryotic cells, is organized into various cytoskeletal structures that provide physical support for the cell and play important roles in numerous cellular processes. Assembly of F-actin into higher-order structures is regulated by over 100 actin binding proteins (ABPs). Although extensive measurements to estimate the mechanical properties of ABP/F-actin networks showed that they are nonlinear and viscoelastic, a full understanding of the origin of such fascinating behaviors is lacking. This thesis presents a multi-scale approach to identify the factors that determine the mechanical properties of F-actin networks from the macroscopic level to the single-molecule level. The mechanical properties of F-actin networks were probed by passive and active methods using optical tweezers. For the passive approach the thermal fluctuations of colloidal spheres are monitored to estimate the frequency-dependent complex shear modulus of an F-actin network. In the active approach, the response of an embedded microsphere to a driving force is tracked to obtain the strain-dependent viscoelasticity. The developed methods were applied to F-actin networks cross-linked with various ABPs such as filamin and a -actinin, with and without gelsolin to control filament length. Microstructures of those networks were also characterized in terms of filament length, mesh size, and degree of bundling.
(cont.) Comparison between cross-linked F-actin with two different length scales of actin filament suggested that network connectivity is another critical parameter in determining mechanical properties. To better understand how the cross-linking protein responds to an external force, a single molecule assay was used to measure the rupture force of a complex formed by an ABP filamin linking two actin filaments. Both force-induced unbinding and unfolding of filamin were observed at the critical force of 70 ± 23pN and 57 ± 19pN, respectively, although unbinding occurred more frequently. Similar pulling experiments were also performed on cross-linked F-actin networks and an abrupt transition was observed in the force trace indicating network rupture. The critical forces at transitions exhibited a similar loading-rate dependence to that observed for rupture forces in the single molecule measurements. Nonlinear behavior observed in strain-dependent microrheology was found to be irreversible. Combined results of molecular unbinding, network rupture, and irreversible network properties suggest that unbinding rather than unfolding is a dominant mechanism governing the mechanical properties of cross-linked F-actin networks. In addition, the mechanical behavior of F-actin networks subjected to an external prestress was investigated using a shear device. Visualization of sheared F-actin networks showed the structural evolution including mesh deformation, filament alignment, and network rupture.
(cont.) Measurement of mechanical properties as a function of external strain demonstrated that some regions exhibited strain-hardening while the others showed strain-softening. Aligned stretching of actin filaments observed at high strain seemed to play a role in strain-stiffening. By comparing the behaviors of an F-actin network cross-linked with wildtype and mutant FLNa, it was demonstrated how molecular structure of the ABP alters the mechanical behavior of F-actin network.
by Hyungsuk Lee.
Ph.D.
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Müller-Nedebock, Kristian Kurt. "Statistical mechanical properties of polymer networks". Thesis, University of Cambridge, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.627493.
Testo completo
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Comley, Kerstyn Sigerith Clara. "The mechanical properties of adipose tissue". Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608929.
Testo completo
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Morris, Julia Kathleen. "Mechanical properties of phospholipid coated microbubbles". Thesis, University of Edinburgh, 2014. http://hdl.handle.net/1842/9979.
Testo completoAbstract (sommario):
Phospholipid coated, inert gas filled microbubbles (MBs) are currently in widespread use in medical applications for the enhancement of diagnostic ultrasound images, and they are promising candidates for use in the area of targeted drug/gene delivery and uptake. As phospholipid coated MBs were developed for use with diagnostic ultrasound, their behaviour under acoustic loading is well investigated, however much less is known about their response to direct mechanical loading, which will potentially prove important as the range of uses of MBs expands. This is particularly true of the existing commercially available MB products. In this thesis, atomic force microscopy (AFM) was used to investigate the mechanical behaviour of three types of commercially produced phospholipid coated MBs, Definity®, BR14 and Sonovue®, at small deformations. Force spectroscopy was used to produce force-deformation (F-Δ) curves showing how the MBs deform under mechanical loading. Definity® MBs were deformed with tipless cantilevers at high deformations (though still less than 30% of the initial height of the MB); BR14 and Sonovue® MBs were probed with both tipless and tipped cantilevers to investigate both whole-bubble deformation and also shell indentation. BR14 was limited to low deformations; Sonovue® included both low and high deformations. The F-Δ curves were used to evaluate MB stiffness and also in combination with up to four mechanical models to predict the Young’s modulus of the MBs. The suitability of Reissner, Hertz, Elastic Membrane and De Jong theories for the prediction of the Young’s modulus of the MBs was explored. In the case of Definity® MBs no correlation between MB size and stiffness was observed; however an unexpected size dependence was observed in the Young’s modulus values, possibly due to variations in the thickness of the phospholipid shell. The membrane stretching component of elastic membrane theory was found to be the most applicable model on these MBs in this higher deformation regime. However, in this regime, gas compressibility could play a role and this is not included in the model. We studied the mechanical properties of BR14 MBs at very low deformations using ‘soft’ cantilevers. In this regime, gas compressibility should play a minimal role and there are several mechanical models which may be used. These MBs demonstrated decreasing stiffness with increasing diameter, and little variation in Young’s modulus with diameter. Hertz and De Jong theories showed more realistic Young’s modulus values (compared to other models) with little observable trend. Sonovue® MBs were used for a more comprehensive study of the small and very small deformation regimes using ‘soft’, ‘hard’ and tipped cantilevers. They showed no definitive trend in MB stiffness with MB diameter. Hertz and De Jong theory were again found to be most suitable. Analysis of curves acquired with tipped cantilevers indicated that the stiffness of a localised area of the shell membrane is similar to the overall stiffness of the MB and that the apparent Young’s modulus of the membrane according to the Hertz theory is also similar to that of the MB as a whole. Generally, considering all systems, Reissner theory was found to produce large overestimates of Young’s modulus, exceeding expected values by several orders of magnitude. Hertz and De Jong theories produced underestimates, though by a much smaller margin. Elastic membrane theory worked well and produced realistic Young’s modulus values only at relatively high deformation (the stretching term) in spite of the fact that gas compressibility is not taken into account. The suitability of the models is therefore very dependent on the deformation regime of the experiment. It seems that there is scope for better models at low deformation taking into account the soft shell of the MB and possibly its specific structure. Precise structural information of the MB shells does not exist; it is not trivial to attain and should certainly be a topic of future work with additional instrumentation.
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Bin, Kamaruddin Shamsul. "Long-term mechanical properties of rubber". Thesis, University of Southampton, 2013. https://eprints.soton.ac.uk/360430/.
Testo completoAbstract (sommario):
Natural rubber has a good potential to be used as a material for the development of wave energy converters (WECs). Generally, rubber has the ability to withstand very large strains without permanent deformation or fracture and is not much affected by exposure to water. This makes it ideal for applications related to wave energy converter (WECs). However, there is a need to predict the efficiency of performance over the full lifetime of such an application given that WECs will represent large, expensive and novel products that must remain operational in an at-sea environment for 10-15 years. Pertaining to that criterion, fracture mechanics of rubber is an important aspect as well as strain-history and environmental effects. The objective of this study is to gain a fundamental understanding of several factors that contribute to service lifetime: the effect of ozone and oxidation, stress-strain behaviour including hysteresis, set, and cyclic stress relaxation, mechanical fatigue and the possible role of poorly dispersed filler agglomerates in nucleating failure. The work includes studies of the effect of protective coating layers and of naturally aged rubber artefacts using a mathematical model for diffusion limited oxidation. A literature review has been performed to address rubber science and associated technology, including, the durability of rubber. Experiments encompassing the effect of ozone, cyclic stress-strain behaviour and crack growth & fatigue of rubber were performed to interpret the relevant properties for the consideration of the development of wave energy converters. New parameters for characterizing macro dispersion in rubber are introduced in an attempt to seek a correlation with the life span of rubbers. All these observations and outcomes give an opportunity to enhance our understanding of the factors that determine long-term mechanical properties of rubber, to the general benefit of rubber science and technology.
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Drozdetski, Aleksander Vladimirovich. "Unexpected mechanical properties of nucleic acids". Diss., Virginia Tech, 2016. http://hdl.handle.net/10919/71660.
Testo completoAbstract (sommario):
Mechanical deformations of nucleic acids (NA) play a very important role in many biological life processes. The bending persistence length of DNA is of specific interest, because so much eukaryotic DNA that stores genetic information is tightly packed inside cell nuclei, even though DNA is considered to be a relatively stiff biopolymer. However, recent experiments suggest that DNA may be more flexible than its persistence length (~ 150 bp or ~ 47 nm) suggests, especially for fragments shorter than 100 bp. It is important to reconcile these two seemingly competing pictures of DNA bending by providing a model that can explain the novel results without discrediting old experiments and the widely-accepted worm-like chain model. Another factor that influences both molecular geometry as well as mechanical properties is the ionic atmosphere surrounding the NA. It is known that multivalent ions with charge of +3e and higher can condense DNA into aggregates at high enough concentration. However, most conventional models cannot explain why RNA and DNA condense at different concentrations. Furthermore, our recent simulation results suggest that even though DNA persistence length decreases with multivalent ion concentration due to increasing electrostatic screening, RNA actually becomes stiffer due to a structural transition from the internal binding of the counterions.Ph. D.
46
Palos, Artemio. "Mechanical Properties of Polymer Modified Mortar". Thesis, University of North Texas, 2002. https://digital.library.unt.edu/ark:/67531/metadc3173/.
Testo completoAbstract (sommario):
The mechanical properties of the polymer-modified mortar are markedly improved over conventional cement mortar. We utilized recycled ABS in powder form and a polymer latex emulsion, polymer percentage ranges from 0 to 25 percent by polymer/cement ratio were investigated. The mechanical properties investigated were compression strength and adhesion strength. Compression strength effects did not have an impact on adhesion strength. Adhesion strength was calculated with pullout testing apparatus designed by the author. Results indicate that recycled ABS had a lower adhesive strength than the acrylic latex emulsion and the base mortar, but did increase in adhesive strength when mixed with maleic-anhydride. The adhesive strength was investigated for a Fiber Reinforced Polymer (FRP) made of an "E" glass fiber that is a continuous strand roving oriented and pre-tensioned longitudinally in an isopthalic polyester matrix material. The FRP rebar was compared to standard steel rebars, and found that the standard steel corrugated rebar had a higher adhesive strength, due to mechanical interlocking. This was clarified by measurements using a smooth steel rebar. Characterization of the polymer-modified mortar was conducted by pore analysis and scanning electron microscopy. Scanning Electron Microscopy was implemented to view the polymer particles, the cement fibrils formed by the hydration, and to prove Ohama's theory of network structure.
47
Soliman, Hazem. "Mechanical Properties of Cellular Core Structures". Diss., Virginia Tech, 2016. http://hdl.handle.net/10919/70456.
Testo completoAbstract (sommario):
Cellular core structures are the state-of-the-art technology for light weight structures in the aerospace industry. In an aerospace product, sandwich panels with cellular core represent the primary structural component as a given aerospace product may contain a large number of sandwich panels. This reveals the necessity of understanding the mechanical behavior of the cellular core and the impact of that behavior on the overall structural behavior of the sandwich panel, and hence the final aerospace product. As the final aerospace product must go through multiple qualification tests to achieve a final structure that is capable of withstanding all environments possible, analyzing the structure prior to testing is very important to avoid any possible failures and to ensure that the final design is indeed capable of withstanding the loads. To date, due to the lack of full understanding of the mechanical behavior of cellular cores and hence the sandwich panels, there still remains a significant lack of analytical capability to predict the proper behavior of the final product and failures may still occur even with significant effort spent on pre-test analyses. Analyzing cellular core to calculate the equivalent material properties of this type of structure is the only way to properly design the core for sandwich enhanced stiffness to weight ratio of the sandwich panels. A detailed literature review is first conducted to access the current state of development of this research area based on experiment and analysis. Then, one of the recently developed homogenization schemes is chosen to investigate the mechanical behavior of heavy, non-corrugated square cellular core with a potential application in marine structures. The mechanical behavior of the square cellular core is then calculated by applying the displacement approach to a representative unit cell finite element model. The mechanical behavior is then incorporated into sandwich panel finite element model and in an in-house code to test the predicted mechanical properties by comparing the center-of-panel displacement from all analyses to that of a highly detailed model. The research is then expanded to cover three cellular core shapes, hexagonal cores made of corrugated sheets, square cores made of corrugated sheets, and triangular cores. The expansion covers five different cell sizes and twenty one different core densities for each of the core shapes considering light cellular cores for space applications, for a total of 315 detailed studies. The accuracy of the calculated properties for all three core shapes is checked against highly detailed finite element models of sandwich panels. Formulas are then developed to calculate the mechanical properties of the three shapes of cellular cores studied for any core density and any of the five cell sizes. An error analysis is then performed to understand the quality of the predicted equivalent properties considering the panel size to cell size ratio as well as the facesheet thickness to core thickness ratio.
The research finally expanded to understand the effect of buckling of the unit cell on the equivalent mechanical property of the cellular core. This part of the research is meant to address the impact of the local buckling that may occur due to impact of any type during the manufacturing, handling or assembly of the sandwich panels. The variation of the equivalent mechanical properties with the increase in transverse compression load, until the first folding of the unit cell is complete, is calculated for each of the three core shapes under investigation.Ph. D.
48
Trevett, Adrian S. "The mechanical properties of hydrogel polymers". Thesis, Aston University, 1991. http://publications.aston.ac.uk/9692/.
Testo completo
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Lawson, Nathaniel C. "Mechanical properties of dental impression materials". Birmingham, Ala. : University of Alabama at Birmingham, 2007. https://www.mhsl.uab.edu/dt/2008r/lawson.pdf.
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50
Ajwani, Anita. "Mechanical properties of bio-absorbable materials". Thesis, This resource online, 1994. http://scholar.lib.vt.edu/theses/available/etd-12042009-020133/.
Testo completo