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1
Dahlberg, Johan. "On the asperity point load mechanism for rolling contact fatigue". Doctoral thesis, Stockholm : Hållfasthetslära, Kungliga Tekniska högskolan, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4569.
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Cai, Feng. "Evaluation of the mechanism of hypersensitivity to contact lens preseratives". Thesis, McGill University, 1988. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=61754.
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Alfredsson, Bo. "A study on contact fatigue mechanisms". Doctoral thesis, Stockholm, 2000. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3028.
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Gallagher, Christopher T. "Contact force control for continuous scanning coordinate measuring machines". Thesis, Georgia Institute of Technology, 1995. http://hdl.handle.net/1853/17319.
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Yoshida, Mutsumi. "Mechanism of biomaterial adjuvant effect phenotype of dendritic cells upon biomaterial contact /". Diss., Available online, Georgia Institute of Technology, 2005, 2005. http://etd.gatech.edu/theses/available/etd-07152005-141108/.
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Thesis (Ph. D.)--Biomedical Engineering, Georgia Institute of Technology, 2006.Babensee, Julia, Committee Chair ; Andres Garcia, Committee Member ; Mary Marovich, Committee Member ; Barbara Boyan, Committee Member ; Elliot Chaikof, Committee Member ; Cheng Zhu, Committee Member.
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Torres, James Ph D. Massachusetts Institute of Technology. "Large gain amplification mechanism for piezoelectric actuators utilizing a rolling contact joint". Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/74948.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 55-56).
Due to the limited displacement of piezoelectric stack actuators, common practice is to use some form of displacement amplification mechanism. An efficient, heavy-duty displacement amplification mechanism for piezoelectric stack actuators is presented in this thesis. The displacement amplification gain is increased by a factor of more than 100 in a single stage by using a buckling mechanism combined with a novel rolling contact design. Unlike traditional flexure-type monolithic mechanisms, which are accurate but inefficient and fragile, the new mechanism consists of all rolling contact couples, providing high stiffness, durability and energy efficient characteristics. Furthermore, a new design of pre-loading mechanism using shape memory alloy doubles the possible cyclic work output and provides a desirable restoring force for constraining the rolling contact mechanism stably and efficiently. This mechanism is intended to be interfaced with a sinusoidal gear cam that acts as the load. The dynamics of the system are derived and are shown to be fifth order. Due to the significantly nonlinear amplification caused by the buckling phenomenon and the gear, the dynamics are run in simulation to gain insight into the dynamic performance of the actuator. There is shown to be an optimal speed at which to run the actuator to maximize the possible power output. Furthermore, due to the simple binary control significant benefits are achieved by varying the control timing based on the velocity to ensure the force and velocity of the output are in phase. Finally, a prototype was constructed to compare to the static model. The prototype had a peak to peak displacement of 6.8 mm, an amplification of over 150, and produced a peak charged force of 56 Newtons.
by James Torres.
S.M.
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Jon, Sundh. "On wear transitions in the wheel-rail contact". Doctoral thesis, KTH, Maskinkonstruktion (Avd.), 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-11563.
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Wear transitions in the wheel–rail contact are of increasing interest since the general trend in railway traffic is toward increased velocities and axle loads. Curving increases the risk of flanging, causing the contact to change from an almost pure rolling wheel tread–rail head contact to more of a sliding wheel flange–rail gauge contact on the high rail in curves. Under wheel flange–rail gauge contact conditions, wear transitions to severe or catastrophic wear will occur if the contact is improperly lubricated. Such a transition is the most undesirable transition in the wheel–rail contact, as it represents a very expensive operating condition for railway companies. The contact conditions responsible for this transition are very severe as regards sliding velocity and contact pressure, and thus place high demands on both the lubricant and the wheel and rail materials. The focus of this thesis is on the transitions between different wear regimes in a wheel–rail contact. Wear is discussed both in traditional tribological terms and in terms of the categories used in the railway business, namely mild, severe and catastrophic wear. Most of the work was experimental and was performed at the Royal Institute of Technology (KTH), Department of Machine Design. The effects of contact pressure, sliding velocity, and type of lubricant have been investigated, producing results that resemble those of other studies presented in the literature. The absence of research relating to the wheel flange–rail gauge contact is addressed, and it is concluded that a lubricant film must be present on rails in curves to prevent severe or catastrophic wear. The formulation of this lubricant can further increase its wear- and seizure-preventing properties. To obtain a deeper understanding of wear transitions, methods such as airborne particle measurement and electron microscopy have been used. Paper A presents the test methodology used to detect seizure and discusses the wear-reducing influence of free carbon in highly loaded contacts. Paper B presents the testing of seizure-initiating conditions for a range of environmentally adapted lubricants applied to wheel and rail materials; a transient pin-on-disc test methodology was used for the testing. Paper C presents the use of pin-on-disc methodology to study the wear-reducing effects of a wide range of lubricants. The best performing lubricant was a mineral oil containing EP and AW additives. Paper D relates wear rates and transitions to airborne particles generated by an experimentally simulated wheel–rail contact. The airborne particles generated varied in size distribution and amount with wear rate and mechanism. Paper E relates additional analysis techniques, such as FIB sectioning, ESCA analysis, airborne particle measurements, and SEM imaging of airborne wear particles, to the contact temperature.QC 20100721
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Amuzuga, Kwassi. "Damage mechanism related to plasticity around heterogeneous inclusions under rolling contact loading in hybrid bearings ceramic/steel". Thesis, Lyon, 2016. http://www.theses.fr/2016LYSEI154.
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La durée de vie des pièces mécaniques en contact est fortement affectée par la présence d'hétérogénéités dans le matériau, comme des renforts (fibres, particules), des précipités, des porosités, ou encore des fissures. Des hétérogénéités dures et de formes complexes peuvent créer des surcontraintes locales, initiatrices de fissures par fatigue à proximité de la surface de contact. Une analyse quantitative des surcontraintes créées par les hétérogénéités est nécessaire à la compréhension des mécanismes d'endommagement. Cette étude s'applique à des roulements de ligne d'arbre qui font partie des éléments critiques de moteurs en aéronautique. Elle vise à déterminer précisément la distribution du champ de pression sur l'aire effective de contact et à prédire le profil et l'évolution des champs de contraintes/déformations à chaque passage de la charge sur un volume élémentaire représentatif prenant en compte le gradient de dureté, la présence de carbures et l'existence des contraintes initiales d'origine thermochimique. Une partie de l’étude est consacrée au développement d’un solveur du problème de contact roulant élasto-plastique avec présence d’hétérogénéité par les méthodes semi analytiques assurant un excellent gain en temps et ressources de calculs. Ensuite, un algorithme homogénéisation a été conçu pour analyser le comportement effectif d’un massif élasto-plastique hétérogène sous indentation. Enfin une partie expérimentale est dédiée à la caractérisation microstructurale des aciers étudiés dans le but de déterminer leurs propriétés. Les analyses des résultats de cette étude concourent à soutenir que bien que les inclusions de particules non métalliques soient responsables de la haute résistance de ces matériaux, certaines d’entre elles (celles de longueur dépassant les dizaines de micromètre ou celles qui forment des chaines dans une direction particulière) deviennent, au cours des cycles de fatigue, les principales sources d’endommagement depuis l’échelle locale jusqu’à la rupture globale de la structureThe lifetime of contacting mechanical parts is strongly affected by the presence of heterogeneities in their materials, such as reinforcements (fibers, particles), precipitates, porosities, or cracks. Hard heterogeneities having complex forms can create local overstress that initiating fatigue cracks near the contact surface. The presence of heterogeneities influences the physical and mechanical properties of the material at microscopic and macroscopic scales. A quantitative analysis of the over-stresses generated by heterogeneities is necessary to the comprehension of the damage mechanisms. The present study is applied to rolling bearings which are the critical elements of the aero-engine's mainshaft. The performance required for these bearings, led SKF Aerospace to introduce a new technology of hybrid bearing with ceramic rolling elements on high-strength steels having experienced a double surface treatment (carburizing followed by nitriding). The study aims to precisely determine the pressure field distribution on the effective contact area and to predict the profile and the evolution of the stress/strain fields at each loading cycle on a representative elementary volume that takes into account the gradient of hardness, the presence of carbides and the existence of an initial compressive stress from thermochemical origin. A major part of this study is devoted to develop a heterogeneous elastic-plastic rolling contact solver, by semi-analytical methods ensuring an excellent saving of calculation time and resources. Thereafter, a homogenization algorithm was built to analyze the effective behavior of a heterogeneous elastic-plastic half-space subjected to an indentation loading. Finally, an experimental part is dedicated to the microstructure characterization of the studied steels with intent to determine their properties. A description of the carbides behavior inside the matrix during micro-tensile tests was carried out under SEM in-situ observation. In the scheme of all analyses conducted in the present work, it can be argued that, although the heterogeneities (such as carbides or nitrides) are responsible for the high resistance of the studied materials, some of them (those whose length exceeds tens of micrometer or those which form stringers in a particular direction) become, over fatigue cycles, the main sources of damage, from their local scale up to the macroscopic failure of the structure
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Lin, Yun Materials Science & Engineering Faculty of Science UNSW. "Contact deformation mechanism of complex carbon nitride and metal nitride based bi-layer coatings". Awarded by:University of New South Wales. Materials Science & Engineering, 2009. http://handle.unsw.edu.au/1959.4/44544.
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Hard wear-resistant coatings have been widely applied 10 cutting tools to increase their durability and improve tribological properties. Physical vapour deposited TiAIN-based coatings. used in dry cutting performances. Have shown excellent hot hardness and oxidation resistance. The main handicap of these coatings is their brittleness. Another type of coatings arc amorphous. rather than crystalline. for example those based on fullerene-like carbon nitride (CNx). Such coatings possess high elasticity. but relatively low hardness. In this study. bi-Iayer coatings. designed as 1I CNx-based outer layer and a TiAlN-based inner layer. were deposited on either a silicon substrate or a high speed steel M2 substrate. The CNx outer layer incorporated either Ti/AI or Cr to improve its hardness. Cr was also added to the TiAIN based inner layer in some of the coatings. Detailed microstructural characterization and nanoindentation. to assess contact damage. \\CTC carried out on these coatings. The results showed that the microstructure of these coatings plays a critical part in the contact response. The addition ofCr into the CNx layer improves the toughness of the CNx layer. It acts as an inhibitor to the propagation of shear cracks initiated from the inner TiAIN layer. The incorporation of Cr into the TiAIN-based inner layer results in refinement of grain size and solid solution hardening. subsequently, this hinders inter-columnar sliding. Which results in the plastic deformation occurring lit a higher load. The application of a Cr interlayer was found to enhance the adhesion strength between the coating and the substrate. It was found the thicker. and harder. coatings on the ductile steel substrate generated inclined and lateral cracking. In contrast. thinner coatings initiated inter-columnar sliding and shear cracking followed by substrate plastic now. In turn. significant edge cracking (circumferential cracking) occurred in the CNx outer layer along the periphery of indent. It was also noted that nano-indentation testing using both Berkovich and spherical indenters produced different mechanical response and deformation microstructures in the coatings deposited on silicon substrates.
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Cain, Jason James. "Collision Analysis of the Reversible Crankshaft Mechanism in a Convertible Refrigeration Compressor". Thesis, Virginia Tech, 2000. http://hdl.handle.net/10919/33479.
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The purpose of this study is to analyze the behavior of a reversible two-cylinder refrigerant compressor manufactured by Bristol Compressor Incorporated. This compressor contains a specialized linkage that causes the compressor to transition from a two-cylinder compressor to a single-cylinder compressor when the direction of rotation of the crankshaft is reversed. The linkage accomplishes this by reducing the throw of one cylinder to zero. Of interest are the conditions to which this linkage is subjected when the direction of rotation is again reversed, causing the compressor to return to its two-cylinder functioning. When this reversal takes place, a collision occurs within the linkage. These repeated collisions are thought to be the cause of fatigue failure of the linkage in many of these compressors.
To verify that this collision is the problem, an understanding of the stress state during the collision is needed. This thesis begins the work necessary to determine the dynamic stress state present within the system.
A FORTRAN program was developed that modeled the kinematic behavior of the system under operating conditions. The program predicts the accelerations, velocities, positions, and internal forces present within the system during startup conditions.
Also, a method has been developed to model rotary sliding contact between two cylindrical surfaces. This method is developed and investigated in hopes that it will facilitate the modeling of the behavior of the compressor linkage in a dynamic finite element analysis.Master of Science
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Hannes, Dave. "Modelling of surface initiated rolling contact fatigue crack growth using the asperity point load mechanism". Licentiate thesis, KTH, Hållfasthetslära (Avd.), 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-34005.
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Oberhardt, Tobias. "A micromechanical model for the nonlinearity of microcracks in random distributions and their effect on higher harmonic Rayleigh wave generation". Thesis, Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/54365.
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This research investigates the modeling of randomly distributed surface-breaking microcracks and their effects on higher harmonic generation in Rayleigh surface waves. The modeling is based on micromechanical considerations of rough surface contact. The nonlinear behavior of a single microcrack is described by a hyperelastic effective stress-strain relationship. Finite element simulations of nonlinear wave propagation in a solid with distributed microcracks are performed. The evolution of fundamental and second harmonic amplitudes along the propagation distance is studied and the acoustic nonlinearity parameter is calculated. The results show that the nonlinearity parameter increases with crack density and root mean square roughness of the crack faces. While, for a dilute concentration of microcracks, the increase in acoustic nonlinearity is proportional to the crack density, this is not valid for higher crack densities, as the microcracks start to interact. Finally, it is shown that odd higher harmonic generation in Rayleigh surface waves due to sliding crack faces introduces a friction nonlinearity.
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Hannes, Dave. "On fatigue crack growth modelling of surface initiated rolling contact fatigue using the asperity point load mechanism". Doctoral thesis, KTH, Hållfasthetslära (Avd.), 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-141151.
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Load transfer in applications or between machine components is generally achieved through contact. In case of recurrent high contact loads in combination with a rolling motion, i.e. with a relatively small amount of slip, the contact surface may eventually suffer from rolling contact fatigue (RCF). The damage consists then of cracks and craters or spalls, which can cause dysfunctionality of the application leading to inefficiency or increased maintenance costs. Ultimately the damage may cause total failure of the machine component. The damage process is still not fully understood due to the complexity of the problem. Different mechanisms have been suggested to explain initiation and propagation of RCF damage. The current work focused on crack growth modelling of surface initiated RCF in case hardened gear steel. The study was based on the asperity point load mechanism, which emphasizes the importance of the surface roughness in the damage process. Asperities on the contact surface act as stress raisers inducing locally high tensile surface stress when entering the contact. Improved understanding of the damage process and further validation of the asperity point load mechanism was achieved. In Paper A, the crack path of surface initiated RCF was simulated in the symmetry plane of the damage with the trajectory of the largest principal stress in the uncracked material. The mode I fracture mechanism was found applicable as well as linear elastic fracture mechanics (LEFM). The evolvement of the asperity contact parameters during the load cycle was determined through a finite element (FE) contact model based on an equivalent contact geometry. The predicted RCF crack path agreed with experimental spall profiles both in entry details as in overall shape. An experimental series was performed in Paper B to investigate the crack closure behaviour in presence of large negative minimum loads. The experimental results suggested a crack closure limit close to zero. The choice of the equivalent mixed-mode stress intensity factor range and especially the crack closure limit had a significant effect on the predicted RCF or spalling life. The two-dimensional crack growth model was further developed in Paper C and used to investigate the influence of asperity size, friction and residual surface stress on the simulated RCF damage. The simulations agreed qualitatively with experimental observations where reduced surface roughness, improved lubrication and compressive residual surface stress increased RCF resistance. In Paper D, a three-dimensional stationary crack was studied using an FE model and a simplified RCF load. A new crack geometry was proposed allowing the investigation of the spall opening angle of the typical vshaped damage. Crack arrest through crack closure was suggested as explaining mechanism. A qualitative study indicated increased spread of the surface damage with increased friction. The results also depended on the crack inclination angle. The different studies supported the asperity point load mechanism to explain not only fatigue initiation but also fatigue crack propagation.QC 20140210
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Ngan, Choi-chik, e 顔才績. "A hidden Markov model approach to force-based contact recognition for intelligent robotic assembly". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2002. http://hub.hku.hk/bib/B31243496.
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Javadi, Ali. "METHACRYLATED POLY(ETHYLENE GLYCOL)S AS PRECURSORS FOR SUPERPLASTICIZERS AND UV-CURABLE ELECTRICAL CONTACT STABILIZATION MATERIALS". University of Akron / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=akron1508331641641639.
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Cannon, Jesse R. "Compliant Mechanisms to Perform Bearing and Spring Function in High Precision Applications". BYU ScholarsArchive, 2004. https://scholarsarchive.byu.edu/etd/229.
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An increasing number of mechanical systems are being designed on the micro and meso scales. Assembly and maintenance become increasingly difficult as the size of mechanisms decrease, and the minimum size of traditional elements such as bearings and springs is limited. The backlash of bearings also limits their usefulness in applications where high precision and repeatability are needed. At small scales and for high precision applications, alternative, non-traditional elements are needed. The objective of this thesis is to develop reliable and scalable compliant components to replace bearings and helical springs. Components replacing springs must be able to produce specified torque/motion requirements. Components replacing bearings must permit sufficient motion about the axis of rotation, bear specified loads in the lateral directions, and fit within roughly the same design space as a bearing. Additionally, all components will be designed to be manufactured using in-plane fabrication processes. Practical application of the components will be demonstrated by their use in Sandia National Laboratory's Stronglink assembly. The concepts discussed in this thesis fall into three categories: mechanisms that replace 1) the helical spring, 2) the bearing, and 3) both the helical spring and the bearing. The serpentine flexure belongs to the first category, the compliant rolling-contact element (CORE), CORE bearing, and elliptical CORE bearing belong to the second, and the compliant contact-aided revolute (CCAR) joint belongs to the third category.
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Barksby, Helen Emma. "Bax interactions with VDAC-ANT mitochondrial intermembrane contact sites during apoptosis : implications for a mechanism of outer mitochondrial membrane permeabilisation". Thesis, University College London (University of London), 2005. http://discovery.ucl.ac.uk/1444510/.
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The Bcl-2 family of proteins regulates the mitochondrial apoptotic pathway and consists of both pro- and anti-apoptotic members. Bax-cc is present in the cytosol of healthy cells. Apoptotic stimuli induce the translocation of Bax-oc to the mitochondria leading to the permeabilisation of the outer mitochondrial membrane (OMM) and the release of downstream pro-apoptotic proteins from the intermembrane space (IMS). The mechanism by which Bax permeabilises the OMM remains unclear. Recent evidence suggests Bax alone might be sufficient to permeabilise the OMM. However, other models indicate the involvement of the mitochondrial permeability transition pore complex or the voltage-dependent anion channel (VDAC) present in the OMM. In this study Bax and C-terminally truncated Bax were expressed as GST-fusion proteins and were immobilised on agarose-GSH. The binding of mitochondrial membrane proteins that might be involved in the Bax mediated release of proteins from the IMS was investigated. The results showed that VDAC and the adenine nucleotide translocase (ANT) were retained by GST-Bax. Exogenous cyclophilin D (Cyp D) was added in the presence of VDAC and ANT and was also retained indicating that Bax interacts with the components of the permeability transition pore complex. The anti-apoptotic Bcl-2 protein Bcl-XL was expressed with a hexahistidine tag at its N-terminus. This was used to investigate its effects on Bax interaction with VDAC and ANT. The ANT ligands atractyloside and bongkrekic acid which promote and inhibit apoptosis respectively were shown to change the relative amounts of VDAC and ANT that bind GST-Bax. Apoptotic cell death has been identified in cardiomyocytes subjected to ischaemia. In this investigation cardiomyocytes transfected with GFP-Bax were treated with cyanide to simulate ischaemia and GFP-Bax translocation was observed using fluorescence microscopy. GFP-Bax co-immunoprecipitated with VDAC and ANT after translocation to mitochondria but not with Cyp D. The implications of these findings are discussed in this thesis.
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Parel, Kurien Stephen. "An analysis of contact stiffness and frictional receding contacts". Thesis, University of Oxford, 2017. http://ora.ox.ac.uk/objects/uuid:3c29863a-b0cf-4870-851d-261be72f457f.
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The tangential contact stiffness for ground Ti-6Al-4V surfaces is measured to linearly decrease with the application of tangential load. At the beginning of the application of tangential load, for ground surfaces, the ratio of the tangential contact stiffness to the normal contact stiffness is seen to be approximately half the Mindlin ratio. This is consistent with many other published experimental studies. Measurements of normal contact stiffness for ground surfaces conform to a model that posits a linear relationship between normal contact stiffness and normal load. An equivalent surface roughness parameter is defined for two surfaces in contact; and the normal contact stiffness for ground surfaces is observed to be inversely proportional to this parameter. Single asperity models were constructed to simulate the effect of different frictional laws and plasticity on the tangential displacement of an asperity contact. Further, multi-asperity modelling showed the effect of different normal load distributions on the tangential behaviour of interfaces. In addition, normal contact stiffness was modelled for a grid of asperities taking into account asperity interactions. A receding contact problem for which the required form of the distributed dislocations is bounded-bounded was solved. Then, a fundamental 2D frictional receding contact problem involving a homogeneous linear elastic infinite layer pressed by a line load onto a half-plane of the same material was analysed. This was done by the insertion of preformed distributed dislocations (or eigenstrains), which take into account the correct form of the separation of the interface at points away from the area of loading, along with corrective bounded-bounded distributions. The general method of solution was further refined and adapted to solve three other receding contact problems. The solutions demonstrated the robustness and applicability of this new procedure.
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Schiwy, Sabrina [Verfasser]. "Development and application of sediment contact bioassays for the investigation of acute and mechanism specific toxicity with Danio rerio embryos / Sabrina Schiwy". Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2014. http://d-nb.info/1067265228/34.
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Everitt, Carl-Magnus. "Initiation of rolling contact fatigue from asperities in elastohydrodynamic lubricated contacts". Licentiate thesis, KTH, Hållfasthetslära (Avd.), 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-222371.
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Rolling contacts are utilized in many technical applications, both in bearings and in the contact between gear teeth. These components are often highly loaded, which makes them susceptible to suffer from rolling contact fatigue. This work focuses on the rolling contact fatigue mechanism of pitting. In order to attain a better understanding of why pitting initiates and grows, detailed simulations of rolling contacts have been performed. In particular the contact between two gears in a truck retarder was here used as a case study. The investigated contact experienced elastohydrodynamic lubrication conditions since the load was high enough to causes the surfaces in contact to deform and the viscosity of the lubricant to increase significantly. In Paper A it was investigated if surface irregularities in the size of the surface roughness are large enough to cause surface initiated fatigue. The investigation focused on the pitch line since small surface initiated pits were found here even though there was no slip present. Since there were pits present at the pitch line, it is important that the theories of pitting can explain the development of pits also in the absence of slip. The conclusion of the work was that surface irregularities of the size of normal surface roughness are enough to cause surface initiated fatigue at the pitch line. In Paper B it was investigated why pits are more likely to initiate in the dedendum of pinion gears than in the addendum. In both areas slip is present but in different directions. In the dedendum the friction from slip is against the rolling direction which enhances the risk for pitting. The investigation was performed by studying the effect of the temperature rise in the contact caused by the slip. The conclusion drawn was that the temperature rise in the contact explained why pitting was more common in the dedendum than in the addendum.Rullande kontakter förekommer i många applikationer, till exempel i lager och mellan kugghjulständer. Både lager och kugghjul utsätts ofta för höga laster vilket gör att dess ytor löper stor risk att utmattas, vilket kallas rullande kontaktutmattning. Denna studie fokuserar på pitting, även kallat spalling, vilket är en typ av rullande kontaktutmattning där en utmattninsspricka växer fram som får delar av ytan att ramla av. För att få en bättre förståelse varför pittingskador uppkommer har noggranna simuleringar utförts av rullande kontakter. Kontakten mellan två tänder på kugghjul i en lastbilsretarder har används som underlag då många pittingskador påträffats på dem. För att minska friktionen och nötningen i kontakten mellan kuggtänderna användes smörjmedel. De höga lasterna lastbilsretardern utsattes för deformerade kuggarnas ytor elastiskt samtidigt de kraftigt ökade viskositeten hos smörjmedlet. Dessa förhållanden gör att kontakten kallas för elastohydrodynamiskt smord, vilket på engelska förkortas till EHL. I Artikel A undersöktes om små ytojämnheter kan orsaka ytinitierade pittingskador. Eftersom skadan påträffats i friktionslösa kontakter, så som vid rullcirkeln på de undersökta kugghjulen, är det viktigt att teorierna som förklarar uppkomsten inte är beroende av friktion. Undersökningen fokuserade därför på förhållandena vid rullcirkeln. Slutsatsen från arbetet var att små ytojämnheter, av samma storleksordning som ytojämnheterna på de undersökta kugghjulen, är tillräckligt stora för att orsaka utmattningsskador. I Artikel B undersöktes varför det är vanligare att pitts initieras i dedendum än addendum på drivande kugghjul. Kontakten på båda sidorna om rullcirkeln slirar svagt åt olika håll. Att kontakten slirar skapar friktion som är motriktad rullriktningen i dedendum vilket ökar risken för pittingskador. För att undersöka varför dessa förhållanden ökar risken för skador fördjupades analysen av kontakten genom att inkludera temperaturfältet. Simuleringarna visade att temperaturen ökar genom kontakten vilket orsakar en asymmetrisk spänningsfördelning. Denna asymmetriska spänningsfördelning gör att ytojämnheter i dedendum är troligare att orsaka skador än ytojämnheter i addendum.
QC 20180213
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Pickert, Janko. "Untersuchungen zur Bindung kontaktallergener Substanzen an nukleophile Aminosäureseitenketten". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2004. http://nbn-resolving.de/urn:nbn:de:swb:14-1104831124156-50838.
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In der heutigen Zeit sind ca. 4000 Verbindungen bekannt, denen die Fähigkeit nachgesagt wird, eine Kontaktallergie auslösen zu können. Die Entscheidung, ob ein Stoff hautsensibilisierende Eigenschaften besitzt, wird dabei meist auf der Grundlage von Beobachtungen am Menschen und/oder von tierexperimentellen Befunden getroffen. Bedingt durch den vermehrten Einsatz exotischer Pflanzen und neu entwickelter synthetischer Substanzen im Bereich der Kosmetikindustrie besteht der Bedarf an einer einfachen Methode zur Vorhersage des kontaktallergenen Potentials einer Verbindung. Als zentraler Schritt bei der Manifestierung einer Kontaktallergie wird die Bildung eines Hapten-Carrier-Komplexes aus dem niedermolekularen Kontaktallergen (Hapten) und Hautproteinen (Carrier) angesehen. Zur Abschätzung der Sensibilisierungsfähigkeit kann daher auch die Reaktivität der Substanz oder ihrer Metaboliten gegenüber Proteinen herangezogen werden. Im Rahmen dieser Arbeit werden neben dem bedeutenden Phytoekzematogen Tulipalin A die bisher wenig untersuchten kontaktallergenen Duftstoffe Geraniol, 7-Hydroxycitronellal, Benzaldehyd, Salicylaldehyd, Vanillin, Zimtaldehyd, a-Amyl-zimtaldehyd und Benzylcinnamat hinsichtlich ihrer Reaktivität gegenüber peptidgebundenen Aminosäuren untersucht. Verwendet werden für diese Modellversuche Hippuryl-Derivate und Acetyl-Derivate der Aminosäuren Lysin, Histidin, Arginin bzw. Cystein sowie zusätzlich Glutathion. Die dabei gewählten Bedingungen sollen eine Adaption an physiologische Gegebenheiten erlauben. Ziel ist es, zu klären, ob die mit diesen Modellversuchen zu gewinnenden Ergebnisse mit dem bekannten kontaktallergenen Potential der eingesetzten Haptene korrelieren und somit geeignet sind, die Sensibilisierungsfähigkeit einer Substanz vorherzusagen. Über Konjugationsprodukte von Kontaktallergenen mit Peptiden oder Proteinen ist in der Literatur sehr wenig bekannt. Daher ist es ein weiteres Ziel dieser Arbeit, individuelle Produkte der Reaktionen der kontaktallergenen Substanzen mit den nukleophilen Aminosäureseitenketten zu isolieren und zu charakterisieren, um so definierte Hapten-Carrier-Konjugate, die unter physiologisch relevanten Bedingungen entstehen können, zu beschreiben. Aufbauend auf den gefundenen Strukturen sollte es auch möglich sein, Hinweise auf eventuelle Reaktionsmechanismen zu erhalten.
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Delebarre, Corentin. "Etude expérimentale des systèmes d'étanchéité de type labyrinthe pour turbomachine par l'étude du contact grande vitesse". Thesis, Toulouse, INPT, 2015. http://www.theses.fr/2015INPT0130.
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Une des solutions envisagées par les motoristes pour améliorer le rendement des turbomoteurs consiste à réduire le jeu en fonctionnement dans les deux systèmes d’air des turbomachines. Plus particulièrement dans le système d’étanchéité d’air secondaire, la réduction du jeu entre les parties fixes et tournantes améliore le contrôle des étanchéités du moteur et les niveaux de refroidissement entre les différents modules des turbomoteurs, mais entraînent des interactions indésirables entre les différents composants. Les systèmes d’étanchéité dynamiques sont composés de joints labyrinthes couplés à un revêtement abradable sacrificiel en vis-à-vis, qui offre la particularité de s’user préférentiellement en préservant la majeure partie du système d’étanchéité en évitant une usure de la partie tournante. L’objectif de cette thèse est de reproduire et d’étudier, par l’intermédiaire d’un banc technologique haute vitesse spécialement conçu pour l’étude, les interactions labyrinthe-abradable appliquées au couple acier inoxydable/Al-Si 6%, dans des conditions de fonctionnement d’un turbomoteur. Une instrumentation spécifique est développée sur le banc et apporte à l’étude les données expérimentales manquantes visant à la caractérisation des interactions labyrinthe-abradable. Une analyse tribologique, basée sur le concept de troisième corps, est menée afin d’identifier les différents débits de matière puis de décrire les étapes successives des mécanismes d’usure (circuit tribologique) du revêtement Al-Si 6%. L’influence des paramètres de pilotage de l’interaction et de la géométrie du joint a été étudiée. Enfin, une étude statistique basée sur l’iconographie des corrélations, permet de modéliser le comportement global du système labyrinthe-abradable, d’identifier les paramètres influents du système et d’apporter des critères pour une potentielle surveillance des interactions. Ces travaux de thèse ont été réalisés dans le cadre d’une collaboration entre TURBOMECA Bordes (groupe SAFRAN), et le Laboratoire Génie Production (LGP) de TarbesS of turbomachinery. Especially in the secondary air sealing system, the tight clearance between the stationary and rotating parts improves control of engine seals and cooling levels between the different modules of the engines but may cause undesirable interactions between the static and rotating components. Dynamic sealing systems are composed of labyrinth seals coupled to a sacrificial abradable coating, which can accommodate interactions to preserve the global geometrical seal integrity. The objective of this thesis is to study and reproduce, through a technological high speed test rig specifically designed for the study, the labyrinth-abradable interactions applied to stainless steel/Al-Si 6% couple, under similar turbo-engine operating conditions. A suitable instrumentation is developed on the test ring to complete missing experimental data to characterize abradable labyrinth interactions. A tribological study, based on the third body concept, is proposed to identify wear mechanisms process of the Al-Si 6% coating. The influence of interaction control parameters and the seal geometry was investigated. Finally, a statistical study, based on the iconography of the correlation, is proposed to model the overall labyrinth-abradable system behavior and to identify influential parameters of the system and provide suitable criteria to monitor labyrinth seal interactions. This work was achieved within the framework of cooperation between TURBOMECA Bordes (SAFRAN Group), and the Laboratoire Génie de Production (LGP) de Tarbes
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Cadieu, Marie-Claude. "Reactions de contact des octanes sur des catalyseurs a base de platine et de palladium". Université Louis Pasteur (Strasbourg) (1971-2008), 1988. http://www.theses.fr/1988STR13023.
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Etude des differences de comportements catalytiques entre des catalyseurs supportes pt et pd a partir d'un nouveau type d'hydrocarbure test: les octanes. Mecanisme de cyclisation et d'isomerisation des aliphatiques et d'hydrogenolyse des cyclopentanes
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Wang, Yu. "Multi-scale investigation and analysis of the friction behavior and wear in the manufacturing of the woven composite reinforcements". Electronic Thesis or Diss., Mulhouse, 2024. http://www.theses.fr/2024MULH7126.
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Dans le processus de fabrication des renforts textiles, la friction entre les fils torsadés joue un rôle clé dans les propriétés mécaniques des composites renforcés de fibres, une friction excessive pouvant les détériorer. Cette étude développe un cadre analytique pour comprendre ce comportement frictionnel. Deux modèles analytiques ont été introduits pour décrire la friction des fils, prenant en compte la dynamique de contact à grande échelle. Les validations expérimentales confirment l'efficacité des modèles, offrant une caractérisation détaillée de la friction dans différentes conditions et révélant l'influence de paramètres critiques sur le comportement des architectures textiles. En outre, un modèle numérique innovant a été développé pour prédire la friction et l'usure des fils, en intégrant des composantes géométriques et mécaniques. Ce modèle prend en compte l'endommagement des fibres selon la théorie de la poutre de Timoshenko et aborde les problèmes de pénétration via un auto-codage. Il a été validé par des résultats expérimentaux et une imagerie micro-CT, confirmant sa précision. Cette méthode fournit des informations précieuses sur la réponse mécanique des fils pendant la fabrication des renforts, permettant une compréhension approfondie des effets des paramètres géométriques et mécaniques sur la friction et l'usure. Des améliorations futures de ce modèle pourront affiner la caractérisation du comportement frictionnel lors de la formation des composites et ainsi optimiser les processus de fabrication pour prolonger la durée de vie des produits tout en réduisant les déchetsIn the manufacturing process of textile reinforcements, friction between twisted yarns is a critical factor that significantly influences the mechanical properties of fiber-reinforced composites. Excessive friction can deteriorate these properties. This study develops a comprehensive analytical framework to understand the friction behavior in the manufacturing of textile reinforcements. Two novel analytical models were introduced to describe friction behaviorof yarns. The models account for the full-scale contact dynamics influenced. Experimental validations confirm the model's efficacy by providing a detailed characterization of friction under various conditions, which reveal the influence of critical parameters on friction behavior of fabric architectures. Additionally, an innovative numerical model for predicting yarn friction and wear behavior was introduced, incorporating both geometric and mechanical components. A yarn model,considering fiber damage behavior based on the Timoshenko beam, was developed to explore friction and wear during reinforcement manufacturing. The model effectively addresses penetration problems through self-coding and was validated by experimental results and microCT imaging, confirming its accuracy. This method provides valuable insights into the mechanical response of yarns during reinforcement manufacturing, allowing for an in-depth understanding of the effect of yarn geometrical and mechanical parameters on friction and wear behavior. Future improvements can further characterize friction behavior during composite forming and optimize the manufacturing process, promoting longer product lifespans and reduced waste
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Vyplašil, Jan. "Návrh horizontální výměnné hlavy - VA1 –C". Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2011. http://www.nusl.cz/ntk/nusl-229757.
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Diploma thesis deals with the concept of interchangeable straight spindle head type VA1-C, used for automatic clamping in 80 kW spindle head from production of TOS Kuřim. Thesis contains survey of interchangeable heads, analysis of the spindle housing, strenght analysis of the spindle with the safety calculations in critical sec-tions and analysis of the clamping mechanism together with the calculation of the camping force, which holds the tools in the spindle. Analysis of the camping mechanisms contains calculation of the contact stress between balls and housings. All results are shown in the graphs. In the last part of the thesis are described the construction elements using the model of the interchangeable head. Enclosed is the assembly drawing of the spindle head, bill of material, detail drawing of the spindle and also 3D model of the interchangeable spindle head.
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Marston, Daniel John. "Mechanisms for EPHB4 regulated contact repulsion". Thesis, University College London (University of London), 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.407925.
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Haider, Christine Irmingard. "Particle contact mechanisms in pressure agglomeration". Thesis, University of Sheffield, 2014. http://etheses.whiterose.ac.uk/6995/.
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Ma, Lifeng. "Contact mechanics for coated systems". Thesis, University of Oxford, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.409112.
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Fathi, Ali. "Dégradation des aspérités des joints rocheux sous différentes conditions de chargement". Thèse, Université de Sherbrooke, 2015. http://hdl.handle.net/11143/8176.
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Résumé: L’objectif de cette thèse est d’interpréter la dégradation des aspérités des joints rocheux sous différentes conditions de chargement. Pour cela, la variation des aspérités durant les différentes étapes du cisaillement d’un joint rocheux est observée. Selon le concept appelé “tiny windows”, une nouvelle méthodologie de caractérisation des épontes des joints a été développée. La méthodologie est basée sur les coordonnées tridimensionnelles de la surface des joints et elles sont mesurées après chaque essai. Après la reconstruction du modèle géométrique de la surface du joint, les zones en contact sont identifiées à travers la comparaison des hauteurs des “tiny windows” superposées. Ainsi, la distribution des zones de la surface en contact, endommagées et sans contact ont été identifiées. La méthode d’analyse d’image a été utilisée pour vérifier les résultats de la méthodologie proposée. Les résultats indiquent que cette méthode est appropriée pour déterminer la taille et la distribution des surfaces du joint en contact et endommagées à différentes étapes du cisaillement. Un ensemble de 38 répliques ont été préparées en coulant du mortier sans retrait sur une surface de fracture obtenue à partir d’un bloc de granite. Différentes conditions de chargement, incluant des chargements statiques et cycliques ont été appliquées afin d’étudier la dégradation des aspérités à différentes étapes du procédé de cisaillement. Les propriétés géométriques des “tiny windows” en contact en phase pré-pic, pic, post-pic et résiduelle ont été analysées en fonction de leurs angles et de leurs auteurs. Il a été remarqué que les facettes des aspérités faisant face à la direction de cisaillement jouent un rôle majeur dans le cisaillement. Aussi, il a été observé que les aspérités présentent différentes contributions dans le cisaillement. Les aspérités les plus aigües (“tiny windows” les plus inclinées) sont abîmées et les aspérités les plus plates glissent les unes sur les autres. Les aspérités d’angles intermédiaires sont définies comme “angle seuil endommagé” et “angle seuil en contact”. En augmentant la charge normale, les angles seuils diminuent d’une part et, d’autre part, le nombre de zones endommagées et en contact augmentent. Pour un petit nombre de cycles (avec faible amplitude et fréquence), indépendamment de l’amplitude, une contraction apparaît ; par conséquent, la surface en contact et les paramètres de résistance au cisaillement augmentent légèrement. Pour un grand nombre de cycles, la dégradation est observée à l’échelle des aspérités de second ordre, d’où une baisse des paramètres de résistance au cisaillement. Il a été aussi observée que les “tiny windows” avec différentes inclinaisons contribuent au processus de cisaillement, en plus des “tiny windows” les plus inclinées (aspérités plus aigües). Les résultats de la méthode proposée montrent que la différenciation entre les zones en contact et celles endommagées s’avère utile pour une meilleure compréhension du mécanisme de cisaillement des joints rocheux.Abstract: The objective of the current research is to interpret the asperity degradation of rock joints under different loading conditions. For this aim, the changes of asperities during different stages of shearing in the three-dimensional joint surface are tracked. According to a concept named ‘tiny window’, a new methodology for the characterization of the joint surfaces was developed. The methodology is based on the three-dimensional coordinates of the joints surface that are captured before and after each test. After the reconstruction of geometric models of joint surface, in-contact areas were identified according to the height comparison of the face to face tiny windows. Therefore, the distribution and size of just in-contact areas, in-contact damaged areas and not in-contact areas are identified. Image analysis method was used to verify the results of the proposed method. The results indicated that the proposed method is suitable for determining the size and distribution of the contact and damaged areas at any shearing stage. A total of 38 replicas were prepared by pouring non-shrinking cement mortar on a fresh joint surface of a split granite block. Various loading conditions include monotonic and cyclic loading were applied to study the asperities degradation at different stages of shearing. The geometric properties of the in-contact tiny windows in the pre-peak, peak, post-peak softening and residual shearing stages were investigated based on their angle and height. It was found that those asperities facing the shear direction have the primary role in shearing. It is remarkable that different part of these asperities has their own special cooperation in shearing. The steepest parts (steeper tiny windows) are wore and the flatter parts (flatter tiny windows) are slid. The borderlines between these tiny windows defined as “damaged threshold angle” and “in-contact threshold angle”. By increasing normal load, both the amounts of threshold angles are decreased and contact and damaged areas increased. During low numbers of cycles (with low amplitude and frequency), independent of the type of cycle, contraction occurs and consequently the contact area and the shear strength parameters slightly increased. During larger number of cycles, degradation occurred on the second order asperities, therefore the shear strength parameters slowly decreased. It was also observed that tiny windows with different heights participate in the shearing process, not just the highest ones. The results of the proposed method indicated that considering differences between just in-contact areas and damaged areas provide useful insights into understanding the shear mechanism of rock joints.
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Votsios, Vasilis. "Contact mechanics and impact dynamics of non-conforming elastic and viscoelastic semi-infinite or thin bonded layered solids". Thesis, Loughborough University, 2003. https://dspace.lboro.ac.uk/2134/11815.
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The thesis is concerned with the contact mechanics behaviour of non-conforming solids. The geometry of the solids considered gives rise to various contact configurations, from concentrated contacts with circular and elliptical configuration to those of finite line nature, as well as those of less concentrated form such as circular flat punches. The radii of curvature of mating bodies in contact or impact give rise to these various nonconforming contact configurations and affect their contact characteristics, from those considered as semi-infinite solids in accord with the classical Hertzian theory to those that deviate from it. Furthermore, layered solids have been considered, some with higher elastic modulus than that of the substrate material (such as hard protective coatings) and some with low elastic moduli, often employed as tribological coatings (such as solid lubricants). Other bonded layered solids behave in viscoelastic manner, with creep relaxation behaviour under load, and are often used to dampen structural vibration upon impact. Analytic models have been developed for all these solids to predict their contact and impact behaviour and obtain pressure distribution, footprint shape and deformation under both elastostatic and transient dynamic conditions. Only few solutions for thin bonded layered elastic solids have been reported for elastostatic analysis. The analytical model developed in this thesis is in accord with those reported in the literature and is extended to the case of impact of balls, and employed for a number of practical applications. The elastostatic impact of a roller against a semi-infinite elastic half-space is also treated by analytic means, which has not been reported in literature. Two and three-dimensional finite element models have been developed and compared with all the derived analytic methods, and good agreement found in all cases. The finite element approach used has been made into a generic tool for all the contact configurations, elastic and viscoelastic. The physics of the contact mechanical problems is fully explained by analytic, numerical and supporting experimentation and agreement found between all these approaches to a high level of conformance. This level of agreement, the development of various analytical impact models for layered solids and finite line configuration, and the development of a multi-layered viscoelastic transducer with agreed numerical predictions account for the main contributions to knowledge. There are a significant number of findings within the thesis, but the major findings relate to the protective nature of hard coatings and high modulus bonded layered solids, and the verified viscoelastic behaviour of low elastic modulus compressible thin bonded layers. Most importantly, the thesis has created a rational framework for contact/impact of solids of low contact contiguity.
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Ross, G. M. "Ocular biotribology and contact lens lubrication mechanisms". Thesis, Aston University, 2009. http://publications.aston.ac.uk/10925/.
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The work described in this thesis is concerned with mechanisms of contact lens lubrication. There are three major driving forces in contact lens design and development; cost, convenience, and comfort. Lubrication, as reflected in the coefficient of friction, is becoming recognised as one of the major factors affecting the comfort of the current generation of contact lenses, which have benefited from several decades of design and production improvements. This work started with the study of the in-eye release of soluble macromolecules from a contact lens matrix. The vehicle for the study was the family of CIBA Vision Focus® DAILIES® daily disposable contact lenses which is based on polyvinyl alcohol (PVA). The effective release of linear soluble PVA from DAILIES on the surface of the lens was shown to be beneficial in terms of patient comfort. There was a need to develop a novel characterisation technique in order to study these effects at surfaces; this led to the study of a novel tribological technique, which allowed the friction coefficients of different types of contact lenses to be measured reproducibly at genuinely low values. The tribometer needed the ability to accommodate the following features: (a) an approximation to eye lid load, (b) both new and ex-vivo lenses, (c) variations in substrate, (d) different ocular lubricants (including tears). The tribometer and measuring technique developed in this way was used to examine the surface friction and lubrication mechanisms of two different types of contact lenses: daily disposables and silicone hydrogels. The results from the tribometer in terms of both mean friction coefficient and the friction profiles obtained allowed various mechanisms used for surface enhancement now seen in the daily disposable contact lens sector to be evaluated. The three major methods used are: release of soluble macromolecules (such as PVA) from the lens matrix, irreversible surface binding of a macromolecule (such as polyvinyl pyrrolidone) by charge transfer and the simple polymer adsorption (e.g. Pluoronic) at the lens surface. The tribological technique was also used to examine the trends in the development of silicone hydrogel contact lenses. The focus of the principles in the design of silicone hydrogels has now shifted from oxygen permeability, to the improvement of surface properties. Presently, tribological studies reflect the most effective in vitro method of surface evaluation in relation to the in-eye comfort.
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Lundvall, Olle. "Contact mechanics and noise in gears /". Linköping : Univ, 2004. http://www.bibl.liu.se/liupubl/disp/disp2004/tek862s.pdf.
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Pitteroff, Roland. "Contact mechanics of the bowed string". Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.387739.
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Siles, Brügge Oscar. "Contact mechanics at the molecular scale". Thesis, University of Sheffield, 2017. http://etheses.whiterose.ac.uk/18786/.
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A better understanding of the adhesive interactions between surfaces at the molecular scale is of growing importance as miniaturization efforts continue. To this end, Lifshitz theory of continuum mechanics was used to calculate the interaction energies between hydrocarbon surfaces in over 200 liquids, and compared to those obtained from the Hunter model of hydrogen bond solvation thermodynamics. In alkanes, amines, and primary alcohols, both theories yielded comparable results. However, in cases where the refractive index between interacting phases diverges greatly, a large disparity between the Lifshitz work of adhesion and Hunter free energy of complexation was found. In addition to some of the liquids showing differing results between the two theories, binary mixtures of benzyl alcohol and methanol were also identified for further experimental analysis. Slight modifications were applied to Lifshitz to allow for predictions of polar surfaces, and these too were compared to those provided by the Hunter model. Using force spectroscopy and friction force microscopy the tribological properties of hydrocarbon self-assembled monolayers, in the liquids identified previously, were investigated. While interactions in non-polar liquids were well described by both Lifshitz theory and the Hunter model, the former was found to consistently underestimate the work of adhesion in polar liquids, especially in water (Wad > 40 mJ/m2). In contrast, good agreement was generally obtained between the Hunter model and the experimentally obtained interaction energies. This was also true for binary mixtures of benzyl alcohol and methanol, where Lifshitz theory was completely unable to predict the form of the interaction. Friction-load plots were also obtained for the same systems of non-polar surfaces, and the form of their relation in different media was found to be dependent on the previously obtained adhesive energies. At interaction energies below 6 mJ/m2 linear friction-load relationships were observed, while yielding sublinear plots at work of adhesion values above this, corroborating the idea that friction can be considered to consist of load- and area-dependent terms. Mechanochemical removal of NPEOC photoprotecting groups from surfaces with adsorbed OEG-NPEOC-APTES monolayers using an AFM probe was also performed, with feature sizes up to 20 nm being achieved. The dependence on the width and depth of the patterned features on the applied load was investigated, with a positive relation being found for both, up to a critical load; no such change was observed with increasing write speeds. Changing the tip chemistry and environment (i.e. via immersion in different liquids) yielded no change in the size and quality of the patterns obtained, suggesting the lithographic process relies solely on the physical interaction between tip and sample surface. Modification of the surface through derivatization using TFAA and GFP indicates that only the NPEOC protecting group is being removed. Density functional theory was employed to investigate possible reaction pathways of the usual photodeprotection pathway of NPEOC-APTES, and how the mechanical interaction of the tip with the surface may promote one of these to occur without a high energy photon. It was discovered that a compression of the NPEOC leads to a shift in the UV/Vis absorbance spectrum towards higher wavelengths, and it is suggested that the mechanochemical deprotection of OEG-NPEOC-APTES SAMs occurs via this mechanism.
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King, Christopher David S. M. Massachusetts Institute of Technology. "A coupled contact-mechanics computational model for studying deformable human-artifact contact". Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/118672.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 199-205).
Gas-pressurized spacesuits are necessary for human spaceflight, most notably for extravehicular activity (EVA). Legacy EVA suits have been primarily rigid, and operation in such suits can result in significant metabolic expense, or even injury, for the wearer. To reduce these effects, modern spacesuits are more flexible, through the incorporation of more softgood materials and specially designed joint interfaces such as hip bearings. However, modeling the effects of human-suit interaction for these softgood materials is challenging due to the highly deformable nature of the suit coupled with the deformable nature of the human. To enable improved analysis and design of modern spacesuits, a computational model that can resolve the structural deformations of the suit and human resulting from contact interactions is developed. This thesis details the development and validation of a coupled contact-mechanics solver architecture for use in studying the effects of human-artifact interaction, particularly with respect to pressurized softgood exosuit design. To resolve contact and structural mechanics interactions for a deformable human and artifact, a finite element model is developed. First, the SUMMIT computational framework is employed for resolving the structural deformations of the system, and is coupled to an explicit contact mechanics scheme. The explicit contact scheme is implemented so as to resolve both external- and self-contact problems. Next, the model architecture is integrated to enable parallelization of both the structural deformation and contact systems, and computational scaling investigated. A computational trade study is performed to benchmark the coupled contact-mechanics method against a simpler rigid body model employing a penalty method. Following this, the model is validated against experimental data for various artifact contact problems. The explicit coupled contact-mechanics model is found to effectively capture contact interactions of the experimental data, with improved fidelity for deformable contact interactions. With careful tuning of the system properties, the coupled contact-mechanics model enables an architecture for an integrated human-suit analysis and design model.
by Christopher David King.
S.M.
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Neukirch, Sebastien. "Enroulement, contact et vibrations de tiges élastiques". Habilitation à diriger des recherches, Université Pierre et Marie Curie - Paris VI, 2009. http://tel.archives-ouvertes.fr/tel-00667562.
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Ce mémoire présente plusieurs études sur l'équilibre, la stabilité et les vibrations de poutres élastiques en grande rotation. Le modèle utilisé est d'abord présenté de deux manières différentes : les équations de Kirchhoff sont introduites (i) comme une théorie directe de Cosserat, (ii) par une approche asymptotique 3D->1D. Une étude relativement complète des équilibres avec et sans auto-contact d'une poutre sous contrainte de tension et torsion et encastrée en ses deux extrémités est ensuite exposée. Le modèle est d'autre part appliqué au sur-enroulement de la molécule d'ADN et aux expériences sur molécule individuelle. Le cas d'une poutre naturellement courbe enroulée autour d'un obstacle cylindrique est ensuite traité. Les équations d'équilibre obtenues sont appliquées au cas d'une plante grimpante autour d'un tuteur ainsi qu'aux configurations dimériques de la protéine kératine. Enfin la dynamique d'une poutre plane est analysée sous deux angles différents : (i) le relâchement d'une poutre console qui donne lieu au phénomène de renforcement de courbure, et (ii) les vibrations d'une poutre post-flambée, encastrée en ses deux extrémités, avec comparaison des cas extensibles et inextensibles. La conclusion mentionne quelques problèmes d'intérêt qui seront l'objets d'études futures.
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Wilson, W. Everett Jackson Robert L. "Surface separation and contact resistance considering sinusoidal elastic-plastic multiscale rough surface contact". Auburn, Ala, 2008. http://hdl.handle.net/10415/1490.
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Almqvist, Andreas. "Rough surface elastohydrodynamic lubrication and contact mechanics". Licentiate thesis, Luleå, 2004. http://epubl.luth.se/1402-1757/2004/035.
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Walls, Kenneth Cline. "Multi-material contact for computational structural mechanics". Birmingham, Ala. : University of Alabama at Birmingham, 2008. https://www.mhsl.uab.edu/dt/2008m/walls.pdf.
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Liu, Shubin Carleton University Dissertation Engineering Aerospace. "Boundary element analysis in contact fracture mechanics". Ottawa, 1994.
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Christensen, Peter W. "Computational nonsmooth mechanics : contact, friction and plasticity /". Linköping : Department of mechanical engineering, 2000. http://catalogue.bnf.fr/ark:/12148/cb40921031z.
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Bonari, Jacopo. "Novel interface discretisation methods for contact mechanics". Thesis, IMT Alti Studi Lucca, 2021. http://e-theses.imtlucca.it/326/1/Bonari_phdthesis.pdf.
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This thesis’ main scope is the presentation of two different
methodologies for the analysis of contact problems involving morphologically complex or rough surfaces. Both approaches rely on the Finite Element Method (FEM) as the
chosen computational framework. They hinge on the definition of an interface finite element used to model the space encompassed by two solids incontact. This kind of interface element is shared with the field of non-linear fracture
mechanics, employed for the simulation of non-linear crack
growth according to Cohesive Zone Model (CZM). Here, for
the first time, the formulation is extensively applied to contact mechanics. With no further modifications, the interface
element is suited for the solution of contact problems involving smooth and conformal interfaces, exploiting a node-to node
approach and a penalty formulation for the enforcement
of the contact constraints. The element is enriched with specific characteristics that allow for the solution of rough contact problems yet maintaining a very simple mesh discretisation, both using a single-scale and a multiscale approach. In
the single-scale approach, a novel methodology is exploited
that considers an equivalent flat interface and accounts for
the actual geometry by a suitable correction of the standard
normal gap. In the multi-scale approach, the Boundary Element Method (BEM) is exploited for solving, at a micro-scale,
the normal contact problem of a rough rigid indenter making contact with an elastic half-space, according to a far-field displacement determined by the deformation imposed at a
macro-scale. The solution in terms of averaged pressure and
mean separation is then passed back to the macro-scale.
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Brunetti, Jacopo. "Mechanical energy balance of frictional contacts : From surface to solid energy dissipation in contact dynamic instabilities". Thesis, Lyon, INSA, 2015. http://www.theses.fr/2015ISAL0118/document.
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Chaque fois que se produit un mouvement relatif entre deux systèmes, avec une interface à contact sec, le contact frottant induit des vibrations. La dynamique locale au contact (ruptures et la génération d'ondes) se couple avec la dynamique du système, donnant origine à des vibrations et affectant le comportement frictionnel macroscopique du système. Dans cette thèse, afin de développer une approche globale pour l'investigation des phénomènes multi-physiques, l'énergie a été utilisée comme une caractéristique physique universelle du couplage. La formulation de un bilan énergétique mécanique est utilisé pour identifier deux termes dissipatifs différents, i.e. la dissipation par amortissement matériel/système et la dissipation au contact. Les flux d'énergie, provenant des surfaces en contact et dus aux vibrations induites par frottement, excitent la réponse dynamique du système et, vice versa, l'influence de la réponse dynamique du système sur la dissipation d'énergie locale à l'interface de contact affecte les phénomènes tribologiques connexes. Dans cette thèse, les vibrations induites par le frottement ont été analysées en utilisant: l'approche par éléments finis pour étudier, par l'analyse des flux d'énergie, le couplage entre le contact et la dynamique du système; l'approche expérimentale pour valider les résultats numériques et observer l'influence des phénomènes pas encore inclus dans les modèles numériques; une approche avec une modèle à paramètres concentrés pour évaluer rapidement les effets des paramètres du système. L'analyse numérique par le modèle éléments finis 2D permet une répartition de l’énergie introduite dans le système mécanique entre les deux termes dissipatifs (amortissement matériau et contact), au cours de la réponse transitoire aussi bien en conditions stables qu’instables. En particulier, les vibrations induites par frottement modifient la capacité globale du système à absorber et dissiper l’énergie; une estimation de la puissance dissipée au contact, sans prendre en compte le comportement dynamique du système (flux d’énergie par les vibrations induites par frottement) peut conduire à des erreurs significatives dans la quantification de l’énergie dissipée au contact, ce qui affecte directement plusieurs phénomènes tribologiques. Les mesures expérimentales de crissement montrent comment les mêmes modes instables sont reproduits soit expérimentalement soit numériquement, validant l’utilisation de la simulation 2D transitoires pour l’analyse des vibrations instables induites par le frottement. L’équilibre énergétique a été utilisé sur le modèle à paramétrés concentrés, pour approcher le problème de la surestimation d’instabilité, qui est caractéristique d’une analyse des valeurs propres complexes. Un nouvel indice d’instabilité (MAI) a été défini, par des considérations énergétiques, pour comparer les différents modes instables et pour sélectionner le mode qui devient effectivement instable pendant le crissementWhenever relative motion between two system components occurs, through a dry contact interface, vibrations are induced by the frictional contact. The local dynamics at the contact (ruptures and wave generation) couples with the system dynamics, giving origin to vibrations and affecting the macroscopic frictional behavior of the system. In this thesis, in order to develop an overall approach to the investigation of the multi-physic phenomenon, the energy has been pointed out as a coupling physical characteristic among the several phenomena at the different scales. The formulation of a mechanical energy balance is used for distinguishing between two different dissipative terms, i.e. the dissipation by material/system damping and the dissipation at the contact. The energy flows coming from the frictional surfaces, by friction induced vibrations, excites the dynamic response of the system, and vice versa the influence of the system dynamic response on the local energy dissipation at the contact interface affects the related tribological phenomena. The friction-induced vibrations have been analyzed using three different approaches: the finite element approach, to investigate the coupling between the contact and system dynamics by the analysis of the energy flows; the experimental approach to validate the numerical results and observe the influence of phenomena not still included into the numerical model; a lumped parameter model approach to quickly investigate the effects of the system parameters. The numerical analysis by the 2D finite element model allowed investigating the repartition of the energy introduced into the mechanical system between the two dissipative terms (material damping and contact) during both stable and unstable friction-induced vibrations. In particular, it has been shown how the friction-induced vibrations modify the overall capacity of the system to absorb and dissipate energy; an estimation of the power dissipated at the contact, without considering the dynamic behavior of the system (energy flows by friction induced vibrations) can lead to significant error in the quantification of the dissipated energy at the contact, which affects directly several tribological phenomena. The experimental squeal measurements show how the same unstable modes are recovered both experimentally and numerically, validating the use of the 2D transient simulations for the reproduction of the unstable friction-induced vibrations. Once the energy balance formulated, it has been used on the lumped model to approach the instability over-prediction issue characteristic of the complex eigenvalue analysis. By energy considerations, a newer instability index (MAI) has been defined to compare the different unstable modes and to select the mode that becomes effectively unstable during the transient response. The Modal Absorption Index allows quantifying the capability of each mode to exchange energy with the external environment
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Jelagin, Denis. "Frictional Effects on Hertzian Contact and Fracture". Doctoral thesis, KTH, Hållfasthetslära (Avd.), 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4535.
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This thesis addresses normal axisymmetric contact of dissimilar elastic solids at finite interfacial friction. It is shown that in the case of smooth and convex but otherwise arbitrary contact profiles and monotonically increasing loading a single stick-slip contour evolves being independent of loading and profile geometry. This allows developing an incremental procedure based on a reduced problem corresponding to frictional rigid flat punch indentation of an elastic half-space. The reduced problem, being independent of loading and contact region, was solved by a finite element method based on a stationary contact contour and characterized by high accuracy. Subsequently, a tailored cumulative superposition procedure was developed to resolve the original problem to determine global and local field values for two practically important geometries: flat and conical profiles with rounded edges and apices. Results are given for relations between force, depth and contact contours together with surface stress distributions and maximum von Mises effective stress, in particular to predict initiation of fracture and plastic flow. It is also observed that the presence of friction radically reduces the magnitude of the maximum surface tensile stress, thus retarding brittle fracture initiation. Hertzian fracture through indentation of flat float glass specimens by steel balls has been examined experimentally for a full load cycle. It has been observed that if the specimen survived during loading to a maximum level it frequently failed at decreasing load. It has been proposed by Johnson et al. (1973) that the underlying physical cause of Hertzian fracture initiation during load removal is that at unloading frictional tractions reverse their sign over part of the contact region. Guided by these considerations a robust computational procedure has been developed to determine global and local field values in particular at unloading at finite friction. In contrast to the situation at monotonically increasing loading, at unloading invariance properties are lost and stick-slip regions proved to be severely history dependent and in particular with an opposed frictional shear stress at the contact boundary region. This causes an increase of the maximum tensile stress at the contour under progressive unloading. It is shown that the experimental observations concerning Hertzian fracture initiation at unloading are at least in qualitative correlation with the effect friction has on the maximum surface tensile stress. A contact cycle between two dissimilar elastic bodies at finite Coulomb friction has been further investigated analytically and numerically for a wider range of material parameters and contact geometries. With the issue of Hertzian fracture initiation in mind, results concerning the influence of the friction coefficient and compliance parameters on the absolute maximum surface tensile stress during a frictional contact cycle are reported along with the magnitudes of the relative increase of maximum tensile stresses at unloading. Based on a critical stress fracture criterion it is discussed how the predicted increases will influence the critical loads required for crack initiation. Fracture loads are measured with steel and tungsten carbide spherical indenters in contact with float glass specimens at monotonically increasing loading and during a load cycle. Computational predictions concerning the fracture loads are given based on Hertz and frictional contact theories combined with a critical stress fracture criterion. The computational results obtained for frictional contact are shown to be in better agreement with experimental findings as compared to the predictions based on the Hertz theory. The remaining quantitative discrepancy was attributed to the well-known fact that a Hertzian macro-crack initiates from pre-existing defects on the specimen’s surface. In order to account for the influence of the random distribution of these defects on the fracture loads at monotonic loading, Weibull statistics was introduced. The predicted critical loads corresponding to 50% failure probability were found to be in close agreement with experimentally observed ones.QC 20100729
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Azeggagh, Nacer. "Damage mechanisms in silicon nitride materials under contact loading". Thesis, Lyon, INSA, 2015. http://www.theses.fr/2015ISAL0075/document.
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Ces travaux de thèse portent sur la détermination des propriétés mécaniques à différentes échelles ainsi que les mécanismes d'endommagement des nitrures de silicium denses ou avec différents taux de porosités. Ces céramiques techniques présentent des propriétés mécaniques forte intéressantes : une faible densité, une dureté élevée, une bonne résistance à la corrosion et un faible coefficient de dilatation thermique. Elles sont notamment utilisées dans la fabrication des billes de roulement pour des applications dans les industries automobiles et aéronautiques. La caractérisation du comportement local sous un chargement de contact est donc un enjeu majeur. Les matériaux étudiés ont été obtenus par frittage flash d'une poudre de Nitrure de Silicium avec différents pourcentages d'oxide d'yttrium comme additif. Le contrôle de la température et de la pression de frittage a permis d'obtenir des matériaux denses avec différentes tailles de grain mais aussi avec une porosité résiduelle variable. La première partie de ce travail consistait à caractériser l'influence des conditions d'élaboration (température, pression, pourcentage d'additif) sur la microstructure (taille des grains, compositions ...) et les propriétés mécaniques à l'échelle macroscopique (module élastique, dureté Vickers, résistance à la flexion ...) des matériaux frittés. Des essais de contact de Hertz ont été ensuite réalisés afin d'identifier les mécanismes d'endommagement. L'utilisation de sphères de différents rayons a mis en évidence un important effet d'échelle : des fissures circonférentielles à échelle macroscopique (mode fragile) et des déformations plastique localisées à l'échelle mesoscopique avec des micro fissures distribuées aléatoirement (mode quasi-ductile). Les tests de nanoindentation permettent de solliciter localement les échantillons pour obtenir les propriétés élastiques des courbes force-déplacement. Des méthodes d'identification inverses permettent aussi d'extraire les paramètres d'écoulement. Le comportement non linéaire des céramiques a été modélisé en utilisant une loi bilinéaire où Sy est la limite d'élasticité et K un paramètre d'écrouissage. Afin d'identifier ces deux paramètres, un modèle éléments finis axisymétrique avec une pointe sphérique déformable a été construit sous Abaqus. Le modèle a été couplé à un module d'identification inverse fondé sur l'algorithme de Levemberg-Marquart pour minimiser l'écart (au sens des moindres carrés) entre les courbes expérimentales et numériques. Les simulations avec le code Isaac développé au sein du laboratoire LaMCoS ont permis de suivre d'évolution de la zone plastique lors d'un chargement de roulementThis work deals with the mechanical properties and damage mechanisms under contact loading of dense and porous silicon nitrides materials. These technical ceramics exhibit a very interesting combination of mechanical properties: low density, high hardness and strength, good corrosion resistance and a low thermal coefficient. They are used in many applications including ball bearings for the automotive and aerospace industries. The characterization of the local behaviour under contact loading is then a crucial issue. Spark plasma sintering technique is used to process silicon nitride ceramics with addition of different amount of yttrium oxide as sintering aid. Controlling the sintering temperature and the applied pressure has permitted to obtain materials with fine, medium and coarse microstructures. In addition, materials with different porosity contents have been obtained. First, we have investigated the influence of processing conditions (temperature, pressure, amount of yttria on the microstructure and mechanical properties at the macroscopic scale (elastic parameters, Vickers hardness, flexural resistance ...). Hertzian contact tests were then performed to identify the damage mechanisms at the surface and subsurface of the sintered materials. The use of indenting spheres of different radii permitted to observe a significant size effect. Brittle mode consisting of surface ring cracks were observed at large scale (macroscopic scale) while localized plastic deformation with microcracks randomly distributed was observed at small scale (mesoscopic scale). Transmission electron microscopy observations of thin foils machined by ion milling were performed to investigate the subsurface damage. Numerical simulations with a code developed internally in LaMCoS laboratory enabled to follow the evolution of the plastic zone under pure rolling conditions. In these simulations, the nonlinear behaviour of ceramics was modelled using a bilinear law where Sy is the yield stress and K a hardening parameter of the ceramic specimen. Instrumented indentation tests were performed using a diamond spherical tip of radius 42 µm. Experimental load versus displacement curves were used as input data for an inverse identification purpose. Levemberg-Marquart algorithm was used to minimize the gap in the least squares sense
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Shellock, Matthew R. "Contact analysis of nominally flat surfaces". Thesis, Monterey, Calif. : Naval Postgraduate School, 2008. http://handle.dtic.mil/100.2/ADA483610.
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Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, June 2008.Thesis Advisor(s): Kwon, Young W. "June 2008." Description based on title screen as viewed on September 2, 2008. Includes bibliographical references (p. 51). Also available in print.
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Ajibose, Olusegun K. "Nonlinear dynamics and contact fracture mechanics of high frequency percussive drilling". Thesis, Available from the University of Aberdeen Library and Historic Collections Digital Resources, 2009. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?application=DIGITOOL-3&owner=resourcediscovery&custom_att_2=simple_viewer&pid=61011.
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Man, Kim Wai. "Boundary element analysis of contact in fracture mechanics". Thesis, University of Portsmouth, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.317864.
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Burke, Dave. "The mechanics of the contact phase in trampolining". Thesis, Loughborough University, 2015. https://dspace.lboro.ac.uk/2134/16744.
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During the takeoff for a trampoline skill the trampolinist should produce sufficient vertical velocity and angular momentum to permit the required skill to be completed in the aerial phase without excessive horizontal travel. The aim of this study was to investigate the optimum technique to produce forward somersault rotation. A seven-segment, subject-specific torque-driven computer simulation model of the takeoff in trampolining was developed in conjunction with a model of the reaction forces exerted on the trampolinist by the trampoline suspension system. The ankle, knee, hip, and shoulder joints were torque-driven, with the metatarsal-phalangeal and elbow joints angle-driven. Kinematic data of trampolining performances were obtained using a Vicon motion capture system. Segmental inertia parameters were calculated from anthropometric measurements. Viscoelastic parameters governing the trampoline were determined by matching an angle-driven model to the performance data. The torque-driven model was matched to the performance data by scaling joint torque parameters from the literature, and varying the activation parameters of the torque generators using a simulated annealing algorithm technique. The torque-driven model with the scaled isometric strength was evaluated by matching the performance data. The evaluation produced close agreement between the simulations and the performance, with an average difference of 4.4% across three forward rotating skills. The model was considered able to accurately represent the motion of a trampolinist in contact with a trampoline and was subsequently used to investigate optimal performance. Optimisations for maximum jump height for different somersaulting skills and maximum rotation potential produced increases in jump height of up to 14% and increases of rotation potential up to 15%. The optimised technique for rotation potential showed greater shoulder flexion during the recoil of the trampoline and for jump height showed greater plantar flexion and later and quicker knee extension before takeoff. Future applications of the model can include investigations into the sensitivity of the model to changes in initial conditions, and activation, strength, and trampoline parameters.
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Reisse, Franziska. "Effect of malalignment on knee joint contact mechanics". Thesis, Anglia Ruskin University, 2014. http://arro.anglia.ac.uk/550363/.
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Osteoarthritis (OA) is a debilitating joint disease that leads to significant pain, loss of mobility and quality of life. Knee malalignment results in increased joint pressure, which is a primary cause for OA progression. High Tibial Osteotomy (HTO) is a surgical procedure to correct malalignment and redistribute load in the knee joint, reduce peak pressure and delay OA progression. However, clinical outcomes have been unpredictable. Therefore, the aim of this study was to determine the relationship between malalignment and knee contact mechanics. A 3D computational model was created from magnetic resonance images of a cadaveric knee joint. A ligament tuning process was conducted to determine material properties. Finite element analyses were conducted, simulating end of weight acceptance during walking. Different wedge geometries were virtually removed to simulate malalignments from 14° valgus to 16° varus. Contact mechanics were sensitive to soft tissue material properties. In-vitro experiments were compared with computational modelling of the same specimen. Percent full-scale errors for contact force and pressure were less than 8%, demonstrating a unique subject-specific model validation. The native alignment of the cadaveric knee (1° varus) had medial and lateral compartment peak pressures of 4.28 MPa and 2.42 MPa, respectively. The medial:lateral force ratio was 70%:30%. Minimum contact stress did not occur at a Mechanical Axis Deviation (MAD) of zero millimetres nor at the Fujisawa Point, which are common targets for HTO correction. Results showed very strong correlations (r >0.94) between MAD and joint contact loading. This study is the first to demonstrate the relationship between stress (normal, shear, contact pressure) and MAD in a subject-specific model. This is a prerequisite for the development of a tool that could help surgeons make informed decisions on the degree of realignment required to minimise peak joint loading, thereby delaying OA progression.