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Tesi sul tema "Deformations (Mechanics)"

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Autore: Grafiati

Pubblicato: 4 giugno 2021

Ultima modifica: 29 luglio 2024

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1

Student, James John. "The Box Ankle and Ocmulgee shear zones of central Georgia: a study of geochemical response to Southern Appalachian deformation events". Thesis, This resource online, 1992. http://scholar.lib.vt.edu/theses/available/etd-09192009-040411/.

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2

Wentworth, Stephen Thomas. "A qualitative study of planar elastic deformations". CSUSB ScholarWorks, 1994. https://scholarworks.lib.csusb.edu/etd-project/909.

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3

Pelillo, Enrico. "Scratch deformations of polymers". Thesis, Imperial College London, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.286438.

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4

Wadwalkar, Saurabh Sunil Jackson Robert L. "A study of elastic plastic deformation of heavily deformed spherical surfaces". Auburn, Ala., 2009. http://hdl.handle.net/10415/1957.

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5

Jiang, Tianci. "Impact & penetration studies simplified models and and materials design from AB initio methods /". Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/10443.

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In recent impact and penetration mechanical tests, steel projectiles (AISI4340) were impacted into targets like concrete with striking velocities (1200 m/s to 1500 m/s). Results indicated a material removal from the nose of the projectile, phase changes of the projectile materials, a reduction in the length of the projectile, and a blunting of the nose shape. These observations cannot be explained by current theories and numerical integration code that are used to study impact and penetration mechanics. Thus, the objectives of the thesis research are to (a) formulate and characterize the mechanisms responsible for the material erosion of the impacting projectile and the mass loss from the nose region; and (b) to determine the physical properties of alloy steels that are important to penetration mechanics from ab initio methods. The results can be used to design new projectile materials that can provide the desired penetration characteristics. These objectives are accomplished by investigating two related problems. The first problem is to formulate simplified models that can explain the penetration mechanics. The new models include the varying cross-section nose, changes of yield stress behind the shock wave and high strain rate phase transitions. Nose erosion effects, and time-dependent penetration path can be determined by integrating ODEs. A cavity expansion theory model is used to obtain the target resistance that is responsible slowing and deforming the penetrating projectile. The second problem concerns the determination of the constitutive relations from ab initio methods. The equation of state (EOS) and magnetic moments for alloy steels are investigated by using a special quasirandom structure technique and ab initio methods. Specifically, EOS for an interstitial disordered alloy Fe1-x-yNixCy is developed. First, the EOS of iron and phase transition of iron are studied and validated. Second, Nickel is considered to investigate the substitutional disordered alloy Fe1-x-yNixCy. Third, Carbon is placed at an interstitial position in the substitutional disordered alloy. These investigations will form foundation for future work involving new projectile with steel nose and shank made of multifunctional structural energetic materials.

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6

Okada, Hiroshi. "Boundary element methods for linear and nonlinear solid mechanics problems : and fracture toughness enhancement mechanisms in ceramic materials". Diss., Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/19169.

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7

Wang, Changxin. "Large deformation and no-tension analysis of selected problems in soil mechanics". Thesis, The University of Sydney, 2001. https://hdl.handle.net/2123/27804.

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The research carried out in this thesis is two-fold, covering the application of large deformation analysis in soil mechanics, and the problem of tensile failure of soil. The work may be conveniently divided into four sections. (1) Four large deformation analysis approaches have been implemented in the finite element package AFENA. Discussions of the advantages and limitations of the four approaches are given, based on the formulations adopted in these approaches and the results of analyses of some test problems. The applicability of the four approaches for some selected soil mechanics problems was investigated, by comparing the load—displacement curves predicted by these four approaches. (2) Large deformation analyses of strip and circular footings penetrating into layered clay, and of strip anchors uplifted in homogeneous and non—homogeneous clay, were carried out. The bearing or uplift capacity factors, load—displacement curves, failure zones, and stress distributions in the soil were investigated in the analyses. A wide range of soil parameters was adopted in the analysis, and the results presented cover a large variety of circumstances. The necessity of large deformation analysis of these problems is discussed, based on the comparisons of the results predicted by both small and large deformation analyses. (3) Soil tensile failure models were established for the problem of strip anchors uplifted in unsaturated and saturated soil, in terms of both total stress and effective stress. (4) The analyses adopting the soil tensile failure models were carried out for the problem of strip anchors uplifted in homogeneous or non-homogeneous soil. The significance of soil tensile failure on the uplift behaviour of anchors was investigated in a wide range of circumstances. It was found that the stress and strain formulations adopted in the four large deformation approaches make little difference to the predictions for the soil mechanics problems considered in this work. However, a distorted finite element mesh may affect the predictions of the numerical analysis significantly after a relatively large deformation occurs. Therefore a reliable large deformation analysis should handle mesh distortion properly. A large deformation analysis is able to capture the softening and hardening behaviour of the soil—structure system, while small deformation analysis can not predict this type of response. Whether a large deformation analysis is important for engineering practice depends on many factors, which include the magnitude of the geometry change, the soil strength profile, the initial position of the structure, the stiffness of soil and the soil self-weight. The analysis considering soil tensile failure for a strip anchor in homogeneous and non-homogeneous soil showed that the tensile failure reduces the uplift capacity from slightly to very significantly, depending on the particular conditions. The significance of tensile failure depends on the embedment depth of the anchor, the ratio of overburden pressure to shear strength, the soil strength gradient and the capacity of the pore fluid to sustain suction.

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8

Li, Jianxiong. "The lamellar structure and deformation mechanisms of [beta]-polypropylene". Click to view the E-thesis via HKUTO, 1997. http://sunzi.lib.hku.hk/hkuto/record/B31237368.

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9

Graham, Samuel Jr. "The stress state dependence of finite inelastic deformation behavior of FCC polycrystalline materials". Thesis, Georgia Institute of Technology, 1995. http://hdl.handle.net/1853/16802.

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10

Tanner, Albert Buck. "Modeling temperature and strain rate history effects in OFHC CU". Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/17143.

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11

Horstemeyer, Mark F. "Physically-motivated modeling of deformation-induced anisotropy". Diss., Georgia Institute of Technology, 1995. http://hdl.handle.net/1853/17262.

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12

Siu, Allen H. P. "Development of three-dimensional least-square finite element technique for inelastic deformation analysis". Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/17546.

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13

Shah, Ritesh P. "Plastic deformation and microcracking behavior of polycrystalline NiAl(Zr)". Diss., Georgia Institute of Technology, 1994. http://hdl.handle.net/1853/18928.

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14

李建雄 e Jianxiong Li. "The lamellar structure and deformation mechanisms of {221}-polypropylene". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1997. http://hub.hku.hk/bib/B31237368.

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15

Li, Jianxiong. "The lamellar structure and deformation mechanisms of b-polypropylene /". Hong Kong : University of Hong Kong, 1997. http://sunzi.lib.hku.hk/hkuto/record.jsp?B18865410.

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16

Padbidri, Jagan. "Minimal boundary conditions for simulations of disordered materials". WSU all campuses online access, 2003. http://www.dissertations.wsu.edu/Thesis/Fall2003/j%5Fpadbidri%5F121503.pdf.

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17

Forsberg, Fredrik. "Measurements of deformations and flows inside optically nontransparent materials". Licentiate thesis, Luleå, 2005. http://epubl.luth.se/1402-1757/2005/26.

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18

Liu, Ming. "Multidimensional damage state identification using phase space warping /". View online ; access limited to URI, 2005. http://0-wwwlib.umi.com.helin.uri.edu/dissertations/dlnow/3188065.

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19

Russell, Adrian Robert Civil &amp Environmental Engineering Faculty of Engineering UNSW. "Cavity expansion in unsaturated soils". Awarded by:University of New South Wales. School of Civil and Environmental Engineering, 2004. http://handle.unsw.edu.au/1959.4/22263.

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The problem of cavity expansion in unsaturated soils is investigated. A unified constitutive model for unsaturated soils is presented in a critical state framework using the concepts of effective stress and bounding surface plasticity theory. Consideration is given to the effects of suction and particle crushing in the definition of the critical state. A simple isotropic elastic rule is adopted. A loading surface and bounding surface of the same shape are defined using simple and versatile functions. A limiting isotropic compression line exists, towards which the stress trajectories of all isotropic compression load paths approach. A non-associated flow rule is assumed for all soil types. Isotropic hardening/softening occurs due to changes in plastic volumetric strains as well as suction for some unsaturated soils, enabling account of the phenomenon of volumetric collapse upon wetting. Results of isotropic compression tests, oedometric compression tests and drained and undrained triaxial compression tests performed on Kurnell (quartz) sand in saturated and unsaturated states and subjected to stresses sufficient to cause particle crushing are presented and used to calibrate the model. The model is also calibrated using results reported in the literature for triaxial tests performed on saturated and unsaturated speswhite kaolin and three load paths. For both soils the model leads to a much improved fit between simulation and experiment compared to that for models based on conventional plasticity theory. The model is implemented into a cavity expansion analysis using the similarity technique, extended for application to unsaturated soils. Cylindrical and spherical cavities are considered, as are drained and undrained conditions. Cavity expansion results for the bounding surface model and conventional plasticity models are compared for saturated conditions. Substantial differences highlight the importance of adopting a model that accurately describes stress-strain behaviour. Cavity expansion results for the bounding surface model and saturated and unsaturated conditions are also compared. Substantial differences, particularly in the limit pressure, highlight the major influence of suction and the importance of accounting for this when using cavity expansion theory to interpret results of the cone penetration and pressuremeter tests.

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20

Bai, Jie. "A homogenization based continuum plasticity-damage model for ductile fracture of materials containing heterogeneities". Columbus, Ohio : Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1211910660.

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21

Aroonsiri, Nontavat. "Design of reinforced concrete buildings for restrained and imposed deformations /". Title page, contents and abstract only, 2000. http://web4.library.adelaide.edu.au/theses/09ENS/09ensa769.pdf.

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22

Pacheco, Alejandro Andres. "Molecular Mechanics Simulations of Instabilities in 3D Deformations of Gold Nanospecimens". Diss., Virginia Tech, 2009. http://hdl.handle.net/10919/37734.

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We use molecular mechanics (MM) simulations with the tight-binding (TB) potential to study local and global instabilities in initially defect-free finite specimens of gold crystals deformed in shear, simple shear, tension/compression, simple tension/compression, and triaxial tension/compression. The criteria used to delineate local instabilities in a system include the following: (i) a second order spatial derivative of the displacement field having large values relative to its average value in the body, (ii) the minimum eigenvalue of the Hessian of the potential energy of an atom becoming nonpositive, (iii) and structural changes represented by a high value of the common neighborhood parameter. A specimen becomes globally unstable when its potential energy decreases significantly with a small increase in its deformations. It is found that the three criteria for local instability are satisfied essentially simultaneously at the same atomic position. Deformations of a specimen are quite different when it is deformed with some bounding surfaces free from external forces as opposed to essential boundary conditions prescribed on all bounding surfaces. It is found that the initial unloaded configuration (or the reference configuration) of the minimum potential energy has significant in-plane stresses on the bounding surfaces and nonzero normal stresses at interior points. In tensile/compressive deformations of a rectangular prismatic nanobar the yield stress defined as the average axial stress when the average axial stress vs. the average axial strain curve exhibits a sharp discontinuity depends upon the specimen size; a similar result holds for simulations of shear deformations. Specimens deformed with essential boundary conditions on all bounding surfaces experience instabilities at a higher value of the average strain than identical specimens deformed similarly but with one or more pairs of opposite bounding surfaces traction free. For the former set of deformations, the response of a specimen prior to the onset of instability is the same as that of a hyperelastic body with the strain energy derived from the TB potential and deformations obeying the Cauchy-Born rule. Specimens with some traction free bounding surfaces experience local instabilities prior to the onset of a global instability but the two instabilities occur simultaneously in specimens with essential boundary conditions prescribed on all bounding surfaces. It is believed that because of residual stresses in the reference configuration, the average axial stress at yield in compression is nearly one-half of that in tension.
Ph. D.

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23

Sun, Ning. "Development and validation of a design method coupling block theory and three-dimensional discontinuous deformation analysis". Click to view the E-thesis via HKUTO, 2005. http://sunzi.lib.hku.hk/hkuto/record/B31789122.

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24

Sun, Ning, e 孫宁. "Development and validation of a design method coupling block theory and three-dimensional discontinuous deformation analysis". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2005. http://hub.hku.hk/bib/B31789122.

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25

Diehl, Ted. "Modeling of elastic-viscoplastic behavior and its finite element implementation /". Online version of thesis, 1988. http://hdl.handle.net/1850/10461.

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26

Chiao, Ling-Yun. "Membrane deformation rate and geometry of subducting slabs /". Thesis, Connect to this title online; UW restricted, 1991. http://hdl.handle.net/1773/6814.

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27

Nanavati, Hemant. "Molecular modeling of the elastic and photoelastic properties of crosslinked polymer networks: a statistical segment approach / by Hemant Nanavati". Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/8227.

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28

Hu, Qiang. "Effects of nano structure on the deformation behavior of polymeric networks". Diss., Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/10179.

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29

Whitelaw, Roberts S. III. "Experimental determination and constitutive modeling of the deformation behavior of lead-free solders". Thesis, Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/17224.

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30

Carpenter, Charles E. "On-line consolidation mechanisms for thermoplastic composites". Thesis, Georgia Institute of Technology, 1992. http://hdl.handle.net/1853/19266.

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31

Antolovich, Bruce F. "Deformation response of ARALL under uni-axial loading conditions". Thesis, Georgia Institute of Technology, 1990. http://hdl.handle.net/1853/19456.

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32

Marangou, Maria G. "Thermoforming of polystyrene sheets deformation and tensile properties". Thesis, McGill University, 1986. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=65953.

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33

Gu, Rui, e 顧瑞. "Size effect on deformation of aluminum and duralumin micro-pillars". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2014. http://hdl.handle.net/10722/206336.

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Abstract (sommario):

It is by now well-known that micron-sized metallic crystals exhibit a smaller-being-stronger size effect: the yield strength σvaries with specimen size D approximately as a power law σ~D^(-m)_, and the exponent m has been found to vary within a range of ~0.3 to ~1.0 for different metals. However, little is known about why such a power law comes into play, and what determines the actual value of the exponent m involved. In this study, the power-law scaling of size effect on strength in micro-crystals is explained in terms of the Taylor-type resistance in the dislocation network distribution in the specimen. Theoretical analysis shows that the power-law dependence of yield strength of metallic micro-specimens is derived from a fractal geometry of the initial dislocation network, with m = 3/(q + n) where q is the fractal dimension and n the stress exponent of dislocation velocity. Moderate departures of the initial dislocation structure from an exact fractal geometry may also yield approximate power-law dependence of strength on size. The plastic deformation of micro-pillars is also known to be affected by whether dislocations can escape easily from the material volume, and the extent to which they mutually interact during the deformation. In the present work, pre-straining and coating are used to modify the initial dislocation content and the constraints on the escape of dislocations. Aluminum micro-pillars with or without thin coating by tungsten deposition and pre-straining, were compressed using a flat-punch nanoindenter to study their plasticity behavior. The results reveal very different behavior between specimens in the size regime of a few microns and that about one micron, suggesting that the dominant hardening mechanisms are different. As mentioned above, pure and pristine metal micro-specimens have been found to exhibit very strong size dependence of strength, but alloyed counterparts with a much refined microstructural length scale due to the precipitates present are unknown in this aspect. Here, compression tests on duralumin (aluminum 2025 alloy) micro-pillars reveal a much weaker size dependence of strength compared to pure Al, indicating the predominance of the internal length scale in determining strength. Moreover, two-dimensional dislocation dynamics simulations are used to study precipitate strengthening effects in duralumin micro-pillars. The results show that a refined microstructure may resist and slow down the movement of dislocations inside the confined volume, leading to hardening and weak size dependence of strength. In addition to the compression behavior, the size dependence of the creep behavior of duralumin micro-pillars is also investigated at room temperature. The effects of an internal grain boundary are also investigated. The results reveal that peak-aged duralumin pillars show increasingly significant creep with increasing pillar size, with a typical creep rate of ~〖10〗^(-4) S^(-1) which is drastically larger than that of bulk at room temperature. The bi-crystalline pillars creep even faster than the single crystalline counterparts. TEM examination of the deformed microstructures reveals that the creep rate depends on the residual dislocation density, indicating that dislocations are the agents for creep. Theoretical modeling suggests that the steadystate creep rate is proportional to the lifetime of mobile dislocations, which rises with specimen size in the microns range due to the fact that the dislocations are not easily pinned in this range, therefore they spend longer time in viscous motion across the specimen, leading to a higher strain rate according to the Orowan equation.
published_or_final_version
Mechanical Engineering
Doctoral
Doctor of Philosophy

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34

Chen, Xiao Yu. "Feature matching of deformable models /". View abstract or full-text, 2008. http://library.ust.hk/cgi/db/thesis.pl?MECH%202008%20CHENX.

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35

Zamiri, Amir Reza. "Computationally efficient crystal plasticity models for polycrystalline materials". Diss., Connect to online resource - MSU authorized users, 2008.

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36

ALSAYED, SALEH HAMED. "INELASTIC BEHAVIOR OF SINGLE ANGLE COLUMNS". Diss., The University of Arizona, 1987. http://hdl.handle.net/10150/184041.

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The study examines the behavior of pinned-end, centrally loaded columns of monosymmetric and asymmetric cross sections, with emphasis on angle shapes. The investigation covers flexural and flexural-torsional buckling in the elastic and inelastic ranges, which the aim of developing a rational method of predicting the buckling load for cross sections with low torsional rigidity and single or no axes of symmetry. The computer program that was developed takes into account the effect of residual stresses. The properties of the cross section were determined in the laboratory and utilized in the computer model. Full-scale column tests were run to verify the theoretical model. The results shows that equal-legged angles with low width-to-thickness ratio have flexural and flexural-torsional buckling loads that are less than 2% different. It is therefore suitable to continue using a flexural buckling solution for such shapes. This is also true for equal-legged angles with a high width-to-thickness ratio that fail in the elastic range, but in the inelastic range the flexural-torsional buckling load was about 11% less than the flexural buckling load. When the angle is unequal-legged, the flexural-torsional buckling load is always smaller than the corresponding flexural buckling load, in both the elastic and the inelastic ranges. The average difference between the flexural and flexural-torsional load for unequal-legged angle ranges from 3% in the elastic range to 10% in the inelastic range. The average ratio of the experimental results to the minimum of the theoretical results was 0.95 and the coefficient of variation was 0.053. Comparison with the results of other researchers show that it is possible to formulate an empirical formula that can be used in designing columns that are made of monosymmetric or asymmetric cross sections. However, due to the scarcity of data at this stage, it is recommended that the development of such a formula be postponed until additional test data are available. Moreover, in designing any cross section that does not have two axes of symmetry, it is advisable to check the possibility of flexural and flexural-torsional buckling.

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37

Jane, Kuo Chang. "Buckling, postbuckling deformation and vibration of a delaminated plate". Diss., Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/19975.

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38

Gregory, P. W. "Finite elastic-plastic deformations of highly anisotropic materials". Thesis, University of Nottingham, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.282601.

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39

Manjunath, Deepak Rosenblad Brent L. "Point target interferometry as applied to the characterization of localized deformation features". Diss., Columbia, Mo. : University of Missouri--Columbia, 2008. http://hdl.handle.net/10355/7118.

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Title from PDF of title page (University of Missouri--Columbia, viewed on Feb. 23, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Dr. Brent Rosenblad, Dissertation Supervisor. Vita. Includes bibliographical references.

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40

Peña-Ramos, Carlos Enrique 1962. "CONCRETE PONDING EFFECTS IN COMPOSITE FLOOR SYSTEMS". Thesis, The University of Arizona, 1987. http://hdl.handle.net/10150/276421.

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41

Wagner, Till Jakob Wenzel. "Elastocapillarity : adhesion and large deformations of thin sheets". Thesis, University of Cambridge, 2013. https://www.repository.cam.ac.uk/handle/1810/244939.

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This thesis is concerned with the deformation and adhesion of thin elastic sheets that come into contact with an underlying substrate. The focus of this work is on the interplay between material and geometric properties of a system and how this interplay determines the equilibrium states of sheet and substrate, particularly in the regime of geometrically nonlinear deformations. We first consider the form of an elastic sheet that is partially adhered to a rigid substrate, accounting for deflections with large slope: the Sticky Elastica. Starting from the classical Euler Elastica we provide numerical results for the profiles of such blisters and present asymptotic expressions that go beyond the previously known, linear, approximations. Our theoretical predictions are confirmed by desktop experiments and suggest a new method for the measurement of material properties for systems undergoing large deformations. With the aim to gain better understanding of the initial appearance of blisters we next investigate the deformation of a thin elastic sheet floating on a liquid surface. We show that, after the appearance of initial wrinkles, the sheet delaminates from the liquid over a finite region at a critical compression, forming a delamination blister. We determine the initial blister size and the evolution of blister size with continuing compression before verifying our theoretical results with experiments at a macroscopic scale. We next study theoretically the deposition of thin sheets onto a grooved substrate, in the context of graphene adhesion. We develop a model to understand the equilibrium of the sheet allowing for partial conformation of sheet to substrate. This model gives phys- ical insight into recent observations of ‘snap-through’ from flat to conforming states and emphasises the crucial role of substrate shape in determining the nature of this transition. We finally present a theoretical investigation of stiction in nanoscale electromechanical contact switches. Our model captures the elastic bending of the switch in response to both electrostatic and van der Waals forces and accounts for geometrically nonlinear deflections. We solve the resulting equations numerically to study how a cantilever beam adheres to a fixed bottom electrode: transitions between free, pinned and clamped states are shown to be discontinuous and to exhibit significant hysteresis. The implications for nanoscale switch design are discussed.

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42

Ng, Kwok-sing. "Plastic deformation of aluminium micro-specimens". Click to view the E-thesis via HKUTO, 2008. http://sunzi.lib.hku.hk/hkuto/record/B4175802X.

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43

Vasudevan, S. "Development of new spatially curved non-linear frame finite element using a mixed variational principle and rotations as independent variables". Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/13069.

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44

Miller, Matthew P. "Improved constitutive laws for finite strain inelastic deformation". Diss., Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/16098.

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45

Scott, Daniel T. "Phenomenological modeling of the deformation behavior of solders". Thesis, Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/17889.

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46

Milligan, Walter W. Jr. "Deformation modeling and constitutive modeling for anisotropic superalloys". Diss., Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/19922.

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47

Chen, Yongjin, e 陳永進. "Effects of rolling conditions on texture and microstructure development in [alpha] brass". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2000. http://hub.hku.hk/bib/B31239997.

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48

Siu, Kai-wing, e 蕭啟穎. "Effects on plastic deformation by high-frequency vibrations on metals". Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2013. http://hub.hku.hk/bib/B50534087.

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Abstract (sommario):

The effect of softening due to vibrations induced on metals has been used in many industrial processes such as forming, machining and joining. These industrial applications utilize ultrasonic vibrations in addition to quasi-static stresses in order to deform metals more easily. The phenomenon of ultrasonic softening is also called the Blaha effect or acoustoplastic effect. Besides the macro-scale softening due to ultrasonic vibrations imposed on quasi-static deformation stress, sub-micron level softening due to vibrations was also observed in nanoindentation experiments in recent years. These experiments made use of the oscillatory stresses of the vibrations provided by the continuous stiffness measurement (CSM) mode of nanoindentation. Lowering of loading and hardness data has been observed at shallow indent depths where the amplitude of vibration is relatively large. Despite the common industrial usages of acoustoplastic effect and the observation of softening in CSM mode nanoindentation, the physical principle underlying is still not well understood. For acoustoplastic effect the existing understanding is usually one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal. For the softening observed in CSM mode nanoindentation, the effect is either attributed to instrumental errors or enhancement of nucleation of dislocations which makes them move faster. Investigations on the link between microscopical changes and the softening have been rare. In this thesis, indentation experiments in both macro and micro scales were performed on aluminium, copper and molybdenum samples with and without the simultaneously application of oscillatory stresses. Significant softening was observed, and the amount of softening from macro to micro scale indentation of similar displacement/amplitude ratios is similar. The deformation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. Electron microscopy analyses reveal subgrain formation under the vibrated indents, which implies intrinsic changes. To further give physical insight into the phenomenon, dislocation dynamics simulations were carried out to investigate the interactions of dislocations under the combined influence of quasi-static and oscillatory stresses. Under a combined stress state, dislocation annihilation is found to be enhanced leading to larger strains at the same load history. The simulated strain evolution under different stress schemes also resembles closely certain experimental observations previously obtained. The discovery here goes far beyond the simple picture that the effect of vibration is merely an added-stress one, since here, the intrinsic strain-hardening potency of the material is found to be reduced by the oscillatory stress, through its effect on enhancing dislocation annihilation. The experimental and simulation results collectively suggest that simultaneous application of oscillatory stress has the ability to enhance dipole annihilation and cause subgrain formation. The superimposed oscillatory stress causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation. In addition, microscopic observations showed that subgrain formation and reduction in dislocation density generally occurred in different metals when stress oscillations were applied. These suggest that the intrinsic oscillation-induced effects of softening and dislocation annihilation are a rather general phenomenon occurring in metals with different stacking fault energies and crystal structures.
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Mechanical Engineering
Doctoral
Doctor of Philosophy

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49

Jenkins, Brent Allan. "Phase transformations and deformation metallography in dilute uranium alloys". Thesis, University of Oxford, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.670343.

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Dano, Marie-Laure. "SMA-induced deformations in unsymmetric cross-ply laminates". Thesis, This resource online, 1993. http://scholar.lib.vt.edu/theses/available/etd-09122009-040504/.

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