Дисертації з теми "Strain mechanisms"
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1
Johnson, David Thomas. "Deformation mechanisms in ABS polymers." Thesis, Cranfield University, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.341042.
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Turner, Stephen George. "Strain relaxation mechanisms and stress-balancing of SiGe heterostructures." Thesis, Imperial College London, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491115.
Повний текст джерелаАнотація:
Until now, progress of the microelectronic industry has been maintained by scaling devices. Physical limits now necessitate alternative approaches such as enhancing material properties through strain engineering or increasing functionality through use of Si/SiGe heterostructures (e.g. optoelectronic integration), requiring control of strain, which may enhance electrical properties but can also lead to deleterious structural and electronic effects. Strain can be controlled via growth of stress-balanced structures on a fully strain relaxed virtual substrate (VS) which acts as a growth template for, and maintains strain in, the active layers, which need to be ordered and defect free. The crystalline quality of the VS therefore impacts directly upon the device. These basic elements are realised in a molecular beam epitaxy system using gaseous hydride precursors. Compositionally graded VS are used to investigate strain-relaxation processes. High resolution x-ray diffraction (XRD) allowed examination of lattice reciprocal space, where compositional and strain information is dispersed. Localised distortions of the lattice revealed the mosaic nature of the structure. Nomarski contrast microscopy and atomic force microscopy enabled quantitative analysis of surface defects and roughness, including characteristic crosshatch. VS grading rates, growth temperatures and precursor fluxes were varied to assess relaxation kinetics. The nucleation and propagation of strain-relieving dislocations were investigated through the growth of different VS structures on vicinal Si(001) substrates. XRD analysis showed tilting of the epitaxial layer with respect to the substrate. A correlation of tilt angle with offcut angle was observed, following energy barrier asymmetries on the available slip systems. Development of the tilt was seen to be a non-linear function of both Ge composition and grading rate, and was explained by interdislocation interactions. Stress-balancing was investigated by designing distributed Bragg reflector stacks. Structures without detrimental strain relief were achieved, with a reflectance in excess of those reported in the literature, demonstrating the applicability of the VS growth and stress-balance methods. The elastic properties of SiGe could also be probed via this approach, and small deviations from linear interpolation of bulk Si and Ge values were found, supported by theoretical modelling.
3
Krasner, Paul. "Strain Localization Mechanisms in the Scituate Granite, Rhode Island." University of Akron / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=akron1497285708350254.
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Xu, Lili Sh D. Massachusetts Institute of Technology. "High temperature deformation mechanisms and strain heterogeneities in calcite rocks." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45602.
Повний текст джерелаАнотація:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
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Includes bibliographical references.
In nature, carbonates often accumulate large amounts of strain in localized shear zones. Such marble sequences play a key role in crustal deformation processes. Despite extensive field and laboratory investigation, many questions remain concerning the mechanical behavior of these rocks. For example, the mechanical behavior of different limestones and marbles differ greatly, possibly owing to the presence of chemical impurities or solid-solutes. Thus, Chapter 2 examines the effect of Mg solute, a common impurity, on the mechanical behavior of calcite rocks. The results indicate that increasing Mg content increases the strength of calcite rocks during dislocation creep. The anisotropic nature of crystal slip usually entails variations in reorientation of individual grains and heterogeneous deformation within the polycrystalline material. In Chapter 3, a new technique including a series of sample preparation and image analysis algorithms is developed to provide quantitative measurements of the scale of heterogeneities produced, and to gain fundamental insight into polycrystalline plasticity. We place particular attention on quantifying variations of strain within grain interiors and at grain boundaries, and on recognizing the relative activities of different slip systems. The quantification of grain-to-grain interactions during straining is relevant for the improvement and verification of models of polycrystalline plasticity. The strain measurements obtained from Chapter 3 are compared with predictions of grain strain and reorientation obtained from the self-consistent viscoplastic method (Chapter 4). The results suggest that the self-consistent model gives a good description of global texture, but does not always predict lattice rotation and deformation within individual grains. To predict the actual deformation of individual grains will require a quantitative consideration of the effects on local strain of grain-boundary misorientation, local strain/stress state, grain-boundary sliding, and deviations in grain geometry.
by Lili Xu.
Ph.D.
5
Adelmann, Hans Christoph. "Growth and strain relaxation mechanisms of group III nitride heterostructures." Université Joseph Fourier (Grenoble), 2002. http://www.theses.fr/2002GRE10039.
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Ce travail a porté sur les mécanismes de croissance épitaxiale et de relaxation de contrainte d'hétérostructures de nitrures d'éléments III, GaN et AIN, en épitaxie par jets moléculaires assistée par plasma d'azote. Nous avons étudié les structures de la surface (0001) de GaN et d'AIN en conditions d'excès de métal. Nous avons montré que la quantité de Ga absorbée sur GaN peur être évaluée par RHEED. Cette méthode a été appliquée à l'absorption de Ga et à la croissance de GaN en conditions riche Ga. Dans les deux cas, les résultats ont permis de tracer un diagramme qui décrit la quantité de Ga présente sur GaN en fonction du flux de Ga et de la température de croissance. Les morphologies de surface obtenues après la croissance de GaN dans les différentes parties du diagramme sont discutées. Nous avons ensuite abordé la relaxation des contraintes d'hétérostructures GaN/AIN. En conditions d'excès d'azote, la relaxation se produit de façon élastique, soit par la formation d'îlots plats à basse température, soit par la formation d'îlots pyramidaux selon le mode Stranski-Krastanow à haute température. En conditions d'excès de métal, la croissance est bidimensionnelle et la relaxation se produit par introduction de dislocations. Nous avons démontré que l'épaisseur critique de cette relaxation plastique dépend fortement des conditions de croissance. L'étude de la nucléation d'îlots de GaN sur AIN en mode Stranski-Krastanowa permis de contrôler leur taille et leur densité. Ces deux paramètres peuvent être variés de façon indépendante dans une large gamme de valeurs. Nous avons trouvé que l'optimisation de la distribution de taille des îlots est compliquée par une distribution bimodale à haute température. Finalement, nous avons étudié l'adsorption de Ga sur les surfaces (0001) d'AIN et d'AlGaN. Ceci a permis de démontrer la faisabilité d'épitaxie en flux alternés de GaN sur AIN et AlGaN.
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Degli, Alessandrini Giulia. "Deformation mechanisms and strain localization in the mafic continental lower crust." Thesis, University of Plymouth, 2018. http://hdl.handle.net/10026.1/12799.
Повний текст джерелаАнотація:
The rheology and strength of the lower crust play a key role in lithosphere dynamics, influencing the orogenic cycle and how plate tectonics work. Despite their geological importance, the processes that cause weakening of the lower crust and strain localization are still poorly understood. Through microstructural analysis of naturally deformed samples, this PhD aims to investigate how weakening and strain localization occurs in the mafic continental lower crust. Mafic granulites are analysed from two unrelated continental lower crustal shear zones which share comparable mineralogical assemblages and high-grade deformation conditions (T > 700 °C and P > 6 Kbar): the Seiland Igneous Province in northern Norway (case-study 1) and the Finero mafic complex in the Italian Southern Alps (case-study 2). Case-study 1 investigates a metagabbroic dyke embedded in a lower crustal metasedimentary shear zone undergoing partial melting. Shearing of the dyke was accompanied by infiltration of felsic melt from the adjacent partially molten metapelites. Findings of case-study 1 show that weakening of dry and strong mafic rocks can result from melt infiltration from nearby partially molten metasediments. The infiltrated melt triggers melt-rock reactions and nucleation of a fine-grained (< 10 µm average grain size) polyphase matrix. This fine-grained mixture deforms by diffusion creep, causing significant rheological weakening. Case-study 2 investigates a lower crustal shear zone in a compositionally-layered mafic complex made of amphibole-rich and amphibole-poor metagabbros. Findings of case-study 2 show that during prograde metamorphism (T > 800 °C), the presence of amphibole undergoing dehydration melting reactions is key to weakening and strain localization. Dehydration of amphibole generates fine-grained symplectic intergrowths of pyroxene + plagioclase. These reaction products form an interconnected network of fine-grained (< 20 µm average grain size) polyphase material that deforms by diffusion creep, causing strain partitioning and localization in amphibole-rich layers. Those layers without amphibole fail to produce an interconnected network of fine grained material. In this layers, plagioclase deforms by dislocation creep, and pyroxene by microfracturing and neocrystallization. Overall, this PhD research highlights that weakening and strain localization in the mafic lower crust is governed by high-T mineral and chemical reactions that drastically reduce grain size and trigger diffusion creep.
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Da, Re Gregory 1970. "Physical mechanisms controlling the pre-failure stress-strain behavior of frozen sand." Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/31088.
Повний текст джерелаАнотація:
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2000.Includes bibliographical references.
The physical mechanisms controlling the pre-failure stress-strain behavior of frozen sands are investigated in triaxial compression. The pre-failure, or small strain behavior (Ea
Ph.D.
8
Allred, Jacob D. "An Investigation into the Mechanisms of Formation of the Hard Zone in FSW X65." BYU ScholarsArchive, 2013. https://scholarsarchive.byu.edu/etd/3806.
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Friction stir welding (FSW) of HSLA steel commonly produces a hard zone (HZ) on the advancing side (AS) of the weld. Despite its detrimental effects on weld toughness, the mechanisms of its formation have not been thoroughly investigated and are not well understood. This paper investigates the various mechanisms in FSW believed to affect the weld HZ, namely: strain, strain-rate, peak temperature and cooling rate. Gleeble tests indicate that strain and strain rate have negligible effects on weld HZ with cooling rate and peak temperature as dominant effects. Jominy tests resulted in cooling rate having 270% greater influence than peak temperature on the formation of lath ferrite microstructures similar to what is observed in the HZ of FSW X65. Comparing weld HZ microstructures to Jominy tests, it is estimated that cooling rates on the AS of the weld are at least 150°C/s higher than the retreating side. Reducing the cooling rate on the AS will likely lead to an improved microstructure at the weld HZ.
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Acosta, Matias [Verfasser], Jürgen [Akademischer Betreuer] Rödel, and Wolfgang [Akademischer Betreuer] Donner. "Strain Mechanisms in Lead-Free Ferroelectrics for Actuators / Matias Acosta ; Jürgen Rödel, Wolfgang Donner." Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2015. http://d-nb.info/1122286163/34.
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Aldea, Maria Ramona. "Identification of novel regulatory mechanisms controlling heterocyst development in Anabaena Sp. strain PCC 7120." [College Station, Tex. : Texas A&M University, 2008. http://hdl.handle.net/1969.1/ETD-TAMU-2996.
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Howes, Meghan K. "Injury Mechanisms, Tissue Properties, and Response of the Post-Mortem Human Abdomen in Frontal Impact." Diss., Virginia Tech, 2013. http://hdl.handle.net/10919/24382.
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Motor vehicle collisions (MVCs) are a leading cause of injuries and injury-related fatalities in the United States. The National Highway Traffic Safety Administration (NHTSA) reported over 21,250 vehicle occupant fatalities in 2011, with 1,240,000 injuries sustained by passenger car occupants alone. MVCs are a common cause of blunt abdominal injuries. It has been reported that approximately 9,000 front seat occupants sustain moderate to severe abdominal injuries in frontal MVCs in the United States each year. A detailed understanding of the occurrence and mechanisms of abdominal injuries, as well as knowledge of the biomechanical response and tolerance of the abdomen in crash-specific loading modes, could benefit the reduction of abdominal organ injury incidence in MVCs. Therefore, the objective of the research presented in this dissertation was to characterize abdominal injury mechanisms, tissue failure thresholds, and internal organ response to blunt impacts of the abdomen. Field accident data from the Crash Injury Research and Engineering Network (CIREN) database were analyzed to determine the occupant and crash characteristics associated with crash-induced hollow abdominal organ injuries. Dynamic equibiaxial tension tests were conducted on tissue samples obtained from the human post-mortem stomach, small intestine, and colon to characterize the material properties and failure tolerance of these tissues. The effects of cadaver orientation on the relative position of the abdominal organs of two cadavers were quantified, and high-speed biplane x-ray imaging was used to investigate the relative kinematics of the thoracic and abdominal organs of four cadavers in response to crash-specific loading modes. Test configurations included blunt abdominal and thoracic impacts and driver-shoulder seatbelt loading. The motivation for this research was to advance efforts toward abdominal organ injury mitigation in MVCs, with each aspect of this research generating novel injury biomechanics data with applications for future experimental testing and finite element modeling.Ph. D.
12
McMillan, Duncan George Glenn, and n/a. "Proton and iron capture mechanisms of Bacillus sp. strain TA2.A1 at alkaline pH values." University of Otago. Department of Microbiology & Immunology, 2008. http://adt.otago.ac.nz./public/adt-NZDU20081104.090840.
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The thermoalkaliphilic Bacillus sp. strain TA2.A1 was able to grow in pH-controlled batch culture containing a fermentable growth substrate (i.e. sucrose) from pH 7.5 to 10.0 with no significant change in specific growth rate, suggesting that this bacterium is a facultative alkaliphile. However, when strain TA2.A1 was grown on non-fermentable carbon sources like succinate, no growth was observed until the external pH was > 9.0, suggesting this bacterium is an obligate alkaliphile. Growth on succinate at pH 9.5 was sensitive to both carbonyl cyanide m-chlorophenylhydrazone (CCCP) and monensin revealing that both the proton and sodium motive force ([Delta][mu][H⁺] and [Delta][mu][Na⁺], respectively) were obligate requirements for growth at alkaline pH values. Transport of succinate was driven by a chemical gradient of Na⁺ ([Delta]pNa⁺) that was strictly coupled to [Delta][Psi]. A single transport system was detected for the uptake of succinate, with an apparent K[m] of 19 [mu]M and V[max] of 0.45 nmol succinate/min/mg protein. Succinate transport was pH-dependent, and showed optimal activity at pH values greater than 8.5. Other C₄-dicarboxylates (e.g. malate, fumarate) inhibited the uptake of succinate suggesting that the permease was general for other C₄-dicarboxylates.
Cytochrome content, succinate dehydrogenase oxidoreductase, and F₁F₀-ATPase activities were lower in membranes from strain TA2.A1 cells grown at pH 7.5 compared to those cultured at 9.5. These data suggest that oxidative phosphorylation-linked processes are down-regulated at neutral pH values, an observation that mirrored oxygen consumption profiles of strain TA2.A1 in whole cells. To study this phenomenon at a molecular level, we measured ATP synthesis by the F₁F₀-ATP synthase from strain TA2.A1 as a function of pH. The strain TA2.A1 F₁F₀-ATP synthase had a pH optimum for ATP synthesis of 9.0-9.5, and significantly lower rates of ATP synthesis observed below pH 9.0. Analysis of the atp operon from the thermoalkaliphilic Bacillus sp. strain TA2.A1 and comparison with other atp operons from alkaliphilic bacteria reveals the presence of a conserved lysine residue at position 180 (Bacillus sp. strain TA2.A1 numbering) within the a subunit of these F₁F₀-ATP synthases. We hypothesize that the basic nature of this residue is ideally suited to capture protons from the bulk phase at high pH. To test this hypothesis, a heterologous expression system for the ATP synthase from Bacillus sp. TA2.A1 (TA2F₁F₀) was developed in Escherichia coli DK8 ([Delta]atp). Amino acid substitutions were made in the a subunit of TA2F₁F₀ at position 180. Lysine (aK180) was substituted for the basic residues histidine (aK180H) or arginine (aK180R), and the uncharged residue glycine (aK180G). ATP synthesis experiments were performed in ADP plus P[i]-loaded right-side out membrane vesicles energized by ascorbate-phenazine methosulfate. When these enzyme complexes were examined for their ability to perform ATP synthesis over the pH range from 7.0 to 10.0, TA2F₁F₀ and aK180R showed a similar pH profile having optimum ATP synthesis rates at pH 9.0 to 9.5 with no measurable ATP synthesis at pH 7.5. Conversely, aK180H and aK180G showed maximal ATP synthesis at pH�s 8.0 and 7.5, respectively. ATP synthesis under these conditions for all enzyme forms was sensitive to DCCD. These data strongly imply that amino acid residue K180 is a specific adaptation within the a subunit of TA2F₁F₀ to facilitate proton capture at high pH. At pH values near the pK[a] of K180, the trapped protons readily dissociate to reach the subunit c binding sites but this dissociation is impeded at neutral pH values causing either a blocking of the proposed H⁺ channel and/or mechanism of proton translocation, and hence ATP synthesis is inhibited.
The mechanisms where by alkaliphilic bacteria obtain iron remains unknown. Growth of strain TA2.A1 at pH 9.5 in the presence of the artificial iron chelators ethylenediamine O-hydroxyphenylacetic acid (EDDHA) and 2�2� dipyridal revealed that iron is an important requirement for aerobic growth at alkaline pH values. Furthermore, biochemical analysis showed that Bacillus alcalophilus and Bacillus pseudofirmus both synthesized orange catecholate siderophores, whilst Bacillus halodurans synthesized a hydroxamate siderophore. These tests showed that strain TA2.A1 synthesized both orange catecholate and hydroxamate siderophore/s. Attempts to purify the catecholate were unsuccessful. No homologues of previously identified non-ribosomal peptide synthase (NRPS) genes in Bacillus subtilis and B.halodurans were detected in the genome of strain TA2.A1 using both PCR and Southern hybridization using known non-ribosomal peptide synthase genes.
13
Bagchi, Tamishraha. "Immune mechanisms in murine brucellosis : studies with strain RB51, a rough mutant of Brucella abortus /." Diss., This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-09162005-115020/.
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Schayes, Claire. "Low cycle fatigue of the Fe-3Si steel : damage mechanisms and strain localisation by EBSD." Thesis, Lille 1, 2016. http://www.theses.fr/2016LIL10002/document.
Повний текст джерелаАнотація:
L’objectif de ce travail de thèse était d’étudier le comportement en fatigue oligocyclique de l’acier Fe-3Si utilisé pour les rotors des nouveaux alterno-démarreurs développés par Valeo. L’étude pris en compte la relation entre la réponse mécanique et la microstructure, les mécanismes de fatigue, l’évaluation de la déformation à une échelle mésoscopique et une optimisation du dimensionnement du rotor. Les essais de fatigue, menés sur des éprouvettes d’épaisseur 350μm, ont montré un durcissement cyclique du matériau et le mode de propagation de la fissure, fragile, souligne sa faible ténacité. La formation de structure de dislocations observées par Microscopie Electronique en Transmission (TEM) et par cECCI (Controlled Electron Channelling Contrast Imaging) est dépendante de la déformation et la variation de déformation totale Δεt=0.7% apparaît comme une valeur de transition. En-dessous de Δεt =0.7%, des arrangements homogènes de dislocations et un amorçage de la fissure transgranulaire sont observés. Au-dessus de Δεt =0.7%, des structures veines-canaux et murs-canaux sont observés et l'amorçage de fissure est intergranulaire. Une étude EBSD (Electron Back-Scattered Diffraction) basée sur les paramètres KAM, GROD et GOS menée sur les éprouvettes fatiguées montre un gradient de déformation dans les grains et une localisation de la déformation aux joints de grain au-dessus de Δεt =0.7%. Enfin, en prenant en compte les propriétés en fatigue de l’acier Fe-3Si dans la base de données matériaux de la modélisation par éléments finis, des légères modifications de la géométrie du rotor ont été effectuées ce qui améliore les performances électromagnétiques de la machineThe objective of the PhD work was to investigate the low cycle fatigue (LCF) behaviour of Fe-3Si steel employed for rotors of new starter-alternators developed by Valeo. The study took into account the relation between mechanical response and microstructure, the fatigue mechanisms, the assessment of strain at a mesoscopic scale and an optimisation of the rotor design. Low cycle fatigue tests performed on 350μm thick specimen pointed out a cyclic hardening of the material. The brittle crack propagation mode underlined the low toughness of the material. The formation of dislocations structures observed by Transmission Electron Microscopy (TEM) and controlled Electron Channelling Contrast Imaging (cECCI) was strain dependant. The total strain variation Δεt =0.7% appeared as a transition value. Below Δεt =0.7%, homogeneous arrangement of dislocations and transgranular crack initiation are observed. Above Δεt =0.7%, a mixture of veins-channels and wall-channels were observed inside the grains and cracking became intergranular. An Electron Back-Scattered Diffraction (EBSD) study was then performed on the specimen fatigued above Δεt=0.7% and approached with KAM, GROD and GOS parameters. It pointed out the strain gradient within grain and highlighted the strain localisation at grain boundaries. Finally, by taking into account LCF properties of the Fe-3Si steel in the material database of the finite element modelling, slight modifications of the rotor geometry have been performed which improves the electromagnetic performances of the machine
15
Jiang, Hao. "A Micromechanical Computational Frameworks for Dynamic Failure Mechanisms in Polycrystalline Materials at High Strain Rates." Case Western Reserve University School of Graduate Studies / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=case1554472837553259.
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Baratoux, Lenka. "Petrology, deformation mechanisms, and fabric anisotropy of metabasites deformed at natural strain and metamorphic gradient." Lyon 1, 2004. http://www.theses.fr/2004LYO10077.
Повний текст джерелаАнотація:
La pétrologie, les mécanismes de déformation, la rhéologie ainsi que les propriétés sismiques de roches métabasiques déformées sous des conditions allant du faciès amphibolite au faciès amphibolite supérieur ont été étudiés. Les amphiboles et les plagioclases montrent une évolution progressive de leur composition et des microstructures avec une augmentation des conditions métamorphiques. A 600-650 ʿC, les deux minéraux se déforment par fracturation accompagnée d'une déformation plastique et par un mécanisme de diffusion fluage. A 700-750 ʿC, la déformation plastique est accompagnée par la nucléation et la croissance. La rhéologie des amphiboles et plagioclases devient similaire grâce aux mécanismes de déformation tels que la diffusion fluage ou la croissance syn-déformation. Un désaccord entre les vitesses sismiques obtenues par une méthode directe et une méthode numérique est interprété en termes de stratification minérale et présence de phases rétrogrades
17
Guros, Frankie. "Emotion Regulation and Strain in Corrections Officers: Examining the Role of Recovery Experiences and Coping Mechanisms." PDXScholar, 2013. https://pdxscholar.library.pdx.edu/open_access_etds/1121.
Повний текст джерелаАнотація:
Research has begun to identify recovery experiences during nonwork time as an important mechanism explaining the relationship between job characteristics and strain (Geurts & Sonnentag, 2006; Kinnunen, Feldt, Siltaloppi, Sonnentag, 2011). Corrections officers face challenges unique to their occupation (Armstrong & Griffin, 2004) that may contribute to the high levels of strain that currently characterize their occupation (i.e., short life expectancy, high suicide rates; Spinaris & Denhof, 2011; Stack & Tsoudis, 1997). Though previous research has not examined emotion regulation, recovery experiences, and coping within corrections officers, these constructs may be of particular importance to an occupation that requires employees to navigate the complex demands of managing an incarcerated population of individuals. Emotion regulation - managing one's emotions - is an important aspect of many jobs, and has been linked with employee strain, such as burnout, psychological strain, and psychosomatic complaints (Hülshege & Schewe, 2011). However, research only begun to assess nonwork recovery and its relationship with emotion regulation strategies at work, and additionally the influence of individual coping strategies has not been considered in past studies. I offered that recovery experiences (i.e., psychological detachment, relaxation, and mastery) mediated the relationship between emotion regulation at work (i.e., reappraisal and suppression) and strain (i.e., disengagement, emotional exhaustion, and psychological distress). I further offered, based on the matching hypothesis (de Jonge & Dormann, 2006), that emotion-focused coping strategies (i.e., emotional social support and venting emotions) moderated the relationships between emotion regulation, recovery experiences, and strain. To examine these hypotheses, data were collected via a survey of corrections officers in Oregon (N = 1317). Results indicated that emotion regulation at work was associated with strain, and this relationship was partially mediated by recovery experiences. The results further suggested that the coping strategies examined in this study did not appear to moderate relationships between the study variables. These findings are discussed in the context of the present sample, and implications for future research are considered.
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Netangaheni, Phumudzo T. "An investigation of the failure mechanisms of thermoplastic composites at various rates of strain and temperatures." Master's thesis, University of Cape Town, 2003. http://hdl.handle.net/11427/8577.
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Includes bibliographical references .The focus of this project is on the deformation behaviour of thermoplastic composites. The materials used were polypropylene and polyamide resins with glass fibres and talc as fillers. These materials were provided by PLASTAMID (pty) ltd. The injection moulded specimens of polypropylene, polyamide 6-6, 30% talc filled polypropylene, 30% short glass fibre reinforced polypropylene and 30% short glass fibre reinforced polyamide 6-6 were tested in tension, flexure and impact (lzod, Charpy and drop-weight). Two different injection-moulding machines were used for specimen manufacture. These were an automated injection moulding machine simulating good control of processing conditions and a simple hand operated injection moulding machine simulating different processing and cooling (crystallisation) conditions. The mechanical tests were performed at different rates of strain and temperatures. The mechanical results show higher ductility of the unfilled polypropylene material. The strength and the elastic modulus of the materials are different for different materials. Addition of short glass fibres enhances the mechanical properties of polypropylene and polyamide 6-6. Talc fillers reduce the tensile, Izod and Charpy impact properties of polypropylene while the flexural and dropweight impact strength is increased. Optical and scanning electron microscopy were used to observe the microstructural features and deformation behaviour such as matrix plastic deformation, matrix crazing and tearing, fibre-matrix debonding, fibre fracture, fibre orientation and crack propagation. These deformation behaviours are influenced by the test conditions such as strain rate, temperature and the type of the test conducted. The deformation behaviour is also dependent on the constituents of the material. The mechanical test response together with the macro- and microscopic features observed on the fracture surfaces of tested specimens are evidence of the various mechanisms of failure that take place in different thermoplastic composites. The understanding of the mechanical response and the failure mechanisms of thermoplastic matrix composites is important in the design and processing stages.
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Brown, Nathan Chandler. "Characterizing Behaviors and Functions of Joints for Design of Origami-Based Mechanical Systems." BYU ScholarsArchive, 2021. https://scholarsarchive.byu.edu/etd/9269.
Повний текст джерелаАнотація:
This thesis addresses a number of challenges designers face when designing deployable origami-based arrays, specifically joint selection, design, and placement within an array. In deployable systems, the selection and arrangement of joint types is key to how the system functions. The kinematics and performance of an array is directly affected by joint performance. This work develops joint metrics which are then used to compare joint performances, constructing a tool designers can use when selecting joints for an origami array. While often a single type of joint is used throughout an array, this work shows how using multiple types of joints within the same array can offer benefits for motion deployment, and array stiffening.
Origami arrays are often used for their unique solutions for stowing and deploying large planar shapes. Folds, enabled through joints, within these patterns allow the arrays to fold compactly. However, it can be difficult to fully deploy arrays, particularly array designs with a high number of joints. In addition, it is a challenge to stabilize a fully deployed array from undesired re-folding. This work introduces a strain-energy storing joint that is used to deploy and stiffen foldable origami arrays, the Lenticular Lock (LentLock). Geometry of the LentLock is introduced and the deploying and stiffening performance of the joint is shown.
Folds within an origami array create the constraints that link motion between panels, and can be used to create kinematic benefits, such as creating mechanisms with a single degree-of-freedom. While many fold-constraints are required to define motion, this work shows that origami-based system contain many redundant constraints. The removal of redundant joints does not affect the motion of the array nor the observed mobility, but may decrease the likelihood of binding, simplify the overall system and decrease actuation force. This work introduces a visual and iterative approach designers can use to identify redundant constraints in origami patterns, and techniques that can be used to remove the identified redundant constraints. The presented techniques are demonstrated by removing redundant constraints from prototyped origami mechanisms.
As a result of this work, designers will be better able to approach and design deployable origami-based mechanisms.
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Mok, Benjamin Chit Man. "An investigation of strain localization in cemented sands and mechanisms of stiffness anisotropy using the dem simulations /." View abstract or full-text, 2007. http://library.ust.hk/cgi/db/thesis.pl?CIVL%202007%20MOK.
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Ross, Ian Lindsay. "Mechanisms of biocontrol of Gaeumannomyces graminis var. tritici by Pseudomonas corrugata strain 2140 : genetic and biochemical aspects." Title page, table of contents and summary only, 1996. http://web4.library.adelaide.edu.au/theses/09PH/09phr824.pdf.
Повний текст джерелаАнотація:
Bibliography: leaves 207-220. Pseudomonas corrigata strain 2140 (Pc2140), isolated from wheat field soil in Australia, antagonises the take-all fungus, Gaeumannomyces graminis var. tritici (Ggt) in vitro and significantly reduces take-all symptoms on wheat in pot trials. This study investigates the mechanisms by which the biocontrol agent reduces the disease symptoms. Biochemical analysis of metabolites of P. corrugata 2140 reveal a number of compounds potentially antagonistic to Ggt and which may play a role in disease control. These include water-soluble antibiotics, siderophores, proteases, peptides and volatiles including hydrogen cyanide.
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LeBlanc, Justin Christian. "Mechanisms of survival against desiccation and associated stresses of the soil-residing actinomycete Rhodococcus jostii strain RHA1." Thesis, University of British Columbia, 2014. http://hdl.handle.net/2429/51756.
Повний текст джерелаАнотація:
Actinomycetes are an abundant bacterial group in soil, with a critical role in the decomposition of organic matter. Rhodococcus jostii strain RHA1 is of particular interest to the field of bioremediation because it can degrade a broad range of organic compounds, both natural and xenobiotic. Understanding the factors contributing to the desiccation resistance of RHA1 will enrich our basic knowledge of this common soil stress and may help advance bioremediation technologies for contaminated soils subject to droughts.
Here I report the first transcriptomic analysis of a Gram-positive bacterium during desiccation. Filtered RHA1 cells incubated at either low relative humidity, as an air-drying treatment, or high relative humidity, as a control, were transcriptionally profiled over a comprehensive time series. Also, the morphology of RHA1 cells was characterized by cryofixation scanning electron microscopy during each treatment. Desiccation resulted in a transcriptional response of 819 differentially regulated genes, 8-times more than in the control. Included among the highly up-regulated desiccation-specific genes was dps1 (induced 33-fold), encoding an oxidative stress protection protein which has not previously been directly associated with desiccation, as well as sigF3 (induced 58-fold), encoding a sigma factor possibly involved in the regulatory response to desiccation.
RHA1 mutants with dps1 or both of its dps homologs deleted were challenged with oxidative stressors under a variety of assay conditions. The mutants were also exposed to physiological stresses that generate reactive oxygen species intracellularly, including desiccation. In all cases, the dps− mutants did not have impaired oxidative stress resistance – a novel finding with respect to bacterial dps-null strains. Additionally, the RHA1 dps-null mutant did not have substantially lower survival compared to the wild type when challenged with metal toxicity or DNA-damaging agents or when they were cocultured through multiple cycles of starvation. Nevertheless, expression of RHA1 dps1 in an Escherichia coli dps– mutant restored its hydrogen peroxide resistance. Purified RHA1 Dps1 was shown to have ferroxidase activity and thereby to protect DNA from oxidative damage. The general insensitivity of the RHA1 dps-null mutant may be representative of a large group of Actinobacteria for which robust oxidative stress tolerance is an important adaptation.Science, Faculty of
Microbiology and Immunology, Department of
Graduate
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Ntogwa, Mpumelelo. "Mechanisms of HIV-induced peripheral neuropathic pain by focusing on Schwann cell-macrophage interaction." Doctoral thesis, Kyoto University, 2021. http://hdl.handle.net/2433/263602.
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Volmer, Jan [Verfasser]. "Strain and process engineering to exploit solvent tolerance mechanisms of Pseudomonas taiwanensis VLB120 for asymmetric styrene epoxidation / Jan Volmer." Aachen : Shaker, 2016. http://d-nb.info/1098039130/34.
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Reza, Gholizadeh. "Formation Mechanisms of Fine/Ultra-fine Grains in Metals Highly Deformed by Torsion at Various Temperatures and Strain Rates." Kyoto University, 2017. http://hdl.handle.net/2433/227626.
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Markurt, Toni. "Transmission electron microscopy investigation of growth and strain relaxation mechanisms in GaN (0001) films grown on silicon (111) substrates." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2016. http://dx.doi.org/10.18452/17405.
Повний текст джерелаАнотація:
In dieser Arbeit untersuchen wir die grundlegenden Wachstums- und Relaxationsprozesse, die es erlauben den Verzerrungszustand von GaN (0001) beim Wachstum auf Silizium (111) Substraten einzustellen und die resultierende Dichte an Durchstoßversetzungen zu reduzieren. Zu deren Analyse werden GaN (0001) Schichten, die mittels metallorganischer Gasphasenepitaxy abgeschieden worden sind, hauptsächlich mit transmissionselekronenmikroskopischen Methoden untersucht. Die wesentlichen Erkenntnisse der Arbeit sind: (i) Der Aufbau einer kompressiven Verzerrung von GaN (0001) Filmen mittels AlGaN Zwischenschichten beruht auf einer Asymmetrie der plastischen Relaxation an den beiden Grenzflächen der AlGaN Zwischenschicht. Fehlpassungsversetzungen bilden sich zwar an beiden Grenzflächen aus, jedoch ist der mittlere Abstand zwischen Versetzungslinien an der unteren Grenzfläche kleiner, als an der oberen. (ii) Plastische Relaxation von verzerrten (0001) Wurtzit Schichten erfolgt im Wesentlichen durch Bildung von a-Typ Fehlpassungsversetzungen im 1/3 |{0001} Gleitsystem. Diese bilden sich aber nur dann, wenn die verzerrten Schichten eine 3-D Morphologie aufweisen. Eine quantitative Modellierung dieses Prozesses zeigt, dass die kritische Schichtdicke für das Einsetzen der plastischen Relaxation wesentlich vom Wachstumsmodus bestimmt wird. (iii) Eine Silizium Delta-Dotierung der GaN (0001) Oberfläche führt zum Wachstum einer kohärenten Sub-Monolage SiGaN3, die eine periodisch Anordnung von Silizium- und Galliumatomen, sowie Galliumvakanzen aufweist. Da das Wachstum von GaN direkt auf der SiGaN3-Monolage unterdrückt ist, tritt ein Übergang zu 3-D Inselwachstum auf, das zunächst ausschließlich in Löchern der SiGaN3-Monolage anfängt. Eine hohe Konzentration von Silizium auf der GaN (0001) Oberfläche wirkt also als Anti-Surfactant beim epitaktischen Wachstum von GaN. Rechnungen mittels der Dichtefunktionaltheorie liefern Erklärungen für das beobachtete Wachstumsverhalten.In this work we study the basic growth and relaxation processes that are used for strain and dislocation engineering in the growth of GaN (0001) films on silicon (111) substrates. To analyse these processes, samples, grown by metalorganic vapour phase epitaxy were investigate by means of transmission electron microscopy. Our investigations have revealed the following main results: (i) Strain engineering and build-up of compressive strain in GaN (0001) films by means of AlGaN interlayer is based on an asymmetry in plastic relaxation between the two interfaces of the AlGaN interlayer. Although misfit dislocation networks form at both interfaces of the interlayer, the average spacing of dislocation lines at the lower interface is smaller than that at the upper one. (ii) Plastic relaxation of strained (0001) wurtzite films is caused mainly by formation of a-type misfit dislocations in the 1/3 |{0001} slip-system. These a-type misfit dislocations form once the strained films undergo a transition to a 3-D surface morphology, e.g. by island growth or cracking. Quantitative modelling of this process reveals that the critical thickness for nucleation of a-type misfit dislocations depends next to the lattice mismatch mainly on the growth mode of the film. (iii) Silicon delta-doping of the GaN (0001) surface leads to the growth of a coherent sub-monolayer of SiGaN3 that shows a periodic arrangement of silicon and gallium atoms and gallium vacancies. Since growth of thick GaN layers directly on top of the SiGaN3-monolayer is inhibited a transition towards 3-D island growth occurs, whereby GaN islands exclusively nucleate at openings in the SiGaN3-monolayer. A high concentration of silicon on the GaN (0001) surface thus acts as an anti-surfactant in the epitaxial growth of GaN. Our density functional theory calculations provide an explanation for both the self-limited growth of the SiGaN3-monolayer, as well as for the blocking of GaN growth on top of the SiGaN3-monolayer.
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Whitezell, Marc A. "Development of a Strain Energy Storage Mechanism Using Tension Elements to Enhance Golf Club Performance." Diss., CLICK HERE for online access, 2006. http://contentdm.lib.byu.edu/ETD/image/etd1258.pdf.
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Harris, Matthew Bradley. "Elucidating the Mechanisms of Rate-Dependent Deformation at Ambient Temperatures in a Model Metallic Glass." BYU ScholarsArchive, 2015. https://scholarsarchive.byu.edu/etd/6145.
Повний текст джерелаАнотація:
In this work, the Shear Transformation Zone (STZ) dynamics model is adapted to capture the transitions between different regimes of flow serration in the deformation map of metallic glass. This was accomplished by scaling the STZ volume with a log-linear fit to the strain rate, and also adjusting the activation energy of an STZ with a log-linear fit to maintain constant yield strength at differing strain rates. Twelve simulations are run at each of six different strain rates ranging from 10-5 to 100 s-1, and statistics are collected on simulation behavior and shear band nucleation and propagation rates. The simulations show shear band nucleation has a positive correlation to strain rate, and shear band propagation has a negative correlation to strain rate. This shows that in STZ dynamics, the regime of reduced flow serration arises due to competing rates of nucleation and propagation, supporting the hypothesis proposed by Schuh. A positive correlation between critical shear band nucleus size and strain rate is proposed as an underlying cause of these rate dependencies.
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Torabiandehkordi, Noushin. "High and very high cycle fatigue behavior of DP600 dual-phase steel : correlation between temperature, strain rate, and deformation mechanisms." Thesis, Paris, ENSAM, 2017. http://www.theses.fr/2017ENAM0020/document.
Повний текст джерелаАнотація:
Ce travail vise à améliorer notre compréhension du comportement en fatigue à grand et très grand nombre de cycles d’un acier ferrito-martensitique dual phase, notamment les effets de la température et de la vitesse de déformation résultant de chargements cycliques à haute fréquence. L'effet de la fréquence sur la réponse en fatigue de l'acier DP600 a été étudié en effectuant des essais de fatigue sur une machine ultrasonique travaillant à 20 kHz et sur une machine conventionnelle travaillant à des fréquences inférieures à 100 Hz. Des études de fractographie et des observations microscopiques à la surface des échantillons ont été effectuées pour étudier les mécanismes de déformation et de rupture. De plus, la thermographie infrarouge in situ a été utilisée pour étudier la réponse thermique et les mécanismes dissipatifs du matériau lors des essais de fatigue. Les courbes S-N ont été déterminées à partir de chargements de fatigue ultrasoniques à 20 kHz et d’essais conventionnels à 30 Hz. La durée de vie pour une amplitude de contrainte donnée est plus élevée dans le cas de la fatigue ultrasonique bien que la limite de fatigue soit identique dans les deux cas. L’augmentation inévitable de la température en fatigue ultrasonique à fortes amplitudes de contraintes, ainsi que le comportement dépendant de la vitesse de déformation de la ferrite, en tant que structure CC, ont été trouvés comme les paramètres clés expliquant le comportement observé en fatigue, et la réponse thermique sous les fréquences faibles et ultrasoniques. Les écarts observés entre l’essai de fatigue conventionnel et celui ultrasonique ont été évalués à travers les mécanismes de mobilité des dislocations vis dans la phase ferritique de structure cubique centrée (CC). La durée de vie plus élevée et l’amorçage de la fissure principale sur une inclusion observés en fatigue ultrasonique ont été attribués au vieillissement dynamique résultant du fort auto-échauffement du matériau aux fortes amplitudes de contraintes. L'existence d'une transition du régime thermiquement activé au régime athermique avec l’augmentation de l'amplitude de contrainte a été mise en évidence. Au-dessous de la limite de fatigue, la déformation a lieu dans un régime thermiquement activé alors qu'elle est dans un régime athermique au-dessus de la limite de fatigue. En fatigue conventionnelle, la déformation est athermique pour toutes les amplitudes de contrainte. Une carte de transition a été produite en utilisant les résultats expérimentaux pour l'acier DP600 ainsi que les données disponibles dans la littérature pour d'autres aciers à base de ferrite, montrant ainsi la corrélation entre le mouvement des dislocations vis thermiquement activé et l'absence de rupture en fatigue à très grand nombre de cycleThis work is an attempt towards a better understanding of the high cycle and very high cycle fatigue behaviors of a ferritic-martensitic dual-phase steel, with a regard to temperature and strain rate effects, resulting from accelerated fatigue loading frequencies. The influence of frequency on fatigue response of DP600 steel was investigated by conducting ultrasonic and conventional low frequency fatigue tests. Fractography studies and microscopic observations on the surface of specimens were carried out to study the deformation and fracture mechanisms under low and ultrasonic frequencies. Moreover, in situ infrared thermography was carried out to investigate the thermal response and dissipative mechanisms of the material under fatigue tests. The S-N curves were determined from ultrasonic 20-kHz fatigue loadings and conventional tests at 30 Hz. Fatigue life for a given stress amplitude was found to be higher in the case of ultrasonic fatigue whereas the fatigue limit was the same for both cases. Moreover, crack initiation was always inclusion-induced under ultrasonic loading while under conventional tests it occurred at slip bands or defects on the surface. The inevitable temperature increase under ultrasonic fatigue at high stress amplitudes along with the rate dependent deformation behavior of ferrite, as a body centered cubic (BCC) structure, were found as the key parameters explaining the observed fatigue behavior and thermal response under low and ultrasonic frequencies. The discrepancies observed between conventional and ultrasonic fatigue tests were assessed through the mechanisms of screw dislocation mobility in the ferrite phase as a BCC structure. The higher fatigue life and inclusion-induced crack initiations in the case of ultrasonic loading were attributed to the dynamic strain aging, which resulted from the high temperature increases at high stress amplitudes. The existence of a transition in deformation regime from thermally-activated to athermal regime under ultrasonic fatigue loading by increasing the stress amplitude was confirmed. Below the fatigue limit, deformation occurred in thermally-activated regime while it was in athermal regime above the fatigue limit. Under conventional loading deformation occurred in athermal regime for all stress amplitudes. From the analysis of the experimental data gathered in this work, guidelines were given regarding the comparison and interpretation of S-N curves obtained from conventional and ultrasonic fatigue testing. A transition map was produced using the experimental results for DP600 steel as well as data available in the literature for other ferrite based steels, showing the correlation between thermally-activated screw dislocation movement and the absence of failure in very high cycle fatigue
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Soundararajan, Manonmani [Verfasser], and Tobias [Gutachter] Ölschläger. "Investigations into the mechanisms behind the antagonistic effects and phage resistance of probiotic Escherichia coli strain Nissle 1917 / Manonmani Soundararajan ; Gutachter: Tobias Ölschläger." Würzburg : Universität Würzburg, 2020. http://d-nb.info/1221063162/34.
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Lu, Xuekun. "Characterisation of the anisotropic fracture toughness and crack-tip shielding mechanisms in elephant dentin." Thesis, University of Manchester, 2015. https://www.research.manchester.ac.uk/portal/en/theses/characterisation-of-the-anisotropic-fracture-toughness-and-cracktip-shielding-mechanisms-in-elephant-dentin(5f0a739b-e30a-401e-905a-b38e9224ac0a).html.
Повний текст джерелаАнотація:
Teeth trauma has become one of the most serious physical problems that people are suffering from in the past years. Early diagnosis and management are desperately needed to improve tooth survival, functionality and avoid the tooth loss. However, the diagnosis of cracked tooth could be challenging due to the small size of the crack. Therefore, a mechanics understanding of the tooth fracture is demanding from the perspective of developing a framework for failure prediction in clinical research and bio-mimetic restorative materials. This study focuses on characterising the anisotropic fracture behaviour and the crack shielding mechanisms in elephant dentin. This is often used as a structural analogue for human dentin due to the similarities in microstructure and chemical composition, in order to avoid the test-piece size restrictions, given the larger size of ivory than human teeth. Compact tension test-pieces were extracted from different locations on the ivory tusk so as to have different crack growth directions relative to the microstructure to inspect the fracture anisotropies. The fracture toughness as a function of the crack extension was assessed in terms of fracture resistance curves (R-curve). The accumulative crack-tip strain fields were also measured for the first time in dentin using digital image correlation technique (DIC) to investigate the capability of crack-tip elastic/plastic deformation before material failure. Investigation of crack morphologies, the interaction between crack and the microstructures, the fracture surfaces using both 2-D and 3-D techniques could provide with insights into extrinsic shielding mechanisms. Surface and volume crack opening displacement (COD) were measured for the first time optically and by X-ray computed tomography to investigate the effect of extrinsic crack-tip shielding. The displacement fields around the crack-tip obtained by DIC were fitted using Westergaard’s analytical solution to extract the effective stress intensity factor, by comparing this to the applied load, the efficiency of the crack-tip shielding could be evaluated. A novel cohesive element model (traction-separation law) was then established based on the COD results to simulate the physical process of crack-tip shielding. It is the first time the cohesive model has been adapted to studying the direct crack behaviour measured by in-situ experiment to predict the crack growth. This model was then validated using the crack-tip strain field and R-curve obtained from the experiment measurement.
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Strömbro, Jessica. "Micro-mechanical mechanisms for deformation in polymer-material structures." Doctoral thesis, KTH, Hållfasthetslära (Inst.), 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4626.
Повний текст джерелаАнотація:
In this thesis, the focus has been on micro-mechanical mechanisms in polymer-based materials and structures. The first part of the thesis treats length-scale effects on polymer materials. Experiments have showed that the smaller the specimen, the stronger is the material. The length-scale effect was examined experimentally in two different polymers materials, polystyrene and epoxy. First micro-indentations to various depths were made on polystyrene. The experiments showed that length-scale effects in inelastic deformations exist in polystyrene. It was also possible to show a connection between the experimental findings and the molecular length. The second experimental study was performed on glass-sphere filled epoxy, where the damage development for tensile loading was investigated. It could be showed that the debond stresses increased with decreasing sphere diameter. The debonding grew along the interface and eventually these cracks kinked out into the matrix. It was found that the length to diameter ratio of the matrix cracks increased with increasing diameter. The experimental findings may be explained by a length-scale effect in the yield process which depends on the strain gradients. The second part of the thesis treats mechano-sorptive creep in paper, i.e. the acceleration of creep by moisture content changes. Paper can be seen as a polymer based composite that consists of a network of wood fibres, which in its turn are natural polymer composites. A simplified network model for mechano-sorptive creep has been developed. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre-fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys a constitutive law that is non-linear in stress. Fibre kinks are included in order to capture experimental observations of larger mechano-sorptive creep effects in compression than in tension. Furthermore, moisture dependent material parameters and anisotropy are taken into account. Theoretical predictions based on the developed model are compared to experimental results for anisotropic paper both under tensile and compressive loading at varying moisture content. The important features in the experiments are captured by the model. Different kinds of drying conditions have also been examined.QC 20100910
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Djukic, Stéphanie. "Damage mechanisms under tensile stress of amorphous and low semi-crystalline polymers." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSE1041.
Повний текст джерелаАнотація:
Ces dernières années, une nouvelle classe de polyamides a été développée pour des applications spécifiques nécessitant de meilleures propriétés mécaniques et thermiques, telles que l’électronique ou l’industrie automobile. Les polyphtalamides (PPA) sont des polyamides semi-aromatiques, contenant des cycles aromatiques dans leur chaine principale. Récemment, les mécanismes d’endommagement ont été étudiés dans le cas de polymères semi-cristallins (PA66) et amorphes (acétate de cellulose). Le but de la thèse a été d’étudié les mécanismes d’endommagement de polymères amorphes et semi-cristallins. Par ailleurs, peu d'études ont été réalisées pour caractériser les propriétés des PPA purs. Des données plus détaillées concernant leurs propriétés mécaniques sont nécessaires. L'étude des propriétés de cette classe de polymères est importante, d'autant plus que leurs applications sont différentes des polyamides aliphatiques. Nous avons dans un premier temps caractérisé ces polymères puis étudié leur comportement mécanique en traction, compression et choc. Nous avons ainsi pu mettre en évidence l’apparition d’un phénomène de striction et d’un régime de durcissement plastique (strain hardening) dès 20% de déformation pour trois PPA étudiés. Le durcissement plastique a également été observé dans d’autres polymères amorphes ductiles tels que le polycarbonate (PC) ou le poly(méthyle methacrylate) (PMMA). Afin de décrire les mécanismes microscopiques liés à l’initiation et la propagation de l’endommagement sous traction de nos polymères, nous avons réalisé des observations par microscopie électronique à balayage en transmission (STEM) et microscopie optique ainsi que par diffusion des rayons X aux très petits angles (USAXS). L’analyse de ces différents polymères (PPA amorphes et semi-cristallins, PC, PMMA) par USAXS met en évidence différents modes d’endommagement. La nucléation simultanée de craquelures nanométriques autour des défauts préexistants (défauts liés au processus d’injection), puis la croissance limitée de ces craquelures sont observées pour les PPA amorphes, le PC et le PMMA étudiés, dans un mécanisme similaire à celui étudié dans le cas de l’acétate de cellulose. L’endommagement est bloqué dans un premier temps par le durcissement plastique. Dans le cas du polycarbonate et du PMMA, lorsque la contrainte appliquée devient suffisamment élevée, une petite fraction de ces craquelures croît plus rapidement jusqu’à entrainer la rupture de l’échantillon, ce qui permet d’observer l’évolution d’une deuxième famille de craquelures de plus grandes tailles. On observe également la croissance d’une deuxième famille de craquelures pour l’un des PPA amorphe, mais leur croissance s’arrête à l’apparition de la striction. Aucune rupture n’est observée. Les deux PPA semi-cristallins se déforment par striction sans rupture. Aucune cavitation n’a été observée par USAXS. L’endommagement des polymères étudiés se classe en trois catégories. La première catégorie concerne les polymères qui ont un comportement similaire à l’acétate de cellulose. On observe la nucléation de craquelures dont la croissance est dans un premier temps bloquée par le durcissement plastique. Lorsque la contrainte appliquée devient suffisamment élevée une fraction de ces craquelures croît plus rapidement jusqu’à entrainer la rupture de l’échantillon, ce qui permet d’observer l’évolution d’une deuxième famille de craquelures de plus grandes tailles. Cette catégorie concerne le polycarbonate et le PMMA. La deuxième catégorie concerne l’un des PPA amorphe ayant des craquelures dont la croissance est dans un premier temps bloquée par le durcissement plastique. La croissance d’une deuxième famille de craquelures s’arrête à l’apparition de la striction, et aucune rupture n’est observée. La dernière catégorie concerne les polymères ne présentant pas d’endommagement (PPA semi-cristallins). Nous proposons dans cette thèse une interprétation de ces différents mécanismes d’endommagementIn the last few years, a new class of polyamides has been developed for specific applications requiring better mechanical and thermal properties, such as electronics or automotive industry. Polyphthalamides (PPA) are semi-aromatic polyamides containing aromatic rings in their main chain. Recently, the damage mechanisms have been studied in the case of semi-crystalline (PA66) and amorphous (cellulose acetate) polymers. The aim of the PhD thesis was to study the damage mechanisms of amorphous and semi-crystalline polymers. In addition, few studies have been conducted to characterize the properties of pure PPA. More detailed data regarding their mechanical properties are needed. The study of the properties of this class of polymers is important, especially since their applications are different from aliphatic polyamides. We first characterized these polymers and then studied their mechanical behavior in traction, compression and Charpy impact strength. We have thus been able to highlight the appearance of a necking phenomenon and a strain hardening regime from 20% deformation for three PPA studied. Strain hardening has also been observed in other amorphous ductile polymers such as polycarbonate (PC) or poly(methyl methacrylate) (PMMA). The strain hardening stabilizes the deformation by avoiding the localization of the damage. In order to describe the microscopic mechanisms associated with the initiation and propagation of damage under tensile deformation of our polymers, we have carried out observations by scanning transmission electron microscopy (STEM) and optical microscopy as well as by Ultra-small angle X-ray scattering (USAXS). The analysis of these different polymers (amorphous and semi-crystalline PPA, PC, PMMA) by USAXS highlights different modes of damage. The simultaneous nucleation of nanometric crazes around the pre-existing defects (defects related to the injection process), then the limited growth of these crazes are observed for the amorphous PPA, the PC and the PMMA studied, in a mechanism similar to that studied in the case of cellulose acetate. The damage is blocked in the first place by the strain hardening. However, in the case of polycarbonate and PMMA, when the stress applied becomes sufficiently high, a small fraction of these crazes grows faster to cause the rupture of the sample, which allows to observe the evolution of a second family of larger crazes. We also observe the growth of a second family of crazes for one of the amorphous PPA, but their growth stops at the appearance of necking. No failure of the sample is observed. The second amorphous PPA is damaged by cavitation up to about 5% deformation. The cavities stop to grow at the appearance of shear bands. When the shear bands propagated on either side of the sample, the failure is observed. The two semi-crystalline PPA deform by necking without failure. No cavitation was observed by USAXS. The damage of the polymers studied can be classified into three categories. The first category concerns polymers that behave similarly to cellulose acetate. Crazing nucleation is observed, which growth is initially blocked by strain hardening. When the stress applied becomes sufficiently high, a fraction of these crazes increases more rapidly until the sample is broken, which makes it possible to observe the evolution of a second family of larger crazes. This category concerns polycarbonate and PMMA. The second category concerns one of the amorphous PPA with crazes whose growth is initially blocked also by strain hardening. The growth of a second family of crazes stops at the appearance of necking, and no failure of the specimen is observed. The last category concerns polymers that do not show any damage. These are the two semi-crystalline PPA. We propose in this thesis an interpretation of these different mechanisms of damage
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Fowler, Rebecca M. "Identification of deformation mechanisms during bi-axial straining of superplastic AA5083 material." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2004. http://handle.dtic.mil/100.2/ADA432796.
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Casalena, Lee. "Multimodal Nanoscale Characterization of Transformation and Deformation Mechanisms in Several Nickel Titanium Based Shape Memory Alloys." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1499568013015563.
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Mott, Jason M. "Identification and characterization of survival mechanisms of the Human Granulocytic Ehrlichiosis Agent and identification and characterization of an Ehrlichia Risticii Strain isolated from Aquatic Insects and Horse isolates /." The Ohio State University, 2001. http://rave.ohiolink.edu/etdc/view?acc_num=osu1486399451962255.
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Markurt, Toni [Verfasser], Henning [Akademischer Betreuer] Riechert, Sir Colin [Akademischer Betreuer] Humphreys, and Armin [Akademischer Betreuer] Dadgar. "Transmission electron microscopy investigation of growth and strain relaxation mechanisms in GaN (0001) films grown on silicon (111) substrates / Toni Markurt. Gutachter: Henning Riechert ; Sir Colin Humphreys ; Armin Dadgar." Berlin : Mathematisch-Naturwissenschaftliche Fakultät, 2016. http://d-nb.info/1081418443/34.
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Barhum, Rabea. "Mechanisms of the interaction between continuous and short fibres in textile-reinforced concrete (TRC)." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-143501.
Повний текст джерелаАнотація:
This thesis reports on experimental investigations of the mechanisms inherent in the joint action of short and continuous fibres in high-performance, cement-based composites. Experiments on different levels of observation (macro- meso- and micro-levels) were performed to provide detailed insights into the various effects of adding different types of short fibres (dispersed AR glass, integral AR glass and dispersed carbon fibres) on the strength, deformation, and failure behaviour of textile-reinforced concrete (TRC) subjected to tensile loading. Moreover, visual inspections of the specimens' surfaces and microscopic investigation of the fracture surfaces and the interface zone between fibre and matrix were performed and evaluated. Subsequently, the mathematical descriptions for TRC with short fibres under deformation controlled tensile loading conditions were derived based on a multi-scale rheological-statistical modelling approach. Based on a literature review, the state of the art is presented and discussed to identify key questions that are yet to be answered satisfactorily. This provides the starting point for the investigations presented in this thesis. The experimental program on the macro-level included uniaxial tension tests performed on thin, narrow plates reinforced by: a) only textile reinforcement, b) only short fibres, and c) hybrid reinforcement (both textile reinforcement with the addition of short fibres). Special attention was directed toward the course of the stress-strain relationship, crack pattern development, and fibre failure behaviour. The stress-strain curves resulting from uniaxial tension testing demonstrated clearly the positive influence of all types of short fibre on the mechanical performance of TRC. While the first-crack stress in TRC specimens increased significantly due to the addition of short fibres, an expansion of the strain region, where multiple cracks form, was observed for the stress-strain curves for TRC with added short fibres. The visual inspection of the specimens\' surfaces showed a higher number of cracks and finer crack patterns for given strain levels in the cases when short fibres were added to TRC. Moreover, depending on fibre type, the positive effects of the addition of short fibres on both tensile strength and work-to-fracture of the composite were found to vary significantly. The findings at the micro- and meso-levels of observation provided to a great extent a core of understanding of some particular mechanical behavioural properties of TRC with short fibres at the macro-level of observation. Thus, in addition to the experimental testing performed on composite materials with different parameter combinations, investigations of the action of individual material components, i.e., multifilament-yarns and single short fibres, embedded into cement-based matrices were carried out. It was found that short fibres indeed improve the bond between multifilament-yarns and the surrounding matrix. By their random positioning on the yarn\'s surface, short fibres built new adhesive cross-links which provided extra connecting points to the surrounding matrix. Furthermore, the water-to-binder ratio of the matrix influenced bond quality between fibre and matrix, i.e., various degrees of matrix-fibre bond were observed. As a result, the mechanical behaviour of the composite varied with w/b: While the good bond of the fibre embedded in a matrix with a low water-to-binder ratio leads to increase in stiffness and strength of the composite, fibres with weak bonding can be considered as defects with respect to stiffness as they lead to a decrease in the value. The thesis further derives the mathematical relationships for TRC with the addition of short fibres under deformation-controlled tensile loading. A physically based rheological model consisting of simple rheological elements was developed based on the experimental results on the micro-scale, using single-fibre pullout tests. Special attention was paid to the gradual de-bonding process and the resulting force-displacement branch. The model adequately reproduced both relevant fibre failure scenarios: fibre fracture and fibre pullout. By means of statistical procedures the combination of these models led to description of the stress-crack opening behaviour of an individual crack bridged by the given number of short fibres. The stress-strain relation for TRC with short fibres subjected to tensile loading was then derived. The concept followed at the macro-level of observation was modelling separately the three main regions of the characteristic stress-strain curve. The regions of crack-free material and crack-widening were considered linear and described based on the corresponding characteristic values of each region. The behaviour of the multiple cracking region was derived by considering an increasing number of cracks in serial interconnection and the contribution of the uncracked matrix in between. The stress transfer, i.e., bridging stress, across the crack was determined based on the contribution of both short fibres and multifilament-yarns. Behaviour of individual cracks was adjusted by varying the number of bridging fibres in different cracks and by varying the yarn bridging stress according to range observed in the pullout experimentsIn der vorliegenden Arbeit wird über Untersuchungen zu den Mechanismen der Wechselwirkungen zwischen Kurz- und Endlosfasern in zement-basierenten Hochleistungskompositen berichtet. Hierzu wurden experimentelle Untersuchungen auf verschiedenen Betrachtungsebenen (Makro-, Meso- und Mikroebene) durchgeführt mit dem Ziel, detaillierte Erkenntnisse zu den Auswirkungen der Zugabe von verschiedenen Arten von Kurzfasern (disperse und integrale AR-Glasfasern, Kohlenstofffasern) hinsichtlich des Festigkeits-, Verformungs- und Bruchverhaltens von Textilbeton (engl.: textile-reinforced concrete = TRC) unter Zugbeanspruchung zu gewinnen. Die Bruchflächen sowie die Gestalt der Interphase zwischen der Bewehrung aus Textilien oder Kurzfasern und der umhüllenden zemengebundenen Matrix wurden mit optischen und elektronenmikroskopischen Verfahren hinsichtlich der Wechselwirkungsphänomene ausgewertet. Die Ergebnisse der experimentellen Arbeiten bildeten den Ausgangspunkt für die mathematischen Beschreibungen für TRC mit Kurzfasern unter verformungsgesteuerter Zugbelastung. Die Formulierungen erfolgten auf Grundlage multiskalarer rheologisch-statistischer Modellansätze. In einer Literatursichtung wurde zunächst der Kenntnisstand zu den Materialien und zum Verhalten von TRC und Faserbeton unter Zugbeanspruchung dargestellt und diskutiert. Die noch zu erforschenden Fragen wurden präzisiert und die Grundlagen für deren Untersuchung geschaffen. Bei den Experimenten auf der Makroebene wurden drei Bewehrungsvarianten betrachtet: a) textile Bewehrung, b) Kurzfaserbewehrung, und c) hybride Bewehrung (Textil und Kurzfasern). An Dehnkörpern wurde die Spannungs-Dehnungsbeziehung unter einachsiger Zugbelastung studiert und dabei das Rissbild und die Phänomene des Faserversagens detailliert beobachtet. Anhand der Spannungs-Dehnungsbeziehungen konnte gezeigt werden, dass die Zugabe von Kurzfasern bei allen untersuchten Kurzfaserarten zu einer erheblichen Verbesserung der Leistungsfähigkeit von Textilbeton führt. Dies zeigte sich unter anderem in einer ausgeprägten Anhebung der Erstrissspannung sowie der Entwicklung von zahlreicheren und damit feineren Rissen, die zu einer Verbesserung der Duktilität führten. Ebenso wurden Steigerungen der Zugfestigkeit und der Energiedissipation festgestellt. In welchem Maß diese Änderungen stattfinden, hängt von der Art der Kurzfasern ab. Die Experimente auf der Mikro- und Mesoebene wurden so konzipiert, dass sie die Erkundung der Mechanismen, die den auf der Makroebene beobachteten Phänomenen zugrunde liegen, unterstützten. Auf der Mesoebene wurden Mulitifilamentgarnauszugversuche (mit und ohne Kurzfasern in der Matrix) und auf der Mikroebene Einzelfaserauszugsversuche für alle betrachteten Kurzfasertypen durchgeführt. Es wurde festgestellt, dass die Kurzfasern den Verbund zwischen Matrix und Multifilamentgarn verbessern. Kurzfasern können bei zufälliger Positionierung an der Garnoberfläche zusätzliche Haftbrücken bzw. Verbindungsstellen zu umgebender Matrix bilden. Für die Verbundqualität zwischen Faser und Matrix ist der Wasser-Bindemittel-Wert (W/B-Wert) von entscheidender Bedeutung. Bei einer Matrix mit niedrigem W/B-Wert führt die gute Qualität des Verbunds der eingebetteten Fasern zu einer Erhöhung der Steifigkeit sowie der Festigkeit des Komposites. Bei hohem W/B-Wert haben die Fasern einen schlechten Verbund zur Matrix und müssen überwiegend als Fehl- bzw. Schwachstellen betrachtet werden. Festigkeit und Steifigkeit des Komposits nehmen daher ab. Die Ableitung mathematischer Beziehungen für Textilbeton mit Zugabe von Kurzfasern unter verformungsgesteuerter Zugbelastung erfolgte aufbauend auf den Ergebnissen der experimentellen Untersuchungen auf der Mikroebene. Die Einzelfaserauszugsversuche wurden mit Hilfe eines physikalisch basierten Modelles nachgebildet, das aus einfachen rheologischen Elementen besteht. Phänomene wie die graduelle Ablösung der Faser, Faserbruch und Faserauszug wurden durch eine entsprechende Kombination und Parametrierung der rheologischen Elemente abgebildet. Im Ergebnis wurden zutreffende Kraft-Rissöffnungsbeziehungen modelliert. Auf der Mesoebene wurde ein einzelner Riss modelliert, der sowohl durch Multifilamentgarne als auch Kurzfasern überbrückt werden kann. Der rissüberbrückenden Wirkung der zahlreichen Kurzfasern wurde mit Hilfe statistischer Methoden rechnung getragen, die unterschiedliche Faser-Risswinkel und Einbindelängen berücksichtigen. Die resultierende Spannungs-Rissöffnungskurve umfasst die rissüberbrückende Wirkung von Multifilamentgarnen und Kurzfasern. Auf der Makroebene kann die charakteristische Spannungs-Dehnungsbeziehung von TRC unter Zugbelastung in 3 Bereiche (Zustände I, IIa, IIb) unterteilt werden. Die Kurvenverläufe im Zustand I (ungerissenen) sowie Zustand IIb (abgeschlossenes Rissbild) wurden als linear betrachtet und basierend auf den entsprechenden charakteristischen Werten des jeweiligen Zustands beschrieben. Das Verhalten im Zustand IIa (multiple Rissbildung) wurde durch die Reihenschaltung einer zunehmenden Anzahl von Rissen sowie den Beitrags der ungerissenen Matrix zwischen den Rissen modelliert
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Metelka, Jiří. "Analýza konstrukce otevíracího mechanismu podlahového poklopu." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2012. http://www.nusl.cz/ntk/nusl-230280.
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In a family house not far from Brno, there is an entryway hole for getting into the space under the floor. The hole is covered by a high weight concrete hatch. For easier manipulation with the hatch an opening mechanism was designed and manufactured. This mechanism had been installed under the hatch and nowadays it works well. This master’s thesis deals with description of main components of the mechanism and their function. Load analysis and stress-strain analysis are the major part of this thesis. At the end of this work the analysis results are commented and some suggestions for further research are proposed.
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Fung, Kam-sang. "Fatigue crack propagation with strain energy density approach /." [Hong Kong] : University of Hong Kong, 1989. http://sunzi.lib.hku.hk/hkuto/record.jsp?B12827204.
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Johansson, Denny. "Autoproteolysis accelerated by conformational strain : a novel biochemical mechanism /." Göteborg : Institute of Biomedicine, Department of Medical Biochemistry and Cell Biology, University of Gothenburg, 2008. http://hdl.handle.net/2077/17808.
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Shehadeh, Mu'Tasem A. "Modeling of high strain rate and strain localization in FCC single crystals multiscale dislocation dynamics analyses /." Online access for everyone, 2005. http://www.dissertations.wsu.edu/Dissertations/Spring2005/M%5FShehadeh%5F050405.pdf.
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Wong, Koon-yui, and 黃冠睿. "Strain analysis of displacement data from the pos selim landslide." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2010. http://hub.hku.hk/bib/B45161811.
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馮錦生 and Kam-sang Fung. "Fatigue crack propagation with strain energy density approach." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1989. http://hub.hku.hk/bib/B31209713.
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Bergström, Jonathan, and Christoffer Åhman. "Shear strain rate dependency of expanded polystyrene foam." Thesis, KTH, Hållfasthetslära (Inst.), 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-254623.
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Wong, Siu-wah. "Predicition of fatigue crack propagation using strain energy density method /." [Hong Kong : University of Hong Kong], 1989. http://sunzi.lib.hku.hk/hkuto/record.jsp?B12751601.
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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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Gumusoglu, M. Cetin. "Analysis of underground excavations in strain softening rock masses." Thesis, Imperial College London, 1987. http://hdl.handle.net/10044/1/38339.
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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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Butterworth, Simon Richard. "Anelastic strain recovery of rock core and crustal stress measurements." Thesis, University of East Anglia, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.482046.
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