Dissertations / Theses on the topic 'Hypothèse du gradient de stress'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Hypothèse du gradient de stress.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
Muhamed, Hassan. "Le rôle des interactions biotiques dans la régénération des chênes au niveau des communautés de forêts dunaires de la région Aquitaine (Sud-Ouest de la France)." Thesis, Bordeaux 1, 2012. http://www.theses.fr/2012BOR14576/document.
Although biotic interactions are known to be important determinants of species establishment, it is uncertain what factors determine the net balance between positive and negative interactions thus, under what conditions biotic interactions could enhance or impede species regeneration. Bien que les interactions biotiques soient connues pour être This thesis aims to study the role of biotic interactions of shrubs with oak seedlings for regeneration of three oak species on the Aquitaine coastal dune forests, by testing how the net effect of these interactions vary along aridity gradient, between two overstory canopies and in respect to the functional strategies of three oak species in the context of climate change. This was done by using two approaches, descriptive approach using spatial point pattern data and experimental approach by transplanting the target seedlings. The results show that the spatial variation in the nature of biotic interactions is strongly relate to environmental severity conditions, where the shrub-oak seedling interactions were very sensitive to increasing summer drought and canopy opening, the interactions strength was facilitative under gap plots in the dry northern dunes in Soulac and switch on competitive under forest plots in the wet southern dunes in Seignosse. The nature of the interactions was constant across the functional strategies of the targets species of oak. For the most part, results of this thesis show general support to the original formulation of SGH which predicts increasing facilitation with increasing severity and underscore the fact that atmospheric water stress reduction by shrubs is required for oak seedling regeneration. In this perspective, silviculturist should conserve understory shrubs, in particular in gaps, in order to allow a better regeneration niche of oak seedlings. This thesis highlights the importance of considering biotic interactions in oak regeneration under current harshness climatic conditions and with expectation to have an ambitious role in alleviation future climatic change consequence in this region
Rande, Hugo. "Effets du niveau de pollution métallique et des stratégies fonctionnelles sur les types d’effets en jeu dans les interactions entre plantes au niveau d’anciens sites miniers des Pyrénées." Electronic Thesis or Diss., Bordeaux, 2024. http://www.theses.fr/2024BORD0032.
Plant-plant interactions have been overlooked in metal/metalloids-impacted environments and are likely driven by several factors whose influence is barely known. First, plant-plant interactions depend on the level of metal pollution, but also on the functional plant strategies of the interacting plants. Furthermore, plants can have several type of effects on their immediate environment, acting at different timescales. Plants canopy and roots have an instantaneous influence on the microclimate and available resources in their immediate vicinity. Then, during a growing season, the production of litter and its decomposition beneath their canopy can influence soil chemical and physical properties. In the longer term, when this cycle of litter production/decomposition is repeated over the years, the dynamics of the organic matter will influence soil conditions even more. In this thesis, our main objective was to delineate these effects, and to understand how plant functional strategies can influence these various effects along metal pollution gradients. We studied these effects during three consecutive years (from 2020 to 2022) in a former mining valley in the French Pyrenees (Sentein, Ariège, France). In this area, we studied interactions between plants using observational and target transplantation methods controlling for the presence of plant canopy and/or plant litter, in three study sites: a slag heap with homogeneous pollution and two mine tailings areas with heterogeneous pollution creating a gradient of pollution. Along these gradients, short-term canopy and root-uptake effects followed the Stress Gradient Hypothesis, switching from competition to facilitation as pollution increased. This facilitation was stronger when the species producing the effect were acquisitive (in relation with soil resources and the Leaf Economic Spectrum), and benefits more the low metal-tolerant plants. These positive effects were mainly due to the improvement of micro-climatic conditions during hot and dry episodes in summer. Concerning the effects linked to litter production and decomposition, negative effects on target plants were found, suggesting the so-called “elemental allelopathic” effects, in relation with the high concentration of metallic elements in the decomposing litter. These negative litter effects were more important in the least polluted environments, where metal-accumulating metallophyte plants (which have high concentration of metals in their leaves) and less metal-tolerant plants interacts. They were particularly marked for targets sensitive to metal pollution. The results of this thesis give important perspectives regarding the use of facilitation for the phyto-management of metals/metalloids-polluted environments, given that the functional strategies of interacting plants and the level of pollution involved are explicitly considered. Additionnaly, the results obtained during the 2022 heatwave provide useful insights regarding the expected evolution of the different effects driving plant interactions in metalliferous ecosystems in a climate change context
Karkkainen, Ryan. "Stress-gradient failure theory for textile structural composites." [Gainesville, Fla.] : University of Florida, 2006. http://purl.fcla.edu/fcla/etd/UFE0013704.
Hopkinson, David P. "Development of stress gradient enhanced piezoelectric composite unimorph actuators." Thesis, Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/16372.
Wolfe, Christopher Edward. "Damage accumulation in a gradient stress field in graphite/epoxy laminates." Thesis, Massachusetts Institute of Technology, 1989. http://hdl.handle.net/1721.1/39360.
Bonello, Kenneth John. "Damage accumulation in graphite/epoxy laminates due to cyclic gradient stress fields." Thesis, Massachusetts Institute of Technology, 1990. http://hdl.handle.net/1721.1/42999.
Batista, Daniela Miranda. "" Impacts of Warming on Freshwater Decomposers Along a Gradient of Cadmium Stress"." Master's thesis, Instituto de Ciências Biomédicas Abel Salazar, 2009. http://hdl.handle.net/10216/26239.
Ferry, Barbara Marie Céline. "Study of the stress gradient and the size effect in fretting fatigue." reponame:Repositório Institucional da UnB, 2017. http://repositorio.unb.br/handle/10482/31283.
Texto parcialmente liberado pelo autor. Conteúdo restrito: Capítulos 4 e 5.
Submitted by Raquel Almeida (raquel.df13@gmail.com) on 2018-01-04T18:22:52Z No. of bitstreams: 1 2017_BarbaraMarieCélineFerry_PARCIAL.pdf: 3017479 bytes, checksum: b23c85c14a5f22650fc965f93bbbf89d (MD5)
Approved for entry into archive by Raquel Viana (raquelviana@bce.unb.br) on 2018-02-21T18:37:40Z (GMT) No. of bitstreams: 1 2017_BarbaraMarieCélineFerry_PARCIAL.pdf: 3017479 bytes, checksum: b23c85c14a5f22650fc965f93bbbf89d (MD5)
Made available in DSpace on 2018-02-21T18:37:40Z (GMT). No. of bitstreams: 1 2017_BarbaraMarieCélineFerry_PARCIAL.pdf: 3017479 bytes, checksum: b23c85c14a5f22650fc965f93bbbf89d (MD5) Previous issue date: 2018-02-20
A fadiga por fretting está relacionada com o processo de dano, localizado na fronteira de contato entre dois corpos submetidos a cargas de fadiga. A previsão desse fenômeno é de grande importância na análise, por exemplo, da vida de lâminas de discos de turbinas. Na fadiga por fretting, o campo de tensão é máximo na superfície de contato, sendo observado um alto gradiente de tensão sob essa superfície. A diferença de escala entre experimentos realizados em laboratórios e os sistemas industriais, motivaram, nessa pesquisa, uma investigação da influência do efeito de tamanho na previsão de vida em fadiga por fretting. Para quantificar o efeito do gradiente de tensão e o efeito de tamanho, diferentes testes foram realizados na Universidade de Brasília em um equipamento servo-hidráulico com dois atuadores especialmente desenvolvido para ensaios de fadiga por fretting. Os parâmetros de testes foram definidos de forma que induzissem diferentes gradientes de tensão nos corpos de prova e posteriormente foram ajustados de modo que o volume solicitado sob a região de contato fosse o mesmo. Após os ensaios, para investigar os mecanismos de dano, os corpos de prova foram analisados por meio de um microscópio confocal laser. Foi demostrado nesta tese que para a liga Ti-6Al-4V, uma abordagem não local, baseada no campo de velocidades em uma determinada área ao redor da região de contato consegue prover boas previsões de vida em fadiga por fretting. Uma análise da influência das forças de fadiga nos processos de iniciação e propagação de trincas foi realizada e indicaram que essas forças não podem ser negligenciadas. De fato, se cerca de 75% dos mecanismos de iniciação de trinca em fadiga por fretting são controlados por tensões de contato (cisalhantes e pressão de contato) a consideração das tensões normais possibilita realizar previsões de vida mais realistas. O estudo do efeito do tamanho foi dividido em duas fases. A primeira investigou a influência do volume solicitado, reduzindo a largura de contato, porém, o gradiente de tensão sob a área de contato e a área de dano dentro da zona de deslizamento foram mantidas constantes. Na segunda, mantendo os parâmetro experimentais σB,max/p0 e Q/fP constantes, a influência da área de dano no interior da zona de deslizamento foi isolada enquanto que a área de dano sob a zona de deslizamento foi reduzida. Os resultados experimentais foram analisados utilizando o critério de fadiga da Curva Modificada de Wöhler em conjunto com a Teoria da Distância Crítica. Foi observado que nenhum desses dois parâmetros influenciam significativamente a vida em fadiga por fretting. Assim, o termo “efeito de tamanho” deve ser analisado somente com base no efeito do gradiente.
Fretting fatigue refers to the damage process localized at the frontier of the contact between two contacting bodies subjected to fatigue loadings. The prediction of this phenomenon is of major importance in determining, for instance, the lifetime of fan’s disk. In the vicinity of the contact front, the stress field inherited from the contact loads is maximal at the surface and displays a strong gradient under the contact. The difference of scale between the laboratory’s experiments and the industrials’ system motivated the study of the impact of the size effect for the determination of the lifetimes. To quantify the effect of the stress gradient and of the size effect, tests were carried out on a two vertical-actuators fretting-fatigue rig at the University of Brasilia, with experimental conditions ensuring different stress gradient and later different volume solicited under the contact. Damage mechanisms were studied using post-mortem analysis with a confocal microscope on some contact elements tested. It was shown on this thesis, for a Ti-6Al-4V alloy, that a nonlocal approach, based on equivalent velocity field on a determined area around the contact, leads to good expectation for the determination of fretting fatigue lives. The influence of the bulk stress for the description of the fretting fatigue crack initiation and propagation was also determined and it appears that it could not be neglected for the determination of the crack initiation boundary. As a matter of fact, if around 75% of the crack initiation mechanism in fretting fatigue is controlled by the contact stresses, i.e. shear stress and contact pressure, the consideration of the normal stress allows to obtain more realistic prediction. The study of the size effect was divided into two phases. First the influence of the volume stressed was investigated by reducing the width of the contact but maintaining the stress gradient under the contact and the damaged area within the slip zone constant. Then, the influence of the damaged area within the slip zone was isolated by maintaining the experimental parameters, i.e. σB,max/p0 and Q/fP, constant while the damaged area under the slip zone was reduced. The experimental results were analysed by applying a fatigue criterion, the Modified Wöhler Curve Method, in conjunction with the Theory of the Critical Distance. It was found that none of these two parameters influences significantly the fretting fatigue lifetimes, and so the term ‘size effect’ usually referenced in the literature as a damaging effect should refer only to the gradient effect.
Batista, Daniela Miranda. "" Impacts of Warming on Freshwater Decomposers Along a Gradient of Cadmium Stress"." Dissertação, Instituto de Ciências Biomédicas Abel Salazar, 2009. http://hdl.handle.net/10216/26239.
Bellecave, Johan. "Stress Gradients In Fretting Fatigue." Thesis, Cachan, Ecole normale supérieure, 2015. http://www.theses.fr/2015DENS0036/document.
This thesis is part of an international research program (IRG Cognac) initiated by the engine manufacturer SNECMA (SAFRAN group) involving ENS Cachan, UnB, ENSMA, CNRS, Snecma, Turbomeca et Messier Bugatti Dowty. The thesis focuses on the effect of a stress gradient in fretting fatigue. Fretting-fatigue refers to the damage process localized at the frontier of the contact between two contacting bodies subjected to fatigue loadings. The prediction of this phenomenon is of major importance in determining, for instance, the lifetime of fan's disc. In the vicinity of the contact front, the stress field inherited from the contact loads is maximal at the surface and displays a strong gradient from the surface. It was shown in this thesis, for a Ti-6AL-4V alloy, that local approaches, based on local stresses at the most critical point, are not appropriate to predict fretting fatigue lives. As a matter of fact, short cracks initiated at the most critical point may stop if the stress decay from the surface is strong enough or may continue their growth, up to the failure of the component, if the stress gradient from the surface is not string enough. A second difficulty is the multiaxial and non-proportional nature of the loading conditions. Fatigue-fretting stems from the combination of loads that have neither the same spatial distribution nor the same time-dependency. In fretting-fatigue tests, three loading components are considered, the fatigue loading of the component (cyclic), the normal part (assumed to be constant) and the in-plane part (cyclic) of the loads between the two contacting components. To quantify the effect of the stress gradient, tests were carried out on a fatigue testing contact bench developed at the University of Brasilia, with experimental conditions ensuring different stress gradient while keeping the maximal stress the same. Damage mechanisms were studied using post-mortem analysis and optical microscopy on the contact elements tested. The prediction of the fretting fatigue life was done using different approaches. The first one is based on the Critical Distance Method and a fatigue criterion. The second is based on a K-based short crack arrest method. Finally, a new criterion was proposed. This method considers a generalized von Mises yield criterion for the crack tip region and accounts for the T-stresses in the asymptotic LEFM development
Ting, Chi Man Roger. "Controlled gradient consolidation of soft soils with reference to the development of Kâ†o." Thesis, University of East London, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.258762.
Kumar, Abhishek. "RKEM implementation for strain gradient theory in multiple dimensions." [Tampa, Fla.] : University of South Florida, 2007. http://purl.fcla.edu/usf/dc/et/SFE0002163.
Strasser, Erin Hennegan. "Reproductive failure and the stress response in American kestrels nesting along a human disturbance gradient." [Boise, Idaho] : Boise State University, 2010. http://scholarworks.boisestate.edu/td/86/.
Růžičková, Kateřina. "Mezidruhové interakce v mokřadním společenstvu v závislosti na vodním režimu: dlouhodobý test stress-gradient hypotézy." Master's thesis, Česká zemědělská univerzita v Praze, 2016. http://www.nusl.cz/ntk/nusl-258647.
Liang, Y., C. Zeng, J. J. Wang, M. W. Liu, Yeh T. C. Jim, and Y. Y. Zha. "Constant Gradient Erosion Apparatus for Appraisal of Piping Behavior in Upward Seepage Flow." AMER SOC TESTING MATERIALS, 2017. http://hdl.handle.net/10150/625214.
Keogh, Teri M. "Changes in competition intensity, herbivory and stress along a soil depth gradient in an old field." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape2/PQDD_0021/MQ58467.pdf.
Peng, Jun, and 彭軍. "Effects of strain gradient on maximun concrete stress and flexural capacity of normal-strength RC members." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2009. http://hub.hku.hk/bib/B43085787.
Peng, Jun. "Effects of strain gradient on maximun concrete stress and flexural capacity of normal-strength RC members." Click to view the E-thesis via HKUTO, 2009. http://sunzi.lib.hku.hk/hkuto/record/B43085787.
Goulding, A. "Small fatigue crack growth in a near alpha titanium alloy : crack closure, stress gradient and temperature considerations." Thesis, Swansea University, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637082.
Montebello, Claudio. "Analysis of the stress gradient effect in Fretting-Fatigue through a description based on nonlocal intensity factors." Thesis, Université Paris-Saclay (ComUE), 2015. http://www.theses.fr/2015SACLN019/document.
In this manuscript a new method to describe the stress gradient effect in fretting-fatigue is proposed. It is based on the description of the mechanical fields arising close to the contact edges through nonlocal intensity factors. For this purpose, the kinetic field around the contact ends is partitioned into a summation of multiple terms, each one expressed as the product between intensity factors, Is, Ia, Ic, depending on the macroscopic loads applied to the mechanical assembly, and spatial reference fields, ds, da, dc, depending on the local geometry of the part. This description is obtained through nonintrusive post-processing of FE computation and is conceived in order to be easily implementable in the industrial context. As a matter of fact, for any given macroscopic load and geometry, a set of nonlocal intensity factors is computed that permits to characterize the mechanical fields close to the contact edges. Such nonlocal description has the advantage of being (i) geometry independent so that the nonlocal intensity factors can be used to compare laboratory test with real-scale industrial assembly, (ii) applicable to industrial FE models usually characterized by rougher meshes compared to the ones used to describe fretting-fatigue in the academic context. The procedure is applied to fretting-fatigue test data in order to verify whether the nonlocal intensity factors can be used to transpose experimental results to different contact geometries from the one in which they have been obtained
Akther, S. M. Lovely. "Effects of water stress on Alnus glutinosa populations across the species distribution range." Master's thesis, ISA, 2019. http://hdl.handle.net/10400.5/19572.
Alnus glutinosa (Black alder) is paramount species in the riparian ecosystem for supporting ecosystem functioning and the services it provides. This species is declining in an alarming rate which is a prominent threat to devastate native priority forests across Europe, so understanding population responses to environmental change is key for its proper management. In this study, we characterized vegetative phenology and investigated morphological, structural and physiological responses to imposed drought across five populations from countries ranging the species distribution limits (Sweden, Italy, Spain, Portugal, and Morocco). First, we registered 5 stages of budburst of the 120 seedlings of alder (24 by population) in open space. Then, we established a greenhouse experiment where we imposed progressive (Field capacity-FC, 75%FC, 50%FC and 25%FC) water stress (17.06.2019 to 01.08.2019) on 40 seedlings (S), keeping 40 at field capacity, as control until the end of experiment (CE). During 45 days, we performed physiological and morphological measurement at different percentages of FC to compare treatments and populations. Destructive harvest was performed on 40 seedlings (from the total 120) at the onset of the experiment (CO), and then, at the end of the experiment control (CE) and stress (S) seedlings were also destroyed to analyze the structural and functional responses of alder among CE, S and CO and also to compare the variations among populations. We found significant differences among populations on the number of days to reach each phenological stage where northern populations displayed delayed budburst than the southern. As a result of imposed drought, growth and development of A. glutinosa was generally reduced under water restrictions compared to control plants, yet none of the parameters reflected severe plant stress. Conversely, we observed that several of the studied parameters were significantly different among the studied populations likely reflecting intraspecific diversity and environmental conditions
N/A
Li, Maoxin. "Seepage induced instability in widely graded soils." Thesis, University of British Columbia, 2008. http://hdl.handle.net/2429/862.
Zhang, Bohua. "The Influence of Porosity and Its Modeling on Fatigue Behavior of High Pressure Die Cast Aluminum including the Effects of Mean Stress, Stress Gradient and Specimen Size." University of Toledo / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1556259677391458.
Braccia, Amy. "Quantifying the environmental factors that determine benthic macroinvertebrate assemblages in streams by analyzing stressors associated with a gradient of cattle grazing." Diss., Virginia Tech, 2005. http://hdl.handle.net/10919/29326.
Ph. D.
Twycross-Lewis, Richard. "Micro flow chambers for studies on effects of the spatial gradient of the fluid shear stress on cultured endothial cells." Thesis, Queen Mary, University of London, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.417841.
Nguyen, Le Thu Ha. "Effet bottom-up du stress hydrique sur la gamme d’hôtes des parasitoïdes de pucerons." Thesis, Université Côte d'Azur (ComUE), 2017. http://www.theses.fr/2017AZUR4132/document.
Biological control (BC - the use of natural enemies to control pests) are sustainable, environmentally friendly and cost-effective methods to counteract pest resistance by increasing pesticide use. Aphid parasitoids are common natural enemies of aphids, the major worldwide pests in agriculture. The study of parasitoid host specificity contributes to (1) understanding ecological and evolutionary mechanisms driving the ecosystem and (2) evaluating the efficiency of biocontrol agents and the ecological risks for non-target species. This study focuses on the parasitoids fundamental host specificity on individual levels, in terms of resource requirements and in the context of multi-trophic interactions under environmental abiotic stress, i.e.water limitation. Aphidius ervi (Hymenoptera: Braconidae: Aphidiinae) was chosen; this aphid parasitoid is used widely as an ecological model and commercial biological control agent (BCA). On the one hand, A. ervi host specificity index was measured on a broad range of aphid species. On the other hand, the indirect impacts of water limitation were investigated on the host specificity of the parasitoid. Furthermore, water stress-induced modifications in the plant and the aphid life-history traits were measured. A. ervi was shown to be an intermediate specialist species who attacked all aphid species at high rates but was unable to develop well on all of them. The few that developed well were phylogenetically close and belong to the Macrosiphini tribe. Interestingly, a positive correlation preference – performance was found. Under water stress, both preference and performance of parasitoids were affected causing loss of the correlation. Water limitation negatively altered the plant nutritional quality resulting in low aphid performance on host plants. This in turn decreased the suitability of aphid hosts for the parasitoid. The impacts of water limitation were not similar across all plant-aphid combinations and depended on several factors, namely stress-adapted plant mechanisms and the host specialization of both aphids and parasitoids
Amargier, Rémi. "Amorçage de fissures et gradient de contrainte en fretting et en fatigue." Phd thesis, Ecole Centrale de Lyon, 2011. http://tel.archives-ouvertes.fr/tel-00728236.
Bray, Jonathan Peter. "The ecology of algal assemblages across a gradient of acid mine drainage stress on the West Coast, South Island, New Zealand." Thesis, University of Canterbury. Biological Sciences, 2007. http://hdl.handle.net/10092/1492.
Rhymer, Donald William. "Stress Intensity Solutions of Thermally Induced Cracks in a Combustor Liner Hot Spot Using Finite Element Analysis." Diss., Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7515.
Nguyen, Le Thu Ha. "Effet bottom-up du stress hydrique sur la gamme d’hôtes des parasitoïdes de pucerons." Electronic Thesis or Diss., Université Côte d'Azur (ComUE), 2017. http://www.theses.fr/2017AZUR4132.
Biological control (BC - the use of natural enemies to control pests) are sustainable, environmentally friendly and cost-effective methods to counteract pest resistance by increasing pesticide use. Aphid parasitoids are common natural enemies of aphids, the major worldwide pests in agriculture. The study of parasitoid host specificity contributes to (1) understanding ecological and evolutionary mechanisms driving the ecosystem and (2) evaluating the efficiency of biocontrol agents and the ecological risks for non-target species. This study focuses on the parasitoids fundamental host specificity on individual levels, in terms of resource requirements and in the context of multi-trophic interactions under environmental abiotic stress, i.e.water limitation. Aphidius ervi (Hymenoptera: Braconidae: Aphidiinae) was chosen; this aphid parasitoid is used widely as an ecological model and commercial biological control agent (BCA). On the one hand, A. ervi host specificity index was measured on a broad range of aphid species. On the other hand, the indirect impacts of water limitation were investigated on the host specificity of the parasitoid. Furthermore, water stress-induced modifications in the plant and the aphid life-history traits were measured. A. ervi was shown to be an intermediate specialist species who attacked all aphid species at high rates but was unable to develop well on all of them. The few that developed well were phylogenetically close and belong to the Macrosiphini tribe. Interestingly, a positive correlation preference – performance was found. Under water stress, both preference and performance of parasitoids were affected causing loss of the correlation. Water limitation negatively altered the plant nutritional quality resulting in low aphid performance on host plants. This in turn decreased the suitability of aphid hosts for the parasitoid. The impacts of water limitation were not similar across all plant-aphid combinations and depended on several factors, namely stress-adapted plant mechanisms and the host specialization of both aphids and parasitoids
Belluau, Michaël. "Traits fonctionnels, tolérances et distributions des espèces herbacées sur un gradient de disponibilité en eau : une approche prédictive par modèle d'équation structurale." Thèse, Université de Sherbrooke, 2017. http://hdl.handle.net/11143/11584.
Abstract : Species assembly (their presence/absence) in a natural community is the consequence of several filtering mechanisms made by the environment. Among these filters, the abiotic filter selects species able to tolerate local environmental conditions. Variation in water availability in the soil is one of the main environmental gradients according to which plant species are differently distributed. Considering the hypothesis that functional traits and their relationships are hierarchical, habitat preferences of species along environmental gradients should be determined by a combination of hierarchical physiological and morpho-anatomical traits. During this PhD, my overall goal is to identify morphological, anatomical and physiological drought tolerance functional traits that can predict the presence of species along a soil hydrology gradient. More specifically : (i) What are the physiological traits that best reflect drought tolerance? (ii) What are the relationships between morpho-anatomical traits and physiological traits of tolerance? (iii) What are the optimal morpho-anatomical traits for predicting tolerance of herbaceous species to drought? (iv) What forms of relationships exist between optimal morpho-anatomical traits of tolerance and the presence of species in drought condition? (v) Can the presence of species in drought condition be predicted from their morpho-anatomical features? Our results show (1) that it is possible to predict the distribution of species on a soil hydrology gradient from five physiological traits of drought tolerance. These five traits are maximum net photosynthesis, maximum stomatal conductance, water potential of the soil at the wilting point, stomatal conductance at the wilting point, and efficiency of water use at the wilting point. We have shown that (ii) the physiological traits of drought tolerance are predicted by optimal morpho-anatomical traits (leaf area, leaf dry matter content, leaf nitrogen content, root length and stomatal surface). (iii) Morpho-anatomical features alone are not good predictors of species hydrology and (iv) the sequence “morpho-anatomical traits physiological traits species hydrology” gives the best predictions. However (v) the model does not provide reliable predictions using morpho-anatomical traits measured under natural conditions. These results confirm, at least partially, the hypothesis that the distribution of species on a hydrological gradient can be predicted from their drought tolerance traits themselves predicted by their morpho-anatomical features. In summary, we used a functional approach by constructing a predictive causal model that allowed us to focus on environmental filtering mechanisms and more specifically on the role of the species hydrological niche in assembling plant communities.
Todd, Nicolas. "Effets sanitaires à long terme des stress de la Première Guerre mondiale." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066452/document.
This thesis explores the First World War as a historical model in early life psychological stress. The Developmental Origins of Health and Disease (DOHaD) hypothesis predicts increased susceptibility to chronic diseases in adulthood for those exposed to an extreme psychological trauma in very early life. We collected vital information on French orphans born 1914-1916 thanks to the “pupille de la Nation” distinction, a legal status created in 1917 and granted upon request to all orphans. Notification of “adoption by the Nation” was by law inscribed on the birth certificate of a newly adopted child. Birth registers thus provided a census of all pupilles born in the included cities during the inclusion period as well as long-term mortality follow-up. The birth certificates of 7,250 pupilles have been digitized. Call to the Died for France Database enabled us to retrieve the paternal date of death. Matched non-orphans (MNOs) were drawn from the same birth registers. For each orphan, his MNO was therefore chosen born in the same district at the same time. The outcome of interest was longevity of those who survived to 31 y. An orphan-MNO difference in adult longevity of ~2.5 years was found for orphans who had lost their father before) birth (prenatal orphans), but no difference in adult longevity could be measured between postnatal orphans and their MNOs. These two results suggest early trauma in utero has programming effects on biological susceptibility in adulthood strong enough to alter longevity. The fact that no loss of lifespan was found in the case of a postnatal loss of father further suggests efficient buffers to early postnatal stress existed in French society
Lai, Zhi Cheng. "Finite element analysis of electrostatic coupled systems using geometrically nonlinear mixed assumed stress finite elements." Diss., Pretoria : [s.n.], 2007. http://upetd.up.ac.za/thesis/available/etd-05052008-101337/.
Coelho, Fabricio dal Cero. "Maîtrise de la tenue en fatigue des cordons de soudure." Thesis, Chasseneuil-du-Poitou, Ecole nationale supérieure de mécanique et d'aérotechnique, 2014. http://www.theses.fr/2014ESMA0016/document.
Automotive chassis are composed mostly of welded parts subjected to complex multiaxial loadings.Welds are potential sites to initiation and propagation of fatigue cracks because of their particular geometry andtransformations induced by manufacturing process. A reliable and tractable fatigue design methodology is achallenge for automotive industry designers because it allows detecting the critical points from the upstreamphase, avoiding oversizing and reducing the number of physical prototypes.This study is divided into two parts allowing the distinction of effects induced by multiaxial loadingsand particular characteristics of the welded joints. The first part focuses on the complex multiaxial loading. Thebehavior of a “well known material”, the 1045 steel, is studied under "automobile" spectrum loading. Differentfatigue life prediction methods are compared under two criteria: quality of prediction and simplicity of use. Thesecond part focuses on the study of behavior and fatigue life simulation of welded structures. To achieve thisgoal, a representative specimen of chassis parts and manufacturing process has been designed. A representativeFE model is constructed, based on micrographic observations and instrumented tests. Two loading modes areused in fatigue tests in order to activate different damage mechanisms at constant amplitude and under"automobile" spectrum.Finally, the methodology "Weld Stress Gradient" is proposed to incorporate in fatigue analysis theeffect of the stress gradient at the vicinity of weld joints critical points. This methodology, combined with Vucriterion (taking into account complex loadings) and "DCA" nonlinear cumulative damage rule (taking intoaccount interaction between spectrum loading blocks), provides encouraging results for the studied specimen
Kulkarni, Anish Niranjan. "Energy and Strength-based Criteria for Intralaminar Crack Growth in Regions with High Stress Gradients." Thesis, Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-87384.
Grau, Fernàndez Oriol. "Interaccions planta-planta en gradients d'estrès en ecosistemes freds / Plant-plant interactions along stress gradients in cold ecosystems." Doctoral thesis, Universitat de Barcelona, 2013. http://hdl.handle.net/10803/101146.
In this thesis I present four chapters, and in all of them I discuss how dwarf shrubs interact with co‐occurring plants under varying regimes of stress. This research involved ecosystems of great environmental, nature conservation and ecological value, yet highly sensitive to environmental changes, in four contrasting cold regions at high altitude or high latitude. Following a latitudinal order, the selected ecosystems were: 1) a temperate alpine treeline in the Central Pyrenees; 2) a primary succession gradient in a boreal ecosystem in Finland; 3) a subarctic alpine treeline in Lapland; and 4) a high‐arctic tundra in north‐eastern Greenland. The first two chapters are based on an experimental approach and focus on how shrubs commonly found near the treeline interact with tree seedlings of treelineforming species in two contrasting environments, i.e. in a subarctic forest‐tundra ecotone in Lapland, northern Sweden, and in a more southern, temperate forestalpine pasture ecotone in the Central Pyrenees. In addition, since trees living near their limit of distribution are very sensitive to environmental changes, especially to increased temperature during the growing season (Körner 2003), we also assessed how distinct environmental change scenarios may affect tree seedling growth and survival across the ecotone. The first chapter is entitled ‘Shrub‐tree interactions and environmental changes drive treeline dynamics in the Subarctic’, where we explain the three‐year‐long experiment performed in the Abisko National Park, in the subarctic Scandes, Northern Sweden; the species studied were Betula pubescens tree seedlings and the shrub Vaccinium myrtillus. In this study we showed the importance of facilitative and competitive shrub‐tree interactions as drivers of subarctic treeline dynamics. Furthermore, we demonstrated that the great sensitivity of tree seedlings to warming had strong implications for treeline dynamics under the predicted warmer scenario at high latitudes, and we identified that complex interactions between shrubs and herbivores are critical to predicting future changes. The second chapter is entitled ‘Similar tree seedling responses to shrubs and to simulated environmental changes at Pyrenean and subarctic treelines’. Here we presented a comparison between the results obtained in the experiment presented in the first chapter and those obtained in a parallel experiment performed during a similar period near the treeline in the Alt Pirineu Natural Park, in the Central Pyrenees, Catalonia. This experiment was based on the same factorial design but with different species (i.e. Pinus uncinata tree seedlings and the shrub Rhododendron ferrugineum). To our knowledge, it is the first study which experimentally tests the responses of plants to distinct environmental scenarios in a high mountain ecosystem in the Pyrenees. In this chapter we presented some mechanisms for understanding the recently observed variability of local responses of both subarctic and alpine treelines to currently changing climate while identifying some commonalities that can be used to generalise large scale response of treelines to climate warming. The third chapter focuses on the effects of a dominant dwarf shrub (i.e. Empetrum nigrum) on Pinus sylvestris tree seedlings along a primary succession within a boreal ecosystem on an uplifting island in Bothnian Bay, Finland. This chapter is called ‘An ericoid shrub plays a dual role in recruiting both pines and their fungal symbionts along primary succession gradients’. Here we showed that facilitative and competitive effects of shrubs markedly determined tree seedling establishment and their fungal colonisation along this succession gradient, but in this chapter we did not relate these findings to any environmental changes. As far as we know, we presented the first finding that an ericoid mycorrhizal shrub may enhance both the performance of the ectomycorrhizal host tree and the tree’s fungal symbionts. The study presented in the fourth chapter was performed along a snow‐depth gradient in an extreme arctic tundra ecosystem in the Northeast Greenland National Park, the largest national park in the world. The chapter is entitled ‘Plant interactions and higharctic vegetation composition along a snow‐depth gradient in NE Greenland’. This ecosystem is probably the most sensitive and fragile among the ecosystems studied in this thesis as the eastern coast of Greenland is expected to experience substantial changes in climate due to marked changes in snow precipitation and temperature regimes (Brown and Mote 2009). Here we assessed plant species richness, establishment and composition patterns in distinct growth forms occurring in common arctic plant communities associated with varying snow‐depth during the winter season. This study will help to predict potential diversity and vegetation changes in the high Arctic if snow precipitation regime changes in the future as anticipated.
Lévêque, Louis. "Nouveaux matériaux composites à gradient de permittivité structurés par un champ électrique et leur application pour la gradation de potentiel." Thesis, Toulouse 3, 2017. http://www.theses.fr/2017TOU30004/document.
New developments in power electronics allow increasing the power density of the conversion systems. This means that the insulating materials, such as the encapsulation in power modules, are more are more stressed. If the electric field reinforcements in insulating polymers reach critical values, this can lead to a partial discharge activity, electrical treeing and eventually a complete breakdown of the insulation. The objective of this thesis is to study the appropriate matching of the dielectric properties of insulating polymer composites in order to reduce the electrical stress in the regions of field reinforcement. A new approach to minimize the reinforcements is proposed through a local structuration of the composite material allowing an auto-adaptive permittivity gradient where the largest stresses are present. This structuration is achieved thanks to the application of a DC electric field during the elaboration process of the composite material, leading to the displacement of the particles by electrophoresis. The field grading material is an epoxy matrix filled with high permittivity particles (strontium titanate SrTiO3 or barium titanate BaTiO3). Applying a DC electric field on the liquid resin containing the particles induces their accumulation on the high voltage electrode, building an accumulated layer highly concentrated in particles, conferring to this region a higher permittivity. Each region of the structured composite (accumulated layer and low concentrated region) was characterized in terms of dielectric properties (permittivity and losses). While the low concentrated region of particles keeps a permittivity close to that of homogeneous composites one, the accumulated layer exhibits a significant increase due to the increase in the local particle content. The particle concentration in each region of the structured material were precisely determined, and the related permittivity values are in good agreement with the permittivity values of the homogeneous composite materials of the same filler content. This shows that the accumulated layer was not organized in a particular way. Regarding the dielectric strength of the accumulated layer, its values are large enough for the applications and these values follow the typical power law versus thickness. Finite element methods prove that these materials are appropriate for minimizing the electric field reinforcements at the triple point, between the metal, the ceramic and the encapsulation. These results highlight the interests of these new kind of field grading materials. They could allow improving the reliability and the robustness of power modules or other electrical systems working at high field
Dieltiens, Baptiste. "Contributions à la gestion des risques en assurance vie." Electronic Thesis or Diss., Lyon, 2021. http://www.theses.fr/2021LYSE1135.
Risk management is a major issue for the piloting of an insurance company. The increasing amount of data, the sophistication of models and the growing computing power now allow actuaries, risk managers and data scientists to refine the knowledge of their policyholder portfolios and the underlying risks. This is the context of this thesis, which aims to contribute to the understanding and modeling of biometric and behavioral risks in life insurance, through three chapters introduced and contextualized in a general introduction. Chapter 1 focuses on free payments on life insurance contracts. We propose a methodology based on machine learning to pilot them efficiently: the model, based on the Gradient Boosting algorithm, relies on variables related to past payments as well as variables related to the product in question and its business plan, and we show that it gives better results than a more classical methodology based on the use of time series. In addition, the analysis of the model via the framework proposed by SHAP (Shapley Additive Explanations) makes it possible to highlight certain stylized facts; finally, the study at a finer scale completes the work and questions the relationship between payments and surrenders or arbitrages. Chapter 2 deals with life insurance transfers, which allow a saver to invest money in a new contract while retaining some of the advantages of the original contract. In particular, we are interested in the Fourgous and PACTE transfers, which we present; we highlight the main common points and major differences of those transfers. We then propose a model of the Fourgous amendment using dynamic logistic regression and analyze, given the initial observations, to what the extent the lessons that can be drawn from it are applicable to the PACTE law. Finally, we broaden the reflection by discussing the legislative framework and its potential impacts in terms of policyholder behavior. Finally, Chapter 3 is devoted to the risk of longevity, and focuses in particular on an extreme assumption, not really considered in actuarial science: transhumanism. This assumption considers a potential gigantic improvement in longevity through the use of science and technology. After discussing the state of the art on longevity and all the related subjects (life expectancy, maximum biological age in particular) and the main hypotheses on its future evolution, thus highlighting the lack of consensus and the complexity of this subject, we analyze the transhumanist assumption in more details and discuss its ins and outs
Villaret, Flore. "Développement d’une jonction austéno-martensitique à gradient de composition chimique par fabrication additive." Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEI104.
This PhD work concerns the problem of bimetallic austenitic/martensitic steel connections. This research action focuses on a 316L austenitic steel (X2 CrNiMo 18-12-02) / Fe-9Cr-1Mo (X10 CrMo 9-1) martensitic steel connection. The objective is to understand the metallurgical problems related to the assembly of these two steels and to evaluate the possibilities of using powder metallurgy and additive manufacturing to produce austenitic/martensitic steel transitions. A weld obtained by electron beam is used as a reference for this study which focuses on the interest of powder metallurgy to achieve a transition between two steels. Materials with a chemical composition gradient have been consolidated by HIP and SPS and show very good mechanical properties and an excellent junction between the two steels. By additive manufacturing (DED-LB or PBF-LB), we also obtain very good bonds between the two steels, but the microstructures are much more complex. Curiously, we observe that the higher the cooling rate, the higher the ferrite fraction in the martensitic steel. Different calculations based on the nucleation and growth of the austenitic phase have made it possible to propose a coherent scenario to explain the phase fractions present in the materials. The transition zone between the two steels shows strong variations in hardness. These variations are explained by changes in chemical composition, leading to modifications in phase change temperatures, and the particular thermal cycles seen during building. From a technological point of view, materials obtained by additive manufacturing have tensile performances very similar to those obtained by electron beam welding. It is shown that additive manufacturing also makes it possible to control the composition gradient between a martensitic and an austenitic steel
Fernandez, Rodrigo da Silva. "Diversidade florística e estrutura filogenética de ilhas arbustivas em uma restinga subtropical." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2012. http://hdl.handle.net/10183/55910.
In this work we tested how pioneer shrubs regulate community structure in woody islands of a subtropical sand-dune (restinga) environment. Shrubby species show a great ability to modify ecosystems, since they change the flux of nutrients and water in the soil, they retain under their crowns seeds carried by water or wind, and the seeds in turn attract new dispersers. The combination of these processes leads to the formation of fertility islands in areas with herbaceous vegetation, increasing the local diversity. Biological interactions are determinant factors in the coexistence of species in these shrubby islands. So we used vegetative traits of shrubs and mean phylogenetic distance (MPD) between species to evaluate how richness, abundance and diversity, i.e., Rao‟s quadratic entropy (RQE), are regulated in sand-dune woody islands. We also evaluated the same parameters in the surrounding areas with predominantly herbaceous vegetation. In order to understand the phylogenetic structuring in this environment we estimated the net relatedness index (NRI) in each woody thicket. Our data reinforce the importance of pioneer shrubs in a matrix of the predominantly open grassland vegetation because of their ability to modify the microhabitat under their crowns and their interactions with other species which colonize the shrubby islands.
Dona, Marco. "Static and dynamic analysis of multi-cracked beams with local and non-local elasticity." Thesis, Loughborough University, 2014. https://dspace.lboro.ac.uk/2134/14893.
Deboucq, Julien. "Caractérisation ultrasonore de structures à couche et à gradient de contraintes par ondes de surface haute fréquence générées par capteurs MEMS de type IDT -SAW." Thesis, Valenciennes, 2012. http://www.theses.fr/2012VALE0017/document.
The use of coatings and thin layers deposited on substrates is highly sought in many applications. The objectives of these coatings and deposits are multiple (improve the durability of structures, their wear resistance and fatigue, etc.). On the other hand, gradient materials are being developed to meet new functional requirements, such as a better resistance to temperature, wear and corrosion. For all of these applications, the characterization of these coatings and gradient materials, in order to determine their properties (thickness, elastic constants, adherence, residual stresses, etc…), is decisive for the health control of pieces and for their optimum operation during their use. To characterize these structures, wechose to exploit the dispersion of surface acoustic waves over a wide frequency range (10 to 60 MHz).To excite these waves, SAW-IDT MEMS sensors have been carried out at different frequencies covering the entire frequency range we considered. The quasi-harmonic excitation was preferred to obtain accurate measures of phase velocities. We showed the potential of this approach by characterizing, first, thin layers structures (500 nm) and second, amorphous structures with a stressesgradient
Pham, Tuan Hiep. "Modélisation analytique et simulation numérique de la nucléation et de la propagation de la fissure cohésive couplée avec la plasticité." Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLX001/document.
The aims of this work is to study the cracks evolution under plasticity and nonuniform stress field effects by using cohesive zone model. Firstly, basing on the variational approach, the crack evolution in the elastoplastic material is investigated. The solutions for 1D beam under simple tension is expressed explicitly through the first and the second orders stability conditions of energy. This study shows us the plasticity effects on the material softening behavior as soon as crack appears. In fact, the global solution of the beam under described displacement is stable only if the beam length is lower than a characteristic length. This length is independent of plasticity hardening module but depends on Young modulus and on the second derivative of crack energy density. The energy formulations can be generalized for 3D structure. In this case, the plasticity and cohesive criteria become two curves in Mohr’s stresses plane. The comparison between theses curves allows us to consider the crack nucleation in the plastified domain. Secondly, the non-uniform stress field effects on the crack nucleation in the elastic material is highlighted. The analytical solution is established by using two-scales techniqueand complex analysis. The evolution of fully cohesive crack and partially non-cohesive crack is controlled by the stress gradient, which is related to a characteristic length. Different cohesive laws are used in our study. The sensitivity of solution to preexisting imperfection size is also explored. Finally, analytical results are validated by numerical simulations and the cohesive zone model in mixed mode is implemented in Code_Aster
Castanho, Camila de Toledo. "Facilitação entre plantas e suas implicações para a dinâmica e restauração de restingas." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/41/41134/tde-24082012-151906/.
In the last two decades, empirical studies have shown the importance of plant interaction, especially under harsh environmental conditions. These observations led to the proposition of the stress gradient hypothesis (SGH), a conceptual model predicting that the relative frequency of facilitation and competition will vary inversely across gradients of stress, with facilitation being the dominant interaction under highly stressful conditions. Coastal dunes (known as restinga in Brazil) are environments with limiting conditions for plant performance, which makes them suitable for facilitation predominance among plant interactions. Furthermore, there is a gradient of stress and disturbance related to the presence of the sea, making this environment an ideal model to test the SGH. The general goal of this thesis was to study plant facilitation in coastal dunes with the following specific objectives: i) to perform a systematic review of published studies on plant facilitation in costal dunes around the world as well as to investigate, using a meta-analysis approach, factors that affect the existence and magnitude of plant facilitation; ii) to test if spatial association between isolated adult trees and understory plants of distinct life forms changes along a beach-to-inland gradient, as expected by the SGH; iii) to test the SGH through field experiments involving target species transplantation coupled with manipulation of limiting resource along a beach-to-inland gradient; iv) to test if shading is a facilitation mechanism in coastal dunes. The empirical tests (objectives ii-iv) were conducted in an open scrub coastal dune located at Ilha do Cardoso State Park, Cananéia, São Paulo, Brazil. Our results indicate that: i) several aspects and conditions of plant facilitation in coastal dunes have been little studied. For instance, few studies were conducted in tropical regions and few of them studied the effect of neighbor presence on seed and adult life stage of the target species. Precipitation, geographic region and target life stage affect the existence and magnitude of plant facilitation; ii) For most of the understory life forms, the spatial association between adult trees and understor plants do not change along the environmental gradient as predicted by the SGH. Among all life forms, only young trees were positively associated with adult trees, suggesting a nurse plant effect among trees in coastal dunes; iii) Both intensity and importance of the interaction among trees were negatively related to stress, rejecting the classic proposition of the SGH. Furthermore, the signal of the net interaction depends on the life stage of the target species since the adult neighbor had mostly negative effects on target species seeds but positive effects on seedlings survival; iv) The nurse plant effect among trees was, at least partly, due to the shading provided by the facilitator crown. Overall, we concluded that facilitation driven by shade is a relevant interaction between adult and young trees in the studied coastal dune. However, contrary to what was expected by the early propositions of the SGH, the observed nurse plant effect tends to be more intense under mild conditions. Our results suggest a positive feedback relationship started with the establishment of the first isolated trees. This positive feedback has important implications to understand the vegetation organization on open scrub vegetation, where patches of woody species are scattered in a matrix of herbaceous plants. Furthermore, the facilitation between trees and their dependency on local environmental conditions could be fundamental to explain the gradient of vegetation physiognomies from open vegetation to forest observed along the beach-to-inland gradient, as well as to understand the dynamics of this vegetation over time. Finally, our results also have practical implications, highlighting the potential use of nurse plants and artificial shade as techniques to restore shrubland or forest physiognomies in coastal dunes.
Landron, Thomas. "effets des gradients de porosité et de contrainte sur le comportement en fatigue à grand nombre de cycles de l’alliage d’aluminium AlSi9Cu3 coulé sous pression." Electronic Thesis or Diss., Paris, HESAM, 2023. http://www.theses.fr/2023HESAE091.
This research deals with the effect of porosity and stress gradients on the high-cycle fatigue behavior (HCF) of the high-pressure die-cast (HPDC) aluminum alloy AlSi9Cu3 used by the automotive industry. The components produced by HPDC are characterized by a high level of porosity compared to other casting processes. In addition, the porosity or defect distribution within a component is highly heterogeneous and the existence of a porosity gradient between the surface and the in-bulk material is notable. Defects of different nature and size are present, including defects formed by complex three-dimensional pore networks.To study the effects of the porosity and stress gradients on the HCF behavior, a test methodology based on reducing the thickness of as-cast specimens by machining was developed. Four geometries machined from the same as-cast specimens were tested in HCF under different loading modes: plane bending and tension-compression with a load ratio R=-1. The analysis of the fatigue failure surfaces, combined with fatigue results in the form of Wöhler curves shows that fatigue behavior is controlled by the interaction between the porosity gradient and the stress gradient. The position of the crack initiation site (surface or in-bulk), the nature of the critical defects, as well as the fatigue strength are entirely determined by these two gradients.The effect of defects on the fatigue behavior was analyzed using an approach based on the Kitagawa-Takahashi diagram and the stress intensity factor threshold (Kth). This analysis shows that the fatigue strength of the material is controlled by the size of the defects in terms of the Murakami parameter (√area). The wide range of defect size and the variety of defect types makes it possible to characterize the behavior of the material in both the short crack and long crack regimes. It is observed that the position of the crack initiation site (surface or in-bulk) has only a small effect on the fatigue behavior. Thanks to a comparison between the studied alloy and other gravity-cast aluminum alloys, it is shown that both the fatigue strength and the crack propagation behavior, is partly controlled by (a) the microstructural characteristics of alloys (grain size and secondary dendrite arm spacing (SDAS)) and by (b) the monotonic and cyclic mechanical behavior of the materials.The complexity of the defect distribution (type, size and spatial distribution) complicates the characterization and modeling of the kinetics of crack propagation and their interactions with defects. To investigate this, in-situ image correlation was used to monitor crack growth during fatigue tests, with the aim of observing the crack interaction with natural defects, in bending at R=-1. The analysis of the crack growth kinetics and the fatigue failure surfaces reveals a strong interaction between fatigue cracks and defects. These interactions cause rapid crack accelerations, which drastically reduce the fatigue life. A HCF crack propagation model is proposed using an averaging approach with the Paris law. The coefficients are adjusted using the crack monitoring tests. The comparison between the simulated Wöhler curves at initial iso-defect-size and the experimental Wöhler curves shows that the propagation approach is not sufficient to account for all the stages leading to fatigue failure. It is clearly highlighted that the initiation phase must be taken into account and that the fatigue damage mechanisms, associated with defects comprised of complex three-dimensional pore networks, are extremely complex.Keywords: High cycle fatigue (HCF), microstructural heterogeneity, porosity gradient, stress gradient, aluminum alloy, high-pressure die-casting, porosity networks, fatigue life, crack – defect interaction
von, Stillfried Florian. "Computational fluid-dynamics investigations of vortex generators for flow-separation control." Doctoral thesis, KTH, Turbulens, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-94879.
QC 20120511
Ferré, Romain. "Etude expérimentale et modélisation de la durabilité d'un contact représentatif de l'interface aube / disque de soufflante grenaillé soumis à des chargements de fretting / fatigue / usure." Thesis, Ecully, Ecole centrale de Lyon, 2013. http://www.theses.fr/2013ECDL0012.
This work aims at studying the crack initiation risk of a blade/disk contact under fretting/fatigue loading. The fan stage of a civil engine is studied and the material used is a titanium alloy: Ti-6Al-4V. This complex issue concerns small contacts which are subjected to high contact pressures and micro-displacements. Thus, a complex multiaxial loading occurs on the structure. Fretting, static stressed fretting and fretting fatigue testing are performed in order to quantify the crack nucleation thresholds. In addition, several geometries which present different stress gradient values are used. The experimental results show a stress gradient effect on the crack initiation. Thus, using a wide range of stress gradient, it has been confirmed that stress gradient delays crack nucleation. A multiaxial fatigue criterion is used to determine the equivalent stress field under the contact. Then, a non-local approach, identified thanks to one fretting experimental condition, is employed in order to consider the stress gradient effect. This approach provides the prediction of the whole experimental results. In this way, a predictive method of the initiation fatigue life has been introduced. This numerical approach takes into account the multiaxial loading, the stress gradient effect, the plastic-elastic behavior of the interface and the fatigue strength limits of the material. The blade/disk contacts of the engine are shot-peened. During the flight, interfaces are subjected to low displacement amplitudes leading to crack initiation. On the other hand, during landing and take-off, contacts are submitted to high displacement amplitudes leading to the interface wear. As a consequence, competition between wear kinetic and nucleation one is studied and “bell curves” are plotted (i.e. fatigue life time versus displacement amplitude).Moreover, the effect of the shot-peening residual stresses on fatigue life time is observed. In case of un-treated material, a beneficial impact on the fatigue life time is observed due to wear process. Compressive residual stresses of shot-peening increase the fatigue life when fretting/fatigue on partial slip regime occurs. Nevertheless, a decrease of the fatigue life time is observed when wear process is activated by the gross slip condition. Finally, the complex loading of the engine blade/disk contact has been reproduced in the laboratory. To achieve this, a new testing, using an experimental machine composed of three hydraulic actuators is developed. Thus, the global life time of the representative interface of the fan stage is studied. Oligocyclic (low frequency) and polycyclic (high frequency) solicitations interact themselves and lead, firstly, to a rearrangement of the residual stresses, and secondly, to an increase of the wear kinetics. This research work highlights the interest to consider the polycyclic loadings to design the blade/disk structure. These solicitations reproduce the “cracking” fatigue phenomenon, the cumulative damages and the wear kinetics of the interface
Parmigiani, Renan. "Diversidade funcional ao longo de um gradiente de estresse: um estudo de caso na restinga." Universidade de São Paulo, 2018. http://www.teses.usp.br/teses/disponiveis/41/41134/tde-23102018-141149/.
Understanding processes underlying community assembly is one of the main questions in community ecology. The influence of processes such as environmental filtering and competition can be observed in patterns of functional diversity patterns in plant communities. Competition, through competitive exclusion, limits similarities in ecological strategies in a given community. Environmental filtering, on the other hand, constrains the species that can be established in a given community, restricting the functional diversity. One can reasonably predict that the influence of such processes changes across environmental gradients, where the environmental filtering will exert more influence in more stressful environments, whereas competition will exert more influence in less stressful places. This study aimed to understand the influence of environmental filtering and competition on functional diversity in a plant community across a stress gradient. We expected an inverse relationship between functional diversity and stress. The stress gradient studied occurs in the restinga of the Cardoso Island State Park (Cananeia, SP). We sampled 41 sites, in which we found 104 species of vascular plants. We measured three traits: life form, leaf area and leaf economic spectrum (LES). We represented the environmental filter using edaphic variables that represent restinga environmental restrictions. We used Grime\'s strategies classification (CSR), to extract the component related to competitiveness of each species, and therefore, calculated the competition community weighted mean (CWM) of each plot as proxy of competition. We built linear mixed models (LMM) to represent different hypothesis related to functional diversity and selected the best models by Akaike Criterion (AIC). We evaluated functional diversity through three response variables in the models: functional richness (FRic), functional dispersion (FDis) and CWM. In the model selection of CWM for each trait, FRic for life form and FRic for all traits were predicted by the environmental filtering. The FRic of LES, FRic of leaf area and all models of FDis had the null model as the most plausible, discarding the influence of competition and environmental filter in functional diversity. The fact that there is a concentration of abundance around certain strategies explains why there is no difference in functional dispersion. We infer that environmental filter restricts some strategies, reducing functional richness or displacing functional space of the communities. The absence of competition affecting functional diversity suggests that limiting similarity exerts little influence on community assembly in the studied gradient, or that the consequences of similarity limitation is compensated by other process
Tardif, Antoine. "Prédiction des taux de décomposition des litières végétales par les traits fonctionnels agrégés." Thèse, Université de Sherbrooke, 2014. http://savoirs.usherbrooke.ca/handle/11143/84.
Niamchaona, Wichian. "Modélisation de l'influence des défauts de surface sur le comportement en fatigue de nuances d'acier innovantes." Thesis, Université Clermont Auvergne (2017-2020), 2019. http://www.theses.fr/2019CLFAC086.
The steel manufacturers develop nowadays high strength steels as CP800 grade for automotive applications for the purpose of lightening vehicles. Such steels are strongly sensitive from the fatigue behaviour point of view to the surface defects generated by metal forming or cutting of steel sheets. Surface defects of different types and sizes were machined by electroerosion on CP800 specimens so that they are similar to the surface defects observed on steel sheets after stamping or cutting. The present study deals with the numerical and the experimental fatigue behaviour simulation of these specimens.The defect influence modelisation about the steel fatigue behaviour uses either the critical plane approach or the integral approach in multiaxial fatigue.The stress gradient influence contributes also to the fatigue life prediction of the defective simples.The numerical simulation aims to assess stress states and stress gradient fields within the tested specimens in the vicinity of their own surface defects. Accounting for stress gradients strongly improves the ability of multiaxial fatigue criteria to accurately predict the actual fatigue resistance of defective specimens. It shows also that multiaxial criteria have to be calibrated over fatigue test results with high stress gradients to properly predict the fatigue behaviour of high strength steel with surface small defect