La majoria de les fallades en elements estructurals són degudes a càrrega per fatiga. En conseqüència, la fatiga mecànica és un factor clau per al disseny d'elements mecànics. En el cas de materials compòsits laminats, el procés de fallada per fatiga inclou diferents mecanismes de dany que resulten en la degradació del material. Un dels mecanismes de dany més importants és la delaminació entre capes del laminat. En el cas de components aeronàutics, les plaques de composit estan exposades a impactes i les delaminacions apareixen facilment en un laminat després d'un impacte. Molts components fets de compòsit tenen formes corbes, superposició de capes i capes amb diferents orientacions que fan que la delaminació es propagui en un mode mixt que depen de la grandària de la delaminació. És a dir, les delaminacions generalment es propaguen en mode mixt variable. És per això que és important desenvolupar nous mètodes per caracteritzar el creixement subcrític en mode mixt per fatiga de les delaminacions. El principal objectiu d'aquest treball és la caracterització del creixement en mode mixt variable de les delaminacions en compòsits laminats per efecte de càrregues a fatiga. Amb aquest fi, es proposa un nou model per al creixement per fatiga de la delaminació en mode mixt. Contràriament als models ja existents, el model que es proposa es formula d'acord a la variació no-monotònica dels paràmetres de propagació amb el mode mixt observada en diferents resultats experimentals. A més, es du a terme un anàlisi de l'assaig mixed-mode end load split (MMELS), la característica més important del qual és la variació del mode mixt a mesura que la delaminació creix. Per a aquest anàlisi, es tenen em compte dos mètodes teòrics presents en la literatura. No obstant, les expressions resultants per l'assaig MMELS no són equivalents i les diferències entre els dos mètodes poden ser importants, fins a 50 vegades. Per aquest motiu, en aquest treball es porta a terme un anàlisi alternatiu més acurat del MMELS per tal d'establir una comparació. Aquest anàlisi alternatiu es basa en el mètode dels elements finits i virtual crack closure technique (VCCT). D'aquest anàlisi en resulten importants aspectes a considerar per a la bona caracterització de materials utilitzant l'assaig MMELS. Durant l'estudi s'ha dissenyat i construït un utillatge per l'assaig MMELS. Per a la caracterització experimental de la propagació per fatiga de delaminacions en mode mixt variable s'utilitzen diferents provetes de laminats carboni/epoxy essencialment unidireccionals. També es du a terme un anàlisi fractogràfic d'algunes de les superfícies de fractura per delaminació. Els resultats experimentals són comparats amb les prediccions del model proposat per la propagació per fatiga d'esquerdes interlaminars. / Most of the failures in structural elements in use are a consequence of mechanical fatigue. Therefore, fatigue is a decisive factor in designing durable mechanical elements. In laminated composite materials, the fatigue process involves different damage mechanisms that result in the degradation of the material. One of the most important damage mechanisms is the delamination between plies of the laminate. In aeronautical applications, composite plates are sensitive to impact and delamination occurs readily in composite laminates on impact. Many composite components have curved shapes, tapered thickness and plies with different orientations, which make the delamination grow with a mode mix that depends on the extent of the crack. Thus, delaminations generally grow under varying mode mix. It is therefore important to develop methods that can characterise subcritical, mixed-mode growth in fatigue delamination. The main objective of the present investigation is the characterisation of the variable mixed-mode delamination in composite laminates under fatigue conditions. To this end, a mixed-mode fatigue delamination model is proposed. Oppositely to the mixed-mode fatigue delamination models present in the literature, the proposed model takes into account the non-monotonic variation of the propagation parameters with the mode mix observed in different experimental data. Moreover, the mixed-mode end load split (MMELS) test, which main characteristic is that the propagation mode of the interlaminar crack varies with the crack extent, is analysed. Two theoretical approaches present in the literature are considered. However, the resulting expressions for the MMELS test are not equivalent and the differences between approaches can be up to 50 times. A more accurate alternative analysis of the MMELS test is carried out in the present study for comparison. The alternative analysis is based on the finite element method and the virtual crack closure technique. Significant findings are found for precise materials characterisation using the MMELS test. A MMELS test rig is also designed and built. Different specimens of essentially unidirectional carbon/epoxy laminates are tested for the experimental characterisation of fatigue delamination under varying mode mix. A fractographic analysis is also conducted in some of the delaminated fracture surfaces. The experimental results are compared to the predictions of a proposed model for the fatigue propagation of interlaminar cracks.

Cette thèse porte sur la fissuration par fatigue sous chargement variable en mode mixte I+II+III et sur les effets d'histoire induits par la plasticité confinée et les contraintes internes. Les essais réalisés montrent qu’il existe des effets antagonistes d’histoire du chargement à courte distance et à longue distance, que la forme de la séquence de chargement est cruciale et, par comparaison, que les effets de contact et de frottement sont d’importance moindre. Les contraintes internes jouent un rôle majeur sur la vitesse de fissuration par fatigue et sur le mode de fissuration. Une démarche a été mise en place pour étudier le comportement élasto-plastique d’une section représentative du front de la fissure par éléments finis. Pour ne retenir des calculs réalisés que le minimum d’information nécessaire, on se donne une approximation cinématique, le champ de vitesse est partitionné en composantes de modes I, II, III élastiques et plastiques, chaque composante étant caractérisée par un facteur d’intensité et une distribution spatiale fixe. Les calculs réalisés ont permis de sélectionner 7 trajets de chargement différents en mode I+II et en mode I+II+III, qui présentent les mêmes amplitudes pour chaque mode, les mêmes maxima, minima et valeurs moyennes. Ces trajets, censés être équivalents au sens des critères de rupture usuels, ne le sont pourtant pas lorsqu’on considère le comportement élasto-plastique du matériau et conduisent à des vitesses et des trajets de fissuration expérimentaux très différents les uns des autres. Les simulations numériques et la modélisation simplifiée sont en accord avec les résultats expérimentaux. Les calculs réalisés ont également permis de discuter le rôle des phases de chargement en mode III sur la fissuration. Le comportement du matériau étant non-linéaire, la direction du chargement nominal ne coïncide pas nécessairement avec celle de l’écoulement plastique. Ajouter une phase de chargement en mode III peut, dans certains cas, modifier très significativement le comportement de la fissure (direction de propagation, vitesse de fissuration, écoulement plastique). / This thesis deals with fatigue crack growth in non-proportional variable amplitude mixed mode I + II + III loading conditions and analyses the effects of internal stresses stemming from the confinement of the plastic zone in small scale yielding conditions. The tests showed that there are antagonistic long-distance and short-distance effects of the loading history on fatigue crack growth. The shape of loading path, and not only the maximum and minimum values in this path, is crucial and, by comparison, the effects of contact and friction are of lesser importance. Internal stresses play a major role on the fatigue crack growth rate and on the crack path. An approach was developed to analyze the elastic-plastic behavior of a representative section of the crack front using the FEA. A model reduction technic is used to extract the relevant information from the FE results. To do so, the velocity field is partitioned into mode I, II, III elastic and plastic components, each component being characterized by an intensity factor and a fixed spatial distribution. The calculations were used to select seven loading paths in I + II and I + II + III mixed mode conditions, which all have the same amplitudes for each mode, the same maximum, minimum and average values. These paths are supposed to be equivalent in the sense of common failure criteria, but differ significantly when the elastic-plastic behavior of the material is accounted for. The results of finite element simulations and of simulations using a simplified model proposed in this thesis are both in agreement with experimental results. The approach was also used to discuss the role of mode III loading steps. Since the material behavior is nonlinear, the nominal loading direction does not coïncide with the plastic flow direction. Adding a mode III loading step in a mode I+II fatigue cycle, may, in some cases, significantly modify the behaviour of the crack (crack growth rate, crack path and plasti flow).

Este trabalho apresenta os principais fenômenos que regem a propagação de trincas em anéis de rolamentos, por meio da aplicação de conceitos da Mecânica da Fratura Elástica Linear e Fadiga. O estado complexo de tensões atuante sobre o anel do rolamento culmina em carregamento modo misto sobre as paredes da trinca. Para o problema de rolamento proposto, são considerados os modos de abertura e de cisalhamento puro no plano do defeito, sob regime cíclico. O fator de intensidade de tensões K e a integral J são parâmetros que estabelecem condições de fratura e são suficientes, para caracterizar a criticidade de trincas instaladas em anéis de rolamentos. A avaliação da criticidade de defeitos é baseada na aplicação de Critérios de Falha distintos, oriundos de literaturas especializadas. A partir dos parâmetros de fratura, K e J , e da avaliação da criticidade de defeitos é possível determinar a vida remanescente à fadiga, por meio de modelos que descrevem a taxa de propagação de trincas. Os resultados indicam que o modo misto de carregamento de trinca considerado na dissertação, é um dos parâmetros mais importantes a considera-se na nucleação e propagação de defeitos em rolamentos, e são responsáveis por uma redução significativa da vida à fadiga originalmente estipulada. / This work presents the main phenomenon that governs the crack propagation in bearing rings, through the application of Linear Elastic Fracture Mechanics and Fatigue concepts. The complex stress state acting on the bearing ring produces a mixed mode loading over the crack faces. The proposed bearing problem considers a tensile mode and pure shear mode in the plane of the defect, both under a cyclic regime. The stress intensity factor K and the J -integral parameters define the fracture conditions and suffice to characterize the severity of cracks developed in bearing rings. The procedure for defect assessment is based on the application of different failure criteria. Using parameters, K and J, coupled to the defect assessment procedures, it is possible to estimate the remaining fatigue life, through models that describe the crack propagation rate. The results indicate that the mixed mode loading considered in this work is one of the most important parameters in nucleation and propagation of bearing defects and are responsible for a significant reduction in the original estimation of fatigue life.

It is widely recognised that one's ability of adaptation is remarkable and thermal comfort is significantly related to such adaptations. This study proposes an alternative method of predicting adaptive thermal comfort based on the availability of adaptations, in particular behavioural adaptations, which needs quantifications of individual adaptation processes and of interactions between them. The fundamental argument of this method is that exercising an adaptive behaviour leads to an increase in comfort temperature, which is termed adaptive increment in this study. Apart from adaptive increments, this method also determines a baseline thermal comfort temperature (the thermal comfort temperature without adaptations) and a correction factor that considers the factors affecting adaptive behaviours, based on which, the highest operative temperature at which people may still feel thermally comfortable. This may be applied in mixed mode (MM) buildings to achieve a higher air-conditioning (AC) setpoint which may lead to a significant reduction in cooling energy. This method is believed to be flexible in dealing with different environments with various levels of adaptations and likely to be advantageous over the steady-state and adaptive models in predicting thermal comfort temperature of an environment with abundant adaptive opportunities. This study also evaluates ways of promoting the use of adaptive opportunities. It explores how adaptive thermal comfort theories may be used for behaviour modelling and in turn be applied to enhance the energy performances and comfort levels of real buildings. To improve the feasibility of this method key effective adaptive behaviours are studied in detail through lab experiments and field studies. The lab experiment has found the adaptive increment of taking cold water to be 1.5°C which is more significant than the previous literature suggests. When all the studied adaptive behaviours are exercised, the overall adaptive increment is as high as 4.7°C. However, the research has identified some issues associated with the adaptive opportunities studied. These include the existence of constraints on the use of adaptive behaviours, the low availability of some effective adaptive opportunities, the low operation frequency of desk fans and the misuse of windows and AC systems. Despite this, the availability of more adaptive opportunities has been verified to be capable of increasing the highest operative temperature at which people may still feel thermally comfortable: the lab experiment shows that over 80% of the participants can still find it thermally comfortable at an operative temperature of 30°C on the condition that adequate adaptive opportunities are provided; the field study shows that the thermal comfort temperature of occupants increases by at least 1°C when desk fans and cool mats are available. Based on these analyses, it proposes an MM system which encourages occupants to exercise adaptive opportunities and improves both comfort levels and energy efficiency. Building performance simulation results show that the proposed MM system is effective in reducing the reliance on AC systems and promotes effective uses of windows and AC systems. By applying the MM system and the associated passive energy-saving strategies, an office can cut cooling energy by about 90% and the peak cooling load by over 80% during transitional seasons.

Cette thèse s’effectue au sein de l’université de Bordeaux. Ce projet concerne la construction en bois et en particulier la compréhension des phénomènes à l’origine des fissures dans les structures bois. L’un des objectifs étant de concevoir des méthodes efficaces de renforcement local pour les éléments de structure. Pour cela, l’étude se décompose en plusieurs étapes, la compréhension des phénomènes mis en jeu dans la création des fissures sur des bois de construction. Ce travail s’orientera sur la modélisation de fissure en mode mixte, la recherche de solution de renforcement avec compréhension fine de l’impact de types de renforts sur la propagation de la fissure. Cette étude sera accompagnée d’une campagne d’essais, afin de vérifier l’efficacité des renforcements choisis et d’identifier l’impact de l’effet d’échelle sur les modèle de prédiction. Des essais de grandes dimensions seront donc réalisés pour mieux appréhender les effets de groupes et les effets d’échelle sur du matériau d’emploi. On vise ensuite à définir des outils prédictifs de la résistance des systèmes renforcés et de moyens de contrôles pour les Plan d’Assurance Qualité. / The purpose of reinforcing assemblies and structural elements inwood is to overcome the resistance limits of the material, by transferring greaterefforts in areas which can lead to premature cracking in structures. The reinforcementsused can be made of steel, composite materials or wood. Their hook can bemechanical (screwed bodies) or by adhesion (structural bonding like glued-in rodsfor example). In both cases, the transfer of solicitations remains poorly known, andthe effect of the beginning and the deflection of crack are not well apprehended. Inengineering techniques, the wood resistance in the reinforced area is neglected, whichis in line with the precautionary principle. Currently, the scientific investigations areinterested in the resistance of those kind of techniques without considering the interactionsbetween the quasi-brittle behavior of the wood and the reinforcementswhich govern the gain in mechanical performance. However, these solutions can leadto a failure caused by the progressive splitting of the wood and the anchor loss ofthe reinforcement. So it seems accurate to propose predictions of the short-termstrength for splitting of reinforced and unreinforced beams, which can be used tofurther exploration of the long-term failure mechanism. That is why, in this study, aglobal prediction model of the ultimate strength of structural components subjectedto splitting, reinforced and unreinforced ones, was developed. It considers the quasibrittlebehavior of the wood and crack propagation in mixed mode, using a mixinglaw established on the R-curves. The relevance of this modeling was then comparedto the current dimensioning methods of the Eurocodes 5, for notched beams, withexperimental campaigns conducted at different scales.

No description available.

<p>Traditionally, rolling mills use emulsions based on a mixture of oil and water for lubrication. Since two years ago SAPA has been using (instead of oil) a synthetic lubricant so called conditional emulsion for hot-rolling of aluminum. This lubricant is water based and homogenous at ambient temperature, but switches to a two-phase system at heating above the cloud point.</p><p>This project aims to validate and if necessary modify an existing HPLC method for quantifying two out of three key components (A, B and C) in the conditional emulsion. Attempts to develop a method to quantify the pH adjusting components, X and Y were also made. These two methods are required to optimize the lubricant.</p><p>Due to the complexity of the components, it has been difficult to present a method for quantification, and HPLC with ELS detection was chosen after a long series of trials. Due to a few uncontrollable parameters the proposed analysis method has tendencies to be unstable. The column used is sensitive to changes in equilibrium and ELSD is also less sensitive and less reproducible than the commonly used UV-detector.</p><p>While the proposed assay method shows somewhat large relative standard deviations the method has been shown to produce sufficiently precise and accurate data for the intended purpose.</p><p>Development of a method for the pH-adjusting components X and Y was more difficult than expected. For some reason their difference in chemical properties does not show satisfying impact in the chromatograms.</p><p>This method is still in its cradle and needs further development.</p><p> </p>

Traditionally, rolling mills use emulsions based on a mixture of oil and water for lubrication. Since two years ago SAPA has been using (instead of oil) a synthetic lubricant so called conditional emulsion for hot-rolling of aluminum. This lubricant is water based and homogenous at ambient temperature, but switches to a two-phase system at heating above the cloud point. This project aims to validate and if necessary modify an existing HPLC method for quantifying two out of three key components (A, B and C) in the conditional emulsion. Attempts to develop a method to quantify the pH adjusting components, X and Y were also made. These two methods are required to optimize the lubricant. Due to the complexity of the components, it has been difficult to present a method for quantification, and HPLC with ELS detection was chosen after a long series of trials. Due to a few uncontrollable parameters the proposed analysis method has tendencies to be unstable. The column used is sensitive to changes in equilibrium and ELSD is also less sensitive and less reproducible than the commonly used UV-detector. While the proposed assay method shows somewhat large relative standard deviations the method has been shown to produce sufficiently precise and accurate data for the intended purpose. Development of a method for the pH-adjusting components X and Y was more difficult than expected. For some reason their difference in chemical properties does not show satisfying impact in the chromatograms. This method is still in its cradle and needs further development.

For a large mixed-mode database, how to discretize its continuous data into interval events is still a practical approach. If there are no class labels for the database, we have nohelpful correlation references to such task Actually a large relational database may contain various correlated attribute clusters. To handle these kinds of problems, we first have to partition the databases into sub-groups of attributes containing some sort of correlated relationship. This process has become known as attribute clustering, and it is an important way to reduce our search in looking for or discovering patterns Furthermore, once correlated attribute groups are obtained, from each of them, we could find the most representative attribute with the strongest interdependence with all other attributes in that cluster, and use it as a candidate like a a class label of that group. That will set up a correlation attribute to drive the discretization of the other continuous data in each attribute cluster. This thesis provides the theoretical framework, the methodology and the computational system to achieve that goal.

This dissertation focuses mainly on three topics: (1) mixed-mode branching and segmentation of hydraulic fractures in brittle materials, (2) hydraulic fracture propagation in particulate materials, and (3) hydraulic fracturing in water flooding conditions. Mixed-mode loading is one of the primary causes of fracture branching and segmentation in brittle materials. We conducted the first laboratory experiments on the mixed mode I+III hydraulic fracturing. We found that a KIII/KI ratio as small as ~1% is sufficient for fracture front segmentation. In reality, such a small mode III component is always expected, for example, due to the small deviations of the fracture shape from planar. Thus, we concluded that fracture segmentation is likely to accompany growth of most, if not all, real hydraulic fractures. We also proposed a theoretical model that captures the main features of experimental observations and indicates the importance of the hydraulic effect of segmentation. Particulate materials often exhibit pronounced non-linear behavior and yielding even at relatively small loads. In order to adequately describe hydraulic fracturing in particulate materials with low or no cohesion, plasticity at the crack tip must be explicitly considered. We investigated the shear band mechanism of strain localization at the fracture front. This mechanism takes into account the fact that cohesionless material can not bear tension, and is in compression everywhere, including near the fracture front. To verify the shear band hypothesis, we conducted numerical simulations of the plastic deformation at the tip of a fracture in particulate material with strain softening. Our model describes the shear bands by properly placed and oriented dislocations. The model results are consistent with experimental observations. Water flooding, which in certain important cases, can result in processes resembly hydraulic fracturing by a low-viscosity fluid with extremely high leak-off. It is difficult to simulate this process in the laboratory. To investigate the fracture initiation mechanism in water flooding conditions, we conducted a numerical simulation of fluid injection into particulate material by using the discrete element code PFC2D. We also considered an analytical model of cavity initiation based on the fluidization mechanism. The estimates given by this model fit remarkably well with the numerical simulation results.