Dissertations / Theses on the topic 'Hardening-Softening'

Ce manuscrit présente des récentes extensions au modèle PFVD, basé sur l'hétérogénéité de la dynamique des polymères vitreux à l'échelle de quelques nanomètres et résolu par simulation en 3D, afin de fournir une description physique unifiée des propriétés mécaniques et dynamiques des polymères vitreux soumis à déformation plastique. Trois sujets principaux sont traités : La plastification. Sous déformation, les polymères atteignent le seuil de plasticité (yield) à quelques pourcents de déformation et quelques dizaines de MPa. Nous proposons que l'énergie élastique absorbée à l'échelle des hétérogénéités dynamiques accélère la dynamique locale. On observe contraintes ultimes de quelques dizaines de MPa à quelques pourcents de déformation et que la plastification est due à un nombre relativement petit d'événements locaux. Il a été observé que la dynamique devient plus rapide et homogène dans le régime plastique et que la mobilité moyenne atteint une valeur stationnaire, linéaire avec le taux de déformation. Nous proposons que la contrainte locale stimule la diffusion de monomères des domaines lents à ceux rapides (mécanisme de facilitation) et accélère dynamique locale. Ceci permets d'observer l'homogénéisation de la dynamique, avec des caractéristiques proches de l'expérience. L'écrouissage, dans les polymères enchevêtrés ou réticulés. A grande déformation, la contrainte augmente avec une pente caractéristique d'ordre 10 – 100 MPa au-dessous de la transition vitreuse. De manière analogue à une théorie récente, nous proposons que la déformation locale oriente les monomères dans la direction d'étirage et ralentie la dynamique, suite à l'intensification des interactions locales. Les modules d'écrouissage mesurés, les effets de la réticulation et du taux de déformation sont comparables aux données expérimentales. En outre, on trouve que l'écrouissage a un effet stabilisateur sur les phénomènes de localisation et sur les bandes de cisaillement
This manuscript presents recent extensions to the PFVD model, based on the heterogeneity of theh dynamics of glassy polymers at the scale of a few nanometers et solved by 3D numerical simulation, which aim at providing a unified physical description of the mechanical and dynamical properties of glassy polymers during plastic deformation. Three main topics are treated: Plasticization. Under applied deformation, polymers undergo yield at strains of a few percent and stresses of some 10 MPa.We propose that the elastic energy stored at the scale of dynamical heterogeneities accelerates local dynamics. We observe yield stresses of a few 10 MPa are obtained at a few percent of deformation and that plastification is due to a relatively small amount of local yields. It has been observed that dynamics becomes faster and more homogeneous close to yield and that the average mobility attains a stationary value, linear with the strain rate. We propose that stress-induced acceleration of the dynamics enhances the diffusion of monomers from slow domains to fast ones (facilitation mechanism), accelerating local dynamics. This allows for obtaining the homogeneisation of the dynamics, with the same features observed during experiments. Strain-hardening, in highly entangled and cross-linked polymers. At large strain, stress increases with increasing strain, with a characteristic slope (hardening modulus) of order 10 – 100 MPa well below the glass transition. Analogously to a recent theory, we propose that local deformation orients monomers in the drawing direction and slows dows the dynamics, as a consequence of the intensification of local interactions. The hardening moduli mesured, the effect of reticulation and of strain rate are comparable with experimental data. In addition, strain-hardening is found to have a stabilizing effect over strain localization and shear banding

It is impossible to improve the quality of the machines, to increase their reliability and lifetime if the working conditions and the properties of the material are not analyzed. We must know the type of the material (hardening, softening or cyclically stabile), what is chosen for the constructions in low cycle loading, because strain and stress change during the exploitation and depend on this type. If we know the type of the material, we can determine the possibility of its application in concrete exploitation conditions. Real working conditions of the most constructions are close to loading with limited strain (hard straining), because elastic and plastic deformation is met in the zones of crack and stress concentration, that are surrounded with elastically deformed material. The low cycle loading curves parameters A, and are used for the computation of elastic plastic strain curves. These parameters are obtained from the soft low cycle loading results in many cases. The other possible ways for the determination of parameters A, and are shown in this work. The most investigated materials had the initial instability in the interval . For more objective evaluation of stress strain curves parameters A, and , all values of width of hysteresis loop up to semicycle were rejected as insignificant in comparison with the rest lifetime in cycles range . The parameter for the evaluation of hardening (softening) intensity was determined, when the values of... [to full text]

Le travail s'intéresse aux effets de l'histoire de chargement sur le comportement et la durée de vie en fatigue de deux nuances (THYSSEN et CLI)d'un acier inoxydable austénitique 304L à la température ambiante. Les essais ont été réalisés en utilisant deux catégories d'éprouvettes. Les éprouvettes de la première catégorie (vierges) ont été soumises à des essais classiques de fatigue,alors que celles de la deuxième ont subi, avant les essais de fatigue, un pré-écrouissage monotone ou cyclique en déformation imposée. Les éprouvettes vierges manifestent un adoucissement cyclique suivi d'un durcissement cyclique alors que les éprouvettes pré-écrouies ne présentent qu'un durcissement cyclique. Les résultats montrent une grande influence du pré-écrouissage qui semble bénéfique en contrainte imposée, mais néfaste en déformation imposée,même en présence d'une contrainte moyenne de compression. Ces résultats sont discutés en termes d'évolution cyclique du module d'élasticité, des écrouissages isotropes et cinématiques, et de la densité d'énergie absorbées par cycle, dans différentes configurations : avec ou sans pré-écrouissage, en contrainte ou déformation imposées...

Magnesium alloys have been of growing interest to various engineering applications, such as the automobile, aerospace, communication and computer industries due to their low density, high specific strength, good machineability and availability as compared with other structural materials. However, most Mg alloys suffer from poor plasticity due to their Hexagonal Close Packed structure. Grain refinement has been proved to be an effective method to enhance the strength and alter the ductility of the materials. Several methods have been proposed to produce materials with nanocrystalline grain structures. So far, most of the research work on nanocrystalline materials has been carried out on Face-Centered Cubic and Body-Centered Cubic metals. However, there has been little investigation of nanocrystalline Mg alloys. In this study, bulk coarse-grained and nanocrystalline Mg alloys were fabricated by a mechanical alloying method. The mixed powder of Mg chips and Al powder was mechanically milled under argon atmosphere for different durations of 0 hours (MA0), 10 hours (MA10), 20 hours (MA20), 30 hours (MA30) and 40 hours (MA40), followed by compaction and sintering. Then the sintered billets were hot-extruded into metallic rods with a 7 mm diameter. The obtained Mg alloys have a nominal composition of Mg–5wt% Al, with grain sizes ranging from 13 μm down to 50 nm, depending on the milling durations. The microstructure characterization and evolution after deformation were carried out by means of Optical microscopy, X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Scanning Probe Microscopy and Neutron Diffraction techniques. Nanoindentaion, compression and micro-compression tests on micro-pillars were used to study the size effects on the mechanical behaviour of the Mg alloys. Two kinds of size effects on the mechanical behaviours and deformation mechanisms were investigated: grain size effect and sample size effect. The nanoindentation tests were composed of constant strain rate, constant loading rate and indentation creep tests. The normally reported indentation size effect in single crystal and coarse-grained crystals was observed in both the coarse-grained and nanocrystalline Mg alloys. Since the indentation size effect is correlated to the Geometrically Necessary Dislocations under the indenter to accommodate the plastic deformation, the good agreement between the experimental results and the Indentation Size Effect model indicated that, in the current nanocrystalline MA20 and MA30, the dislocation plasticity was still the dominant deformation mechanism. Significant hardness enhancement with decreasing grain size, down to 58 nm, was found in the nanocrystalline Mg alloys. Further reduction of grain size would lead to a drop in the hardness values. The failure of grain refinement strengthening with the relatively high strain rate sensitivity of nanocrystalline Mg alloys suggested a change in the deformation mechanism. Indentation creep tests showed that the stress exponent was dependent on the loading rate during the loading section of the indentation, which was related to the dislocation structures before the creep starts. The influence of grain size on the mechanical behaviour and strength of extruded coarse-grained and nanocrystalline Mg alloys were investigated using uniaxial compression tests. The macroscopic response of the Mg alloys transited from strain hardening to strain softening behaviour, with grain size reduced from 13 ìm to 50 nm. The strain hardening was related to the twinning induced hardening and dislocation hardening effect, while the strain softening was attributed to the localized deformation in the nanocrystalline grains. The tension–compression yield asymmetry was noticed in the nanocrystalline region, demonstrating the twinning effect in the ultra-fine-grained and nanocrystalline region. The relationship k tensions < k compression failed in the nanocrystalline Mg alloys; this was attributed to the twofold effect of grain size on twinning. The nanocrystalline Mg alloys were found to exhibit increased strain rate sensitivity with decreasing grain size, with strain rate ranging from 0.0001/s to 0.01/s. Strain rate sensitivity of coarse-grained MA0 was increased by more than 10 times in MA40. The Hall-Petch relationship broke down at a critical grain size in the nanocrystalline region. The breakdown of the Hall-Petch relationship and the increased strain rate sensitivity were due to the localized dislocation activities (generalization and annihilation at grain boundaries) and the more significant contribution from grain boundary mediated mechanisms. In the micro-compression tests, the sample size effects on the mechanical behaviours were studied on MA0, MA20 and MA40 micro-pillars. In contrast to the bulk samples under compression, the stress-strain curves of MA0 and MA20 micro-pillars were characterized with a number of discrete strain burst events separated by nearly elastic strain segments. Unlike MA0 and MA20, the stress-strain curves of MA40 micro-pillars were smooth, without obvious strain bursts. The deformation mechanisms of the MA0 and MA20 micro-pillars under micro-compression tests were considered to be initially dominated by deformation twinning, followed by dislocation mechanisms. For MA40 pillars, the deformation mechanisms were believed to be localized dislocation activities and grain boundary related mechanisms. The strain hardening behaviours of the micro-pillars suggested that the grain boundaries in the nanocrystalline micro-pillars would reduce the source (nucleation sources for twins/dislocations) starvation hardening effect. The power law relationship of the yield strength on pillar dimensions in MA0, MA20 supported the fact that the twinning mechanism was correlated to the pre-existing defects, which can promote the nucleation of the twins. Then, we provided a latitudinal comparison of the results and conclusions derived from the different techniques used for testing the coarse-grained and nanocrystalline Mg alloy; this helps to better understand the deformation mechanisms of the Mg alloys as a whole. At the end, we summarized the thesis and highlighted the conclusions, contributions, innovations and outcomes of the research. Finally, it outlined recommendations for future work.

Die Arbeit widmet sich dem Fließverhalten von Stählen bei zyklisch inkrementeller Torsion. Dazu werden vergleichend Reineisen und der ferritisch-perlitische Stahl 42CrMo4N bei unterschiedlichen Verformungspfaden betrachtet. Vor allem in zyklischen Torsionsversuchen mit bleibendem Verformungsinkrement je Zyklus werden Fließkurvenverläufe und Verformungsverfestigung analysiert. Die Ergebnisse belegen den Einfluss des lamellaren Zementit auf das Fließverhalten des Stahls 42CrMo4N, während die Eigenschaften des Reineisens von der entstehenden Versetzungszellstruktur bestimmt werden. Die Richtungsabhängigkeit der Fließspannung und die Verläufe der Fließkurven unterscheiden sich für die betrachteten Werkstoffe deutlich. Fließortkurven dienen der quantitativen Beschreibung der Verfestigung. Die Vorgehensweise zu ihrer Ermittlung und ihre Abhängigkeit von den Versuchsbedingungen und den Verformungszuständen werden gezeigt. Bei Reineisen dominieren isotrope, bei dem Stahl 42CrMo4N kinematische Verfestigungsanteile das Fließverhalten
Ferritic-pearlitic steel 42CrMo4N and pure iron under different strain paths are compared regarding their flow behaviour. Mainly in cyclic torsion tests with resulting strain increment per cycle shear stresses and strain hardening are analysed. The results show, that the cementite lamellae determine the flow behaviour of the steel 42CrMo4N, whereas the properties of pure iron are governed by the evolving dislocation cell structure. The dependency of flow stress on the strain direction is different for the two materials. Yield surfaces describe strain hardening quantitatively. The procedure for yield point detection and the dependency of subsequent yield surfaces on experimental conditions and strain states is shown. For pure iron isotropic hardening, for steel 42CrMo4N kinematic strain hardening dominates the flow behaviour

Questa tesi di dottorato analizza la dinamica non lineare di una trave omogenea e isotropa. Sono state inizialmente ricavate le equazioni differenziali alle derivate parziali del moto per un modello di trave rettilinea nel caso piano, tenendo conto degli effetti dell’estensibilità, di taglio e dell’inerzia longitudinale, trasversale e rotazionale, e per diverse definizioni di curvatura. Per le particolari condizioni al contorno scelte (trave incernierata – semplicemente appoggiata con una molla assiale), è stato utilizzato il metodo alle scale temporali multiple (multiple time-scales) per cogliere i differenti fenomeni dinamici nel sistema, dove le vibrazioni dinamiche libere e forzate con smorzamento sono rappresentate rispettivamente da curve backbone e da curve non lineari di risposta in frequenza. In questo lavoro sono state accoppiate vibrazioni longitudinali e trasversali, mostrando ed analizzando l'importanza dell’interazione di due modi, ad esempio per fenomeni di risonanze interne. I risultati analitici sono state integrati con un modello numerico realizzato nel software commerciale Abaqus_CAE. Inizialmente, il comportamento lineare del modello della trave è stato esaminato nel modulo in frequenza e successivamente, utilizzando il modulo dinamico esplicito, sono state ottenute le curve backbone e le curve non lineari di risposta in frequenza stabili. Le simulazioni numeriche hanno usato i metodi di path-following e shooting. I risultati delle simulazioni sono stati confrontati con le soluzioni analitiche e dimostrano la conformità di entrambi i metodi. Oltre alle sofisticate analisi teoriche, sono stati effettuati alcuni test preliminari di laboratorio su un prototipo di trave di Eulero-Bernoulli al variare delle condizioni al contorno in direzione longitudinale. La dinamica della struttura reale è stata testata sperimentalmente forzando il sistema cinematicamente nella direzione trasversale con diverse ampiezze di eccitazione e condizioni al contorno. Sono stati osservati fenomeni di non linearità quali hardening e softening e salti tra soluzioni stabili. Tali effetti sono stati riscontrati sia negli studi analitici sia in quelli numerici, nonché negli esperimenti sul prototipo. Infine, il lavoro è completato dalle conclusioni e dai possibili sviluppi futuri del problema.
In the thesis, studies on nonlinear dynamics of a homogeneous and isotropic beam are performed. Partial differential equations of motion of a straight planar beam model are derived, taking into account extensibility and shearing effects, longitudinal, transverse and rotational inertia as well as different curvature definitions. For special boundary conditions (like hinged-simply supported beam with an axial spring) approximate method of multiple time-scales is implemented to catch various dynamic phenomena of the system, wherein free dynamics and forced-damped vibrations are represented by backbone curves and nonlinear frequency response curves, respectively. In the structure axial-transverse oscillations are coupled and the importance of combination of two modes, for instance in internal resonance, is shown. Analytical considerations are supported by numerical model elaborated in commercial software Abaqus_CAE. Initially, the linear behaviour of the beam model in frequency module is tested and then backbone curves and stable frequency response curves with use of transient dynamic explicit module are obtained. Time-dependent simulations with path-following and shooting methods are used and outcomes are compared with analytical results. Apart from sophisticated theoretical approaches, a basic validation tests on the Euler-Bernoulli beam prototype are done. Nonlinear dynamics of kinematically excited prototype with various boundary conditions in axial direction is executed experimentally. Hardening/softening phenomena and jumps between stable solutions are observed in analytical and numerical solutions, as well as in experiments. Study on nonlinear dynamics of a beam ends with conclusions and further developments are suggested.

Combining the most recent technologies in concrete, Ultra-High-Performance Fibre-Reinforced Concrete (UHPFRC) arises as a promising material for the near future. UHPFRC have shown how flexible concrete can be to adapt to the ever-changing social and environmental demands. With its high flexibility composition and its mechanical properties, UHPFRC is full of both unexplored and unexploited possibilities. Engineers should take responsibility for this task. However, it is fair to acknowledge that this is not an easy task and it requires the development of reliable and widely accepted design standards provided by the scientific community. A major concern about durability, long-lasting structures and reduction of maintenance cost, as well as the development of new concrete technologies, improved knowledge of fibre effect and a huge growth in the fibre industry accompanied by fibre price reduction have led, among other factors, to the development of new types of concrete whose mechanical behaviour substantially differs from conventional fibre-reinforced concrete. This is why current characterisation methodologies and design standards must be reviewed and adjusted to these newer materials. However, design standard revision cannot disregard former milestones achieved thanks to decades of hard work. It must offer an integrated view in which new types of concrete comprise existing ones in a broader group, because at the end of the day and despite having newer and improved properties, new types of concrete are still concrete. That is how it should be understood and how it must be reflected in newer codes and standards. The work presented herein is focused on one of these recently developed materials that embraces major advanced technologies in concrete: Ultra-High-Performance Fibre-Reinforced Concrete (UHPFRC). This work is specifically focused on those crucial requirements for the development and widespread use of it, such as constitutive tensile characterisation and classification. This work includes a deep revision of the uniaxial tensile behaviour of concrete and its development as fibre technology has evolved. In addition, traditional characterisation standard methods as well as those recently developed for its specific use on UHPFRC are reviewed and called into question. Throughout the document, the development of different methodologies to determine the uniaxial constitutive tensile behaviour of UHPFRC from bending tests are shown, together with a simplified characterisation proposal specially developed for being included in a standard. All developed methodologies presented herein are checked and validated. These methods are specifically designed for their application on experimental results obtained from a special type of four-point bending test, whose standardisation proposal for UHPFRC is also shown. Finally, a classification proposal is presented as a function of more relevant UHPFRC tensile parameters necessary for design that can be directly obtained from the standard characterisation test method suggested. Proposed classification encompasses the existing classification for conventional reinforced and fibre-reinforced concrete. In it, both plain concrete and fibre-reinforced concrete are presented as a particular case of a more general tensile constitutive response for concrete. Standard methodology and classification proposed are in accordance with the evolution of concrete and unify historic milestones achieved by the international research community.
El Hormigón de Muy Alto Rendimiento (HMAR) combina los últimos avances tecnológicos en hormigón y se erige como un material prometedor para el futuro. El HMAR ha demostrado su gran capacidad para adaptarse a las cada vez más exigentes demandas sociales y medioambientales. Con un gran abanico de posibilidades en su dosificación para conseguir las propiedades mecánicas deseadas, el HMAR es un material lleno de posibilidades aún sin explorar y sin explotar. Los ingenieros tienen la responsabilidad de esta tarea. Sin embargo, es justo reconocer que no se trata de una tarea fácil y que requiere de un desarrollo previo de códigos de diseño adecuados y ampliamente aceptados por parte de la comunidad científica. La aparición de nuevas tecnologías, el mayor conocimiento sobre la aportación de las fibras así como su industrialización y bajada de precios, las mayores preocupaciones sobre la durabilidad estructural, incremento de la vida útil o la reducción de los costes de mantenimiento, entre otros factores, han derivado en el desarrollo de nuevas tipologías de hormigones cuyo comportamiento mecánico difiere de manera sustancial de los tradicionales hormigones con fibras. Es por ello que tanto la readaptación de las metodologías de caracterización como las metodologías de diseño deben ser reformuladas. Y esto debe hacerse de manera no disruptiva, es decir, manteniendo la línea de los hitos alcanzados en los hormigones con fibras convencionales de manera que queden integrados en metodologías de caracterización y de diseño que los engloben, porque al fin y al cabo, y aunque con nuevas y mejores propiedades mecánicas, los nuevos hormigones siguen siendo hormigones. Así debe ser entendido y así debe quedar reflejado en las nuevas normativas. El presente trabajo se centra en uno de esos nuevos materiales desarrollados con el avance de las nuevas tecnologías como es el HMAR. En especial, este documento se centra en ese aspecto tan fundamental para el desarrollo de nuevos hormigones como es la caracterización mecánica y la tipificación. Este trabajo incluye una revisión del comportamiento mecánico uniaxial a tracción del hormigón y de su evolución con la aparición de las diferentes tecnologías. Además, se revisan y se ponen en cuestión los sistemas tradicionales de caracterización, así como los nuevos sistemas desarrollados en los últimos años para su empleo específico en el HMAR. A lo largo del documento se desarrollan diferentes metodologías para la obtención del comportamiento constitutivo a tracción del HMAR, así como la propuesta de una metdología simplificada de caracterización especialmente diseñada para ser incluida en una norma, todas ellas debidamente validadas. Estas metodologías son de aplicación específica a los resultados experimentales obtenidos mediante un ensayo a cuatro puntos sin entalla, cuya propuesta de estandarización para el HMAR ha sido también desarrollada. Finalmente, se presenta una propuesta de tipificación de acuerdo a los parámetros más relevantes del comportamiento a tracción del HMAR que son necesarios para el diseño y que pueden ser directamente obtenidos del ensayo de caracterización propuesto. Esta clasificación engloba a la clasificación existente para el hormigón armado convencional y los actuales hormigones con fibras, de manera que se presenta la actual definición de hormigón con fibras como un caso particular de estos nuevos hormigones, respetando al máximo la evolución de este material y aunando los logros conseguidos por la comunidad científica.
Dins de les combinacions de les tecnologies més recents en el formigó, el formigó de molt alt rendiment (UHPFRC) sorgeix com un material prometedor per al futur pròxim. L'UHPFRC ha demostrat poder ser un formigó flexible per adaptar-se a les sempre canviants demandes socials i mediambientals. Amb una gran flexibilitat en la seua composició i les seues propietats mecàniques, l`UHPFRC està ple de possibilitats de ser explorades i explotades. Els enginyers han de prendre la responsabilitat d'aquesta tasca. No obstant això, és just reconèixer que això no serà fàcil i requerirà el desenvolupament de normes de disseny fiables i àmpliament acceptades per la comunitat científica. Hi ha una gran preocupació al voltant de la durabilitat, la vida útil de les estructures i la reducció del cost de manteniment, juntament amb el desenvolupament de noves tecnologies de formigó, un millor coneixement de l'efecte de la fibra i un enorme creixement en la indústria de la fibra acompanyat per la reducció del preu de la fibra, han conduït, entre altres factors, al desenvolupament de nous tipus de formigons, el comportament mecànic dels quals es diferencia substancialment dels formigons reforçats amb fibres convencionals. És per això que les metodologies de caracterització actuals i les normes de disseny han de ser revisades i ajustades a aquests nous materials. No obstant això, la revisió del codis de disseny no pot prescindir de les antigues fites aconseguides gràcies a dècades de treball dur. S'ha d'oferir una visió integrada en la qual els nous tipus de formigons integren els ja existents en un grup més ampli, ja que, al cap i la fi i malgrat tenir propietats noves i millorades, els nous tipus de formigons són encara un tipus de formigó. Així es com s'hauria d'entendre i reflectir-se en els nous codis i normes. El treball presentat en aquest document es centra en un d'aquests materials que s'han desenvolupat recentment i que abasta les principals tecnologies avançades en el formigó: el Formigó de Molt Alt Rendiment Reforçat amb Fibres (UHPFRC). Aquest treball se centra específicament en els requisits fonamentals per al desenvolupament i l'ús generalitzat d'aquest, com ara la caracterització i classificació del comportament constitutiu a tracció. Aquest treball inclou una revisió profunda del comportament a tracció uniaxial del formigó i els seus canvis al temps que la tecnologia de les fibres ha evolucionat. A més, els mètodes tradicionals estàndard de caracterització, així com els recentment desenvolupats per al seu ús específic en l'UHPFRC són revisats i qüestionats. Al llarg del document, es mostra el desenvolupament de diferents metodologies per a determinar el comportament constitutiu a tracció uniaxial de l'UHPFRC, juntament amb una proposta de caracterització simplificada especialment desenvolupada per poder ser inclosa en normativa. Totes les metodologies desenvolupades presentades en aquest document han estat comprovades i validades. Aquests mètodes estan dissenyats específicament per a la seva aplicació en els resultats experimentals obtinguts a partir d'un tipus especial d'assaig de flexió a quatre punts, a més també s'inclou una proposta d'estandardització per a l'UHPFRC. Finalment, es presenta una proposta de classificació en funció dels paràmetres més rellevants del comportament a tracció de l'UHPFRC que són necessaris per al disseny i que es poden obtindre directament del mètode d'assaig estàndard suggerit per a la caracterització de l'UHPFRC. La classificació proposada té amb compte la classificació existent per al formigó armat convencional i el reforçat amb fibres. En ella, tant el formigó en massa com el formigó reforçat amb fibres es presenten com un cas particular d'una resposta constitutiva a tracció més general per al formigó. La metodologia estàndard i la classificació proposada estan d'acord amb l'evolució de formigó i unifica l
López Martínez, JÁ. (2017). CHARACTERISATION OF THE TENSILE BEHAVIOUR OF UHPFRC BY MEANS OF FOUR-POINT BENDING TESTS [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/79740
TESIS

Dans un contexte d’amélioration des dispositifs pour la réduction de bruit, l’étude sur le transfert d’énergie irréversible en utilisant des résonateurs purement acoustiques à comportement non linéaire a été réalisée. Les résonateurs acoustiques classiques en régime linéaire agissent comme un Amortisseur de Masse Accordée (TMD, en anglais) et ils sont efficaces pour une gamme de fréquence très étroite. Cependant, lorsqu’ils sont soumis à des excitations très fortes (régime non-linéaire) ils peuvent devenir efficaces pour une plus large gamme de fréquences si des termes non linéaires peuvent être activés. Dans un premier temps, une étude sur ce comportement non-linéaire d’un résonateur d’Helmholtz modifié a été réalisée expérimentalement. Ensuite, l’équation dynamique gouvernante de tels résonateurs ont été développées en prenant en compte les non-linéarités de la force de rappel et d’amortissement. Une approximation de la solution analytique de l’équation gouvernante du résonateur acoustique a été déterminée en utilisant les méthodes des échelles multiples du temps et de transformation du temps non régulière. Dans un deuxième temps, une étude du couplage entre un mode acoustique en basses fréquences et un résonateur (celui étudié précédemment) à comportement non-linéaire a été réalisée. Pour ce faire, des mesures expérimentales avec un montage du système couplé ont permis de vérifier l’atténuation acoustique produite par le résonateur en régime forcé et libre. Une modélisation analytique du couplage a permis d’identifier l’expression de la variété invariante lente, ce qui a permis d’étudier les possibles points d’équilibre et points singuliers du système. Les modèles analytiques développés ont également été vérifiés par des simulations numériques
Nowadays, there is a need of new types of technologies for sound reduction because of the growing of different industries. In this context, we have studied the targeted energy transfer using a purely acoustic resonator. These acoustic resonators act, in the linear regime, as a Tuned Masse Damper (TMD) and they are efficient for a narrow frequency band. But, when they are excited with high forces, in the nonlinear regime, they are efficient for a wider frequency band if the nonlinear terms are activated. First, an experimental study about the nonlinear behavior of a modified Helmholtz Resonator was done. Then, the governing equation of such resonators were developed considering the nonlinearities in the restitution force and damping. An approximation of the analytical solution of the governing equation of the acoustical resonator is derived using the multiples scales of time method and the non-smooth time transformation method. In a second part, a study about the coupling between an acoustic mode in low frequencies and a resonator (the one studied in the previous part) with a nonlinear behavior is done. In order to do this, experimental measurements of the coupled system to confirm acoustic attenuation by the resonator in forced and free regime were done. Then, an analytical modelling of the coupled system allowed to derive the expression of the Slow Invariant Manifold (SIM), in order to identify the possible equilibrium points and singular points of the system. Derived analytical models were verified by numerical simulations

Multiple step tests under cyclic strain control have been performed using cylindrical specimens of cast polycrystalline Inconel 738LC superalloy at 23, 700, 500, 800 and 900 °C in laboratory atmosphere to obtain cyclic stress-strain curves. During cyclic straining of specimen were obtained cyclic hardening-softening curves. Their progress changed with temperature and strain amplitude. Evaluated cyclic stress-strain curves are shifted to lower stresses with increasing temperature. Surface relief was observed in fatigued specimens under SEM and metalography under optic microscopy. Slip markings were studied on specimen surface fatigued at 700 °C .Stress-strain response is compared and discussed in relation to the surface observations - persistent slip markings.

Le traitement des sols, notamment par des liants hydrauliques ou de la chaux, est une technique très répandue pour améliorer les caractéristiques mécaniques d'un sol de qualité insuffisante. Après leur construction, les ouvrages en sols traités sont soumis à des sollicitations environnementales qui peuvent potentiellement altérer les effets du traitement, et donc conduire à la dégradation de la performance du sol traité. Dans ce contexte, l'objectif de la thèse est d'étudier l'impact de l'accumulation de cycles hydriques sur le comportement mécanique d'un sable traité au ciment. Une méthodologie multi-échelles et multi-physiques a été mise en place pour comprendre et quantifier la dégradation du comportement mécanique macroscopique des sables traités au ciment au cours de cycles de séchage/humidification. La caractérisation mécanique à court terme du sable a été réalisée en menant une campagne d'essais triaxiaux. Le taux de liaison défini à partir de la dilatance du sol traité, a été introduit pour apprécier la mobilisation puis la dégradation des liaisons cimentaires au cours des essais triaxiaux. Afin de caractériser la durabilité du sable traité, l'impact de deux types de cycles hydriques d'intensité variable a été considéré. À la suite des cycles, des essais triaxiaux ont été réalisés constituant une base de données de 130 résultats d'essais. Le suivi du taux de liaison a permis de quantifier explicitement la dégradation des éprouvettes cimentées en fonction du type et du nombre de cycles. Les cycles hydriques ont pour effet principal d'altérer les liaisons cimentaires et donc de diminuer les performances mécaniques. Cette altération dépend du dosage en ciment mais aussi de l'intensité des cycles. Les cycles de type I conduisent à une dégradation plus importante que les cycles de type II. La cinétique de l'altération ainsi que son ampleur dépendent du dosage de ciment. L'investigation des processus intervenant à l'échelle microscopique après 24 cycles des 2 types de cycles via des observations microscopiques (MEB et MET) et des analyses physico-chimiques (DRX et ATG) a mis en évidence des transformations minéralogiques intenses dont la carbonatation des phases cimentaires à divers degrés selon l'intensité du cycle ainsi que la formation d'aiguilles d'ettringite dans la porosité. Pourtant, au niveau du comportement macromécanique, une stabilisation des performances mécaniques après une diminution modérée (-20 à -30% du déviateur) a été observée après 12 cycles hydriques. Cette approche a permis de montrer que décrire les transformations minéralogiques pour décrire la durabilité n'est pas suffisant. Il est nécessaire d'évaluer la contribution de chaque phase dans la résistance puisque des transformations minéralogiques intenses ne sont pas synonymes de fortes dégradations des sols traités. Une loi de comportement pour modéliser le comportement déviatorique observé du sable traité a été proposée en combinant le Hardening Soil Model pour la partie pré-pic et une fonction sigmoïde inverse pour la partie post-pic. Cette approche a montré une bonne précision pour simuler le comportement et notamment le radoucissement des sols traités à des pressions de confinement supérieures à 50 kPa car dans ce cas une fonction exponentielle est probablement plus adaptée pour décrire le comportement post-pic. De plus, l'optimisation a permis de déterminer les valeurs les plus adéquates pour 5 paramètres du modèle (ψ, Rf, m, λ et ecrit) en fonction du dosage en ciment. La modélisation a permis de compléter l'étude expérimentale et apporter une réflexion sur la manière de simuler le comportement post-pic. Cette étude a permis une quantification mécanique à partir d'observations macroscopiques couplée à une quantification physico-chimique des différents processus se produisant lors de cycles hydriques. Un cadre conceptuel permettant de prendre en compte l'effet de la cimentation dans le comportement des sols traités a été proposé
Soil treatment, especially with hydraulic binders or lime, is a widespread technique to improve the mechanical characteristics of poor-quality soils. After their construction, engineering structures are subjected to environmental solicitations that can potentially alter the effects of the treatment, and thus lead to the degradation of the performance of the treated soil. In this context, the objective of this thesis was to study the impact of the accumulation of hydric cycles on the mechanical behaviour of a cement-treated sand. A multi-scale and multi-physics methodology was set up to understand and quantify the degradation of the macroscopic mechanical behavior of cement-treated sands during drying/wetting cycles.Short-term mechanical characterization of the sand was performed by conducting a triaxial testing campaign. The bonding ratio, η_bondmax, defined according to the stress-dilatancy theory, was introduced to assess the mobilization and the degradation of cementitious bonds during the triaxial tests. In order to characterize the durability of the treated sand, the impact of two types of hydric cycles of different intensity was evaluated. Following the cycles, triaxial tests were performed constituting a database of 130 test results. The bonding ratio monitoring allowed to explicitly quantify the degradation of the cemented specimens as a function of the type and number of cycles. The main effect of the hydric cycles is to alter the cementitious bonds and subsequently to decrease the mechanical performances. This alteration depends on the cement content but also on the intensity of the cycles. Type I cycles lead to a more important degradation than type II cycles. The kinetics of the alteration as well as its extent depend on the cement content.The investigation of the processes occurring at the microscopic scale after 24 cycles of the two types via microscopic observations (SEM and TEM) and physicochemical analyses (XRD and GTA) has highlighted intense mineralogical transformations including carbonation of the cementitious phases to various degrees depending on the intensity of the cycle as well as the formation of ettringite needles in the pore-space. However, in terms of macromechanical behaviour, a stabilization of mechanical performance after a moderate decrease (-20 to -30% of the deviatoric stress) was observed after 12 hydric cycles. This approach has shown that relating mineralogical transformations to durability is not sufficient. It is necessary to evaluate the contribution of each phase to the strength since intense mineralogical transformations are not necessarily associated with strong degradation of the treated soils.A constitutive law to model the observed deviatoric behaviour of treated sand was proposed by combining the Hardening Soil Model for the pre-peak hardening and an inverse sigmoid function for the post-peak softening. This approach showed a satisfactory accuracy to simulate the behaviour and in particular the softening of treated soils under relatively high confining pressures. Under low confining pressures an exponentially decreasing function was however more suitable. In addition, parameters optimization determined the most appropriate values for 5 model parameters (ψ, Rf, m, λ, and ecrit) as a function of cement content. The modeling permitted to complete the experimental study and broughtsome reflexions on the way to simulate the post-peak behaviour.This study allowed a mechanical quantification from macroscopic observations coupled with a physicochemical quantification of the various processes occurring during hydric cycles. A conceptual framework allowing to take into account the effect of cementing in the behaviour of treated soils was proposed

Within the framework of presented PhD, friction stir spot welding (FSSW) ofmultiple ultrathin sheets of AA 5754 – H111 (AlMg3) alloy 0.3 mm thick, wasstudied. The influence of tool geometry and process parameters such as rotationalspeed and axial load have been analysed using numerous techniques. It has beenunderstood that during the welding at low rotational speeds weld zone undergoesstrain hardening, while at high rotational speeds weld zone undergoes thermalsoftening. It was observed that during FSSW at low rotational speeds a complexlayer at weld interface is present, which causes delamination when welded samplesare subjected to load.
У оквиру ове докторске дисертације испитивано је тачкасто заваривање трењем са мешањем ултратанких лимова дебљине 0.3 mm од легуре АА 5754 – H111 (AlMg3). Утицај геометрије алата и параметара као што су угаона брзина и аксијално оптерећење су детаљно анализирани уз помоћ бројних техника. Установљено је да приликом заваривања ниским угаоним брзинама долази до деформационог ојачавања, док на високим угаоним брзинама долази до термичког омекшавања зоне завара. Код узорка завареног са најмањим бројем обртаја долази до формирања комплексног слоја на међуконтактној површини који изазива деламинацију приликом испитивања механичких особина.
U okviru ove doktorske disertacije ispitivano je tačkasto zavarivanje trenjem sa mešanjem ultratankih limova debljine 0.3 mm od legure AA 5754 – H111 (AlMg3). Uticaj geometrije alata i parametara kao što su ugaona brzina i aksijalno opterećenje su detaljno analizirani uz pomoć brojnih tehnika. Ustanovljeno je da prilikom zavarivanja niskim ugaonim brzinama dolazi do deformacionog ojačavanja, dok na visokim ugaonim brzinama dolazi do termičkog omekšavanja zone zavara. Kod uzorka zavarenog sa najmanjim brojem obrtaja dolazi do formiranja kompleksnog sloja na međukontaktnoj površini koji izaziva delaminaciju prilikom ispitivanja mehaničkih osobina.

Ground anchors represent an important structural element in the area of ground engineering. Despite an extensive usage of these elements, their design is usually performed using simple empirical and semi-empirical methods. An application of these procedures brings to the design a number of simplifying assumptions. The goal of the dissertation is to refine the computational design of ground anchors, analysis and quantification of selected factors significantly affecting their load displacement behavior. Firstly, the finite element method is applied. Two novel constitutive models are used: Multilaminate Constitutive Model for Stiff Soils (Schädlich, 2012) involving post peak shear softening of overconsolidated cohesive soils and Shotcrete Model (Schädlich, 2014) involving tension softening after tensile strength is reached. First constitutive model was used in order to simulate progressive decrease of skin friction along the anchor fixed length. Second constitutive model was applied for the grout material. In the next step, experimental program including several anchor load tests was carried out. The goal of the experimental program was to confirm conclusions from numerical studies and to obtain relevant data for further back – analysis. A newly developed application, in which all the findings from numerical computations and experimental measurements are incorporated, is described afterwards. The application is based on the use of so-called load-transfer functions. The program verification was conducted through series of back analysis of investigation anchor load tests realized on four different construction sites in two different types of fine-grained soils. The verification of the application is followed by series of parametric studies in which an influence of input parameters values is analyzed. Dissertation is concluded by the summarization of the most important findings.

Gegenstand der Dissertation ist die Entwicklung eines Stoffmodells für duktile Salzgesteine, mit dem alle drei Kriechphasen, abhängig von einem inneren Zustandsparameter, beschrieben werden können. Die Modellierung der Schädigungs- bzw. Dilatanzentwicklung und deren Rückkopplung auf das Kriechverhalten ist das Kernstück der Stoffmodellentwicklung. Es wird eine Beziehung abgeleitet, die die Dilatanzentwicklung abhängig vom Manteldruck und der spezifischen Formänderungsarbeit beschreibt. Durch diese Formulierung und deren Verknüpfung mit dem o. g. Zustandsparameter lassen sich tertiäres Kriechen, Kriechbruch, Nachbruchverhalten und Restfestigkeit geschwindigkeitsabhängig beschreiben. Es erfolgte eine Validierung des Modells anhand von Laboruntersuchungen. Weiter wurden Parametersätze für zwei unterschiedliche Steinsalztypen abgeleitet. Anhand von drei realen Problemstellungen konnte gezeigt werden, dass das gemessene In-situ-Verhalten mit dem Modell berechnet werden kann und Prognoseberechnungen zu plausiblen Ergebnissen führen.

Gegenstand der Dissertation ist die Entwicklung eines Stoffmodells für duktile Salzgesteine, mit dem alle drei Kriechphasen, abhängig von einem inneren Zustandsparameter, beschrieben werden können. Die Modellierung der Schädigungs- bzw. Dilatanzentwicklung und deren Rückkopplung auf das Kriechverhalten ist das Kernstück der Stoffmodellentwicklung. Es wird eine Beziehung abgeleitet, die die Dilatanzentwicklung abhängig vom Manteldruck und der spezifischen Formänderungsarbeit beschreibt. Durch diese Formulierung und deren Verknüpfung mit dem o. g. Zustandsparameter lassen sich tertiäres Kriechen, Kriechbruch, Nachbruchverhalten und Restfestigkeit geschwindigkeitsabhängig beschreiben. Es erfolgte eine Validierung des Modells anhand von Laboruntersuchungen. Weiter wurden Parametersätze für zwei unterschiedliche Steinsalztypen abgeleitet. Anhand von drei realen Problemstellungen konnte gezeigt werden, dass das gemessene In-situ-Verhalten mit dem Modell berechnet werden kann und Prognoseberechnungen zu plausiblen Ergebnissen führen.

[ES] El principal objetivo de la presente tesis es el desarrollo de una completa metodología para el modelado numérico del UHPFRC desde el material hasta el elemento estructural. Se pretende contribuir al avance del conocimiento del comportamiento mecánico del UHPFRC obteniendo como resultado un procedimiento para la modelización numérica que permita el modelado y diseño estructural que permitiría hacer que este material fuera competitivo para ser utilizado en el mercado de la construcción. En la metodología de modelado propuesta, se considera un comportamiento constitutivo del UHPFRC optimizado por medio de un procedimiento directo y fiable con el que se aprovechan las ventajas del material, resultando en un diseño estructural eficiente desde el punto de vista mecánico y económico. ¿Es necesario producir SH-UHPFRC para conseguir grandes propiedades mecánicas? ¿Es posible generar SS-UHPFRC de manera que queden reducidos los costos iniciales y se mantengan unas propiedades mecánicas y de durabilidad competitivas que comporten un diseño estructural efectivo? El desarrollo de UHPFRC con bajo endurecimiento por deformación y de SS-UHPFRC puede reducir sus propiedades mecánicas, pero si son adecuadamente estudiadas y controladas, éstos podrían ser optimizados. La tesis aborda algunas de estas cuestiones a través del estudio del comportamiento a tracción que va desde SH-UHPFRC hasta SS-UHPFRC. Se pretende llevar a cabo una propuesta de procedimiento fiable para caracterizar el comportamiento constitutivo a tracción y definir un modelo numérico de elementos finitos fiable para modelar con precisión la respuesta de probetas y elementos estructurales armados de UHPFRC. Para definir el procedimiento directo para caracterizar a tracción tanto SH-UHPFRC como SS-UHPFRC, se ha llevado a cabo una campaña experimental y numérica en la que se ha analizado el resultado de ensayar 227 probetas sin armadura fabricadas con UHPFRC con cantidades de fibras cortas y lisas de acero de 120-130kg/m3 y 160kg/m3, ensayadas a flexión a través del ensayo a cuatro puntos (4PBT). El desarrollo y la validación de dicho proceso se respaldan mediante un modelo no lineal de elementos finitos (NLFEM) fiable. La validación numérica llevada a cabo ha sido decisiva para que este procedimiento sea preciso, simple y fiable. Utilizando esta campaña experimental, se ha desarrollado una aplicación predictiva para estimar los parámetros que definen el comportamiento constitutivo a tracción del UHPFRC. Esta aplicación es simple y directa y evita la posible variabilidad producida por malas interpretaciones en la aplicación del proceso. Además, se ha llevado a cabo una segunda campaña experimental constituida por vigas de UHPFRC armadas a flexión con diferentes escalas: 36 vigas cortas con 130 y 160kg/m3 de fibras y dos vigas largas. Esta campaña experimental se ha modelado con el NLFEM aquí desarrollado teniendo en cuenta efectos importantes debidos a la interacción del UHPFRC con las barras de armado. También se han modelado con el NLFEM tirantes de UHPFRC armados de una campaña experimental de otra investigación. El modelo considera efectos debidos a la retracción, al 3D y comportamiento tensión stiffening que generan resultados muy precisos cuando se comparan con los resultados experimentales. Como resultado de la presente tesis doctoral, se ha obtenido un modelo de elementos finitos capaz de modelar con precisión elementos estructurales de UHPFRC armados. Los resultados no sólo demuestran la fiabilidad del NLFEM llevado a cabo sino también la coherencia del procedimiento desarrollado para caracterizar el comportamiento constitutivo a tracción del UHPFRC para los dos casos, tanto SH-UHPFRC como SS-UHPFRC, tanto en elementos estructurales armados a flexión como en elementos estructurales armados a tracción directa. Consecuentemente se ha propuesto una metodología completa y efectiva para el modelado numérico del UHPFRC
[CA] El principal objectiu de la present tesi es el desenvolupament d'una completa metodologia per al modelat numèric de l'UHPFRC des del nivell material fins arribar als elements estructurals. Es pretén contribuir a l'avanç del coneixement del comportament mecànic de l'UHPFRC per mitjà d'un procediment per al modelat numèric útil per al modelat i disseny estructural que permeta fer que aquest material siga competitiu al mercat de la construcció. En la metodologia de modelat proposta, es considera un comportament constitutiu de l'UHPFRC optimitzat per mitjà d'un procediment directe i fiable amb el qual s'aprofiten els avantatges del material, resultant en un disseny estructural eficient des del punt de vista mecànic i econòmic. És necessari produir SH-UHPFRC per a aconseguir grans propietats mecàniques? És possible generar SS-UHPFRC amb el qual queden reduïts els costs inicials mantenint unes propietats mecàniques i de durabilitat competitives que comporten un disseny estructural efectiu? El desenvolupament d'UHPFRC amb baix enduriment per deformació i de SS-UHPFRC pot reduir les seues propietats mecàniques però, si són adequadament estudiades i controlades, aquests podrien ser optimitzats. La tesi aborda algunes d'aquestes qüestions per mitjà de l'estudi del comportament a tracció de l'UHPFRC que va des de SH-UHPFRC fins SS-UHPFRC. Es pretén dur a terme una proposta de procediment fiable per a caracteritzar el comportament constitutiu a tracció i definir un model numèric d'elements finits fiable per a modelar amb precisió la resposta de provetes i elements estructurals armats d'UHPFRC. Per a definir el procediment directe per a caracteritzar a tracció tant SH-UHPFRC com SS-UHPFRC, s'ha dut a terme una campanya experimental i numèrica en la que s'ha analitzat el resultat d'assajar 227 provetes sense armadura fabricades amb UHPFRC amb quantitats de fibres curtes i llises d'acer de 120-130kg/m3 i 160kg/m3, assajades a flexió per mitjà de l'assaig a quatre punts (4PBT). El desenvolupament i la validació de l'esmentat procés són assegurats per mitjà d'un model no lineal d'elements finits (NLFEM) fiable. La validació numèrica duta a terme ha estat decisiva per a que aquest procediment siga precís, simple i fiable. Utilitzant aquesta campanya experimental, s'ha desenvolupat una aplicació predictiva per a estimar els paràmetres que defineixen el comportament constitutiu a tracció de l'UHPFRC. Aquesta aplicació és simple i directa i evita la possible variabilitat produïda per males interpretacions en l'aplicació del procés. A més a més, també s'ha dut a terme una segon campanya experimental constituïda per bigues d'UHPFRC armades a flexió amb diferents escales: 36 bigues curtes amb 130 i 160kg/m3 de fibres i dos bigues llargues de gran escala. Aquesta campanya s'ha modelat amb el NLFEM ací desenvolupat incloent efectes importants deguts a la interacció de l'UHPFRC amb les barres d'armat. Addicionalment, també s'han modelat amb el NLFEM tirants d'UHPFRC armats a tracció provinents d'una campanya experimental d'altra investigació. El model considera efectes deguts a la retracció, al 3D i comportament tensió stiffening que generen resultats molt precisos quan es comparen amb els resultats experimentals. Per tant, com a resultat de la present tesi doctoral, s'ha obtingut un model d'elements finits capaç de modelar amb precisió elements estructurals d'UHPFRC armats. Els resultats del model comparats amb els resultats experimentals no sols demostren la fiabilitat del NLFEM dut a terme sinó que també la coherència del procediment directe desenvolupat per a caracteritzar el comportament constitutiu a tracció de l'UHPFRC als dos casos, tant per a SH-UHPFRC com SS-UHPFRC, tant en elements estructurals armats a flexió com amb elements estructurals armats a tracció directa. Conseqüentment, s'ha proposat una metodologia completa i efectiva per al modelat numèric de l'UHPFRC des del niv
[EN] The main objective of the present PhD thesis is to develop a complete methodology for the numerical modelling of UHPFRC from the material level to structural elements. It intends to contribute to advanced knowledge of mechanical UHPFRC behaviour to lead to a numerically modelling proposal that is useful for structural modelling and design that allows options for this material to be competitive in the construction market. Optimised UHPFRC material constitutive behaviour, characterised by a direct reliable defined procedure, is considered in the proposed modelling methodology to take advantage of these properties, and to lead to an efficient structural design from the mechanical and economical points of view. Is it necessary to produce SH-UHPFRC to obtain excellent properties? Is it possible to develop SS-UHPFRC that leads to lower initial costs and to maintain competitive mechanical and durability properties that result in an effective structural design? The development of low strain-hardening and SS-UHPFRC would lead to reduce its mechanical properties, but they can be optimised if they are studied and controlled. The thesis addresses some of these questions by studying tensile UHPFRC behaviour to cover a wide range of tensile constitutive behaviours from SH-UHPFRC to SS-UHPFRC. It intends to propose a reliable tensile characterisation process and a reliable finite element model capable of accurately simulating the response of UHPFRC specimens and reinforced structural elements. An extensive experimental and numerical campaign with 227 unreinforced four-point bending test (4PBT) specimens with amounts of smooth-straight (13/0.20) steel fibres of 1.53-1.66% (120-130kg/m3) in volume and with 2.00% (160kg/m3), which represents SS-UHPFRC and SH-UHPFRC tensile behaviours, was carried out to set up a direct tensile characterisation procedure involving SS-UHPFRC and SH-UHPFRC. The direct procedure's development and validity are ensured by a reliable non-linear finite element model (NLFEM). Numerical validation was carried out and is decisive for performing the direct procedure to characterise the tensile behaviour of both SS and SH-UHPFRC herein developed accurately, simply and reliably. With the experimental programme herein, a predictive application for estimating tensile UHPFRC parameters was developed. The prediction offers reliable results. The application is simple and direct, and avoids variability in the characterisation procedure due to possible misinterpretations in its application. In addition, a second experimental programme, which includes reinforced concrete flexural beams on different scales, with 36 UHPFRC reinforced short beams with 130 and 160kg/m3 of steel fibres and two full-scale long beams, was carried out and modelled with the NLFEM herein developed including major effects due to the interaction between UHPFRC and reinforcement bars. Additionally, reinforced UHPFRC tensile bars from a recent experimental campaign performed by other researchers were modelled with the NLFEM. The model considers shrinkage effects, tension stiffening behaviour and 3D effects due to the particularities of the test, which provide very accurate results compared to those obtained with the experimental tests. As a result of this PhD thesis, an accurate NLFEM was obtained to model reinforced UHPFRC structural elements. The results of the model compared to the experimental ones demonstrate not only the reliability of the developed NLFEM, but also the coherence of the developed direct procedure to characterise tensile UHPFRC behaviour in both strain-softening and strain-hardening in reinforced flexural and direct tensile structural elements. Consequently, a complete and effective methodology for numerical UHPFRC modelling from the material level to structural elements is proposed.
Mezquida Alcaraz, EJ. (2021). Numerical Modelling of UHPFRC: from the Material to the Structural Element [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/167017
TESIS

碩士
大同大學
材料工程學系(所)
95
In this study thermomechanical processings were applied to four alloys, including AZ31 and L9 Mg alloys, 6061 Al alloy and Cu-30%Zn alloy, for the strain softening and anneal hardening studies. By using differential scanning calorimetry (DSC), optical microscopy (OM), and elevated temperature compression test. The relationships between microstructure evolution and the behaviors of strain softening and anneal hardening were examinated. The results showed that the elevated temperature strength of alloys is inversely proportional to the applied pre-strain, which leads to the strain softening effect. An anneal treatment, on the other hand, increases the elevated temperature strength of alloys, and the magnitude of anneal hardening was found to be proportional to the magnitude of pre-strain.

Compliant foams are usually characterized by a wide range of desirable mechanical properties. These properties include viscoelasticity at different temperatures, energy absorption, recoverability under cyclic loading, impact resistance, and thermal, electrical, acoustic and radiation-resistance. Some foams contain nano-sized features and are used in small-scale devices. This implies that the characteristic dimensions of foams span multiple length scales, rendering modeling their mechanical properties difficult. Continuum mechanics-based models capture some salient experimental features like the linear elastic regime, followed by non-linear plateau stress regime. However, they lack mesostructural physical details. This makes them incapable of accurately predicting local peaks in stress and strain distributions, which significantly affect the deformation paths. Atomistic methods are capable of capturing the physical origins of deformation at smaller scales, but suffer from impractical computational intensity. Capturing deformation at the so-called meso-scale, which is capable of describing the phenomenon at a continuum level, but with some physical insights, requires developing new theoretical approaches.

A fundamental question that motivates the modeling of foams is ‘how to extract the intrinsic material response from simple mechanical test data, such as stress vs. strain response?’ A 3D model was developed to simulate the mechanical response of foam-type materials. The novelty of this model includes unique features such as the hardening-softening-hardening material response, strain rate-dependence, and plastically compressible solids with plastic non-normality. Suggestive links from atomistic simulations of foams were borrowed to formulate a physically informed hardening material input function. Motivated by a model that qualitatively captured the response of foam-type vertically aligned carbon nanotube (VACNT) pillars under uniaxial compression [2011,“Analysis of Uniaxial Compression of Vertically Aligned Carbon Nanotubes,” J. Mech.Phys. Solids, 59, pp. 2227–2237, Erratum 60, 1753–1756 (2012)], the property space exploration was advanced to three types of simple mechanical tests: 1) uniaxial compression, 2) uniaxial tension, and 3) nanoindentation with a conical and a flat-punch tip. The simulations attempt to explain some of the salient features in experimental data, like
1) The initial linear elastic response.
2) One or more nonlinear instabilities, yielding, and hardening.

The model-inherent relationships between the material properties and the overall stress-strain behavior were validated against the available experimental data. The material properties include the gradient in stiffness along the height, plastic and elastic compressibility, and hardening. Each of these tests was evaluated in terms of their efficiency in extracting material properties. The uniaxial simulation results proved to be a combination of structural and material influences. Out of all deformation paths, flat-punch indentation proved to be superior since it is the most sensitive in capturing the material properties.

碩士
國立臺灣大學
土木工程學研究所
100
There are some severe restrictions in the traditional limit analysis. For the load type, the load space is one-dimensional and monotonic by imposing the restriction of uni-directional proportional loading. On the other hand, only can the constitutive law be perfectly elastic, either rigid-perfectly plastic or elastic-perfectly plastic. However, the load space is usually high dimensional in true situation, and the hardening/softening behavior prevails in almost all engineering structures. To deal with these problems, it is important to loose the above restrictions. The greatest advantage of limit analysis is that it can obtain collapse loads directly without giving loading paths. In the field of limit analysis, the most common approach is applying mathematical programming to calculating the collapse loads of elastoplastic structures, and the problem becomes an optimization problem of maximizing the collapse load. Collapse loads form a collapse surface in high dimensional load space; we deem a collapse surface to be not merely numerical results of optimization, but an equivalent model of yield surfaces once the structure in equilibrium loses its static indeterminancy and forms mechanisms. That is, we observe that each piece of a collapse surface represents a collapse mode of the structure and is corresponding to the yield surfaces of those structural members that form a collapse mechanism.Therefore, by using the relationship between the collapse surface and the yield surface in load space, we define the conditions of mechanism vectors which have mathematical and physical meaning. After searching each mechanism, the model of collapse surface can be constructed, and then we can construct the safety region in load space. Finally, some examples for truss structures with hardening and softening are given to verify this method.

Research Doctorate - Doctor of Philosophy (PhD)
In this thesis, the undrained and drained spherical cavity expansions in clays are analysed according to a new viewpoint, in which this boundary problem is treated as the integration of stress-strain following cavity expansion load paths. The widely adopted substepping scheme numerical integration technique is used in developing the undrained and drained solution of cavity expansion problems based on the modified UH model and the original and modified two-surface bubble models. This bounding surface plasticity series model has been proved to have inherent advantages in capturing the overall soil behaviour for clays with different consolidation histories than the other critical state models under the classical plasticity framework, especially for predicting nonlinear soil response at the early stage of cavity expansion for a soil stress state initially located beneath the yield surface for previous consolidation. Since the embedded hardening law of the modified UH model and the original and modified two-surface bubble model are related to the degree of overconsolidation, the influence of consolidation history (OCR) on the expansion responses for spherical cavities in clays under both undrained and drained conditions can be fully demonstrated. In addition, the solution developed in this study will then be used to predict soil behaviour in the compaction grouting test and the pressuremeter test. The advantage of using this method over the conventional interpretation method for the results of the compaction grouting test and pressuremeter test is also outlined. Therefore, the newly developed solution can serve as a useful tool in many geotechnical engineering problems, such as interpreting the results of cone penetration tests, as well as predicting the excess pore-pressure generated during pile installation. Additionally, the solution system developed in this study can be used as a platform to develop the cavity expansion solution, based on another more sophisticated constitutive model, to consider effects such as creep and local consolidation.

碩士
大同大學
材料工程學系(所)
93
In this study thermomechanical processings were applied to a Zn-22 wt.% Al alloy to produce fine- and coarse-grained structures for the strain softening and anneal hardening studies. The microstructure, hardness and compression curve of Zn-22 wt.% Al alloy were studied by using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), microhardness measurements and compression tester. The results showed the occurrence of a strain softening phenomenon in the Zn-Al alloy in the temperature range from -10℃ to 250℃ and an anneal hardening behavior in the softened Zn-Al alloy upon high temperature annealing. The mechanism of the strain softening behavior was found to be the DRX-induced softening during hot working, which can be facilitated by the formation of ultra-fine β grains with high-angle-boundaries; the mechanism of the anneal hardening behavior, on the other hand, was the annihilation of the ultra-fine β grains by a high temperature grain coarsening treatment to retard the occurrence of the DRX-induced softening and to restore the “normal strength” of the alloy. A composite stress-strain curve model was proposed in this study to resolve the mechanism of strain softening in the Zn-22 wt.％ Al alloy. By using this model detailed informations regarding to the deformation and the DRX (i.e. the strain softening behavior) in individual α and β phases were disclosed from the experimental compressive stress-strain curve.

碩士
大同大學
材料工程學系(所)
94
In this study thermomechanical processings were applied to three Zn-Al alloys, namely the micro-duplex Zn-22 wt.% Al alloy, pseudo-single α phase Zn-95 wt.% Al and pseudo-single β phase Zn-1wt.% Al for the strain softening and anneal hardening studies. The microstructure, hardness and compression S-S curves of these Zn-Al alloys were studied by using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), microhardness measurements and compression tester. The results showed the occurrence of a strain softening phenomenon in Zn-Al alloys containing a substantial amount of β phase in the temperature range from -10℃ to 250℃ and an anneal hardening behavior in the strain softened Zn-Al alloys upon high temperature annealing. The mechanism of the strain softening behavior was found to be a DRX-induced softening during hot working, which can be facilitated by the formation of ultra-fine β grains with high-angle-boundaries; the mechanism of the anneal hardening behavior, on the other hand, was found to be the annihilation of the ultra-fine β grains by a high temperature grain coarsening treatment to retard the occurrence of the DRX-induced softening and to restore the “normal strength” of the alloy. A composite stress-strain curve model was proposed in this study to resolve the strain softening and anneal hardening behaviors in the dual-phase Zn-22 wt.％ Al alloy. By using this model detailed informations regarding to the deformation, the DRX (i.e. the strain softening) and the grain coarsening (i.e. the anneal hardening) behaviors in the β phase can be disclosed.

This research focuses on the investigation of the mechanical properties of plantation Eucalyptus nitens timber and the understanding of its underlying failure mechanism above and below the fibre saturation point both experimentally and numerically. Fast-growing eucalypts are now considered potential building materials; one such re-source is plantation Eucalyptus nitens. To use such a resource as structural or engineered timber, that is to establish design codes and specifications, its compressive and bending strengths need to be studied as they are two important mechanical properties for structural elements in buildings, bridges and decks. For timber members when sub-merged in water or immersed in soil near the river but above the water table, water can be drawn into the wood by capillary action and lead to moisture contents (MC) above fibre saturation point, FSP, which is normally at MC of 25 - 30%. In this study, com-pression and four-point bending experiments were undertaken with a Universal Testing Machine to examine for the first time the anisotropic and/or nonlinear mechanical behaviour of fibre-managed plantation Eucalyptus nitens (E. nitens) samples for low and high MC. Here, low MC means that the MC is less than 15%, which is the normal situation for E. nitens timber as used in the building industry, and high MC means that the MC is above FSP, which is the extreme situation for E. nitens in structural applications when exposed to water. As the effect of the moisture dependent material properties are very difficult to examine separately using an experimental approach, sophisticated methods, such as finite ele-ment method (FEM) techniques combined with constitutive modelling, are needed to predict ultimate strength more accurately and to assess failure mechanisms in timber under complex loading situations. A newly developed constitutive model extended from an existing constitutive model, namely, a moisture-dependent anisotropic elasto-plasticity model, was proposed in order to display more realistically the short-term de-formation and more accurately the failure mechanisms in E. nitens timber with different moisture contents and varying grain angles. In this model, a criterion which combined the Hill yield criterion with the Drucker–Prager yield criterion was introduced to exam the anisotropic ultimate capacity of wood with consideration of differences in tension and compression. Variation of material properties of E. nitens timber with varying moisture contents is included in a new moisture-dependent anisotropic elasto-plasticity model in order to predict the short-term anisotropic material behaviour of E. nitens timber more accurately when moisture content changes. All these parameters can be derived from experimental data. A UMAT (User-defined mechanical material behaviour in ABAQUS) subroutine within the commercial software ABAQUS was coded to execute the constitutive model for compression, tension and bending. The observed agreement between numerical predictions and experimental investigations shows that the developed constitutive model can be used for E. nitens timber with varying moisture contents. The underlying anisotropic behaviour of E. nitens timber was also examined numerically. The key findings of this research were that fibre-managed plantation E. nitens timber was highly anisotropic with respect to grain angle and its ductility increased as MC increased. The E. nitens timber with high moisture content exhibited considerably larger deflections at lower maximum loads, while that with low moisture content showed quite abrupt failures at relatively higher ultimate loads. For both tension and compression, stiffness and strength were dependent on moisture content and loading direction, while the failure mode depended mainly on loading direction. That the anisotropy of E. nitens timber was dependent on moisture content sensitivity was proven both numerically and experimentally. The yield surface of E. nitens timber enlarged with a decrease in moisture content, while different failure strengths under tension and compression were found. Compared with moisture content, grain angle had a stronger effect on the anisotropic behaviour of E. nitens timber due to a difference in the stresses state. At a grain angle of 0°, the normal stress along the grain was dominant compared with the corresponding shear stress and the normal stress across the grain. At loading directions of 30°, 45° and 60°, shear stresses played an important role, and for 90°, the effect of normal stress across the grain was significant. Parametric investigation showed that Young’s modulus parallel to the grain and yield stress affected the stiffness and strength significantly when loads were at grain angles between 0° and 15°. Their effects weakened as the grain angle increased from 15° to 45° with almost no effect on compressive strength after 45°. The effect of shear modulus and shear yield stress along wood fibre became the largest contributing factor from 15° to 30°, while after 45° the radial Young’s modulus and yield stress took a dominant role in determining stiffness and strength, respectively, compared with the effects of longitudinal Young’s modulus and shear modulus. The risk of damage to an E. nitens member is expected to increase in high moisture conditions. The moisture modification factors of Eucalyptus timber at mean level are higher than those of the traditional construction material, Pinus radiata, implying that E. nitens was promising as a material to be used for compressive or bending members in the construction industry, especially in water saturated conditions, for example, in wood piling foundations of buildings near the coast for a long time. This research will provide basic data for E. nitens in structural applications, and the approaches introduced can be used in future research for assessing the vulnerability of E. nitens members in normal or fully water-saturated states for building and structural applications. The moisture dependent composite plastic constitutive model, by providing results for a comprehensive study of anisotropic plasticity material behaviour incorporating anisotropic and moisture effects, overcame the limitations of the von Mises model and the Hill model with nonlinear isotropic hardening, as they were not always appropriate for E. nitens timber. For example, experiments found that the load deformation behaviour changed from unsteady-state “softening” behaviour at compression parallel to the grain to steady-state “hardening” behaviour at compression perpendicular to the grain. The numerical investigation into the short-term response of E. nitens under compression, tension and bending with high and low moisture contents, compared with the experimental data, demonstrated acceptable agreement, showing that the constitutive model works well for Eucalyptus wood.

Die Arbeit widmet sich dem Fließverhalten von Stählen bei zyklisch inkrementeller Torsion. Dazu werden vergleichend Reineisen und der ferritisch-perlitische Stahl 42CrMo4N bei unterschiedlichen Verformungspfaden betrachtet. Vor allem in zyklischen Torsionsversuchen mit bleibendem Verformungsinkrement je Zyklus werden Fließkurvenverläufe und Verformungsverfestigung analysiert. Die Ergebnisse belegen den Einfluss des lamellaren Zementit auf das Fließverhalten des Stahls 42CrMo4N, während die Eigenschaften des Reineisens von der entstehenden Versetzungszellstruktur bestimmt werden. Die Richtungsabhängigkeit der Fließspannung und die Verläufe der Fließkurven unterscheiden sich für die betrachteten Werkstoffe deutlich. Fließortkurven dienen der quantitativen Beschreibung der Verfestigung. Die Vorgehensweise zu ihrer Ermittlung und ihre Abhängigkeit von den Versuchsbedingungen und den Verformungszuständen werden gezeigt. Bei Reineisen dominieren isotrope, bei dem Stahl 42CrMo4N kinematische Verfestigungsanteile das Fließverhalten.
Ferritic-pearlitic steel 42CrMo4N and pure iron under different strain paths are compared regarding their flow behaviour. Mainly in cyclic torsion tests with resulting strain increment per cycle shear stresses and strain hardening are analysed. The results show, that the cementite lamellae determine the flow behaviour of the steel 42CrMo4N, whereas the properties of pure iron are governed by the evolving dislocation cell structure. The dependency of flow stress on the strain direction is different for the two materials. Yield surfaces describe strain hardening quantitatively. The procedure for yield point detection and the dependency of subsequent yield surfaces on experimental conditions and strain states is shown. For pure iron isotropic hardening, for steel 42CrMo4N kinematic strain hardening dominates the flow behaviour.