Teses / dissertações sobre o tema "Brittle behaviour"

The mechanical behavior of fractures in geological media is important to geotechnical and geoenvironmental engineering. Considerable investigations have been conducted on, firstly, the characterization of the fracture topography and secondly, on the assessment of the influence of fracture topography on the mechanical behavior, particularly the dilatancy of the discontinuity. The studies of the mechanical behavior of fracture surfaces have invariably been concerned with the examination of the mechanics of the contact surfaces, to the virtual exclusion of the progress of failure zones into regions adjacent to the contacting fracture surfaces. This thesis conducts a computational assessment of the role of geomaterial plasticity and surface topography on the behavior of a fracture. The computational modelling takes accounts of the irregularity of the joint surface, the frictional and elasticity characteristics of the contact zones, the elasto-plastic failure of the material and incompatible deformations that arise during shear of an irregular fracture surface. The computational shear responses are compared for the cases where a regular fracture surface exhibits identical shear behavior in the presence of geomaterial plasticity. For an irregular joint, it is observed that the shear behavior is relatively unaffected by material plasticity. Variation of dilatancy with shear cycles, however, can be directly attributed to the presence of material plasticity. Plastic energy dissipation is related to the normal restraints specified. Shear behavior of a specific joint appears to depend mainly on the interfacial behavior of the limited number of asperity contact during shear. The surface geometry of these asperities governs the dilatancy and their slopes control the peak shear resistance. The thesis also examines briefly the influence of initial separation of joints on the shear behavior.

Includes bibliographical references.
The brooding behaviour and brooded young are described for Ophioderma wahlbergii Müller & Troschel 1842, a large, common brittle star from the coastal waters of South Africa. Twenty specimens were collected each month from June 2013 – May 2014 (n = 240). The species was found to be gonochoric.

The thesis describes the development of a new continuum damage mechanics (hereafter, CDM) model for the deformation and failure of brittle matrix composites reinforced with continuous fibres. The CDM model is valid over sizes scales large compared to the spacing of the fibres and the dimensions of the damage. The composite is allowed to sustain damage in the form of matrix micro-cracking, shear delamination, tensile delamination and fibre failure. The constitutive equations are developed by decomposing the composite compliance into terms attributable to the fibre and matrix, and modelling the competing failure modes by intersecting failure surfaces based on maximum stress theory. The fibres are treated as being weakly bonded to the matrix so that the fibres only transmit axial loads, and fail in tension. The matrix is modelled as isotropic linear elastic and is treated as transversely-isotropic after damage has initiated. The effect of multiple matrix cracking on the stiffness was determined from experimental data, while failure was modelled by a rapid decay in the load bearing capacity. Although the model is motivated largely to proportional loading, matrix unloading and damage closure has been modelled by damage elasticity. During compression, the matrix stiffness is identical to the undamaged state with the exception that the fibres are assumed not to transmit compressive loads. The model was implemented computationally through a FORTRAN subroutine interfaced with the ABAQUS/Standard finite element solver. The CDM model was validated by comparing experimental and computational results of test specimens with unidirectional and balanced 0°-90° woven fibres of a brittle matrix composite, fabricated from polyester fibres in a polyester matrix. This composite system exhibits low elastic mismatch between fibres and matrix, and has similar non-dimensionalised stress-strain response to a SiC/SiC composite proposed for the exhaust diffuser unit of the Rolls-Royce EJ200 aero-engine.

Dans ces travaux de thèse, des approches micromécaniques et numériques sont utilisées pour prédire les propriétés thermiques et thermoélastiques effectives d’un milieu microfissuré. Les développements sont effectués pour les cas bidimensionnel et tridimensionnel. Une attention particulière est portée à l’anisotropie induite par l’orientation des fissures et à l’effet unilatéralassocié à leur ouverture/fermeture. Plus précisément, les fissures sont modélisées comme des inclusions ellipsoïdales, sans rigidité et thermiquement isolantes dans le cas des fissures ouverteset constituées d’un matériau isotrope fictif dans le cas des fissures fermées. L’approche théorique tire profit de différents schémas et bornes obtenues par homogénéisation pour déterminer lesexpressions analytiques des propriétés effectives. Sur le plan numérique, le travail s’appuie sur la modélisation par éléments finis et se base sur les mêmes géométrie et propriétés des fissures quecelles retenues pour la démarche micromécanique. La comparaison des résultats permet de montrer la cohérence entre ces deux approches
Micromechanical and numerical methods are explored to predict the effective thermal and thermoelastic properties of a microcracked media. The effective properties are given in 2D and3D. In this thesis, special attention is paid to the anisotropy, induced by the orientation of the cracks and the unilateral effect related to the opening and closing of the cracks. The cracks aremodelled as ellipsoidal inclusions. The open cracks are considered to have no stiffness and to be thermally insulating, whereas the closed cracks are represented by a fictitious isotropic material.The theoretical approach takes advantage of various homogenization schemes and bounds to derive closed-form expressions of effective properties. The numerical approach considers finiteelement modelling and is based on the same geometry and properties of cracks as in the theory. Finally, results are compared to demonstrate the consistency between the two approaches

Research area and topicality of the work. Mechanical properties and their evolution under loading are the most significant factors for the development of various mechanical structures, technologies and equipment. It seems to be natu-ral that deeper understanding of the behaviour of existing and design of new materials presents a challenge in different research areas. It should be noted, that all the materials are heterogeneous in meso- and micro- scales. They exhibit essential differences, compared to the macroscopic continuum behaviour. Basically, both experimental and numerical simulation methods are extensively applied for investigation purposes. Experimental techniques, capable of giving a realistic view of the inside of the material and extracting the real data, are very expensive. Therefore, the nu-merical simulation tools are extensively used as an alternative for investigation purposes. They have considerable advantages allowing the reproduction of multiple experiments and providing comprehensive data about ongoing phe-nomena. Recently, numerical technologies have become highly multidisciplinary subjects. They comprise phenomenological and statistical ideas, while mathe-matical models employ the relations of continuum mechanics, classical discre-tization methods and molecular dynamics. The Discrete Element Method (DEM) is one of new methods. It is aimed at simulating the dynamic behaviour of the contacting particles. Variable topology of the system of particles is an... [to full text]
Tyrimų sritis ir darbo aktualumas. Kuriant modernias ��vairios paskirties mechanines sistemas, technologijas ir įrangą, svarbiomis tampa jas sudarančios medžiagos. Savaime suprantama, kad žinomos ir naujai kuriamos medžiagos dabar kur kas išsamiau nagrinėjamos daugelyje mokslo šakų, įskaitant ir me-džiagų mechaniką. Visos medžiagos mezo- ir mikrostruktūros požiūriu yra ne-vienalytės. Jų mikroskopinės savybės skirtingos, lyginant su įprastu kontinuu-mu. Medžiagų savybėms tirti dažniausiai taikomi eksperimentiniai metodai. Eksperimentiniais metodais ištirti medžiagos struktūras ir jose vykstančius procesus ir įvertinti tam tikras jų savybes labai brangu. Tai viena priežasčių, kodėl skaitinis modeliavimas tampa realia tyrimų alternatyva. Skaitinį eksperi-mentą galima kartoti daug kartų, valdant bandinio parametrus, išlaikant tas pa-čias sąlygas, ir stebėti reiškiniui būdingus rodiklius visame tūryje. Šiuolaikiniai modeliavimo metodai yra kompleksiniai. Jie jungia fenome-nologines ir statistines idėjas, o matematiniai modeliai sudaromi taikant konti-nuumo mechanikos ir jų diskrečiųjų modelių bei molekulinės dinamikos pri-klausomybes. Diskrečiųjų elementų metodas (DEM) taip pat priskiriamas šiuo-laikinių metodų kategorijai. Jis skirtas kontaktuojančių dalelių sistemų dinami-niam modeliavimui. Kintanti dalelių sistemos topologija – būdingas metodo požymis. Pastaruoju metu DEM jau taikomas kontinuumui modeliuoti ir praktikoje aktualiems irimo uždaviniams spręsti. Reikia pastebėti... [toliau žr. visą tekstą]

Steel hollow section members filled with concrete have been frequently used in recent construction industry as columns and beams and beam-columns because of their superior performance and constructability. Previous research demonstrated that such system has large energy absorption capacity which is critical in the event of an earthquake. By filling steel RHS with concrete, the failure of the steel shell due to local buckling can be delayed and the ductility of the concrete core can be improved as a result of the confinement of the steel shell. This type of composite section may be used in various structures including frames of high rise buildings, bridges, offshore structures, cast-in-situ piles in foundation etc. Design methods for concrete-filled steel tubular sections are recommended in a number of code of practices. Due to the significant differences in the material properties between normal strength concrete and high strength concrete, there is a need to study the behaviour of composite sections with higher strength concretes. The study emphasises ultimate strength, ductility, post-failure strength reserve and interface bond. It also emphasises ductility and post-failure strength of the composite beams due to the brittle behaviour of higher strength concretes when compared to normal strength concrete. Spreadsheet graph were used to present the results such as load versus strains, load versus deflections etc. In this thesis analytical study is presented on the calculation of ultimate moment of resistance of the concrete-filled RHS beams. Among the main considerations of the derivation, the steel portion was assumed either elastic-perfectly plastic or perfectly plastic and concrete carries no strength in the tensile zone. At the interface both full bond and partial bond were assumed for comparison. Efforts were also made to calculate the midspan deflections of the composite beams. Simple analytical expressions derived from this study can be coded to a prgrammable calculator or in a small spreadsheet program for design use. Finite element studies were carried out by using a proprietorship software package ANSYS. In the analysis of concrete-filled, three types of elements with large deformation and nonlinear capabilities were used. A plastic shell element, a solid concrete element with cracking and crushing capabilities, and a nonlinear spring contact element were used to model the steel shell, the concrete core and the interface respectively.
Doctor of Philosophy (PhD)

Steel hollow section members filled with concrete have been frequently used in recent construction industry as columns and beams and beam-columns because of their superior performance and constructability. Previous research demonstrated that such system has large energy absorption capacity which is critical in the event of an earthquake. By filling steel RHS with concrete, the failure of the steel shell due to local buckling can be delayed and the ductility of the concrete core can be improved as a result of the confinement of the steel shell. This type of composite section may be used in various structures including frames of high rise buildings, bridges, offshore structures, cast-in-situ piles in foundation etc. Design methods for concrete-filled steel tubular sections are recommended in a number of code of practices. Due to the significant differences in the material properties between normal strength concrete and high strength concrete, there is a need to study the behaviour of composite sections with higher strength concretes. The study emphasises ultimate strength, ductility, post-failure strength reserve and interface bond. It also emphasises ductility and post-failure strength of the composite beams due to the brittle behaviour of higher strength concretes when compared to normal strength concrete. Spreadsheet graph were used to present the results such as load versus strains, load versus deflections etc. In this thesis analytical study is presented on the calculation of ultimate moment of resistance of the concrete-filled RHS beams. Among the main considerations of the derivation, the steel portion was assumed either elastic-perfectly plastic or perfectly plastic and concrete carries no strength in the tensile zone. At the interface both full bond and partial bond were assumed for comparison. Efforts were also made to calculate the midspan deflections of the composite beams. Simple analytical expressions derived from this study can be coded to a prgrammable calculator or in a small spreadsheet program for design use. Finite element studies were carried out by using a proprietorship software package ANSYS. In the analysis of concrete-filled, three types of elements with large deformation and nonlinear capabilities were used. A plastic shell element, a solid concrete element with cracking and crushing capabilities, and a nonlinear spring contact element were used to model the steel shell, the concrete core and the interface respectively.

Ce travail concerne des expériences mécaniques, des analyses numériques et des modélisations analytiques de la rupture cohésives (Mode I), vis-à-vis de l’étude du comportement mis en évidence par la courbe de Résistance (Courbe-R) et l’effet d’échelle de structures entaillées en bois massif. Des expériences de fissuration sont combinées à des analyses numériques pour déterminer les propriétés de rupture au moyen d’une procédure appelée Théorie de la Mécanique de la Rupture Linéaire Élastique équivalente (TMRLE), basée sur la complaisance de la structure. La courbe-R, obtenue à partir des expériences, selon une méthode de correction du poids propre, montre l’existence d’un domaine endommagé (Zone de Processus de Rupture) de taille non négligeable se développant en fond de fissure. Dans des conditions de fissuration stationnaire, ce domaine atteint une taille critique, et l’énergie nécessaire pour faire propager la fissure avec ce domaine endommagé (par unité de surface de rupture), reste constante. Le taux de libération de l’énergie de fissuration ainsi attendu, joue un rôle important en Mécanique de la Rupture, car il est possible simuler le comportement quasi-fragile du matériau en combinaison avec les autres propriétés de cohésion. La loi d’effet d’échelle de Bažant, utilisée pour prévoir l’influence de la taille sur la contrainte nominale, est estimée à partir de la réunion de deux comportements asymptotiques réalisés sur de petites tailles (Analyse limite ou RdM) et des grandes tailles. Une procédure analytique est présentée pour évaluer le comportement asymptotique additionnel exhibé par la contrainte nominale dans le régime intermédiaire, de façon plus exacte. Une validation numérique est présentée, et l’information expérimentale vient confirmer ce comportement asymptotique
This work concerns the mechanical testing, numerical analysis and modelling of cohesive fracture (Mode I) on the purpose to study the Resistance-curve behaviour and the size effect in wooden notched structures. The mechanical testing is combined with the numerical analysis to evaluate fracture properties by means of an equivalent LEFM approach based on the structure compliance. The Resistance-curve being revealed from the experiments, by means of a self-weight compensation method, correction puts into evidence that a non-negligible damaged domain (Fracture Process Zone) is under development in the crack front during the loading process. This being the case, among other fracture parameters issued from the Resistance-curve, the critical (asymptotic) energy release rate is determined, turning possible to use it in combination with other cohesive crack properties in the crack modelling (in Mode I). Thus, for a given geometry it is possible to monitor the critical dimension being revealed by the Fracture Process Zone (FPZ) during the crack propagation. The well known Bažant’s size effect law provides the scaling of the nominal strength through the asymptotic matching performed both on the small (Strength Theory) and on the large (LEFM) structure sizes. An analytical procedure is proposed to determine an additional asymptotic regime in the intermediate size range through a more accurate manner. Numerical validations of the proposed procedure are made and experimental data is presented revealing the scaling of the nominal strength through an envelop of values

‘Coupled process’ implies that one process affects the initiation and progress of the others and vice versa. The deformation and damage behaviors of rock under loading process change the fluid flow field within it, and lead to altering in permeable characteristics; on the other side inner fluid flow leads to altering in pore pressure and effective stress of rock matrix and flow by influencing stress strain behavior of rock. Therefore, responses of rock to natural or man-made perturbations cannot be predicted with confidence by considering each process independently. As far as hydro-mechanical behavior of rock is concerned, the researchers have always been making efforts to develop the model which can represent the permeable characteristics as well as stress-strain behaviors during the entire damage process. A brittle low porous granite was chosen as the study object in this thesis, the aim is to establish a corresponding constitutive law including the relation between permeability evolution and mechanical deformation as well as the rock failure behavior under hydro-mechanical coupled conditions based on own hydro-mechanical coupled lab tests. The main research works of this thesis are as follows: 1. The fluid flow and mechanical theoretical models have been reviewed and the theoretical methods to solve hydro-mechanical coupled problems of porous medium such as flow equations, elasto-plastic constitutive law, and Biot coupled control equations have been summarized. 2. A series of laboratory tests have been conducted on the granite from Erzgebirge–Vogtland region within the Saxothuringian segment of Central Europe, including: permeability measurements, ultrasonic wave speed measurements, Brazilian tests, uniaxial and triaxial compression tests. A hydro-mechanical coupled testing system has been designed and used to conduct drained, undrained triaxial compression tests and permeability evolution measurements during complete loading process. A set of physical and mechanical parameters were obtained. 3. Based on analyzing the complete stress-strain curves obtained from triaxial compression tests and Hoek-Brown failure criterion, a modified elemental elasto-plastic constitutive law was developed which can represent strength degradation and volume dilation considering the influence of confining pressure. 4. The mechanism of HM-coupled behavior according to the Biot theory of elastic porous medium is summarized. A trilinear evolution rule for Biot’s coefficient based on the laboratory observations was deduced to eliminate the error in predicting rock strength caused by constant Biot’s coefficient. 5. The permeability evolution of low porous rock during the failure process was described based on literature data and own measurements, a general rule for the permeability evolution was developed for the laboratory scale, a strong linear relation between permeability and volumetrical strain was observed and a linear function was extracted to predict permeability evolution during loading process based on own measurements. 6. By combining modified constitutive law, the trilinear Biot’s coefficient evolution model and the linear relationship between permeability and volumetrical strain, a fully hydro-mechanical coupled numerical simulation scheme was developed and implemented in FLAC3D. A series of numerical simulations of triaxial compression test considering the hydro-mechanical coupling were performed with FLAC3D. And a good agreement was found between the numerical simulation results and the laboratory measurements under 20 MPa confining pressure and 10 MPa fluid pressure, the feasibility of this fully hydro-mechanical coupled model was proven.

La démarche actuelle des industries aérospatiales est de diminuer le coût de lancement des engins spatiaux par une réduction de la masse des composants. Dans l’optique de cette démarche, de nouveaux matériaux sont élaborés et permettent de satisfaire aux exigences de densification, de dissipation thermique et de réduction de masse des équipements électroniques embarqués dans les satellites. Cette thèse est une contribution à l’étude de deux de ces matériaux, l’AlSi CE9F et une nuance d’alumine cofrittée à température, destinés à réaliser des boitiers hybrides de protections de composants électroniques, initialement conçus en Kovar. Les objectifs sont d’affiner les connaissances sur propriétés mécaniques des deux matériaux et de mettre en place des règles de conceptions propres à leurs comportements mécaniques. En effet, l’AlSi CE9F et l’alumine ont un comportement à rupture fragile. La détermination de leurs résistances à la rupture est alors réalisée dans le cadre de la théorie de Weibull. Des séries d’essais de flexion quatre points et trois points sont effectués. Elles permettent d’identifier les paramètres de Weibull des deux matériaux à température ambiante et de mettre en évidence les effets de volume. L’étude expérimentale est poursuivie sur l’AlSi CE9F afin de déterminer l’influence de la température sur ses propriétés mécaniques à travers deux approches. La première s’intéresse à une variation monotone de la température et la seconde à des cycles thermiques entre -50 et 125°C. Si la première étude ne montre qu’une faible évolution du module d’élasticité, la seconde démontre que les cycles thermiques contribuent à l’amélioration de la résistance à la rupture de l’AlSi CE9F. Cette augmentation de la contrainte à la rupture se traduit également par une évolution de sa microstructure. Dans un second temps, un modèle de Weibull est numériquement mis en place à partir des paramètres identifiés et du critère de la contrainte équivalente de Freudenthal. Ce critère est analysé et validé à travers l’étude de trois éprouvettes en AlSi CE9F à chargements complexes. Le modèle validé est enfin utilisé pour décrire le comportement mécanique de deux composants dans différentes configurations de sollicitation, réalisés respectivement en alumine HTCC et en AlSi CE9F. Une méthodologie de dimensionnement est alors mise en place et permettra de disposer de nouvelles règles de conception équivalentes à celles existant sur les matériaux classiques
The aerospace companies currently want to decrease the price of spacecraft launching with a reduction of the mass components. New materials were recently developed to satisfy the rising requirements of thermal dissipation, densification and weight decrease of on-board electronic equipment intended to satellite. This thesis is a contribution to the characterization of two of these innovative materials: AlSi CE9F and a grade of alumina HTCC. These materials are designed to manufacture hybrid boxes for computing chips, originally made in Kovar. The objectives are to improve the mechanical properties knowledge of these materials and to develop a know-how design specific to their mechanical behaviours. Indeed, AlSi CE9F and alumina have brittle fracture behaviour. The strength analysis is also realized in connection with the Weibull theory. The Weibull’s parameters are identified from the four points and three points bending strength and the volume effects are highlighted. The experimental study is completed by the analysis of the temperature influence on the mechanical properties of AlSi CE9F through two approaches. The first one considers a monotonic variation of temperature and shows a minor evolution of the elastic modulus. The second one proves that thermal cycles between -50 and 125°C improve the strength value of AlSi CE9F. This increase is also reflected by an evolution of its microstructure. Secondly, a Weibull’s model is numerically established based on identified parameters and the Freudenthal’s equivalent stress criterion. The Freudenthal’s criterion is analysed and confirmed through the study of complex loading samples made in AlSi CE9F. The confirmed model is finally used to describe the mechanical behaviours of two components respectively made in AlSi CE9F and alumina HTCC, thoroughly in several loading configurations. A design methodology is developed and will bring new rules in modelling and design, closed to those existing in conventional materials

The present dissertation presents an investigation of the bond behavior of newly developed fiber reinforced composite systems applied to quasi-brittle material interfaces. Direct shear tests were performed on steel reinforced polymer (SRP) and steel reinforced grout (SRG) composite strips applied to both concrete and masonry substrates. Different types of cementitious matrices and different densities of steel fiber sheets were employed. Tests were performed investigating several parameters, i.e. bonded width, bonded length, loading rate, and face to which the composite strip was applied. Failure modes and load responses were presented and discussed. It was observed that the fracture energy GF of SRP-concrete joints is independent of the composite density but varies as the composite is bonded to different faces of the concrete prism. The width effect was considered in the evaluation of the load-carrying capacity of SRP-concrete joints, while the loading rate influenced the peak load of both SRP and SRG specimens. The behavior of SRP-concrete joints was also investigated through a numerical analysis, using lattice discrete particle model (LDPM), obtaining an excellent match with the experimental results. Some concrete prisms were reinforced and tested using a geopolymer matrix that showed interesting results. Some masonry specimens were subjected to artificially weathering cycles to investigate the durability performances of SRG strips with respect to salt attack. Furthermore, monotonic compressive tests were performed on concrete columns confined with both SRP and SRG composites. Several parameters were investigated, i.e. the density of steel fiber sheets, the concrete corner condition, the overlapping length, the number of confinement layers, the scale effect, and the shape effect. Failure modes and load responses were presented and discussed. Finally, several applications of basalt-fiber reinforced cementitious matrix (B-FRCM) composites bonded to masonry substrates were showed, including full scale tests on existing masonry arches.

[EN] A simplified analytical method ("FAST") for the estimation of large-scale vulnerability of Reinforced Concrete (RC) Moment Resisting Frames with masonry infills is proposed and subsequently tested by using real damage scenario caused by the 2011 Lorca earthquake as a benchmark. FAST is a spectral-based approach that allows predicting the average non-structural Damage State expected for each class of building (defined by number of storeys, age of construction, infills ratio in plan and location) for a given demand level. It accounts for non-uniformity of infills in elevation, i.e. a reduction of infills ratio of the ground floor. FAST is based on: (i) the definition of approximated capacity curves of the infilled building, assuming that the RC frame is designed according to the corresponding seismic code; and on (ii) the assumption of "a priori" deformed shapes in accordance with the attainment of each non-structural damage state at 1st storey, estimated through experimental and numerical correlations. Two versions of FAST are proposed: a "simplified" approach aimed at the evaluation of uniformly infilled frames; and a "generalised" version which can account for any intermediate situation between uniformly infilled frames and pilotis frames (i.e. without infills at 1st storey). Also, some extensions of the method are highlighted. Aimed at testing FAST, the real damage scenario after the earthquake of Lorca (2011) is used as a benchmark, despite its impulsivity and directivity. In order to define the specific input parameters for the case study, information regarding ground motion, post-earthquake damage scenario and also building design practice must be collected. Hence, a detailed review of historical Spanish seismic codes and a critical analysis of current Spanish seismic code NCSE-02 in comparison with current reference performance-based codes such as Eurocode 8 are provided. Special emphasis is placed on provisions which can prevent a proper capacity design and that, in turn, can cause brittle failures or favour the interaction with infills. Also, the prescription of lower behaviour factor for wide-beam frames with respect to deep-beam frames -which is not present in most codes¿ is discussed; outcomes of several case studies suggest that such prescription is obsolete. Finally, FAST is applied to Lorca earthquake and predicted damage scenarios are obtained, considering different assumptions for input values. Results show proper agreement between predicted and real damages. Structural collapses were rarely observed, even though the PGA was three times higher than the typical acceleration of design, so FAST proves that masonry infills provided additional strength to RC frames.
[ES] Se propone un método analítico simplificado ("FAST") para la estimación de la vulnerabilidad a gran escala de edificios porticados de hormigón armado con tabiquería de fábrica, posteriormente testeado mediante la adopción del escenario de daño real correspondiente al terremoto de Lorca de 2011 como patrón de comparación. FAST es un procedimiento espectral que permite predecir el nivel de daño no estructural medio esperado para cada clase de edificio (definido por su número de plantas, año de construcción, densidad de tabiquería en planta y localización geográfica), considerando un nivel de demanda dado. El método tiene en cuenta la irregularidad de la tabiquería en alzado, es decir, la posible reducción relativa de tabiquería en planta baja. FAST se basa en: (i) la definición de curvas de capacidad aproximadas para los edificios tabicados, asumiendo que la estructura de HA se ha proyectado según la norma sísmica correspondiente en cada caso; y en (ii) la asunción de deformadas "apriorísticas" coherentes con cada grado de daño (suponiendo que éste se alcanza siempre en planta baja), estimadas a través de correlaciones experimentales y numéricas. Se proponen dos versiones de FAST: una "simplificada" para la evaluación de edificios uniformemente tabicados en altura, y otra "generalizada", que es capaz de tener en cuenta cualquier situación intermedia entre el prototipo uniformemente tabicado y el de planta baja diáfana. Además, se proponen ciertas extensiones al método. A fin de validar FAST, se elige el escenario de daño real correspondiente al terremoto de Lorca (2011) como patrón de comparación, a pesar de su impulsividad y directividad. Para definir los parámetros de input correspondientes al caso de estudio, es necesario recopilar previamente la información concerniente a la señal sísmica, el escenario de daño y las características del parque construido. Por tanto, se lleva a cabo una revisión exhaustiva de las normas sísmicas históricas en España y un análisis crítico de la norma sísmica española actual NCSE-02 en comparación con otras normas actuales de referencia basadas en el desempeño, como el Eurocódigo 8, haciendo énfasis en las provisiones que no garantizan el diseño por capacidad y que por tanto pueden provocar mecanismos frágiles o favorecer la excesiva influencia de la tabiquería. Además, se discute sobre la restricción del coeficiente de ductilidad en estructuras de vigas planas, cuestión que no se refleja en otras normas. Los resultados obtenidos mediante análisis de casos de estudio muestran que dicha prescripción resulta obsoleta para normas actuales. Finalmente, FAST se aplica al caso del terremoto de Lorca, obteniéndose predicciones de daño medio para diferentes asunciones. Los resultados muestran una coincidencia aceptable entre la predicción y los daños reales. FAST confirma que la causa principal de la práctica ausencia de colapsos (ante un terremoto con PGA triple que la típica de proyecto) hay que buscarla en la contribución estructural de la tabiquería de fábrica.
[CAT] Es proposa un mètode analític simplificat ("FAST") per a l'estimació de la vulnerabilitat a gran escala d'edificis porticats de formigó armat amb envans de fàbrica. Posteriorment, el mètode ha estat testejat mitjançant l'adopció de l'escenari de dany real corresponent al terratrèmol de Lorca de 2011 com a patró de comparació. FAST és un procediment espectral que permet predir el nivell de dany no estructural mitjà esperat per a cada classe d'edifici (definit pel seu nombre de plantes, any de construcció, densitat d'envans en planta i localització geogràfica), considerant un determinat nivell de demanda. El mètode té en compte la irregularitat de la distribució de envans al llarg de les diferents plantes del edifici. Es a dir, es pot tenir en compte que, freqüentment, hi ha una menor quantitat de d'envans a la planta baixa. FAST es fonamenta en: (i) la definició de corbes de capacitat aproximades que tenen en compte no sols la estructura del edifici sinó també els envans i assumint que l'estructura de HA s'ha projectat segons la norma sísmica corresponent en cada cas; (ii) l'assumpció de deformades "apriorístiques" coherents amb cada grau de dany (suposant que aquest es dona sempre a la planta baixa) que han estat estimades a través de correlacions experimentals i numèriques. Es proposen dues versions de FAST: una "simplificada" per a l'avaluació d'edificis amb envans uniformement repartits per totes les plantes, i una altra "generalitzada", que és capaç de tenir en compte qualsevol situació intermèdia entre el prototip uniformement paredat i el de planta baixa diàfana. A més, es proposen certes extensions al mètode. Per tal de validar FAST, es tria l'escenari de dany real corresponent al terratrèmol de Lorca (2011) com a patró de comparació, malgrat la seva impulsivitat i directivitat. Per definir els paràmetres de entrada corresponents al cas d'estudi, cal recopilar prèviament la informació concernent al senyal sísmica, l'escenari de dany i les característiques del parc construït. Per tant, es porta a terme una revisió exhaustiva de les normes sísmiques històriques a Espanya i una anàlisi crítica de la norma sísmica espanyola actual (NCSE-02) comparant-la amb altres normes actuals de referència, com l'Eurocodi 8, fonamentat en el concepte d'acompliment. També es fa èmfasi a les provisions que no garanteixen el disseny per capacitat i que, per tant, poden provocar mecanismes de col·lapse fràgils o afavorir la interacció de la estructura amb els envans. A més, es discuteix sobre la restricció del coeficient de ductilitat de les estructures de bigues planes ja que es una qüestió que no aborden la majoria de les normes. Els resultats obtinguts mitjançant l'anàlisi de casos d'estudi mostren que aquesta restricció resulta obsoleta a les normes actuals. Finalment, FAST s'aplica al cas del terratrèmol de Lorca, obtenint prediccions de dany mitjà per a diferents combinacions del paràmetres de entrada. Els resultats mostren una coincidència acceptable entre la predicció i els danys reals. FAST confirma que la causa principal de la pràctica absència de col·lapses (davant un terratrèmol amb PGA triple que la típica de projecte) cal buscar-la en la contribució estructural dels envans.
Gómez Martínez, F. (2015). FAST simplified vulnerability approach for seismic assessment of infilled RC MRF buildings and its application to the 2011 Lorca (Spain) earthquake [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/54780
TESIS
Premiado

Propagation des fissures microscopiques, est représentée par des variables d’endommagement. L’évolution de la variable d’endommagement est généralement formulée sur la base d’observations expérimentales. De nombreux modèles phénoménologiques d’endommagement ont été proposés dans la littérature. L’objet de cette thèse est de développer une nouvelle procédure pour obtenir des lois d’évolution macroscopique d’endommagement,dans lesquelles l’évolution de l’endommagement est entièrement déduite de l’analyse de la microstructure. Nous utilisons une homogénéisation basée sur des développements asymptotiques pour décrire le comportement global à partir de la description explicite d’un volume élémentaire microfissuré.Nous considérons d’une part un critère quasi-fragile (indépendant du temps) puis un critère sous-critique(dépendant du temps) pour décrire la propagation des microfissures. De plus, le frottement entre les lèvres des microfissures est pris en compte. Une analyse énergétique est proposée, conduisant à une loi d’évolution d’endommagement qui intègre une dégradation de la rigidité, un adoucissement du comportement du matériau, des effets de taille et d’unilatéralité, mettant en avant un comportement différent à la rupture en contact avec et sans frottement. L’information sur les micro-fissures est contenue dans les coefficients homogénéisés et dans la loi d’évolution de l’endommagement. Les coefficients homogénéisés décrivent la réponse globale en présence de micro-fissures (éventuellement statiques), tels qu’ils sont calculées avec la(quasi-) solution microscopique statique. La loi d’endommagement contient l’information sur l’évolution des micro-fissures, résultant de l’équilibre énergétique dans le temps pendant la propagation microscopique.La loi homogénéisée est formulée en incrément de contrainte. Les coefficients homogénéisés sont calculées numériquement pour des longueurs de fissures et des orientations différentes. Cela permet la construction complète des lois macroscopiques. Une première analyse concerne le comportement local macroscopique, pour des trajets de chargement complexes, afin de comprendre le comportement prédit par le modèle à deux échelles et l’influence des paramètres micro structuraux, comme par exemple le coefficient de frottement. Ensuite, la mise en œuvre en éléments finis des équations macroscopiques est effectuée et des simulations pour différents essais de compression sont réalisées. Les résultats des simulations numériques sont comparés avec les résultats expérimentaux obtenus en utilisant un nouvel appareil triaxial récemment mis au point au Laboratoire 3SR à Grenoble (France)
In continuum damage models, the degradation of the elastic moduli, as the results of microscopic crackgrowth, is represented through damage variables. The evolution of damage variable is generally postulatedbased on the results of the experimental observations. Many such phenomenological damage modelshave been proposed in the literature. The purpose of this contribution is to develop a new procedurein order to obtain macroscopic damage evolution laws, in which the damage evolution is completelydeduced from micro-structural analysis. We use homogenization based on two-scale asymptotic developmentsto describe the overall behaviour starting from explicit description of elementary volumes withmicro-cracks. We consider quasi-brittle (time independent) and sub-critical (time dependent) criteria formicro-cracks propagation. Additionally, frictional contact is assumed on the crack faces. An appropriatemicro-mechanical energy analysis is proposed, leading to a damage evolution law that incorporates stiffnessdegradation, material softening, size effect, and unilaterality, different fracture behaviour in contactwithout and with friction. The information about micro-cracks is contained in the homogenized coefficientsand in the damage evolution law. The homogenized coefficients describe the overall response inthe presence of (possibly static) micro-cracks, as they are computed with the (quasi-) static microscopicsolution. The damage law contains the information about the evolution of micro-cracks, as a result ofthe energy balance in time during the microscopic propagation. The homogenized law is obtained in therate form. Effective coefficients are numerically computed for different crack lengths and orientations.This allows for the complete construction of the macroscopic laws. A first analysis concerns the localmacroscopic behaviour, for complex loading paths, in order to understand the behaviour predicted bythe two-scale model and the influence of micro structural parameters, like for example friction coefficient.Next, the FEM implementation of the macroscopic equations is performed and simulations for variouscompression tests are conducted. The results of the numerical simulations are compared with the experimentalresults obtained using a new true-triaxial apparatus recently developed at the Laboratory 3SRin Grenoble (France)

Dynamic tensile characterization of geo-materials is critical to the modeling and design of protective structures that are often made of concrete. One of the most commonly used techniques currently associated with this type of testing is performed with a Kolsky bar and is known as the spall technique. The validity of the data from the spall technique is highly debated because the necessary boundary conditions for the experiment are not satisfied. By using a technique called pulse shaping, a new “controlled” spall technique was developed to satisfy all boundary conditions so that the analyzed data may be useful in modeling and design. The results from this project were promising and show the potential to revolutionize the way Kolsky bar testing is performed.

The work described herein examines the fracture behavior of steels used in bridge applications. As part of Transportation Pooled Fund (TPF) Project 5-238, Design and Fabrication Standards to Eliminate Fracture Critical Concerns in Steel Members Traditionally Classified as Fracture Critical, researchers aim to take advantage of advances made in both steel production technology and in the field of fracture mechanics. Testing and analysis of both conventional and High Performance Steel (HPS) grades of bridge steel was conducted as part of this study. This includes both Charpy V-Notch testing, as well as more rigorous elastic-plastic fracture toughness testing. Analysis includes the application of the master curve methodology to statistically characterize fracture behavior in the ductile to brittle transition region. In addition, a database of historic bridge fracture toughness data was compiled and re-analyzed using plasticity corrections to estimate elastic-plastic fracture toughness. Correlations between Charpy V-Notch impact energy and fracture toughness, which forms the basis for the current material specification, were also examined. Application of fracture toughness characterization of both new and historic data results in updated methodologies for addressing fracture in bridge design.
Ph. D.

A novel, general, numerical model is described that is capable of predicting the load-displacement relationship up to and at failure of multiple-bolt joints in timber of various configurations. The model is not tied to a single input function and bolt holes are permitted to be drilled oversize resulting in a slack system. The model consists of five parts. A new mathematical hysteresis model describes the stiffness of the individual bolt at each time step increment and accounts for non-linear and slack behavior; a mechanically-based structural stiffness model explains the interaction of one bolt with another bolt within a joint; an analytically-based failure model computes the stresses at each time step and initiates failure if crack length equals fastener spacing; a stochastic routine accounts for material property variation; and a heuristic optimization routine estimates the parameters needed. The core model is a modified array of differential equations whose solution describes accurate hysteresis shapes for slack systems. Hysteresis parameter identification is carried out by a genetic algorithm routine that searches for the best-fit parameters following evolutionary principles (survival of the fittest). The structural model is a linear spring model. Failure is predicted based on a newly developed 'Displaced-Volume-Method' in conjunction with beam on elastic foundation theory, elastic theory, and a modified Tsai-Wu Failure criterion. The devised computer model enhances the understanding of the mechanics of multiple-bolt joints in timber, and yields valid predictions of joint response of two-member multiple-bolt joints. This research represents a significant step towards the simulation of structural wood components.
Ph. D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 139-143).
This research study investigates the cracking processes in a brittle material associated with inclusions of varying shape, orientation and materials. Specifically, this study summarizes a series of uniaxial compression tests on gypsum specimens with varying inclusion materials, shapes and pair configurations using high speed imagery to determine cracking behavior. The inclusions in the study consisted of differing materials, of contrasting Young's Modulus (higher and lower than the matrix), shapes (hexagon, diamond, ellipse), and relative pair orientations (bridging angle). In addition, single ellipse inclusions were tested to investigate the cracking behavior associated with an ellipse inclusion in a brittle material. Similar to previous research regarding the coalescence of cracks propagating from inclusion pairs, the inclusion material did not affect the coalescence patterns. The coalescence behavior trended from indirect or no coalescence, to direct shear coalescence, to combined direct tensile-shear coalescence as the inclusion bridging angle was increased, similar to past studies on circular and square inclusion pairs, as well as flaw pairs. An analogy was proposed relating the debonded inclusion interfaces to corresponding flaw pairs to compare coalescence behavior. Although the general coalescence trends regarding the effect of bridging angle on inclusion pairs were comparable to those of flaw pairs, the coalescence based on a debonded interface representation did not appear to be similar to that of corresponding flaw pairs. Along with previous work conducted by the MIT rock mechanics group, this research provides detailed experimental observations regarding both the cracking and coalescence behaviors of inclusions in a brittle material.
by Stephen Philip Morgan.
S.M.

Gorgonocephalus eucnemis, Ophiothrix suensonii and Ophiothrix spiculata are aerobic Echinoderms. Previous observations on the anatomy of these two genera state five pairs of radial shields and genital plates are responsible for regulating the position of the roof of the body disc and the flushing of water in and out of the bursae. Rates of bursal ventilation increase by an average 60-64% when the ophiuroid is exposed to an increase in food or a decrease in dissolved oxygen in Gorgonocephalus. When exposed to hypoxic oxygen concentrations O. suensonii and O. spiculata increased bursal-ventilation rates by (means of) 35% and 28%. Measurements of DO from inside and outside the bursae show that DO is being absorbed during bursal-ventilations. These findings suggest bursal ventilation is a means of respiration and increased rates of bursal-ventilation may help meet increased oxygen demands during feeding and some periods of hypoxia. This dissertation includes unpublished, co-authored material.

The use of FRCM composites (Fiber Reinforced Cementitious Matrix) is becoming more and more widespread. The inorganic matrix guarantees many advantages, especially when dealing with masonry substrates, including a good compatibility from both a physical and a chemical point of view and the lower sensitivity to debonding phenomena at the interface. Compared to FRP composites, which presents many data in the literature, FRCM composites must be studied in detail and research in this field has only begun in recent years. This work deals with an important problem: the realization of an anchorage system to improve the strength of composites and allow their use even in the absence of adequate development length. In this study, the effectiveness of the anchorage system and the interaction with an externally bonded FRCM were studied on masonry columns. The columns were tested until failure condition in the Laboratory of Structural and Geotechnical Engineering (DICAM – LISG) of the University of Bologna, via del Lazzaretto 15/5, Bologna. Test results demonstrate that the introduction of additional anchorages improves the effectiveness of the FRCM composites in terms of resistance and loading capacity.

Laser beam welding has gained a significant interest during the last two decades. The suitability of the process for high volume production has the possibility to give a strong advantage compared to several other welding methods. However, it is important to have the process in full control since various quality issues may otherwise occur. During laser welding of boron steels quality issues such as imperfections, changes in local and global geometry as well as strength reduction can occur. The aspects that need to be considered are strongly depending on alloy content, process parameters etc. These problems that can occur could be fatal for the construction and the lowest level of occurrence is wanted, independent of industry. The focus of this study has been to investigate the properties of laser welded boron steel. The study includes laser welding of boron alloyed steels with strengths of 1500 MPa and a recently introduced 1900 MPa grade. Focus has been to investigate weldability and the occurrence of cracks, porosity and strength reducing microstructure that can occur during laser welding, as well as distortion studies for tolerances in geometry. The results show that both conventional and 1900 MPa boron alloyed steel are suitable for laser welding. Due to the martensitic structure of welds the material tends to behave brittle. Cracking and porosity do not seem to be an issue limiting the use of these steels. For tolerances in geometry for larger structures tests has been done simulating laser welding of A-pillars and B-pillars. Measurements have been done with Vernier caliper as well as a more advanced optical method capturing the movements during the welding sequence. Results from the tests done on Ushaped beams indicates that depending on the geometry of the structure and heat input distortions can be controlled to give distortions from 1 to 8 mm, at a welding length of 700 mm. This means that important geometry points can be distorted several millimeters if the laser welding process not is controlled.

La présente étude porte sur le comportement mécanique de deux types de supports de catalyseurs utilisés industriellement en hydrotraitement des résidus. Ces supports extrudés, fabriqués par IFPEN, sont constitués d’alumine de transition γ avec un taux de porosité proche de 70%. La porosité du premier matériau est uniquement constituée de mésopores (< 50 nm). La porosité du second matériau est constituée de mésopores et de macropores (jusqu’à 20 µm). Les niveaux de sollicitation en service étant très peu connus, cette étude s’attache à décrire de manière précise et exhaustive le comportement mécanique de ces supports sous une large gamme de sollicitations, et à identifier les différents mécanismes de ruine possibles. L’objectif final est de mieux comprendre les relations entre les paramètres microstructuraux et les propriétés mécaniques afin d’identifier des leviers d’amélioration de la tenue mécanique des supports. Dans un premier temps, une méthodologie adaptée de caractérisation mécanique est établie. Le comportement des supports est étudié d’une part en traction, à l’aide d’essais de flexion trois points et d’écrasement diamétral, et d’autre part, en compression sous différents taux de triaxialité, à l’aide d’essais de compression uniaxiale et hydrostatique et d’essais de micro-indentation sphérique. Les différents mécanismes responsables de la ruine des supports sont identifiés au moyen de techniques d’imagerie telles que la microscopie électronique à balayage et la micro-tomographie à rayons X. En traction, le comportement est fragile avec l’amorçage de la rupture sur un défaut critique. En compression, une transition fragile / quasi-plastique du comportement est observée avec l’augmentation du taux de confinement. Cette quasi-plasticité s’exprime en particulier à travers un phénomène de densification de la macroporosité. Dans un deuxième temps, un critère de rupture est identifié pour chaque type de matériau en vue de représenter sur une même surface de charge les différents types de comportement et phénomènes physiques observés. Cette identification est réalisée en couplant les essais d’indentation sphérique à une analyse numérique. Des critères faisant intervenir la pression hydrostatique permettent de rendre compte de la forte dissymétrie du comportement des matériaux en traction et en compression. Enfin, dans un souci de se rapprocher des sollicitations subies par les supports de catalyseurs dans un réacteur en service, le comportement d’un empilement de supports est étudié en compression œdométrique. L’analyse de cet essai par tomographie à rayons X permet de déterminer les différents mécanismes de ruine intervenant au sein d’un empilement, en particulier ceux responsables de la génération de fines. Les résultats illustrent la pertinence de la caractérisation en flexion et en indentation des supports de catalyseurs seuls pour prévoir leur comportement au sein d’un empilement en compression
In this work, we study the mechanical behaviour of two types of catalysts supports produced by IFPEN and industrially used in residues hydrotreating. Those extruded supports are made of transition γ-alumina with about 70% of porous volume. The first material’s porosity is exclusively composed of mesopores (< 50 nm). The porosity of the second material is composed of both mesopores and macropores (up to 20 µm). Because of the limited knowledge of the stress fields in embedded catalysts supports in use in a reactor, this study aims at precisely and exhaustively describing the mechanical behaviour of those supports under a wide range of stresses, and identifying the possible damage mechanisms. The final objective is to better understand the influence of microstructural parameters on the mechanical properties of the supports in order to propose some leads about how to improve their mechanical strength. First, an adequate mechanical characterization methodology is set. On one hand, the tensile mechanical behaviour of the supports is studied with three-point bending and diametrical crushing tests. On the other hand, their compressive behaviour under various triaxiality rates is characterized in uniaxial and hydrostatic compression, and by spherical micro-indentation. The different damaging mechanisms are identified by imaging techniques such as scanning electronic microscopy and X-ray micro-tomography. Under tensile stresses, the supports exhibit a brittle behaviour and fracture initiates at a critical flaw. Under compressive stresses, a brittle/quasi-plastic transition is observed with increasing the triaxiality rate. The quasi-plasticity is mainly due to the densification of the macroporosity. The second part of the study consists in identifying, for each material, a fracture criterion able to represent every types of behaviour and physical phenomena observed on the same yield surface. This identification is achieved by coupling the spherical indentation tests to a numerical analysis. Fracture criteria involving hydrostatic pressure are well suited to describe the highly dissymmetric mechanical behaviour of the materials in tension and in compression. The last part of this work aims at studying the mechanical behaviour of a stack of supports under œdometric compression in order to produce stress fields more representative of those existing within the supports stacked in a reactor. This test is analysed by X-ray tomography, which allows us to determine/acknowledge the different damaging mechanisms involved in fragments and fines generation. The results illustrate the suitability of the bending and indentation tests to characterize the mechanical properties of a single support and relate them to its mechanical behaviour in a stack of supports under compression

The thesis is focused on the prediction of the fracture toughness temperature dependence through a universal curve of fracture toughness (also known as the master curve). To determine the parameters of the universal curve of fracture toughness, values acquired from the measurement results of fracture toughness and tensile tests of structural steel P91 are used. The theoretical part is based on a summary of the relevant information from the field of fracture mechanics and brittle-ductile fracture behavior of steels that are important for the understanding of fracture-strain response of materials depending on load conditions. The experimental part of the thesis contains the results from practical measurements and analyses, which were used for determining the parameters of a universal curve of fracture toughness as well as for the evaluation of fracture behavior and description of the impact of structural parameters on this behavior in case of steel P91.

Ce travail porte sur la caractérisation des mécanismes de rupture, ductile et fragile, de matériaux tubulaires utilisés pour la fabrication de générateurs de gaz pour airbag. Lors d’un accident, le coussin d’un airbag est gonflé en quelques millisecondes par un générateur de gaz. En cas de défaillance de ce dernier, un mode de rupture ductile doit être assuré jusqu’à des températures de -60°C. Un essai Charpy sur éprouvette anneau a été proposé pour quantifier le risque de rupture fragile des matériaux des générateurs de gaz. Cependant, cette modification de l’essai Charpy soulève deux problématiques : dans quelle mesure cet essai permet de caractériser la transition ductile-fragile et comment déduire le risque de rupture fragile d’un générateur de gaz en fonctionnement à partir des résultats des essais Charpy ? Pour répondre à ces questions, une approche combinant essais et simulations numériques a été mise en place. Différents essais ont été développés afin de caractériser le comportement et l’endommagement ductile du matériau de l’étude. Une approche locale de la rupture ductile a été adoptée pour la modélisation. Ensuite, une campagne d’essais Charpy sur anneau effectuée pour des températures comprises entre -160°C et 23°C a permis de caractériser la transition ductile-fragile. Une étude numérique de cet essai a été menée puis couplées aux essais Charpy, cela a permis de mettre en place une modélisation de la rupture fragile. Le risque de rupture fragile des éléments structuraux de générateurs de gaz est enfin évalué. L’influence du procédé de fabrication sur le risque de rupture fragile a aussi été étudiée
This work deals with the characterization of the ductile and brittle failure mechanisms of tubular materials used for the manufacture of airbag gas generators. During a crash, airbag cushion is inflated in a few milliseconds by a gas generator. In case of gas generator failure, a ductile failure mode must be ensured up to temperatures of -60°C. A Charpy ring test has been proposed to quantify the risk of brittle failure of gas generator materials. However, this modification of the Charpy test raises two issues: to what extent does this test allow to characterize the ductile-fragile transition and how can the risk of brittle failure of an operating gas generator be deduced from the results of the Charpy test? To answer these questions, an approach combining experiments and numerical simulations was implemented. Various experimental configurations were developed in order to characterize material behavior and ductile damage. The ductile failure is modeled with a local approach. Then, ductile to brittle transition was characterized by a Charpy ring test campaign performed for temperatures between -160°C and 23°C. A numerical study of this test was carried out. Coupled with the Charpy tests, it allowed to set up a model of the brittle failure. The risk of gas generators brittle failure is finally evaluated. The influence of the manufacturing process on the risk of brittle failure was also studied

Der Vortrag beschreibt die Grundlagen der Elasto-Plastizität sowie die softwaretechnische Anwendung mit dem FEM-Programm Creo Simulate bzw. Pro/MECHANICA von PTC. Der erste Teil des Vortrages beschreibt die Charakteristika plastischen Verhaltens, unterschiedliche plastische Materialgesetze, Fließkriterien bei mehrachsiger Beanspruchung und unterschiedliche Verfestigungsmodelle. Im zweiten Vortragsteil werden Möglichkeiten und Grenzen der Berechnung elasto-plastischer Probleme mit der Software dargestellt sowie Anwendungstipps gegeben. Im dritten Vortragsteil schließlich werden verschiedene Beispiele vorgestellt, davon besonders ausführlich das Verhalten einer einachsigen elasto-plastischen Zugprobe vor und nach dem Eintreten der Einschnürdehnung
This presentation describes the basics of elasto-plasticity and its application with the finite element software Creo Simulate (formerly Pro/MECHANICA) from PTC. The first part describes the characteristics of plastic behavior, different plastic material laws, yield criteria for multiaxial stress states and different hardening models. In the second part, the opportunities and limitations of analyzing elasto-plastic problems with the FEM-code are described and user information is provided. The last part finally presents different examples. Deeply treated is the behavior of a uniaxial tensile test specimen before and after elongation with necking appears

Delamination is a prevalent composite laminate failure mode. It is of particular concern to the aerospace industry where laminated composites have found widespread usage in critical applications. Delamination growth has been widely studied, with Linear Elastic Fracture Mechanics (LEFM) being the most common approach taken to predict delamination behaviour, typically through global parameters such as specimen geometry and applied load measured away from the actual crack tip. However, internal mechanisms, such as fibre bridging, can occur within the structure thus affecting the transfer of the globally applied conditions to the actual crack tip. Thus, measurements made at the actual crack tip, referred to as local measurements, can be compared to the delamination behaviour predicted from the global analysis. The objective of this thesis is to better understand the relationship between the globally predicted and the actual local crack tip behaviour under mixed mode loading. Of particular interest is the mixed mode region at low shear (mode, II) loads where the global tensile opening (mode I) loads exceed the pure mode I fracture toughness of the material. An experimental loading j ig developed by Paris et al. (2001) is used in this work. The j ig can be used inside a scanning electron microscope (SEM) to allow for simultaneous local and global analysis of a specimen. Previous pure mode experiments (Paris, 1998) showed that in the absence of fibre bridging mode I global predictions match the actual local crack tip conditions. Global mode II loads, however, induce a local mixed mode condition where the mode I component is created due to the surface roughness as the crack surfaces slide over each other. In this work, mixed mode experiments on unidirectional AS4/3501-6 carbon fibre/epoxy laminates show that global applied shear loads are transferred directly to the crack tip. The crack surface roughness effects seen initially under pure mode II loading also manifest themselves under mixed mode loading resulting in a significantly higher local mode I component than predicted globally. As such, current test practices and global data reduction schemes are inadequate and do not provide a complete picture of delamination behaviour. The total local critical strain energy release rate is also significantly higher than that determined globally and observed in the literature. The higher local failure loads observed are consistent with fractographic evidence in the literature that indicates failure at the mixed mode conditions examined here is similar to much tougher mode II dominated failure.

Clay shales are currently being assessed as possible host rock formations for the deep geological disposal of radioactive waste. However, one main concern is that the favourable long-term isolation properties of the intact rock mass could be negatively affected by the formation of an excavation damaged zone (EDZ) around the underground openings. This thesis investigated the deformation and failure process of a clay shale, namely Opalinus Clay, with particular focus on the influence of anisotropy on the short-term response of circular tunnels. To achieve this goal, a hybrid continuum-discontinuum numerical approach was used in combination with new field measurements from the Mont Terri underground research laboratory. The response of Opalinus Clay during the excavation of a full-scale emplacement (FE) test tunnel was characterized by geodetic monitoring of wall displacements, radial extensometers and longitudinal inclinometers. The deformation measurements indicated strong directionality induced by the combined effect of in situ stress field and presence of bedding planes striking parallel to the tunnel axis, with the most severe deformation occurring in the direction approximately perpendicular to the material layering. Computer simulations were conducted using a newly-extended combined finite-discrete element method (FEM/DEM), a numerical technique which allows the explicit simulation of brittle fracturing and associated seismicity. The numerical experimentation firstly focused on the laboratory-scale analysis of failure processes (e.g., acoustic activity) in brittle rocks, and on the role of strength and modulus anisotropy in the failure behaviour of Opalinus Clay in tension and compression. The fracturing behaviour of unsupported circular excavations in laminated rock masses was then analyzed under different in situ stress conditions. Lastly, the modelling methodology was applied to the aforementioned FE tunnel to obtain original insights into the possible EDZ formation process around emplacement tunnels for nuclear waste. The calibrated numerical model suggested delamination along bedding planes and subsequent extensional fracturing as key mechanisms of the damage process potentially leading to buckling and spalling phenomena. Overall, the research findings may have a potential impact on the constructability and support design of an underground repository as well as implications for its long-term safety assessment procedure.

As hard rock mining progresses into higher stress mining conditions through either late stage extraction or mining at depth, the rock mass is driven not just to the peak strength, but often well into the post-peak until complete ‘failure’ occurs and easier mining conditions become evident. Limited research has been accomplished in identifying the transition of the rock mass and its behaviour into the post-peak and this research investigates this behaviour in detail. As the rock mass progressively fails, fractures are initiated through intact rock and extension and shear failure of these and pre-existing features occurs. Associated with this failure are microseismic events, which can be used to give an indication of the strength state of the rock mass. Based on an analogy to laboratory testing of intact rock and measurement of acoustic emissions, the microseismicity can be used to identify, fracture initiation, coalescence of fractures (yield), localization (peak-strength), accumulation of damage (post-peak) and ultimate failure (residual strength) leading to aseismic behaviour. The case studies presented in this thesis provide an opportunity to examine and analyse rock mass failure into the post-peak, through the regional and confined failures at the Williams and the Golden Giant mines, both in the Hemlo camp in Northern Ontario, Canada. At the Williams mine, the progressive failure of a sill pillar region into the post-peak was analysed; relating the seismic event density, combined with numerical modelling and a spatial and temporal examination of the principal components analysis (PCA), to characterize the extent, trend and state of the yielding zone, which formed a macrofracture shear structure. Observations of conventional displacement instrumentation, indicates regional dilation or shear of the rock mass occurs at or prior to the point of ‘disassociation’ (breakdown of stable PCA trends) when approaching the residual strength. At the Golden Giant mine, the complete process from initiation to aseismic behaviour is monitored in a highly stressed and confined pendent pillar. The PCA technique, numerical modelling and focal mechanism studies are used to define significant stages of the failure process, in which a similar macrofracture structure was formed. Temporal observations of key source parameters show significant changes prior to and at the point of coalescence and localization.

碩士
國立交通大學
土木工程學系研究所
85
A series of experimental work were carried out previously in NCTU (National Chiao - Tung University) to investigate the mechanical behavior of brittle rocks under elevated pressures and temperatures. In this study, the experimental data of triaxial compression tests is summarized and re-analyzed. Furthermore , the study develops a constitutive model to describe the mechanical behavior of brittle rocks. The experimental results show that crack-closure siginificantly affect the initial stress - strain relation of a brittle rock during triaxial compression. The initial stress - strain curve reveals that the tangent stiffness gradually increases and reaches an elastic stiffness at a crack- closed strain. As the confining pressure increases, the elastic stiffness raises while the crack-closed strain decreases. The effect of temperature on the elastic stiffness and the crack - closed strain , however, is still not concluded from the experimental data. For the post - peak behavior , the class II behavior is obvious. On the basis of the experimental results, a constitutive model is proposed to d escribe the mechanical behavior of brittle rocks. The presented constitutive model contains a pore/crack closure model and a continuum damage mechanics model. The study also developes a computer program behavior optimiz -ation procedure for calibrating the appropriate material parameters from a set of experimental data. By comparing the simulated and experimental results , it is demonstrated that the proposed model can closely simulate the stress- strain relation of brittle rocks with very limited material parameters. Parametric studies also demonstrate that, with proper combination of parameters,the proposed model is capable of modeling (i) the class I as well as the class II post-peak behavior,and (ii) the dilatancy during a compression test.

‘Coupled process’ implies that one process affects the initiation and progress of the others and vice versa. The deformation and damage behaviors of rock under loading process change the fluid flow field within it, and lead to altering in permeable characteristics; on the other side inner fluid flow leads to altering in pore pressure and effective stress of rock matrix and flow by influencing stress strain behavior of rock. Therefore, responses of rock to natural or man-made perturbations cannot be predicted with confidence by considering each process independently. As far as hydro-mechanical behavior of rock is concerned, the researchers have always been making efforts to develop the model which can represent the permeable characteristics as well as stress-strain behaviors during the entire damage process. A brittle low porous granite was chosen as the study object in this thesis, the aim is to establish a corresponding constitutive law including the relation between permeability evolution and mechanical deformation as well as the rock failure behavior under hydro-mechanical coupled conditions based on own hydro-mechanical coupled lab tests. The main research works of this thesis are as follows: 1. The fluid flow and mechanical theoretical models have been reviewed and the theoretical methods to solve hydro-mechanical coupled problems of porous medium such as flow equations, elasto-plastic constitutive law, and Biot coupled control equations have been summarized. 2. A series of laboratory tests have been conducted on the granite from Erzgebirge–Vogtland region within the Saxothuringian segment of Central Europe, including: permeability measurements, ultrasonic wave speed measurements, Brazilian tests, uniaxial and triaxial compression tests. A hydro-mechanical coupled testing system has been designed and used to conduct drained, undrained triaxial compression tests and permeability evolution measurements during complete loading process. A set of physical and mechanical parameters were obtained. 3. Based on analyzing the complete stress-strain curves obtained from triaxial compression tests and Hoek-Brown failure criterion, a modified elemental elasto-plastic constitutive law was developed which can represent strength degradation and volume dilation considering the influence of confining pressure. 4. The mechanism of HM-coupled behavior according to the Biot theory of elastic porous medium is summarized. A trilinear evolution rule for Biot’s coefficient based on the laboratory observations was deduced to eliminate the error in predicting rock strength caused by constant Biot’s coefficient. 5. The permeability evolution of low porous rock during the failure process was described based on literature data and own measurements, a general rule for the permeability evolution was developed for the laboratory scale, a strong linear relation between permeability and volumetrical strain was observed and a linear function was extracted to predict permeability evolution during loading process based on own measurements. 6. By combining modified constitutive law, the trilinear Biot’s coefficient evolution model and the linear relationship between permeability and volumetrical strain, a fully hydro-mechanical coupled numerical simulation scheme was developed and implemented in FLAC3D. A series of numerical simulations of triaxial compression test considering the hydro-mechanical coupling were performed with FLAC3D. And a good agreement was found between the numerical simulation results and the laboratory measurements under 20 MPa confining pressure and 10 MPa fluid pressure, the feasibility of this fully hydro-mechanical coupled model was proven.

博士
國立清華大學
材料科學工程學系
89
In this thesis, the micronecking mechanism of the craze is correlated with the molecular chain network and the entanglement density of the chain network will affect the deformation of polymer thin film. In addition, a new model of the strain localization of polymer thin film is explored. By suppressing the trend of strain localization the brittle polymer thin film can be toughened and demonstrate a super-plastic behavior. The void fraction in the fully necked craze region was determined and a close-packed fibril structure was then concluded. The local stress and strain within the craze were obtained from AFM topographic data by the Bridgman’s plasticity analysis. The stress / strain curve of craze fibrillation was subsequently determined where an apparent strain softening was found in the initiation of fibrillation. Strain rate was found to peak at the craze boundaries, consistent with the surface drawing mechanism from TEM results. We further exploited the necking characteristic of crazing by sandwiching the craze-forming brittle polymer film between two ductile polymer films to examine the deformation behavior of the brittle polymer which necking is suppressed. The super-plastic behavior is remarkably dependent on the thickness of the outer ductile polymer layers. When the outer-layer thickness is less than a critical thickness, the brittle polymer film in combination with the sandwich structure demonstrated a different degree of strain localization with the critical strain increased with the thickness of the outer-layer. A sharp ductile-brittle transition in the sandwich thin film structure was observed as the PPO thickness decreased. A simple mechanical model built upon the competition between the necking force, associated with crazing, and the constraining force, due to the ductile films, was utilized to analyze the stability of this super-plasticity. The result of the mechanical analysis is in good agreement with the experimental data.

博士
國立臺灣科技大學
營建工程系
103
The early development of Taiwan transportation infrastructure was mostly located on the side of mountain region forming a circumferential turnpike and railway network. The rapid densification of this network continues. At present, National highway design considers environmental and economical impacts. To reduce damage to sensitive ecosystems brought on by construction, bridges and tunnels are considered to be the environmentally sustainable options. Previously, a lot of emphasis has been given to safety and economy of excavation of rock mass without particular attention to excavation in heated rock and to effects of tunnel fires. However, thermo-induced damage affects the structural and material integrity of civil engineering structures; the damage can induce direct or indirect extensive structural collapse. This study aims to give engineers and tunnel designers a reference for protecting the tunnel structure and surrounding rock, increasing construction efficiency as well as decreasing casualties and cost in the project life cycle. In this study, the complicated engineering problem was simplified into two issues; geo-thermal rock and tunnel fire studied under three topics: 1. Pseudo-static mechanics test after one-dimensional heat-driven Fracture(HdF), 2. Dynamic mechanics test after individual heat-driven damage, and 3. Heat-driven damage test with linear transient thermal loading (LTTL) on inner hollow surface (HIS) for investigating the failure mechanism. The intended purpose of the method was to predict the effects of heat treatment on the static and dynamic mechanical properties and characteristic width of process zones, as well as the establishment of theoretical model is to predict fracture occurring time and position on the topic of HdD with LTTL on HIS. The numerical and experimental methods were used to verify the theoretical model. In addition, using the theoretical model discusses the fracture influence of increasing temperature rate and the radius ratio of outer and inner hole. The results of the 1st topic showed that in the macroscopic, all of the results can be regressed by a continuous equation; therefore, the regression equations obtained from the results of continued heat-damaged specimen pieces represented more accurate prediction equations. Moreover, a critical damage temperature is approximately 500-600°C. For a temperature range between room temperature and approximately 500-600°C; the variation of all of the mechanical properties decreased by approximately 7.6-14.5% per 100°C, but they decreased by approximately 29-37% per 100°C between 500-600°C and the highest temperature used in the tests. On investigating the microscopic results, for specimen at room temperature, micro-cracks cluster around the top of SCB sample (near the loading position) before approximate loading level of 80%, then micro-cracks will cluster around the tip of pre-existing crack during loading level of 80% and 100%. The micro-cracks around the tip of pre-existing crack dominate the SCB fracture behavior. However, for the higher temperature specimen (approximately 450°C), before loading level of 80%, micro-cracks do not cluster at the top of Semi-circular Bend (SCB) sample, because the thermal-induced defects existed within the SCB sample, the stress field near top of SCB sample is a compressive situation, the micro-cracks will be closing, hence the acoustic emission sensors will receive new signals from the top of SCB sample during this stage. Moreover, the study attempted to calculate fracture toughness of quasi-brittle material using synchronized nondestructive techniques. Currently, only some sample results were obtained. The results on 2nd topic showed that the dynamic mechanical parameters increase with the loading rate and decrease with the heat treatment in general. The parameters are hardly affected between room temperature and 450°C. the parameters decrease sharply between 450 and 600°C, it is probably related with the phase change of Quartz from the ? to ? phase, in which volume increases by 0.4% at approximately 573°C. In addition, the average velocity of crack propagation increases with the temperature. On investigation of the results of the 3rd topic, comparison of the temperature and stresses between simulation analysis and theoretical results obtained the similar patterns; for experimental results, the temperature of heat conduction, the micro-crack localization position by AE, as well as the macro-crack initiation location from ESPI are the similar as the theoretical results. The localization position clusters between approximately nondimensional radius (?? of 4.5 and 6.5 using acoustical technique and the initiation location emerges at approximately ? of 5. Then, using the theoretical model to understand the fracture behavior with varying increasing temperature rate (M) and the radius ratio of outer- and inner-hole (?搓); the ?搭 decreases and failure temperature increases with the M and ?搓 increases. To investigate the coefficient ?燁, the ?搭 decreases with ?燁 increases, when the ?燁<5, the failure time and the ?搭 increase.

碩士
國防大學理工學院
軍事工程碩士班
102
Split Hopkinson Pressure Bar was initially applied to metallic materials. Recently, this testing method was introduced to the rock and brittle materials, such as ceramics and concrete due to its simple structure and convenience for operation. Lots of studies focused on the relationship between strength and strain rate of materials, but less studies investigated the deformation of the materials. The material strain obtained from traditional SHPB test was calculated by relative shifts between planes of elastic rod and material, and brittle materials were easily damaged, which led to the difference in transmission of stress waves. The issues about if the traditional methods were suitable for brittle materials and the different results obtained from one-wave method and three-wave method should be further discussed. Certain proportion of the gypsum mortar and maintenance procedures were utilized in this study for preparing brittle materials, and they were compared with ductile materials by difference methods of measurement and analysis to investigate the deformation characteristics of brittle materials under SHPB tests, while using high-speed cameras to record development of specimen fracture. Secondly, natural rocks were underwent SHPB tests, except for comparing strength and deformation with ductile materials, observations and discussions were also being made over development of fracture and mechanism of damage. The results showed that dynamic uniaxial compression strength, strain, dynamic deformation modulus and Poisson's ratio of materials were improved with the increased stress rate, and stress balance between both planes of the specimen was relatively worse. When the damaged level of specimen was lower, there is no huge difference in duration of strain and stress between one-wave method and three-wave method. Differences were becoming significantly at high damaged level, and the strains measured by strain gage measurements were below one-wave method and three-wave method. In addition, there were different development of fracture and destruction in brittle materials and rocks. The experimental results confirmed that strains of materials can be obtained by using strain gage and laser displacement measurement, while the latter required models with more stable signals and adjusted positions of measurement to meet the measuring requirements.

碩士
國立彰化師範大學
工業教育與技術學系
97
This article coordinates instrument of the rigid test brittle materials by the 100 metric tons of universal testing machines. The brittle materials were each kind of different intensity concrete, cement mortar, figuline brick and brick of common structural. The experiment was compared with different characteristics of brittle materials that were dry and wet two conditions, beside the concrete. Concrete and brick were principal materials of the building nowadays. The purposes of study were analyzed and compared the characteristics that the experimental mechanical behaviors of brittle materials after failure under the axial pressure. Behaviors of brittle materials after failure were analyzed and compared by the pictures of strain-stress, transformed from experimental data, and provided for design construction in engineering. People were educated to judge security of the landslide environment by the study results. The next step will strive for the escaping time, and bring the biggest safety for the people. According to research results, on the behaviors of brittle materials after failure were showed. First, the strengths of brittle materials were inversed proportional with the E value of brittle materials after failure. Second, the collapsing speed and the escaping time were directed proportional with The E value of brittle materials after failure. Brittle materials of toughness were caught up the strength after failure by steel bars.

Pt-aluminide (PtAl) coatings form an integral part of thermal barrier coating (TBC) systems that are applied on Ni-based superalloy components operating in the hot sections of gas turbine engines. These coatings serve as a bond coat between the superalloy substrate and the ceramic yttrium stabilized zirconia (YSZ) coating in the TBC system and provide oxidation resistance to the superalloy component during service at high temperatures. The PtAl coatings are formed by the diffusion aluminizing process and form an integral part of the superalloy substrate. The microstructure of the PtAl coatings is heavily graded in composition as well as phase constitution. The matrix phase of the coating is constituted of the B2-NiAl phase. Pt, in the coating, is present as a separate PtAl2 phase as well as in solid solution in B2-NiAl. The oxidation resistance of the PtAl bond coat is derived from the B2-NiAl phase. At high temperatures, Al from the B2-NiAl phase forms a regenerative layer of alumina on the coating surface which, thereby, lowers the overall oxidation rate of the superalloy substrate. The presence of Pt is beneficial in improving the adherence of the alumina scale to the surface and thereby enhancing the oxidation resistance of the coating. However, despite its excellent oxidation resistance, the B2-NiAl being an intermetallic phase, renders the PtAl coating brittle and imparts it with a high brittle-to-ductile-transition-temperature (BDTT). The PtAl coating, therefore, remains prone to cracking during service. The penetration of these cracks into the substrate is known to degrade the strain tolerance of the components. Evaluation of the mechanical behavior of these coatings, therefore, becomes important from the point of views of scientific understanding as well as application of these coatings in gas turbine engine components. Studies on the mechanical behavior of coatings have been mostly carried on coated bulk superalloy specimens. However, since the coating is brittle and the superalloy substrate more ductile when compared to the coating, the results obtained from these studies may not be representative of the coating. Therefore, it is imperative that the mechanical behavior of the coating in stand-alone condition, i.e. the free-standing coating specimen without any substrate attached to it, be evaluated for ascertaining the true mechanical response of the coating. Study of stand-alone bond coats involves complex specimen preparation techniques and challenging testing procedures. Therefore, reports on the evaluation of mechanical properties of stand-alone coatings are limited in open literature. Further, no systematic effort has so far been made to examine important aspects such as the effect of temperature and strain rate on the tensile behavior of these coatings. The deformation mechanisms associated with these bond coats have also not been reported in the literature. In light of the above, the present research study aims at evaluating the tensile behavior of free-standing PtAl coatings by the micro-tensile testing technique. The micro-tensile testing method was chosen for property evaluation because of its inherent ability to generate uniform strain in the specimen while testing, which makes the results easy to interpret. Further, since the technique offers the feasibility to test the entire graded PtAl coating in-situ, the results remain representative of the coating. Using the above testing technique, the tensile behavior of the PtAl coating has been evaluated at various temperatures and strain rates. The effect of strain rate on the BDTT of the coating has been ascertained. Further, the effect of Pt content on the tensile behavior of these coatings has also been evaluated. Attempts have been made to identify the mechanisms associated with tensile deformation and fracture in these coatings. The thesis is divided into nine chapters. Chapter 1 presents a brief introduction on the operating environment in gas turbine engines and the materials that are used in the hot sections of gas turbine engines. The degradation mechanisms taking place in the superalloy in gas turbine environments and the need for application of coatings has also been highlighted. The basic architecture of a typical thermal barrier coating (TBC) system applied on gas turbine engine components has been presented. The constituents of the TBC system, i.e. the ceramic YSZ coating, MCrAlY overlay as well as diffusion aluminide bond coats and, the various techniques adopted for the deposition of these coatings have been described in brief. Chapter 2 presents an overview of the literature relevant to this study. This chapter is divided into four sub-chapters. The formation of diffusion aluminide coatings on Ni-based superalloys has been described in the first sub-chapter. Emphasis has been laid on pack cementation process for the formation of the coatings. The fundamentals of pack aluminizing process, including the thermodynamic and kinetic aspects, have been mentioned in brief. The microstructural aspects of high activity and low activity plain aluminide and Pt-aluminide coatings have also been illustrated. The techniques applied for the mechanical testing of bond coats have been discussed in the second sub-chapter. The macro-scale testing techniques have been mentioned in brief. The small scale testing methods such as indentation, bend tests and micro-tensile testing have also been discussed in the context of evaluation of mechanical properties of bond coats. Since the matrix in the aluminide bond coats is constituted of the B2-NiAl phase, a description of the crystal structure and deformation characteristics of this phase including the flow behavior, ductility and fracture behavior has been mentioned in the third sub-chapter. In the fourth sub-chapter, reported literature on the tensile behavior and brittle-to-ductile-transition-temperature (BDTT) of diffusion aluminide bond coats has been discussed. In Chapter 3, details on experiments carried out for the formation of various coatings used in the present study and, their microstructural characterization, are provided. The method for extraction of stand-alone coating specimens and their testing is discussed. The microstructure and composition of the various coatings used in the present study are discussed in detail in Chapter 4. Unlike in case of bulk tensile testing, for which standards on the design of specimens exist, there are no standards available for the design of micro-tensile specimens. Therefore, as part of the present research work, a finite element method (FEM)-based study was carried out for ascertaining the dimensions of the specimens. The simulation studies predicted that failure of the specimens within the gage length can be ensured only when certain correlations between the dimensional parameters are satisfied. Further, the predictions from the simulation study were validated experimentally by carrying out actual testing of specimens of various dimensions. Details on the above mentioned aspects of specimen design are provided in Chapter 5. The PtAl coatings undergo brittle fracture at lower temperatures while ductile fracture occurs at higher temperatures. Further, the coatings exhibit a scatter in the yielding behavior at temperatures in the vicinity of BDTT. Therefore, the BDTT, determined as the temperature at which yielding is first observed in the stress-strain curves, may not be representative of the PtAl coatings. In Chapter 6, a method for the precise determination of BDTT of aluminide bond coats, based on the variation in the plastic strain to fracture with temperature, has been demonstrated. The BDTT determined by the above method correlated well with the variation in fracture surface features of the coating and was found representative of these coatings. In Chapter 7, the effect of temperature and strain rate on the tensile properties of a PtAl bond coat has been evaluated. The temperature and strain rate was varied between room temperature (RT)-1100°C and 10-5 s-1-10-1 s-1, respectively. The effect of strain rate on the BDTT of the PtAl bond coat has been examined. Further, the variation in fracture surface features and mechanism of fracture with temperature and strain rate are illustrated. The micro-mechanisms of deformation and fracture in the coating at different temperature regimes have also been discussed. The coating exhibited brittle-to-ductile transition with increase in temperature at all strain rates. The BDTT was strain rate sensitive and increased significantly at higher strain rates. Above BDTT, YS and UTS of the coating decreased and its ductility increased with increase in the test temperature at all strain rates. Brittle behavior occurring in the coating at temperatures below the BDTT has been attributed to the lack of operative slip systems in the B2-NiAl phase of the coating. The onset of ductility in the coating in the vicinity of BDTT has been ascribed to generation of additional slip systems caused by climb of dislocations onto high index planes. The coating exhibited two distinct mechanisms for plastic deformation as the temperature was increased from BDTT to 1100°C. For temperatures in the range BDTT to about 100°C above it, deformation was controlled by dislocations overcoming the Peierls-Nabarro barrier. Above this temperature range, non-conservative motion of jogs by jog dragging mechanism controlled the deformation. The transition temperature for change of deformation mechanism also increased with increase in strain rate. For all strain rates, fracture in the coating at test temperatures below the BDTT, occurred by initiation of cracks in the intermediate single phase B2-NiAl layer of the coating and subsequent inside-out propagation of the cracks across the coating thickness. Ductile fracture in the coating above the BDTT was associated with micro-void formation throughout the coating. The effect of Pt content on the tensile behavior of PtAl coating, evaluated at various temperatures ranging from room temperature (RT) to 1100°C and at a nominal strain rate of 10-3 s-1, is presented in Chapter 8. Irrespective of Pt content in the coating, the variation in tensile behavior of the coating with temperature remained similar. At temperatures below BDTT, the coatings exhibited linear stress-strain response (brittle behavior) while yielding (ductile behavior) was observed at temperatures above BDTT. At any given temperature, the elastic modulus decreased while the strength increased with increase in Pt content in the coating. On the other hand, the ductility of the coating remained unaffected with Pt content. The BDTT of the coating also increased with increase in Pt content in the coating. Addition of Pt did not affect the fracture mechanism in the coating. Fracture at temperatures below BDTT was caused by nucleation of cracks at the intermediate layer and their subsequent inside-out propagation. At high temperatures, fracture occurred in a ductile manner comprising void formation, void linkage and subsequent joining with cracks. The deformation sub-structure of the coating did not get affected with Pt incorporation. Short straight dislocations were observed at temperatures below BDTT, while, curved dislocations marked by jog formation were observed at temperatures above BDTT. The factors controlling fracture stress and strength in the PtAl coatings at various temperatures have also been assessed. The overall summary of the present research study and recommendations for future studies are presented in the last chapter, i.e. Chapter 9.

碩士
國立臺灣科技大學
營建工程系
105
For investigating mechanical behaviors of tunnel structure (like tunnel lining) is subjected to a coupling of temperature (such as fire, geothermal, and the other thermal gradient) and stress (geostress and working stress) treatment. This study was focusing on the investigation of macroscopical residual mechanical behavior and microscopical fracture evolution utilizing acousto-optical nondestructive technique. The acoustical nondestructive technique is also divided by active and inactive means which is like supersonic and acoustic emission method, respectively. The measurein laboratory and in-situ emplying nondestructive techaniques are eager to obtain the macro- and microscopical results for judging the damage severity of material.This research paid attention on the linking material such as concrete (fc’ = 420 kgf/cm2) that due to serious fire-induced damage, by applying the uniaxial compressive stress (working stress ratio = 0 ~ 0.4) and complete thermo history (maximum temp. = 25 ~ 600 ℃) in the same time to simulate the critical failure of tunnel lining, with constant the rate of heating (10 ℃/min), exposure time that calculated by one dimensional heat transfer equation (8 min), and way of cooling (cooling in air). The macro-scale mechanical parameters and complete loading history were obtained from compressive test which using lateral displacement as a feedback signal, during the compressive test, associated with synchronized acousto-optic nonintrusive technique (AE & ESPI) to investigate the 3 types of macro-scale mechanical parameters (Stiffness, Strength and Tougness) and 3 types of micro-scale failure evolution (Localization, Crack initiation and propagation). Build up the relationship of macro and micro-scale mechanical parameters and ‘‘velocity ratio, VS/VP’’ which obtained from ultrasonic pulse (UP).Based on the result of measurement temp. inside concrete, the one dimensional heat transfer equation could simplify the temp. history for obtaining the intial exposure time. In macro-scale, the stiffness and strength of specimens stressed in compression during heating (< 530 ℃) were geneally 15 to 60 % higher than those of companion spcimens which were not stressd during heating, but the specimens which stress in working stress ratio 0.45 and max temp. 600 ℃ were decreased 6 % of stiffness and strength. In micro-scale, as the working stress ratio increase, AE events before the peak load were decreased .In velocity ratio, the working stress was obvious effect in max temp. 400 ~ 530 ℃ and displaced the reverse trend compared with non-working stress condition.