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In this thesis, the possibility to numerically describing the behaviour that signifies shear type cracking in concrete is studied. Different means for describing cracking are evaluated where both methods proposed in design codes based on experiments and advanced finite element analyses with a non-linear material description are evaluated. It is shown that there is a large difference in the estimation of the crack width based on the calculation methods in design codes. The large difference occurs due to several of these methods do not account for shear friction in the crack face. The finite element method is an important tool for analysing the non-linear behaviour caused by cracking. It is especially of importance when combined with experimental investigations for evaluating load bearing capacity or establishing the structural health. It is shown that non-linear continuum material models can successfully be used to accurately describe the shear type cracking in concrete. A method based on plasticity and damage theory was shown to provide accurate estimations of the behaviour. The methods based on fracture mechanics with or without inclusion of damage theory, overestimated the stiffness after crack initiation considerably. The rotated crack approach of these methods gave less accurate descriptions of the crack pattern and underestimated the crack widths. After verification of the material model, realistic finite element models based on plasticity and damage theory are developed to analyse the cause for cracking in two large concrete structures. The Storfinnforsen hydropower buttress dam is evaluated where the seasonal temperature variation in combination with the water pressure have resulted in cracking. With the numerical model the cause for cracking can be explained and the crack pattern found in-situ is accurately simulated. The model is verified against measurements of variation in crest displacement and crack width with close agreement. The construction process of a balanced cantilever bridge, Gröndal Bridge, is numerically simulated and a rational explanation of the cause for cracking is presented. It is shown that large stresses and micro-cracks develop in the webs during construction, especially after tensioning the continuing tendons in the bottom flange. Further loads from temperature variation cause cracking in the webs that is in close agreement with the cracking found in-situ. The effect of strengthening performed on this bridge is also evaluated where the vertical Dywidag tendons so far seem to have been successful in stopping further crack propagation. / QC 20100730

L’utilisation des matériaux constitués de fibres synthétique ou naturelle est en pleine expansion et concerne de nombreux secteurs : industrie automobile, aéronautique, électrique, filtration de l’air ou applications médicales. Malgré des procédés de fabrication et des natures de fibres différents, ces matériaux ont pour point commun d’être constitués d’un réseau de fibres liées entre elles par des liaisons. Les papiers et les cartons sont, par exemple, constitués de fibres de cellulose naturelles liées chimiquement. A l’heure actuelle, les mécanismes de fissuration dans de tels milieux sont encore mal compris. Ils dépendent fortement (a) des propriétés des constituants : géométrie et propriétés mécaniques des fibres et des contacts fibre-fibre, (b) des caractéristiques des réseaux fibreux : géométrie et arrangement des fibres, et des caractéristiques du réseau poreux induit : porosité, distribution de taille des pores, répartition spatiale des pores, etc. et (c) des modes de sollicitations mécaniques. Dans ce type de matériaux, les effets d’échelles doivent être pris en compte pour compléter les approches mécaniques traditionnelles. Les récents progrès en mécanique expérimentale et en simulation numérique permettent de mener une telle étude de l’échelle de la fibre à celle du réseau fibreux.Cette thèse a donc pour but de mettre en place des outils d’analyse des microstructures et des mécanismes de fissuration dans les milieux fibreux à faible densité. Pour cela, (i) des essais de micromécaniques seront couplés à des méthodes d’imagerie (ESEM, microtomographie à rayons X, stéréocorrélation) afin de caractériser expérimentalement les milieux et leur endommagement (ii) Cette étude vient compléter les travaux expérimentaux menés dans les deux laboratoires 3SR et LGP2 (ANR ANAFIB http://anafib.hmg.inpg.fr/spip.php?rubrique1) et sera complétée par des simulations numériques des essais réalisés en collaboration avec Per Isaksson de l’Université d’Uppsala (Suède). / Materials made up of synthetic or natural fibres are increasingly developed in various domains: papermaking, composite, automotive and aeronautic industries for structural, packaging, air filtration or medical applications. Despite the variety of manufacturing processes of such materials, all of them can be considered as being formed by a network of fibres interconnected via bonds. For instance, in the case of materials made up of natural cellulosic fibres such as papers or boards, fibres are chemically linked.Crack propagation phenomena in such materials remain poorly understood even though it can be presumed that such mechanisms depend on:- (i) the geometrical and mechanical properties of the constituents of individual fibres and fibre-fibre bonds,- (ii) the architecture of the fibrous network, for example the spatial distributions of fibres, bonds and pores and the size distributions of pores and bonds,- (iii) the applied mechanical loadings.In such materials, scale effects must be investigated in order to improve the classical approaches used to understand crack propagation mechanisms. Recent progresses in both experimental mechanics and numerical simulation approaches allow such a study from the fibre scale up to the fibre network scale to be carried out.The proposed PhD aims first at developing an original experimental approach to analyse microstructure changes and crack propagation phenomena for low density papers. For that purpose x-ray microtomography or ESEM, and stereo-correlation experiments will be carried out to investigate microstructural changes and deformation mechanisms at all relevant scales (see the illustration given in ).

Simulation of crack growth in the internal cooling system of a blade in a Siemens gas turbine has been studied by inserting and propagating cracks at appropriate locations. The softwares used are ABAQUS and FRANC3D, where the latter supports finite element meshing of a crack and calculation of the stress intensities along the crack front based on the results from an external finite element program. The blade is subjected to thermo-mechanical fatigue and the cracks are grown subjected to in-phase loading conditions.   The material of the blade is STAL15SX, a nickel-base single-crystal superalloy. The <001> crystalline direction is aligned with the loading direction of the blade, while the secondary crystalline directions are varied to examine how it affects the thermo-mechanical crack propagation fatigue life of the blade.   The finite element model is set up using a submodeling technique to reduce the computational time for the simulations. Investigations to validate the submodeling technique are conducted.   From the work it can be concluded that a crack located at a critical location in the cooling lattice reach above the crack propagation target life. Cracks located at noncritical locations have crack propagation lives of a factor 5.2 times the life of the critical crack.

Dans ces travaux, nous avons étudié numériquement la croissance du décollement de l'interface fibre / matrice d'un composite UD avec garnissage hexagonale de fibre sous charge longitudinal et transversal. Nous avons mis l'accent en particulier sur l'influence des fibres voisines sur sa croissance. Dans la présente étude, le taux de libération d'énergie (ERR) est considéré comme la force motrice de la croissance du décollement et a été calculé sur la base de Integral J et de la technique de fermeture virtuelle de fissures (VCCT) à l'aide du logiciel de calcul par éléments finis ANSYS. Dans la présente recherche de thèse, nous avons étudier d’abord l'influence des fibres voisines sur ERR d'une décohésion émanant d'une rupture de fibre en condition de chargement longitudinal. Dans le cas du chargement longitudinal, la croissance du décollement est gouvernée par le mode II. Comme point de départ l’étude, nous avons mis place un modèle axisymétrique composé de 5 cylindres concentriques représentant la fibre endommagée, la matrice environnante, les fibres voisines, la matrice environnante et le composite effectif généré. On constate qu'il y a deux stades de croissance, la première étape correspond à une longueur courte du décollement, l'ERR diminue à mesure que l'angle du décollement augmente, et la présence de voisins augmente significativement la décohésion de l'ERR. Pour une décohésion relativement longue, le décollement se situe dans une région de croissance en état stationnaire lorsque l'ERR est pratiquement constant quelle que soit la longueur du décollement. À l’état stationnaire de la croissance du défaut, la présence de fibres voisines n'a que peu d'effet sur l'ERR. Les travails ultérieurs, nous avons mis en place un modèle 3-D (explicite) avec la fibre endommagée et ses 6 fibres les plus proches dans un composite UD compacté hexagonal, entourées par le composite homogénéisé. Sur la base des résultats obtenus, nous avons montré que l'ERR varie le long de la face frontale et a son maximum à l'endroit circonférentiel où la distance entre deux centres de fibre est la plus petite. Cela indique que le front du décollement n’est pas circulaire. Pour l'état stable du décollement, la présence de fibres a peu d'effet sur l'ERR qui progresse le long du front du décollement. Pour un décollement court, la présence de fibres augmente l'ERRS moyenné, et cette augmentation est plus significative lorsque la distance entre fibre est la plus petite. Après l’étude du la décollement fibre / matrice en charge longitudinale, nous avons commencé à étudier la croissance du décollement fibre / matrice le long de la circonférence de la fibre sous charge transversale. On constate que la croissance de la du décollement est en mode mixte, et les composants ERR du mode I et du mode II augmentent avec l'augmentation de l'angle de déformation puis diminuent. La croissance du décollement démarre principalement en mode I pour les petits angles de décollement et se poursuit en mode II. La présence de fibres voisines a un effet d’accroissement sur la croissance du décollement jusqu'à certains petits angles et change ensuite en effet protecteur. En fin, nous avons étudié l'interaction entre deux décollement sous chargement transversale. Nous avons constaté que lorsque deux décollements sont proches l'un de l'autre, l'interaction entre devient beaucoup plus forte et conduit à l'augmentation significative de l'ERR de chaque décollement, ce qui facilite la croissance du décollement / In the presence thesis, the growth of fiber/matrix interface debond of a UD composite with hexagonal fiber packing under longitudinal and transverse tensile loading was investigated numerically, with the special focus on the influence of neighboring fibers on its growth. In the current study, energy release rate (ERR) is considered as the driving force for debond growth and was calculated based on J Integral and Virtual Crack Closure Technique (VCCT) using finite element software ANSYS. In the present thesis research, we started with investigating the influence of neighboring fibers on ERR of a debond emanating from a fiber break in longitudinal loading condition. In longitudinal loading case, debond growth is mode II dominated. As the starting point for the research, an axisymmetric model consisting 5 concentric cylinders that represent broken fiber with debond, surrounding matrix, neighboring fibers, surrounding matrix and effective composites was generated. It’s found that there are two stages of debond growth, the first stage is when debond length is short, the ERR decreases with increasing debond angle, and the presence of neighboring significantly increase the ERR of debond. For relatively long debond, the debond is in a steady state growth region when ERR is almost constant regardless of debond length. In steady state of debond growth, the presence of neighboring fibers have little effect on the ERR. In the later research, a 3-D model was generated with broken fiber and its 6 nearest fibers in a hexagonal packed UD composite were modelled explicitly, surrounded by the homogenized composite. Based on the obtained results, it’s shown that ERR is varying along debond front, and has its maximum at the circumferential location where the distance between two fiber center is the smallest. This indicates the debond front is not a circle. For steady state debond, the presence of fibers have little effect on ERR that averages along debond front. For short debond, the presence of fibers increases the averaged ERRS, and that the increase is more significant when inter-fiber distance are the smallest. When we conclude our investigation on fiber/matrix debonding under longitudinal loading, we began studying the growth of a fiber/matrix debond along fiber circumference under transverse loading. It’s found that debond growth is mixed-mode, and both mode I and mode II ERR components increase with increasing debond angle and then decreases. Debond growth is mode I dominated for small debond angle and then switch to mode II dominated. The presence of neighboring fibers have an enhancement effect on debond growth up to certain small debond angle and then changes to a protective effect. Finally, the interaction between two arc-size debond under transverse loading is investigated. It’s found that when two debonds are close to each other, the interaction between two debond becomes much stronger, and that interaction leads to the increase of ERR of each debond significantly, which facilitates further debond growth for both debond

Cortical bone tissue - one of the most intriguing materials found in nature - demonstrate some fascinating behaviours that have attracted great attention of many researchers from all over the world. In contrast to engineering materials, bone has its unique characters: it is a material that has both sufficient stiffness and toughness to provide physical support and protection to internal organs and yet adaptively balanced for its weight and functional requirements. Its structure and mechanical properties are of great importance to the physiological functioning of the body. Still, our understanding on the mechanical deformation processes of cortical bone tissue is rather limited. Penetration into a bone tissue is an intrinsic part of many clinical procedures, such as orthopaedic surgery, bone implant and repair operations. The success of bone-cutting surgery depends largely on precision of the operation and the extent of damage it causes to the surrounding tissues. The anisotropic behaviour of cortical bone acts as a distinctive protective mechanism and increases the difficulty during cutting process. A comprehensive understanding of deformation and damage mechanisms during the cutting process is necessary for improving the operational accuracy and postoperative recovery of patients. However, the current literature on experimental results provides limited information about processes in the vicinity of the cutting tool-bone interaction zone; while; numerical models cannot fully describe the material anisotropy and the effect of damage mechanisms of cortical bone tissue. In addition, a conventional finite-element scheme faces numerical challenges due to large deformation and highly localised distortion in the process zone. This PhD project is aimed at bridging the gap in current lack of understanding on cutting-induced deformation and fracture processes in the cortical bone tissue through experimental and numerical approaches. A number of experimental studies were accomplished to characterise the mechanical behaviour of bovine cortical bone tissue and to analyse deformation and damage mechanisms associated with the cutting process II along different bone axes in four anatomic cortices, namely, anterior, posterior, medial and lateral. These experiments included: (1) a Vickers hardness test to provide initial assessments on deformation and damage processes in the cortical bone tissue under a concentrated compressive load; (2) uniaxial tension and compression tests, performed to understand the effect of orientation and local variability of microstructure constituents on the macroscopic material properties of cortical bone; (3) fracture toughness tests, aimed at elucidating the anisotropic character of fracture toughness of cortical bone and its various fracture toughness mechanisms in relation to different orientations; (4) penetration tests, conducted to evaluate and validate mechanisms involved in bone cutting as well as orientation associated anisotropic deformation and damage processes at various different cortex positions. Information obtained in these experimental studies was used to assist the development of advanced finite-element models: (1) the effective homogenised XFEM models developed in conjunction with three-point bending test to represent a macroscopically, anisotropic elasticplastic fracture behaviour of cortical bone tissue; (2) three microstructured XFEM models to further investigate the effect of the randomly distributed microstructural constituents on the local fracture process and the variability of fracture toughness of cortical bone; (3) a novel finite-element modelling approach encompassing both conventional and SPH elements, incorporating anisotropic elastic-plastic material properties and progressive damage criteria to simulate large deformation and damage processes of cortical bone under penetration. The established models can adequately and accurately reflect large deformations and damage processes during the penetration in bone cutting. The results of this study made valuable contributions to our existing understanding of the mechanics of cortical bone tissue and most importantly to the understanding of its mechanical behaviours during the cutting process.

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The employment of composite materials in the aerospace industry has been gradually considered due to the fundamental lightweight and strength characteristics that these type of materials offer. The science material and technological progress that has been reached, matches perfectly with the requirements for high-performance materials in aircraft and aerospace structures, thus, the development of primary structure elements applying composite materials became something very convenient. It is extremely important to pay attention to the failure modes that influence composite materials performances, since, these failures lead to a loss of stiffness and strength of the laminate. Delamination is a failure mode present in most of the damaged structures and can be ruinous, considering that, the evolution of interlaminar defects can carry the structure to a total failure followed by its collapse. Different techniques are usually adopted to accurately predict the behavior of damaged structures but, due to the complex nature of failure phenomena, there is not an established pattern. The present research project aims to develop a delamination propagation model to estimate a progressive interlaminar delamination failure in laminated composite materials and to allow the prediction of material\'s degradation due to the delamination phenomenon. Experimental tests assisted by ASTM Standards were performed to determine material\'s parameter, like the strain energy release rate, using GFRPs laminated composites. The delamination propagation model proposed was implemented as subroutines in FORTRAN language (UMAT-User Material Subroutine) with formulations based on the Fracture Mechanics. Finally, the model was compiled beside with the commercial Finite Element program ABAQUSTM. / O emprego de materiais compósitos na indústria aeroespacial tem sido gradualmente utilizado devido às suas características fundamentais, como peso leve e alta rigidez, que este tipo de material oferece. Tanto a ciência do material como o desenvolvimento tecnológico que se tem logrado, possibilitaram que estes materiais cumprissem com os requisitos de desempenho para aplicações em estruturas aeronáuticas e aeroespaciais, por tanto, o desenvolvimento de elementos de estruturas primárias usando materiais compósitos, passou a ser muito conveniente. É de extrema importância prestar atenção aos modos de falha que comprometem a performance dos materiais compósitos, uma vez que, estas falhas levam a uma perda de resistência e rigidez do laminado. A delaminação é um modo de falha presente na maioria de estruturas danificadas e pode ser desastroso, considerando que, a evolução dos defeitos interlaminares podem levar a estrutura a falhar seguido pelo colapso estructural. Diferentes técnicas são geralmente adotadas para prever, de maneira correta, o comportamento de estruturas danificadas, porém, devido à natureza complexa do fenômeno de falha, não existe um padrão estabelecido. O presente trabalho de pesquisa visa desenvolver um modelo de delaminação e de propagação da delaminação para estimar a evolução da falha interlaminar em materiais compósitos laminados e permitir a predição do comportamento do material com a evolução da delaminação. Ensaios experimentais auxiliados por normas ASTM foram realizados para determinar parâmetros do material, tais como, as taxas de liberação de energia de deformação, usando materiais compósitos laminados de matriz polimérica reforçada com fibra de vidro. O modelo de propagação da delaminação proposto, foi implementado como uma sub-rotina em linguagem FORTRAN (UMAT – User Material) com formulações baseadas na Mecânica da Fratura. Finalmente, o modelo foi compilado com o software comercial de Elementos Finitos, ABAQUSTM.

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Orientador: Itamar Ferreira / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-13T02:49:59Z (GMT). No. of bitstreams: 1 Carnio_MarcoAntonio_D.pdf: 4129193 bytes, checksum: 27ed4b4a807f05043017033527f2d6f8 (MD5) Previous issue date: 2009 / Resumo: O concreto reforçado com fibras (CRF) é um compósito constituído de duas fases: concreto (matriz) e fibras (reforço). As fibras são elementos descontínuos de vários aspectos geométricos e materiais, destacando-se atualmente as fibras metálicas (aço) e as fibras sintéticas (polipropileno). Em geral, os teores de fibras em volume adicionados à matriz se dividem em: baixos teores - até 0,5%; médios teores - entre 0,5% e 1% e altos teores - acima de 1%. Dentre as aplicações do CRF com baixos teores, a utilização em pavimentos de concreto é a mais significativa, cuja solicitação predominante é cíclica. O estudo da fadiga se divide em iniciação da trinca, propagação da trinca e fratura final, e a vida em fadiga (Nf) é caracterizada pela soma do número de ciclos até a fratura. Dessa forma, como no estudo da fadiga do CRF a maioria dos trabalhos se concentra na obtenção de seu comportamento por meio das curvas S-N (tensão em função do número de ciclos até a fratura), este trabalho tem como objetivo estudar experimentalmente a propagação da trinca por fadiga dos CRFs com baixos teores, por meio de ensaios de flexão em corpos-de-prova prismáticos de seção quadrada, submetidos a carga cíclica com freqüência de 20 Hz e razão de carga R = 0,1, representando seu comportamento pelas curvas da/dN-_K (taxa de propagação de trinca por fadiga em função da variação do fator de intensidade de tensão), ensaios de flexão em corpos-de-prova prismáticos de seção quadrada, submetidos a carga monotônica, representando seu comportamento pelas curvas carga monotônica-deslocamento de abertura da boca da trinca (CMOD) e o estudo macroscópico da seção fraturada desses concretos, após cargas cíclicas e monotônica para avaliação dos mecanismos de fratura da seção, visando melhor entender a contribuição dessas adições nos CRFs submetidos a cargas cíclicas. Os resultados indicam que o CRF com fibras de polipropileno de 54 mm apresenta melhor desempenho à propagação de trinca por fadiga, com comportamento equivalente aos CRFs com fibras de aço de 60 mm e que, o CRF com fibras de aço de 35 mm apresenta melhor desempenho dentre todos tipos e teores de fibras estudadas. Quanto ao comportamento à carga monotônica, o desempenho dos CRFs com fibras de aço de 60 mm se mostrou melhor dentre os estudados, desempenho esse não confirmado quanto à propagação de trinca por fadiga. Também se conclui que a ancoragem das fibras interfere no comportamento dos CRFs às cargas monotônica e cíclica, sendo que a degradação da ancoragem mecânica nas fibras de aço submetidas a cargas cíclicas penaliza mais os CRFs com fibras de aço de 60 mm quanto ao desempenho à propagação de trinca por fadiga. Na avaliação da superfície de fratura dos CRFs, verificou-se que a adição de fibras interfere nessa superfície, apresentando-se mais plana para os CRFs com fibras sintéticas e mais irregular para os CRFs com fibras de aço. / Abstract: The fibers reinforced concrete (FRC) is a composite formed by two phases: concrete (matrix) and fibers (reinforce). The fibers are discontinuous elements of several geometric aspects and materials, currently detaching the metal fibers (steel) and the synthetic fibers (polypropylene). In general, the contents of fibers in volume added to the matrix are separated in: low contents - up to 0.5%; medium contents - between 0.5% and 1% and high contents - above 1%. Among the applications of the FRC with low contents, the usage in concrete pavements is the most significant, which predominant request is cyclic. The study of fatigue is divided into initialization of crack, propagation of crack and final fracture and the life in fatigue (Nf) is characterized by the sum of number of cycles up to the fracture. In this way, as in the study of fatigue of FRC the majority of the work focuses on obtaining their behavior through the S-N curves (tensile due to the number of cycles up to the fracture) this work has as objective to study experimentally the fatigue crack propagation of the low contents FRC, by means of testing the flexure in prismatic samples of square section, subjected to a cyclic load of 20 Hz frequency and load ratio R=0.1, representing their behavior through the da/dN-_K curves (rate of fatigue crack propagation in lights of the variation of the intensity of the tensile), by testing the flexure in prismatic samples of square section, subjected to a monotonic load, representing their behavior through the curves monotonic load - cracking mouth opening displacement (CMOD) and the macroscopic study of the fracture section, to better understand the contribution of these additions into the FRC subjected to a cyclic load. The results indicate that the FRC with synthetic fibers of 54 mm size shows a better performance for fatigue crack propagation, with equivalent behavior as of the FRCs with steel fibers of 60 mm size and that the FRC with steel fibers of 35 mm size shows even a better performance among all types and contents of fibers studied. About the behavior of the monotonic load, the performance of the FRCs with steel fibers of 60 mm size was better among all studied behaviors, but that performance is not confirmed regarding the fatigue crack propagation. Also concludes that the anchorage of the fibers interfere in the behavior of both FRC subjected to monotonic load and cyclic load, and the degradation of the mechanic anchorage in the steel fibers, subjected to a cyclic loads punish more the FRCs with the steel fibers of 60 mm regarding to their performance of fatigue crack propagation. Evaluating the fracture surfaces of the FRC, verify that the addition of fibers interfere on these, resulting in a more flat to the FRC with synthetic fibers and more irregular to the FRCs with steel fibers. / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica