[pt] Após identificar que uma trinca de fadiga permanecia fechada durante parte do ciclo, Elber assumiu que o dano era induzido apenas pela fração do carregamento acima da carga necessária para abrir a trinca. Diversos modelos foram propostos utilizando o Delta Keff como força motriz da propagação, como os modelos da faixa plástica (strip-yield), que são amplamente utilizados para prever vida residual de componentes trincados. Embora o fenômeno do fechamento da trinca esteja provado, sua real importância na propagação da trinca de fadiga ainda é controversa. Outros mecanismos, além do fechamento da trinca, foram utilizados na tentativa de explicar os efeitos de sequência do carregamento na propagação em amplitude variável como o campo de tensão residual à frente da trinca. Mesmo após mais de 50 anos de pesquisas desde a proposição da primeira regra de propagação por Paris ainda não há consenso nem sobre o mecanismo nem sobre a modelagem. Esse trabalho tem como objetivo apresentar uma modelagem para prever propagação da trinca de fadiga com base na hipótese de que o dano acumulado por deformação plástica seria a força motriz para propagação. A modelagem proposta se diferença de outros modelos de acúmulo de dano por permitir que o contato existente entre as superfícies da trinca exerça influência sobre as deformações plástica à frente de sua ponta. Os resultados mostram que a modelagem proposta possui capacidade de reproduzir curvas de propagação semelhante ao modelo strip-yield. / [en] After identify that a fatigue crack remains closed during part of the load cycle, Elber assumed the damage was induced only by the cycle part over the load required to open the crack. Several models were developed based on Delta Keff as the strip-yield ones, which are widely used to predict residual lives of cracked components. Although the crack closure phenomenon is well proven its actual significance for the propagation is still controversial. Others mechanisms, beyond the crack closure, were used in trying to explain the sequence effects on variable amplitude crack propagation like the residual stress field ahead of the crack tip. However even after more than 50 years of research since the first propagation rule proposed by Paris there is no neither about the mechanism neither about modelling. This work has the aim of present a modelling to predict fatigue crack growth based on the hypothesis that the damage accumulated by cyclic plastic strain would be propagation the drive force. The modelling proposed differs from others damage accumulation models by allowing the existed contact between the crack surfaces to exercise its influence on plastic strain ahead of the crack tip. The results show that the proposed model is able to reproduce propagation curves similar to the model strip-yield.

Matériaux incontournables des matériaux fonctionnels, les alliages à mémoire de forme(AMF) peuvent présenter de très larges déformations réversibles. La Transformation de Phase Martensitique (TPM), ayant lieu lorsqu’il est soumis à une action mécanique ou thermique, lui confère des caractéristiques particulières. Le comportement thermomécanique des AMF est à présent bien maîtrisé. Cependant la connaissance de leur comportement `a la rupture reste un enjeu majeur pour leur dimensionnement dans le cadre de leur industrialisation pérenne. Ces travaux de recherche se sont attachés `a la connaissance, la description et la quantification du phénomène de localisation en pointe de fissure liée à la TPM induite sous contrainte, au travers du développement d’un modèle prédictif et de sa corrélation expérimentale par mesures de champs simultanées lors d’essais de rupture sur des éprouvettes fissurées de NiTi. Deux modèles analytiques basés sur la mécanique linéaire de la rupture, intégrant le caractère dissymétrique du comportement des AMF en traction/compression, ont été développés pour la prédiction des zones de transformation au voisinage de la pointe de fissure en tenant compte des différents modes de rupture ( élémentaires et mixtes I+II) et du rayon de courbure en pointe de fissure. Un banc de caractérisation par mesures simultanées de champs cinématiques par corrélation d’images (DIC) et thermique par thermographie infrarouge a été développé pour cartographier les champs expérimentaux d’essais de rupture en mode I sur des éprouvettes pré-fissurées. Cette bonne corrélation des modèles analytiques ouvre de nombreuses perspectives concernant l’analyse du couplage thermo mécanique associé à la TPM en pointe de fissure, l’enrichissement des modèles analytiques initiaux, et la confrontation avec les résultats expérimentaux pour des modes de rupture plus complexes (II et mixte I+II). / Major player among functional materials, Shape Memory Alloys (SMA) may undergo verylarge reversible strain. SMA exhibit a Martensitic Phase Transformation (MPT) when they aresubmitted to mechanical or thermal actions, and that gives them some specific characteristics.The thermomechanical behavior of SMA is now well controlled. However, the knowledge of theSMA fracture behavior is a major challenge for their design and sizing for their sustainableindustrialization. This research project has focused on the understanding, describing and quantifyingof the phenomenon of localization at the crack tip due to stress-induced MPT. The study includestwo main aspects: the development of an analytical model and its experimental correlation bysimultaneous field’s measurements during tests on cracked NiTi specimens. Two analytical modelsbased on the linear fracture mechanics and those introduce the asymmetrical nature of the SMAbehavior in tension/compression, were developed for the prediction of transformation zones in thevicinity of the crack tip, taking into account the fracture mode (elementary and mixed ones)and the radii of curvature of the crack tip. A testbench with the measurement of simultaneouskinematic field with Digital Image Correlation (DIC) and thermal field with infrared thermographywas designed for mapping the experimental fields during fracture tests in mode I on pre-crackedspecimen. This good correlation of analytical models opens up many perspectives on the analysisof thermomechanical coupling associated with the MPT at the crack tip, the enrichment of the initialanalytical models, and comparison with experimental results for more complex failure modes (II andmixed I+II).

The main goal of this work is the comparison of the size of the plastic zone at the crack tip for two analysis methods: an analytical linear method and an elastic-plastic analysis employing the Finite Element method (ANSYS software). All calculations were made for a crack loaded under pure shear modes. These types of loading are not sufficiently described in the literature. The first part of this work introduces the problem with the crack tip plastic zone using both linear and nonlinear fracture parameters. The second part is dedicated to the construction of the Finite Element model in the ANSYS software. The geometry of the samples and the loading levels were chosen to match an existing experimental test of the impact of shear modes on the crack behavior. In the third part of this thesis, the plastic zone radii for pure shear modes II and III are estimated using several methods and the results are compared. In the last part of this work, the same procedure as in the previous part is applied on a mixed-mode II+III loading. A result of this thesis is the assessment of the application limits of the linear analysis method used to estimate the size of the plastic zone at the crack tip for a specific geometry and material model.

This thesis is on fatigue crack growth experiments and assessments of fatigue crack growth rates. Both constant and variable amplitude loads in two different materials are considered; a nickel based super-alloy Inconel 718 and a stainless steel 316L. The considered load levels extend from small scale yielding (SSY) to large scale yielding (LSY) for both materials. The effect of different load schemes on the fatigue crack growth rates is investigated on Inconel 718 and compact tension specimens in Paper A. It is concluded that load decreasing schemes give a to high Paris law exponent compared to constant or increasing load amplitude schemes. Inconel 718 is further analyzed in Paper B where growth rates at variable amplitude loading in notched tensile specimens are assessed. The predictions are based on the fatigue crack growth parameters obtained in Paper A. The crack closure levels are taken into consideration and it is concluded that linear elastic fracture mechanics is incapable of predicting the growth rates in notches that experience large plastic cyclic strains. Even if crack closure free fatigue parameters are used and residual stresses due to plasticity are included. It is also concluded that crack closure free and nominal fatigue crack growth data predict the growth rates equally well. However, if the crack closure free parameters are used, then it is possible to make a statement in advance on the prediction in relation to the experimental outcome. This is not possible with nominal fatigue crack growth parameters. The last three papers consider fatigue crack growth in stainless steel 316L. Here the load is defined as the crack tip opening displacement parameter. Paper C constitutes an investigation on the effect of plastic deformation on the potential drop and consequently the measured crack length. It is concluded that the nominal calibration equation obtained in the undeformed geometry can be used at large plastic deformations. However, two conditions must be met: the reference potential must be taken in the deformed geometry and the reference potential needs to be adjusted at every major change of plastic deformation. The potential drop technique is further used in Paper D and Paper E for crack length measurements at monotonic LSY. Constant amplitude loads are considered in Paper D and two different variable amplitude block loads are investigated in Paper E. The crack tip opening displacement is concluded in Paper D to be an objective parameter able to characterize the load state in two different geometries and at the present load levels. Furthermore, if the crack tip opening displacement is controlled in an experiment and the local load ratio set to zero, then only monotonic LSY will appear due to extensive isotropic hardening, i.e. elastic shake-down. This is also the reason why the linear elastic stress-intensity factor successfully could merge all growth rates, extending from SSY to monotonic LSY along a single line in a Paris law type of diagram, even though the generally accepted criteria for SSY is never fulfilled. For the variable amplitude loads investigated in Paper E, the effect of plastic deformation on measured potential drop is more pronounced. However, also here both the crack tip opening displacement parameter and the linear elastic stress-intensity factor successfully characterized the load state. / <p>QC 20130108</p>

This diploma thesis deals with the different possible calculations of crack path. Specifically, it focuses on criteria based on maximum tangential stress, minimal strain energy density, crack tip displacement, and local symmetry. These criteria are used for calculations in ANSYS software to estimate possible crack paths on four simple structures. The thesis also contains the codes created in ANSYS. Using these, the crack trajectory of a given structure can be calculated by any of the four criteria described.

The presented dissertation thesis is focused, as the title suggests, on the analysis of stress state aspects of quasi-brittle fracture. That means the fracture of composite materials with cement matrix (such as concrete, mortar, plaster, etc.), ceramics and other composites. Used methods are based on the theory of multi-parameter linear elastic fracture mechanics, which highlights the importance of considering of several initial terms of Williams power series, approximating the stress and displacement fields in a cracked body, within conducted fracture analyses. Determination of values of coefficients of terms of this series, recalculated into the shape functions serving in most of the conducted stress state analyses, is performed via the so called over-deterministic method. Another tool for the problem solving is nonlinear fracture mechanics, represented primarily by the cohesive crack model, namely the crack band model implemented in the used ATENA software. For the backward reconstruction of stress field in the cracked bodies the application ReFraPro is used. The analytical part deals with various aspects of wedge-splitting test – from the boundary conditions, though various possibilities of nodal selection (required as input variables for the over-deterministic method) up to the advanced (automated) analysis of numerical model. Special chapter includes atypical test specimens designed for adjusting of various levels of constraint of stress and deformation at the propagating crack tip. The study of this geometry and also the subsequent detail analysis reveals important information for real experiments. Backward reconstruction of stress field presents analysis on suitable possibilities of nodal selections as inputs into the procedure of approximation of the crack tip fields and answers the question of the necessity of application of the multi-parameter linear elastic fracture mechanics for certain fracture analyses of specimens from quasi-brittle materials. The th