The presented master’s thesis deals with description of short fatigue cracks and can be divided to the several parts. In the first part, theoretical background of fatigue of materials and fracture mechanics parameters which can describe fatigue cracks is presented. Following part describes how to determine fracture mechanics parameters by finite elements method. Due to the shape of propagated fatigue crack, numerical model was established as a three-dimensional. Results obtained based on this model, was compared with literature with very good agreement. Than real shape of propagated short crack was used for description of the experimental data. It was shown, that plastic part of J-integral is suitable parameter for description of the short cracks.

The main goal of this diploma thesis is the examination of the influence of structure directionality on fatigue properties of formed aluminium alloy 6082/T6. The main attention is focused on the study of the influence of structure directionality on kinetics of short fatigue cracks growth. The measurement of short fatigue cracks growth was performed on cylindrical samples. The samples were made in two different directions; one parallel with the forming direction and second perpendicular to the forming direction. Servo hydraulic machine MTS 880 was used for the cyclic loading. The samples were cycled at two different constant stress amplitudes. Cyclic loading was systematically interrupted in order to measure the length of short cracks by a light microscope. Tensile tests, measuring of cycling hardening-softening curves, observation of microstructure, observation of surface relief, measuring of microhardness and fractographical analysis of fracture surfaces were used for further examination of the influence of the structure directionality. Some of these measured characteristics did not show any influence of the structure directionality (microhardness, fatigue life curve, Young modulus). In other cases is this influence measurable, however insignificant (yield stress, ultimate stress, cyclic hardening-softening curves and kinetics of short fatigue cracks growth). It can be summarised that the material, though the directionality of its microstructure is apparent, shows relatively isotropic mechanical behaviour.

This Ph.D. thesis was written under the supervision of Assoc. prof. Pavel Hutař, Ph.D., and Assoc. prof. Luboš Náhlík, Ph.D. The thesis is focused on the effect of plasticity induced crack closure, its characteristic and ways of numerical modelling. Premature fatigue crack closure has a significant effect on the fatigue crack propagation rate and therefore on the residual lifetime of a structure. A three-dimensional numerical model allows a detailed look at the stress and strain distribution along the crack front, and particularly it allows a local description of parameters along the crack front which governs the fatigue crack propagation rate. In the first part of the thesis, the study is focused on the influence of a singular stress field at the vicinity of the free surface on the crack front curvature without crack closure being involved. In the second part, a numerical model in 2D of plasticity induced crack closure was created and verified by experimental results. In the final part, a 3D numerical model is used to describe the influence of the crack closure on its fatigue propagation rate and explains typical crack front curvature. The suggested technique allows quantitative accuracy improvement of numerical simulation of the fatigue crack propagation and therefore, more reliable estimation of the residual lifetime of the cracked structure.

As vigas celulares ocupam um lugar de destaque crescente na construção de pontes em concreto protendido. Dentro desta opção, a tendência moderna é de se construírem pontes unicelulares cada vez mais largas, o que traz como conseqüência um aumento considerável da flexão transversal em seus elementos, especialmente em suas almas, submetendo-as a uma combinação de cisalhamento com flexão transversal que pode atingir valores importantes. Este trabalho tem por finalidade apresentar um novo critério de dimensionamento das almas das vigas de seção celular, incluindo o caso do estado limite último de fadiga. Este critério foi idealizado a partir de uma análise crítica dos modelos vigentes, os quais são analisados e comparados por meio de gráficos de interação relacionando força cortante com flexão transversal, que permitem a escolha da melhor opção para as situações de projeto. Desenvolveu-se uma investigação experimental, a fim de verificar a validade do critério de dimensionamento desenvolvido. Foram analisados os seguintes modos de colapso: esmagamento das bielas comprimidas de concreto, alongamento plástico excessivo dos estribos e ruptura dos estribos por fadiga. Os resultados experimentais mostraram uma boa aproximação do Critério de Dimensionamento Proposto e revelaram novidades nos ensaios de fadiga: a ruptura dos estribos por fadiga se deu por etapas, um estribo de cada vez, num processo gradual. A ruptura por fadiga ocorreu sistematicamente próximo à ligação da alma com a mesa inferior e não no dobramento dos estribos. / Box-girders have received a growing attention in the field of prestressed concrete bridges. The modern trend is to build wider unicellular bridges, which leads to a considerable increase in the transverse bending moment acting mainly in their webs. These are subjected to a combination of shear force and transverse bending moments, which may reach important values. The purpose of this thesis is to introduce a new design approach of box-girder webs, including the Ultimate Limit State due to fatigue. This design approach is derived from a critical analysis of the current criteria. The different criteria for the design of box-girder webs are analyzed and compared by means of shear-bending moment interaction diagrams as an attempt to identify the more realistic one. An experimental investigation has been undertaken with the purpose of verifying the validity of the new developed approach. The following failure modes have been considered: crushing of the compressed struts, excessive plastic deformation of the stirrups and rupture of the stirrups due to fatigue. The experimental results have shown good agreement with those predicted by the proposed approach. Furthermore, the tests have revealed new aspects of the fatigue behavior: the rupture of the stirrups due to fatigue occurred in stages, one at a time in a gradual manner. In all cases the failure took place near the top face of the lower flange and not at the corner of the stirrups.

As vigas celulares ocupam um lugar de destaque crescente na construção de pontes em concreto protendido. Dentro desta opção, a tendência moderna é de se construírem pontes unicelulares cada vez mais largas, o que traz como conseqüência um aumento considerável da flexão transversal em seus elementos, especialmente em suas almas, submetendo-as a uma combinação de cisalhamento com flexão transversal que pode atingir valores importantes. Este trabalho tem por finalidade apresentar um novo critério de dimensionamento das almas das vigas de seção celular, incluindo o caso do estado limite último de fadiga. Este critério foi idealizado a partir de uma análise crítica dos modelos vigentes, os quais são analisados e comparados por meio de gráficos de interação relacionando força cortante com flexão transversal, que permitem a escolha da melhor opção para as situações de projeto. Desenvolveu-se uma investigação experimental, a fim de verificar a validade do critério de dimensionamento desenvolvido. Foram analisados os seguintes modos de colapso: esmagamento das bielas comprimidas de concreto, alongamento plástico excessivo dos estribos e ruptura dos estribos por fadiga. Os resultados experimentais mostraram uma boa aproximação do Critério de Dimensionamento Proposto e revelaram novidades nos ensaios de fadiga: a ruptura dos estribos por fadiga se deu por etapas, um estribo de cada vez, num processo gradual. A ruptura por fadiga ocorreu sistematicamente próximo à ligação da alma com a mesa inferior e não no dobramento dos estribos. / Box-girders have received a growing attention in the field of prestressed concrete bridges. The modern trend is to build wider unicellular bridges, which leads to a considerable increase in the transverse bending moment acting mainly in their webs. These are subjected to a combination of shear force and transverse bending moments, which may reach important values. The purpose of this thesis is to introduce a new design approach of box-girder webs, including the Ultimate Limit State due to fatigue. This design approach is derived from a critical analysis of the current criteria. The different criteria for the design of box-girder webs are analyzed and compared by means of shear-bending moment interaction diagrams as an attempt to identify the more realistic one. An experimental investigation has been undertaken with the purpose of verifying the validity of the new developed approach. The following failure modes have been considered: crushing of the compressed struts, excessive plastic deformation of the stirrups and rupture of the stirrups due to fatigue. The experimental results have shown good agreement with those predicted by the proposed approach. Furthermore, the tests have revealed new aspects of the fatigue behavior: the rupture of the stirrups due to fatigue occurred in stages, one at a time in a gradual manner. In all cases the failure took place near the top face of the lower flange and not at the corner of the stirrups.

O Jateamento com granalha (shot peening, em inglês) é um processo de fabricação amplamente utilizado em indústrias mecânicas, automotivas, navais, aeronáuticas e outras indústrias metalúrgicas. O objetivo principal é induzir tensões residuais de compressão para melhorar a vida à fadiga das peças e estruturas. A modelagem e simulação do processo são muito difíceis, pois envolvem a consideração de muitos e complexos aspectos. Este trabalho propõe um método de modelagem tridimensional utilizando o método dos elementos finitos para a simulação numérica desse processo assim como a sua influência na propagação de trinca, considerando efeitos pouco estudados, como o amolecimento do material devido ao aumento da temperatura na peça durante os impactos, ao comportamento elasto-plástico das granalhas e a rugosidade da peça tratada nos parâmetros da trinca. A modelagem proposta para simular o processo de jateamento com granalhas inclui: (i) análise dinâmica explícita, (ii) modelagem tridimensional, (iii) cálculo da velocidade real das granalhas, (iv) modelo de contato com coeficientes de atrito estático e dinâmico entre as áreas em contato, (v) o modelo constitutivo de Johnson-Cook para o comportamento do material alvo, (vi) um comportamento elasto-plástico multilinear para as granalhas, (vii) a consideração da cobertura real de uma área dada da superfície jateada, (viii) a consideração do aumento da temperatura no material durante o processo e (ix) o relaxamento de tensões residuais devido a este aumento de temperatura. As simulações numérico-computacionais para investigar a influência do processo em fenômenos, tais como fadiga e propagação de trinca, necessitou uma nova abordagem para a incorporação das tensões residuais num modelo de elementos finitos. Além disso, foram realizados vários ensaios quase-estáticos e dinâmicos para a determinação dos parâmetros do modelo constitutivo de Johnson-Cook para o aço AISI 5160. Para a avaliação das metodologias propostas, as tensões residuais foram introduzidas e avaliadas em uma mola parabólica automotiva, sendo que as tensões residuais foram avaliadas através da técnica de difração de raios-X, a intensidade e a cobertura do processo de jateamento com granalha na mola foram avaliadas com Lupa e rugosímetro. Também, dados experimentais de ensaios de fadiga relatados na literatura, realizados em corpos de prova tipo CT foram utilizados. A modelagem numérica do processo de jateamento com granalha desenvolvida neste trabalho consegue prever com sucesso os perfis das tensões residuais, com valores bem próximos aos obtidos experimentalmente. A investigação numérica do efeito do processo na vida à fadiga e fratura das peças comprovou a sua influência nos parâmetros de propagação de trinca nas regiões afetadas pelo processo, ocasionando um amplo aumento da vida à fadiga, e confirmou o relaxamento das tensões residuais devido aos ciclos de carregamento conforme as observações experimentais na literatura. / Shot peening is a manufacturing process widely used in the mechanical, automotive, marine, aeronautical and other metallurgical industries. The main purpose is to induce residual compression stresses to improve fatigue life. The modeling and simulation of the process are very difficult as they involve the consideration of many and complex aspects. This work proposes a three-dimensional modelling method using the finite element method for the numerical simulation of this process as well as its influence on crack propagation, considering little studied effects such as softening of the material due to the increase in temperature in the part during impacts , the elastic-plastic behavior of the shot and the roughness of the treated part in the crack parameters. The proposed modelling approach to simulate the shot peening process includes: (i) explicit dynamic analysis, (ii) three-dimensional modeling, (iii) calculation of the real velocity of the shot, (iv) contact model with static and dynamic friction coefficients between the contact areas, (v) the Johnson-Cook constitutive model for target material behavior, (vi) a multilinear elasto-plastic behavior for the shot, (vii) consideration of real coverage in a given area of the peened surface, (viii) consideration of temperature increase in the material during the process and (ix) relaxation of residual stresses due to this rise in temperature. Numerical-computational simulations to investigate the influence of the shot peening process on phenomena, such as fatigue and crack propagation, required a new approach for the incorporation of residual stress in a finite element model. In addition, several quasi-static and dynamic tests were carried out to determine the parameters of the constitutive Johnson-Cook model for AISI 5160 steel. To assess the proposed methodologies, the residual stresses were introduced and evaluated in an automotive parabolic spring, with the residual stresses being evaluated by the X-ray diffraction technique, the intensity and coverage of the shot peening process for the spring were evaluated with magnifying glass and roughness gauge. Also, fatigue tests available in the literature, carried out on types of CT specimens, were used. The numerical modeling of the shot peening process developed in this work can successfully predict the residual stress profiles, with values close to those obtained experimentally. The numerical investigation of the effect of the process in the fatigue life and fracture of the pieces proved its influence in the parameters of crack propagation in the regions affected by the process, causing a wide increase of the fatigue life, and confirmed the relaxation of the residual stresses due to the cycles loading according to experimental observations in the literature.