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Made available in DSpace on 2014-06-11T19:24:47Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-01-20Bitstream added on 2014-06-13T20:32:14Z : No. of bitstreams: 1 calcada_ftb_me_guara.pdf: 2051959 bytes, checksum: ca1589717e6afa4ab2801767fb3f5869 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / No presente trabalho foi feita a analise quantitativa das superfícies de fratura obtidas através da propagação de trinca em fadiga. As trincas em fadiga são o modo de falha que ocorre por uma carga cíclica e repetida aplicada a um corpo de prova, o que irá causar a nucleação, propagação da trinca até a sua ruptura. Utilizou-se a ferramente da dimensão fractal para analisarmos correlações com a propagação de trincas. Os materiais que foram ensaiados são: liga de alumínio AA7475 e aço 300M, ambos materiais de uso aeronáutico. Foram realizados ensaios de propagação de trinca em fadiga em corpos de prova do tipo C(T) para a obtenção da superfície de fratura para análise. Posteriormente as superfícies foram analisadas em microscopio óptico e em microscópio eletrônico de varredura. Foi realizada a medição da dimensão monofractal (DF), quando a superfície é descrita por um único valor, também os valores de dimensão textural (DT), quando temos macroescalas e a dimensão estrutural (DS), quando temos microescalas. Com os resultados obtidos podemos analisar que o DS é quem melhor representa o comportamento das superfícies de fratura, já que este indica os micromecanismos de fratura presentes. Os valores de DS são menos dispersos na identificação das regiões de fadiga (pré-trinca, propagação estável e propagação instável da trinca),possibilitando a comparação com as imagens obtidas de cada etapa do mecanismo de fratura. Os valores de DT são melhores representados pelas reconstruções em microscópio óptico, pois estas permitem descrição mais precisa da topografia do material, considerando que o DT é característico em macroescalas. Já os valores de DS são melhores descritos em análises de microscopia eletrônica de varredura, já que esta apresenta melhor definição para a observação... / In this paper we make quantitative analysis of fracture surfaces obtained through from fatigue crack propagation. The fatigue cracks are in the mode of failure that occurs for a repetead cycling loading and applied to a specimen, which will cause the nucleation, crack propagation until rupture. We will use the tool of fractal dimension to analyze correlations with the crack propagation. The materials that were tasted are: AA7475 aluminum alloy and 300M steel, both materials for aeronautical use. Tested were performed in fatigue crack propagation inspecimens of type C(T) to obtain the fracture surface for analysis. Subsequently the surfaces were examines in optical microscope and scanning electron microscope. Was performed to measure the monofractal dimension (DF), when the surface is described by a single value, so the textural dimension values (DT), and when we involved macroescale and finally structural dimension (DS) when was presente microscale. With the results we can analyze the DS is who represents the best value for the fractal fracture surface, as this indicates the micromechanics of fracture surfaces. The DS values are less dispersed in the identification of regions of fatigue (pre-crack propagation stable and unstable crack propagation), allowing comparison with the images obtained from each step of the mechanism of fracture. The values of DT are better represented when we do analysis in an optical microscope, because it a achieves the best results of the topography of the material, whereas the DT is characteristic macroscale. The values of DS are best described in the analysis of scanning electron microscopy, since it show better the micromechanics of fracture. So we can say the the DS would be the best representative of the fractal dimension to the analysis of micromechanics of fracture surfaces to their good representation at the microscale

An alternative point of view with regard to understanding the mechanism of energy transfer involved to create new surface is considered in this study. A combination of transport equation and cohesive element is presented. A practical demonstration in 1-D is presented to simulate the mechanism of energy transfer in a damage zone model for both elastic and elastic-plastic materials. The combination of transport and cohesion element shows the extent elastic energy plays to supply the energy required for crack growth. Meanwhile, plastic energy dissipation for an elastic-plastic material is shown to be well described by the transport approach. The cohesive zone model is one of many alternative approaches used to simulate fatigue crack growth. The model incorporates a relationship between cohesive traction and separation in the zone ahead of a crack tip. The model introduces irreversibility into the constitutive relationships by means of damage accumulation with cyclic loading. The traction-separation relationship underpinning the cohesive zone model is not required to follow a predetermined path, but is dependent on irreversibility introduced by decreasing a critical cohesive traction parameter. The approach can simulate fatigue crack growth without the need for re-meshing and caters for constant amplitude loading and single overloading. This study shows the retardation phenomenon occurring in elastic plastic-materials due to single overloading. Plastic materials can generate a significant plastic zone at the crack which is shown to be well captured by the cohesive zone model approach. In a cohesive zone model, fatigue crack growth involves the dissipation of separation energy released per cycle. The crack advance is defined by the total energy separation dissipated term equal to the critical energy release rate or toughness. The effect of varying toughness with the assumption that the critical traction remains fixed is investigated here. This study reveals that varying toughness does not significantly affect the stress distribution along the crack path. However, plastic energy dissipation can significantly increase with toughness. A new methodology called the fast-track method is introduced to accelerate the simulation of fatigue crack growth. The method adopts an artificial material toughness. The basic idea of the proposed method is to decrease the number of cycle for computation by reducing the toughness. By establishing a functional relationship between the number of cycles and variable artificial toughness, the real number of cycles can be predicted. The proposed method is shown to be an excellent agreement with the numerical results for both constant amplitude loading and single overloading. A new approach to predict fatigue crack growth curves is presented. The approach combines the fast-track method and an extrapolation methodology. The basic concept is to establish a function relationship using the curve fitting technique applied to data obtained from preliminary calculation of fast-track methodology. It is shown in this thesis that the new methodology provides excellent agreement with an empirical model. The methodology is limited to constant amplitude loading and small scale yielding conditions. It is shown in the thesis that fatigue crack growth curves for variable amplitude loading can be predicted by using the data set for fatigue crack growth rate for constant amplitude loading. A retardation parameter can be deduced from the number of cycles delayed using the cohesive zone model. The retardation parameter is established by performing calculation for different toughness. This methodology is shown to give good agreement with results from empirical models for different variable amplitude loading conditions.

The effect of surface finish on fatigue limit of two types of austenitic stainless steels (AISI 304L and AISI 316L) has been investigated. Fatigue specimens having two different surface conditions were obtained by changing the final cutting condition; annealing was performed to separate the residual stress effects from surface roughness. Electropolished samples were tested as a reference for each material. A generic mechanistic model for short fatigue crack propagation proposed by Navarroand Rios (N-R model) was implemented to assess its suitability for predicting the fatigue behaviour of specimens with various controlled surface conditions, obtained by machining. The surface/material properties required to implement this model were obtained by electron backscatter diffraction (EBSD), surface profilometry, hardness testing and X-ray diffraction residual stress measurement. The fatigue limits were determined using rotating-bending by means of the staircase method. The fatigue limits predicted by the N-R fatigue model were compared with the results of the fatigue tests. There was no agreement between the prediction and observations, indicating that the original form of the N-R model is not appropriate for austenitic stainless steels. In AISI 304L, the surface residual stresses are the dominant parameter, allowing prediction of the effects of machining on fatigue resistance while, the surface roughness developed by machining has no significant effect. In AISI 316L, the effect of surface roughness is found to be negligible, with a weaker effect of surface residual stress than has been observed for AISI 304L. Crack nuclei in run-out (>107 cycles) fatigue tests were observed to arrest at twins and martensite packets, developed by fatigue in AISI 316L and AISI 304L, respectively. Good agreement with experiments was achieved by using a modification to the fatigue model, which takes account of the observed effect of the plastic deformation on the microstructure.

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Crack propagation in structures when subjected to fatigue loading, follows three different phases namely - short crack growth, stable crack growth and unstable crack growth. Accurate fatigue life prediction demands the consideration of every crack propagation phase rather than only the stable crack growth stage. Further, the use of existing crack growth laws in structures with small cracks under-predicts the growth rate compared to experimentally observed ones, thereby leading to an unsafe design and keeping the structure in a potentially dangerous state. In the present work, an attempt is made to establish fatigue crack propagation laws for plain concrete, reinforced concrete and concrete-concrete jointed interfaces from first principles using the concepts of dimensional analysis and self-similarity. Different crack growth laws are proposed to understand the behavior in each of the three regimes of the fatigue crack growth curve. Important crack growth characterizing material and geometrical parameters for each zone are included in the proposed analytical models. In real life applications to structures, the amplitude of cyclic loading rarely remains constant and is subjected to a wide spectrum of load amplitudes. Furthermore, the crack growth behavior changes in the presence of high amplitude load spikes within a constant amplitude history and this is incorporated in the model formulation. Using scaling laws, an improved understanding of the scaling behavior on different parameters is achieved. The models describing different regimes of crack propagation are finally unified to obtain the entire crack growth curve and compute the total fatigue life. In addition, crack growth analysis is performed for a reinforced concrete member by modifying the model derived for plain concrete in the Paris regime. Energy dissipation occurring due to shake-down phenomenon in steel reinforcement is addressed. The bond-slip mechanism which is of serious concern in reinforced concrete members is included in the study and a method is proposed for the prediction of residual moment carrying capacity as a function of relative crack depth. The application of the proposed analytical model in the computation of fatigue crack growth is demonstrated on three practical problems – beam in flexure, concrete arch bridge and a patch repaired beam. Through a sensitivity study, the influence of different parameters on the crack growth behavior is highlighted.