<p>QC 20110523</p>

Fatigue in concrete is a complex phenomenon involving formation of microcracks, their coalescence into major crack and simultaneous formation of the fracture process zone ahead of the crack tip. Complex phenomena are best dealt through an energy approach and hence it is reasonable to use the theory of thermodynamics. Fracture mechanics and damage mechanics are two theories that are based on physically sound principles and are used to describe failure processes in materials. The former deals with the study of macroscopic cracks, whereas the latter defines the state of microcracking. In this study, the concepts from these theories are utilized to improve our understanding and modeling of fatigue process in concrete. In this thesis, a closed form expression for the thermodynamic function entropy is proposed and examined for its size independency and its use as a material property to characterize failure of concrete under fatigue. In the thermodynamic formalism, dissipative phenomena are described by a dissipation potential or its dual, from which evolution laws for internal variables could be defined. In this work, closed form expressions for dual of dissipation potential are derived using concepts of dimensional analysis and self-similarity within the framework of fracture mechanics and damage mechanics. Consequently, a fatigue crack propagation law and a fatigue damage evolution law are proposed respectively. A method is proposed in this study to correlate fracture mechanics and damage mechanics theories by equating the potentials obtained in each theory. Through this equivalence, a crack could be transformed into an equivalent damage zone and vice versa. Also, damage state corresponding to a given crack in a member can be quantified in terms of a damage index. An analytical way of computing size independent S-N curves is proposed, using a nonlocal damage theory by including aggregate size and specimen size in the formulation. It is realized from this study that fracture mechanics and damage mechanics theories should be used in a unified manner in order to accurately model the process of fatigue in concrete. Furthermore, based on the models developed in this study, several damage indicators for fatigue of concrete are proposed. The advantages and limitations of each of these indices are presented such that, the relevant damage index could be used, based on available parameters. Additionally, deterministic sensitivity studies are carried out to determine the most important parameters influencing fatigue life of a concrete member.

A análise da vida em fadiga em amplitude variável associada à corrosão é bastante complexa, devido a combinações entre as interações eletroquímicas ocorridas, tendo em vista a exposição da frente da trinca à névoa salina somada aos efeitos deletérios do carregamento de amplitude variável. Esta combinação de efeitos ainda não é modelada matematicamente, e os dados experimentais observados em outros trabalhos ainda geram hipóteses inconclusivas. Neste contexto, este estudo tem como objetivo avaliar a vida em fadiga da liga 7475 T7351, utilizando carregamento de voos simulados TWIST e FALSTAFF e submetida a névoa salina a 3,5 e 5%. Foram executados ensaios eletroquímicos, para verificar o potencial de corrosão dessa liga e análise microestrutural para verificar a formação de pites. O crescimento de trinca por fadiga com carregamento de voo foi simulado, sem considerar os efeitos do meio, nos programas NASGRO 4.0, AFGROW e CRACK 2000, com parâmetros calibrados no NASGRO 4.0, no intuito de verificar quais modelos matemáticos e programas descrevem a curva experimental em ar com menor erro. Os resultados mostraram que os programas NASGRO 4.0 e AFGROW se aproximaram com menor erro à curva experimental em ar. Os ensaios de crescimento de trinca por fadiga foram executados com carregamento de voos simulados e os parâmetros para o sistema de geração de névoa salina foram determinados experimentalmente. As curvas de crescimento de trinca e taxa de propagação em névoa salina foram comparadas às curvas em ar, as quais também foram obtidas sob as mesmas condições de carregamento. Observou-se que para os ensaios em névoa salina, a trinca rompeu com maior número de voos em relação às curvas em ar. / The corrosion fatigue life analysis under variable amplitude loading is very complex due electrochemistry effects in crack tip exposed to salt spray matched to harmful effects of amplitude variable loading. This effects combination is not modeling mathematically actually, and the experimentally data observed in other research generates inconclusive theories. In this context, the main aim of this research to evaluate the corrosion fatigue life of 7475 T7351 under spectrum loading TWIST and FALSTAFF and exposed to 3.5% and 5% NaCl. Electrochemistry test were performed to verify the corrosion potential followed of microstructural analysis by SEM, to verify the pitting on 7475 T7351 alloy. The fatigue crack growth was simulated, without environmental effects, using the codes NASGRO, AFGROW and CRACK 2000, the simulation parameters was fit on NASGRO 4.0, to analyze whether the crack growth models and codes are able to represent properly the air experimental crack growth data. The results showed that NASGRO 4.0 and AFGROW codes described the fatigue crack growth in air with less error. The fatigue crack growth tests were performed under spectrum loadings and the parameters to obtain salt spray were determined experimentally. The corrosion fatigue crack growth and rate data were compared to air data, under the same loading conditions. The corrosion fatigue tests showed that the specimens cracked under a larger number of flights than specimens in air.

Fatigue damage of aluminium alloys is one of the key concerns in transport industries, particularly in the aerospace industry. The purpose of the project is to develop new knowledge and techniques against fatigue failure for these industries through a systematic investigation of fatigue resistance and crack growth behaviours of aluminium alloys. Fatigue and fracture mechanics have been investigated analytically, numerically and experimentally in this project. Overload transient effect on fatigue crack growth has been examined by considering various parameters including crack closure, overload ratio (OLR), load ratio (R ratio), baseline stress intensity factor range, (∆K)_BL and geometry. It was found that crack closure can be correlated qualitatively and quantitatively to all other parameters associated with overload transient behaviour. It is proposed that the effect of crack tip plasticity on the non-linearity of the compliance curve can be separated to obtain reliable crack closure measurement. In this project, different methods are used to better understand the transient retardation process so that the damage tolerance design (DTD) of the components made of aluminium alloys can be enhanced. Another important parameter for fatigue and damage tolerance design (DTD) of engineering components is the threshold stress intensity factor range for fatigue crack growth, ∆K_th. A small variation in identification of ∆K_th can lead to a big change in overall estimation of fatigue life. In this project, an analytical model has been developed for aluminium alloys by fitting an analytical curve with raw crack growth data in order to identify the ∆K_th. This model has the capacity to identify ∆K_th for different aluminium alloys at various R ratios. There is a great demand for enhanced fatigue life of aluminium alloys in the transport industry. This project has carried out a detailed investigation of electromagnetic treatment (ET) in the form of electropulsing treatment to develop an efficient technique for fatigue resistance enhancement. ET parameters including the treatment intensity, treatment time and the number of applications have been optimised. It is suggested that the duration of ET treatment can be used as the main parameter among all these to control the fatigue resistance of the aluminium alloy. The improvement in fatigue resistance has been explained by the change in microhardness and conductivity of aluminium alloy due to ET. Additionally, the fracture morphology was analysed using scanning electron microscopy (SEM). The precipitates and dislocation characteristics were also studied using transmission electron microscopy (TEM). The outcomes of this investigation will help improve structural integrity by enhancing fatigue resistance of aluminium alloys.

A análise da vida em fadiga em amplitude variável associada à corrosão é bastante complexa, devido a combinações entre as interações eletroquímicas ocorridas, tendo em vista a exposição da frente da trinca à névoa salina somada aos efeitos deletérios do carregamento de amplitude variável. Esta combinação de efeitos ainda não é modelada matematicamente, e os dados experimentais observados em outros trabalhos ainda geram hipóteses inconclusivas. Neste contexto, este estudo tem como objetivo avaliar a vida em fadiga da liga 7475 T7351, utilizando carregamento de voos simulados TWIST e FALSTAFF e submetida a névoa salina a 3,5 e 5%. Foram executados ensaios eletroquímicos, para verificar o potencial de corrosão dessa liga e análise microestrutural para verificar a formação de pites. O crescimento de trinca por fadiga com carregamento de voo foi simulado, sem considerar os efeitos do meio, nos programas NASGRO 4.0, AFGROW e CRACK 2000, com parâmetros calibrados no NASGRO 4.0, no intuito de verificar quais modelos matemáticos e programas descrevem a curva experimental em ar com menor erro. Os resultados mostraram que os programas NASGRO 4.0 e AFGROW se aproximaram com menor erro à curva experimental em ar. Os ensaios de crescimento de trinca por fadiga foram executados com carregamento de voos simulados e os parâmetros para o sistema de geração de névoa salina foram determinados experimentalmente. As curvas de crescimento de trinca e taxa de propagação em névoa salina foram comparadas às curvas em ar, as quais também foram obtidas sob as mesmas condições de carregamento. Observou-se que para os ensaios em névoa salina, a trinca rompeu com maior número de voos em relação às curvas em ar. / The corrosion fatigue life analysis under variable amplitude loading is very complex due electrochemistry effects in crack tip exposed to salt spray matched to harmful effects of amplitude variable loading. This effects combination is not modeling mathematically actually, and the experimentally data observed in other research generates inconclusive theories. In this context, the main aim of this research to evaluate the corrosion fatigue life of 7475 T7351 under spectrum loading TWIST and FALSTAFF and exposed to 3.5% and 5% NaCl. Electrochemistry test were performed to verify the corrosion potential followed of microstructural analysis by SEM, to verify the pitting on 7475 T7351 alloy. The fatigue crack growth was simulated, without environmental effects, using the codes NASGRO, AFGROW and CRACK 2000, the simulation parameters was fit on NASGRO 4.0, to analyze whether the crack growth models and codes are able to represent properly the air experimental crack growth data. The results showed that NASGRO 4.0 and AFGROW codes described the fatigue crack growth in air with less error. The fatigue crack growth tests were performed under spectrum loadings and the parameters to obtain salt spray were determined experimentally. The corrosion fatigue crack growth and rate data were compared to air data, under the same loading conditions. The corrosion fatigue tests showed that the specimens cracked under a larger number of flights than specimens in air.

No description available.

In the past, the disagreement of near-threshold fatigue-crack growth (FCG) rate data generated from constant Kmax tests, high load ratio (minimum to maximum load) constant R tests, and ΔKeff based data was a mysterious issue. Because of the disagreement, a variety of test or analysis methods were created to correlate FCG rate data. It was suspected that the ASTM threshold test method using load reduction was inducing remote crack closure due to plastically deformed material, which caused elevated thresholds and slower rates than steady-state behavior. The first goal of this study was the development of a test method to eliminate remote closure during threshold testing. In order to avoid/minimize remote closure effect, compression-precracking methods were used to initiate a crack from a starter notch on compact specimens. Two materials with different fatigue crack surface profiles (flat or very rough) were tested and the results generated from the conventional ASTM precracking method and the compression-precracking test method were compared. In order to understand the disagreement of near-threshold data, crack-opening load measurements were performed from locally (near crack tip) installed strain gages instead of the remote gage (i.e., back face gage). Some careful specimen preparations were performed to avoid out-of-plane bending, to maintain straight crack fronts, and to ensure testing system linearity. It was known that remote gages, such as crack-mouth- opening-displacement-gages were insensitive to measuring load-strain records near threshold. By using local gages, the crack closure effects were clearly observed even in high load ratio (R) tests, like or higher than R = 0.7, and constant Kmax tests, which were believed to be crack closure free. By measuring load-reduced-strain records from local gages, crack-opening loads were able to correlate FCG rate data and showed that ΔKeff-rate data was unique for a wide variety of materials. By comparing (ΔKeff)th values, it may provide reasonable guidance for the material resistance against FCG. Because of “high R crack closure”, some theories considered in the past may need to be reconsidered. First, constant Kmax tests are not entirely crack-closure free. Second, there is no critical load ratio, Rc, to indicate the transition from crack-closure affected to crack-closure free data, and Kmax effects that appear in ΔKth-Kmax relations. Research has shown that the three dominate crack-closure mechanisms (plasticity-, roughness- and debris-induced crack closure) FCG rate behavior in the threshold regime from low to high load ratios.

Titanium (Ti) alloys have been commonly used in the aerospace industry, not only because they have a high strength-to-weight ratio (comparing to the steels) but also their satisfactory corrosion resistance. Furthermore, they can be assembled with the carbon fibre composite parts. However, conventional manufacturing methods cause high material scrap rate and require lots of machining to obtain the final shape and size, which increases both the manufacturing time and cost. In order to improve the efficiency and reduce the cost of Ti parts, Additive Manufacturing (AM) has been developed. Rolled Wire and Arc Additive Manufacturing (rolled WAAM) is one of the AM processes. The main characteristics of this technology is the reduced β grain size to refine the alloy's microstructure. Both the ultimate tensile strength and yield strength of Ti alloy made by rolled WAAM are at least 10% higher than traditional wrought Ti. This project is to investigate the fatigue crack growth rates of the Ti-6Al-4V built by rolled WAAM process in both the longitudinal and transverse orientations to study the effect of rolling on fatigue crack growth rate of WAAM processed Ti. The project was carried out by testing the fatigue crack growth rates for 4 compact tension specimens. The test results of different orientations were compared with each other, and scatters in fatigue life and fatigue crack growth rate were found. Fatigue crack growth rate is lower in the longitudinal specimens. The results are also compared with those of the unrolled WAAM specimens tested in a previous project. It was found that rolling can significantly improve the fatigue crack growth behaviour in WAAM processed Ti, and can reduce the difference between the two orientations, i.e. achieving better isotropic material properties. Recorded scatters may be caused by the process induced residual stresses, error in measurement, and the test machine load range being much higher than the applied loads. More specimens can be tested to validate above observations further.

The application of fracture mechanics to engineering design has provided significant advances in understanding of the causes and mechanisms of failure and crack growth. Despite this, there are still some aspects that remain incompletely understood, such as the crack closure/crack shielding effect. The presence of crack closure/shielding acts to reduce . The mechanisms of crack closure/shielding are complicated, and have not been fully understood. This work focuses on the plasticity-induced crack tip shielding mechanism and presents a novel approach to characterise the elastic stress fields under the influence of the plastic enclave surrounding the crack tip. The model is successfully applied to determine the four stress parameters experimentally using full-field photoelastic stress analysis on polycarbonate CT specimens, following studies of the effect of the crack tip position and the valid data collection zone giving the best fit between the model predictions and the experimental data. The predicted values from the model demonstrate good data repeatability, and exhibit sensible trends as a function of crack length and load ratio that are interpretable in terms of physically meaningful changes to the plastic enclave. In addition, the model is proven to describe the stress field around a crack more accurately than classic Williams‟ stress solution. The model is also extended to AL 2024-T3 specimens using a full-field displacement measurement technique, digital image correlation. Using the Sobel edge detection method to identify the crack tip from the displacement fields with a rectangular shaped data collection zone employed in the current study, reasonable trends were again demonstrated in the experimental results as a function of crack length.

Cette thèse porte sur la fissuration par fatigue sous chargement variable en mode mixte I+II+III et sur les effets d'histoire induits par la plasticité confinée et les contraintes internes. Les essais réalisés montrent qu’il existe des effets antagonistes d’histoire du chargement à courte distance et à longue distance, que la forme de la séquence de chargement est cruciale et, par comparaison, que les effets de contact et de frottement sont d’importance moindre. Les contraintes internes jouent un rôle majeur sur la vitesse de fissuration par fatigue et sur le mode de fissuration. Une démarche a été mise en place pour étudier le comportement élasto-plastique d’une section représentative du front de la fissure par éléments finis. Pour ne retenir des calculs réalisés que le minimum d’information nécessaire, on se donne une approximation cinématique, le champ de vitesse est partitionné en composantes de modes I, II, III élastiques et plastiques, chaque composante étant caractérisée par un facteur d’intensité et une distribution spatiale fixe. Les calculs réalisés ont permis de sélectionner 7 trajets de chargement différents en mode I+II et en mode I+II+III, qui présentent les mêmes amplitudes pour chaque mode, les mêmes maxima, minima et valeurs moyennes. Ces trajets, censés être équivalents au sens des critères de rupture usuels, ne le sont pourtant pas lorsqu’on considère le comportement élasto-plastique du matériau et conduisent à des vitesses et des trajets de fissuration expérimentaux très différents les uns des autres. Les simulations numériques et la modélisation simplifiée sont en accord avec les résultats expérimentaux. Les calculs réalisés ont également permis de discuter le rôle des phases de chargement en mode III sur la fissuration. Le comportement du matériau étant non-linéaire, la direction du chargement nominal ne coïncide pas nécessairement avec celle de l’écoulement plastique. Ajouter une phase de chargement en mode III peut, dans certains cas, modifier très significativement le comportement de la fissure (direction de propagation, vitesse de fissuration, écoulement plastique). / This thesis deals with fatigue crack growth in non-proportional variable amplitude mixed mode I + II + III loading conditions and analyses the effects of internal stresses stemming from the confinement of the plastic zone in small scale yielding conditions. The tests showed that there are antagonistic long-distance and short-distance effects of the loading history on fatigue crack growth. The shape of loading path, and not only the maximum and minimum values in this path, is crucial and, by comparison, the effects of contact and friction are of lesser importance. Internal stresses play a major role on the fatigue crack growth rate and on the crack path. An approach was developed to analyze the elastic-plastic behavior of a representative section of the crack front using the FEA. A model reduction technic is used to extract the relevant information from the FE results. To do so, the velocity field is partitioned into mode I, II, III elastic and plastic components, each component being characterized by an intensity factor and a fixed spatial distribution. The calculations were used to select seven loading paths in I + II and I + II + III mixed mode conditions, which all have the same amplitudes for each mode, the same maximum, minimum and average values. These paths are supposed to be equivalent in the sense of common failure criteria, but differ significantly when the elastic-plastic behavior of the material is accounted for. The results of finite element simulations and of simulations using a simplified model proposed in this thesis are both in agreement with experimental results. The approach was also used to discuss the role of mode III loading steps. Since the material behavior is nonlinear, the nominal loading direction does not coïncide with the plastic flow direction. Adding a mode III loading step in a mode I+II fatigue cycle, may, in some cases, significantly modify the behaviour of the crack (crack growth rate, crack path and plasti flow).