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DIN 35NiCrMoV125 (equivalent to AISI 4340), which is a high strength low alloy steel (HSLA), is mainly used at military applications in the production of gun barrels. The main aim of this study was to determine the low cycle fatigue (LCF) behaviour and the influence of temperature on low cycle fatigue failure properties of that steel. Three different temperatures (room temperature, 2500C and 4000C) were used in the experiments in order to analyze the effect of temperature. For each temperature, five strain amplitudes (in the range of 0.2% offset yield point to 2% strain) were applied and the duplicates of each experiment were performed to obtain more accurate results. Strain amplitudes and the corresponding stresses were calculated from tension tests performed at each temperature. Strain amplitude versus fatigue life (e- N) curves for three different temperatures predicted that fatigue life at a given strain increases with increasing temperature. The transition lives of those three curves were observed at 1 % strain amplitude and no significant effect of temperature on transition lives was observed. For stress based analysis, stress versus fatigue life (S-N) curves were drawn. These curves pointed that fatigue strength at a given number of cycle decreases with increasing temperature. Fractographic analyses of the fracture surfaces were performed to examine the effects of load and temperature on the specimens. It was observed that the number of crack initiation sites increases with increasing strain.

Damage to steel moment frames in the 1994 Northridge and 1995 Hyogken-Nanbu earthquakes subsequently motivated intensive research and testing efforts in the US, Japan, and elsewhere on moment frames. Despite extensive past research efforts, one important problem remains unresolved: the degree of panel zone participation that should be permitted in the inelastic seismic response of a steel moment frame. To date, a fundamental computational model has yet to be developed to assess the cyclic rupture performance of moment frames. Without such a model, the aforementioned problem can never be resolved. This dissertation develops an innovative way of predicting cyclic rupture in steel moment frames by employing artificial neural networks. First, finite element analyses of 30 notched round bar models are conducted, and the analytical results in the vicinity of the notch root are extracted to form the inputs for either a single neural network or a competitive neural array. After training the neural networks, the element with the highest potential to initiate a fatigue crack is identified, and the time elapsed up to the crack initiation is predicted and compared with its true synthetic answer. Following similar procedures, a competitive neural array comprising dynamic neural networks is established. Two types of steel-like materials are created so that material identification information can be added to the input vectors for neural networks. The time elapsed by the end of every stage in the fracture progression is evaluated based on the synthetic allocation of the total initiation life assigned to each model. Then, experimental results of eight beam-to-column moment joint specimens tested by four different programs are collected. The history of local field variables in the vicinity of the beam flange - column flange weld is extracted from hierarchical finite element models. Using the dynamic competitive neural array that has been established and trained, the time elapsed to initiate a low cycle fatigue crack is predicted and compared with lab observations. Finally, finite element analyses of newly designed specimens are performed, the strength of their panel zone is identified, and the fatigue performance of the specimens with a weak panel zone is predicted.

Dans une optique d'allègement, les alliages d'aluminium de fonderie représentent une excellente solution technique. Ainsi, pour les culasses automobiles, les alliages d'aluminium Al-Si-Cu (Aluminium-Silicium-Cuivre) sont souvent choisis pour leurs performances mécaniques ainsi que leur bonne coulabilité. L'objectif de cette étude est d'améliorer la compréhension des mécanismes en fatigue oligocyclique pour définir un critère de fatigue adapté. Cet objectif est réalisé en combinant des essais (en fatigue oligocyclique à température ambiante et élevée), de nombreuses observations (microscopie à balayage électronique, microtomographie 3D rayons-X de laboratoire, suivi de fissure) et des simulations (génération de population de défauts et calculs éléments finis). À l'aide d'outils spécialement développés, la population de défauts (défauts de fonderie) est analysée en détails : la taille de défaut est modélisée par une distribution de Pareto généralisée et leurs positions par un processus ponctuel de type agrégé (démontrant que les défauts ne sont pas spatialement distribués de manière totalement aléatoire). Les processus ponctuels marqués permettent de montrer que tailles et positions de défauts ne sont pas corrélées. En utilisant ces informations statistiques, il est démontré que seuls les défauts proches de la surface sont critiques pour la fatigue oligocyclique. Un critère de fatigue basé sur l'énergie de déformation est utilisé pour prévoir la durée de vie en fatigue. Par la suite, un modèle de propagation de fissure permet de prendre en compte la taille de défaut dans la prévision de la durée de vie. Combiné à la statistique des défauts, ce modèle permet d'estimer la dispersion des essais. En dernier lieu, des essais et des simulations sur éprouvettes entaillées montrent que l'étude de la population de défauts est d'autant plus critique que le volume fortement sollicité mécaniquement devient plus faible. / In a search for lightweight materials, cast aluminium alloys have become an efficient solution for the automotive industry. For cylinder heads, cast aluminium alloys (Aluminium-Silicon-Copper based) are often chosen for their mechanical performances combined with their good casting abilities. This study aims at better understanding the damage mechanisms of Low-Cycle Fatigue (LCF) to help define a suitable design criterion. This is achieved by combining experiments (LCF tests at ambient and high temperature), various observations (Scanning electron microscopy, laboratory 3D X-ray microtomography, crack propagation monitoring) and numerical techniques (Finite element method simulations and defect population generation). Specifically designed statistical tools allow clearly identifying the defect (casting defects) population: the defect size can be modelled by a generalized Pareto distribution and their positions by a clustered point process (thus showing the defect locations are not completely random). Using marked point processes, it is also shown that defect size and defect position are uncorrelated. Using this statistical information and by systematic observations, it is proven that only defects close to the surface are critical for the fatigue life. The different interactions between cracks and defects are also studied, especially with respect to temperature. A strain energy based fatigue criterion is introduced and allows estimating the fatigue life. Then, a crack propagation model provides a mean of taking the defect size into account, which combined with the statistics of defects, gives an estimate of the fatigue life dispersion. Finally, notched specimens tests and simulations show the study of the defect population is even more critical as the highly loaded volume becomes smaller.

Cette Thèse a porté sur l’étude en fatigue oligocyclique et en fissuration des joints soudés des aciers inoxydables superausténitique (Alloy28) et duplex (UNS S32550). L’effet de la présence du joint soudé dans la structure a été examiné.La première étape de ce travail a consisté à étudier le comportement monotone et cyclique du métal de base et du métal d’apport de chaque nuance d’acier inoxydable de l’étude. Les lois de comportement à température ambiante ont été ainsi identifiées.Des essais de fatigue oligocyclique sous différents niveaux de déformation imposée ont été effectués sur les aciers inoxydables et leurs joints soudés. Les résultats indiquent que la durée de vie des soudures est plus faible que celle du métal de base. Les analyses des mécanismes d’endommagement montrent que la durée de vie réduite des joints soudés est attribuée à l’accélération de la phase d’amorçage.Enfin, les essais de propagation de fissure sur l’Alloy28 montrent que le rapport de charge a un effet sur la cinétique de propagation dû au phénomène de fermeture par rugosité et par plasticité. Les vitesses de propagation de fissure dans le métal d’apport sont très similaires à ceux du métal de base. Ceci est dû à la compensation de plusieurs effets comme le phénomène de fermeture, la rugosité et les contraintes résiduelles. L’acier inoxydable duplex ne présente pas d’effet de rapport de charge sur la cinétique de fissuration. Les vitesses de propagation de fissure du métal d’apport sont aussi comparables à celles du métal de base. / This thesis focused on the study of low cycle fatigue and crack growth of welded joints of superaustenitic (Alloy28) and Duplex (UNS S32550) stainless steels. The effect of the presence of the welded joint in the structure was also examined.The first part of this work was to study the monotonous and cyclic behavior of the base metal and weld metal of each stainless steel. The behavior laws of each material were identified at room temperature.Low cycle fatigue tests at different levels of deformation were performed on stainless steels and their welded joints. The results indicate that the fatigue life of welded joints is lower than the base metal. The analyses of damage mechanisms show that the short life of welded joints is attributed to the acceleration of the initiation phase.Finally, crack propagation tests in Alloy28 stainless steel show that load ratio has an effect on the crack growth rate due to the closure phenomenon by roughness and plasticity. The crack growth rate in the weld metal is very similar to that of the base metal. This is due to the compensation of various effects such as the roughness, the residual stress and the closure phenomenon. The duplex stainless steel does not present load ratio effect on the crack growth rate. The crack growth rates of the weld metal of the duplex stainless steel are also comparable to those of the base metal.

Strength Reduction Factor (R-Factor), often referred to as Response Modification Factor, is commonly used in the design of lateral force resisting systems under seismic loading. R-Factors allow for a reduction in design base shear demands, leading to more economical designs. The reduction of strength is remedied with ductile behavior in members of proper detailing. Modern seismic codes and provisions recommend R-Factors for many types of lateral force resisting systems. However these factors are independent of the system fundamental frequency and many other important system properties, resulting in factors that may result in an unfavorable seismic response. To evaluate the validity of prescribed R-Factors an extensive analytical parameter study was conducted using a FEM single degree-of-freedom Concentrically Braced Frame (CBF) under incremental dynamic analysis over a suite of ground motions. Parameters of the study include brace slenderness, fundamental frequency, increment resolution, FEM mesh refinement, effects of leaning columns, and effects of low-cycle fatigue. Results suggest that R-Factor can vary drastically for CBF systems with differing properties.

The master’s thesis is focused on the application of the technique of oriented foils in transmission electron microscopy. Dislocation structures were identified by this technique in both phases of polycrystalline austenitic-ferritic stainless steel SAF 2507 formed during low cycle fatigue at two plastic strain amplitudes ap. In individual grains the stress axis and the Schmid factors of active slip systems were determined. In austenitic grains, the planar structures were determined at both ap. In ferritic grains, the dislocation arrangement was different for the two observed ap. Individual screw dislocations and pile-up´s of edge dislocations were found at low ap = 1x10-4. Mixture of vein and wall dislocation structures were formed at high ap = 2x10-3. Observed dislocation structures were discussed in relation with the cyclic plastic response of the duplex stainless steel.

This thesis describes innovative methods used to predict high-stress amplitude, low cycle fatigue (LCF) behavior of a material commonly used in gas turbine blade design with the absence of such data. A combination of extrapolation and estimation techniques from both prior and current studies has been explored with the goal of developing a method to accurately characterize such high-temperature fatigue of IN738LC, a dual-phase Ni-base superalloy. A method capable of predicting high-stress (or strain) amplitude fatigue from incessantly available low-stress amplitude, high cycle fatigue (HCF) would lower the costs of inspection, repair, and replacement on certain turbine components. Three sets of experimental data at different temperatures are used to evaluate and examine the validity of extrapolation methods such as anchor points and hysteresis energy trends. Stemming from extrapolation techniques developed earlier by Coffin, Manson, and Basquin, the techniques exercised in this study purely implement tensile test and HCF data with limited plastic strain during the estimation processes. A standard practice in engineering design necessitates mechanical testing closely resembling planned service conditions; for design against fatigue failure, HCF and tensile data are the experiments of choice. High stress amplitude data points approaching the ultimate strength of the material were added to the pre-existing HCF base data to achieve a full-range data set that could be used to test the legitimacy of the different prediction methods. While some methods proved to be useful for bounding estimates, others provided for superior estimation.

A series of full-scale beam-to-column moment connection tests were completed to determine the effects of powder actuated fasteners (PAF) and puddle welds on the seismic behavior of steel moment connections.  In seismic regions, PAF are currently prohibited in the connection region (referred to as the protected zone) due to the concern of low-cycle fatigue fracture.  There is almost no information available in the literature regarding the seismic behavior of moment connections with PAF or puddle welds. Full-scale connection testing is the most accurate way to investigate the behavior of different moment connections with common defects and fasteners applied in the protected zone.  However, it is cost prohibitive to conduct full-scale testing programs that are sufficiently comprehensive to investigate a wide range of defect types, severity, and locations.  For this reason, it is desired to develop alternative methods of investigation.  A finite element (FE) model capable of simulating both the global deformation patterns and local buckling effects in a moment connection has been developed.  Validated FE models will allow for further evaluation through numerical simulation of additional configurations.  Furthermore, alternate, more economical, test configurations to experimentally investigate the effect of defects on steel moment connections were explored.  This report discusses the full-scale test setup, results and analysis of completed experimental testing, the development of an FE connection model, and the preliminary development of alternate test configurations. / Master of Science

Titanium and its alloys are widely used in applications ranging from aeroengines and offshore equipment to biomedical implants and sporting goods, owing to their high ratio of strength to density, excellent corrosion resistance, and biomedical compatibility. Among the titanium alloys used in aerospace, Ti-6Al-4V (an α+β alloy) is the most widely used, in applications in which the temperature may reach 350°C, at which point it retains good fatigue and fracture properties as well as moderate tensile strength and ductility. These alloy properties are dependent on variables such as crystalline structure, alloy chemistry, manufacturing techniques and environmental conditions during service. These variables influence the microstructure and mechanical properties of titanium alloys. With regard to the alloy chemistry and operating environment, the focus of the present work is to understand the influence of boron and hydrogen on the microstructure and selected mechanical properties of cast Ti-6Al-4V. The addition of boron to cast Ti-6Al-4V (0.06 and 0.11 wt% in this work) refines the coarse “as cast” microstructure, which is evaluated quantitatively using FoveaPro image analysis software. Compression testing was performed using a Gleeble 1500 instrument, by applying a 10% strain at different strain rates (0.001, 0.1 and 1 s-1) for temperatures in the range 25-1100°C. The tests were performed to evaluate the effect of boron on the mechanical properties of the alloy. It was observed that there is an increase in the compressive strength, predominantly at room temperature, of cast Ti-6Al-4V after the addition of boron. Metallographic evaluation showed that this increase in strength is a likely result of reductions in both the prior β grain and α colony dimensions, which is caused by boron addition. Studies in a hydrogen environment at 150 bar showed that cast Ti-6Al-4V exhibited lower yield strength and lower ultimate tensile strength in comparison with those properties measured in an air environment. No significant change in the ductility was observed. It was also noted that in a high strain range (≈2%) the low cycle fatigue (LCF) life was significantly reduced in hydrogen compared with air. Microstructural and fractographic characterization techniques were used to establish the role of hydrogen on the deformation mechanism by analysing the crack propagation path through the microstructure. It is seen that cracks tend to propagate along the interface between prior β grain boundaries and/or along the α colony boundaries