Sulfide Stress Cracking (SSC) resistance of the newly developed API-X100 High Strength Low Alloy (HSLA) steel was investigated in the NACE TM0177 "A" solution. The NACE TM0177 "A" solution is a hydrogen sulfide (H2S) saturated solution containing 5.0 wt.% sodium chloride (NaC1) and 0.5 wt.% acetic acid (CH3COOH). The aim of this thesis was to study the effect of microstructure, non-metallic inclusions and alloying elements of the X100 on H2S corrosion and SSC susceptibility. The study was conducted by means of electrochemical polarization techniques and constant load (proof ring) testing. Microstructural analysis and electrochemical polarization results for X100were compared with those for X80, an older generation HSLA steel. Uniaxial constant load SSC testing was conducted using X100 samples and the results were compared with those reported for older generation HSLA steels. Addition of H2S to the NACE TM0177 "A" solution increased the corrosion rate of X100from 51.6 to 96.7 mpy. The effect of H2S on the corrosion rate was similar for X80. The corrosion rate for X80 increased from 45.2 to 80.2 mpy when H2S was added to the test solution. Addition of H2S enhanced the anodic kinetics by forming a catalyst (FeHSads) on the metal surface and as a result, shifted the anodic polarization curve to more current densities. Moreover, the cathodic half cell potential increased due to the decrease in pH, from 2.9 to 2.7, which shifted the cathodic polarization curve to more current densities. The increase in both the anodic and cathodic currents, after H2S addition, caused the rise in the corrosion current density. In H2S saturated NACE TM-0177 "A" solution, the X100 steel corrosion rate was higher than the X80 steel by 20%. Longer phase boundaries and larger nonmetallic inclusions in the X100 microstructure generated more areas with dissimilar corrosion potentials and therefore, a stronger driving force for corrosion. Higher density of second phase regions and larger nonmetallic inclusions acted as an increased cathode area on the X100 surface which increased the cathodic current density and consequently, increased the corrosion current density. Proof ring tests on the X100 gave a threshold stress value, C5th, of 46% YS, 343.1 MPa(49.7 ksi). The main failure was caused by SSC cracking. SSC nucleated at corrosion pits on the metal surface and microcracks in the metal body and propagated perpendicular to the applied stress. Hydrogen Induced Cracking (HIC) was observed in the X100. HIC cracks nucleated at banded martensite-ferrite interfaces and propagated along the rolling direction parallel to the applied tensile stress through the softer ferrite phase. When compared to older HSLA grades, the X100 tested in this study had a high SSC susceptibility and therefore, is not be recommended for H2S service applications. The high X100 SSC susceptibility was caused by the material high corrosion rates in H2Smedia which formed corrosion pits that acted as crack initiation sites on the metal surface and provided more hydrogen that migrated into the steel. In addition, the X100 inhomogeneous microstructure provided a high density of hydrogen traps in front of the main crack tip which promoted SSC microcrack formation inside the metal. Microcracks in the metal body connected with the main crack tip that originated from corrosion pits which assisted SSC propagation.

Aunque el hormigón de alta resistencia se está utilizando de manera creciente en los últimos años para la construcción de estructuras, la norma Española vigente, la Instrucción EHE, sólo abarca hormigones de resistencias características a compresión inferiores a 50 MPa. El aumento de resistencia del hormigón está directamente asociado a una mejora en la mayoría de sus prestaciones, especialmente de la durabilidad, aunque también produce un aumento en la fragilidad y una disminución de la rugosidad de las fisuras, lo que afecta de forma muy especial a la resistencia a cortante.El objetivo principal de este trabajo es contribuir al avance del conocimiento del comportamiento frente a la rotura por cortante de vigas de hormigón de alta resistencia. Para ello, y en primer lugar, se ha llevado a cabo una extensa revisión del estado actual del conocimiento de la resistencia a cortante, tanto para hormigón convencional como para hormigón de alta resistencia, así como una profunda investigación de campañas experimentales anteriores. Se ha realizado una campaña experimental sobre vigas de hormigón de alta resistencia sometidas a flexión y cortante. La resistencia a compresión del hormigón de las vigas variaba entre 50 y 87 MPa. Las principales variables de diseño eran la cuantía de armadura longitudinal y transversal. Los resultados obtenidos experimentalmente han sido analizados para estudiar la influencia de las distintas variables en función de la resistencia a compresión del hormigón.Con el objetivo de tener en cuenta, no sólo los resultados de nuestros ensayos, sino también la gran cantidad de información disponible en la bibliografía técnica, se ha preparado una base de datos con vigas de hormigón convencional y de alta resistencia a partir del banco de datos de la Universidad de Illinois. Los resultados empíricos han sido comparados con los cortantes últimos calculados según la Instrucción EHE, las especificaciones AASHTO LRFD, el Código ACI 318-99 y el programa Response-2000, basado en la teoría modificada del campo de compresiones.Se han construido dos Redes Neuronales Artificiales (RNA) para predecir la resistencia a cortante en base a la gran cantidad de resultados experimentales. La principal característica de las RNA es su habilidad para aprender, mediante el ajuste de pesos internos, incluso cuando los datos de entrada y salida presentan un cierto nivel de ruido. Con los resultados de la RNA se ha realizado un análisis paramétrico de cada variable que afecta la resistencia última a cortante.Se han propuesto nuevas expresiones que tienen el cuenta el comportamiento observado para el diseño frente al esfuerzo cortante de vigas tanto de hormigón convencional como de alta resistencia con y sin armadura a cortante, así como una nueva ecuación para la determinación de la armadura mínima a cortante. Las nuevas expresiones presentan resultados que se ajustan mejor a los resultados experimentales que los obtenidos mediante la utilización de las normativas vigentes.Finalmente se han planteado varias sugerencias de futuras líneas de trabajo, que son resultado de la propia evolución del conocimiento sobre el tema de estudio durante el desarrollo de esta tesis. / Although High-Strength Concrete has been increasingly used in the construction industry during the last few years, current Spanish Structural Concrete code of practice (EHE) only covers concrete of strengths up to 50 MPa. An increase in the strength of concrete is directly associated with an improvement in most of its properties, in special the durability, but this also produces an increase in its brittleness and smoother crack surfaces which affects significantly the shear strength. The aim of this research is to enhance the understanding of the behaviour of high-strength concrete beams with and without web reinforcement failing in shear. In order to achieve this objective, an extensive review of the state-of-the-art in shear strength for both normal-strength and high-strength concrete beams was made, as well as in-depth research into previous experimental campaigns.An experimental programme involving the testing of eighteen high-strength beam specimens under a central point load was performed. The concrete compressive strength of the beams at the age of the tests ranged from 50 to 87 MPa. Primary design variables were the amount of shear and longitudinal reinforcement. The results obtained experimentally were analysed to study the influence of those parameters related to the concrete compressive strength.With the aim of taking into account, in addition to the results of our tests, the large amount of information available, a large database was assembled based on the University of Illinois Sheardatabank for normal-strength and high-strength concrete beams. These test results were compared with failure shear strengths predicted by the EHE Code, the 2002 Final Draft of EuroCode 2, the AASHTO LRFD Specifications, the ACI Code 318-99, and Response-2000 program, a computer program based on the modified compression field theory. Furthermore, two Artificial Neural Networks (ANN) were developed to predict the shear strength of reinforced beams based on the database beam specimens. An ANN is a computational tool made up of a number of simple, highly-interconnected processing elements that constitute a network. The main feature of an ANN is its ability to learn, by means of adjusting internal weights, even when the input and output data present a degree of noise. Based on the ANN results, a parametric study was carried out to study the influence of each parameter affecting the failure shear strength.New expressions are proposed, taking into account the observed behaviour for the design of high-strength and normal-strength reinforced concrete beams with and without web reinforcement. A new equation is given for the amount of minimum reinforcement as well. The new expressions correlate with the empirical tests better than any current code of practice.Finally, as a natural corollary to the evolution of our understanding of this field, some recommendations for future studies are made.

The current research has considered the characterization of the high strain rate constitutive response of three steels: a drawing quality steel (DDQ), a high strength low alloy steel (HSLA350), and a dual phase steel (DP600). The stress-strain response of these steels were measured at seven strain rates between 0. 003 s^-1 and 1500 s^-1 (0. 003, 0. 1, 30, 100, 500, 1000, and 1500 s^-1) and temperatures of 21, 150, and 300 °C. In addition, the steels were tested in both the undeformed sheet condition and the as-formed tube condition, so that tube forming effects could be identified. After the experiments were performed, the parameters of the Johnson-Cook and Zerilli-Armstrong constitutive models were fit to the results. <br /><br /> In order to determine the response of the steels at strain rates of 30 and 100 s^-1, an intermediate rate tensile experiment was developed as part of this research using an instrumented falling weight impact facility (IFWI). An Instron tensile apparatus was used to perform the experiments at lower strain rates and a tensile split-Hopkinson bar was used to perform the experiments at strain rates above 500 s^-1 <br /><br /> A positive strain rate sensitivity was observed for each of the steels. It was found that, as the nominal strength of the steel increased, the strain rate sensitivity decreased. For an increase in strain rate from 0. 003 to 100 s^-1, the corresponding increase in strength at 10% strain was found to be approximately 170, 130, and 110 MPa for DDQ, HSLA350, and DP600, respectively. <br /><br /> The thermal sensitivity was obtained for each steel as well, however no correlation was seen between strength and thermal sensitivity. For a rise in temperature from 21 to 300 °C, the loss in strength at 10% strain was found to be 200, 225, and 195 MPa for DDQ, HSLA350, and DP600, respectively for the 6 o?clock tube specimens. <br /><br /> For all of the alloys, a difference in the stress ? strain behaviour was seen between the sheet and tube specimens due to the plastic work that was imparted during forming of the tube. For the DP600, the plastic work only affected the work-hardening response. <br /><br /> It was found that both the HSLA350 and DDQ sheet specimens exhibited an upper/lower yield stress that was amplified as the strain rate increased. Consequently the actual strength at 30 and 100 s^-1 was obscured and the data at strain rates above 500 s^-1 to be unusable for constitutive modeling. This effect was not observed in any of the tube specimens or the DP600 sheet specimens <br /><br /> For each of the steels, both the Johnson-Cook and Zerilli-Armstrong models fit the experimental data well; however, the Zerilli-Armstrong fit was slightly more accurate. Numerical models of the IFWI and the TSHB tests were created to assess whether the experimental results could be reproduced using the constitutive fits. Both numerical models confirmed that the constitutive fits were applied correctly.

This multi-scale investigation aimed to provide new knowledge and understanding of creep and shrinkage of high performance lightweight concrete (HPLC) by assessing prestress losses in HPLC prestressed members in a large-scale study; by quantifying the effect of the constituent materials and external conditions on creep and shrinkage in a medium-scale study; and by improving the fundamental understanding of creep and shrinkage in a small-scale study. Creep plus shrinkage prestress losses were between two and eight times lower than those estimated for the design standards and approximately 50% of those measured in similar strength normal weight high performance concrete girders. The lower creep and shrinkage exhibited by HPLC was found to be caused by a synergy between the pre-soaked lightweight aggregate and the low water-to-cementitious material ratio matrix. That is, the water contained in the lightweight aggregate contributes to enhance hydration by providing an internal moist curing. The water in the aggregate also contributes to maintain a high internal relative humidity which reduces or eliminates autogenous shrinkage. This higher internal relative humidity also reduces creep by preventing load-induced water migration. Finally, lightweight aggregate exhibits a better elastic compatibility with the paste than normal weight aggregate. This improved elastic matching and the enhanced hydration are believed to reduce peak deformations at the ITZ which further decreases creep and shrinkage.

Building fuel efficient automobiles is increasingly important due to the rising cost of energy. One way to improve fuel efficiency is to reduce the overall automobile weight. Weight reductions using steel components are desirable because of easy integration into existing manufacturing systems. Designing components with Advanced High Strength Steels (AHSS) has allowed for material reductions, while maintaining strength requirements. Two Advanced High Strength steel microstructures investigated in this research utilize different strengthening mechanisms to obtain a desired tensile strength grade of 590MPa. One steel, HR590, utilizes precipitation strengthening to refine the grain size and harden the steel. The other steel, HR590DP, utilizes a dual phase microstructure consisting of hardened martensite constituents in a ferrite matrix. The steels are processed to have the same tensile strength grade, but exhibit different fatigue behavior. The central objective of this research is to characterize and compare the fatigue behavior of these two steels. The results show the dual phase steel work hardens at a low fatigue life. The precipitation strengthened microstructure shows hardening at low strain amplitudes, softening at intermediate strain amplitudes and little to no effect at high strain amplitudes. These different fatigue responses are characterized and quantified in this research. Additionally, observations showing the fracture surfaces and the bulk microstructure are analyzed.

Die vorliegende Arbeit zeigt Forschungsergebnisse zur Materialveränderung normaler und hochfester Betone unter Impulsbelastung gegenüber einer quasi-statischen Beanspruchung. Die Untersuchung wurde an speziellen eingeschnürten Prismen unter zentrischem Zug vorgenommen. Die Untersuchungen belaufen sich auf die generelle Festigkeitssteigerung und Modifikation der Spannungs-Dehnungs-Beziehung infolge veränderter Belastung. Neben den Betrachtungen des Materialverhaltens wurde auf die entstehenden Bruchflächen ein genaueres Augenmerk gerichtet, um Unterschiede des Materialverhaltens durch Änderungen beim Bruchvorgang zu erkennen. Zur Beurteilung der Bruchfläche werden die fraktale Analyse und ein Ortsfrequenzspektrum genutzt. / The present paper exhibits results of the latest research into the modification of the behaviour of normal and high-strength concrete on impact versus quasi-static loads. This examination has been carried out at tensile stress on special constricted specimens. The enquiries amount to the general strength increase and modification of the stress-strain-relationship due to a changing of load history. Further on, a special attention was turned to the crack surface to get differences of the material behaviour during the cracking process. The fractal analysis and a spatial frequency spectrum were used for the description of the surface.

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 120-125). Also available on the World Wide Web.

An understanding of the damage evolution prior to crack initiation in advanced structural materials is of vital importance to the fatigue community in both academia and industry. Features known as the Persistent Slip Bands (PSBs) play an integral role in this damage evolution. Therefore, PSBs have been the focus of a lot of science-based investigations over the years. However, most existing studies in this area are restricted to analysis of PSBs in single crystal face centered cubic (FCC) materials. Moreover, these studies lack a quantitative analysis of the evolution of the fatigue damage (or PSBs) as a function of the material microstructure. This is especially true for relatively modern materials such as the Advanced High Strength Structural (AHSS) steels that are gaining a lot of importance in the automotive sector. Accordingly, the objective of this research is to quantitatively characterize evolution of PSBs in three AHSS steels having different microstructures as a function of number of fatigue cycles and strain amplitude. For this purpose strain controlled interrupted fatigue tests have been performed on two dual phase steels (DP-590 and DP-980) having different relative amounts of tempered martensite and a ferritic high strength low alloy steel (HR-590). Digital image analysis and Stereology have been used for unbiased quantitative characterization of the evolution of global geometry of the PSB colonies as function of number of fatigue cycles and strain amplitude. Evolution of PSB colonies has been couched in terms of variation of PSB colony volume fraction and total surface area unit volume, and total surface area of individual PSBs per unit volume and three-dimensional angular orientation distribution of the PSBs. For this purpose, new stereological techniques have been developed for estimation of the three-dimensional angular orientation distribution. The stereological data reveal that during strain controlled in these AHSS steels, volume fraction of the PSB colonies varies linearly with the their total surface area per unit volume. Detailed analysis of the stereological data leads to a simple geometric model for evolution of the PSB colonies in the three AHSS steels, which accounts for all observed data trends.

Continuous beams represent main structural elements in most reinforced concrete (RC) structures such as parking garages and overpass bridges. Deterioration of such structures due to corrosion of steel reinforcement is common in North America. To overcome the corrosion problems, the use of fiber-reinforced polymer (FRP) bars and stirrups becomes a viable alternative to steel reinforcement. However, to date, the shear behaviour of FRP-RC continuous beams has not been explored yet. As such, the objective of this study is to investigate the shear behaviour of such beams. In this study, twenty four full-scale continuous concrete beams were constructed and tested. The test beams had rectangular cross section with 200-mm width and a height of 300, 550 or 850 mm and were continuous over two equal spans. The main investigated parameters were concrete strength, type and ratio of longitudinal reinforcement, type and ratio of transverse reinforcement and beam effective depth. Moreover, a 3-D nonlinear finite element model (FEM) was constructed to simulate the behaviour of FRP-RC continuous beams. The model was verified against the experimental results and validated against test results from previous studies. Then, the verified/validated model was used to conduct a parametric study to investigate the effect of a wide range of the parameters on the shear behaviour of GFRP-RC beams. The experimental and FEM results showed that shear-critical GFRP-RC continuous beams exhibited moment redistribution. Also, it was observed that increasing the concrete strength and the longitudinal reinforcement ratio increased the shear strength significantly. Moreover, the presence of GFRP stirrups significantly enhanced the shear strength of the tested beams. Regarding the size effect, test results showed that there was adverse or no size effect on the shear strength of GFRP-RC continuous beams when they failed in the interior shear span while beams failed in the exterior shear span exhibited clear size effect. Furthermore, a comparison between the test results and the provisions of the available models and FRP standards and design guidelines in North America revealed that these design provisions can be safely applied to continuous beams. / February 2016

published_or_final_version / Civil Engineering / Master / Master of Philosophy