Browsing by Subject "Collapse"
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Item Case-based drilling curricula using integrated HIL simulator and remote collaboration center(2016-12) Hoss, Ashton Ashkan; Van Oort, Eric; Pryor, Mitchell WayneThe university educational system has raised many concerns in recent years regarding the effectiveness of its curricula and implementation. The focus on course-based training in engineering programs does not provide students sufficient opportunities to apply the attained knowledge and skills to demonstrate their competency. To address this deficiency of academia, industry spends millions of dollars building development programs and on-the-job training. This creates an opportunity for the universities to address this deficiency and increase their students’ marketability, while also addressing problem solving in their curricula. Inspired by a successful program developed and offered at Harvard Business School, the advantages and disadvantages of the case-based method was investigated. It was concluded that the students can benefit the most from a combination of existing educational and case-based curricula elements. Further research expressed the engineering students’ interest and positive feedbacks towards utilization of this method supported by statistical analysis. The aviation industry experienced a great training cost reduction and eliminated the on-the-training accidents after adopting simulators to train their workforce. This encouraged the Drilling & Automation team at University of Texas at Austin to develop the existing surface simulator further and utilize it as a tool to train the next generation of engineers to carry out the appropriate performance at the time of failure and emergencies. By considering various effective skills development methods such as Triadic method and Kolb’s Four-Stage Learning Cycle, ten case-based laboratories were designed and proposed. These open-ended student-led laboratories provide the opportunity for students to experience life-like challenges associated with drilling operations using a realistic up-to-date virtual drilling simulator. Students are divided in teams and assigned to different roles (drilling engineer, remote supervising engineer, etc.) where they are required to make decisions and communicate with one another. This creates a realistic work environment where depending on difficulty of each case, different amounts of stress are experienced. To implement the proposed laboratories, down-hole physics models were identified and developed. These mathematical models were then simulated in MATLAB programing language and integrated with one another to form the down-hole simulator. An Application Program Interface, API, was developed to access the surface simulator data and to connect the surface and the down-hole simulators. The integrated developed simulator has potential for future research including automated rig design.Item Contingency on the Korean peninsula : collapse to unification(2010-05) O, Tara C.; Galbraith, James K.A collapsed North Korea would pose a momentous test to the future of the region. The five regional powers—South Korea, China, Japan, Russia, and the United States—are ill-prepared for such an event, partly because of the act of planning for it would upset North Korea. However, the potential challenges of a collapse are too great to ignore. This study presents an historical and political analysis of the increasing risk that North Korea may collapse. A comparison with earlier cases suggests that triggers and indicators of collapse can be identified, including increasing cross-border information flows, defections, and the possible death or incapacitation of North Korea’s leader. Further, the large and growing economic disparity between North Korea and its neighbors, South Korea and China, points to likely consequences of collapse, including possible mass migration. The study then examines the roles of South Korea, China, the U.S., Japan, and Russia in the future of the Korean peninsula; it concludes with a further consideration of the paradox of collapse planning, but argues that it would be better to run the risks entailed in the exercise than to be caught flatfooted when a collapse occurs. The analysis is based on interviews, surveys, and documents in English and Korean.Item On the crushing of honeycomb under axial compression(2010-12) Wilbert, Adrien; Kyriakides, S.; Ravi-Chandar, KrishnaswamyThis thesis presents a comprehensive study of the compressive response of hexagonal honeycomb panels from the initial elastic regime to a fully crushed state. Expanded aluminum alloy honeycomb panels with a cell size of 0.375 in (9.53 mm), a relative density of 0.026, and a height of 0.625 in (15.9 mm) are laterally compressed quasi statically between rigid platens under displacement control. The cells buckle elastically and collapse at a higher stress due to inelastic action. Deformation then first localizes at mid-height and the cells crush by progressive formation of folds; associated with each fold family is a stress undulation. The response densifies when the whole panel height is consumed by folds. The buckling, collapse, and crushing events are simulated numerically using finite element models involving periodic domains of a single or several characteristic cells. The models idealize the microstructure as hexagonal, with double walls in one direction. The nonlinear behavior is initiated by elastic buckling while inelastic collapse that leads to the localization observed in the experiments occurs at a significantly higher load. The collapse stress is found to be mildly sensitive to various problem imperfections. For the particular honeycomb studied, the collapse stress is 67% higher than the buckling stress. It was also shown that all aspects of the compressive behavior can be reproduced numerically using periodic domains with a fine mesh capable of capturing the complexity of the folds. The calculated buckling stress is reduced when considering periodic square domains as the compatibility of the buckles between neighboring cells tends to make the structure more compliant. The mode consisting of three half waves is observed in every simulation but its amplitude is seen to be accented at the center of the domains. The calculated crushing response is shown to better resemble measured ones when a 4x4 cell domain is used, which is smoother and reproduces decays in the amplitude of load peaks. However, the average crushing stress can be captured with engineering accuracy even from a single cell domain.Item On the effect of Lüders bands on the bending of steel tubes(2011-12) Hallai, Julian de Freitas; Kyriakides, S.; Engelhardt, Michael D.; Landis, Chad M.; Liechti, Kenneth M.; Ravi-Chandar, KrishnaswaIn several practical applications, hot-finished steel pipe that exhibits Lüders bands is bent to strains of 2-3%. Lüders banding is a material instability that leads to inhomogeneous plastic deformation in the range of 1-4%. This work investigates the influence of Lüders banding on the inelastic response and stability of tubes under rotation controlled pure bending. It starts with the results of an experimental study involving tubes of several diameter-to-thickness ratios in the range of 33.2 to 14.7 and Lüders strains of 1.8% to 2.7%. In all cases, the initial elastic regime terminates at a local moment maximum and the local nucleation of narrow angled Lüders bands of higher strain on the tension and compression sides of the tube. As the rotation continues, the bands multiply and spread axially causing the affected zone to bend to a higher curvature while the rest of the tube is still at the curvature corresponding to the initial moment maximum. With further rotation of the ends, the higher curvature zone(s) gradually spreads while the moment remains essentially unchanged. For relatively low D/t tubes and/or short Lüders strains, the whole tube eventually is deformed to the higher curvature entering the usual hardening regime. Subsequently it continues to deform uniformly until the usual limit moment instability is reached. For high D/t tubes and/or materials with longer Lüders strains, the propagation of the larger curvature is interrupted by collapse when a critical length is Lüders deformed leaving behind part of the structure essentially undeformed. The higher the D/t and/or the longer the Lüders strain is, the shorter the critical length. This class of problems is analyzed using 3D finite elements while the material is modeled as an elastic-plastic solid with an “up-down-up” response over the extent of the Lüders strain, followed by hardening. The analysis reproduces the main features of the mechanical behavior provided the unstable part of the response is suitably calibrated. The uniform curvature elastic regime terminates with the nucleation of localized banded deformation. The bands appear in pockets on the most deformed sites of the tube and propagate into the hitherto intact part of the structure while the moment remains essentially unchanged. The Lüders-deformed section has a higher curvature, ovalizes more than the rest of the tube, and develops wrinkles with a characteristic wavelength. For every tube D/t there exists a threshold of Lüders strain separating the two types of behavior. This bounding value of Lüders strain was studied parametrically.Item Ratcheting, wrinkling and collapse of tubes due to axial cycling(2011-12) Jiao, Rong; Kyriakides, S.; Landis, Chad; Liechti, Kenneth M.; Ravi-Chandar, K.; Tassoulas, JohnThe first instability of circular tubes compressed into the plastic range is axisymmetric wrinkling, which is stable. Compressed further the wrinkle amplitude grows, leading to a limit load instability followed by collapse. The two instabilities can be separated by strain levels of a few percent. This work investigates whether a tube that develops small amplitude wrinkles can be subsequently collapsed by persistent cycling. The problem was first investigated experimentally using SAF 2507 super-duplex steel tubes with D/t of 28.5. The tubes are first compressed to strain levels high enough for mild wrinkles to form and then cycled axially under stress control about a compressive mean stress. This type of cycling usually results in accumulation of compressive strain; here it is accompanied by growth of the amplitude of the initial wrinkles. The tube average strain initially grows nearly linearly with the number of cycles, but as a critical value of wrinkle amplitude is approached, wrinkling localizes, the rate of ratcheting grows exponentially and the tube collapses. Similar experiments were then performed for tubes involving axial cycling under internal pressure and the combined loads cause simultaneous ratcheting in the hoop and axial directions as well as a gradual growth of the wrinkles. The rate of ratcheting and the number of cycles to collapse depend on the initial compressive pre-strain, the internal pressure, and the stress cycle parameters all of which were varied sufficiently to generate vii a sufficient data base. Interestingly, in both the pressurized and unpressurized cases collapse was found to occur when the accumulated average strain reaches the value at which the tube localizes under monotonic compression. A custom shell model of the tube with initial axisymmetric imperfections, coupled to the Dafalias-Popov two-surface nonlinear kinematic hardening model, are presented and used to simulate the experiments performed. It is demonstrated that when suitably calibrated this modeling framework reproduces the prevalent ratcheting deformations and the evolution of wrinkling including the conditions at collapse accurately for all experiments. The calibrated model is then used to evaluate the ratcheting behavior of pipes under thermal-pressure cyclic loading histories experienced by axially restrained pipelines.