# Browsing by Subject "Buckling"

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Item Case-based drilling curricula using integrated HIL simulator and remote collaboration center(2016-12) Hoss, Ashton Ashkan; Oort, Eric van; Pryor, Mitchell WayneShow more The 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.Show more Item Compressive forces causing rod buckling in sucker rod pumps and using sinker bars to prevent buckling(2016-05) Eritici, Cagdas; Bommer, Paul Michael; Sepehrnoori, Kamy, 1951-Show more Sucker rod pumps has been the most commonly used pumps in the petroleum industry. Therefore, many studies associated with sucker rods focuses on maximizing the rod life. Rod buckling is a leading problem which causes concentrated wear on tubing wall, immediate failure in rod strings, and shortens fatigue life of the string. This study fundamentally consists of a review of the literature on compressive forces causing rod buckling in sucker rod pumps and using sinker bars to prevent buckling. The study initially addresses defining rod buckling, and then continues with the studies on analyzing the static forces acting near pump in the literature. Subsequently, the critical loads causing rod buckling, and the various approximations to estimate these critical loads are discussed. Then, the comparisons of the measured and calculated critical loads in the literature are presented. Next, the two of the most commonly experienced buckling types in the sucker pumps, sinusoidal buckling and helical buckling, are discussed. An example study on developing a model to estimate compressive forces acting on the pump plunger is reviewed to illustrate the importance of the parameters, such as surface roughness of pump, valve diameter, and pump geometry. Lastly, using sinker bar which is the most practiced method in the industry to prevent rod buckling is extensively discussed and demonstrated.Show more Item Extreme energy absorption : the design, modeling, and testing of negative stiffness metamaterial inclusions(2013-08) Klatt, Timothy Daniel; Seepersad, Carolyn; Haberman, Michael R. (Michael Richard), 1977-Show more A persistent challenge in the design of composite materials is the ability to fabricate materials that simultaneously display high stiffness and high loss factors for the creation of structural elements capable of passively suppressing vibro-acoustic energy. Relevant recent research has shown that it is possible to produce composite materials whose macroscopic mechanical stiffness and loss properties surpass those of conventional composites through the addition of trace amounts of materials displaying negative stiffness (NS) induced by phase transformation [R. S. Lakes, et al., Nature, 410, pp. 565-567, (2001)]. The present work investigates the ability to elicit NS behavior without employing physical phenomena such as inherent nonlinear material behavior (e.g., phase change or plastic deformation) or dynamic effects, but rather the controlled buckling of small-scale structural elements, metamaterials, embedded in a continuous viscoelastic matrix. To illustrate the effect of these buckled elements, a nonlinear hierarchical multiscale material model is derived which estimates the macroscopic stiffness and loss of a composite material containing pre-strained microscale structured inclusions. The nonlinear multiscale model is then utilized in a set-based hierarchical design approach to explore the design space over a wide range of inclusion geometries. Finally, prototype NS inclusions are fabricated using an additive manufacturing technique and tested to determine quasi-static inclusion stiffness which is compared with analytical predictions.Show more Item On the crushing of honeycomb under axial compression(2010-12) Wilbert, Adrien; Kyriakides, S.; Ravi-Chandar, KrishnaswamyShow more This 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.Show more Item On the implosion of underwater composite shells(2011-12) Leduc, Mathieu; Liechti, K. M.; Kyriakides, S.Show more The aim of this study was to investigate the dynamic collapse of composite shells in a constant external pressure water environment that is representative of a naval underwater structure. Laminated carbon/epoxy composite shells with diameters of 1.735 in., wall thickness of 0.041 in, length-to-diameter ratios ranging for 2.8 to 12 and [55/-55/(90)3/-55/55] layup were collapsed in a custom pressure testing facility that provided a constant pressure water environment. Buckling was sudden, dynamic, led to failure and fragmentation of the shells; the whole event lasted only a couple of ms. The dynamic collapse of the shells was recorded using high-speed digital imaging and dynamic pressure sensors synchronized with the camera were used to monitor the emanating pressure waves. All shells buckled in mode 2 at pressure levels predicted by models adopted. Collapse led to a localization zone in the central section of the shells, approximately spanning on a 4D length for the longer ones, and shorter for the shorter shells. A single axial crack developed in the collapsing section, which propagated 2 to 4 diameters depending on the length of the specimen. The axial crack was located on the extrados for long shells, and on the intrados for shorter ones. Helical cracks initiated from the tips of the axial crack, propagated outwards, and were responsible for the collapse and fragmentation of the two outer sections. The receding walls of the central localizing zone caused a dynamic drop in pressure that lasted until the inward motion was arrested by contact. This was followed by a sharp, short duration positive pressure pulse associated with an outward expansion wave. The pressure pulse varied to some degree around the circumference with the highest peak occurring opposite the initial crack. The final result of such dynamic events was catastrophic failure and fragmentation of the shell into small shreds.Show more Item Plastic buckling of circular tubes under combined internal pressure and axial compression(2004-12-18) Paquette, J. (Joshua); Kyriakides, S.Show more This thesis deals with the problem of plastic buckling and collapse of moderately thin-walled tubes under combined internal pressure and axial compression. The problem is investigated through a combination of experiments and analysis. Experiments were conducted on stainless steel tubes with D/t ratios of 28.3 and 39.8. The specimens were designed to mimic an infinitely long tube. The tubes were pressurized to pressures ranging from 0-70% of the yield pressure and then compressed under constant pressure. All tubes buckled in the plastic range The first buckling consisted of axisymmetric wrinkling which occurs at increasing stress. Further compression caused the wrinkle amplitude to grow. This reduced the axial rigidity of the tube and eventually caused a limit load instability representing the onset of collapse. The onset of buckling and the onset of collapse were established for each tube D/t ratio as a function of pressure. The bifurcation problem was analyzed using deformation plasticity to find the critical stress and wavelength, and flow theory plasticity to find the critical strain. Both plasticity theories were modified to include plastic anisotropy, which was found to be present in the pipe stock from which the tube specimens were cut. Inclusion of the anisotropic plasticity was essential in making the predictions agree with the measured values. The postbuckling behavior of the tubes was analyzed by considering a shell with a uniform axisymmetric imperfection. Anisotropic flow theory was used in this model. The model reproduces the measured responses up to and including the limit point. For pressurized tubes, predictions of the limit strain were in good agreement with the experiments. For pure compression, the measured and calculated values differed somewhat due to a second nonaxisymmetric bifurcation that was encountered in the experiments but that was not included in the model. The calculated limit stresses were somewhat lower than the measured values due to finite deformation effects not included in the model. Modeling of the anisotropy was essential for the success of this analysis alsoShow more 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, JohnShow more The 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.Show more Item Stretch-induced compressive stress and wrinkling in elastic thin sheets(2010-08) Nayyar, Vishal; Huang, Rui, doctor of civil and environmental engineering; Chandar, Krishnaswamy R.Show more A finite element analysis approach is used to determine the susceptibility to wrinkles for thin sheets with clamped ends when subjected to tensile loading. The model problem chosen to do this analysis is the stretching of a thin sheet with clamped-ends. In the preliminary analysis, a stress analysis of thin sheets is done to study the stresses that develop under these boundary conditions. The analysis shows that there is a stretch-induced compressive stress in the transverse direction to the applied load that causes wrinkles. Then, the parametric study is conducted to determine the effect of aspect ratio and strain on the compressive stress. Based on the results of the parametric study, a critical strain value for each aspect ratio is determined for which the corresponding compressive stress is zero. Further buckling analysis is performed to find the buckling modes of the model problem that shows a limit of aspect ratio below which buckling is not possible under given conditions. Finally, post-buckling analysis shows the nature of wrinkles observed in the model problem for different aspect ratios.Show more Item Theoretical and numerical study on adhesive interactions between graphene and substrate(2018-06-14) Wang, Peng, Ph. D.; Huang, Rui, Ph. D. in civil and environmental engineering.; Liechti, Kenneth M; Landis, Chad M; Ravi-Chandar, Krishnaswa; Ren, PengyuShow more This dissertation presents a set of theoretical and numerical studies on adhesive interactions between monolayer graphene membranes and their substrates. Both continuum mechanics models and molecular dynamics simulations are developed to investigate deformation of graphene membranes depending on the adhesive interactions with the substrates. First, a numerical study on snap transitions of gas-filled graphene blisters is presented, based on a continuum model combining a nonlinear plate theory with a nonlinear traction–separation relation. The numerical results may be used in conjunction with experiments for quantitative characterization of the interfacial properties of graphene and other two-dimensional (2D) membrane materials. Next, a statistical mechanics analysis on thermal rippling of monolayer graphene supported on a rigid substrate is presented and compared with molecular dynamics simulations to reveal the entropic effects of thermal rippling on van der Waals interactions between graphene and the substrate. While the amplitude of thermal rippling is reduced by the adhesive interactions, the entropic contribution of thermal rippling leads to an effective repulsion, thus reducing the effective adhesion. Moreover, the effect of a biaxial pre-strain in graphene is considered, and a buckling instability is predicted at a critical compressive strain that depends on both the temperature and the adhesive interactions. This motivates a systematic study on morphological transitions of monolayer graphene on a substrate under uniaxial compressive strain, from rippling to wrinkling/buckling and to folding. The presence of water at the interface has significant influence on the adhesive interactions between graphene and its substrate. Molecular dynamics simulations are performed to study the interactions between graphene and a wet substrate that is covered by a thin layer of water. Four stages of the traction-separation relations are identified and they are analyzed approximately by simple continuum models. When the thickness of water layer is below 1 nm, the water molecules form discrete monolayer or bilayer structures, leading to different traction-separation behaviors. Finally, with a finite number of water molecules trapped between a monolayer graphene and its substrate, water-filled graphene blisters form spontaneously. Based on molecular dynamics simulations and a simple theoretical model, the work of adhesion for the graphene/substrate interface may be estimated by measuring the aspect ratios of the graphene blisters. Unlike gas-filled graphene blisters in previous studies, the shape and size of the water-filled graphene blister depend on the wetting properties of graphene and the substrate. The results on wet adhesion and water-filled blisters can be readily extended to other 2D materials.Show more Item Viscous lava flows(2009-03) Barker, Daniel S.Show more