# Browsing by Subject "Buckling (Mechanics)"

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Item On the design of slip-on buckle arrestors for offshore pipelines(2007-12) Lee, Liang-hai, 1973-; Kyriakides, S.Show more Offshore pipelines are susceptible to the damage that leads to local collapse. If the ambient pressure is sufficiently high, local collapse can initiate a buckle that propagates at high velocity catastrophically destroying the pipeline. Buckle arrestors are circumferential local stiffeners that are placed periodically along the length of the pipeline. When properly designed, they arrest an incoming buckle thus limiting the damage to the structure to the distance between two adjacent arrestors. Slip-on type buckle arrestors are tight-fitting rings placed over the pipe. They are relatively easy to install and do not require welding. As a result they have been widely used in shallow waters. It has been known that such devices often cannot reach higher levels of arresting efficiency. The somewhat deficient performance is due to the fact that a buckle can penetrate such devices via a folded-up U-mode at pressures that are lower than the collapse pressure of the intact pipe. Because of this they have not seen extensive use in deeper waters. The aim of this study is to quantify the limits in arresting performance of slip-on buckle arrestors in order to enable expanded use in pipelines installed in moderately deep and deep waters. The performance of slip-on buckle arrestors is studied through a combination of experiments and analysis. The study concentrates on pipes with lower D/t values (18-35) suitable for moderately deep and deep waters. The arresting efficiency is studied parametrically through experiments and full scale numerical simulations. The results are used to generate an empirical design formula for the efficiency as a function of the pipe and arrestor geometric and mechanical properties. The performance of slip-on arrestors is shown to be bounded by the socalled the confined propagation pressure. That is the lowest pressure that U-mode pipe collapse propagates inside a rigid circular cavity. Therefore, a quantitative study of this critical pressure is undertaken using experiments and numerical simulations. A new expression relating this critical pressure to the material and geometric parameters of the liner pipe is developed. This in turn is used to develop quantitative limits for the efficiency of slip-on buckle arrestors.Show more Item Plastic buckling and collapse of circular cylinders under axial compression(2006) Bardi, Francois C.; Kyriakides, S.Show more This study is concerned with the plastic buckling of relatively thick tubes and the ensuing succession of instabilities leading to their failure. The first instability is uniform axisymmetric wrinkling that is treated as a plastic bifurcation. As the wrinkles grow, the axial rigidity of the shell is gradually reduced. This eventually leads to a limit load instability beyond which failure in the form of localized deformation takes place. The problem is studied using experiments and analyses. Stainless steel specimens with D/t of 23-52 were custom-designed to avoid stress concentrations and reproduce long uniform pipe conditions. The specimens were compressed to failure under displacement control. In all cases, a second bifurcation involving nonaxisymmetric mode of deformation preceded the limit load. The bifurcation into axisymmetric wrinkling was determined by monitoring the development of wrinkles on the surface of the tubes. This critical state was successfully predicted using an anisotropic deformation theory of plasticity. The anisotropy of the material was established experimentally and modeled using Hill's quadratic anisotropic yield criterion. The problem was first modeled as uniform axisymmetric wrinkling. The model uses Sanders’ shell kinematics assuming small strains and moderately small rotations and includes a modified flow theory of plasticity to accommodate the anisotropy observed in the tubes. Small axisymmetric imperfections based on the critical halfwavelength were integrated into the model. The problem was formulated through the principle of virtual work and solved using Newton’s method. The solution correctly simulates the growth of wrinkles resulting in a limit load instability. The model included second bifurcation calculations from axisymmetric to non-axisymmetric configuration. Second bifurcation instabilities were found to occur before the limit load developed. For this reason, a second model was developed in which non-axisymmetric deformation of the shell was simulated by introducing both axisymmetric and nonaxisymmetric imperfections. Non-axisymmetric responses were found to be highly sensitive to the imperfections. Each experiment was first reproduced accurately by choosing the right combination of imperfections. However, to achieve a satisfactory prediction of the limit state over the whole range of D/t, a thorough parametric study of the imperfection sensitivity was performed. The relative amplitude of the axisymmetric imperfection to the non-axisymmetric imperfection was found to define whether the shell deforms axisymmetrically or not. Furthermore, if one of the imperfections governs the deformation configuration, then the effect of the second onto the response is negligible. Thus, a constant axisymmetric imperfection of 0.05% of the pipe wall thickness and a non-axisymmetric imperfection proportional to (D/t) 2 / m 3 yielded accurate predictions of both mode of deformation and limit load.Show more Item Stability bracing behavior for truss systems(2011-05) Wongjeeraphat, Rangsan; Helwig, Todd Aaron, 1965-; Frank, Karl H.; Engelhardt, Michael D.; Tassoulas, John L.; Ravi-Chandar, KrishnaswamyShow more The stability bracing behavior of trusses was investigated using experimental testing and computational modeling. The laboratory experiments were conducted on twin trusses fabricated with W4x13 sections for the chord and web members. Spans of 48 and 72 feet were used in the tests that included both lateral load tests and buckling tests. Most of the tests were done on the regular (Howe) truss, except the lateral stiffness tests which were also done on the inverted (Pratt) truss. Computational models were developed using the three-dimensional finite element program, ANSYS, which were validated using the laboratory test data. A variety of models were used to simulate both as-built and idealized truss models. The experiments demonstrated that the buckling capacity of the truss with torsional bracing largely depended on the brace stiffness and the number of intermediate braces. Similar behavior was observed in the truss with lateral bracing. The tests results demonstrated that cross sectional distortion dramatically reduces the effectiveness of the torsional braces. The experiments provided valuable data for validating the finite element models that were used to conduct parametric studies on torsional bracing of truss systems. The results from the parametric studies were used to develop stiffness requirements for torsional bracing of trusses.Show more