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In this paper, ratcheting behavior of stainless steel 304L cylindrical shells under cyclic combined and axial loadings are studied, experimentally. Tests were performed by a servo-hydraulic INSTRON 8802 machine and the shells were fixed normal and oblique under 20 degree and subjected to cyclic loads. In this paper, the effect of length of cylindrical shell and the effect of angle of cylindrical shell on ratcheting behavior were investigated. Based on the experimental results, it was found that bending moment plays a crucial role in waste of energy and increase in plastic deformations. Seen that due to the existence of bending moment in different cross section of oblique cylindrical shell, there are more plastic deformation and accumulation in comparison to normal cylindrical shell. Also, analyzing the loading history of cylindrical shell under combined loading, it has been seen that by keeping the mean force at constant value while increasing the force amplitude, the ratcheting displacement became higher and by the prior load with higher force amplitude retards the ratcheting behavior and plastic deformation with samller force amplitude.

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In this article, an analytical procedure is presented for prediction of linear buckling load of a waffle cylinder stiffened by an array of equilateral triangles. The grid stiffened shell is subjected to axial loading condition. The shell has simply supported boundary conditions at its two edges. The equivalent stiffness of the stiffener and skin is computed by superimposing between the stiffness contributions of the stiffeners and skin with a new method. Total stiffness matrix of the shell is composed of stiffness matrix of skin and grids with special volume fractions. In this analysis, using energy method, equilibrium equations of the grid stiffened shell are extracted based on the thin shell theory of Flugge. The Navier solution is applied to solve the problem. A 3-D finite element model was also built in ANSYS software to show the accuracy and validity of the present solution. The results show that the present new approach has high accuracy and precision. The effect of various geometrical parameters on the critical buckling load is investigated. Due to the stability and accuracy, the present method can be used by many designers and engineers to improve their design quality.

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Today, in order to reduce the damage caused by the collision, energy absorbers are used. Thin-walled structures are most popular as energy absorbent that are used in various forms. In this research, the cylindrical absorber made of expanded metal sheets (expanded metal tube) under impact loading has been examined. Expanded metal sheets due to their low weight, effective collapse mechanism has a high energy absorption capacity. Two types of absorbers with different cells angle were examined. First, the absorber with cell angle α =0 and then the absorber with angle cell α =90. Tests are done by drop hammer device. The output of device is acceleration - time Diagram which is shown by Accelerometer that is located on the picky mass. In this study the type of collapse, force - displacement diagram and effective parameters has been investigated. From the obtained results it was observed that the absorber with cell angle α =0, have symmetric collapse and had high energy absorption capacity but the absorber with cell angle α =90, had global buckling and the energy absorption value was not suitable.

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Thin-walled structures are frequently used as energy absorbers in automotive, railway and aviation industries. This paper deals with the collapse and energy absorption behavior of thin-walled structures under dynamic axial loading Numerical modeling was performed using finite element code LS-DYNA. In order to validate the results of finite element analyses, a square tube was collapsed using universal test machine. This tube was then simulated in LS-DYNA, and the results with those of experiments were compared. There was a good consistency between the numerical and experimental results. The tubes with different cross-sections namely square, hexagonal and octagonal shapes reinforced with inside ribs as well as with different scales (ratio of sectional side length of the inner tube to that of outer tube) 0, 0.25, 0.5, 0.75 and 1 were simulated in LS-DYNA. To determine the suitable cross-section in terms of crashworthiness, multi-criteria decision making method known as Technique of Order Preference by Similarity to Ideal Solution (TOPSIS) was employed. The results demonstrated that the double walled tube with octagonal cross-section possessing the scale between 0.25 and 0.5 had the best crashworthiness behavior. To find the optimum values of scale and wall-thickness, response surface method (RSM) and D-optimal criterion using design of experiments (DOE) were utilized Moreover, the effect of number of inside ribs (4 and 8) on the capability of absorbing energy was also investigated and the octagonal tube with 4 inside ribs was selected as an optimal tube with lower maximum impact force.

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In this research, the experimental and numerical analysis of the Al. alloy 5083-H321 fracture behavior under uniaxial and bi-axial tension has been investigated. The bi-axial tension cruciform specimens are made by electrochemical methods, according to Lionel model, for considering bi-axial fracture behavior of the material. The specimens are gridded by electrochemical etching method. A dependent bi-axial tension mechanism is fabricated with relatively high precision machining methods. The experimental bi-axial tests have been performed by the mechanism on the INSTRON-1343 uniaxial machine, at ambient temperature and strain rate of 0.0003 1. For comparing the experimental and numerical results, how to fracture the material at the beginning and development of it, the location of fracture on the test section of cruciform specimen, and the force diagrams on the cruciform specimen arms are of interest can be mentioned. The finite element method has been used with regard to the damage conditions of ABAQUS software for simulating the fracture behavior. The experimental results show that fracture at the specimen center does not happen. The fracture of cruciform specimen begins in the test section of specimen and in range with the corners of the specimen. Furthermore, the strains are minimal near cruciform specimen arms and in the test section area. Also, the gradient of stress is towards the test section and along the corners. There was an excellent correlation between theoretical and experimental results for location of damage initiation in the test section, how to fracture in the beginning and after that, and arms forces.

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Abstract In addition to the operational and environmental loads, an offshore pipeline may be subjected to accidental transverse loads by falling heavy objects or trawl gears. As a result, the load bearing capacity of the pipeline may be significantly impared by the dents, gouges or other types of damages caused by the impact. Such damage to an offshore structure may have serious environmental and economic consequences. In this study, results of experimental investigations on the residual strength of plain and gouged dented steel pressurized pipes under monotonic axial compression are presented. Some series small-scale specimens were fabricated from API-5L-X80 steel pipes with (D/t) ratio of 22 for the purpose of experimental tests. The specimens were dented by a spherical indenter with (d/D) ratio of 0.45 and gouges were applied along the pipe axis on the outer surface of the middle portion, whose cross section was rectangular. Defected and intact specimens were then collapsed by monotonic axial compression loading whilst subjected to constant internal pressure. In this research, effects of some key non-dimensional parameters such as dent depth, presence of the internal pressure and geometrical parameters of gouges have been studied.

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The members of concrete structures may need to be retrofitted for various reasons, including poor quality of materials, design errors, structural changes, non-compliance with the requirements of design codes and also losing cover concrete due to rebar corrosion. Steel jackets, concrete jackets and fiber reinforced polymers can be mentioned as commonly used retrofit methods for concrete members. There are also several methods for retrofitting concrete columns that have lost their concrete cover. Using concrete jacket has some disadvantages, such as a significant increase in the weight of structure, increasing the element dimensions and the required time for the implementation of the rehabilitation. On the other hand, steel jackets have various difficulties in implementation stage. In this study, specimens of square concrete columns that have lost their cover concrete due to rebar corrosion have been investigated and have been retrofitted by combining a number of methods. These methods include the use of a new concrete layer and the wrapping of columns with carbon fiber. Combining these methods will result in the enhanced performance of the rehabilitation technique since these methods will cover the deficiencies of each other. It is expected that these combined methods will result in increased load capacity, energy absorption and ease of forming. Therefore, in this study, the combined effect of carbon fibers and high performance concrete layer is investigated. The combination of high compressive strength of this type of concrete and high tensile strength of carbon fibers can be used to increase the axial load capacity and energy absorption of square concrete columns. The variables of this study include the type of cover concrete (UHPC, UHPFRC and SCM) and the number of layers of carbon fiber (one or two layers). The total number of specimens in this study was 42, of which 6 were control specimens, 6 were damaged control specimens and 30 were damaged specimens, which were retrofitted with cover layer and carbon fiber. All of the specimens are placed under uniform axial load. The results of the experimental study show that in the retrofitting of the square column, it is better to use the UHPC coating layer. While it is better to use a self-compacting mortar as a coating layer in retrofitting the circular column. Retrofitted columns have significant increase in strength and energy absorption capacity compared to the control columns. The least effect was seen for the columns retrofitted with the coating layer of ultra-high performance fiber reinforced concrete (UHPFRC), which showed an increase by 33% and 85% in terms of strength and energy absorption with respect to the control columns. The greatest effect was seen in the columns retrofitted with self-compacting mortar coating layer with two layers of carbon fiber, which increased the strength and energy absorption by 210 and 480%, respectively. Also, the results show that because the confinement effect in the circular sections is uniform and the entire concrete is effectively confined, the effect of the type of concrete on the coating layer is reduced. A numerical study on a real-dimension column was carried out to verify the results of the laboratory tests and also in order to allow the experimental results from small-sized samples be extended to large-scale columns. The results showed that by increasing the column dimensions, the carbon fiber confinement effect is significantly reduced.

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Thin-walled tubes play a significant role in increasing the energy absorption in energy absorbing systems. Holed thin-walled tubes are a suitable option for use in these systems due to the ease of production and the lack of geometry complexity. In this paper, a new geometric pattern for holed thin-walled cylindrical tubes made of aluminum alloy 6061 is presented, to improve the energy absorption characteristics. To this aim, the Taguchi design of experiment method has been used to find the optimal levels of the geometrical parameters of the tube to achieve the maximum energy-to-weight ratio and the minimum effective equivalent strain. The number of rows of holes, the number of holes in each row, the diameter of the small hole and the diameter coefficient of the small hole were considered as the geometric (input) parameters of the tubes. The initial crushing force, the total absorbed energy, the ratio of energy to weight and the ratio of the maximum initial force to the average force were compared for the optimal layouts. Examining the results showed that the arrangement of the holes in the middle with 3 rows of holes, 8 holes in each row, diameter of the small hole of 5 mm and the diameter coefficient of 1.2 (the large diameter is 6 mm) will lead to the best energy absorption result.