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Today, many researchers have focused to use of composite materials in various industries. Most of the composites used in industry are structurally sandwich composites. Sandwich composites consist of three parts: shells, core and adhesive materials. Sandwich structures are mostly used where the weight and volume of the structure is superior to its strength. Therefore, if sandwich structures can be strengthened in some way without huge increase of weight, they will be a very good alternative for all types of metal and composite structures. One of the issues in the field of pressure vessels and composite bodies is the study of their endurance in the face of external pressure. Due to the creation of unstable buckling in tanks under external pressure, the buckling is always considered as the main mechanism of their failure.The current study aims to investigate the core of sandwich structures by modeling the tanks and bodies under external pressure of these structures using the finite element method. Two modes were examined for modeling: one with a solid foam core and the other with a honeycomb-shaped core (trapezoidal). Each of these modes was modeled in two distinct materials, as well as three various dimensions and sizes, which were first derived linearly by the first buckling mode for each of the modes; The problem was then nonlinearly analyzed for the most crucial situation with coefficient of initial geometric disturbances . By analyzing prior research in this subject, the results of finite element analysis were confirmed. The usage of sandwich composite constructions in the face of external pressure conditions, according to the findings, can be highly useful.

Composites are commonly used in pressure vessels. One of the critical problems in manufacturing composite vessel under the external pressure is sealed opening design and its considerations. It's required to review the body resistance in opening locations. In this paper, a special design process for opening closure presented and the influence of such cutouts on vessel body studied. Because of the importance of body resistance, the vessel material and structure mechanical performance were analytically evaluated. Then, Mechanical behavior of the vessel structure was investigated based on the parameters affecting the definition of different opening designs through finite element simulation and analysis in ABAQUS commercial code/software. Hence, the effect of the presence of openings, including the consequences of creating asymmetry in the structure, has been determined in a large aspect ratio. Finally, the results represents, the importance of complying practical requirements and implementation of reinforcement rings in the opening locations to get the suitable design as one of the most effective solutions to compensate the opening reduction consequences i.e.; buckling strength reduction and highly increasing asymmetry.

Because of the high ratio of strength to weight, composite vessels are widely used in maritime and aerospace industries. The way of composite vessels breaking under hydrostatic pressure is assignable by paying attention to the geometry and material's properties. In composite vessels, using of composite stiffeners has high complexity in manufacturing process. On the other hand, due to the low young's modulus of composite materials, usually cannot use of the materials as a stiffener. In this research, effects of using hoop stiffeners on buckling pressure of composite bodies have been studied. In this paper, mechanical manner of a composite body is studied by Finite Element Method (FEM) in two statuses (with metal stiffener and without metal stiffener). According to results, using of a metal hoop stiffener that has 3.6 percents of the composite body weight increases buckling pressure up to 25 percents. Based on results increasing the buckling pressure of composite bodies by mounting steel hoop stiffeners added lower weight to the suite from increasing of thickness of bodies. For verification of numerical results, a composite vessel of GRP (Glass Reinforced Polymer), reinforced by a hoop stiffener mounted on inner surface has been experimentally under hydrostatic pressure to the breaking step. Finally, it was broken by 16 bar pressure. The results showed that the estimated pressure of software has a little difference with the experimental estimation that represents high accuracy of modeling process in the research.

Today, composite cylindrical shells extensively, have been used in aerospace to increase the performance of the structures. Experience has shown that, when the shells are under compressive loading, Buckling phenomenon will be the most important factor in structural failure. Therefore, the main objective of this research was to study the buckling behavior of composite shells and effective parameters in occurrence of buckling. For this purpose, the finite element simulation by ABAQUS software has been used. First, the estimation of the buckling force for an experimental specimen occurred by using three analyses, including: linear buckle, nonlinear static and dynamic, to compare with experimental findings. Results indicated that the nonlinear static and dynamic analysis, were able to estimate the actual buckling force. The estimation accuracy with respect to the experiment results is about 3~7 percent. After validation process, specimens with different thickness and initial geometric imperfections have been studied and buckling force was measured by using linear buckling and nonlinear static analysis. Finding showed that even slight geometric imperfections, especially for low thickness of shell, could extremely reduced the real buckling force. As a result, high sensitivity of composite shells to initial geometric imperfections, will lead to an unreliable estimation of the buckling force, if there is not enough knowledge of these geometrical imperfections.