فهرست مطالب حسین قره باغی

This paper investigates the mechanical performance and energy absorption capacity of bi-material three-dimensional lattice structures via experimental and numerical approaches. At first, fused deposition modeling was used to manufacture the outer part of the proposed three-dimensional lattice structure with TPU material. Using a syringe, epoxy resin is injected into the inner part of the manufactured lattice structure. Then, quasi-static compression tests were conducted to analyze the mechanical properties and energy absorption capacity of the bi-material three-dimensional lattice structure. As the nonlinear numerical study, the elasto-plasto-damage behavior was implemented in finite element analyses which track the nonlinear response of considered structures. This model is capable to investigate the differences in tensile and compressive properties of the materials as well. The comparison of the load-displacement curve of structures under compressive loading has been compared. The numerical models exhibit an acceptable prediction about the linear and nonlinear responses of the proposed three-dimensional lattice structure. The results reveal that not only does the use of hybrid structures provide more energy absorption and improve mechanical properties, but also the rational combination of two materials makes the bi-material three-dimensional lattice structure with the optimum energy absorption and stiffness, in comparison to those usual lattice structures with a single material.

Welding is very important in the aerospace industry and is widely used in aerospace structures. One of the problems that most industries are facing is created residual stress by the welding process. Residual stresses in the surrounding areas of welding can cause cracks and crack growth so identifying and evaluating residual stresses in the welded structures is necessary. There are different methods for determining residual stress. In this paper, laboratory and numerical methods were presented for determining the residual stress. Then, the welding process of two aluminum sheets of 6061-T6 alloy has been done and the residual stresses have been obtained by drilling method. Welding is done in two passes and by spot welding, the first, the end and the middle of the weld line are connected to prevent the sheets from moving. Also, the welding process of the two aluminum sheets was simulated in 3D in the ABAQUS finite element software and the residual stresses were extracted. All conditions in the finite element analysis are similar to the welding conditions in the laboratory. Results show high accuracy in the modeling of finite element processes in the welding process. Finally, in the finite element method, thermal stress is modeled and optimal parameters are extracted for the above operations.

Welding is very important in the aerospace industry and widely used in aerospace structures. One of the problems that most industries are facing is created residual stress by the welding process. Residual stresses in the surrounding areas of welding can cause cracks and crack growth so identify and evaluate of residual stresses in the welded structures is necessary. There are different methods for determining the residual stress. In this paper, the laboratory and numerical methods were presented for determining the residual stress. Then, the welding process of two aluminum sheets of 6061-T6 alloy has been done and the residual stresses have been obtained by drilling method. Welding is done in two passes and by spot welding the first, the end and the middle of the weld line are connected to prevent the sheets from moving. Also, the welding process of the two aluminum sheets was simulated in 3D in the ABAQUS finite element software and the residual stresses were extracted. All conditions in the finite element analysis are similar to the welding conditions in the laboratory. Results show high accuracy in the modeling of finite element processes in the welding process. Finally, the effect of residual stress in the value of natural frequencies is studied.

Due to the problems in the country (Islamic Republic of Iran) for purchasing aircraft and aircraft old fleet, a solution should be proposedfor this problem. One way to address this need is designing and manufacturing the aircraft in the country.According to the previous investigations carried out, 180-seater passenger planes with a range about 5,000 Km are part of the requirements of the country. In this paper, the costs of design, construction and operation of this type of aircraft has been studied. Also, the inventory costs in the United States of America have been investigated and compared with the costs in the Islamic Republic of Iran. There are three main objectives in this research: the first is to estimate the total expenditures in terms of the different phases of the design, construction and operation; the second concerns reducing design costs to much less than the costs of construction and mining; and finally, the third involves an analogy of the costs of design and production in Iran versus the U.S.

In the first part of this paper، the gas tungsten-arc welding process of a thin cylinder made of Al6061-T6 alloy is simulated using 3-D finite element model in the ABAQUS code and the distribution of residual stress is obtained. Temperature dependent thermo-mechanical properties are considered for the aluminum alloy and for simulating the heat source of tungsten arc welding، Goldak''s double ellipsoid model is also employed. Based on the finite element results، the value of residual stress is considerably positive around the weld line and HAZ area which can increase the risk of crack initiation and propagation in the weldment zone. Hence، in the second part of the numerical analyses of this research a longitudinal semi elliptical crack is considered in the wall of internally pressurized aluminum cylinder and its mode I stress intensity factor (KI) is determined numerically for different crack geometries. Finally the influence of both residual stress field and internal pressure is taken into account on the value of KI. It is observed that the effects of combined internal pressure and tensile residual stresses around the crack can facilitate required conditions for crack propagation in the analyzed cracked thin cylinder.