Sharif Digital Repository

Recently, mechanical properties of advanced materials such as nanostructured and ultra-fine materials and especially nanocomposites were studied by lots of researcher. In this study, fatigue behavior of Al-SiC nanocomposites is observed. Ultra-fine grains Al matrix nanocomposites reinforced with 50 nm diameter SiC nanoparticles were produced by milling and hot extrusion. For this purpose, pure aluminum powder was mixed with 2, 4 and 6 Vol. % of SiC nanoparticles and then milled by planetary mill. The milled powder then were degased in an Argon atmosphere furnace, cold pressed and then hot extruded. Closed die-hot forging was used with about 18.8% reduction in cross section in order to... 

By investigating reports of severe and destructive earthquakes of the past years we have realized that shear-wall structures most of the times had better seismic performance and more reliability than moment resistance frames. Asymmetrical arrangement of shear walls due to architectural limitations caused distortions in the structure. On the other hand, seismic regulation don’t compliance existence of twisting moment in the structures. What was said are all the most important reasons for not employing asymmetric shear wall sets in the structures by designers. The purpose of this study is to compare the seismic behavior of symmetric moment frame structures with seismic behavior of symmetric... 

According to the importance of obstacle and collision avoidance for multiple Autonomous underwater vehicle (AUV) that are synchronized for starting the path following, in this study the coordination path following controller for a group of underactuated underwater vehicle with considering of obstacle and collision avoidance presented. The proposed control method is based on graph theory and back stepping with modeling of obstacle using limit cycle. At first, for controller design the back stepping method for path following of one AUV is used. Then the designed controller using of principles of graph theory, has been extended for multiple AUV and as the result multiple AUV is synchronized and... 

Today, the use of controllable swimmer micro-robots has expanded from micro to macro-scale dimensions due to their many applications. These robots, as a result of their small size and dimensions, have the ability to access limited and complex environments. Considering the various applications of such swimmers, it can be said that in addition to the necessity of well designing and implementation, their maneuverability and controllability are among the most important research areas on these swimmers. Regarding the dimensions and technologies needed for these robots, various methods for designing and implementing them are suggested, which are mostly inspired by the nature of microorganisms. In... 

Ocean waves are a huge, renewable energy resource, and the potential for extracting energy from waves is high. So a lot of different systems have been designed but most of them are hydraulically and have a lot of problems and they are very complex. In this thesis, the mechanism designed is purely mechanical and very simple and innovative. It has some gears in special combination with together. The mechanism can convert any form of oscillation to rotation movement; this event has been shown in results and prototype. According to the results and discussion, Designed mechanism needs very low maintenance and has low lost. So, efficiency is between 80-95%  

Despite the proven advantage of the engineered water flooding technique, a coherent and mechanistic understanding of the fundamental phenomena occurring at pore scale is lacking. Most of the available simulation models have a phenomenological approach and have limited predictive capability. One of the key questions is how to justify and relate large (Darcy) scale observations to effects and phenomena that essentially occur at much smaller scales (i.e. pore and molecular level). Furthermore, two-phase flow dynamics and the effect of complex interplay between wettability, capillary number, and ions dispersion in a heterogeneous porous medium on the trapping and mobilization of oil at pore... 

Cable-driven parallel robots (CDPRs) form a class of robotic manipulator in which the actuation end-effector is transmitted through cables. Due to the unique characteristics and advantages of cable transmission, CDPRs have become increasingly studied in recent years. In this thesis, the study of Geometric Analysis and Dynamic Trajectory of a 5-DOF Cable-Suspended Spatial Parallel Robot is investigated. Geometric analysis is corresponds to the workspace on which the robot is designed. The design of cable mechanisms allows the robot to be used with appropriate configuration in various fields such as industry, medicine, etc. The study of CDPR design has been limited due to several reasons.... 

Aerodynamics loads and aeroheating during reentry, are one of the challenging issues in designing One of the main challenges in the design and application of reusable spacecrafts is the aerodynamic and thermal loads acting on their structures during reentry into the atmosphere. For this reason, all the concepts/prototypes presented in the 20th century were either not fully reusable, or were partially canceled, or did not have economic justification. In this report, the aerodynamic loads on the first stage of a launch vehicle during the launch and return phases have been investigated. Due to the fact that the main part of the first stage of the vehicle includes fuel and oxidizer tanks, the... 

The collision of moderately large asteroids and comets also referred to as Near-Earth Objects (NEO's) with earth would have catastrophic consequences. Such events have occurred in the past and may occur again in the future. However, for the first time in known history, humanity may have the technology required to counter this threat. Methods studied for mitigation of this hazard are based on deflecting asteroid's trajectory and are normally divided into two basic categories: high energy impulsive methods, and long-duration low-thrust methods. Two parts of mission to be optimized are the trajectory of spacecraft to reach the target and the deflection strategy which could be analyzed by the...