Mechanical Design

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http://etd.aau.edu.et/handle/12345678/142

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Browsing Mechanical Design by Author "Alem, Bazezew (PhD)"

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    Optimized Design of a Dynamically-Based Motion Generating Spatial Four-Bar Mechanism on Rigid Multibody Systems
    (Addis Ababa University, 2009-10) Yirga, Kelem; Alem, Bazezew (PhD)
    The four-bar spatial mechanism is the most basic chain that can be composed of four links and can include joints with any combination of rotational and translational freedom used in thousands of applications. This thesis will present some of the techniques and introduce solution tools that were not needed for planar motion. One of the new techniques known as the Euler parameters will be considered in this work. The thesis includes modeling, optimizing computer-aided dynamic analysis and simulation of four-bar spatial mechanism composed of rigid bodies that are used for different applications of spatially moving motion generating mechanisms. The Motions of the rigid bodies are predicted by numerically integrating Differential-Algebraic Equations (DAEs) developed from principles of mechanics by the Newton-Euler’s approach. The computer program, MSC.ADAMS2005 will be used to model, solve, simulate, and optimize the dynamics of the appraised spatial four-bar mechanism as a lens-polishing mechanism by integrating the differential equations. Unlike analytical synthesis, optimization allows direct incorporation of a greater number of design constraints, thus resulting in solutions that are more practical. In this thesis, an efficient algorithm known as the Generalized Reduced Gradient (GRG) is used to synthesize all kinematic linkages of the spatial mechanism. This approach will allow monitoring and controlling objectives and constraints, which will yield practical solutions to realistic mechanism design problems with lower kinematic pairs. In addition to the above mentioned points, a mobility analysis has been done for the RSSR mechanism, which is a one degree of freedom, single loop, and spatial mechanism. Thus, this thesis specifically discusses a practical example of a lens polishing four-bar spatial mechanism that simply substitutes the extremely expensive existing polishing robots. This mechanism is applicable in polishing lenses of military fire control instruments found in the Ethiopian Defence Forces. Thus, the design presented in this thesis provides a relatively low-cost solution for the existing problem as compared to the robots. This can be created with ease of manufacture in a machine shop quickly and simply. Numerical results obtained in this thesis are compared with existing literatures.
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    Stress Analysis of a Composite Material Shell Having a Crack
    (Addis Ababa University, 2008-12) Jiregna, Hirko; Alem, Bazezew (PhD)
    Composite materials are artificially made materials used in various mechanical applications like shells and plates. A composite material cylindrical shell is one of the components used in weight sensitive areas like aeronautics and marine environments. In these applications, the weight of the components is much more related to cost and the mechanical properties of the structural material are of great importance to hinder the damage that could result as a result of failure. On the other hand, strength and stiffness are very important parameters to prevent the associated serious failures. So, in order to prevent failure and save weight composite materials were evolved with currently emerging technology. Composite materials are nowadays commonly used in the aviation industry and marine applications in the form of vessels or shells. In this work, what is targeted at is the carrying out the stress variation analysis of composite material cylindrical shell. The analysis was performed by studying the properties of composites, their areas of application, anisotropic elasticity relations, fracture mechanics approach for composites, and finally finite element method application is applied to investigate the stress distribution at the crack tip in the composite material shell. Mathematical modeling and finite element simulation of the cracked shell was performed and the problem was at the end solved using ANSYS finite element analysis package. Consequently, results were obtained in the post processing stage of ANSYS. The main results obtained were the mode of stress and strain variation, the profile of the cracked surface, and the localized critical stress and strain regions at the crack front which could be cold shielded damaged zone. Ultimately, the stress and strain variation, the crack propagation schemes were compared to isotropic material shell to draw conclusion on the advantages of composites compared to conventional materials with regard to crack advancement.