Browsing by Author "Mulugeta Habtemariam (PhD)"

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    Investigation on the Intralaminar Fracture Toughness of Woven Sisal/Epoxy Composite: Effect of Glass Fiber Hybridization
    (Addis Ababa University, 2024-03) Biruk Hussen; Mulugeta Habtemariam (PhD)
    A robust and safe design of composite structures for load bearing applications requires the knowledge of their damage tolerance and crack resistance capabilities. To study the effect of glass fiber hybridization on the intralaminar fracture toughness of woven sisal/epoxy composite, tensile mode-I intralaminar fracture toughness experimental tests were carried out on doubly-tapered compact tension (2TCT) specimens prepared from pure sisal, two hybrids of sisal and glass, and pure glass fiber reinforced epoxy composites under displacement control. A data reduction technique recommended for composite laminates based on the finite element analysis (FEA) was utilized. Load-displacement responses were obtained, fracture toughness values based on critical energy release rate (𝐺𝐼𝐶) were evaluated, and resistance curves (R-curves) were plotted for each group of composite laminates and compared to examine the hybridization effect. The fractography was also discussed. The results showed that interply hybridization of woven sisal fibers with woven glass fibers in an epoxy matrix resulted in a considerable improvement of intralaminar fracture toughness values. The hybrid laminates showed an intermediate fracture behavior among their monolithic counterparts. The critical energy release rate (𝐺𝐼𝐶) values of the pure sisal, two hybrids of sisal and glass, and pure glass fiber reinforced epoxy composites were found to be 16.32, 25.06, 27.64, and 39.62 𝑘𝐽/𝑚2, respectively. The results of the research provide an experimental data, which can be used for the safe designing of energy absorbing and other low to medium load bearing structural components.
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    Mechanism Design for Simultaneous Removal and Installation of All Wheels of Passenger Car: Synthesis, Analysis and Simulation
    (2023-06) Natenael Fantaye; Mulugeta Habtemariam (PhD)
    This thesis is about the design of a novel mechanism that will be used in tire repair shops for installing/removing all wheels of a passenger car at the same time. Numerous important repair activities in such shops require all wheel removal. Such activities include all-tire replacement, tire rotation, wheel balancing, wheel alignment and associated suspension repair and brake inspection and service. The work technicians perform to accomplish such activities, although assisted by modern tools and equipment still requires manual work as there is lack of machine or equipment for simultaneous all wheel removal/installation. Currently, technicians that work in tire repair shops are exposed to various musculoskeletal injuries such as strains, sprains and overuse injuries from manual activities such as lifting and maneuvering of wheels, repetitive motion such during work having to work and dirty and noisy environment. The methodology followed includes identifying the functions of the device to be, decomposing them to sub functions, generating alternative concepts to meet the decomposed functions and embodiment design of components assemblies and modules. Part of the resulting overall mechanism is a symmetric 16-bar mechanism that performs the wheel-positioning task. This mechanism required a special attention because its motion is complex. Therefore, as a graduate-level work, this thesis rigorously analyzes the motion of every link in this particular mechanism and every joint that connects the links by using specialized kinematic and dynamic multibody simulation process. The results of the simulation part are position, velocity, acceleration and joint force values – data critically important for further design decisions. Results of the synthesis part is geometric shapes and dimensions of 31 major components and 5 major modules that create the overall mechanism. Geometry and dimension of every component, how they assemble, and their relative motion with respect to one another is thoroughly worked out to the point that further steps of the design process (i.e. detail design) have been reduced into a routine engineering task of sizing cross sections of components and selecting materials to prevent component failure during service. In conclusion, the resulting design solution enables simultaneous all-wheel removal/installation without a need for manual work in the process thereby removing the problems on technicians mentioned above. Also, the complex 16-bar mechanism’s motion is fully characterized by determining all motion parameters that describe its motion. More importantly, significant dynamic forces that result from accelerating masses of the links are determined exhaustively through multibody simulation. Having these force data is crucial to further steps in the design process as these forces must be taken into consideration in subsequent stress and deflection analyses.
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    The Influence of Wheel Flat for Low-Speed train Performance due to Contact Load: A Case Study of Ethio-Djibouti Railway (EDR
    (Addis Ababa University, 2023-04) Yared Wondiye; Mulugeta Habtemariam (PhD); Awel Mohammed Seid (Mr.) Co-Advisor
    Wheel flats are the most common defect that occurs during train operation. When a train wheel is locked during braking, wheel sliding happens, and as a result, wheel flat rises this wheel flat causes severe impact load on both the vehicle and track components, which leads to excessively damaged railway vehicles and tracks. In this scenario, the Low-speed performance of a train generates higher friction on the wheel/rail contact, which causes wear and defects like wheel flat. The rigid multi-body system dynamics simulation considered wheel flat defect and track irregularity were established based on SIMPACK software. The present research work is validated based on measurements and relevant research literature. The current work considered the changes in vehicles running speeds from 40km/hr to 100km/hr and wheel flat lengths from 25 mm to 85 mm and studied the dynamic response (vertical force, lateral force & acceleration of the car body), comfort (vertical/lateral), Ride index (vertical/lateral), the influence of speeds. The results of the analysis showed that the impact load typically rises with running low-speeds and wheel flat lengths, while other parameters, such as vertical acceleration along with its amplitude and power spectrum density rise and reach a maximum value at a specific speed of 70 km/hr, after which it begins to significantly decline. Finally, the results of impact force for the railway vehicle running at different low speeds and with different wheel flat lengths were analyzed with the warning and alarm limts of impact force that can damage the vehicle and tarck components to classify the running condition as safe, moderate and severe operating condition based on the maximum impact force due to the defects.