Mechanical Design

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    Fabrication, Mechanical and Physical properties Characterization of Sisal Fiber reinforced Epoxy Composites for Automotive Parts
    (Addis Ababa University, 2021-09) Araya Abera; Anthony N. Sinclair (Prof.); Daniel Tilahun (prof.)
    The demand for natural fibers to replace synthetic fibers in composites manufacturing has grown as a result of the high energy consumption and pollution created in the production and usage of synthetic fibers. Natural fibers have several advantages over synthetic fibers, including low cost, biodegradability, and nontoxicity. In this study, as received Sisal fibers (SF) and those whose surface were chemically modified using 10 wt.% NaOH for 3 h at 60oC and compression molded to produce epoxy matrix composites containing 15, 25, 30, 35, and 40 wt.% of SFs. The composites' morphological, thermal, tensile, flexural, impact physical, and structural characteristics were studied before and after treatment. Scanning electron microscopy (SEM), thermography analysis (TGA), dynamic mechanical analysis (DMA), and Fourier transform infrared spectroscopy (FTIRS) are some of the techniques used. The impact of different fiber loadings and chemical treatments on the mechanical, physical, and thermal characteristics of sisal fiber reinforced Epoxy composites were studied. Mechanical properties of the composites were investigated using InstronTM machine. The effect of chemical modification on water uptake of the composites was also studied. Surface chemical treatment of the SFs by soaking in 10 wt.% NaOH for 3 h at 60oC resulted in 20% increase in the cellulose content of the fiber. In the treated SFs, FTIR spectroscopy revealed a decrease and removal of certain noncellulosic components. Although the treated fibers increased, their tensile strength and water absorption capacity decreased as compared to non-treated fibers. The use of sisal fiber in reinforcing epoxy for both treated and untreated led to increase in tensile modulus, tensile strength, flexural strength, flexural modulus and crystallization temperature of the composite when compared to less fiber loading. The rate of water absorption for composites containing treated fibers is lower than that for composites containing untreated fiber. Results also showed that increasing fiber content decreased the tensile strength and flexural strength after 30wt. percentage fiber loading is reached and the impact strength increase with increase of fiber loading. After characterization of the physical and mechanical properties of sisal fiber reinforced epoxy composite, we have seen good agreement with the literature reviewed materials so that this composite can also be used for substitute of materials for automotive parts application. After all we have developed lowvelocity drop weight impactor and we have seen the results which we found from our test machine agreed with the other machines which has similar features.
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    Performance Analysis of Nano Reinforced Polyether Ether Ketone (PEEK) Gear
    (Addis Ababa University, 2024-04) Tariku Debebe; Tariku Debebe (PhD)
    Current research trend is shifted in finding better materials and enhanced material for gear application. Polymer nanocomposites materials are being used as alternative gear materials for the application of power transmission due to their numerous advantages. Polymer nanocomposites are lighter than conventional materials. They have high degree of stiffness and strength compared with high-density materials. In this study, best performing material is chosen scientifically comparing different materials that are used for power transmission that can sustain high cyclic load & wearing from recently developed literatures. Then the material property is predicted applying machine learning method. Chemical composition consisting of Polyether ether ketone (PEEK) polymer matrix composite material and the 4% nano filler multi-walled carbon nano tube (MWCNT), 3% Zicronia (ZrO2) and 5% Silica (SiO2) by weight is selected and the material mechanical properties is obtained from the trained machine learning model. Using the data obtained analysis of the gear with different loading condition is simulated with ANSYS software and the results are compared with recently developed gear materials for power transmission. The result shows that the addition of nano particles in PEEK polymer can improve the property of the material. The material gear life cycle is found to be one million cycles with loading condition of 1,450 rpm and 21 Nm torque and the maximum bending stress at this load is recorded for at the transient structural FEM analysis which is 75.22 MPa while in static structural FEM analysis is obtained 64.48 MPa. Whereas for contact and frictional stress close results are obtained. The polymer nanocomposite gear material is appeared to be potential gear material with enhanced material property, good strength and durability for the application of power transmission.
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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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    Design and Analysis of Electromagnetic Braking System for LightWeight Vehicle
    (Addis Ababa University, 2024-01) Harnet Tekle,; Daiel Tilahun (PhD)
    In this study, an optimized electromagnetic brake system (EMB) is designed and simulated to replace the conventional hydraulic brake (CHB) system of a lightweight vehicle Lifan 530. The electromagnetic brake system is a pure frictionless electromechanically controlled actuator that has the potential to further reduce braking time and braking distance. The primary goal of this research is to design an electromagnetic braking system that replaces the CHB of the Lifan 530 car to stop as a standalone using a 12v battery source. A theoretical and numerical model is developed in the quarter vehicle model. Using ANSYS Workbench 2020 R2, a 3D finite element model of the EMB was created by SpaceClaim 2020 R2 to simulate and optimize the EMB domain using magnetostatic analysis to study the magnetic flux and braking torque created by the system. Steady-state thermal analysis was performed to investigate heat buildup and the thermal effect of the electromagnet on the brake system. In addition, a static structural analysis is performed to investigate the structural response to temperature changes. The finite element (FEM) has been used to optimize the magnetic circuit design for maximizing the braking torque. The optimized electromagnetic result is achieved by using a wire diameter of 3.2 x 10−4m and 70 turns. Which results in a maximum torque of 338 𝑁𝑚 The simulated result revealed that the braking torque increases with the conducting area and shows the suitability of the concept for the selected vehicle in terms of dynamics, installation space, and energy requirements without demanding extra battery. Finally to validate the numerical result MatLab and ANSYS software are utilized.
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    Fracture Analysis of All Composite Made Fuselage Shell Under Random Vibration Loading
    (Addis Ababa University, 2011-07) Kirubeil Awoke; Idalberto Mendoza (Prof.)
    Aircrafts and its airframes are subjected to various kinds of time dependent loadings; ranging from flight loads to ground maneuvering loads. In this thesis, the dynamic responses such as stress, strain or displacement near the crack tips on the all composite made fuselage structure when landing on various types of take off and landing pavement are assessed. Finite element modeling and analysis for all composite made fuselages is done using the layered shell element 4 node 63. A crack of significant parameter is generated on the outer top part of the shell for initiation of fracture. One leg model of the landing gear is used to drive the required mathematical formulations. The crack tips responses to random excitation caused by road roughness are determined. The excitation includes smooth, pastured and ploughed take off and landing strips. In all curves as the crack length increases the displacement and stresses response near the crack tips increases. We can see from the literature that the applied stress is directly proportional to the square root of the half crack length. This result was verified experimentally by Griffith for a wide range of crack length. This confirms both the analytical and experimental results obtained by Griffith’s and other similar researchers. From the curves we can observe that the shell responds relatively lowest stress and displacement response to class A than Class G and Class H pavement. Class H has the worst stress and strain response near crack tips and much affects and severs the thin structure. More over, stress and strain response to circumferential crack orientation is higher than the corresponding longitudinal one. This is probably due to the stress waves are perpendicular to the orientation of a crack, and which maximizes the local stress. Generally, the shell structure has higher strength to weight ratio and has higher stress and strain carrying capability, trends should look for using shells as their primary structure. Moreover, in the event of forced and emergency landing, the pilot has recommended to land as much as possible on Class G pavement than Class H pavement so that the applied stress is optimized.
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    Simulation of the Effect of Aerodynamics on Quadcopter UAV for Precision Agricultural Sowing
    (Addis Ababa University, 2023-10) Anania Mulugeta; Behailu Mamo (Mr.)
    Nowadays, agricultural aerial vehicles are being broadly applied at the focal point of the farming development. Where crop damage rate raised by 1 % without mechanization at sown area, wind was one of the negative factors to utilization of UAVs for this purpose. This research was directed with the objective of characterizing three quadcopter UAV models and numerical CFD analysis of the models to study the aerodynamic behavior and stability analysis for precise agricultural sowing application. Implementing the finite volumetric analysis using Rynolds Average Navier Stock method, Ansys was used to simulate the fluid flow simulation along the identified horizontal and downwash wind fields for wind speed ranges of 0.3 m/s, 0.6 m/s, 0.9 m/s, and 1.2 m/s, 2.5 m/s considered in case of altitude increase. The objective parameters were velocity magnitude, pressure distribution, drag force, drag coefficient and moment coefficients. Findings of drag coefficients of rectangular solid, crossed helical, mobius ring all with elliptical hopper were 0.274, 1.29, 2.5 respectively, for downwash flow and 1.5, 2.6, 3.7 respectively, for horizontal flow during 2.5 m/s wind flow. Instability in terms of impulse response were more pronounced on mobius ring with elliptical hopper at 2.5 m/s wind gust while rectangular solid with elliptical hopper had revealed a desirable minimum drag coefficient and stability condition on moment coefficients for both flow cases as compared to the other models. In conclusion, shape effects had been observed on the variation of drag and stability values particularly on mobius ring with elliptical hopper
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    Numerical Modeling of the Residual Stress Induced by multi-impact Shot Peening process on Carburized Fe-0.85Mo-0.35C and Fe-1.5Mo-0.3C Steel Materials
    (Addis Ababa University, 2023) Esmael Endris; Samuel Tesfaye (PhD)
    Shot peening is a surface treatment process in which a large number of small shots are impact an engineering component to generate a compressive residual stress layer at the surface of the component. The compressive residual stress increase fatigue life, resistance to stress corrosion, refine grain size, improve microhardness and prevent crack propagation on the component. However, improper shot peening process parameters would affect the surface quality, and have negative effect on the fatigue performance. The experimental assessment of shot peening process parameters is not only very complex but costly as well. In this paper, a sequential model of multiple-shot impacts has been established to investigate the shot peening process on a carburized prealloyed Fe-0.85Mo-0.35C and Fe-1.5Mo-0.3C steels. A commercial Finite Element Method ABAQUS/Explicit was used to model and simulate the process. The sequential model was applied for the predication of residual stress along the depth profile was obtained in the impact region. Furthermore, the numerical results of compressive residual stress were compared with the experimental result obtained using the x-ray diffraction (XRD) analysis. A parametric study is conducted to investigate the effect of shot velocity, diameter, initial stress and coverage on the residual stress profile. The result demonstrated that increasing shot velocity, diameter and coverage results an increase the surface compressive residual stress and decrease depth of compressive residual layer. On the other hand, initial residual stress increase depth of compressive residual stress layer. The simulation result was in good agreement with the measured result for both steels.
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    Modeling of Geometric Pore Parameters for the Sintered Matrix (Fe-0.85Mo-0.35C) to Study the Effect of Pore Size and Shape on Elastic Properties of Steels
    (Addis Ababa University, 2023-06) Fentahun Yihunie; Samuel Tesfaye (PhD)
    Porosity is a void and inherent characteristic of powder metallurgy (PM) material. This thesis work is modeling of representative volume element (RVE) microstructures with square, triangular and rectangular pore shape with different circularity and study the effect of pore shape and pore parameters on material properties for the Sintered steel (Fe-0.85Mo-0.35C) using finite element method (FEM). Solidwork and Digimat were used to model RVE microstructure and ABAQUS to simulate the process. A parametric study is conducted to investigate the effect of neck radius of curvature (R’=0, R’=0.5, R’=1 and R’=1.5), equivalent diameter, circularity and fraction of load bearing section. The low circularity is determined pore which is 0.21, and the high circularity is determined for circularity pore (1). The low circularity 0.21 yield strength is 348.55 𝑀𝑃𝑎 and high circularity of 1 yield strength is 719.24 𝑀𝑃𝑎. Besides, sharpness of pore, triangular pore has good yield strength than square and rectangular pore. For the effect of neck radius of curvature (R’ = 0) square pore have high elastic modulus (160.98 𝐺𝑝𝑎) and triangular pore have low elastic modulus (147.68 𝐺𝑝𝑎). And (R’ = 1.5) triangular pore have high elastic modulus (176.2 𝐺𝑝𝑎). With circularity of pore from all pore model triangular have low circularity of 0.21 with low elastic modulus of 147.68 𝐺𝑝𝑎 and high circularity of 1 with high elastic modulus 184.75 𝐺𝑝𝑎. There is high equivalent diameter in rectangular pore of with low elastic modulus (149.84 𝐺𝑝𝑎) and low in triangular pore with high elastic modulus (184.75 𝐺𝑝𝑎). RVE Microstructure with 8 % square triangular and rectangular pore shows direct relationship between the circularity, neck radius of curvature and fraction of load bearing with the mechanical properties of the material. And inverse relationship between pore size and mechanical properties of material.
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    The Analysis of Creepage at the Wheel-Rail Interface for Light Rail Transit
    (Addis Ababa University, 2023-10) Gemechis Edosa; Samuel Tesfaye (PhD)
    This study focuses on the analysis of creepage at the wheel-rail interface in railway systems. Creepage refers to the relative motion between the wheel and rail when the sliding force exceeds the adhesion force. A multi-body system dynamic simulation approach using Simpack software is employed to capture the dynamics of the wheel-rail system, including interaction forces, contact patch behavior, and resulting creepage. The objectives of this analysis are to investigate the effects of rail irregularity, velocity variation, and adhesion coefficients on creepage and creep forces. The study first examines the impact of rail irregularities on creepage and creep forces by analyzing their presence in the wheel-rail interaction. Subsequently, creepage and creep forces are studied in the absence of rail irregularities to isolate their effects on the overall system dynamics. The influence of velocity variation on creepage and creep forces is also investigated, considering variations in vehicle speed that can significantly affect the behavior and forces at the wheel-rail interface. The results indicate that rail irregularities increase creepage and creep forces, while their absence leads to lower values. Higher velocities generally generate greater creepage and creep forces compared to lower velocities, highlighting the importance of considering velocity variation in the analysis. Furthermore, the adhesion coefficient plays a crucial role in determining the level of creepage and creep forces. Higher adhesion coefficients are associated with lower creepage and creep forces, emphasizing the influence of adhesion on the wheel-rail interface. The study concludes by analyzing creepage and creep force considering different adhesion coefficients. Analytical simulations are conducted to assess the effects of varying adhesion coefficients on creepage and creep force, providing valuable insights into the influence of adhesion on the wheel-rail interface. Overall, this research contributes to a better understanding of creepage at the wheel-rail interface by considering various conditions such as rail irregularity, velocity variation, and adhesion coefficients. The findings have implications for improving the performance and safety of railway systems.
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    Numerical Analysis on the Impact Resistance and Energy Absorption Behavior of Motorcycle Helmet for Different Impact Conditions
    (Addis Ababa University, 2023-10) Tsehayneh Negash; Araya Abera (PhD); Tadesse Nega (Mr.) Co-Advisor
    Head injuries are common during different activities of daily life – one is related to motorcycle vehicle traffic accident. The aim of this research was concerned on the analysis and evaluation of the effect of impact loads on the basis of impact velocities and angles in the case of accidents. The investigations were carried out using numerical analysis. The independent variables were impact velocities – ranged from 7.5 m/s to 17.5 m/s and impact angles – ranged from 00 to 900. Flat anvil and footpath scenarios were also conditions on which the effects of angles were determined. The peak linear acceleration (PLA) and head injury criteria (HIC) as well as energy absorption effect were the output parameters for the study. Solidworks 2022 SP1 and ANSYS – LS DYNA (HyperMesh 2019.1 – LsDyna (Keyword971_R10.1)) were employed for the modellings and numerical analysis. The Economics Commission for Europe (ECE 22.05) specification was selected to investigate the specified parameters related to the standards. Based on the obtained results when using stiff liner component, the PLA ranges from 199G to 622G (PLA per gravity) and HIC ranges from 1927 to 23900 when the impact velocity increases from 7.5 m/s to 17.5 m/s for frontal impact. Similar incremental trends were also recorded on the crown, rear and lateral impact positions. Using soft liner material, the effect of angles on the crown impact position at 16.5m/s were investigated and 153G of PLA and 1708 of HIC recorded at impact angle of 900. But when the value of the angle decreased, PLA and HIC values also continuously decreased. So, the severity of the accident increases with impact velocity and impact angels on flat anvil impacts. The extent of risk for the accidents which happened on footpath curve, minimized as the inclination of the resultant impact velocity was about 450 for the given specified impact velocity; this is because the impact load decomposed towards the two impact points. Using the model for the specified limit can provide the protection; but crossing the limit may result the risk too severe.
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    Mounting Plate, Stress Concentration and Damage Tolerance Analysis for Antenna Installations on Pressurized Transport Aircraft Using The FEM
    (Addis Ababa University, 2021-07) Woldeyesus, Mideksa; Behailu, Mamo (Mr.)
    During the in-service life of a given aircraft, different antennas are installed on the fuselage of the aircraft for different operational and regulatory requirements. These antennas are part of the Communication, Navigation and surveillance (CNS) systems of aircraft. Antennas play the greatest role in enhancing the operational safety and functionality of a given civil or military aircraft by providing reliable communication and navigation systems. During antenna installation, the fuselage skin loses its structural integrity. Taking this into consideration and also fracture mechanics as a basic principle, damage tolerance analysis for antenna installations on pressurized transport airplanes has been studied. For skin modifications, first from a static strength point of view, the elliptical antenna mounting plate is compared with those of circular and rectangular mounting plates. Next, based on the first result, from the stress concentration factor point of view, the best antenna cut-out shape was studied by comparing antenna cut-out shapes such as elliptical, circular, and square cut-outs for mounting plates subjected to biaxial loads. Then, considering the best mounting plate shape and antenna cut-out, i.e., having the lowest stress concentration factor, damage tolerance analysis (DTA) is done using Linear Elastic Fracture Mechanics (LEFM) and the establishing inspection type and inspection threshold are studied. To perform the above activities, a special tool, Finite Element Analysis (FEA), i.e., ANSYS application software, was used to perform static and fatigue simulations along the potential crack growth path for a flaw initiating from the antenna cut-out normal to the stress application direction. The output from finite element modelling and analysis is compared with the analytical for the safe installation of the antenna. With respect to the above framework, the study was done, and the result shows that the circular mounting plate is relatively better than those of the rectangular mounting plate and the elliptical mounting plate. With regard to the cut-out shape, for the applied biaxial tension loads, a circular cut-out is relatively better than those of the square and elliptical cut-out shapes. A circular mounting plate with a circular antenna cut-out is better than an elliptical or rectangular mounting plate. Starting with the initial crack length of a i and the progressive crack length of a i+1 to the critical crack length value ac, which is calculated using the fracture toughness of the skin materials, an estimated inspection type and inspection threshold (damage detection) were established.
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    Modeling the Interfacial Shear Strength of Natural Fibers Epoxy Matrix by Pullout Failure Mode
    (Addis Ababa University, 2022) Fasil, Henok; Samuel, Tesfaye (PhD)
    Recently natural fiber composites are overtaking the place of synthetic fiber composites for many applications, so it seems crucial to model interfacial shear strength of it and study the load transfer efficiency between the fiber and the resin, which plays a significant role in determining the mechanical properties of the fiber reinforcement which the is fiber-epoxy matrix. In this paper two fiber; Pineapple fiber and kapok, which has the highest and lowest young’s modulus are chosen. Twelve models, six for each kind of fiber, are developed using ABAQUS software considering different conditions, such as various fiber embedment lengths, fiber diameter, and void sizes for fiber pullout failure of the interface between the fibers and epoxy. Parameters studied are fiber embedded length of 3 mm and 0.1 mm; fiber diameter 48 for pineapple and Kapok fiber with a length of 1mm and 0.1 mm with a diameter of 33 is taken. The effect of the presence of voids on the interface of fiber and epoxy is studied. The average interfacial shear strength obtained, between epoxy and fiber interface, are for pineapple and for kapok. This study shows the interfacial shear strength is dependent on the elastic modulus and density of the fiber but the change in the interfacial contact surface area due to change in embedded length of fiber has no significant effect on the interfacial shear strength value. But the change in diameter of the fiber and void ratio affects interfacial shear strength. Rather the presence of void does have a positive effect on the incremental of the value interfacial shear strength.
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    Numerical Simulation for High Cycle Fatigue Life Prediction of Pre-tightened Bolts of Connecting Rod: TOYOTA D4D Dolphin Case
    (Addis Ababa University, 2022-03) Bikila, Debela; Samuel, Tesfaye (PhD)
    The connecting rod (Conrod) bolts are integral components of the engine that are used to connect the piston with the crankshaft through the connecting rod. If the conrod bolts pretension reliability is insufficient or excessive, deformation and fatigue failure will occur, resulting in component breakage and engine failure. At local maintenance shops, they are using different replacement materials than the initial material of the new automobile. The use of improper material is also the problem of engine failure. The focus of this thesis is to estimate the fatigue life of TOYOTA D4D Dolphin connecting rod bolts at high cycle using Finite Element Modeling with the help of software—ANSYS 2020 R 2 and MATLAB tools. The high cycle fatigue life was analyzed at different pretension forces to examine the effect of bolt pretensions and determine the appropriate bolt pretension value. The prediction of high cycle fatigue life was estimated using the Stress-Life approach. Additionally, the total deformation at insufficient and excessive bolt pretension was analyzed. Then the proper bolt pretension has been selected based on the analysis. Again, the existing in-use material for conrod bolts was compared with those the materials used by local automotive maintenance shops as a replacement. Their safety factor was identified to suggest which material is suitable for connecting rod bolts. The study results from the numerical simulation show that the value of bolt pretension has a more significant effect on the fatigue life and total deformation of connecting rod bolts, especially at higher cycles. When bolt pretension changed from 22KN to 30KN, the number of cycles of the materials reduced. The Insufficient values (<22KN)(i.e., 20KN to 0KN) of bolt pretension were applied to the model, and the total deformation increased from 0.16494mm to 0.26291mm, respectively. Again, the excessive values (>22KN) (i.e., 23KN to 50KN) of bolt pretension were applied to the model, and the total deformation increased from 0.16451mm to 0.21111mm, respectively. At the value of 22KN bolt pretension, the total deformation showed the lowest number (0.16449mm), which is the appropriate point to apply the clamping torque. In addition, the applied 22KN to 30KN pretension loads on the existing material (ARP2000), ARP625+, and ARP3.5 resulted in the fatigue life of 498.93*10 6 to 489.13*10 6 , 409.71*10 6 to 399.4*10 6, and 407.59*10 6 to 397.29*10 , respectively. For checking the safety factor of the materials, ARP2000 has a good safety factor. The ARP2000 has a greater number of life cycles than the replacement materials. To avoid early failure, therefore, tightening of the bolt needs to be controlled, and a replacement materials selection requires scientific methods of selection.
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    Design, Development and Testing of Drop-weight Impact Test Machine
    (Addis Ababa University, 2021-08) Dagmawe, Tadesse; Daniel, Tilahun (PhD); Araya, Abera (Mr.)
    The aim of this project is to design a drop weight impact test machine and conduct test with the prototype. The testing of materials under dynamic conditions needs an e cient and reliable equipment to experimentally examine and quantify the dynamic behavior of materials under low velocity impact loads. Drop weight impact test machine designed and modeled is vertical, double column, simple, compact, inexpensive and easily transported to desired location. The development process includes conceptual design phase, detailed design phase prototyping and testing the intended testing machine. Both drop weight and height can be varied independently. The measurement system includes piezoelectric accelerometer connected to the drop weight and Model 485B39 digital ICP signal conditioner. ASTM D7136/7136M standard testing method followed. The machine has a striker of xed shape and known mass which has interchangeable 16 mm diameter hemispherical tup, two friction less guide rod assembly to freely fall the impact assembly, 1:2 m wide- anged drop tower which used as a main support structure, a support bracket which attach the guide rod assembly to the drop tower and specimen xture where standard at rectangular test specimen (150 mm 100 mm) sits on. It has maximum drop height of 0.62 m, standard mass of 5.5 Kg with increment of 0.45 and 0.5 Kg, and maximum impact energy and velocity of 33.5 J and 3.49 m/s respectively. iii
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    Numerical Based Parametric Study on the Static Structural and Modal response of Composite UAV Wing
    (Addis Ababa University, 2022-03) Fasil, Erkyhun; Daniel, Tilahun (PhD)
    In this research, an Unmanned Aerial Vehicle (UAV) wing made of carbon epoxy composite material was used to investigate the wing static structural and modal response. The aerodynamic pressure load was calculated on ANSYS fluent by taking into account practical operational conditions. Two parametric studies were conducted where the first study had three models by varying the wing front and rear spar locations. The location of the front spar of model 1, model 2, and model 3 was at 18%, 22%, and 25%, whereas the rear spar was located at 62%, 65%, and 65%, respectively. On the other hand, the second parametric study had five different composite ply orientations of the wing skin, including [0/30/0/30/0], [0/45/0/45/0], [0/60/0/60/0], [0/90/0/90/0], and [-45/45/-45/45/-45]. Results from static structural analysis of varying spar locations showed that deformation has a maximum value at model 3 whereas bending and shear stress were maximum at model 2. On the other hand, deformation, bending, equivalent, and shear stress was minimum at model 1. The result from varying composite ply orientations showed that maximum and minimum deformation occurs at [0/30/0/30/0] and [0/90/0/90/0] ply orientations, respectively. The shear stress value was maximum at [-45/45/-45/45/-45]; Meanwhile, bending and equivalent stress were maximum at [0/30/0/30/0], while all stresses were minimum at [0/90/0/90/0]. From the modal analysis result, varying the spar location shows less effect on the natural frequency of the wing. Moreover, it was observed that model 1 had better structural performance than the other two models. On the other hand, the natural frequency of the first mode was maximum at [0/90/0/90/0] and minimum at [0/30/0/30/0], while in the last mode, the maximum and minimum values occur at [0/30/0/30/0] and [0/90/0/90/0], respectively. In General, the study showed that when the wing front and rear spar are close to each other and when [0/90/0/90/0], wing skin ply orientation applied, the wing had a better structural performance.
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    Numerical Modeling of Thermo-Mechanical Stress Field Associated with Mode-I and Mixed-Mode Fracture in Homogenous Isotropic Materials
    (Addis Ababa University, 2021-11) Wondimu, Dessalegn; Addis, Kidane (PhD); Araya, Abera (PhD) Co-Advisor
    Engineering components exposed to different loading include mechanical, thermal or combined thermo-mechanical loading. Some of the components subjected to thermomechanical loading are gas turbines, engines, reactor components, fuel chambers, and so forth. Defects and flaws are primal causes for materials failure and exist in almost all materials. In this work, the effect of thermomechanical loading on the stress fields was investigated under pure Mode-I and Mixed Mode fracture by assuming a two-dimensional model under plane strain condition. To study the combined effect of thermo-mechanical loading on the stress field around the crack tip, sequentially coupled thermal-stress analysis employed by ABAQUS software was used under different conditions. The parameters used in this work i.e., material dimension selected based on BS standard, the applied remote stress also calculated analytically using limit load formula as 20MPa, 50MPa and 100MPa. The temperature gradient employed at the surface of the model is taken from literature ranges from��20℃ to 89℃. The crack was considered as thermally insulated means adiabatic crack and the temperature field used as an input for the thermomechanical stress. This temperature field was solved by the assumption of steady-state heat transfer and the heat transfer procedure is utilized to solve it. The temperature field is incorporated as an input for the stress analysis as a predefined field in the static general procedure for developing a thermomechanical stress field. The extended finite element method (XFEM) was employed to model the crack for reducing the time of meshing and processing and to define the temperature and displacement discontinuity along the crack. The thermomechanical stress field was solved by applying different parameters i.e., remote stress, crack angle, crack length and temperature gradient. The mechanical and thermo-mechanical stress fields were evaluated and extracted from the crack tip for the radian value of �� �� 0.002�� and angular position value between ��180° ≤ �� ≤ 180° for both Mode–I and Mixed Mode fracture cases. Based on the result, the stress fields i.e.,��������, ������ and ������ around the crack tip were affected by the temperature gradients. The extreme thermo-mechanical stress field values ������ and ������ were rise by 13.4 to 36.39% and 4 to 17.6% respectively while the in-plane stress ������ drop by 8 to 43.49% relative to extreme mechanical stress value for temperature change Δ���� to Δ���� in case of considering the maximum value in pure mode-I loading case. Even though, the phenomenon of the extreme values for the case of mixed and pure mode-I loading cases are differ, the mixed mode thermo-mechanical stress field values ������ and ������ are drop by 6.78 to 25.64% and 12.85 to 30.3% respectively while the in-plane stress ������ rise by 4.75 to 25.09% in rising of temperature change Δ���� to Δ���� in case of considering the maximum value. This extreme values variation was because of the temperature gradient from the boundary of the plate as compared to mechanical stress field. Besides the angular position of the extreme stress field values shifted for a certain angle as compared to the isothermal stress field. The contour of these stress fields around the crack tip developed numerically has a good agreement with the analytical developed results.
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    Investigation on Crashworthiness of Locally Manufactured Bus Structural Frame by using Numerical Approach
    (Addis Ababa University, 2021-12) Alazar, Mulugeta; Haileleoul, Sahle (PhD)
    The thesis aimed to investigate the crashing behavior of locally manufactured 60 seated bus structural frames in case of frontal and side impacts. In addition to studying the crashworthiness of current structure, the most severe impact condition was identified by referring to crash test and an improvement on structure for a better crash performance for that impact condition was proposed. A full frontal and an offset impact position were used for frontal impact testing while analysis for side impact a perpendicular and angled/oblique impact position were used. Full-frontal crash analysis was done according to the ECE-R29 involving a frontal crash pendulum test in which impacting energy is set to 55 kJ and safe space between steering model and manikin is checked. while for offset frontal impact a simulation of bus structure impacting a rigid wall with an initial velocity of 56km/hr was done to check structures response to such kind of accidents. For sideimpact, residual space specified on ECE-R60 was used to check side frame deformation in case of perpendicular and angled/oblique impact. An impacting trolley with an initial speed of 48 km/hr was used for the side-impact test. Commercial software CATIA and ABAQUS were used for geometrical modelling and analysis. The study focused on identifying the crashworthiness of the bus structure by analyzing the structural deformation, crash pulse, crash force efficiency, HIC, and energy absorbing capacity. The results showed that the current structure had failed to meet regulation in the case of both frontal impacts and side perpendicular impact. In full frontal impact an intrusion of 17.32 mm in to manikin is seen and side frame has intruded residual space by 8.22 mm. Amongst the tests most severe impact damages was observed in the case of full-frontal and off-set frontal crash tests. For improving crashworthiness of the structure, three models were proposed and tested according to ECE-R29, the first two were based on increasing the thickness of frames and the last was by adding a reinforcement profile. The simulation results for improved models showed, by adding a reinforcement profile has improved the crashworthiness of the bus by creating a space between manikin and steering model of 36.33 mm structure and meets with regulation.
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    Development of Ni-modified Alloy Steel for Power Transmission Gear Material and Investigation of its Fatigue Failure
    (Addis Ababa University, 2021-11) Hailemariam, Nigus; Daniel, Tilahun (Asst. Prof.)
    Transmission gears have been working under severe working situations of loads and rotations, due to these situations, the properties and qualities of gear materials are greatly influenced. Consequently, contact fatigue failure is instigated. So, improving the mechanical properties of the existing gear material is very vital since these properties have a direct correlation with gear fatigue failure. Investigations were carried out to determine the mechanical properties of Ni-modified alloy steels by adding 1.55%, 1.75%, and 1.95% of the Ni-content to the existing Cr-Mo alloy steel, by ANN modeling that correlates the complex relationship of the input and output parameters and verified by experimental test. The investigation of these material properties with ANN modeling and experimental tests shows that the more Ni-content added to Cr-Mo alloy steel, the higher the ultimate and yield strength achieved. Likewise, fracture toughness, impact toughness, and percent of retained austenite of these materials were also investigated thoroughly. Thus, results showed that 1.55 % Ni- modified Cr-Mo alloy steel has a higher value on both impact toughness and fracture toughness without sacrificing yield strength compared with other Ni-modified counterparts. Therefore, based on both ANN and experimental results, 1.55 % of Ni- modified Cr-Mo alloy steel showed a better fatigue failure resistance. To address the behavior of lower alloying Ni-contents, the study includes 1.15% and 1.35% Ni-contents and explored with the ANN. Then, the results still indicate that a 1.55% Ni-content has better fatigue failure resistance. After exploring the best Ni-modified Cr-Mo alloy steel (1.55% of Ni-modified Cr-Mo alloy), based on ANN and experimental approaches, design and fatigue analysis of a single-speed transmission gear was carried out for further confirmation. The methods of design and analysis employed were KISSsoft gear simulation software and AGMA standard. Explorations have been done by altering gear parameters like helix angle, face-width, and input torque to get fitting safety factors, fatigue stresses, and smooth operation of the transmission gear pair. Comparing the two materials, Ni-modified Cr-Mo alloy steel has a higher impact load-carrying capacity manifested by bending safety factor compared to existing gear material. However, there are no significant differences in terms of contact load-carrying capacity expressed in contact safety factors between the sample materials.So, further investigation was needed to identify the difference in surface durability and to verify the mechanical property results. Then, Rolling contact fatigue (RCF) experimental test was carried out between 1.55% Ni-modified Cr-Mo alloy steel and existing material. In the RCF test of gears, micropitting was found to be the most vital damage property to characterize the rolled surface of disc samples. In this experiment, RCF tests were done on disc samples, two materials of Cr-Mo alloy steel, and 1.55%Ni-modified Cr-Mo alloy steels to evaluate the surface damage and topography of these materials. The methods utilized to determine the existence of micropitting on these materials were done on an adapted twin-disc machine. It is intended through this test to simulate asperities contact on surfaces of mating gear flanks using disc samples. The disc samples used in this experiment were contained low-speed specimens that are cylindrical-shaped discs, and high-speed specimens consist of crowned-shaped discs to attain a minimum effective contact area of 8.5mm. Thereafter, completing the RCF test, surface topographies were examined by employing SEM and OM for each measure of changes in surface topography and morphological alterations. As revealed from the post-processed micrographs, the 1.55% Ni-modified Cr-Mo alloy steel has better contact fatigue failure resistance compared with the Cr-Mo alloy steel in terms of Micropitted area ratio, pitted depth, and the number of pits. As an additional justification, verifying the result with the previous related literature of having a higher Ni-content (2%) was also compared with the 1.55% Ni-modified Cr-Mo alloy steel under the same conditions owes a 5 % micropitting ratio, and this indicates 1.55% Ni-modified has better surface durability. Thus, the 1.55% Ni-modified Cr-Mo alloy steel is recommended to use for transmission gears with high RCF damage suspicious.
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    Characterizing Tensile and Flexural Properties of Synthetic Fibers-Reinforced Epoxy Composite for Foot Prosthetic Application
    (Addis Ababa University, 2021-09) Galana, Abay; Samuel, Tesfaye (PhD)
    Materials has direct and critical impact on the performance of prosthetic foot. The aim of this thesis is to characterize the tensile and flexural properties of the four stacking sequences of E-glass and hybrid (E-glass/carbon) epoxy composite for prosthetic foot using powerful FEA software (ANSYS 19.2) for selecting stacking sequence that yields high strength without experimental cost. Additionally, to compare the design and strength performance of prosthetic foot model for two materials (Homo-polymer-polypropylene and selected composite) by numerical simulation. This starts from determining the elastic property of lamina to modeling of testing samples according to their standards for each stacking configuration using ANSYS workbench and verifying the numerical result with analytical MATLAB solution. The result shows that the ROM and Halpin-Tsai predicts longitudinal and transversal properties of the lamina with an acceptable range of errors from (1.66%-3.04%) and (0.5%-3.02%) respectively compared to experimental result of [34]. In pure E-glass/epoxy and hybrid/epoxy laminate the ultimate tensile strength is increased from 777.44MPa to1475.5MPa and 1865MPa to 1935MPa respectively. Similarly, in flexural testing the ultimate flexural strength of hybrid composite is increased from 1299.2Mpa -1934.3MPa due effect of stacking sequence. Among the all stacking configuration the SS-3 ( ����������������������) results higher tensile and flexural strength and selected for prosthetic foot. The failure loads of laminate in each stacking sequence verified the numerical results with an error less than 2% and 3% for tensile and flexural loading respectively. The composite prosthetic model has higher performance than Homo-polymer-polypropylene model. The model of prosthetic foot from Homo-polymer-polypropylene is not operating under the material stress limits. The deformation resistance, energy storage capacity, safety factor and stiffness of composite foot is increased by 18.9%, 53.2%, 56.2% and 18.8% respectively compared to HPP prosthetic foot. Its weight and is reduced by 30.62%. Finally, this composite is suggested for prosthetic producers (Prosthetic and orthotic center).
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    Structural Optimization, Crashworthiness and Strength Analysis of Midi – Bus Structure in Static and Rollover Condition
    (Addis Ababa University, 2021-10) Hailemichael, Solomon; Ermias, Gebrekidan (PhD)
    Midi-buses are a valuable vehicle to transport services and goods in Ethiopia's rural and urban areas. However, midi-bus are indirectly regulated through inspecting the end product (finished bus) during licensing for the public transport business in Ethiopia. Because of the lack of engineering analysis and testing procedures, the reliability and crashworthiness of a midi-bus are compromised and ultimately costing human lives and the overweight of the bus. Moreover, the weld formation and their types have been done without scientific reasons. Consequently, this method leads to a catastrophic structural failure during accidents. This research aimed to analyze the original midi-bus structure based on the static strength and rollover analysis using United Nations Regulation 66 via numerical investigation (ANSYS & LS-DYNA). Moreover, four design optimizations of the midi-bus structure were conducted: reinforcement design (����������������; numerical optimization (Response Surface Optimization (RSO) in ANSYS DesignXplorer for static case (model – IIstat); Successive Response Surface Method (SRSM) in LS-OPT for rollover case (model – IIroll)); and combined design approach (model - III) by merging of the static and rollover optimized models. Furthermore, the effect of full and spot arc welding on the quasi-static analysis of floor-wall and roof-wall connections was evaluated. The result shows that the maximum deformation in static and tare-weight rollover cases occurs at the baseline structure's roof section and pillar A and bays from one to three, respectively. The bending stiffness of the reinforced design (model – I), model – IIstat, and model – III (combined) was increased by 41.65 % (1911.4 N/m), 55.8 % (2,563.1 N/m), and 58.1 % (2,667 N/m), as compared to the baseline structure. Moreover, compared to the baseline model, the structure's weights of the reinforced model (model – I), model – IIstat, and model – III (combined) were effectively reduced by 5.23 %, 7.73 %, and 2.33 %, respectively. In addition, model – IIroll exhibited the weight of the reinforced model by 5.6 % in the rollover case. During structural connection, full and spot arc welding are formed at the edges and corners of the frames. Accordingly, these types of welds highly affect the energy absorbing capacity of the floor-wall and roof-wall connections. Generally, this research gives vital information on the midi-bus structure weight, stiffness, and crashworthiness capability from slight to severe loading cases for both static and rollover conditions. Moreover, this research suggests the new optimized bus structure and better weld type while welding the structural connections.