Mechanical Design

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Browsing Mechanical Design by Author "Addis, Kidane (PhD)"

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    Mathematical Modeling for Thermo-Mechanical Stress Field Associated with A Propagating Crack in Homogenous Isotropic Materials
    (Addis Ababa University, 2020-09) Haile, Simachew; Addis, Kidane (PhD)
    The main objective of this research is to develop mathematical modeling of the thermos mechanical stress field associated with the propagating crack in homogeneous isentropic materials by using a temperature field that can be replicated in real world application. In most of existing work, the temperature field is obtained by assuming the heat flux to be singular at the crack tip. However, such assumption could lead to a theoretical solution practically generating a temperature field where the heat flux singular at the crack tip is not attainable. The mechanical design of engineering structure in accordance with preexisting flaws is analyzed based on mechanical loading and mixed mode thermo-mechanical stress field by using asymptotic solution of the steady state 2D temperature field equation. The solution is developed for the mixed mode thermomechanical loading condition when the crack propagates at a constant velocity in homogeneous isentropic materials using an asymptotic approach. The solution is obtained by simultaneously solving equation of motion defined as a function of displacement potentials, and perturbation theory for the solution of two-dimensional steady state temperature field equation. The perturbed temperature field equation is used to derive the first three terms of thermo-mechanical stress field equations by superimposing with the mechanical loading equation for the steady state propagating crack. The thermo-mechanical stress field developed by the superimposition and the developed stress fields evaluated for the crack tip introduce at the center single edge when the value of �� expressed by the interval [−��, ��]. Around the crack tip from the crack tip plasticity approximations theory with the distance �� = 0.002�� at �� = 0 from the crack tip the thermo-mechanical stress field is zero. And the graph of stress intensity factors interpreted on the behalf of stress field since the stress and stress intensity factors have direct relationships. As the temperature and the heat flux increases the principal thermo-mechanical stress field such as ������, ������ and ������ increases and similar trend is observed for the stress intensity factors that the material is overstressed to propagate the initial crack.
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    Numerical Determination of The Effect of Filler Volume Fraction, Geometry, And Temperature on Thermal and Mechanical Property of Polymer Nanocomposites
    (Addis Ababa University, 2021-03) Abrahaley, Eukubay; Addis, Kidane (PhD)
    Some of the engineering materials cannot be standalone for specific applications due to certain limitations of their property. To overcome this limitation, different constitutes mix with other materials to produce a composite material with a shared property for desired engineering applications. For example, polymer material’s low thermal and mechanical property is solved by incorporating nanofillers with different volume fraction and shapes. In such cases, the performance is determined by the combined effect of the inclusion’s geometry, volume fraction, and operating temperatures. Understanding the role of nanoparticle distribution, size, geometry, and temperature on polymer nanocomposites' thermal and mechanical properties help to design materials with infallible mechanical and thermal behaviors. In this work, the role of nano silicate particle volume fraction, geometries/aspect ratio, and the working temperature on the effective thermal conductivity and elastic modulus of the epoxy matrix composites are analyzed numerically using the finite element method and mean-field homogenization approaches. The result furtherly validates with analytical models and experimental results taken from the literature. The effective thermal conductivity and young modulus of the polymer nanocomposite material are increased with the increasing of the nano silicate volume fraction within the epoxy matrix. Besides, ellipsoid nano silicate particles give more improved properties than spherical nanoscopic inclusion of the same volume fraction. The proposed finite element method was effective in estimating the effective thermal conductivity of the composite material for both spherical and ellipsoidal geometries of nanoscopic fillers. On the other hand, the analytical model better predicted the composite material’s effective young modulus of spherical inclusions. The effect of temperature on the nanocomposite’s effective thermal conductivity and modulus of elasticity is also estimated following similar approaches. The nanocomposite’s effective thermal conductivity of the polymer composite material increases with temperature, but the effective modulus of elasticity/stiffness decreases with temperature.
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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.