Applied Mechanics
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Item Influence of Nanoclay on Interlaminar Fracture Characterization of GFRP, and Intralaminar Fracture Characterization of SIMS & Glass Mat Reinforced Thermoplastics Materials(Addis Ababa University, 2020-06) Mulugeta Habtemariam; Belingardi, Supervisor: Giovanni (Prof.); Daniel Tilahun (Assoc. Prof.); Ermias G. Koricho (Asst. Prof.)The needs of lightweight and customizable structural materials instigate researchers to conduct rigorous materials characterization, with particular attention toward failure mechanisms and safety standards, and to study the materials’ development. Furthermore, the currently available structural and semi-structural composite materials made up of fiber-reinforced plastic, that are presently considered to keep the environmental regulations, require multidimensional examination and analysis because of its heterogeneous nature a non-isotropic behavior. Consequently, this research aims to improve the knowledge on specific lightweight material and to contribute the confidence that toward their use in spite of its poor nature and suspicious to fail, focusing on some fundamental material properties, such as interlaminar and intralaminar behaviors. The research started from failure characteristics and mechanisms, followed by the analysis of the modifications adopted to enhance its structural properties in advanced level, and finally report the adopted experimental characterization procedures and discuss the main findings. The first part of the study has been focused on composite materials with special targets of enhancement and the structural behavior of these materials was experimentally characterized. The material was manufactured with a plain-woven S-glass fiber-reinforce plastic. The material modification was obtained by adding nanoparticles to the matrix; therefore a nano-modified composite was developed by the appropriate combination of epoxy and nanoclay family particles, Cloisite 20B. Thus, the fundamental experimental work included the effect of nanoclay, Cloisite 20B inclusion on the mechanical behavior of a woven type glass fiber reinforced plastic (GFRP) composite. Specifically, the study examined the effect of nanoclay, added with various weight percentages, on the tensile, compressive strengths, and modulus of elasticity of GFRP in both weft and warp directions. Results showed that depending on the warp and weft directions, the inclusion of nanoclay, Cloisite 20B, altered the mechanical behavior of GFRP. The advanced investigations focus on the interlaminar characteristics of the material. In this work, the effect of meticulous nanoclay, Cloisite 20B, inclusion on the interlaminar fracture toughness glass fiber reinforced plastic composite was investigated using careful experimental procedure. Afterwards the study moved to the fracture mechanics behavior, with particular reference to the mode-I interlaminar behavior. Tests were conducted based on a double cantilever beam (DCB) specimen using the specific American Society of Testing Materials standard (ASTM D5528). Results showed that the inclusion of nanoclays improved the interlaminar fracture toughness of the GFRP composite in the range of 12.65% and 54.07% relative to pristine, with progressive percentage increment of the nanoclays weight percentage content (from 0.5 to 2%). Therefore, the dissemination of this experimental research results contributes to overlook a better understanding of nanoclay fillers and their contribution to mechanical behaviors; this can lead to a better design of novel structural composites. Moreover, it guides how Cloisite fillers contribute to improve the delamination resistance with this special composite material having a better retardant flame propagation property that can be relevant for some structures. The second part dealt with lightweight materials that are intended for the vehicle /automotive industry. The intralaminar behavior of the two types of materials, which were supplied by two international companies, was investigated. The fundamental behavior, impact, and special structural application studies of these two types of innovative materials were examined, once again with particular attention toward the impact response and the fracture nature of these materials. The first material type is semi impregnated micro sandwich structure (SIMS) and it is manufactured with two specific reinforcing fibers (carbon and glass). The other material belongs to the glass mat thermoplastic (GMT) family that has also two types: the conventional GMT and the GMT modified by adding unidirectional fibers (GMT-UD) to stiff the structure. For those materials, the intralaminar fracture and the nature of the crack behavior were experimentally investigated using compact tension specimen test. The intralaminar fracture toughness of each material was determined along with crack propagation behavior. As a result, the output of this research fills the gaps and it can contribute to having a full picture of the GMT and SIMS materials.Item Effect Of Incompressibility in The Analysis of Metal Forming Using Finite Element Method(Addis Ababa University, 2005-09) Ahmed Nurye; Tamrat Tesfaye (PhD)Metal forming is one of the most common metal manufacturing processes used, which is noted for its minimum waste and dimensional precision, and usually improves the mechanical properties of the formed part. Metal forming process is a process that causes changes in the shape of solid metal particles via plastic (permanent) deformations. Hence, knowledge of metallurgy and mechanics combine to provide an insight to its behavior. Its applications are wide in the manufacturing of machinery, automotive, aerospace and other hardware components. The material is modeled as a hyperelastic, viscoplastic solid. A constitutive model with a single scalar variable representing the isotropic resistance to the plastic flow is employed. Many finite elements exhibit the so-called ‘volumetric locking’ in the analysis of incompressible or quasiincompressible problems in metal forming. Nearly incompressible plasticity in metal forming displays severe volume locking problems when low order standard nodal-based displacement methods are used. This means that after deformation each small portion of the medium has the volume as before the deformation. In this thesis a finite element formulation for a frictionless large deformation contact problem in metal forming is presented. It is based on the formulation which introduces the contact constraints via Lagrange multipliers. The stabilized formulation which allows the use of low-order interpolation functions for both displacement and the pressure field is applied to eliminate volumetric locking effects and to circumvent numerical instabilities. Starting from the variational formulation of the constitutive and the kinematic problems, the linearization of the principle of virtual work relation, the contact potential energy, and finally the matrix formulation of the method is derived. To improve the convergence property of the method an augmentation step was included. Then the formulation is converted to a computer code written using Matlab. Finally the program is used to solve a benchmark example.Item Design And Analysis of Thick Partition Wall for Building from Local Composite “ENSET” Fiber(2011-12) Ermiyas Sisay; Eyassu Woldesenbet (Prof.)Lack of resources and increasing environmental pollution has resulted in great interest of research in materials that are friendly to our health and environment. Polymer composites produced from natural fibers is currently one of the most promising areas in polymer industry. Keeping in view the various advantages of natural fibers, in current series of green composites a study on natural fiber reinforced polymer composites has been made. This paper presents the results of an experimental series designed to assess the possibility of local “Enset” fiber as reinforcing material for non-load bearing partition wall. First , optimized epoxy resin was reinforced with employing “Enset” fiber of different forms such as traditional woven mat, and long woven fiber form of reinforcement. Second, the new non-load bearing partition wall was designed and manufactured using hand lay-up and vacuum bagging techniques. Then, experiment was conducted on the new proposed composite material. Experimental results of; three point bending test, compressive test and impact test is demonstrated and discussed in this thesis. The experimental results focused on the mechanical properties such as the bending strength, compressive strength and impact resistance of epoxy resin reinforced with “Enset” fiber. These results suggest that local “Enset” fiber can be potential candidates for use in natural fiber reinforced polymer composites for non-load bearing partition wall.Item Stress And Deflection Analysis of Cracked Composite Pressure Vessel by Finite Element Method(Addis Ababa University, 2015-10) Tekeliye Tasachew; Daniel Tilahun (PhD)The aim of this thesis is to develop crack and investigate techniques and parameters that could be used to identify crack if it exist in a composite pressure vessel. Many researchers discovered formation or propagation of a crack in a composite pressure vessel will cause a catastrophic failure. Thus, health monitoring for a pressure vessel due to crack using crack detection techniques will minimize or reduce the failure that probably to occur. This research first focused on mathematical and numerical relation which, represent the governing equation of composite material winding on in a pressure vessel. For modeling different crack size in the composite overwrapped pressure vessel fracture stress theories applied. Different surface Crack size are considering according to the literature standard values. Model actual size of composite pressure vessel with varied crack size “a” develops on the surface of composite pressure vessel. This research thoroughly analysis the effect of crack on the surface of composite pressure vessel. Those parameters are considered to see the variation of result due to presence of crack. The main parameters are considering stress, principal stress, von mises stress, deflection and fracture stress. According to the parameters it shows that as crack size increase, the average stress will be increase with the given pressure in composite pressure vessel. In contradict; the fracture stress will be decrease as the crack size increase. In this study, optimal angle-ply orientations of symmetric [550, -550] shells designed for maximum burst pressure with allowable crack size were investigated. It is shown that all the strength characteristics of carbon fiber relevant to structural engineering can be explained by the cracks present in the carbon fiber and can be analyzed using fracture mechanics. The stress and deformation are affected due to the presence of crack in composite pressure vessel. These parameter are takes a design crack as the basis for design. Rather than an allowable stress as in current approaches, with which it is compared cracked composite pressure vessel. A full size cylindrical shell of composite pressure vessels is conducted. A mathematical method, A finite element method and compare experimental test are studied to verify a maximum allowable crack size in composite overwrapped pressure vessel with a given optimum winding angles. The roll of crack size design within the wider philosophy of limit state design is discussed.Item Spacing And Diameter Effects of Rivets in The Failure of Laminated Composite Joints(Addis Ababa University, 2010) Tomas Deress; Raman A. (PhD)Composites have become very important engineering material due to their desirable properties such as high specific strength and light weight. In the use of composite materials for practical purposes, the formations of joints such as mechanical joints are unavoidable; and hence the formation of discontinuities such as holes for rivets or bolts. The aim of this research is to investigate the effects of diameter and spacing (pitch and row spacing) of rivet/hole on composite laminate joints. Mechanical fasteners frequently reduce the load carrying capability of the structure and lead to premature failure. In this thesis, initial failure loads are evaluated with respect to rivet diameter and spacing as a measure of joint strength. The effects of these two parameters are studied with consideration of other directly related parameters such as e/d, w/d and s/d ratios. Finite element analysis and the composite failure criterion Tsai-Wu are used in the study to determine failure loads and from the results obtained conclusions are driven.Item Numerical Analysis of the Effect of Surface Finish and Material Properties on the Carburized Moly Based Low Alloy Material Subjected to A Contact Load(Addis Ababa University, 2022-10) Yibeltal Ali; Behailu Mamo (Mr.)Contact between rough surfaces between surfaces affects a variety of phenomena including friction, wear, thermal contact resistance, and electrical contact resistance. Different studies done on the mechanics of contacting bodies using analytical and experimental methods. Even though the experimental method to determine the surface treatment parameters is convenient and effective, the method is very expensive, consume more time to complete and it needs advanced tools. Furthermore, the analytical methods in analyzing real surface topography is a simplifying approach, which has low accuracy. Therefore, numerical modeling of the contact between rough surfaces is the alternative method. the aim of the study was to model the contact pressure contacting bodies to determine the effect of material density variation and surface finish method (machined and powder metallurgy surface) using finite element analysis utilizing ABAQUS software. The plastic contact of a rough surface and smooth flat surface is analyzed by modeling the rough surface as an isotopic, Gaussian, and random process. For running FE analysis an interface MATLAB code and Solid work software were used to create a 3D model from the measurement data. To achieve the objectives of the study particular material were selected and data were collected and rough surface modeling using computational model were performed. Result of the study in terms of contact load, real area of contact, and contact pressure are extracted from each contact analysis depicted that, the lowdensity PM rough surface leads to minimum area of contact but attains the maximum value of contact pressure maxP and cross hardness limit earlier when compared with the low-density machined and highdensity PM rough surface. In the PM surface model, the max PY ratio to cross the hardness limit of 2.22 at n n P A E of 0.0042 and in the machined surface model the max PY ratio to cross the hardness limit of 2.22 at n n P A E of 0.0057. The area of contact decrease whereas the mean P and maxP increases with a decrease in density value. In the machined surfaces, the contact load is shared by most of the contacting asperities and the asperity interactions restrain the asperities to remain in elastic state mostly even at high loads relative to PM surfaces. The real contact area is low and the von mises stresses and contact pressure are high for the low-density surface as compared to high-density surface, for the surface with ρ = 7.35g/cm3, real contact area occurs at 0.6377mm2 or 10.2% of nominal contact area, for the surface with ρ = 7.35g/cm3, real contact area occurs at 1.14115 mm2 or 18.25% of nominal contact area.