Browsing by Publisher "Elsevier Science Ltd"
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Item Exact solutions of boundary layer equations of a special non-Newtonian fluid over a stretching sheet(Elsevier Science Ltd, 1999) Yürüsoy M.; Pakdemirli M.This work focuses on the boundary layer equations of a special third grade fluid over a stretching sheet in which the second grade effects are negligible compared to third grade and viscous effects. As a first step, the general symmetries of the partial differential system are derived using Lie group analysis. Following this, the equations are reduced to an ordinary differential system via similarity transformations. Finally, the resulting ordinary differential systems are solved. The main observation is that, as the non-Newtonian behavior increases, the boundary layer gets thicker.Item Experimental microhardness for AA 1030, Cu, CuSn7, CuZn30 and 6114 alloys and a correlation with the Hall-Petch relation(Elsevier Science Ltd, 1999) Meriç C.; Atik E.; Engez T.In this experimental study, the strip-formed specimens made from aluminum alloy 1030, pure Cu, CuSn7, CuZn30, and low-carbon steel 6114 were cold worked to different ratios. To determine the microhardness values of the materials, microhardness tests were applied. The grain sizes of the materials were determined by the Heyn method, using a metal microscope. The hardness of materials, H, is dependent on the grain diameter, d, in a way similar to the yield stress in the Hall-Petch relation H = HO + KHd- 1/2 , where HO and KH are constants. The microhardness of the materials was found, with reasonable accuracy, to vary with grain size according to the Hall-Petch equation.Item Evaluation of lignite combustion residues as cement additives(Elsevier Science Ltd, 1999) Demirbaş A.; Aslan A.In this study the physical and chemical properties of lignite fly ashes obtained from electrostatic precipitator and cyclone, lignite bottom ash, cement+lignite ash mixtures, and their effects on mechanical properties of concrete were investigated. The ashes were classified into two general types based on total silica, alumina, and iron-III oxide: class A and class B. When 25% of the cement was replaced by class A lignite bottom ash (the combined three oxide contents were 30.2%), the 28-day compressive strength increased by 18.9% compared to the control mix, and when 25% of the cement was replaced by class B lignite ash (the combined three oxide contents were 78.1%), the compressive strength decreased by 3.5% compared to the control mix. The results obtained were compared with the Turkish Standards and, in general, were found to be within the limits. As a result, the lignite fly ash and bottom ash samples may be used as cementitious materials.Item Physical and mechanical properties of cast under vacuum aluminum alloy 2024 containing lithium additions(Elsevier Science Ltd, 2000) Meriç C.Physical and mechanical properties of aluminum alloy 2024 containing lithium additions were investigated. Lithim (2, 3, and 4 wt%) was added to the alloy of 2024. Melting was carried out in an induction furnace under argon gas protection; casting was performed under vacuum. The cast material was hot extruded to produce tension test rods. To obtain the maximum strength and hardness, the specimens were solution heat treated at 495 °C, then quenched in water at room temperature. They were then aged. For the purposes of comparison, some of the specimens were melted under argon gases, but casting was done without vacuum. All the specimens were subjected to tension and hardness tests. In this study, an increase of 6% in the modulus of elasticity and 3% decrease in the density were obtained for 1% lithium added to the alloy. The mechanical properties of the alloys cast under vacuum were found to be better than those cast without vacuum. Whatever the lithium percent in the alloys, all the mechanical properties reached their maximum level in 1 week of natural aging; for artificial aging, maximum levels were reached at 120 °C in 24 h, at 160 °C in 16 h, at 190 °C in 12 h, and 200 °C in 2 h. By increasing the lithium percent, the specific elastic modulus (E/ρ) of the alloy increased rapidly but the specific strength (σ/ρ) remained constant. Thus, aluminum lithium alloys that are difficult to manufacture by known methods can be manufactured safely by the vacuum metallurgy method.Item Elasto-plastic stress analysis of aluminum metal-matrix composite laminated plates under in-plane loading(Elsevier Science Ltd, 2000) Sayman O.; Akbulut H.; Meriç C.The study presents an elasto-plastic stress analysis of symmetric and antisymmetric cross-ply, angle-ply laminated metal-matrix composite plates. Long stainless steel fiber reinforced aluminum metal-matrix composite layer is manufactured by using moulds under the action of 30 MPa pressure and heating up to 600 °C. A laminated plate consists of four metal-matrix layers bonded symmetrically or antisymmetrically. The first-order shear deformation theory and nine-node Lagrangian finite element is used. The in-plane load is increased gradually.Item Improved approach to the solution of inverse kinematics problems for robot manipulators(Elsevier Science Ltd, 2000) Karlik B.; Aydin S.A structured artificial neural-network (ANN) approach has been proposed here to control the motion of a robot manipulator. Many neural-network models use threshold units with sigmoid transfer functions and gradient descent-type learning rules. The learning equations used are those of the backpropagation algorithm. In this work, the solution of the kinematics of a six-degrees-of-freedom robot manipulator is implemented by using ANN. Work has been undertaken to find the best ANN configurations for this problem. Both the placement and orientation angles of a robot manipulator are used to fin the inverse kinematics solutions.Item Investigation of the effect on boride layer of powder particle size used in boronizing with solid boron-yielding substances(Elsevier Science Ltd, 2000) Meriç C.; Sahin S.; Yilmaz S.S.In this study, the effect of particle size of powder used in the boronizing process with solid boron-yielding substances on the boride layer was investigated. Hot-shaped AISI 1020, AISI 1030, AISI 1040, and AISI 1050 structural steels were used as the base materials. EKabor HM powder was used as the boronizing agent, and was classified into four groups according to particle size. The boronizing process was carried out with each group at 900 °C for 2, 3, 4, and 5 h. The microstructure, microhardness, and layer thickness of the boronized materials were investigated.Item An elastic/plastic solution for a thermoplastic composite cantilever beam loading by bending moment(Elsevier Science Ltd, 2000) Sayman O.; Aksoy S.; Aykul H.In this study, an elastic/plastic stress analysis is carried out for a thermoplastic composite cantilever beam loaded by a bending moment at the free end. The composite beam is reinforced by woven steel fibers, at 0, 15, 30 and 45° orientation angles. An analytical solution is performed for satisfying both the governing differential equation in the plane stress case and boundary conditions for small plastic deformations. The solution is carried out under the assumption of the Bernoulli-Navier hypotheses. It is found that the intensity of the residual stress component of σ(x) is a maximum at the upper and lower surfaces. The composite material is assumed to be as hardening linearly. The Tsai-Hill theory is used as a yield criterion. (C) 2000 Elsevier Science Ltd. All rights reserved.; In this study, an elastic/plastic stress analysis is carried out for a thermoplastic composite cantilever beam loaded by a bending moment at the free end. The composite beam is reinforced by woven steel fibers, at 0, 15, 30 and 45° orientation angles. An analytical solution is performed for satisfying both the governing differential equation in the plane stress case and boundary conditions for small plastic deformations. The solution is carried out under the assumption of the Bernoulli-Navier hypotheses. It is found that the intensity of the residual stress component of σx is a maximum at the upper and lower surfaces. The composite material is assumed to be as hardening linearly. The Tsai-Hill theory is used as a yield criterion.Item Non-linear vibrations and stability of an axially moving beam with time-dependent velocity(Elsevier Science Ltd, 2001) Öz H.R.; Pakdemirli M.; Boyaci H.Non-linear vibrations of an axially moving beam are investigated. The non-linearity is introduced by including stretching effect of the beam. The beam is moving with a time-dependent velocity, namely a harmonically varying velocity about a constant mean velocity. Approximate solutions are sought using the method of multiple scales. Depending on the variation of velocity, three distinct cases arise: (i) frequency away from zero or two times the natural frequency, (ii) frequency close to zero, (iii) frequency close to two times the natural frequency. Amplitude-dependent non-linear frequencies are derived. For frequencies close to two times the natural frequency, stability and bifurcations of steady-state solutions are analyzed. For frequencies close to zero, it is shown that the amplitudes are bounded in time.