Browsing by Author "Ozkaya, E"

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    Residual thermal stress analysis in cylindrical steel bars using finite element method and artificial neural networks
    Toparli, M; Sahin, S; Ozkaya, E; Sasaki, S
    In this study, it was proposed that the residual stresses within steel bars after quenching in water from 600 degreesC could be calculated by using the finite element method (FEM) and an artificial neural network (ANN) algorithm. Three modelled cylindrical specimens of AISI 1020 steel were heated and then quenched in water. Using FEM, temperature distribution with time and thermal residual stress values in the samples were calculated after cooling. The analysis was extended to elastic-plastic deformation during the quenching of steel cylinders of various diameters. The calculated temperature and thermal residual stress values were used in training a multi-layer, feed forward, back propagation ANN algorithm. The results obtained via the ANN algorithm method have been compared with the FEM results. Comparison showed good agreement. (C) 2002 Elsevier Science Ltd. All rights reserved.
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    Non-liner vibrations of a beam-mass system under different boundary conditions
    Ozkaya, E; Pakdemirli, M; Oz, HR
    An Euler-Bernoulli beam and a concentrated mass on this beam are considered as a beam-mass system. The beam is supported by immovable end conditions, thus leading to stretching during the vibrations. This stretching produces cubic non-linearities in the equations. Forcing and damping terms are added into the equations. The dimensionless equations are solved for five different set of boundary conditions. Approximate solutions of the equations are obtained by using the method of multiple scales, a perturbation technique. The first terms of the perturbation series lead to the linear problem. Natural frequencies and mode shapes for the linear problem are calculated exactly for different end conditions. Second order non-linear terms of the perturbation series appear as corrections to the linear problem. Amplitude and phase modulation equations are obtained. Non-linear free and forced vibrations are investigated in detail. The effects of the position and magnitude of the mass, as well as effects of different end conditions on the vibrations, are determined. (C) 1997 Academic Press Limited.
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    Vibrations of a beam-mass systems using artificial neural networks
    Karlik, B; Ozkaya, E; Aydin, S; Pakdemirli, M
    The nonlinear vibrations of an Euler-Bernoulli beam with a concentrated mass attached to it are investigated. Five different sets of boundary conditions are considered. The transcendental equations yielding the exact values of natural frequencies are presented. Using the Newton-Raphson method, natural frequencies are calculated for different boundary conditions, mass ratios and mass locations. The corresponding nonlinear correction coefficients are also calculated for the fundamental mode. The calculated natural frequencies and nonlinear corrections are used in training a multi-layer, feed-forward, backpropagation artificial neural network (ANN) algorithm. The algorithm produces results within 0.5 and 1.5% error limits for linear and nonlinear cases, respectively. By employing the ANN algorithm, computational time is drastically reduced compared with the conventional numerical techniques. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.
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    Non-linear vibrations of a slightly curved beam resting on a non-linear elastic foundation
    Oz, HR; Pakdemirli, M; Ozkaya, E; Yilmaz, M
    In this study, non-linear vibrations of slightly curved beams are investigated. The curvature is taken as an arbitrary function of the spatial variable. The initial displacement is not due to buckling of the beam, but is due to the geometry of the beam itself. The ends of the curved beam are on immovable simple supports and the beam is resting on a non-linear elastic foundation. The immovable end supports result in the extension of the beam during the vibration and hence introduces further non-linear terms to the equations of motion. The integro-differential equations of motion are solved analytically by means of direct application of the method of multiple scales (a perturbation method). The amplitude and phase modulation equations are derived for the case of primary resonances. Both free and forced vibrations with damping are investigated. Effect of non-linear elastic foundation as well as the effect of curvature on the vibrations of the beam are examined. It is found that the effect of curvature is of softening type. For sufficiently high values of the coefficients, the elastic foundation may suppress the softening behaviour resulting in a hardening behaviour of the non-linearity. (C) 1998 Academic Press Limited.
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    Effects of Non-ideal Boundary Conditions on Natural Frequencies of Fluid Conveying Micro-beams
    Atci, DC; Ozkaya, E
    In this study, vibrations of fluid conveying micro-beams under non-ideal boundary conditions are investigated. Non-ideal boundary conditions are modeled as a linear combination of ideal clamped and ideal simply supported boundary conditions. The weighting factor k is presented as a rate of non-ideal boundary condition. Non-ideal clamped and non-ideal simply supported beams are both considered to see the effects of the boundary conditions. Hamilton's principle is used to obtain equations of motion of the system and the method of multiple scales which is one of the perturbation techniques is applied to the equation. Approximate solutions of the linear and nonlinear equations of motion are obtained and the effects of non-ideal boundary conditions on natural frequencies are presented.
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    Sensitization pattern of inhalant allergens in children with asthma who are living different altitudes in Turkey
    Ozkaya, E; Sogut, A; Küçükkoç, M; Eres, M; Acemoglu, H; Yuksel, H; Murat, N
    Variability in children's allergic sensitization has been detected not only among different countries but also among cities within the same nation but yet different climatic areas. The aim of this study was to investigate the sensitization pattern of asthmatic children who lived in different altitude areas: the two largest Turkish cities, Istanbul (sea level) and Erzurum (high altitude). Five hundred and twelve asthmatic children (6-15 years old) from Istanbul (western Turkey, at sea level) and 609 from Erzurum (eastern Turkey, at an altitude of 1800 m) were included in the study. All participants underwent skin testing with common inhalant allergens, spirometry, total IgE level, and clinical examination. The positive sensitization ratio to aeroallergens in children with asthma living at sea level was statistically higher than that in children living in the high altitude group [p = 0.001, OR (odds ratio) 4.9 (confidence interval (CI) 3.67-6.459)]. However, pollen sensitization in asthmatic children living in high altitudes was significantly higher than that in children living at sea level [p = 0.00, OR 2.6 (CI 1.79-3.87)]. Children with asthma who live at high altitudes are characterized by higher pollen but lower mite sensitization rates than those living at sea level in Turkey. Different climatic conditions and altitudes may affect aeroallergen sensitization in children with asthma.
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