Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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Subject: search.filters.subject.Acoustic waves

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  • Master Thesis
    Semi-Analytical Finite Element Modeling for Dispersion Analysis of Multilayered Structures
    (Izmir Institute of Technology, 2017) Akyol, Çağrı Gökhan; Özkol, Ünver; Özkol, Ünver; Önen, Onursal; 03.10. Department of Mechanical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of Technology
    Ultrasonic guided waves are frequently employed for Non-destructive tests (NDT), and Structural Health Monitoring (SHM) applications in the industry. Several analytical and numerical approaches have been developed in order to investigate guided wave behavior on multilayered structures. In this thesis, guided waves were investigated using the Semi-Analytical Finite Element (SAFE) approach on planar (plate like) structures. The guided wave theory and dispersive behavior in bounded structure were presented for Lamb waves for isotropic elastic plate model first. The numerical method to solve the SAFE problem was developed using the Matlab 2014b in order to obtain dispersion curves. The curves were compared with the Disperse Software, which utilizes global matrix approach for analytical solution of the same problem. Good agreement was achieved on obtained dispersion curves. As a second part of this study, dispersion analysis was carried out in a multilayer plate model consisting of elastic and viscoelastic materials. The semi-analytical finite element method was solved by adapting the hysteretic damping model so that it can also be applied in a damped plate model. Phase velocity and attenuation dispersion curves were illustrated and the effect of the viscoelastic layer thickness is also discussed. The obtained attenuation dispersion curves in this damped plate configuration are examined for wave modes with low-attenuation. The dispersion curve results obtained using the SAFE method was also compared with the results of studies available in literature.
  • Master Thesis
    Fluid Properties Measurements Using Planar Quasi-Scholte Waveguides
    (Izmir Institute of Technology, 2017) Bostan, Okan; Özkol, Ünver; Özkol, Ünver; 03.10. Department of Mechanical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of Technology
    Interface acoustic waves can propagate in solid-solid and solid-liquid interfaces. They are perturbed by ambient conditions, interface materials and characteristics. The wave energy are shared between two interfacing media, with its distribution depending on these properties. In this study, it was aimed to make liquid characterization (density) using interface acoustic waves. The waves propagating between the plate and the liquid interface are kind of flexural waves and known as quasi-Scholte mode. Firstly, the wave propagation for multilayered system is analytically modelled. A global matrix method is employed for obtaining the dispersion characteristics of the multilayer structure by considering the boundary conditions of the system. In order to develop the dipstick sensor, liquid characterization is performed firstly by test cell method for longitudinal bulk velocity of the liquid with known density. Test cells with two different lengths were used for measurements. In this technique, transmitting and receiving transducers are placed on opposite sides of test cell. Based on the distance between transducers and time of flight, velocity of longitudinal bulk waves was calculated. Mixtures of ethanol-pure water were prepared at various ratios and measurements were taken at different temperatures. Quasi-Scholte mode dipstick sensoring is employed by a simple measurement waveguide or dipstick inserted into the fluid to measure the velocity of the quasi-Scholte mode on a plate. This method, is based on mode conversions of waves. When the shear wave is excited on the aluminum plate, which is used as dipstick, Lamb waves occur at the solid-gas surface. The lamb waves are then converted into different modes in the solidliquid interface. Leaky Rayleigh and Scholte propagating modes exist in immersed part as well. Quasi-Scholte wave propagate on the interface while the Leaky Rayleigh modes lose their energy. By means of dipstick method, alcohol water mixtures which is used in test cell method, were measured at room temperature. The signals obtained as a result of the experiments were processed by the zero phase slope method. Time delay between reflected signals are found from phase slopes of the signals.