فهرست مطالب

Theoretical and Applied Vibration and Acoustics - Volume:5 Issue:1, 2019
  • Volume:5 Issue:1, 2019
  • تاریخ انتشار: 1397/10/11
  • تعداد عناوین: 6
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  • Nima Najafizadeh, Majid Rajabi *, Ramin Hashemi, Saeid Amini Pages 1-10
    Ultrasonic energy is used for applying severe plastic deformation on metal surfaces. In the present work, the effect of ultrasonic vibration on the formability, microhardness and microstructural properties of St14 steel sheet has been investigated. To be precise, a semi-hemispherical-head forming tool had shaped the specimens until the necking started to happen. Conventional as well as the ultrasonic-assisted biaxial stretch forming test has been performed on St14 steel sheets and obtained data has been used to compare the hardness and microstructure of the specimen with and without superimposing the ultrasonic vibration. It was observed that the hardness of the samples which have been shaped by applying ultrasonic vibrations to the tool with an amplitude of 15µm at 20.5 kHz increased significantly in compared with the samples which have been shaped without using ultrasonic vibration, revealing the efficiency of the ultrasonic operation in increasing the hardness.
    Keywords: Ultrasonic Vibration, Hardness, Necking, Deformation Behavior, Grain size
  • Majid Rajaei * Pages 11-20
    The nonlinear system becomes an area with numerous investigations over the past decades. The conventional modal analysis could not be  applied on nonlinear continuous system which makes it impossible to construct the reduced order models and obtain system response based on limited number of measurement points. Nonlinear normal modes provide a useful tool for extending modal analysis to nonlinear systems but the extraction of nonlinear normal modes is cumbersome. In this research by neglecting the damping effect on response shape, the nonlinear joint in frictional beam is replaced by 3 order nonlinear spring. The equation of motion is solved using the method of multiple scales up to third super harmony which are neglected in most system identification practices. It is shown in an experimental test set-up, the general form of solution can regenerate observed response of any point and more important similar to linear system, the single mode would be sufficient in analysis when excitation frequency is close to resonant frequency.
    Keywords: Nonlinear boundary condition, System identification, Multiple Scales Method
  • Neda Baghiee *, Mohammad Reza Esfahani, Kazem Moslem Pages 21-42
    The research described in this paper focuses on the application of modal strain techniques on a multi-girder bridge superstructure with the objectives of identifying the presence of damage and detecting false damage diagnosis for such structures. The case study is a one-third scale model of a slab-on-girder composite bridge superstructure, comprised of a steel-free concrete deck with FRP rebars supported by four steel girders, similar to North Perimeter Red River Bridge in Winnipeg, Manitoba. The modal test data of the slab-on-girder specimen are analyzed by two mathematical methods and Mindlin approach. The Mindlin approach uses a small number of sensors and only the fundamental mode of vibration to obtain the modal strains. The unit-area normalization method produces a more precise damage patternbased on the Mindlin approach than the widely used unit-norm method and is thus a superior method for locating damage in multi-girder bridge structures. A new method is proposed to distinguish the false damage diagnosis that is common in multi-girder systems. Based on the invariant stress resultant theory and direct stiffness assumptions, a level III damage detection process is applied successfully to indicate damage localization and severity estimation of the damaged girder.
    Keywords: Damage, FRC deck, Invariant stress, Modal strains, spurious diagnosis
  • Farzad Rafieian *, Hassan Jalali Pages 43-54

    Town Board Stations (TBS) are usually located in residential areas. Pressure reduction through reducing valves in these stations generates considerable amounts of sound and vibrations. These mechanical waves have undesired effects on the equipment such as reducing the fatigue life of the gas ransmission line. On the other hand, propagation of these wave in the residential area, auses discomfort for the neighbors of these stations. In this paper, vibrations enerated in a TBS and transmitted to the residential building in the area are investigated through modeling and analysis. The aim is to study and propose a method for reducing vibration transmission from the TBS to residential buildings by using a numerical model. A vibration isolator is designed for this purpose with ptimum parameters and considering the practical limitations of the problem. The performance of the isolator is verified with the help of the developed FEmodel. Results show the effectiveness of the designed vibration isolator for the operating range of the TBS.

    Keywords: Town Board Stations (TBS), Vibration transmission, Finite element modeling
  • Hossein Mohammadi *, Soroush Sepehri Pages 55-68
    In the present manuscript, the second strain gradient (SSG) is utilized to investigate the primary resonance of a nonlinear Euler-Bernoulli nanobeam is analyzed in this paper for the first time. To that end, the second strain gradient theory, a higher-order continuum theory capable of taking the size effects into account, is utilized and the governing equation of the motion for an Euler-Bernoulli nanobeam is derived with sixteen higher-order material constants. Then by implementing the Galerkin’s method,the Duffing equation for the vibration of a hinged-hinged nanobeam is obtained and its primary resonance is studied utilizing the method of multiple scales. The size effects and impact of various system parameters on the amplitude of the response are then investigated for three different materials and the results are compared to thatof the first strain gradient and classical theories. The results of this manuscript clearly shows that the nonlinear vibration of a second strain gradient nanobeam is size-dependent and although the difference between the results obtained by the second strain gradient theory and the first strain gradient theory is negligible for thicker beams, as the thickness decreases, the difference becomes more prominent. Also, the effects of nonlinearity on the forced vibration nonlinear response of an SSG beam are investigated and some observations are reported.
    Keywords: Second Strain Gradient Theory, Nonlinear vibration, Euler-Bernoulli Beam, Method of Mul-tiple Scales
  • Amir Hossein Rabiee * Pages 69-84
    An adaptive fuzzy sliding mode controller (AFSMC) is adopted to reduce the 2D flow-induced vibration of an elastically supported square-section cylinder, free to oscillate in stream-wise andtransverse directions in both lock-in and galloping regions. The AFSMC strategy consists of a fuzzy logic inference system intended to follow a sliding-mode controller (SMC), and a robust control system designed to retrieve the variance between the sliding mode and fuzzy controllers.  The sprung square cylinder first experiences vortex-induced vibrations with increasing Reynolds number, and then, after passing the critical flow velocity, it confronts high-amplitude and low-frequencyvibrations of galloping owning to its sharp corners. A co-simulation platform is considered by linking the AFSMC system modeled in Matlab/Simulink to the plant model implemented in Fluent, aiming at the calculation of opposite control force needed for comprehensive annihilation of the cylinder motions. Based on the performed numerical simulations, it becomes clear that the utilized active control system has successfully mitigated the two-degree-of-freedom vibrations of a square cylinder in both the lock-in region and galloping zone. Here, the vibration amplitudes in the transverse andstreamwise directions have decreased by 93% and 94%, for the lock-in region and 93% and 99%, for the galloping zone, respectively.
    Keywords: Galloping, Flow-induced vibration, Square-section cylinder, Intelligent controller, active controller