Showing 10 results for Frequency Response
Volume 13, Issue 56 (10-2015)
Abstract
In the past, the inspection of cracks on eggshell was typically conducted in the industry by floodlighting, however, it gives eye fatigue, makes misjudgment and is not easy to detect hairline crack. Recent research into the automation of the detection of eggshell cracks is focused both on optical and mechanical detection principles. In this study eggs were excited by a light mechanical impact on different locations of the eggshell and vibrational frequency response of the eggshell combined with pattern recognition was attempted to differentiate intact egg and cracked egg. The pattern recognition was conducted by Principal Component Analysis (PCA) and Support Vector Machine (SVM). The optimal number of principal components was obtained 7 according to maximum error for predictive model and high discrimination between intact and cracked eggs. The result was found that the vibrational impulse response method distinguish intact egg and cracked egg with the level of 100% and 87.5% accuracy, respectively.
Nowrouz Nouri, Hamid Reza Gharavian, Ali Valipour,
Volume 14, Issue 10 (1-2015)
Abstract
Systems of recognition and location identification of underwater moving bodies which using acoustic waves are called sonar. Electroacoustic transducers have an important role in underwater communication systems such as Sonar. A set of electroacoustic transducers which is called sonar array, can be used for sending and receiving underwater sound. The most widely used transducer in these arrays are Tonpilz transducer. In this paper, a full simulation of Tonpilz transducer is given and the most important factors for evaluating transducer performance are checked experimentally and numerically. Also for validation of finite element model, the sample of transducer was designed and made. This transducer was tested in two methods, electrically and acoustically. Electrical behavior was tested by Impedance gain analyzer devise. Acoustic test was carried in the acoustic pool. Then the result of FEM compared with experimental results. With comparing FEM results and tested model, it is observed that the finite element model can predict electrical and acoustical behavior of Tonpilz transducer so well. Finally it is tried to improve frequency response of transducer with making changes in the structure. While the addition of damping factors can increase frequency bandwidth.
Mousa Rezaee, Reza Jahangiri,
Volume 14, Issue 13 (3-2015)
Abstract
In this study, static/dynamic instability and nonlinear vibrations of FG plates resting on elastic foundation under parametric forcing excitation, are investigated. Based on CPT, applying the von-Karman nonlinear strain–displacement relation and the Hamilton’s principle, the governing nonlinear coupled partial differential equations are derived. By considering six vibration modes, the Galerkin’s procedure is used to reduce the equations of motion to nonlinear Mathieu equations. In the absence of elastic foundation, the validity of the formulation for analyzing the static buckling, dynamic instability and nonlinear deflection is accomplished by comparing the results with those of the literature. Then in the presence of the foundation and by deriving the regions of dynamic instability, it is shown that as the parameters of the foundation increases, the natural frequency and the critical buckling load increase and the dynamic instability occurs at higher excitation frequencies. The frequency response equations in the steady-state condition are derived by applying the multiple scales method, and the parametric resonance is analyzed. Then the conditions of existence and stability of nontrivial solutions are discussed. Moreover, the effects of the system parameters, including excitation frequency, amplitude of excitation, foundation parameters and damping, on the nonlinear dynamics of the FG plate are investigated. Also it is shown that the presence of the foundation has a considerable influence on the resonance characteristic curves.
Seyed Hossein Monavary, Ehsan Maani, Hossein Nejat Pishkenari, Mehdi Alvandi,
Volume 16, Issue 11 (1-2017)
Abstract
Dynamic analysis of structures is of significant importance in a variety ofapplications. Modal parameters identification can be utilized in resonance frequency estimation, fault detection and its diagnostics in many industrial applications from automobile to aerospace and satellite industries. To perform the vibration tests utilizing shaker, test sample should be connected to shaker using fixture. No fixture can reproduce a perfectly rigid boundary condition; at some frequencies the interaction between the fixture and the structure will become important, causing the modes of the assembly to be considerably different from the fixed-base modes that would be predicted by an idealized finite element model. However, it would be very convenient to be able to estimate the fixed-base modes of a structure experimentally so they could be used to update or validate the model for the structure. In this paper, two degree of freedom model is considered for the system and optimal shape for fixture is designed based on the analytical analysis. Mode shapes and frequencies of fixture are investigated numerically and compared with experimental results. Effect of connection torques on the system dynamics such as power spectral density and natural frequencies is studied by performing different experimental analyses.
Majid Pour Ghasem, Ahmad Naji Ghazvini, Hosein Gholparvar,
Volume 17, Issue 11 (1-2018)
Abstract
Hybrid joints (Metal–Composite) is being used more and more in aerospace industry due to their low weight and high strength. Dynamic study of this joint, owing to limitation of increase in screw’s preload in composite substructure, has certain importance. Effective factors on nonlinear behavior of the joint are low preload of the screw and high excitation force amplitude on the structure. Layer Element Model has been used to better the description of joint’s behavior in recent years. In this study effects of nonlinear behavior of joint on the structure has been investigated using 2D layer element theory in two divisions: increase of damping and decrease of stiffness which result in nonlinearity. Stiffness characteristics of the joint was modeled with normal stiffness and damping characteristics of the joint with structural damping in shear direction. Nonlinear frequency response function for two preload and two excitation force was extracted and nonlinear finite element model for stiffness and damping of the joint is suggested by High-order polynomial approximation in terms of response amplitude. Effects of increase of excitation force amplitude and decrease of screw’s preload on increase of nonlinearity was extracted by this finite element model. Results indicate that presented nonlinear finite element model corresponds closely to nonlinear vibration tests.
Mohammad Mehdi Alina, Seyed Vahid Sepehr Mousavi, Javad Amanabadi,
Volume 18, Issue 3 (5-2018)
Abstract
Damage occurrence in structural and mechanical systems during utilization is an inevitable phenomenon. Death and financial losses could be prevented by health monitoring systems and damage detection processes in structures. In the mentioned framework, damage detection based on dynamics properties, is one of the most important and efficient methods, without concentration on special zones in structure. In this study frequency response functions were analyzed by principle component analysis, then, in order to complete process, dimension reduction and damage indices extraction were conducted. At the end, plate damage detection was introduced as an optimization problem considering extracted damage indices, and solution of the problem were given by PSO and Genetic algorithms. Output results consist of estimation about location and intensity of applied damage. Several scenarios including single, simultaneously dual and triple stiffness losses were figured out to investigate and evaluate the efficiency of the mentioned algorithms. Finally, outcome result around performance and utility of method had been discussed. It's obviously demonstrated that Particle Swarm Optimization algorithm has more accurate result, especially in estimation of damage location than Genetic algorithm optimization solution, during health monitoring processes. The mentioned conclusion has been gotten more explicit with getting scenario complicated.
Habib Ramezannejad Azarboni, Hemad Keshavarzpour,
Volume 18, Issue 5 (9-2018)
Abstract
In this paper based on the Euler-Bernoulli beam model, the primary resonance a curved single carbon nanotube subjected to axial thermal force in the case of low temperature and high temperature and resting on a viscoelastic foundation is analytically investigated. The nonlinear partial differential governing equation is reduced to nonlinear ordinary differential governing equation by using of a single-mode Galerkin approximation along with the sinusoidal curvature for clamped-clamped single walled carbon nanotube under harmonic external force. The method of multiple scales is applied to determine the analytical primary resonance frequency response. Considering the curved geometry and the mid-plane stretching, a quadratic and cubic terms are presented in the governing equation. The effects of temperature change in high temperature and low temperature conditions, viscoelastic coefficients of medium, amplitude of sinusoidal curvature and excitation amplitude are investigated to study the property frequency response and development or elimination of forward and backward jumping phenomenon in primary resonance frequency response. The results show that these parameters have a significant effect on the frequency response of a curved single walled carbon nanaotubes under transvers harmonic force.
Atieh Andakhshideh, Sattar Maleki, Hossien Karamad,
Volume 18, Issue 9 (12-2018)
Abstract
In this article, for the first time, the effect of non-uniformity of microbeam cross section and various boundary conditions on the nonlinear vibration of microbeam is investigated considering the size dependent behavior based on modified couple stress theory. Using the Hamilton’s principle, the governing equation of Euler–Bernoulli microbeam with von Karman geometric nonlinearity based on the modified couple stress theory is derived. The nonlinear vibration governing equation is then solved using the Generalized Differential Quadrature method (GDQ) and direct iterative method to obtain the nonlinear natural frequencies. In this step, the Galerkin method is used to reduce the nonlinear PDE governing the vibration into a time-dependent ODE of Duffing-type. The time domain is then discretized via spectral differentiation matrix operators which are defined based on the derivatives of a periodic base function. Next, the nonlinear parametric equation is solved using pseudo arc-length method and the frequency–response curves of microbeam nonlinear forced vibration is obtained. Finally, nonlinear natural frequency and frequency response of microbeam with various non-uniformity of cross sections and boundary conditions are obtained. Present results show that, the nonlinear free and forced vibration of microbeam is size dependent. Moreover, this size dependency is more significant for non-uniform microbeam and is deferent for various boundary conditions. The result of present method for simple case including uniform section and simply supported boundary condition is validated with that of exact method and have good agreement.
H. Ramezannejad Azarboni, H. Keshavarzpour ,
Volume 19, Issue 1 (1-2019)
Abstract
In this study, based on the nonlocal nonlinear Euler-Bernoulli beam model, the primary and superharmonic resonance of a single carbon nanotube (CNT) resting on a viscoelastic foundation under the magnetic axial loads and temperature as well as transverse harmonic forces was investigated. Using Galerkin approximation along with the trigonometric shape functions, the nonlinear partial differential governing equation is reduced to nonlinear ordinary differential equation. The frequency response of the single walled CNT is derived by implementing the multiple time scale method for the primary and superharmonic resonances. The effects of surface elasticity, change in temperature, magnetic field and the length-to-outer diameter aspect ratio on the frequency response of CNT in the cases of primary and superharmonic resonances were analyzed. The results show that the nonlinearity according to considered geometrical and mechanical parameters in this study, may cause unpleasant jumping phenomenon accompanied by unstable region in the frequency response. In addition to the surface elasticity, magnetic field, smaller temperature changes, as well as larger aspect ratio have positive effects on weakening the jumping phenomenon and extending the stability level of single walled CNT.
Volume 21, Issue 6 (12-2021)
Abstract
Occurrence of the nonlinear behavior can be a sign of changes in structural parameters and the presence of damage in systems. This paper presents a method for detecting and quantifying of nonlinearity, as an indication of damage, using the indicators that are extracted from the frequency response functions (FRFs) and Hilbert transform of FRFs, for steel moment frame structural systems. Using time history analysis under selected harmonic ground motions, the results of FRFs for the studied 4-story system are illustrated and discussed.
Nonlinear behavior is a result of formation plastic hinges under earthquake loading. FRFs and Hilbert transform of FRFs are extracted from both the linear and nonlinear behavior of 4, 8, and 12-stories steel moment frames under fifteen different earthquake records with different characteristics in their time histories. Some near and far field well-known earthquakes records have been selected for the present study as the ground motions input in time history analysis. Different levels of nonlinearity are determined based on the maximum rotation of hinges in column members of structures equal to 2θy, 4θy and 6θy, in which θy is yield limit rotation. The indicators of the studied systems are calculated and evaluated for linear and different levels of nonlinearity based on the mathematical power of changes for FRFs and Hilbert transform of FRFs. The presented indicators are extracted based on the frequency response functions (FRFs) and Hilbert transform of FRFs for the responses of absolute acceleration and relative displacement of stories. The indicators are calculated at the location of acceleration sensors (accelerometer) in four levels of the structural systems, while the formation of plastic hinges in the columns of the structures will occur only at the level of the distance between the adjacent sensors.It is shown that the proposed method and calculated indicators have enough accuracy and sensitivity in detecting the “existence”, “location” and “extent” of damage.
