Abstract: Changing environmental conditions, especially temperature, have been observed to be a complicated factor affecting vibration properties, such as frequencies, mode shapes, and damping, of civil structures. This paper reviews technical literature concerning variations in vibration properties of civil structures under changing temperature conditions. Most of these studies focus on variations in frequencies of bridge structures, with some studies on variations in mode shapes and damping and other types of structures. Statistical approaches to correlation between temperature and frequencies are also reviewed. A quantitative analysis shows that variations in material modulus under different temperatures are the major cause of the variations in vibration properties. A comparative study on different structures made of different materials is carried out in laboratory. Two real structures, the 1,377-m main span Tsing Ma Suspension Bridge and the 600-m-tall Guangzhou New Television Tower, are examined. Both laboratory experiments and field testing, regardless of different construction materials used and structural types, verify the quantitative analysis. Variations in frequencies of reinforced concrete structures are much more significant than those of steel structures.

Abstract: This paper introduces a single-receiver geoacoustic-inversion method based on dispersion analysis and adapted to low-frequency impulsive sources in shallow-water environments. In this context, most existing methods take advantage of the modal dispersion curves in the time-frequency domain. Inversion is usually performed by matching estimated dispersion curves with simulated replicas. The method proposed here is different. It considers the received modes in the frequency domain. The modes are transformed using an operator called modal reversal, which is parameterized using environmental parameters. When modal reversal is applied using parameters that match the real environment, dispersion is compensated for in all of the modes. In this case, the reversed modes are in phase and add up constructively, which is not the case when modal reversal is ill-parameterized. To use this phenomenon, a criterion that adds up the reversed modes has been defined. The geoacoustic inversion is finally performed by maximizing this criterion. The proposed method is benchmarked against simulated data, and it is applied to experimental data recorded during the Shallow Water 2006 experiment.

Abstract: Resonance-mode analysis is effective to determine the contributions of different network components to a resonance in power system harmonic studies. There are two forms of modal sensitivity in harmonic resonance mode analysis, that is, the modal impedance sensitivity and the modal frequency sensitivity. This paper presents a method to compute the modal frequency sensitivity index. The analysis results are compared with those obtained by using the modal impedance sensitivity index to clarify the differences and similarities between the two sensitivity indices. The comparison shows that these indices complement each other and should be used to jointly diagnose power system harmonic resonance problems.

Abstract: In this paper, we propose a vision-based rotational angle measurement system for large-scale civil structures. Despite the fact that during the last decade several rotation angle measurement systems were introduced, they however often required complex and expensive equipment. Therefore, alternative effective solutions with high resolution are in great demand. The proposed system consists of commercial PCs, commercial camcorders, low-cost frame grabbers, and a wireless LAN router. The calculation of rotation angle is obtained by using image processing techniques with pre-measured calibration parameters. Several laboratory tests were conducted to verify the performance of the proposed system. Compared with the commercial rotation angle measurement, the results of the system showed very good agreement with an error of less than 1.0% in all test cases. Furthermore, several tests were conducted on the five-story modal testing tower with a hybrid mass damper to experimentally verify the feasibility of the proposed system.

Abstract: This paper presents a model for the three-dimensional (3D) dynamic response of endmills while considering the actual fluted cross-sectional geometry and pretwisted shape of the tools. The model is solved using the spectral-Tchebychev (ST) technique. The bending and the coupled torsional-axial behavior of four different fluted endmills is compared to finite element model (FEM) predictions and experimental results obtained using modal testing under free-free boundary conditions. For the first eight modes, including six bending and two torsional/axial modes, the difference between the 3D-ST and experimental natural frequencies is shown to be 3% or less for all four tools tested during this study. For the same modes, the 3D-ST and FEM predictions agree to better than 1%. To demonstrate its application, the 3D-ST model for the fluted section of a commercial endmill is coupled to the spindle–holder to predict the tool-point dynamics using receptance coupling substructure analysis (RCSA) with a flexible connection. The coupled model is validated through experiments.

Abstract: Experimental vibration measurement of mechanical components are very important for studying the dynamic properties. Electromagnetic (EM) shakers are the most widely used exciters in mechanical testing because of both the broadband dynamic range of excitation and the excitation power. However, there are applications where these exciters can work inefficiently, and so underperform. This can be caused by an impedance mismatch between the shaker armature and test rig, which causes dissipation of the generated power into heating the armature rather than moving the test structure. Clearly, mechanical components presenting a high level of structural damping will require higher level of power to obtain high levels of vibration. Hence, it is important to minimize as much as possible any unwanted power dissipation due to both the test rig design and the connection between the shaker and/or the test rig. This paper demonstrates that a bladed disc type of structure can be used as a high impedance connector for a test rig in order to increase the excitation force level. This is possible thanks to otherwise an undesirably dynamic characteristic of bladed discs, which is represented by mistuning of the blades. When this mistuning characteristic is enhanced, it is possible to produce several resonances each with a high impedance match between the shaker and the test rig and this can increase the force applied to the specimen and thus its displacement amplitude. Also, the test rig proposed here can be used of several resonance frequencies depending on the number of blades. Hence the proposed test rig can improve both the performance of a shaker and increase the amplitude of vibration of the test structure. Further to this the application of the amplification process can be used for fatigue trials of composite material component. This has been an application which has caused some considerable difficulty: few cases have been successful and the results in this paper show evidence of how to proceed for future trials.

Abstract: The vibrations developed during the work represent one of the main mechanical problems of the electric motors. Their frequencies and levels can cause large damages of different electrical motors parts. This is the reason why even in the design phase it is useful to be known the values of the natural frequencies of the parts. This can be made in two different ways: using modern capabilities of finite element method (FEM) or doing experimental modal analysis. The study developed in the present paper refers to the modal analysis of a three-phase squirrel cage induction of 1.5 kW, for both, for its elements and, for motor and the electric motor as an assembly. The FEM modal analysis was applied using ABAQUS software for assembly of the 7 elements of the motor. The 3D model was developed, mechanical connections being of screw type. The theoretical model was validated by experiments, with impact hammer test method. The validation confirms the assumptions made for the 3D model and the designed model can be use, as example, for other FEM based modal analysis. The natural frequencies obtained both by FEM and experimental method does not overlap on the working frequencies.

Abstract: The aim of the study is to determine the modal parameters of a prototype damaged arch dam by operational modal analysis (OMA) method for some damage scenarios. For this purpose, a prototype arch dam-reservoir-foundation model is constructed under laboratory conditions. Ambient vibration tests on the arch dam model are performed to identify the modal parameters such as natural frequency, mode shape and damping ratio. The tests are conducted for four test-case scenarios: an undamaged dam with empty reservoir, two different damaged dams with empty reservoirs, and a damaged dam with full reservoir. Loading simulating random impact effects is applied on the dam to crack. Cracks and fractures occurred at the middle of the upper part of the dams and distributed through the abutments. Sensitivity accelerometers are placed on the dams` crests to collect signals for measurements. Operational modal analysis software processes the signals collected from the ambient vibration tests, and enhanced frequency domain decomposition and stochastic subspace identification techniques are used to estimate modal parameters of the dams. The modal parameters are obtained to establish a basis for comparison of the results of two techniques for each damage case. Results show that approximately 35-40% difference exists between the natural frequencies obtained from Case 1 and Case 4. The natural frequencies of the dam considerably decrease with increasing cracks. However, observation shows that the filled reservoir slightly affected modal parameters of the dam after severe cracking. The mode shapes obtained are symmetrical and anti-symmetrical. Apparently, mode shapes in Case 1 represent the probable responses of arch dams more accurately. Also, damping ratio show an increase when cracking increases.

Abstract: This paper studies the application of the sliding mode control method to reduce the vibration of flexible structure with piezoelectric actuators and strain gage transducer in practical complex environment. The state- space dynamic model of the system was derived by using finite element method and experimental modal test. The structure is subjected to arbitrary, unmeasurable disturbance forces. Taking into account the uncertain random disturbance and measurement noise, Kalman filter is chosen as the state estimator to obtain the modal coordinates and modal velocities for the modal space control. A sliding mode controller is adopted due to its distinguished robustness property of insensitiveness to parameter uncertainties and external disturbances. The sliding surface is determined by using optimization method, and the sliding controller is designed by applying Lyapunov direct method. That is, along the switching surface, the cost function of the states is minimized. A real-time control system was built using dSPACE DS1103 platform, and vibration control tests were performed to experimentally verify the performances of the proposed controller. The results of experiment show the controller can effectively attenuate elastic vibration of the structure.

Abstract: This article presents active control laws for sound transmission through a stiffened panel in the low-frequency range. A stiffened panel mounted on a metallic box is used to simulate aircraft fuselage and cabin system. A loudspeaker located outside the box is driven by band-limited white noise to exert acoustic excitation. Dynamic properties of the stiffened panel are characterized through a series of modal testing. Sound isolation performance is evaluated both analytically and experimentally for further efficient control algorithms design. Based on the analysis of these results, a hybrid control strategy combining both feedback and feedforward control is proposed, which utilizes a high-authority/low-authority control architecture. The low-authority control loop is positive position feedback control and high-authority control loop is filtered-X control. When positive position feedback is implemented, active damping is added on the secondary path, which makes filtered-X least mean square more robust, accel...

Abstract: Detailed linear finite element simulations and accurate modal testing techniques ensure a reliable validation of linear dynamic response predictions of engineering structures. The good agreement between simulation and measurement for single components is often diminished when an assembly is considered, since nonlinear effects of the joints influence the response behaviour. To re-establish the agreement between analysis and measurement the nonlinear behaviour must be included in the simulation. Analysis tools are available today to take these nonlinear effects into account which require accurate input parameters, to represent the nonlinear contact interface.

Abstract: The invention relates to a device and a method for testing the dynamic characteristic of a combined surface, belonging to the field of mechanical design and manufacture. A vibration exciter (8) is connected with a force sensor (10) by a top rod (4); an upper test piece (12) is connected at the other end of the force sensor (10); and a displacement sensor (11) and an acceleration sensor (3) are symmetrically distributed on the two sides of the upper test piece (12). The vibration exciter is used for exciting and vibrating the upper test piece to acquire an acceleration signal, a displacement signal and a force signal; all the signals are transmitted to an LMS modal testing system and sent to a computer after being processed by the LMS modal testing system; and the computer acquires a frequency response function of a test piece to be tested according to the acceleration signal, the displacement signal and the force signal and can recognize normal dynamic characteristic parameters of thecombined surface between the upper test piece and a lower test piece according to the frequency response function. The invention solves the problem that the dynamic characteristic at the combined surface position is difficult to test, and can be convenient to separate the normal dynamic characteristic of the combined surface from the dynamic characteristics of an experiment device.

Abstract: Modal parameters extracted from test articles with known boundary conditions are useful for model correlation and updating but are often not obtained because of the time and cost associated with moving and installing the test article on a rigid or isolated seismic mass and performing a separate modal test. It would be advantageous if frequency response functions from known boundary conditions, such as fixed base frequency response functions, could be obtained while the structure undergoes testing on fixtures that contain compliance or dynamics in the frequency range of interest, such as base shake tables.

Abstract: Modal synchronized switch damping on inductor control is a vibration damping technique that combines the advantages of both semiactive and active control techniques based on a modal strategy. This method allows targeting any unwanted vibration mode of a structure while using a semiactive autonomous synchronized switch damping on inductor damping technique. This article presents a performance analysis of an improved modal synchronized switch damping on inductor approach called “SSDI-Max.” The particularity of this new approach is to maximize the self-generated voltage amplitude by a proper definition of the switch instants (voltage inversion) according to the chosen targeted mode. Following the basic modal synchronized switch damping on inductor technique, the switch is synchronized with the chosen modal coordinate extremum. In the investigated approach, the voltage is increased by waiting for the next voltage extremum following immediately any targeted modal coordinate extremum in a given time window. Thi...

Abstract: The paper presents the experimental and analytical modal analysis of a crankshaft. The effective material and geometrical properties are measured, and the dynamic behavior is investigated through impact testing. The three-dimensional finite element models are constructed and an analytical modal analysis is thenperformed to generate natural frequencies and mode shapes in the three-orthogonal directions. The finite element modelagrees well with the experimental tests and can serve as a baseline model of the crankshaft.

Abstract: In this paper, the structural health monitoring (SHM) benchmark problem of the Canton tower is studied. Based on the field monitoring data from the 20 accelerometers deployed on the tower, some modal frequencies and mode shapes at measured degrees of freedom of the tower are identified. Then, these identified incomplete modal data are used to update the reduced finite element (FE) model of the tower by a novel algorithm. The proposed algorithm avoids the problem of subjective selection of updated parameters and directly updates model stiffness matrix without model reduction or modal expansion approach. Only the eigenvalues and eigenvectors of the normal finite element models corresponding to the measured modes are needed in the computation procedures. The updated model not only possesses the measured modal frequencies and mode shapes but also preserves the modal frequencies and modes shapes in their normal values for the unobserved modes. Updating results including the natural frequencies and mode shapes are compared with the experimental ones to evaluate the proposed algorithm. Also, dynamic responses estimated from the updated FE model using remote senor locations are compared with the measurement ones to validate the convergence of the updated model.