scholarly journals Modal Testing Using Impact Excitation and a Scanning LDV

2000 ◽  
Vol 7 (2) ◽  
pp. 91-100 ◽  
Author(s):  
A.B. Stanbridge ◽  
D.J. Ewins ◽  
A.Z. Khan
Keyword(s):  
Impact Test ◽  
Natural Frequencies ◽  
Laser Doppler Vibrometer ◽  
Modal Testing ◽  
Impact Excitation ◽  
Mode Shapes ◽  
Natural Mode ◽  
Output Spectrum ◽  
Angular Vibration ◽  
Scan Line

If a laser Doppler vibrometer is used in a continuously-scanning mode to measure the response of a vibrating structure, its output spectrum contains side-bands from which the response mode shape, as defined along the scan line, may be obtained. With impact excitation, the response is the summation of a set of exponentially-decaying sinusoids which, in the frequency domain, has peaks at the natural frequencies and at `sideband' pseudo-natural frequencies, spaced at multiples of the scan frequency. Techniques are described for deriving natural mode shapes from these, using standard modal analysis procedures. Some limitations as to the types of mode which can be analysed are described. The process is simple and speedy, even when compared with a normal point-by-point impact test survey. Information may also be derived, using a circular scan, on the direction of vibration, and angular vibration, at individual points.

scholarly journals Damage Identification by Using a Self-Synchronizing Multipoint Laser Doppler Vibrometer

2015 ◽  
Vol 2015 ◽  
pp. 1-9 ◽  
Author(s):  
Chong Yang ◽  
Yu Fu ◽  
Jianmin Yuan ◽  
Min Guo ◽  
Keyu Yan ◽  
...  
Keyword(s):  
Mode Shape ◽  
Natural Frequencies ◽  
Damage Identification ◽  
Reference Beam ◽  
Laser Doppler Vibrometer ◽  
Laser Doppler ◽  
Vibration Measurement ◽  
Mode Shapes ◽  
Interference Signal ◽  
Short Period

The vibration-based damage identification method extracts the damage location and severity information from the change of modal properties, such as natural frequency and mode shape. Its performance and accuracy depends on the measurement precision. Laser Doppler vibrometer (LDV) provides a noncontact vibration measurement of high quality, but usually it can only do sampling on a single point. Scanning LDV is normally used to obtain the mode shape with a longer scanning time. In this paper, a damage detection technique is proposed using a self-synchronizing multipoint LDV. Multiple laser beams with various frequency shifts are projected on different points of the object, reflected and interfered with a common reference beam. The interference signal containing synchronized temporal vibration information of multiple spatial points is captured by a single photodetector and can be retrieved in a very short period. Experiments are conducted to measure the natural frequencies and mode shapes of pre- and postcrack cantilever beams. Mode shape curvature is calculated by numerical interpolation and windowed Fourier analysis. The results show that the artificial crack can be identified precisely from the change of natural frequencies and the difference of mode shape curvature squares.

Free Vibration Analysis of PZT Glide Heads

1999 ◽  
Vol 121 (4) ◽  
pp. 984-988 ◽  
Author(s):  
Alex Y. Tsay ◽  
Jin-Hui Ouyang ◽  
C.-P. Roger Ku ◽  
I. Y. Shen ◽  
David Kuo
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
Natural Frequencies ◽  
Laser Doppler Vibrometer ◽  
Free Vibration Analysis ◽  
Mode Shapes ◽  
Element Analysis ◽  
The Finite Element Analysis ◽  
Bonding Length ◽  
Measured Displacement

This paper studies natural frequencies and mode shapes of a glide head with a piezoelectric transducer (PZT) through calibrated experiments and a finite element analysis. In the experiments, the PZT transducer served as an actuator exciting the glide head from 100 kHz to 1.3 MHz, and a laser Doppler vibrometer (LDV) measured displacement of the glide head at the inner or outer rail. The natural frequencies were measured through PZT impedance and frequency response functions from PZT to LDV. In the finite element analysis, the glide head was meshed by brick elements. The finite element results show that there are two types of vibration modes: slider modes and PZT modes. Only the slider modes are important to glide head applications. Moreover, natural frequencies predicted from the finite element analysis agree well with the experimental results within 5% of error. Finally, the finite element analysis identifies four critical slider dimensions whose tolerance will significantly vary the natural frequencies: PZT bonding length, wing thickness, slider thickness, and air bearing recess depth.

A Damage Identification Method Using Vibration Modal Parameters Through Finite Element Discretization

2003 ◽  
Author(s):  
Xiaoping Zhou ◽  
Abhijit Gupta
Keyword(s):  
Finite Element ◽  
Natural Frequencies ◽  
Damage Identification ◽  
Least Squares Method ◽  
Modal Testing ◽  
Mode Shapes ◽  
Element Formulation ◽  
Element Discretization ◽  
Damage Indices ◽  
Actual Damage

Natural frequencies and mode shapes of a structure will change whenever the structure has any kind of damage. This paper introduces a technique to quantify and locate the damage when the natural frequencies and mode shapes of undamaged and damaged structure are known. Aluminum beams (with and without damage) are used for numerical simulation and experimental verification. To establish the theoretical basis of this method, finite element formulation is used. A set of undetermined equations involving damage indices and natural frequencies and mode shapes of undamaged and damaged structures are obtained. The damage indices are computed using non-negative least squares method. Impact modal testing was conducted with three aluminum beams and damage indices based on experimental data are compared with actual damage cases to establish the effectiveness of this method to identify the damage.

Experimental Modal Analysis of a Half-Scale Model Twin-Engine Aircraft Rear Fuselage Engine Mount Support Frame

2017 ◽  
Author(s):  
Diego A. Chamberlain ◽  
Chris K. Mechefske
Keyword(s):  
Modal Analysis ◽  
Natural Frequencies ◽  
Testing Procedure ◽  
Modal Testing ◽  
Scale Model ◽  
Mode Shapes ◽  
Component Testing ◽  
Fitting Procedures ◽  
Engine Mount ◽  
Set Up

Experimental modal testing using an impact hammer is a commonly used method for obtaining the modal parameters of any structure for which the vibrational behavior is of interest. Natural frequencies and associated mode shapes of the structure can be extracted directly from measured FRFs (Frequency Response Functions) through various curve fitting procedures. This paper provides an overview of the modal testing conducted on an aerospace component. Testing set-up, experimental equipment and the methodology employed are all described in detail. Further validation of the testing procedure was done by ensuring that the experimental results satisfy the requirements of repeatability, reciprocity and linearity. The relevant ISO standard has been referenced and important concepts to modal analysis are expanded upon. Recorded natural frequencies, coherence and a description of the observed mode shapes are presented along with notable trends.

scholarly journals High Frequency Modal Testing of the Multiblade Packets Using a Noncontact Measurement and Excitation System

2020 ◽  
Vol 2020 ◽  
pp. 1-12
Author(s):  
C. H. Liu ◽  
C. Zang ◽  
F. Li ◽  
E. P. Petrov
Keyword(s):  
High Frequency ◽  
Measurement Method ◽  
Natural Frequencies ◽  
Spectral Lines ◽  
Modal Testing ◽  
Frequency Resolution ◽  
Mode Shapes ◽  
Finite Model ◽  
Frequency Range ◽  
Noncontact Measurement

High cycle failure of blades and vanes caused by the vibration is one of the major causes reducing the lifetime of turbomachines. For multiblade packets, the failure may occur at vibrations with high frequencies that can reach up to tens of kHz. The experimental modal testing of blades is crucial for the validation of numerical models and for the optimization of turbomachine design. In this paper, the test rig and procedure for measurements of dynamic characteristics of lightweight multiblade packets in wide and high frequency ranges are developed. The measurements are based on a noncontact excitation and noncontact measurement method, which allows the determination of the modal characteristics of the packets with high accuracy in wide frequency ranges. The responses of the multiblade packets are measured using a Scanning Laser Doppler Vibrometry (SLDV), while vibrations are excited by the acoustic excitation technique. Modal tests of the blade packet comprising 18 vane blades connected by shrouds are performed. The measurements are performed within the high frequency range of 0–30 kHz, and the natural frequencies and mode shapes are obtained for first 97 modes. To capture the complex high frequency blade mode shapes, each blade in the packet is scanned over 25 reference points uniformly distributed over the blade concave surface. In order to obtain the high frequency resolution, the frequency range used for the measurements is split into several frequency intervals accordingly to the number of spectral lines available in the used data acquisition system, and for each such interval, the test is performed separately. The finite model of the packet is created, and the numerical modal analysis is performed to compare the calculated natural frequencies and mode shapes with the experimental measurements. The comparison shows the satisfactory with those from finite element analysis. It illustrates the measurement method described in this work is effective and reliable.

Excitation of Arbitrary Displacement/Velocity Conditions in Rotationally Periodic Structures

1995 ◽  
Author(s):  
P. Schmiechen ◽  
D. J. Ewins ◽  
I. Bucher
Keyword(s):  
Natural Frequencies ◽  
Periodic Structures ◽  
Search Algorithm ◽  
Simulated Data ◽  
Travelling Wave ◽  
Modal Testing ◽  
Mode Shapes ◽  
Wave Form ◽  
Structure Interaction ◽  
Wave Patterns

Abstract For an investigation into the structural interaction between rotating and non-rotating rotationally periodic turbine components, it was required to be able to generate experimentally prescribed response conditions. In more descriptive terms, conditions were sought to excite wave-patterns such as travelling and standing waves, and to suppress certain modes. In the paper these conditions are derived from modal properties. Simulated data are presented to demonstrate some of the phenomena and to highlight the practical difficulties. For rotationally periodic structures, most natural frequencies are of multiplicity two, and are sometimes called ‘double modes’. Their associated mode shapes can rotate in the plane of symmetry. The responses due to the two modes can be combined and expressed in a wave form, which can be split into travelling and standing wave components. Theoretically, it is possible to excite a pure travelling wave in a perfectly rotationally periodic structure, but there are limits to this in practice as real structures will always exhibit some degree of imperfection. These structures are said to be mistuned, and the imperfection splits the double modes into pairs of close modes. Simulations show the predicted vibration phenomena. In particular, the case of discrete excitations relevant to modal testing is investigated. The simulations show clearly that in this case components of other modes will generally be present. In an experiment, the results for driving the excitations will not give the theoretically expected response due to non-linearities of the shaker-structure interaction. However, the effects can be reduced by employing a computerised search algorithm.

Modal Testing and Finite Element Calculations for Lightweight Aluminum Panels in Car Carriers

2006 ◽  
Vol 43 (01) ◽  
pp. 11-21
Author(s):  
Junbo Jia ◽  
Anders Ulfvarson
Keyword(s):  
Finite Element ◽  
Natural Frequencies ◽  
Damping Ratio ◽  
Element Model ◽  
Modal Testing ◽  
Structural Behavior ◽  
Mode Shapes ◽  
Maintenance Cost ◽  
Car Suspension ◽  
Aluminum Panels

Due to their characteristics and lower maintenance cost, lightweight aluminum structures have been widely used for manufacturing deck structures. When this type of structure is developed, the natural frequencies for the unloaded deck may increase, while the natural frequencies for loaded decks are most likely to decrease and new problems of vibration and damping may appear. In addition, it has already been shown by the authors that compared to the load effects of normal cargo, the dynamic structural behavior of a vehicle-loaded deck is different due to the participation of vehicle vibrations. The current paper presents a modal analysis by both testing and finite element (FE) calculation for a lightweight deck using aluminum panels. By comparing the results between the unloaded and car-loaded cases, it is shown how vehicle loading influences the dynamic structural behavior of the deck structures. The authors report that an aluminum panel mechanically connected to a steel frame may participate in some mode shapes of vibrations that significantly increase the corresponding damping ratio. The reasonably good agreement between modal testing results and FE calculations validates the finite element model, which may then be used for further dynamic analysis. The authors found that the spring-damping systems of car suspension and tires can interfere in the dynamic transmission of the vehicle mass into the deck structure. The study enables structural engineers interested in the design of car carriers to have a better understanding of how the vehicles parked on decks can influence the dynamic characteristics of the vehicle deck systems.

Verification of a Finite Element Model of an Unmanned Aerial Vehicle Wing Torque Box via Experimental Modal Testing

2012 ◽  
Author(s):  
Levent Unlusoy ◽  
Melin Sahin ◽  
Yavuz Yaman
Keyword(s):  
Finite Element ◽  
Finite Element Model ◽  
Unmanned Aerial Vehicle ◽  
Natural Frequencies ◽  
Element Model ◽  
Modal Testing ◽  
Mode Shapes ◽  
Resonance Frequencies ◽  
Aerial Vehicle ◽  
Experimental Modal Testing

In this study, the detailed finite element model (FEM) of an unmanned aerial vehicle wing torque box was verified by the experimental modal testing. During the computational studies the free-free boundary conditions were used and the natural frequencies and mode-shapes of the structure were obtained by using the MSC® Software. The results were then compared with the experimentally obtained resonance frequencies and mode-shapes. It was observed that the frequencies were in close agreement having an error within the range of 1.5–3.6%.

Modal Parameter Identification of Jacket-Type Platforms Using Vibration Testing Data

2012 ◽  
Vol 170-173 ◽  
pp. 2243-2247
Author(s):  
Yu Jing Chen ◽  
Min Zhang ◽  
Yuan Dong Meng ◽  
Qiang Zhao ◽  
Jie Wen Nie
Keyword(s):  
Natural Frequencies ◽  
Measured Data ◽  
Offshore Platform ◽  
Modal Testing ◽  
Mode Shapes ◽  
Modal Parameters ◽  
Modal Parameter ◽  
Modal Assurance Criterion ◽  
Modal Parameter Identification ◽  
Testing Data

How to estimate the modal parameters (including natural frequencies and mode shapes) of an offshore platform accurately is crucial for the health monitoring problems. The purpose of this paper is to identify the modal parameters of a physical jacket-type platform model from measured modal testing data using Prony’s method. In the modal testing, the identified false modes are unavoidable and bring much difficulty to determine the accurate modal parameters. To solve these problems, in this paper, the Modal Phase Collinearity (MPC) and Modal Assurance Criterion (MAC) are applied to distinguish the true and false modal. The measured data are extracted from a physical model of a realistic offshore platform. And the results demonstrate that the modal parameters of the first two modes of each direction can be accurately estimated by using the proposed method.

scholarly journals Comparison Between Calculated and Measured Free-Free Modes for a Flexible Rotor

1998 ◽  
Author(s):  
José A. Vázquez ◽  
Lloyd E. Barrett
Keyword(s):  
Natural Frequencies ◽  
Forced Response ◽  
Modal Testing ◽  
Analytical Models ◽  
Mode Shapes ◽  
Reduced Order Models ◽  
Response Functions ◽  
Flexible Rotor ◽  
Reduced Order ◽  
Response Characteristics

Many industrial machines nowadays are sold based on analysis performed on mathematical models of the rotors, bearings, substructures, and other components. The validity of the analysts therefore depends on the accuracy of the models themselves. When the rotor is available, modal testing may be used to validate the model of the rotor by comparing the calculated and measured free-free natural frequencies and mode shapes. This work presents additional tools for the verification of analytical models against experimental data. These tools use models of the rotor constructed from the measured data and the analytical model. A comparison of the first six calculated and measured free-free natural frequencies and mode shapes for a multi-mass flexible rotor is presented. The natural frequencies compare within 1.8%. The calculated and measured mode shapes were used to construct independent reduced order models of the rotor. These models were used to perform forced response and stability analyses. Forced response functions are presented comparing the forced response characteristics obtained from the two models. This provides a comparison between the measured and calculated forced response functions for the same number of modes. For the stability analysis, identical bearing models were added to both reduced order models. The eigenvalues were calculated using both models for a range of bearing stiffness and damping coefficients and were plotted for comparison.

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