scholarly journals Assignment of Natural Frequencies and Mode Shapes Based on FRFs

Aerospace ◽  
2021 ◽  
Vol 8 (9) ◽  
pp. 262
Author(s):  
Jun Ren ◽  
Qiuyu Cao
Keyword(s):  
Structural Modification ◽  
Optimization Problem ◽  
Natural Frequencies ◽  
Modal Testing ◽  
Mode Shapes ◽  
Original Structure ◽  
Numerical Verification ◽  
Added Masses ◽  
The Masses ◽  
Mass Spring

This paper proposes a method of structural modification for the assignment of natural frequencies and mode shapes based on frequency response functions (FRFs). The method involves the addition of masses or stiffness (supporting stiffness or connection stiffness), the simultaneous addition of masses and stiffness, or the addition of mass-spring substructures to the original structure. Firstly, the proposed technique was formulated as an optimization problem based on the FRFs of the original structure and the masses or stiffness that needed to be added. Next, the required added masses and stiffness were obtained by solving the optimization problem using a genetic algorithm. Finally, numerical verification was performed for the different structural modification schemes. The results show that, compared to only adding either stiffness or masses, adding both simultaneously or adding spring-mass substructures obtained better optimization results. The advantage of this FRFs-based method is that the FRFs can be directly measured by modal testing, without knowledge of analytical or modal models. Furthermore, multiple structural modifications were considered in the assignment of natural frequencies and mode shapes, making the application of this method more applicable to engineering.

scholarly journals The Effect of Inner Reinforcing Cores on Natural Frequencies of the Lathe Tool Body

2013 ◽  
Vol 21 (Special-Issue) ◽  
pp. 186-192 ◽  
Author(s):  
Ladislav Rolník ◽  
Milan Naď
Keyword(s):  
Material Properties ◽  
Machine Tools ◽  
Structural Modification ◽  
Natural Frequencies ◽  
Mode Shapes ◽  
Geometrical Parameters ◽  
Structural Modifications ◽  
Modal Properties ◽  
Machine Productivity ◽  
Lathe Tool

Abstract The contribution is mainly focused on research and development of structural modification of machine tools, lathes in particular. The main aim of the modification is to change the modal properties (mode shapes, natural frequencies) of the lathe tool. The main objective of the contribution will be to formulate, mathematical analyse and evaluate the proposed methods and procedures for structural modifications of the tool, represented by beam body. A modification of modal properties by insertion of beam cores into beam body is studied in this paper. In this paper, the effect of material properties and geometrical parameters of reinforcing cores on natural frequencies of beam body is presented. The implementation will bring benefit on machine productivity, decreasing the machine tool wear and in many cases it will lead to better conditions in the cutting process.

scholarly journals A receptance-based method for frequency assignment via coupling of subsystems

2019 ◽  
Vol 90 (2) ◽  
pp. 449-465
Author(s):  
Sung-Han Tsai ◽  
Huajiang Ouyang ◽  
Jen-Yuan Chang
Keyword(s):  
Assignment Problem ◽  
Structural Modification ◽  
Optimization Problem ◽  
Natural Frequencies ◽  
Coupled System ◽  
Frequency Assignment ◽  
Frequency Assignment Problem ◽  
Extended Technique ◽  
Modification Process ◽  
Receptance Coupling

Abstract This paper presents a theoretical study of the frequency assignment problem of a coupled system via structural modification of one of its subsystems. It deals with the issue in which the available modifications are not simple; for example, they are not point masses, grounded springs, or spring-mass oscillators. The proposed technique is derived based on receptance coupling technique and formulated as an optimization problem. Only a few receptances at the connection ends of each subsystem are required in the structural modification process. The applicability of the technique is demonstrated on a simulated rotor system. The results show that both bending natural frequencies and torsional natural frequencies can be assigned using a modifiable joint, either separately or simultaneously. In addition, an extension is made on a previously proposed torsional receptance measurement technique to estimate the rotational receptance in bending. Numerical simulations suggest that the extended technique is able to produce accurate estimations and thus is appropriate for this frequency assignment problem of concern.

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.

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.

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 Overcoming Effects from Environmental Temperature on the Natural Frequencies of Cable-strut Structures

2020 ◽  
pp. 22-30
Author(s):  
Nasseradeen Ashwear ◽  
Haithem Elderrat ◽  
Mahmud A. Eljaarani
Keyword(s):  
Genetic Algorithm ◽  
Health Monitoring ◽  
Optimization Problem ◽  
Environmental Temperature ◽  
Natural Frequencies ◽  
Dynamic Properties ◽  
Environmental Variable ◽  
Mode Shapes ◽  
Stress Vector ◽  
Temperature Changes

The changes in dynamic properties such as natural frequencies and mode shapes are used in vibration health monitoring as tools for assessing the structures health status. They are, however, also affected by environmental conditions like wind, humidity and temperature changes. Of particular importance is the change of the environmental temperature, and it is the most commonly considered environmental variable that influences the vibration health monitoring algorithms. This paper discusses how cable-strut structures can be designed such that their first natural frequency is less sensitive to the temperature changes. The optimization problem is solved by using a genetic algorithm. The level of pre-stress can be regulated to achieve the solution, particularly when a symmetric self-stress vector is chosen.

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%.

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