Estimating damping in structure composed of different materials (steel, brass, ... The paper presents Structural damping effect on beam vibration by impact ...
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ScienceDirect Procedia Engineering 144 (2016) 110 – 115
12th International Conference on Vibration Problems, ICOVP 2015
Experimental determination of structural damping of different materials Himanshu Mevadaa,Dipal Patelb a
Department of Mechanical Engineering,cspit,India,388421 Department of Mechanical Engineering,cspit,India,388421
b
Abstract Estimating damping in structure composed of different materials (steel, brass, aluminum) and processes still remains as one of the most extremely vast challengers. The paper presents Structural damping effect on beam vibration by impact hammer. Structural damping contributes to about 10-15% of total system damping. The main objective of this work is to estimate the natural frequency and damping ratio of cantilever beams of Aluminum, Brass, and Steel by LabVIEW software and validate the result with vibration analysis and Harmonic analysis utilizing ANSYS. Free vibration analysis was carried out for identifying the natural frequencies and the harmonic analysis was carried out for obtaining frequency replication curves from which damping ratios were estimated utilizing Half- power Band Width Method. It is observed that damping ratio is higher for brass than steel than aluminum. © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICOVP 2015. Peer-review under responsibility of the organizing committee of ICOVP 2015 Keywords: Structural Damping, LabVIEW, FFT, Half-Power Bandwidth Method, Harmonic analysis, Ansys
1. Introduction The concept of damping within a structural system can have different meanings to the various trade branches. Damping is one of many different methods that have been proposed for allowing a structure to achieve optimal performance when it is subjected to seismic, wind storm or other types of transient shock and vibration disturbances. [5] Vibration is an element which is hard to avoid in practice. Excitation of resonant frequencies of some structural parts can occur with existence of vibration even it is only a small insignificant vibration. The number of times that a complete motion takes place during the period of one second is called frequency which is measured in Hertz (Hz).[10] Dynamic analysis aims at understanding, evaluating and modifying the structural dynamic behaviour which involves many terms such as natural frequencies, eigenvalues, eigenvectors, damping ratios, Frequency Response Functions (FRFs) etc. Modal analysis is an effective means for identifying, accepting and simulating dynamic behaviour and responses of structural elements. Modal analysis using ANSYS is an effective method of determining vibration characteristics [14]. Material
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICOVP 2015
doi:10.1016/j.proeng.2016.05.013
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Himanshu Mevada and Dipal Patel / Procedia Engineering 144 (2016) 110 – 115
damping of cantilever beams attracts a lot of work even though extensive literature exists in the area of vibrations of beams. Material damping has not been paid much attention [14]. In this paper the cantilever beam structure, as shown in Fig. 2, has been taken as a part because of its ease of practical ableness and also the ability to exemplify a variety of mechanical products such as wing of an aircraft, rotor blade of a helicopter, blade of a ceiling fan, needle of a clock, shelve of a civil structure, solar panel of a satellite etc. The dimensions of the cantilever beam structure are 370 x 25 x 10 mm, having a material of MS, Aluminium and Brass. The main aim of this project is to investigate highly damped structural material out of various materials of structural beams from finding damping ratio by half-power bandwidth method to reduce vibration level of the system for increase accuracy, safety and machine life. 2. Damping Damping is the phenomenon by which mechanical energy is dissipated (usually converted into internal thermal energy) in dynamic systems. A knowledge of the level of damping in a dynamic system is important in utilization, analysis, and testing of the system. Damping is the energy dissipation of a material or system under cyclic stress. Several types of damping are inherently present in a mechanical system. They are: 1. Internal (material) damping 2. Structural damping 3. . Fluid damping Internal (material) damping results from mechanical-energy dissipation within the material due to various microscopic and macroscopic processes. Structural damping is caused by mechanical energy dissipation resulting from relative motions between components in a mechanical structure that has common points of contact, joints, or supports. Fluid damping arises from the mechanical energy dissipation resulting from drag forces and associated dynamic interactions when a mechanical system or its components move in a fluid 3. Measurement of damping Damping can be represented by various parameters (such as specific damping capacity, loss factor, Q-factor, and damping ratio) and models (such as viscous, hysteretic, structural, and fluid). Before attempting to measure damping in a system, one should decide on a representation (model) that will adequately characterize the nature of mechanical-energy dissipation in the system. There are two general ways by which damping measurements can be made: time-response methods and frequency-response methods. The basic difference between the two types of measurements is that the first type uses a time-response record of the system to estimate damping, whereas the second type uses a frequency-response record 4. Half-power bandwidth method This method is also based in the magnitude curve of the frequency-response function. Bandwidth (Δω) is defined as the width of the frequency response magnitude curve when the magnitude is ͳൗ times the peak value. Then, ξʹ damping ratio can be determined from bandwidth using the expression
ͳ ȟ߱ ߞൌ ʹ ߱
Fig. 1. Half Power bandwidth method
( 1)
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5. Experimental setup of cantilever beam An experimental setup shown in fig. First a beam of various material is to be fix at one end. An Accelerometer has been attached to the cantilever beam at free end to sense the acceleration data of vibration. Impact hammer is use to disturb the frequency or to oscillate the beam. After impact the beam will be oscillate, so accelerometer sense data and signal generated by DAQ device. LabVIEW software used to analysis the signals on laptop.
Fig. 2. Experimental setup of cantilever beam
To calculate the natural frequency of the cantilever beam experimentally, experiment is conducted the with the specified cantilever beam specimen to record the data of time history (Acceleration-Time), and FFT plot. The natural frequencies of the system can be obtained directly by observing the FFT plot. The location of peak values corresponds to the natural frequencies of the system.
Fig. 3. Typical FFT graph plot
6. Result and discussion The Damping ratio is a parameter, usually denoted by ζ (zeta) provides a mathematical means of expressing the level of damping in a system relative to critical damping. Damping of specimens made up of different materials (brass, aluminium and mild steel) was calculated with the help of half power bandwidth method.
Himanshu Mevada and Dipal Patel / Procedia Engineering 144 (2016) 110 – 115
Amplitude [m/s²] 2.50E+04 2.00E+04
1.50E+04 1.00E+04
5.00E+03 1 10 19 28 37 46 55 64 73 82 91 100
0.00E+00
Fig. 4. FFT plot from Experimental result and Harmonic analysis of MS material.
5.00E+04
Amplitude [m/s²]
4.00E+04 3.00E+04 2.00E+04
1.00E+04
1 11 21 31 41 51 61 71 81 91
0.00E+00
Fig. 5. FFT plot from Experimental result and Harmonic analysis of Aluminium material
Amplitude [m/s²] 6.00E+03 5.00E+03 4.00E+03 3.00E+03 2.00E+03 1.00E+03 1 11 21 31 41 51 61 71 81 91
0.00E+00
Fig. 6. FFT plot from Experimental result and Harmonic analysis of Brass material
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Table 1.Comparision between three materials of Cantilever structural beams SR NO
Material
Damping ratio (Theoretical)
Damping ratio (Experimental)
% ERROR
1 2 3
Mild steel Aluminium Brass
0.0077 0.0040 0.010
0.0069 0.0035 0.009
11 14 11
Natural Frequency 70 60 50 40 30 20 10 0
MS
DAMPING RATIO
0.012 0.01 0.008 0.006 0.004
Theoreti cal
Ansys
Experim ental
59.58
59.72
58.44
ALUMINUM
59.79
59.93
59.12
BRASS
39.43
39.54
38.4
0.002 0 MS Theoretical
ALUMI NUM
BRASS
0.0077
0.004
0.01
Experimental 0.0069
0.0035
0.009
Fig. 7. Comparison between three materials of Natural frequency and Damping ration of Cantilever Structural beams
7. Conclusion The comparison concludes that the theoretical calculations based on the Harmonic Analysis with Ansys match with experiments very well for the structural Beam with maximum errors of 14%.The Harmonic analysis has effective, positive and helpful method for estimating of damping characteristic of structural beam. From the fig it concludes that when Density of material increases, Damping Ratio also increases and Natural Frequency decreases of respective material density. It also conclude that the Damping ratio of Brass material is higher than Aluminium. In the future this experiment can be applied for Nano level as well as composite material. Acknowledgements The experimental work done in dynamics of machinery lab at CSPIT with the help of LabView software. Reference [1] [2] [3] [4] [5] [6]
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