Experimental Study of Structural Vibration

Topics: Vibration, Measurement, Impulse response Pages: 9 (2340 words) Published: April 10, 2011
1. Introduction
In this experiment, we learnt that all systems that have mass are capable of free vibration that can take place in the absence of external excitation. Such a system has its natural frequency of vibration. This natural frequency is critical to any system especially when it is fast-vibrating structure with heating masses.

The masses will give rise to vibration excitation and generate fluctuating forces. When the system is moving at its natural frequency, the system will oscillates or vibrates very violently. This effect will have a strong impact on the life span of the system. When this phenomenon occurs, it is called the resonance of the system. Provided a system is vibrating at its resonance, the system will fail after a short span of time. Thus, it is important to avoid running a structure at its resonance.

Basically there are two types of resonance, primary and secondary resonance. However, not every resonance will always occur in all moving structure. This is also applicable to the system that we used to observe in our experiment. The system that we use is test structure (beam) with hammer excites the system on the different parts on the structure. The response of the system will be recorded with the accelerometers that are attached to the system.

After that analysis data will transfer to Dynamic Testing Software (DTS), also called ICATS to obtain circle- fit modal analysis. DTS software can be used to extract useful parameter such as natural frequencies, damping ratios and modeshapes.

2. Objective

a) To understand vibration concept through actual vibration test. b) To learn how to conduct basic vibration test to obtain frequency response functions of test structure. c) To learn how to use vibration analysis software to extract useful parameter such as natural frequencies, damping ratios and modeshapes.

3. Procedure

a) An accelerometer is attached to a test structure (beam) and connects the end of accelerometer to channel analyzer.

b) Impact hammer is used to excite the structure and connects the end of impact hammer to channel analyzer.

c) The accelerometer will be attached to the different part of the structure the measurement is interested. For the first experiment, accelerometer was at point 1 and stroked by hammer at point 1 also to obtain vibration measurement shown in Figure 3.1

d) Strike the mass at the center gently and ensure that there is no double hit. If the strike is clean, the result will show on Multiple-channel Analysis System Type 3550. The system will prompt the user to repeat the process for 10 times on each mass then the average vibration waveform would be captured in the system by an accelerometer.

e) A typical modal analysis set up with Multiple-channel Analysis System Type 3550 to get vibration measurement. Data from multiple-channel analysis system will transfer to personal computer by using Dynamic Testing Software (DTS).

f) From here, extract useful vibration parameter such as natural frequencies, damping ratio and modeshapes by using ICATS software. Print out the graph containing the needed information.

g) Accelerometer position is moved to point 2(mass 2) and point 3(mass3), impact test is repeated at point 1 where other vibration parameters are obtained.

4. Results

Mass 1
Title: Frequency respond
Excitation=1, Respond=1

|O-fit |Mode 1 |Mode 2 |Mode 3 | |Natural frequency |73.60 |253.68 |300.15 | |(Hz) | | | | |% Structural Damping |6.5478 |1.2132 |0.7544 | |MOD CONST MAG (1/kg) |0.697 |0.122...
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