Modal Analysis
What is the difference between Frequency Response Analysis and Response Spectrum Analysis?
Answers
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frequency response excites the model with a frequency varying load (harmonic load, sucha as sinusoidal wave), and then compute the system response due to its dynamics, specially ressonances.
So let's say you apply a 10N load at 1Hz, and mesure the response. Then 10N load at 2Hz, and measure again...and so on.
You will get the Amplitude of your response (displacement, stress, ...) over the applied frequency.
In response spectrum you also want to investigate the response but the excitation is unique (for example a half-sine pulse) and then we want to evaluate what are the maximum responses you will get for different dynamic systems (having different natural frequencies).
Let's say you want to get the response spectrum when subjected to a half-sine pulse (10N, with 100ms duration).
Then essentially you would apply this transient load to a series of spring-mass systems , each one with different frequency, and then compute how each system behaves to this same applied load. Then if you plot the maximum response of each of them, according with the frequency, you will get the Spectrum for this particular load, giving you essentially what level of load amplification you will have for each frequency. Having this, you can identify in your real structure, what are the most contributing frequencies/modes, and calculate your structure response due to each mode,
OptiStruct does this in the background.
http://www.vibrationdata.com/tutorials2/vrs.pdf
@João Marabisa has implemented this using Activate, and i believe he has posted this somewhere in the Forum.
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Altair Employee