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# Sine on Random

## Sine on Random

(OP)
I am doing a study on Combining Sine and Random vibration.
In test bench, component is tested by starting Sinusoidal testing then followed by Random vibration testing. Harmonic effect continues in Random vibration testing.
I wants to compare my test results with simulation. I performed separately Harmonic analysis and Random analysis then combined using square root of sum of squares method ( SRSS - is not advisable because it lost phase information).
Another method I performed a Random vibration analysis in frequency domain by combining sine and random spectra ( combined in time domain )
Is it possible to compare the combined spectra ( sine and random ) with the test results ( Sequence of harmonic followed by random testing).
If possible, how can the results be justified.

Arumuga Pandian

### RE: Sine on Random

Pretty advanced for this forum. You can join Tom Irvine's web page www.vibrationdata.com for \$20 or so and look around for it. If anybody can resolve your issue Tom can. Go to his website for his contact information and consult with him if you can't find what you're looking for by browsing his website. He's got gobs of white papers and useful software on his website though.

Tunalover

(OP)

### RE: Sine on Random

Arumugam09,

You can model very weird vibration modes through numerical analysis. Work out your applied forces and/or displacements. Do the numerical integration. When I did this, I was curious about non-linear damping.

This may be doable on a spreadsheet. It certainly can be done with MathCAD or Octave.

--
JHG

### RE: Sine on Random

What are the frequency ranges for both tests? Certainly the same.

Harmonic effect is due to a too high level of the excitation.
Try to decrease the level of the signal till the harmonics disappear.

To understand, you can imagine a signal with a fixed sinus frequency. The FFT is just one Dirac.
Now, if the gain is too much, then the sinus is notched out. The sinus looks like a square signal and its FFT is a suite of decreasing Dirac (the harmonics).

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