Random on Random

[izax1]

I have random on random vibration profile (A random floor level with narrowbands superimposed) Can anyone tell me how to find the rms level of the complete vibratrion profile?

Thanks.

 

[GregLocock]

RMS composite =sqrt(RMS^2 random +RMS^2 narrowband)

Now, if you actually try and measure the RMS of the composite signal, you may find that is wrong.

That is because at a given frequency line the random and narrowband signals may constructively or destructively interfere. If this is important (it often is not) then you have to work out the RMS of the signal properly by the usual method. RMS=sqrt(V1^2+V2^2...+Vn^2) where n is the number of lines in your FFT



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[izax1]

Thanks a lot Greg.

My next puzzle. Is it possible to combine 5 different Random-on-Random phases into one phase for FE-analysis purposes? Or do I have to run 5 FE-analyses, use Miners to find the contribution from each phase and use Steinberg or Wirsching (or others) to find total fatigue life?

And what about the real testing. 5 NBROR phases in each direction will give 15 tests. Is there a simpler method?

Thanks for any clues

 

[GregLocock]

Sorry, I don't do fatigue.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[izax1]

Thanks for the suport anyway, Greg