Componenet Test Specification

[izax1]

I found on the nasa gsfc home page (

http://femci.gsfc.nasa.gov/random/randomtestspec.html)

a procedure for specifying a random vibration component test specification. I acknowledge that this was done some years ago, but the physics should still be valid. Goddard claims that you can CUT OFF SHARP PEAKS in the system response when specifying a component test specification. It does not expalin why this is valid. Does anyone know why this is so?

Thanks in advance.

 

[IRstuff]

Have you emailed the author at the bottom of the page?

TTFN

FAQ731-376

 

[monkeydog]

Sharp peaks can be cut off at about 1/2 their height (-3 dB)*
e.g., a sharp peak of 0.52 g2/Hz can be cut off at > 0.26 g2/Hz;

* NOTE: 3dB is a factor of 2 for ASD curves (g2/Hz) while 6dB is a factor of 2 for grms values. For example, reducing a peak ASD value of 12g2/Hz -3dB would give you 6g2/Hz; reducing a value of 12grms -3dB results in a value of 9grms and reducing it -6dB results in a value of 6grms. This tends to be confusing for people new to random vibration.

 

[Transient1]

I broke down the answer to your question in statements that hopefully will put together the picture of how that recommendation was likely formulated.

(1) Sharp peaks may at times be difficult to put into a spec because the slope may be to steep for the shaker table. So that leaves two choices if you want to exactly specify a sharp resonance (1)Have extreme (and likely impossible) slopes or (2) over test over a wide range.

(2) For a very sharp peak, the Q factor is very high (Ratio of peak value to the bandwidth between the half power points (-3dB) on eitehr side of the peak).

(3)The amount of energy contained above the -3dB points (in the peak) is very little due the sharpness of the resonance. So the spec is basically saying that energy above the half power points can be neglected.

(4) Most random vibration specs have a tolerance similar to +/- 3dB. Testing may always under or over test what ever is specified. That is why analysis plus testing is important. Analysis for random vibration generally considers only 3-sigma values (accelerations/displacements that will not be exceeded 99.73% of the time). So then the analysis will encompass the peak value (that was cut off) for a nominal random vibration test.

(5) The spectrum which was generated would likely have variation. Everytime the application random vibration spectrum is measured it will not be the same. So you have to do the best you can and determine how important that sharp resonance is. It may be that you specify that the component may not have any natural frequencies within an octave of that peak resonance.