Shock test per MIL-STD-810 - equivalent static load for FEA 1

[DonkeyDude]

Hi all,

An earlier posting in 2007( thread384-192610 )by someone on this very same topic had some good responses out of which I could relate to Spongebob007 's reply the best.

I can extract Natural Frequencies using FEA software. For simple shock tests (such as 40g over 10 ms)could I not make use of this frequency info and come up with an equivalent static G load which could then be applied for a stress analysis ? I understand random vibration is a whole different ball game. I believe there are simple and clever ways of doing just about everything and you just have to keep looking if you don't find them.

Any simple example would be highly appreciated. There was something missing in the earlier Spongebob007's response (don't know how he came up with the 1.667)and I know he is on this planet somewhere but can't locate him though.

thank you,

DonkeyDude

 

[jedward]

Look in Steinberg, Vibration Analysis for Electronic Equipment where he discusses half sine shock pulse effects

 

[GregLocock]

I think spongebob's approach was based on the fundamental frequency of the system, and then looking at the response of an equivalent SDOF system to the pulse.

As such it is very sensitive to the estimate of the damping, if the frequency content of the shock is close to the resonance, which given the shape of the spectrum of a half sine is likely.

Pretty sure Harris mentions it as well, if you happen to have that handy.



Cheers

Greg Locock

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

 

[spongebob007]

Steinberg pg 258 Fig 11.8

Take the ratio of the structure frequency to the pulse frequency. Select the amplification factor from the chart based on damping and multiply that by the peak acceleration of the pulse.

Not sure where 1.667 came from. Probably a typo.

 

[MAGNAFEAMAN]

"I can extract Natural Frequencies using FEA software. For simple shock tests (such as 40g over 10 ms)could I not make use of this frequency info"
Why not? This is done on a transient analysis and you have to create the profile of the signal and apply it to your model.