
Re: Force Units in Linear Dynamic Analysis
Bill McEachern Jan 20, 2015 9:36 AM (in response to Shaun Erasmus)what kind of study are you doing? That hints of a random vibration study  where a unit like that would be "force spectral density"......a force output would normally be an RMS value of force. Could be a software reporting error  the units given by the software are wrong and it is just Newtons. That happens in other places in the software (like remote loads)

Re: Force Units in Linear Dynamic Analysis
Attilio Colangelo Jan 20, 2015 10:25 AM (in response to Shaun Erasmus)Yes, this is typically the loading for random vibration. It can be converted to a force, which is commonly given in terms of G's relative to gravitational force. Are your N^2/Hz values accompanied by a table or graph of N^2/Hz vs Freq(Hz)? This is what a dynamic analysis would require.
Alternatively, you can "integrate" N^2/Hz over Freq and this will leave you with an effective Newtons,rms which can then be applied as a normal static force.

Re: Force Units in Linear Dynamic Analysis
Mike Pogue Jan 20, 2015 11:31 AM (in response to Shaun Erasmus)Everything Attilio said.
But also, this is random vibration. The power spectral density (PSD) is the statistical frequency content of the vibration. You don't know what the force is at any given instant, but in some sense you know the portion of the vibration power that is at any given frequency. This is totally different from time history methods. The reason the frequency content is useful, is because you can get the transfer function from the stiffness matrix and you'll know how the system reacts to the PSD.
The reaction will be a plot of N^2/Hz vs. Hz. If you integrate under this, you get N^2, like Attilio said. This is the RMS force, which is the same as the 1 Std deviation force. Normally at that point, you multiply it by 3 to get the 3sigma force, and then you can say, "Well, 99.7% of the time, the force at this point will be less than this 3sigma force I just calculated." This is normally the force you'd use for analysis.
It depends on what you are doing, but it's probably more useful to look at the stress plot MPa^2/Hz

Re: Force Units in Linear Dynamic Analysis
Attilio Colangelo Jan 20, 2015 1:44 PM (in response to Mike Pogue)Thank you for the VERY important clarification Mike.....If this is random vibration, such as that experienced by aircraft, the RMS is typically multiplied by 3 for 3 sigma.


Re: Force Units in Linear Dynamic Analysis
Jared Conway Jan 20, 2015 11:42 AM (in response to Shaun Erasmus)like bill asked, what type of analysis are you trying to do?
if you're doing a time history dynamic analysis, N is N, you just need to know the time it is applied for

Re: Force Units in Linear Dynamic Analysis
Shaun Erasmus May 11, 2015 1:29 PM (in response to Shaun Erasmus)Thanks everyone, this project is now on hold, but I appreciate all your input and explanation. It has given me some clarity on it all. The analysis type was an explosive test, so basically a decaying sinusoidal wave I think.