6 Replies Latest reply on May 11, 2015 1:29 PM by Shaun Erasmus

    Force Units in Linear Dynamic Analysis

    Shaun Erasmus

      How does one determine the force in a linear dynamic study? The units are N^2/Hz, how does that equate to a regular Newton force load?

        • Re: Force Units in Linear Dynamic Analysis
          Bill McEachern

          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

            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

              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 3-sigma force, and then you can say, "Well, 99.7% of the time, the force at this point will be less than this 3-sigma 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
                Jared Conway

                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

                  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.