Shock influence on natural frequencies

[farspacer]

Hello!

I've got a cylindrical resonator made of steel. I assigned
appropriate material, constraints and ran modal analysis to
get natural frequencies (It resulted in 4 modes in 1000-
10000 Hz freq range). Now I need to find out how shock
(20g, direction - resonator lateral axis) influences on
these natural frequences, do they change at all... So I'm
asking for your help because I don't understand which
analysis to use for this research. Would be very grateful
if anybody could give me some advices and aims.

 

[Ray Ellender]

You need to do a Dynamic analysis for which you need the Advanced Mechanica licence. You must then set an forcing function, e.g. a half sine wave, and decide on the amount of damping, e.g. 2% or 4% modal damping. You must also set up measures on your model, e.g acceleration, displacement, stress, so that you have something to plot. You do an initial run (which will take ages), plot the response then do a second run to obtain stress plots (full results) at the peaks. For a 'shock' input you want to do a Dynamic Time, with an input of say a half-sine wave (Not a Dynamic Shock which is special for earthquakes),.


However if you want to see how the natural frequencies change with damping you should run a Dynamic Frequency, but make sure that you have enough input steps to be able to plot a good response curve. (Which means you need a lot of points, which means the run time will be very long - possibly days, depending on model size). Neither of these will tell you the frequencies as part of the report - you have to deduce it from the plot.

 

[farspacer]

Thank you for the answer.

The main idea is to compare natural frequencies before and
after the shock. If in modal analysis, for instance, I got
nat. freq. nearly 1541 Hz, which frequency would I get
after the shock?
In this case, I suppose, first I have to apply a force to
the model (thus It will be "shocked" and slightly
deformed), run Static Analysis, then do a Prestressed modal
analysis to see how natural frequencies changed. It seems
to be the right way.