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ABAQUS 3D model: mode shape and frequency issues

ABAQUS 3D model: mode shape and frequency issues

ABAQUS 3D model: mode shape and frequency issues

(OP)
Hi all,

I am new to ENG-tips so I am not sure if I am asking this in the right place. Please let me know if you feel like I should post this elsewhere.

Anyway, I am looking at mode shapes and frequencies for a small 3D cylinder. I used a 3D model so that I can also apply PZT properties. Using the linear perturbation analysis, I have issues with mode shapes that look like non sense. I think they may be local mode shapes but was wondering if there was a way I could avoid these as a result such that I am not filtering through a hundred nonsense shapes to find something that makes sense. This doesn't occur when I use a beam element so I am guessing that it has something to do with not adding enough constraints. I am verifying my results with analytical formulas and there were some mode shapes that reflected the yielded analytical values but the shape itself made zero sense.

Another issue I am having is that for the expected mode shapes for mode numbers 1-5, the frequency is the same. For instance, the breathing frequency is the same frequency as mode shapes with 1-3 antinodes. This makes no sense to me at all. I have tried fixing the cylinder in the axial direction and this allows me to get the shapes but not the correct corresponding frequencies. Could this have something to do with consistent mass matrix vs. element lumped mass matrix? I don't know how to specify either, nor do I know which is used, but that is all I found for possible sources of error.

I would be incredibly grateful with any suggestions or input that may help. I have only been working with ABAQUS for a few months now so i apologize if this is a complete rookie error. Thanks!

Kind regards,

Zach

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