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Random vibration
Posted Feb 17, 2021, 10:37 a.m. EST Acoustics & Vibrations Version 5.6 6 Replies
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Last time when I asked about how to plot the stress due to random vibration, Answer: To plot the stress (i.e von Mises) you use the rvib1.q2 operator on a variable that is sigma_mises^2 and then we take the square root of it. the expression will then be sqrt(rvib1.q2(3*solid.ii2s, fL, fU, nF).
Question: When i take a 3D plot and select the random vibration study I set up, I select surface under 3D plot- used expression rvib1.rms(solid.ii2s,10,500,nF) or sqrt(rvib1.q2(3*solid.ii2s, fL, fU, nF).
----I have been getting errors
Can someone suggest me of how can I plot random vibration- von mises stress?
Thank you
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error message: Failed to evaluate operator. - Operator: rvib1.rms - Geometry: geom1
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The first expression, rvib1.rms(solid.ii2s,10,500,nF), is fundamentally wrong since the rms() operator is only valid for expressions that are linear functions of the degrees of freedom, such as individual stress and strain components (and since solid.iis2 is not an existing variable).
The second expression, sqrt(rvib1.q2(3*solid.ii2s, fL, fU, nF), is in principle correct, except that the name of the variable should be solid.II2s and not solid.iis2 and that the number of parentheses should match.
-------------------Henrik Sönnerlind
COMSOL
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Thank you so much. I can now plot the 3sigma expression.
Could you also suggest me on how do I plot the stress PSD response for a point on the structure for the random vibration study. I would like to plot the various response spectrum of the structure.
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I have been trying to replicate this and would like to obtain the same plot and graph
https://www.researchgate.net/publication/338432119_Vibration_Fatigue_Analyses_of_a_Cantilever_Beam_in_Transportation_Based_on_Military_Standard/link/5e14a33d4585159aa4b9e621/download
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You can find PSD plots in all random vibration examples in Application Galleries. You create them from a Global Evaluation Sweep node.
As in many reports similar to the one you reference, the definition of 'stress' as a bit fuzzy. So it may be difficult to reproduce the result. I would guess that they are using the stress tensor component in the x-direction, because luckily in their geometry that one will be critical. If the object had been rotated in the XY-plane, things would be more difficult, since a stress component in a local coordinate system would have to be used. If the stress state is not locally uniaxial, it is even more difficult...
The whole fatigue evaluation approach used in that paper is a bit suspect in my opinion.
-------------------Henrik Sönnerlind
COMSOL
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I would like to show you my model and just verify small thing of plotting the psd response of various kinds and also about how I have extracted them in tabular columns.
Is that even possible for tomorrow?