Compressive stresses with Modal Analysis

[bladesandvanes]

Does anyone know how to set up a model that has compressive stresses ranging from 100 ksi to 0 from the surface up to .006 below the surface and simulate it to see what effect it has on a part's frequency. I use work bench mainly but can also use classic somewhat. Thanks in advance.

 

[cbrn]

Hi,
argh, bad news for you:
"A static analysis with initial stress [entered via INISTATE] cannot be used in a subsequent prestressed modal or buckling analysis"

Regards

 

[bladesandvanes]

This was copied out of the 11.0 release documents for Modal Analysis. From what I read in the following paragraphs this is possible if you run a Structural Analysis first and then run a Modal Analysis while pointing the initial condition for that Analysis to the Structural one which will then use the prestressed conditon. Any thoughts on this?


Modal Analysis
Introduction
A modal analysis determines the vibration characteristics (natural frequencies and mode shapes) of a structure or a machine component. It can also serve as a starting point for another, more detailed, dynamic analysis, such as a transient dynamic analysis, a harmonic response analysis, or a spectrum analysis. The natural frequencies and mode shapes are important parameters in the design of a structure for dynamic loading conditions.

You can also perform a modal analysis on a prestressed structure, such as a spinning turbine blade.


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Points to Remember
Only linear behavior is valid in a modal analysis.

Damping is ignored in a modal analysis.

Any applied loads are ignored.

Prestressed modal analysis requires performing a static structural analysis first. In the modal analysis you can use the Initial Condition object to point to the Static Structural analysis to include prestress effects.

 

[GregLocock]

Stress won't affect true linear modal behaviour, in general. The only exception I can think of is for tensioned wires... I don't know what the fundamental difference is that causes that - that is, I understand what the difference is between a tensioned wire and an elastic solid, but it is not immediately apparent why only one should be unaffected by stresses.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[cbrn]

Hi,
GregLocock, I have to disagree. Stress state does affect the natural frequencies, no matter if it is a wire (what is a wire if not a continuous solid? only its form factors don't allow him to bear compression because it's like if it buckled - elastically - under very low loads). Then, depending on a lot of things, the influence can also be extremely low, so low that one won't calculate any appreciable difference with the FE. But the influence is still there. The equations for damped vibrations of continuous solids allow to see this (though I can't write them right here because I don't have them on hands and remembering them is too much for my poor head, especially on Monday morning...).
Back to the original question: the Ansys Manual states that it is possible to use the resulting stress state from a static analysis as a "prestress condition" for a subsequent modal analysis. On the other hand, the command needed for your task in the static analysis seems to make that operation impossible. At this point, it is really not clear to me if it is allowed to directly enter this INISTATE in the modal analysis, skipping the static one (after all, why would you need the static part if the stress field is already known?). I can only suggest to re-read very carefully all the parts of the manual regarding Initial State and Prestressed modal analysis...

Regards

 

[GregLocock]

/true linear modal behaviour/ ?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.