Compressive stresses with Modal Analysis
Compressive stresses with Modal Analysis
(OP)
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.





RE: Compressive stresses with Modal Analysis
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
RE: Compressive stresses with Modal Analysis
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.
RE: Compressive stresses with Modal Analysis
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Compressive stresses with Modal Analysis
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
RE: Compressive stresses with Modal Analysis
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.