 # Comparing stresses from modal analysis and static analysis

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• Last Post 17 September 2018
Katerina posted this 13 September 2018

Hello everyone,

For my master thesis, I use the Mechanical APDL 15.0 with the help of which I would like to carry out a static or modal analysis of a piston rod.

My goal is to get the stresses at the nodes for the elements and modes of the piston rod.

I define a txt. script with the necessary conditions to perform a modal analysis.

ANSYS returns the nodal stresses.

Now I would like to choose an alternative way for getting the stresses (in order to check if the stresses are equal):

a static analysis with defining the shifts as constraints with the "d" command of the respective modes.

If I choose the way of the static analysis, the values for the nodal stresses are different then the nodal stresses I got with the modal analysis.

How can this be?

Is this a constant factor between these two values of the modal analysis and the static analysis that ANSYS always includes in the calculation?

And if so, how is this factor to be identified? How are nodal stresses normalized in ANSYS?

Best regards

Katerina

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peteroznewman posted this 13 September 2018

Hello Katerina,

Modal analysis computes the shape and frequency of modes and displays a magnitude of displacement and stress that has no meaning for the displacement and stress of a real structure because there is no load applied to the structure in a modal analysis.

The Static Structural solution has an applied load, so the displacements and stresses represent real values. The question for you is what load are you applying, how are you applying the load and what boundary conditions are you using.

Kind regards,

Peter

Katerina posted this 17 September 2018

Hello Peter,

Thank you for your answer! I would like to clarify that I already know that the stresses' magnitude has so meaning in modal analysis. I would just like to know the factor that Ansys is using to normalize the stresses' magnitude. In other words: How is the stresses' magnitude scaled? Since the stresses' magnitudes behave should increase linearly or decrease linearly between static and modal analysis at the respective modes there has to be a factor connecting the stresses of static and modal analysis.

Best regards Katerina

peteroznewman posted this 17 September 2018

Hello Katerina,

Both Modal and Static Structural use the same material, which has a Young's modulus, E.

Both Modal and Static Structural compute deformations, which result in strains, which can be multiplied by E to get stress.

So Deformation > Strain > Stress. That is the commonality.

Perhaps that answers part of your question. But you have another part to your question and that is how does ANSYS normalize the deformation in a Modal analysis.

Your statement below has an incorrect assumption.

Since the stresses' magnitudes behave should increase linearly or decrease linearly between static and modal analysis at the respective modes there has to be a factor connecting the stresses of static and modal analysis.

There is no relationship between the deformation in a Static Structural model and the deformation in a Modal analysis because the Static Structural model has a load and the Modal analysis has no load.

If you want to know how ANSYS decides how much deformation to put into the Mode shape for each mode, that is a different question and unrelated to the deformation of a Static Structural model. I don't know the answer to that question. I can use the results of a Modal analysis without knowing this because the magnitude of the deformation has no meaning to me. It is only the shape and frequency that is important.

Kind regards,

Peter Close 