General Mechanical

General Mechanical

Modal analysis with pre-stress

    • aitor.amatriain
      Subscriber

      My starting point is a plate as follows

      I need to compute the stresses when the plate es deformed into a cylinder shape. For that purpose, I have run a simulation of Static Structural.

      After knowing the stresses, I need to compute the natural frequencies, so I have created the following tree:

      The stresses seem to be transferred, although I have two problems:

      1) The geometry that is passed to the Modal seems to be the undeformed one.

      2) I cannot add additional supports. In the modal analysis all of the four edges should be fixed, while in the Static Structural I cannot do that (because if not, the structure does not deform)

      I have tried another strategy such as this one:

      In this case, the deformed geometry is transferred, and I can add new supports, but the stresses are not transferred.

      Is there any solution to my problem?

       

      Thank you

       

    • Sahil Sura
      Ansys Employee

      Hi @aitor.amatriain,
      Modal analysis is one of the fundamental analyses of linear dynamics. It determines the vibration characteristics of a component and thus no load/forces are involved in the analysis.  However, 'Pre-stressed Modal' analysis is a way to study the effects of the loading on the vibrational characteristics of the component. You can perform a pre-stressed modal by linking the results of structural analysis to the modal, ensuring the model's linearity. 
      'Pre-stress environment' can be set to the previously linked analysis in the 'Modal' analysis ensuring the results are transferred to the same. (eg. Static structural analysis system is linked to Modal analysis system.)

      This considers the supports, load, etc. added in structural analysis to be considered in the current modal analysis.
      The main outcomes of Modal analysis are the modal frequencies and the participation factors. 
      If you want to gain more insights into the Modal analysis, refer to the following course and references.
      Pre-Stress (ansys.com)
      Modal Analysis (ansys.com)
      Modal Analysis in Ansys Mechanical | Ansys Courses 
      If you want to study the vibrational characteristics of the deformed shape just by adding supports, you can export the deformed geometry as .stl file by right-clicking on the 'Total Deformation' and then performing the 'Modal' analysis of the same.
      Hope this helps.

      Thanks,
      Sahil.
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    • aitor.amatriain
      Subscriber

      Hi Sahil,

      Thank you for your help. I have tried to use the pre-stress fro the structutral simulation, but the system takes the undeformed geometry. I want to compute the natural frequencies of the deformed geometry with the stresses of the deformed geometry. I can export the deformed geometry, but in that I case I cannot perform a modal analysis with pre-stress.

      Is there any way to compute the natural frequencies of the system conbsisting of the deforkmed geometry with the stresses of the deformed geometry?

       

      Thank you,

       

      Aitor

    • Sahil Sura
      Ansys Employee

      Hello Aitor,
      When you link the systems, the results you obtained are with considerations of deformations, stresses, and other parameters from the results of stress analysis as an input to the modal analysis. 
      If you want to verify the same, you can model a simple cantilever beam and get its mode shapes and frequencies. Now you can duplicate the system, add forces, to induce stress in the same, and then link the modal to the system, you may want to compare the results.
      What I would suggest is to choose a simple geometry where you can calculate the modes by hand calculations too!
      But be rest assured that when you link the analyses (share a solution of the first system to the model of the second) the results of the first analysis serve as inputs to the second.
      You also might want to check the following resource explaining the pre-stress modal analysis - How To Perform Prestressed Modal Analysis — Lesson 2 - ANSYS Innovation Courses 

      Hope this helps.
      Thanks,
      Sahil.

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    • aitor.amatriain
      Subscriber

      Hello Sahil,

       

      This tree is right?

       

      In this case what Ansys Mechanical takers is the undeformed geometry. Moreover, I cannot add additional supports (the supports for the static structural and the modal are not the same).

      Thank you,

      Aitor

    • Sahil Sura
      Ansys Employee

      Hi Aitor, 
      Yes, the connection is right (06:12 of How To Perform Prestressed Modal Analysis — Lesson 2 - ANSYS Innovation Courses), I would recommend you to go through the video once to gain insights on the pre-stressed modal analyses. 
      As mentioned, Modal analysis is a 'Linear' analysis so non-linearities won't get reflected in the same. But to inculcate those, we use a structural analysis as the preceding analysis. So if you focus on the outcomes of the analysis both are different for structural and modal. Thus there will definitely be a difference in support and no loading in Modal analysis. 


      Thanks!
      Sahil
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