Pre-stressed modal why changing end time causes natural frequency to change?

- October 10, 2023 at 5:35 pm
  
  Weihan Lin
  Subscriber
  
  Hi all,
  I've been using pre-stressed modulue to modal analysis. Under "Static structural" -> "Analysis setting" -> "step controls", I changed the "step end time" from 10 sec to 100 sec and the results of natural frequency changed dramaticlly.
  Does anyone know why is that and what kind of phenomenon behind it? Which simulation results should I trust?
- October 10, 2023 at 6:51 pm
  
  Ankush Choudhary
  Ansys Employee
  
  Hi Weihan,
  Could you please check whether the stress values are changing in static structural with the change in step end time?
- October 10, 2023 at 6:54 pm
  
  Ankush Choudhary
  Ansys Employee
  
  Please also check the below settings in the Modal analysis.
  - October 10, 2023 at 11:45 pm
    
    Weihan Lin
    Subscriber
    
    Hi Ankush,
    Thanks for you information. I double check the static structural stress values at the end time, and both have exactly same results. However, the "pre-stress" section, I have something different settings. Is this where the problem at?
- October 10, 2023 at 11:50 pm
  
  Ankush Choudhary
  Ansys Employee
  
  Hi Weihan,
  Yes, this is the reason you are getting different natural frequencies, as the static structural data is coming at different time in Modal analysis. If you want to have the prestress data at the end of static structural, then you can change the option to program controlled, as I have shown above.
  - October 10, 2023 at 11:52 pm
    
    Weihan Lin
    Subscriber
    
    Got it. It did work! Thank you for the great help!
- October 10, 2023 at 11:54 pm
  
  Ankush Choudhary
  Ansys Employee
  
  I am glad to hear that the issue is resolved now.?