Modal Results not Responding to Changing Pre-Stress

- April 20, 2024 at 12:41 am
  
  Michael Fusaro
  Subscriber
  
  I am trying to apply moment to a beam at one end and solve for vibration modes.
  Each time I change the moment load at the free end of the beam, the modal solution outputs the exact same frequencies for the 20 modes solved. (Almost as if it is not accounting for the pre stress at all)
  When I try the same with an axial tensile load, the solved frequencies increase with increased load. For some reason they do not when I apply moment. Any advice is appreciated, thank you
- April 20, 2024 at 11:04 am
  
  peteroznewman
  Subscriber
  
  The Pre-Stress input to Modal calculates a new stiffness matrix from the solution of the Static Structural model. The axial tension causes the stiffness matrix to change significantly and that is seen in the Modal frequency. It seems the moment load does not change the stiffness matrix.
  I believe the way Pre-Stress works is that it only uses the stiffness matrix, but it does not update the shape of the part prior to running Modal. Try to put a huge moment on a straight beam so that the deformed shape is a semicircle and see what happens in the Modal analysis.
  - April 22, 2024 at 11:38 pm
    
    Michael Fusaro
    Subscriber
    
    Thanks for your reply Peter.
    An additional question I have is, what might cause my 3-sigma equivalent stress from a random vibration to be much lower than the equivalent stress from the static structural pre-stress analysis?
    For example, I have a part at the end of my beam that exhibits high stresses under static structural loading (~300 MPa)
    When performing random vibration with the above pre-stressed static structural, the 3-sigma equivalent stress only reaches about ~40 MPa.
    I understand Random Vibration is a statistical result, but I cannot understand why the stress would be significantly lower than the static case. I just want to make sure I am interpreting these results correctly.
    
    Thanks in advance,
    Michael
    - April 23, 2024 at 12:49 pm
      
      peteroznewman
      Subscriber
      
      This discussion has links that describe how to combine stress from Static Structural and Random Vibration.
- April 22, 2024 at 8:20 am
  
  Erik Kostson
  Ansys Employee
  
  Hi
  
  It does use the deformed shape say obtained from a static analysis also (not ony the stress stiffness matrix) – just we can not see that in the modal results.
  There is an option to see the mode shapes relative the actual deformed shape:
  Under the Solution in the tree and Information set Mesh Source (beta) -> Result file
  All the best
  Erik