About Free-Free Modal Analysis

- September 1, 2025 at 7:40 pm
  
  turgut.ataseven
  Subscriber
  
  Hi.
  Imagine a component inside of the body of a missile. This component is mounted to body by screws and pins, so Bolt Pretension will be present at threads which will change the natural frequencies. Additionally, lift and drag forces on the wing will affect the natural frequencies too. Since we cannot run RBM situation on Static Structural, we cannot add the effect of pre-stress to free-free modal analysis. Is it better to just run a free-free modal with no boundary conditions or run modal analysis with pre-stress effects included by inserting Fixed Support BC and Bolt Pretension as Force to threads?
- September 2, 2025 at 2:46 pm
  
  dlooman
  Ansys Employee
  
  It might depend on how massive and stiff the missle is relative to the mass and stiffness of the component. If the missile is very massive and stiff, then fixing component at the attachment location would be justified. Or you could model a portion of the missile and do a free-free prestressed modal. Weak springs could allow a static analysis without supports. I always wonder whether bolt pretension has much effect on the modal compared with simply bonding a small area which the bolts compressive. I doubt rigid body motion has much affect on the vibratory frequencies also.
- September 4, 2025 at 1:11 pm
  
  cikix11766
  Subscriber
  
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