Learning Forum

[Yosri3]

I performed a non-linear analysis of a ship structure on a static structural system. Then I transferred the plastic strain to a new plastically deformed model (using the command INISTATE). Now I need to perform modal analysis on this plastically strained prestressed hull. To do that I must first solve the model on static structural, but to solve it I must add boundary condition. The modal analysis should be performed without any boundary condition. Then the question is, how can I conduct modal analysis of prestressed free-free beam? is it even possible?

To simplify the problem I replaced the hull with this simple beam.

The command APDL contains the following line

INISTATE , READ , file,ist,'C:FinishedTasksInstateTest_filesdp0SYS-3MECH'

and the boundary condition defined in the pic is Weak spring on. If I turned it off, how can I solve the problem in static structure to just transfer the stresses to the modal analysis

I would be grateful if someone can help

[Ashish Khemka]

Please see if the following link helps:

Free-free and Pre-stressed Modal Analysis | Ansys Courses

Regards Ashish Khemka

[Yosri3]

Hi Ashish
Thanks for the reply, unfortunately in this tutorial, they just explain the meaning of the free-free modal analysis and the prestressed modal analysis separately. The example of the guitar is easy as the boundary condition of the prestressed guitar wires is known. If we take the example of the airplane in this tutorial and assumed the wing is plastically deformed due to excessive turbulent loads, how can we get the modal frequency of this configuration?
It may be a topic that is not addressed before or maybe beyond Ansys modal capabilities.

[Ashish Khemka]

You can use the deformed structure of the wing as geometry input for the modal analysis.
Regards Ashish Khemka

[Yosri3]

Yes, I already did that, but this way we consider the plastic deformation as imperfection only. Can we also transfer the stresses and strains?

[Ashish Khemka]

no you will not be able to transfer the stresses and strains. But the effect of stresses and strain should be captured in the stiffness of the deformed shape.

Regards Ashish Khemka

[Marufyanuareffendi]

How to find out stiffness in modal analysis ?