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# Pressurized Inflatable structures - Frequency analysis

Experimenter

Hi everyone,

I hope that you can help me. I'm trying to do a simulation of a inflatable cylinder (100mm diameter and 500mm length).

I model the cilinder using shell elements (PSHELL). The wall has a thickness of 0.5mm and the material can be PVC or polyurethane.

My problem isn't in the modelation of the structure but the analysis. I want to do a frequency analysis to know the vibrations modes. In these analysis, I have two problems:

1º I can't especify a pressure inside the cylinder using the solver 103

2ºWhen I try to do a static analysis, it occurs always a fatal error saying that there is excessive pivot ratios in matrix kll. And I'm stuck in this step!

Can you help me?

I started to see some tutorials about a Restart analysis in NX simulation. But like I said I can't do the first part which is the static analysis without a fatal error.

https://www.youtube.com/watch?v=AYTx_jZbAcM

If you need any other information please ask

Thank you

Afonso Ferreira

2 REPLIES

# Re: Pressurized Inflatable structures - Frequency analysis

Legend

Hi there,

For the pressurization of the structure, you'd have to create a preload step.  NX NASTRAN will run your pressure static case first, then use the updated stiffness matrix to perform the modal analysis.  As for the excessive ratio, I'm assuming you'd like to look at your inflatable cylinder floating in space, i.e. no constraints...  You'd have to run with inertia relief to obtain the static solution...

# Re: Pressurized Inflatable structures - Frequency analysis

Experimenter

Hi,

Thanks for your answer. I will try to run the simulation with inertia relief but my idea is to fix the cylinder in the base only, and doing that it appears the fatla error excessive ratio :/

How can I create a preload step in the simulation SOL 103? It doesn't allow me to apply forces.

If you need any information about the problem ask me and I will provide it.

Thank you,

Afonso Ferreira