## Free-Free Preloaded Modal Analysis

## Free-Free Preloaded Modal Analysis

(OP)

Hello,

It might sound weird but I would like to make a free-free preloaded modal analysis. Imagine that a rocket flies with its pressurized propellant tanks. I want to include the stiffening effect of the pressure. I tried two things:

1. Restrain the structure during static (preload) load step. Refer to first load step and have no SPC definition in modal analysis.

2. Do an unrestrained analysis (inertia relief). Refer to first load step and have no SPC definition in modal analysis.

In both cases, I end up with only 3 rigid modes - very close to zero. The remaining 3 modes are actually rigid modes when I visualize them, but their frequency value are way high. Can anyone think of how to approach such problem?

I am actually doing trial analysis with a representative tank structure; standard steel properties with 50 mm diameter, 0.4 mm wall thickness, 6 mm cap thicknesses, 250 mm length, internal pressure of 0.7 MPa.

Any help would be highly appreciated.

It might sound weird but I would like to make a free-free preloaded modal analysis. Imagine that a rocket flies with its pressurized propellant tanks. I want to include the stiffening effect of the pressure. I tried two things:

1. Restrain the structure during static (preload) load step. Refer to first load step and have no SPC definition in modal analysis.

2. Do an unrestrained analysis (inertia relief). Refer to first load step and have no SPC definition in modal analysis.

In both cases, I end up with only 3 rigid modes - very close to zero. The remaining 3 modes are actually rigid modes when I visualize them, but their frequency value are way high. Can anyone think of how to approach such problem?

I am actually doing trial analysis with a representative tank structure; standard steel properties with 50 mm diameter, 0.4 mm wall thickness, 6 mm cap thicknesses, 250 mm length, internal pressure of 0.7 MPa.

Any help would be highly appreciated.

## RE: Free-Free Preloaded Modal Analysis

## RE: Free-Free Preloaded Modal Analysis

It is a tank cyclindrical tank as follows: Standard steel properties with 50 mm diameter, 0.4 mm wall thickness, 6 mm cap thicknesses, 250 mm length, internal pressure of 0.7 MPa.

## RE: Free-Free Preloaded Modal Analysis

## RE: Free-Free Preloaded Modal Analysis

## RE: Free-Free Preloaded Modal Analysis

I have now 6 rigid modes and they are all very close to zero.

Edit: I was wrong. Essentially, this gave similar results with Inertia Relief. I still have 4 to 6 modes as finite, but seem rigid.

## RE: Free-Free Preloaded Modal Analysis

NX Nastran even mentions about this problem in its Dynamics User Guide. Despite my sincere efforts, I was unable to solve this problem. If anyone has similar experience, and managed to solve this issue, I would be really appreciated.

I also tried to ground the structure by very soft springs and then doing the analysis, but still, I got only 3 rigid modes...

## RE: Free-Free Preloaded Modal Analysis

DG

## RE: Free-Free Preloaded Modal Analysis

## RE: Free-Free Preloaded Modal Analysis

@jball1 The question is not about pure free-free, but a preloaded free-free analysis. In other words, you apply a loading to structure and then perform modal analysis to stiffened structure with free-free condition.

Apparently, as suggested by dmapguru, PARAM,KDMFILT seems to be the only option. However, the details of this card is not given anywhere.

## RE: Free-Free Preloaded Modal Analysis

So, you are using a non linear solver (otherwise the preload would have no effect)? There is no such thing as a true modal analysis of a non linear structure, at best you get operating deflection shapes (ODS) for a particular excitation force.

One approach might be to inflate the fuel tank statically, model the deformed geometry, and do a modal on that. You might need mass loading as well.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Free-Free Preloaded Modal Analysis

## RE: Free-Free Preloaded Modal Analysis

It seems to me there are several confounding effects (1) dimensional changes due to gas pressure (2) mass loading of the skin by the gas and (c) the effect of the skin stress due to pressurisation.

You seem to be hanging your hat on (c).

https://link.springer.com/article/10.1007/s42452-0...

has some REAL data and an equation that seems to work quite nicely. For air even in a thick walled tank the mass loading effect is significant.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Free-Free Preloaded Modal Analysis

With a bit of fudging here's the fit of the equation and the test results in that paper

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Free-Free Preloaded Modal Analysis

Sorry to respond so late. However, despite my sincere efforts of reading, I still have not managed to understand how the authors was able to do it, as it is acknowledged as a common problem.

Due to the formulation of differential stiffness matrix, we get only 3 rigid modes, the remaining 3 rigid modes are grounded. The author's of the paper do not mention this, which surprises me.