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Stiffness variation approaches in Nastran

Hi all!

I have an issue and I don't know any reason for the problem :(

I have a structure consisting of two structures connected on one edge. In the basis model which is a beam, it all seems to be OK. But now I want to go into more detail. My target body is an axisymmetric structure consisting of shell elements (like a cylinder shaped form). But first of all, I have to understand the basics.

So lets assume a general structure consisting of two substructures (e.g. PSHELL1 & 2)

Now i want to stiffen Structure2 with a factor.

First approach:

Vary Youngs modulus of the Material of Structure2 with factor

Second approach:

Vary the stiffness matrix entries of Structure2 and combine the stiffened matrix of structure2 to the structure1.

How I am doing that at the moment:

Step 1: Split the Structure2 into a standalone structure

Step 2: Generate mass and stiffness matrix of Structure2 using all DOF (extseout without defining an aset)

-> I have the matrices of Structure2

Factorize this stiffness matrix with the factor and add it to the baseline structure using K2GG = factor*KAAX

E.g. if I want to stiffen structure2 by 40%

-> Factorize Young's modulus of structure 2 with 1.4

-> Use matrix and K2GG = 0.4KAAX as 1.0*KAAX isalready in the baseline structure

In some cases this approach is leading to correct results. Sometimes it does not unfortunately. Mainly it is the case on the "boundary" grids. (e.g. circumferential structures)

Softening the structure by 40% using the matrix approach sometimes also leads to negative values (->fatal error) but this is only lsited for r3 (as CQUAD4 has no inplane rotational stiffness). using the Material-stiffening approach, this is not appearing.

Am I missing a basic theory reason behind it? Are there aspects which lead to differences between the material and the matrix stiffening approach?

I have already created a standalone Matrix for Structure2 modified by young's modulus and compared it to a factorized standalone Matrix for Structure2. These matrices are approximately identical.

The errors are appearing when taking the whole structure into consideration. Are there some DOF couplings that "destroy" this opportunity to scale matrices? Or is the Young's modulus also influencing further entries?

could you please help me ?

Regards

Benny

0 Sanjay Patel over 9 years ago

Hi Benny,

Structure stiffness is not only function of the Modulus but Geometry as well. I would suggest you look into using optimization (SOL 200) it will allow you to control the material variables and element properties to meet your design objectives.

Thanks,

Sanjay.
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0 benjamin.spiess over 9 years ago

Hi Sanjay. Thank you for your fast response.

I know that the geometry is also contributing to the stiffness, but at the moment I want to keep to my "abstract" approach, by only regarding the stiffness values of a present structure (thus, youngs modulus and matrix entries).

I am assuming the material variation approach as reference, but my Matrix stiffening approach is leading to different results at the boundary grids. I have the guess that the rotational degrees of freedom when focusing only structure2 are somehow leading to the deviations between material and matrix stiffening. But I am not sure about this
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