stress stiffening effect 1

[gio1]

If a guitar string (or a beam) is subject to tension (within the elastic range) the frequencies of the fundamental modes increase. This phenomenon is known as "stress stiffening".
But... why? The stiffness of the system (and so the stiffness matrix) does not change within the elastic range (or does it?), and so the mass matrix!

I have run a NASTRAN static non linear SOL106 for a cantilevered beam (modeled with HEXA8) subject to axial tensile load, and requested extraction of eigenmodes at the last load increment, as described in

http://www.mscsoftware.com/support/library/conf/auc99/p05199.pdf

The frequencies I obtain are the same as those of the unloaded structure, what have I done wrong?
Thanks!

Gio1

 

[KSshopper]

yeah you're right. The modes should have changed. Apparently there's a problem in your model. Maybe try and simplify the model to a mass-spring system, that you can verify using hand calculations.

 

[brad]

gio1--
The stiffness of the system DOES change in the elastic range due to nonlinear geometric effects. Do a simple calculation of a guitar string in mild tension, and you will realize that it changes frequency while it is still in the elastic range.

I am not very familiar with SOL 106 details, but there is likely some way to invoke geometric nonlinearity. Make sure that you have done this.

By my best guess, geometric nonlinear effects are probably NOT invoked by default (which would explain your observation). But again, this assumption is based on general knowledge of other nonlinear solvers, and not Nastran in particular.

Brad

 

[gio1]

Bradh, Philcondit,

Thanks for your help. I think the problem is that geometric non linearities (calculation of differential stiffness matrix) are supported in Nastran only for 1D and 2D elements, but not for 3D, which explains why my model (Hexa) wouldn't show any difference

 

[sanjay12]

Hi all,
Regarding the stress stiffening effects & spin softening effects, i have collected document which has been done on older version of nastran. The abstract is pasted below.
I was able to include spin softening effects. But was unable to include stress stiffening effects as i could not locate the DMAP alter (RF63D89) or its equivalent. Can anyone help me in this regard.
---------------------------------

Once the static analysis of the model had been com-pleted,
a modified normal modes analysis (solution 63)
was performed as a “restart” job using the MSC/
NASTRAN database files generated and saved from the
solution 66 run. Two modifications were made in the
solution 63 DMAP code to obtain the correct rotating-blade
mode shapes and frequencies of the structure.
The first modification was the inclusion of a standard
MSC/NASTRAN rigid format DMAP alter (RF63D89)
into the solution 63 source code. This DMAP alter
allowed the stiffness matrix generated and saved from
the solution 66 run, which included the differential stiff-ening effects of the radial forces acting on the rotor
blade, to be used instead of the stiffness matrix normally
generated in the solution 63 run. A second DMAP modi-fication (NLGYRO.ALT) was made to include addi-tional
centrifugal softening terms in the stiffness matrix.
NLGYRO.ALT also adds Coriolis terms to the damping
matrix; however, for the normal modes analysis
described here, damping and Coriolis terms can be
ignored. This modified solution 63 DMAP source code,
with the RF63D89 and NLGYRO.ALT DMAP alters
included, was then recompiled and executed as a restart
job using the previously generated solution 66 database
files.
--------------------------------------------

extracted from:

http://usuarios.lycos.es/carmenyped...and Modal Analysis of a Model Helicopter .pdf