Instabilitiy looping?

[t.sautierr]

Is is possible for some instabilities to be looping from one physical node location to another node :

sometimes when cleaning instabilities in a model, espacially that coming from releases, when you fix one in one particuliar node, another can appears in a second stage and wasn't present before :

for example :

first stage :

instability type 3 node 154 176 202

fix node 154

second stage :

instability type 3 node 162 176 202

Usually when it happens it appears in a node close to the one fixed, but I'm always to see that king of phenomenon, because you never know when it ends -> practicaly I get rid off all realeases and redo it.

This leads to another question : for 1 model with no change between two calcullation run, does robot calculates the solution exactly the same way? or is it possible that sometimes (maybe the case if minor change occurs (change in the llocation of one node) it is calculated with different calculation steps ? ( matrix diagonilsation, assembling ...?)

Sorry but at present I have to model to illustrate my topic but I will put one when it occurs.

[Rafal.Gaweda]

I can be like that because of two reasons:

1. instability type 3 is reported only once (for one node) during one run of calculations

2. it can be shifted if mathematical instability  occurs 

[t.sautierr]

And then what is the typical treatment doctor? 😛

[Rafal.Gaweda · Accepted solution]

And then what is the typical treatment doctor? 😛

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Instability/m-p/3855698/highlight/true#M1246...

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/instability-type-3-RX-direction/m-p/3364875/...

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Instabilities-type-2-and-3/m-p/3950099/highl...

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Instabiliyty-of-type-3/m-p/3179932/highlight...

and :

http://forums.autodesk.com/t5/forums/searchpage/tab/message?page=1&location=forum-board%3A351&q=inst...

[t.sautierr]

Calculation report ... of course ... I should know that !
Sorry
Thx

[t.sautierr]

Last questions :

Is there a mean to be sure that a remaining type 3 instability is the results of mathematical bugging and can be ignored ? checking the efforts and displacement?

Maybe the answer is to use all the methods listed in the links above and if it is still remaining and then it can be ignored?

Is there a way to access the data of the stifness matrix ? (I thinking about being able to highlight nodes considering there their stifness for a given dof.

Thx

[Rafal.Gaweda · Accepted solution]

Last questions :

Do not believe 🙂

Is there a mean to be sure that a remaining type 3 instability is the results of mathematical bugging and can be ignored ? checking the efforts and displacement?

Yes, create dummy load case, assing unit load on direction of instability in instable node and check displacement\rotation in this node

example My=1kNm in node with instability type3 on RY direction -> check rotation result in this node -> reasonable=small values -> ignore instability

Is there a way to access the data of the stifness matrix ? 

No.