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Stability

Stability

Stability

(OP)
Hi,
I have the following problem. I start a small symetric model which I analyse in static mode. Everything is stable, convergence is very good.
I then switch to dynamic analysis, without adding anything (no force, no mass, nothing).
The system is very simple, as shown in the picture. It consist of a 4 noded plan-strain quad element of 1x1 dimension to which I attached three beam in trianglular shape. The top node in the center does have a mass (I plan on applying dynamic excitation at the base of the quad element and I want to check the inertia force produced by the mass).


So when I start the dynamic analysis, there is a small acceleration at every node, even thought I apply absolutely no forces (the only displacement are produced by the self-weight of the quad element)!! And since the top node has a mass, it produce very small force. But these acceleration keeps increasing, and so does the force. Eventually, the structure just move on it's own and fly away... . Why is this happening?
I believe it as something to do with the constraint handler (which is the penalty method)? But no mather which value I use (I tryed 1e30, 1e20,1e16, 1e8..etc). it still happens. Any1 experienced this or have a clue for this? Thanks!

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