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Could anyone explain me what contact stifness is and how it works?

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Old   February 12, 2021, 03:40
Question Could anyone explain me what contact stifness is and how it works?
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Francesco Latella
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Hi guys,

I've a good experience in CFD and numerical calculus but I'm kind of new in the FEM world. I'm trying to understand which is the role of the contact stifness during the calculation. I already know that the problem of a non linear contact relies on the fact that, due to contact or non contact, the stifness matrix in that zone can change (non linearity of the problem) so you have to use a Newton method to find the solution. BUT, keeping in mind this, why need I also to apply a virtual stifness (contact stifness) to better reach the convergence in the simulation?

I'm not able to understand which is the lin between a non linear problem (application of Newton) and the necessity to have this virtual stifness...

Talking practically, I already know that a high value of contact stifness simulate better the reality but could leads to non convergence, instead having a low value leads to an easy convergence but inaccurate results. But beyond this, I really want to understand the numerical meaning of this.

Maybe it is a useless question, but this is my defect ahahaI torture myself

Thank in advance for your kind reply!

Last edited by Francesco95; February 15, 2021 at 01:46.
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Old   February 20, 2021, 13:05
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Contact stiffness used in Penalty Method. Penalty method if only one of many contact algorithms. Penalty methods may be used both in linear and nonlinear solutions.

In the penalty method, the solver (or user) sets some contact stiffness value k. If nodes penetrate opposing contact faces by some distance d then solver produce additional force on these nodes calculated like

F = k*d

This force moves nodes away. On the next iteration the solver again checks penetration and if it still occurs, the solver calculates force, based on updated penetration values.

Note that the penalty method requires some nonzero penetration value to work. Therefore contact nodes always penetrate opposing faces by some value but if contact stiffness is adequate then penetration is negligibly small.

https://slideplayer.com/slide/6816372/
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Old   March 8, 2021, 06:30
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It is a numerical gimmick


Think of the contact process as a walk of a person on a irregular floor with a cane. A stiff cane allows for a better probing of the floor irregularities but you'll also get all possible steep reactions which make you stop. If the cane is more flexible though, you won't feel all the bumps from the irregular floor. However, if the cane is too flexible you might miss a hole and fall... on the other hand if it is too stiff you keep stopping for everything you touch to check what it is...


The cane is related to what it is called "gap element" which carries this virtual stiffness.

Thus..

higher stiffness="more accurate" and harder to converge
lower stiffness="less accurate" and easier to converge


both extremes have problems..it depends on the algorithm.. normally, use the stiffness value of the softer surface for the gap stiffness...


yours,
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Old   March 8, 2021, 09:49
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Thanks to you all guys
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ansys, contact, contact stifness, fem, non linearity

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