Soil Structure Interface Modeling

[Lei Lei]

Kn and Kt are penalty coefficients, so there is the typical rule:
It should be a number large enough to enforce the constraint (Kn for non-penetration, Kt for stick condition), but not too large to avoid numerical problems.
As usual, estimate the typical stiffness (Ktyp) of your problem (E*L for solid elements, EA/L for trusses, and so on...).
Then evaluate the order or magnitude: OOM = round(log10(Ktyp))
Now the penalty coefficient should be P = 10^X
where is X > OOM.
2 or 3 orders of magnitude is enough (so X = OOM + 2). At most use X = OOM+8. Why 8? because double-floating-points numbers have 16 significant digits, so OOM+8 means something in between accuracy of the constraint and numerical stability
Regarding mu, it is the friction coefficient (i.e. the tangent of the friction angle). You can find online typical friction coefficients based on the two materials in contact.

Dear STKOteam,

As you mentioned above, Kn stands for non-penetrating, so it may be larger than the typical stiffness. But does this work for kt? As far as I know, kt is subject to Mohr-Coulomb's theory. Is it just defined by the experiment?

Looking forward to your reply!

[Lei Lei]

If both geometries have the same mesh, their nodes should share the same position. In this case you can use a node-to-node interaction.
In this case, it doesn't matter who is the master. However, there is one rule:
The contact normal vector MUST go from the master to the slave (see the documentation). In this case, a node-to-node interaction has default local axes that coincide with the global axes, so the default contact vector is [0;0;1] (i.e. +Z direction).
In this way, the bottom surface should be the master.
If you want to make the top surface the master, then you need to create a LocalAxes (and assign it to the interaction) with a local Z vector pointing to the negative global Z-axis.

Dear STKOteam,
I want to know when to choose between node-to-node links, node-to-element links, or general links. Does that affect the calculation?

The same question arises from who is the master and who is the slave.

Looking forward to your reply1

[STKO Team]

As you mentioned above, Kn stands for non-penetrating, so it may be larger than the typical stiffness. But does this work for kt? As far as I know, kt is subject to Mohr-Coulomb's theory. Is it just defined by the experiment?

In theory the Kt and Kn are both penalty values, so they should be INFINITY. Since a PC does not use exact arithmetic, you use large values (not INF).
Sometimes you can treat Kn and Kt not as penalty values, but as real stiffnesses if your contact layer has a certain deformability (for example because there's a cohesive zone of finite length that you did not model).

[STKO Team]

Dear STKOteam,
I want to know when to choose between node-to-node links, node-to-element links, or general links. Does that affect the calculation?

The same question arises from who is the master and who is the slave.

I would suggest you to look at the webinar about ZeroLengthContactASDImplex.