Technical Differences in the use of surface contact, sliding/no-separation and separation

[arlan_baylonNAVAC]

We are analysing a tank which will be sitting on the platform/pancake for ocean transport, for securing the tank are also equipped with anchor chairs bolted to the pancake this will also serves as restraints for transversal loading and uplift during the sea transport. In order to transmit the vertical load of tank to pancake, and that the anchor chair will take the sliding, which contact we need to use, Sliding/no-separation or separation? 

Thanks and appreciated your comments.

[John_Holtz]

Hi @arlan_baylonNAVAC . Welcome to the Inventor Nastran forum.

You should use the contact type that gives the result you want with the fastest runtime. 

Since you say the "anchor chairs bolted to the pancake this will also serves as restraints for transversal loading and uplift", this sounds like the tank will remain in contact with the pancake and not slide. (Not 100% sure of this.) If this is a reasonable assumption, then bonded contact will be the fastest solution.

If the tank can slide but not lift off the pancake, then contact of "sliding/no separation" would be suitable. It should solve in approximately the same amount of time as bonded contact. Of course, you need some other means of preventing the tank from sliding since "sliding" contact leaves it free to slide. Note: Sliding contact should only be used between two flat surfaces. Sliding contact on curved surfaces tends to "lock up" and not prevent free sliding.

Separation contact will take the longest time to run. Use it only if the contact faces are curved or if the two contact faces can separate.

Let use know if you have any other questions.

John

[arlan_baylonNAVAC]

Hi John,
Thanks for the prompt reply and I agree with your comment and proposal. But I need to give an explanation to our client about the differences of these two contacts so I may need further technical explanation on its use.
I understand that sliding/no separation can transmit compression and tension loads between two separate surfaces in plane, we uses it in the analysis and we get the expected result of tank load transmitted to the pancake and that the anchor chairs take the transversal. But our client disagrees on this option and ask us to do separation, maybe because literally the two object are not technically attached though structurally both has to have fixity.
How about the separation contact, can we expect that tank can transmit the vertical load directly to the pancake if we use it? IN the analysis, it shows that the tank weight and loads are transmitted through the anchor char and bolt.
I just need a technical explanation of the differences in purpose of this two contacts in terms of use in the structural analysis.

Thanks
Arlan

[John_Holtz]

Hi @arlan_baylonNAVAC 

Here are some more comparisons between the two types of contact.

Item	Sliding/No Separation Contact	Separation Contact
In-plane motion	A node can move in the plane of the element face in contact.	A node can move in the plane of the element face in contact.
Out-of-plane motion	A node cannot move perpendicular to the element face in contact. Thus, tension and compression forces are transmitted between the faces.	A node cannot move into the element face in contact, but it can move away from the element. Thus, compression forces are transmitted but there are no tension forces.
Friction	No considered.	Considered only in a nonlinear analysis.
Solution Method	Uses multi-point constraint equations (MPC) to solve for the contact. Solution is linear.	Uses penalty-based contact elements to solve for the contact. Solution is iterative to determine which elements are in compression (contact) versus tension (separating).

Although the solution method is different (MPCs versus contact elements), the results should be "the same" if none of the nodes on the contact faces try to separate. In other words, looking at the contact force contour plot should be the same when the faces remain in 100% contact.

John