Solver contact, how it's works?

[k_musialik]

Hello i need some help to understand how should i use solver contact option properly. I am analyzing models made of many surfaces. And i using solver contact to connect all the surfaces but i find some troubles with how solver worsk.

For surface model which is welded in real life i mainly use offset bonded contact, with proper activation distance. But i have got troubles with understand when contact will be detected and when it won't.

Soo when i use solver contact with "Specify Contact Regions" option unchecked solver will looking for the contacts for entire model. And that's clear.

But please tell me how it's really work when "Specify Contact Regions" option is checked, because i am confused. In my opinion when option is checked, solver should looking for the contacts between only the surfaces i chose. But in fact it's enough to choose only one surface and contact sometimes will be detected and sometimes won't.

Some examples below:
1. (photo 1)
So in this example distance between bottom surface and top surface is 5 mm. So i use solver contact>type:offset bonded>max activation distance: 5mm, and i choose only bottom surface for this contact. I run the analysis and in outcomes i can see that contact was detected and everthing works.

2. (photo 1)
When i change bottom surface for top surface contact won't be detected even if i increase max activation distance.

3. (photo 2)
ok when i change geometry like in the photo 2, distance between surface is about 26.5 mm and i tried the same. When i choose bottom surface solver doesn't detect contact but when i change bottom for top contact detects.

Of course those are very easy set ups and i can crealy detect what happens, but i have problem with more complicated models, especially with joints.

For example i have got model which is made by many plate sheets which are welded together by offset bonded contact. And between my welded elements i have got rotation joints which i realizes by using conectors and 1d elements. So i have got 2 part both are welded, and at the end i have to conect them using rigid conectors and 1d elements which is axis bolt. So i doesn't want to have contact between the individual parts. For example distance between part number 1 and part number 2 is 10 mm. And activation distance in part 1 in offset contact has to be about 12 mm, so it's grater then distance between parts. Of course in part 1 contact i chose only surfaces of part 1 but as we can see in previous examples sometimes solver catches also unchecked surfaces.

So my questions are:
1. How really solver works, how it looking for surfaces and when it detect contact and when it doesn't?
2. How can I be sure that the contact from part number 1 will not catch the surface of part number 2.

Best,
Kacper

[Anonymous]

This is an excelent question !!
I encountered exacly the same problem, the solution for me was just to guees the "good" distance and hope that solver finds the proper conctats that i requier, but it is just gueessing game, it can be pretty time consuming and very misleading/problematic in bigger projects analysis. 
I wonder if there is any other solution from autodesk to use this solver properly ? Maybe there is an option while chosing contatc surfaces to avoid certain ones ?

Kind regards,

Szymon 

[John_Holtz]

Yes, this is a good question. I just happen to be discussing this topic with the developers for another case. I should have some feedback later this week.

[John_Holtz]

Hi. Sorry for the delay.

The current software (version 2021.2) uses the elements in the selection and creates contact with any node in the model that is within the activation distance. The node does not need to be in the selected surfaces.

After discussing this with the developers, they will change the behavior for a future release. The contact will only be created between nodes and elements that are included in the selected surfaces.

[k_musialik]

Hello John,
Are there any news about this change? Maybe in version 2022 it already works properly? I do quite lot of analysis on shell elements and setting up a contacts without this functionality is a nightmare.
Best,
Kacper