When to define contact ?
When to define contact ?
(OP)
Hi,
I would like to ask when contact should be defined in structural mechanics analyses ? Could you explain it as for a beginner ?
I've heard that every assembly model needs contacts applied but I think that it is possible to avoid them in many cases. I understand that when contact between 2 faces appears during simulation (initially faces are separated) then it should be defined properly because otherwise there could be wrong penetration/collision result (let's say we are modelling hardness testing or ball being dropped on something) and we don't know the area of contact.
But what about situation when faces are already touching each other with their whole area at the beginning ? For example we can imagine tower consisting of a few blocks put on top of each other and we simulate compression or tension treating it as fully welded/glued construction with no chance of separation between blocks. Can contacts be avoided in such case ?
I've also heard that contacts should be used mostly when there is motion in our model (for example gearbox). Is it true ?
Thanks in advance for your reply
I would like to ask when contact should be defined in structural mechanics analyses ? Could you explain it as for a beginner ?
I've heard that every assembly model needs contacts applied but I think that it is possible to avoid them in many cases. I understand that when contact between 2 faces appears during simulation (initially faces are separated) then it should be defined properly because otherwise there could be wrong penetration/collision result (let's say we are modelling hardness testing or ball being dropped on something) and we don't know the area of contact.
But what about situation when faces are already touching each other with their whole area at the beginning ? For example we can imagine tower consisting of a few blocks put on top of each other and we simulate compression or tension treating it as fully welded/glued construction with no chance of separation between blocks. Can contacts be avoided in such case ?
I've also heard that contacts should be used mostly when there is motion in our model (for example gearbox). Is it true ?
Thanks in advance for your reply





RE: When to define contact ?
The computational speed order (from fast to slow): bonded > frictionless > frictional
https://welsim.com
RE: When to define contact ?
According to this you need to do the modeling and input so the output is reasonable.
Basically the better output you want the more input ( modelling e.g. contact) you need to do
There is no general rule for this........
RE: When to define contact ?
Could you tell me which contact should be applied when bodies are initially separated and one hits another (bullet and plate or ball and floor etc.) ?
What about threaded connection ? Should contact be applied to all touching surfaces here ? For example here it's automatic bonded contact everywhere: https://www.youtube.com/watch?v=p1GcedxmnlI Is it enough ?
And one more example: if we want to simulate spring being subjected to compression and we know that coils will touch each other st some point of simulation then contact should be applied ? If yes then which one ?
RE: When to define contact ?
For sure you shouldn't define bonded or tie contact definition between parts which move between each other. You can use friction or frictionless contact (it depends on your simulation). Algorithm check it in all iteration if parts are in contact then contact will be work (Let say in a very simplified form ;)) Regarding that thread connection, I'm not really sure what mean "automatic bonded contact" here. Maybe is it something like "general contact" (all parts in model are in contact) If yes, it's enough :) Same situation in spring case.
BR,
NecroSPEL
RE: When to define contact ?
So in case of spring with coils separated at the beginning of simulation bonded contact is not a good idea and friction/frictionless should be used ?
And in case of threaded connection bonded contact should be enough when we assume that thread is stuck in place ?
And last question - do you know anything about Augmented Lagrangian and Penalty methods ? These types of contact are available in COMSOL from what I’ve seen on my university.
RE: When to define contact ?
2) Yes
3) Lagrangian and Penalty method it's very width topic, generally penalty method is always use in symmetric or self contact when you use a lot of parts in the same contact zone. Lagrangian is used in 2D models and when obtain big material non-linearity. But of course it's always DEPENDS WHAT YOU SIMULATE! :)
BR,
NecroSPEL
RE: When to define contact ?
And which type (penalty or Lagrangian) to use when simulating ball hitting or pressing against flat surface (second one is like in hardness testing) ?
RE: When to define contact ?
RE: When to define contact ?
I would like to ask you one more question concering contacts. I've seen some YouTube tutorials on how to perform impact analysis (explicit dynamics) of car or bullet hitting obstacle. When car was already modelled as touching (connected with) obstacle then bonded contact was used. But when car was separated from obstacle and applied velocity was causing him to hit the wall during simulation then no contact was defined. Like here: https://youtu.be/-cu__bX3pg8?t=8m26s
So is contact definitione unneecessary in such case ? I thought that contact must be defined whenever 2 parts touch each other, even if they are separated from the beginning. Otherwise I thnik that objects should pass each other like ghost would walk through wall. My theory is that if objects deform so much (like in case of car or bullet impact)in explicit dynamics simulation then it's not treated like regular static contact case in which faces interact (generate stress/pressure) but don't penetrate each other physically.
Thanks in advance for your help
RE: When to define contact ?
This also includes the contact between car and obstacle. Using bonded here is wrong in a real crash analysis.
RE: When to define contact ?