Learning Forum

[Lilly_belmont]

Hello,

I was working my way through the cardiovascular stent exercise: /courses/index.php/courses/cardiovascular-stent/ and am now attempting lesson 9. At first, I tried applying frictionless support on the 8 edges of the stent. However, this gave me rigid body motion errors. I'm not sure why.

I am afraid I do not know how to solve this now. Additionally, is it possible to do this exercise without the balloon? What supports/ constraints would work here? Any help is appreciated.

[peteroznewman]

Please insert an image into your reply showing the 8 edges you selected.
Instead of a balloon, you could apply a pressure load to the inside face of the stent.

[Lilly_belmont]

Hello!
I have removed the balloon from the simulation for simplicity and then tried applying a force to the inner face of the stent. I will explain all the ideas I have tried:
1) Initially, I attempted to apply frictionless support to the edges as highlighted in image 1. However, this resulted in rigid body motion error saying that there aren't enough constraints.
2) So next, I tried to fix the two bottom edge faces by applying a fixed support. However, this is struggling to converge. I am confused as to what the right constraints would be for such a geometry.
Thankyou for your answer.
-Lilly

[peteroznewman]

You need a Kinematic Mount. There are many forms. The simplest form is called 3-2-1 and uses three non-collinear vertices. Pick one vertex and hold it with 3 constraints: X,Y,Z = 0. Pick a second vertex, some distance along X from the first point and hold it with 2 constraints: Y,Z = 0. Pick a third vertex, some distance in Y from the first point and hold it with 1 constraint: Z = 0. All expansion will be relative to the first point.
Another form of Kinematic Mount uses three vertices around a circle and requires a cylindrical coordinate system. The three vertices all have Theta and Z = 0 and Radial is Free. That is exactly six constraints, same as the 3-2-1 but all expansion is relative to the center of the circle. That would be nice for a stent which is a cylindrical object, but either one will allow free expansion of the metal stent with a pressure applied to the inner surface.

[Lilly_belmont]

Hello! Thank you for your answer. I think I understand the gist of what you mean. However, I am afraid I am just learning so I was wondering which boundary conditions I should use in static structural to achieve that. I was able to create the polar coordinates. But I am unsure what type of constraint (fixed or frictionless etc.) I should use for this type of setup.

[peteroznewman]

The type of constraint is called Displacement. Select a Vertex, and apply a Displacement constraint. You can assign a zero value to one, two or three axis directions. When I say set Z = 0, that means to leave the other two directions Free.
We could do a Google Meet or Skype call where we can talk and screen share ANSYS so you can more quickly learn what you need to know to be successful with this model. If that is of interest, please leave an email address and I will contact you to set up a time.

[Lilly_belmont]

That would be amazing! is the email thankyou sm :))

Email deleted as per request -

[Lilly_belmont]

Alternatively, could you please show displacement points on a diagram?

[hesamkeshavarzz]

Set two remote points, then define remote displacement for the two remote point as Z=0 , something like the attached image, Or enable the weak spring option in order to avoid rigid body motion .

[peteroznewman]

Here is a cylindrical coordinate system.
(So a cylindrical system just in the middle of a cylindrical structure, e.g., a stent)
Here is a nodal named selection that has three nodes around the circle that have a Nodal Displacement defined.
(nodes on the circumference lying on an arc)
Here is the Nodal Displacement definition which takes the Nodal Named Selection and uses the cylindrical coordinate system to constrain the Y Component (theta) and Z Component (axial) on those three points. This prevents the rigid body motion and allows the Pressure to expand the Stent.
You are welcome to schedule a time to discuss this live if you want.

[Lilly_belmont]

I would love to schedule a time for sure! Also very sorry, I was successful in setting up the system as you suggested but I am still getting a rigid body motion error. I think this may be because I am missing the remote displacement constraint. I am unsure of this. If the nodal displacement constraint controls the movement, what is the purpose of the remote displacement? I apologize for my inexperience, but this has been a great help so far thank you :)

[peteroznewman]

Are you getting a rigid body motion error or just a warning? I often get a warning, yet I can get the solver to converge.
In the screen snapshot, the stent was in an artery and the Remote Displacement was to hold the artery.
I have time to talk on Monday September 13 so reply to the email I sent you and choose a time or a day later in the week.

[Lilly_belmont]

Warning yes! And I can't get it to converge. I have replied to the e-mail. Thank you :)