Learning Forum

[Saketh Bharadwaj Kopparthy]

Hello,
I am trying to model deformation of hollow rectangular pipe. Screenshot below. The red block is concrete (150mm*400mm*400mm). A steel plate is fixed using M12 anchor screws with 5kN pretension. Load applied on the edge of rectangular pipe is 13kN. The backside surface of concrete block is taken as fixed support and the other faces of concrete block are "frictionless support" to arrest the movement in respective directions.

How to model pretension in the screws in this case? I tried to split the screw geometry into screw head and shank, created a new coordinate system, applied pretiension once on shank surface, once on shank body and few other combinations. But I always see the message that "Pretension load was not able to be applied".

Since the shank is completely inside the concrete block, I think pretension has to be handled differently. I am new to Ansys. Any idea on how to get around this problem?

[Saketh Bharadwaj Kopparthy]

Exact error message is as follows "The pretension load was not able to be applied. Make sure the load was not applied to a hole and that the load is not applied more than once per surface or on different surfaces of the same body."

[peteroznewman]

In the Mechanical app, on the Display tab, there is a slider on the Explode tool. Slide that over to visually separate the bolt from the concrete block and flange.  Please reply with that image and show what face was selected for Bolt Pretension.

In a Geometry editor, you should split the face of the shank of the bolt at a plane that is the flange thickness below the underside of the bolt head. That will provide a long face that will have bonded contact to hold the bolt into the concrete block and a short face that will have the bolt preload applied.

[Saketh Bharadwaj Kopparthy]

Thanks @peteroznewman.

Here is the exploded view. Now I split the shank surface into 2 parts. The short face length is same as flange thickness. Still I am unable to apply pretension. Eventhough the short face is available for selection in exploded view, the selection is not successful and the geometry for pretension shows "no selection"

[Saketh Bharadwaj Kopparthy]

[peteroznewman]

Using a Frictionless Support on the top face of the concrete block is a mistake, delete that.  Insert a Frictional Contact between the top face of the concrete block and the bottom face of the flange (unless you already have this in your Connections folder).

Please show the items in your connections folder. Do you use Bonded or Frictional contact of the head to the flange?  Is the bonded contact between the hole and the new lower part of the shank?  Sometimes previously defined contacts only appear on one of the two faces when that face was split into two pieces, and it is sometimes not on the face you want.

Bolt pretension can only be applied to cylindrical faces. Where did the geometry come from?  Check that the face is actually a cylinder.

[Saketh Bharadwaj Kopparthy]

I created the geometry in SpaceClaim and the surfaces of the shank are cylindrical. The frictionless support was given on all 4 side faces of concrete to arrest the movement. Bottom face is fixed support.

Yes, there exits a frictional contact between the concete block and the steel flange. But running this case is taking longer time than the case without friction. So for the time being, I removed the frictional contact and continued with bonded contacts everywhere.

Here is what I did.

In space claim I created a plane at 20mm distance below the screw head and split the screw shank surface into 2 parts using "split" command. This gave me 2 shank faces..long face and short face. Length of long face is exactly equal to length the part of screw that goes into the concrete block and short face length is equal to thickness of steel flange. 

Here is the screenshot of contact regions. Contact region is 1 is in between concrete block and steel flange. Eventhough friction exists, it is taken as bonded contact for time being to run the simulation quickly. Regions 2 to 5 are between concrete and long face on screw shank. 
Region 7-10 are between short steel flange and shank short face plus underside of screw head.
Even now I am unable to apply pretension. 

I believe I am missing something related to contacts between surfaces and the coordinate axis to define the bolt pretension. Can you help on how to proceed further?

 

[Saketh Bharadwaj Kopparthy]

Also, I am seeing this warning message very often. When I use the scoping wizard, I see a green tick mark for branch is simulation tree. I do not know if this creating any problem.

[Saketh Bharadwaj Kopparthy]

Hello @peteroznewman,

Can you help me solving this? Also I tried something further. After splitting the screw shank into 2 surfaces, I created a new co-ordinate system for each short face on each screw. Now instead of applying the pretension on the surface, I applied it on the complete screw (using body selector) and selected the corresponding new coordinate system. Also, I included the frictional contact between concrete block and the steel flange with coefficient 0.45. 

Then I run 2 cases.
1. All other contacts are bonded. 
2. frictional contact between shank short face and steel flange with coefficient 0.2

I run the case in 2 steps. In first step only bolt pretension(5 kN) is applied. The load of 13kN is applied in the second step.
Now the simulations did run successfully but gave me this warning "One or more contact pairs are detected with a friction value greater than 0.2. If convergence problems arise, switching to an unsymmetric Newton Raphson option may aid in convergence."

I got this information from this youtube video Bolt pretension tutorial. Even after these steps, I was never able to apply pretension on the surface. 

Is this approach correct? I can share my archive file but I couldnt attach it here.

Thanks in advance.