# Limitation or bug with shell-offset and contact?

**URL:** <https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990>\
**Category:** General Questions\
**Created:** [July 15, 2023, 3:15pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990 "2023-07-15T15:15:01Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![Gunnar](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@Gunnar](https://prepomax.discourse.group/u/Gunnar)\
**Post date:** [July 15, 2023, 3:15pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/1 "2023-07-15T15:15:01Z")

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Hi all,

maybe i found a limitation or bug with shell-models and contact.  
It seems by using the shell-offset (\> or \< 0) with contact leads to no convergence by specific geometries. Only after modifying the geometry and setting all offsets to zero, the shown model was running successful.

So I made a test-model to understand and clarify the problem.  
With shell-offset \>/\< 0 only model 1 and 2 running successful (no matter if using tie constrain or contact). Obviously the only different are the additional “flange” faces on the top. So i used a separate section with shell-offset = 0 only for this additional surfaces. With this modification all models run successful.

Can anyone confirm this and has a solution?

Thank you

 ![1](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/2/25a2a00f0e447d08ea13fcbe8024f9031728777e.png)

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**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\
**Post date:** [July 15, 2023, 4:40pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/2 "2023-07-15T16:40:44Z")

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Strange. So it doesn’t converge if you just use non-zero offset for the top horizontal face (even though the bottom one already uses it) ? What if you change the way this flange is connected to the web (switch master/slave in tie constraint or use tied contact) ? I can’t test it now but it might be a CalculiX bug (not a PrePoMax one so I moved this to General questions), potentially worth reporting on the CalculiX discourse group.

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<div class="post-metadata">

**Author:** ![Gunnar](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@Gunnar](https://prepomax.discourse.group/u/Gunnar)\
**Post date:** [July 15, 2023, 6:06pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/3 "2023-07-15T18:06:38Z")

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Yes it doesn’t converge even if these faces have nothing to do with the contact/tie constrains.  
Each part has it’s own coincident mesh, tie constraints are only used between the lower and upper parts, alternative to contact.

Switching master/slave does not help.

I am also wondering why it’s no problem to use 0,5 offset for the lower flange… The only different is that this part has fixed BCs (U123=0).

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**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\
**Post date:** [July 15, 2023, 6:27pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/4 "2023-07-15T18:27:56Z")

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> [@Gunnar](#):
>
> Switching master/slave does not help.

What about tied contact ? To use it just change the contact interaction property from hard to tied.

> [@Gunnar](#):
>
> The only different is that this part has fixed BCs (U123=0).

The bottom one (the one that works with offset) ? CalculiX is quite susceptible to overconstraint so this could be a reason if the upper part was fixed.

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<div class="post-metadata">

**Author:** ![Gunnar](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@Gunnar](https://prepomax.discourse.group/u/Gunnar)\
**Post date:** [July 15, 2023, 7:16pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/5 "2023-07-15T19:16:48Z")

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i mean switching master/slave makes no difference for tie constrain and all types of contact.

Yes the bottom one. I don’t think overconstraining is the problem because then model 1+2 would also be overconstraint (only the edges are constraint). All upper parts share the same BC: U1,U3,UR1,UR2,UR3=0 , U2= -1

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**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\
**Post date:** [July 15, 2023, 7:28pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/6 "2023-07-15T19:28:14Z")

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What if you run a frequency analysis for the model with the top flange having a non-zero offset ? Does it fly away ?

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<div class="post-metadata">

**Author:** ![Gunnar](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@Gunnar](https://prepomax.discourse.group/u/Gunnar)\
**Post date:** [July 15, 2023, 8:09pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/7 "2023-07-15T20:09:53Z")

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Its a bit confusing but i think i found the reason… On the left model (which does not converge) I applied the BC U2=-1 to the middle edges which splits the upper flange-surfaces. When i apply the same BC to the outer edges, the model runs successful. Because that middle edge is part of two faces with different shell offsets (flange offset=0,5 and web offset=0), it seems to be a problem for tie constraints and contact… strange

 ![3](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/9/9c106f0e6b545bddc5a6c3aff461d0266ded1492.png)

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<div class="post-metadata">

**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\
**Post date:** [July 15, 2023, 10:38pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/8 "2023-07-15T22:38:26Z")

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So it’s an overconstraint issue, as I suspected. Just a weird one, triggered by shell offset. Might be worth discussing on the CalculiX forum and reporting in its GitHub repository.

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<div class="post-metadata">

**Author:** ![Gunnar](https://avatars.discourse-cdn.com/v4/letter/g/ed655f/32.png) [@Gunnar](https://prepomax.discourse.group/u/Gunnar)\
**Post date:** [July 15, 2023, 10:53pm UTC](https://prepomax.discourse.group/t/limitation-or-bug-with-shell-offset-and-contact/990/9 "2023-07-15T22:53:55Z")

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Confirmed! After removing the sharing nodes from the BCs the original model works.

 ![4](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/4/4da1acb35a414d1912a4a71b8d9539cbc177148c.png)