# Shell plasticity

**URL:** <https://prepomax.discourse.group/t/shell-plasticity/2997>\
**Category:** General Questions\
**Created:** [October 7, 2025, 10:36am UTC](https://prepomax.discourse.group/t/shell-plasticity/2997 "2025-10-07T10:36:06Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![acal](https://avatars.discourse-cdn.com/v4/letter/a/e79b87/32.png) [@acal](https://prepomax.discourse.group/u/acal)\
**Post date:** [October 7, 2025, 10:36am UTC](https://prepomax.discourse.group/t/shell-plasticity/2997/1 "2025-10-07T10:36:06Z")

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Dear all, I am playing with shell contact and plasticity, and I am experiencing sturdy convergence problems.  
I am controlling in displacement, but the contact force doesn’t converge; doesn’t depend on geometrical instability (branching, symmetries, …), contact property parameters, mesh size, shell thickness, plasticity curve or step time increment sizes.

I tried to thicken the shell mesh with a single solid in the thickness and redefine the sections as solids; everything went fine and converged reasonably rapidly, plasticity included.

Is it common to have convergence problems with shell, contact and large displacements in Calculix?

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**Author:** ![acal](https://avatars.discourse-cdn.com/v4/letter/a/e79b87/32.png) [@acal](https://prepomax.discourse.group/u/acal)\
**Post date:** [October 7, 2025, 10:39am UTC](https://prepomax.discourse.group/t/shell-plasticity/2997/2 "2025-10-07T10:39:02Z")

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I am doing the “classical” pipe crossing simulation.

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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:** [October 7, 2025, 10:40am UTC](https://prepomax.discourse.group/t/shell-plasticity/2997/3 "2025-10-07T10:40:23Z")

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> [@acal](#):
>
> Is it common to have convergence problems with shell, contact and large displacements in Calculix?

Short answer - yes. The most common combination leading to non-convergence is shells + rigid body constraints + Nlgeom, but I’ve heard about issues with contact too. Internal conversion of shells to solids is nonlinear (generation of knots) and may interfere with other nonlinearities in some cases. Thus, it’s often better to use the Thicken Shell Mesh tool that you’ve mentioned.

> **[Rigid-body constraint convergance problems](https://calculix.discourse.group/t/rigid-body-constraint-convergance-problems/369/25)**
>
> Hi all, I have not read this topic in detail, but if I am right, it is about the coupling of shells with keywords such as \*RIGID BODY or \*COUPLING. The latter constraints frequently are nonlinear. Since the expansion of shells into solids is...

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**Author:** ![acal](https://avatars.discourse-cdn.com/v4/letter/a/e79b87/32.png) [@acal](https://prepomax.discourse.group/u/acal)\
**Post date:** [October 7, 2025, 10:44am UTC](https://prepomax.discourse.group/t/shell-plasticity/2997/4 "2025-10-07T10:44:35Z")

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Thank you for the INSTANTANEOUS reply! 👍

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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:** [October 7, 2025, 3:46pm UTC](https://prepomax.discourse.group/t/shell-plasticity/2997/5 "2025-10-07T15:46:27Z")

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Also, keep in mind that sometimes it’s the fault of BCs on rotational DOFs of shell elements leading to overconstraint. Here’s such a case: [Contact of two shell pipes - no convergence in newer ccx releases - CalculiX (official versions are on www.calculix.de, the official GitHub repository is at https://github.com/Dhondtguido/CalculiX).](https://calculix.discourse.group/t/contact-of-two-shell-pipes-no-convergence-in-newer-ccx-releases/2865)