# Rigid-body constraint convergance problems

**URL:** <https://calculix.discourse.group/t/rigid-body-constraint-convergance-problems/369>\
**Category:** Uncategorized\
**Created:** [December 22, 2020, 11:04pm UTC](https://calculix.discourse.group/t/rigid-body-constraint-convergance-problems/369 "2020-12-22T23:04:05Z")\
**Posts on this page:** 1\
**Showing post:** 25

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**Author:** ![dhondt](https://avatars.discourse-cdn.com/v4/letter/d/7bcc69/32.png) [@dhondt](https://calculix.discourse.group/u/dhondt)\
**Post date:** [August 6, 2022, 9:44am UTC](https://calculix.discourse.group/t/rigid-body-constraint-convergance-problems/369/25 "2022-08-06T09:44:16Z")

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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 also nonlinear (generation of knots) you are trying to couple nonlinear constraints. This should be avoided since it will not always work in CalculiX. I strongly recommend not to use any nonlinear constraints in combination with shells/beams/plane stress/plane strain/axisymmetric elements in CalculiX. This does not apply to (penalty) contact, since contact leads to the creation of spring elements, not to the formulation of nonlinear (constraint) equations.

Best Greetings,

Guido

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