14.1.8. Interactions - 2D Gap Contact Options

Type - External Gap

When a 2D model contains structured elements only, the 2D gap contact must be used to define Lagrange/Lagrange interactions.

An analysis model that contains a mix of structured (IJK) Lagrangian Parts and unstructured Lagrangian Parts can currently not be run with Gap contact and also not with Trajectory contact.

To be able to use Trajectory contact the analysis model must contain unstructured Lagrangian volume Parts only. Similar, to be able to use Gap contact the analysis model must contain structured Lagrangian Parts only.

Interaction Gap

A gap is required for the interaction logic. The size of this gap must be in the range 1/10 to 1/2 the dimension of the smallest element face of Parts involved in interactions.

For most models you should set the gap size close to the smallest allowed value (Parts using a Lagrange or Shell Solver).


Retain inertia of eroded nodes

Check this box if you want to retain the inertial of eroded nodes (otherwise eroded nodes are removed from the model).

Prevent erosion of degenerate elements

Check this box if you want the interaction calculation to be used to stop elements from becoming degenerate. This option only works for Lagrange volume elements.

Warning:  Activating this option can lead to unphysical results, particularly if a large Gap size is used.

Interaction by Part


Sets interactions between Parts.

Add All

Sets all Parts to interact with each other.


Removes interactions between Parts.

Remove All

Removes all interactions between Parts.


Sets interactions using a Part matrix.


Sets the index range of each Part that will be checked for interactions.


Sets static friction coefficients between different Parts.


Enables a review of all of your interaction settings.

Self Interaction

If the self-interaction option is switched on the contact detection algorithm will also check for external nodes of a part contacting with faces of the same part in addition to other parts. This is the most robust contact setting since all possible external contacts should be detected.

Interaction Timestep

Autodyn automatically calculates an Interaction Timestep based on the Gap size and the maximum velocity of the Parts involved in interactions.

Safety Factor

The safety factor you want to use to calculate the Interaction Timestep.

You cannot enter a value greater than the default value (0.2).

Release 16.2 - © SAS IP, Inc. All rights reserved.