### 14.1.1. Interactions - Lagrange/Lagrange - Introduction

Autodyn offers 2 types of Lagrange/Lagrange interaction (contact) methods.

- Gap contact

  In this contact method each surface segment is surrounded by a contact detection zone. The dimensions of this contact zone is called Gap size. This means that initially the outside surfaces of the parts in the contact should be separated by the gap distance.

  Any nodes entering the contact detection zone of a surface segment are repelled by a force normal to the segment surface and proportional to the depth of penetration of the node into the contact detection zone.

  The interaction conserves linear and angular momentum.

  To assure a stable interaction process the gap contact algorithm uses a timestep restriction such that during 1 computational time step a surface node cannot travel more than 20% into the contact detection zone.

  where δ is the gap size and V the velocity of the penetrating node.

  The gap contact is available for all structured as well as unstructured Lagrange Parts, with exception for 2D unstructured Parts.

- Trajectory contact

  The Trajectory contact algorithm is available for all 3D Unstructured solvers, and has three main potential benefits over the Gap based contact algorithm.

  - There is no requirement to specify a contact detection zone or leave a physical gap between Parts at the start of a simulation. This makes model generation for complex 3D geometries significantly easier.

  - There is no constraint on the timestep due to contact. The algorithm detects node to face contact by tracking the trajectory of the nodes and faces over time. The removal of the timestep constraint on contact can give very significant performance improvements.

  - The algorithm is energy conserving, in addition to momentum conserving.

  The trajectory of nodes and faces included in frictional or frictionless contact are tracked during the computational cycle. If the trajectory of a node and a face intersects during the cycle a contact even is detected.

  Note: The use of SPH parts with Trajectory Contact is not supported. To use SPH parts with Trajectory Contact (in a serial solution only), please contact ANSYS Technical Support.

If a contact event is detected the node involved in the contact will be pushed back towards the true contact position during the computational cycle.

There are two methods available:

1. Penalty

   If a contact event is detected, a local penalty force is calculated to push the node back to the face. Equal and opposite forces are calculated on the nodes of the face in order to conserve linear and angular momentum. The applied penalty force will push the node back towards the true contact position during the cycle. However, it will take several cycles to satisfy the contact condition. Kinetic energy is not necessarily conserved. The conservation of energy can be tracked using the energy time history.

2. Decomposition Response (DCR)

   All contacts that take place at the same point in time are first detected. The response of the system to these contact events is then calculated to conserve momentum and energy. During this process, forces are calculated to ensure that the resulting position of the nodes and faces does not result in further penetration at that point in time. The decomposition response algorithm is more impulsive (in a given computational cycle) than the penalty method. This can give rise to large hourglass energies and energy errors.

The trajectory contact can handle multiple contact of a node with different faces of a contact surface during a computational timestep.

Current Limitations

- The trajectory contact option cannot currently be used in parallel simulations when an SPH part is present in the model.

- The trajectory contact option cannot be used in combination with joined unstructured parts. To circumvent this limitation the option to merge all joined unstructured nodes can be used.

- Interaction of SPH nodes, accounting for their full natural thickness/size is currently not available.

- The decomposition response algorithm cannot currently be used in combination with bonded contact regions. The formulation will automatically switch to the Penalty method if bonded regions are present in the model.

Release 18.2 - © ANSYS, Inc. All rights reserved.