Displacement Check against Elements (CHECK_ELGR)

Limit node growth with CHECK_ELGR. Element surfaces/lines can be defined as limiting surfaces/lines to check node displacements against any contour. This option offers more flexibility than the check for the absolute displacement or the check against geometric primitives.

This page discusses:

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In Other Guides
DVCON_BEAD

The limiting surfaces are formed by beam, shell, or solid structures. The limiting surfaces are generated in the FE preprocessor and loaded through the interface (FEM_INPUT) in the optimization preprocessor.

The parameter


CHECK_ELGR = elgr_name

specifies an element group whose elements might not be penetrated (contact condition) by the nodes of the node group (ND_GROUP parameter).

Activation of the element check represents a collision control. If a node attempts to penetrate an element, the node displacement is scaled back so that the affected node remains on the side of the element where it is intended to be. If the element group is not already defined in the input file, it must be defined with GROUP_DEF before it can be referenced with CHECK_ELGR.

Up to six CHECK_ELGR parameters can be defined in every DVCON_BEAD command. They are executed in the order of their declaration within the DVCON_BEAD command.

If a node starts exactly on a shell element, there is a high probability that it is seen on either side of the shell. If the design nodes have an offset from the element group, the current implementation works fine. Dependent on which side of the shell element a node is seen by the algorithm, every one of the four situations can be right or wrong.

The problem is that, if the node is placed on the shell element, it cannot be said beforehand on which side the node will be seen. As a solution to this situation, the element group should get permeability: nodes can penetrate the element from one side but not from the other side. The new command 


CHECK_ELGR_BLOCKING = <BOTH | POS | NEG>
can have the following values:
BOTH (Default) element group is not penetrable from any side. Like current behavior with the known problems.
POS Element group is penetrable in negative normal direction and blocking in positive normal direction.
NEG Element group is penetrable in positive normal direction and blocking in negative normal direction.

Considering Shell Thickness and Offset

During the optimization, the geometry can be changed to such an extend that, taking into account the shell thickness leads to penetration of the constraining geometry (CHECK_ELGR item). In order to avoid this, Tosca Structure can consider the shell thickness and offset in penetration checks. The image below demonstrates the effect of this feature.



This behavior can be activated and deactivated using the command item


DVCON_BEAD
...
CONSIDER_SHELL_THICKNESS = <YES | NO>
...
END_

Important:
  • Thickness and Offset values taken into account only on design elements
  • Offset definitions only supported with Abaqus and ANSYS®.
    • Abaqus

      Supported offset definition:

      *SHELL SECTION, ..., OFFSET = <numeric> | SPOS | SNEG

      No distribution definitions are supported.

    • ANSYS®

      Supported offset definition:

      SECOFFSET, <numeric> | BOT | TOP | MID

Remarks

Tip: To simplify the definition of the contact check, it is useful to divide the limiting surfaces by assigning various element property numbers (or materials). This simplifies the selection and assembly of groups in the optimization preprocessor.

Important:
  1. All nodes of the selected node group are checked against all elements of the element group, elgr_name. To keep the number of control conditions within limits, only those nodes and elements that have the potential for penetration should be checked against one another. The groups should be correspondingly defined.
  2. The nodes (ND_GROUP parameter) and the elements (CHECK_ELEM parameter) should have a definite minimum distance in the initial model in order to verify on which side of the element a node must remain.
  3. If the elements being used in the check are loaded with the ADD_FILE parameter (FEM_INPUT), take care that node or element IDs are not used twice as Tosca Structure cannot process duplicated IDs.