Penetration checks

Displacement check against solids (CHECK_SOLID)

It is possible to define geometric primitives (solids) as a restriction of the node displacements. Geometric primitives are defined using the SOLID parameter. The SOLID command allows the definition of:

• circles
• circle segments
• ring segments and rectangles in two-dimensional models and cylinders
• cylinder segments
• tubes
• cubes and cube segments in three-dimensional models

There is a differentiation between a variation solid and a restriction solid (variation area or restriction area).

The parameter:

        CHECK_SOLID = solid_name
      

specifies a solid whose borders may not be penetrated.

The following figure shows a displacement check against a solid:

Defining a solid

1. To define a solid in Tosca ANSA® environment, choose New > SOLID command on a CHECK_SOLID item found below DV_CONSTRAINTS.
2. To define a solid in Tosca Structure.gui, choose Command SOLID.
   
   

Important: The CHECK_SOLID restrictions can be performed for surface nodes as well as for inner nodes. To limit the exterior form of a component, it only makes sense to restrict surface nodes. However, the node displacements in the MESH_SMOOTH area should be limited. Up to six different CHECK_SOLID parameters can be defined in every DVCON_SHAPE command. They are executed in the order of their declaration within the DVCON_SHAPE command. In Tosca ANSA® environment this corresponds to the order of the solid entries in the task manager. If the solid is a variation solid, all nodes of the node group in the start model (see ND_GROUP parameter) must be located inside the variation solid. If nodes are located outside the variation solid, the DVCON_SHAPE definition will be rejected. If the solid is a restriction solid, all nodes of the node group in the start model (see ND_GROUP parameter) must be located outside the restriction solid. If nodes are located inside the restriction solid, the DVCON_SHAPE definition will be rejected.

Penetration check (CHECK_ELGR, PENETRATION_CHECK)

Element surfaces and lines as well as solid elements can be defined as limiting surfaces, lines or solids in order to check node displacements against any contour. This option offers more flexibility than the check for the absolute amount of displacement or the check against geometric primitives. The limiting surfaces are formed by beam structures in 2D models and by shell structures or solid structures in 3D models. The limiting surfaces are generated in the FE preprocessor and loaded via the interface (MODEL_LINK Folder or FEM_INPUT command, ADD_FILE parameter) in the optimization preprocessor.

The parameter

        CHECK_ELGR = elgr_name 
      

can be defined in the CHECK_GROUP field of the PENETRATION_CHECK menu in Tosca ANSA® environment or in the Element Group field of Tosca Structure.gui.

It specifies an element group whose elements may not be penetrated (contact condition) by the nodes of the node group specified by the ND_GROUP parameter for shell or beam elements. For solid elements, all nodes inside the solids specified by the CHECK_ELGR parameter are frozen and for all nodes outside the penetration into the solid is avoided. The following figure provides a graphic illustration.

Activation of the element check represents a collision control. If a node attempts to penetrate an element, the node displacement is scaled back so the effected node remains on the side of the element where it is intended. The element group must be defined with GROUP_DEF or in the analysis file before it can be referenced with CHECK_ELGR.

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:

Note:

• CHECK_ELGR restrictions can be performed for surface nodes as well as for inner nodes. To limit the exterior form of a component, it only makes sense to restrict surface nodes. However, the node displacements in the MESH_SMOOTH area should be limited.

• Up to six CHECK_ELGR parameters can be defined in every DVCON_SHAPE command. They are executed in the order of their declaration within the DVCON_SHAPE command. In Tosca ANSA® environment for each group a new PENETRATION_CHECK entry has to be created.
• 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 greatly simplifies the selection and assembly of groups in the optimization preprocessor (when using manual selection).

Important: The nodes (ND_GROUP parameter) and the elements (CHECK_ELEM parameter) should have a definite minimum distance to ensure that a node must remain on the right element side and the initial model is feasible. For determining penetration by beam elements (without lateral dimension), a hypothetical tolerance area amounting to 1% of the beam length surrounds each beam element. A beam element is considered penetrated when a node is moved into the tolerance area. A node displacement moving alongside the tolerance area is not considered as a penetration. If the elements being used in the check are loaded with the ADD_FILE parameter of FEM_INPUT command, attention should be paid that node or element IDs are not used twice since SIMULIA Tosca Structure cannot process duplicated IDs.

• In sensitivity based optimization mode, the node restrictions CHECK_SOLID and CHECK_ELGR are applied as box constraints on the design variables. These commands can be used as an alternative or in addition to CHECK_SHRINK and CHECK_GROW to restrict the design nodes.