Page 5: Conchk (Contact Check)

Purpose:

This interface is used for contact checking which fall under two main categories: initial penetration and tied contact. Initial penetration checking allows the user to check the defined contacts for initial penetrations and crossed edge. A group of parts can also be checked for initial penetrations using the ByParts option. Upon identifying initial penetration, they can be fixed using the fix button. Tied contact checking helps to determine if tied contacts will be successfully tied or not. Often tied surfaces are too far from each other, and do not get tied by LS-DYNA.

What is Penetration? The following discussion attempts to illustrate the various forms of penetration that the Conchk function is designed to handle:

Node-Surface Penetration

If a node is within contact distance to a shell (as shown in Figure 1) there is a penetration. If a node is inside a solid element (as shown in Figure 2) there is also a penetration.

Node-Edge Penetration

All *CONTACT_AUTOMATIC contacts in LS-DYNA include a virtual contact cylinder around each edge. When a node is located in the interior of one of these cylinders, it is considered as a penetration.

Edge-Edge Penetration

Some contact definitions in LS-DYNA, such as *CONTACT_GENERAL, include edge to edge contact. For these kinds of contact, figures 4 and 5 illustrate edge to edge penetrations.

Crossed Edges

A crossed edge is where an edge of an element crosses another shell element’s mid-surface or another solid element’s face. These are not penetrations in a classical sense in LS-DYNA, but they may lead to severe problems when running an analysis and so should be avoided. Figure 1 shows a case where two shell edges cross. The contact thickness of the elements including the virtual contact cylinders on the edges of the shells are displayed. Crossed edges are shown in red. This is not considered as a penetration, but Inipene is able to deal with these as well.

Finding Crossed Edges Example:

The Inipene panel is shown in Figure 7. As an example, the hood of the C2500 NCAC model (shown in Figure 8 below) will be used to demonstrate its functions.

• 
• Figure 8

Check for crossed edges

This is what is set as the default action in the Inipene interface. Select the parts you wish to check for crossed edges and click Check at the bottom of the interface. Note that even self intersecting parts will be detected in LS-PrePost. In this example, 21 crossed edges are found between the two components of the hood. The edges that cross other elements are highlighted as shown in Figure 9. The highlighted edges can be visually turned off by untoggling Show: CE.

Remove crossed edges

Toggle Flip Node and pick the nodes on the wrong side (node 84462 and node 84464 shown highlighted in Figure 10). Nodes picked in Flip Node mode are moved to the other side of the closest visible element. They are moved in the direction of the closest point on the closest visible element (t1+t2)/2 past the mid surface. The Undo button can be used to undo the last flip operation.

Select all nodes that should be flipped to the opposite side and redo the crossed edge by selecting parts and clicking Check (to ensure all crossed edges are removed). The hood is now free of crossed edges as shown in Figure 11

Penetration Check:

In LS-PrePost you can choose to check penetrations on selected parts by toggling Select Parts or you can check existing contact definitions in the keyword file by toggling the *CONTACT_ and selecting the contact you wish to check from the list. Penetrations are checked for shell, beam, and solid elements.

Penetration Check by Select Parts

This option is useful if you have not yet created any contact definitions. Contact thickness for this check is taken from section card data unless the Thickness toggle is activated. If Thickness is toggled, all shell and beam elements will get the user-specified contact thickness (solid elements have zero thickness). The check that is performed follows the same rules as *CONTACT_GENERAL would, i.e. it will check for node-to-surface, free_edge-to-free_edge, and node-to-edge penetrations between all parts (including self contact).

Penetration Check by CONTACT

By selecting a contact from the list of defined contacts in the model, the elements included in the contact are displayed and the check is performed by taking into account all parameters that affect contact thickness (SST, MST, SFST, SFMT, SHLTHK, SLDTHK, SSTHK, OPTT, SFT, TH, TH_SF) and which nodes/elements are to be checked against penetration to which elements.

Penetration Check on the C2500 Hood

Selecting the two parts on the hood and activating the Penetration toggle displays penetrating nodes with white squares and arrows proportional to the penetrating distance. 25 node to surface and 12 edge to edge penetrations are reported together with the maximum penetration distance. Nodes on edge-to-edge penetrations are reported in the same way as node-to-surface penetrations. The displayed penetrations can be visually turned off by untoggling Show: Pen.

Removing Initial Penetrations:

Penetrations can automatically be removed by moving the penetrating nodes in the direction away from the penetration. In LS-PrePost you can move the penetrating nodes a percentage of the per node penetrating distance. The default is 100%. This will move a node (such as shown in Figure 1) exactly out of penetration. But if two shell elements are parallel and penetrating (such as in Figure 13) and all penetrating nodes are moved 100%, there will be a gap between the two elements. In this case, moving 50% of the penetration distance moves the nodes just out of penetration. To be sure that no gap remains after removing penetrations, use Move 50% of pene and let LS-PrePost iterate until all penetrations are removed.

Any nodes can be prohibited from moving during automatic fixing. This is done by toggling Lock Nodes and selecting the nodes that should be locked in place. For the hood example, all nodes on the outer part are locked by toggling Lock Nodes and selecting the nodes By Part. When Fix is clicked, LS-PrePost will move all penetrating nodes that are not locked out of penetration. If a node is locked and penetrates another element, the nodes on the penetrated element are moved instead. For example, the penetrating node in Figure 14 is locked by the user, then all nodes on the lower elements are moved down out of penetration the same distance as the locked node would had been if it was not locked. This is the only case where non-penetrating nodes are moved.