4.2. Performing a Node-to-Surface Contact Analysis

CONTA175 follows the contact pair concept used by surface-to-surface elements CONTA171 through CONTA174. CONTA175 is paired off with target elements TARGE169 and TARGE170. See Identifying Contact Pairs for more information. CONTA175 uses most of the same element KEYOPTS and real constants as the surface-to-surface contact elements. These are described below.

You should avoid midside-noded underlying elements of the contact surface, especially in 3-D. The “effective stiffness” at the contact surface nodes is very nonuniform. For instance, for a 20-node brick (SOLID186), the corner nodes have a negative stiffness associated with them. However, the node-to-surface contact algorithm assumes that the stiffness is uniformly distributed across all the surface nodes when contact is made. This condition can lead to convergence difficulties when using midside-noded elements in contact. The midside-noded elements can only be used when bonded or no-separation contact is defined. You can still use midside nodes on 2-D contact surfaces or on 2-D/3-D target surfaces.

The basic steps for performing a node-to-surface contact analysis using CONTA175 are the same as those used for a typical surface-to-surface analysis using CONTA171 through CONTA174. See Steps in a Contact Analysis for details.

The Contact Manager provides an easy-to-use interface to help you construct and manage contact definitions. You can access the manager via the Contact Manager icon   in the ANSYS Standard Toolbar, or via the menu path Main Menu> Preprocessor> Modeling> Create> Contact Pair. See GUI Aids for Contact Analyses for more information on using the Contact Manager.

As mentioned, CONTA175 uses the ESURF command to generate elements between corresponding contact pairs, similar to the surface-to-surface contact elements. The GUI path is:

Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Node to Surf

Since CONTA175 is a one node element, you are not able to plot the contact results. However, you can list the results using the PRESOL,CONT or PRETAB commands.

4.2.1. CONTA175 KEYOPTS

CONTA175 uses most of the same KEYOPTS that are used by the surface-to-surface contact elements CONTA171 through CONTA174. KEYOPT(3) and KEYOPT(4) are used but have different meanings when used with CONTA175. See Element KEYOPTS for a listing of the remaining KEYOPTS.

4.2.1.1. KEYOPT(3)

KEYOPT(3) in CONTA175 allows you to choose between a contact force-based model (KEYOPT(3) = 0, default), and a contact traction-based model (KEYOPT(3) = 1). For the contact traction-based model, ANSYS can determine the area associated with the contact node. For the single point contact case, a unit area will be used which is equivalent to the contact force-based model.

When the traction-based model is defined, the real constants FKN, FKT, TCC, ECC, and MCC have the same units used in surface-to-surface contact elements (CONTA171 through CONTA174), as do postprocessing items PRES, TAUR, and TAUS.

When the force-based model is defined, the units of these quantities have a factor of AREA with respect to those used in the traction-based model. For instance, contact stiffness FKN has units FORCE/LENGTH for the force-based model, but FORCE/LENGTH3 for the traction-based model. PRES is the contact normal force in the force-based model, but contact pressure in the traction-based model.

4.2.1.2. KEYOPT(4)

KEYOPT(4) in CONTA175 allows you to choose the contact normal direction. The contact normal can be either perpendicular to the target surface (KEYOPT(4) = 0, default or KEYOPT(4) = 3), or perpendicular to the contact surface (KEYOPT(4) = 1, 2). When contact occurs on the bottom surface of a shell or beam, and shell thickness effect is included (KEYOPT(11) = 1), or CNOF is defined, KEYOPT(4) = 2 or 3 should be used in order to capture the contact.

Real constant TOLS is used to add a small tolerance that will internally extend the edge of the target surface. TOLS is useful for problems where contact nodes are likely to lie on the edge of the target (as at symmetry planes or for models generated in a node-to-node contact pattern). In these situations, the contact node may repeatedly “slip” off the target surface and so completely out of contact, resulting in convergence difficulties from oscillations.

Units for TOLS are percent (1.0 implies a 1.0% increase in the target edge length). A small value of TOLS will usually prevent this situation from occurring. The default value is 10 for small deflection and 2 for large deflection (NLGEOM,ON).

4.2.2. CONTA175 Real Constants

CONTA175 uses the same real constants used by the surface-to-surface contact elements CONTA171 through CONTA174), except the units of TCC, ECC, and MCC for the contact force-based model. See a listing of the real constants in Real Constants.


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