6.2. Performing a 3-D Line-to-Surface Contact Analysis

Contact element CONTA177 follows the contact pair concept used by the surface-to-surface elements (CONTA171 through CONTA174). You must pair CONTA177 with 3-D target surface segments (TARGE170) to model 3-D beam-to-surface or 3-D shell edge-to-surface contact. See Identifying Contact Pairs for more information. CONTA177 uses most of the same element KEYOPTs and real constants as the surface-to-surface contact elements. Any differences are described in the next section.

The basic steps for performing a 3-D line-to-surface contact analysis 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.

When the underlying elements are beams or edges of shell elements you can use the ESURF command to create CONTA177 elements. (In the GUI, pick Main Menu> Preprocessor> Modeling> Create> Elements> Surf/Contact> Surf to Surf.) This procedure is similar to that used for the surface-to-surface contact elements.

When using line-to-surface contact elements to form the contact surface, the nodes must be entered in a sequence that defines a continuous line, as shown in the figure below.

Figure 6.2  Continuous Line Segments

Continuous Line Segments

The line can be made up of linear or parabolic segments, depending on whether the attached beam or shell edge is made up of first order or second order elements. If the nodal ordering of the underlying beam elements is not consistent, you must either change them by reversing the node number order of the selected elements (ESURF,,REVERSE command) or make a consistent element ordering (ENORM command).

You can list the CONTA177 results using the PRESOL,CONT or PRETAB commands. Since CONTA177 is a 3-D line element, you can also use the PLLS command to display element table items.

6.2.1. KEYOPTs and Real Constants

CONTA177 uses most of the same KEYOPTs that are used by the surface-to-surface contact elements, CONTA171 through CONTA174. KEYOPT(4) differs from the other contact elements. KEYOPT(3) and KEYOPT(8) are not used by CONTA177. See Element KEYOPTS for a listing of the remaining KEYOPTs.

CONTA177 uses the same real constants used by the surface-to-surface contact elements (CONTA171 through CONTA174), except for the units of FKN and FKT. See Real Constants for a complete list.

6.2.1.1. Real Constants FKN and FKT

CONTA177 uses the contact force-based model. (Since this is the only model used for this element, KEYOPT(3) is not used.) The units of real constants FKN and FKT have a factor of AREA with respect to those used in the surface-to-surface contact elements. For instance, contact stiffness FKN has units of FORCE/LENGTH in the force-based model, but FORCE/LENGTH3 in the surface-to-surface contact elements. PRES is the contact normal force in the force based model, but contact pressure in CONTA171 to CONTA174.

6.2.1.2. Accounting for Thickness Effect (CNOF and KEYOPT(11))

For line-to-surface contact, the thickness effect of underlying beam elements (on the contact side) is accounted for through the contact offset real constant, CNOF. The thickness effect of underlying shell elements (on the target side) is accounted for by setting KEYOPT(11).

For the case of a beam contacting a solid, input half of the beam thickness for CNOF; KEYOPT(11) is ignored. For the case of a beam contacting a shell surface, input half of the beam thickness for CNOF and set KEYOPT(11) = 1 to include the shell thickness effect. CNOF remains constant for the entire contact pair during the solution. For more information on using CNOF, see Adjusting Initial Contact Conditions. For more information on using KEYOPT(11), see Using KEYOPT(11).

6.2.1.3. Real Constant TOLS

For CONTA177, the contact normal is uniquely defined and is perpendicular to the target surfaces. 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 be 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).

6.2.1.4. KEYOPT(4)

You can use CONTA177 with the multipoint constraint (MPC) approach (KEYOPT(2) = 2) to define surface-based constraints. The KEYOPT(4) setting will determine the type of surface-based constraint. Set KEYOPT(4) = 1 for a force-distributed constraint, or set KEYOPT(4) = 0 for a rigid surface constraint. See Surface-based Constraints for more information.


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