5.3. Performing a 3-D Beam-to-Beam Contact Analysis

Contact element CONTA176 follows the contact pair concept used by the surface-to-surface elements (CONTA171 through CONTA174). You must pair CONTA176 with 3-D target line segments (TARGE170) to model 3-D beam-to-beam contact. See Identifying Contact Pairs for more information. CONTA176 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 beam-to-beam 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.

Use the ESURF command to generate CONTA176 elements between corresponding contact pairs. (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. If the underlying elements of target element TARGE170 are part of shell edges, use ESURF,,,LINE to generate 3-D line or parabola target segments.

When using the line segments to form the target surface or using line-to-line 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 5.5:  Continuous Line Segments

Continuous Line Segments

The line can be made up of linear or parabolic segments, depending on whether the attached beam 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 CONTA176 results using the PRESOL,CONT or PRETAB commands. Since CONTA176 is a 3-D line element, you can also use the PLLS command to display element table items.

5.3.1. KEYOPTs and Real Constants

CONTA176 uses most of the same KEYOPTs that are used by the surface-to-surface contact elements, CONTA171 through CONTA174. KEYOPT(3) and KEYOPT(4) differ from the other contact elements, and KEYOPT(11) is not used by CONTA176. See Element KEYOPTS for a listing of the remaining KEYOPTs.

CONTA176 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 a listing of the real constants in Real Constants.

5.3.1.1. Real Constants R1, R2

For beam-to-beam contact, an important assumption is that of constant circular beam cross section. The contact radius is assumed to be the same for all CONTA176 elements in the contact pair. Likewise, the target radius is assumed to be the same for all TARGE170 elements in the contact pair. You supply the target and contact radii through real constants R1 and R2, respectively.

For a general beam cross section, you can use an equivalent circular beam in the contact definition (see figure below). Use these guideline to define the equivalent circular cross section:

  • Determine the smallest cross section along the beam axis.

  • Determine the largest circle embedded in that cross section.

Figure 5.6:  Equivalent Circular Cross Section

Equivalent Circular Cross Section

Use the first real constant, R1, to define the radius on the target side (target radius rt). Use the second real constant, R2, to define the radius on the contact side (contact radius rc).

The contact radius R2 is always a positive number. The target radius R1 can be entered as either a negative or positive value. Use a negative value when modeling internal contact (a beam or pipe sliding within another beam or pipe), with the input value equal to the inner radius of the outer beam. Use a positive value when modeling external contact between the exterior surfaces of two cylindrical beams.

For beam-to-beam contact, the thickness effect is accounted for through the contact and target radii. Therefore, KEYOPT(11) (which is used for the surface-to-surface contact elements to include or exclude thickness effects) is not used for CONTA176.

5.3.1.2. KEYOPT(3)

For CONTA176, KEYOPT(3) determines the type of beam-to-beam contact as follows:

  • Use KEYOPT(3) = 0 or 2 to model external contact between parallel beams or internal contact between beams (a beam inside a hollow beam or a pipe inside a pipe). For this case, the sign of the target radius, R1, will determine whether you are modeling parallel beams (R1 > 0) or a beam inside a beam (R1 < 0).

  • Use KEYOPT(3) = 1 or 3 to model beams that cross. The target radius must be a positive value.

KEYOPT(3) also allows you to choose between a contact force-based model (KEYOPT(3) = 0 or 1), and a contact traction-based model (KEYOPT(3) = 2 or 3). For the contact traction-based model, the program determines the area (based on beam element length and the contact radius, R2) associated with the contact node.

When the traction-based model is specified, certain real constants (FKN, FKT, TNOP) and postprocessing items (PRES, TAUR, TAUS, SFRIC, and so on) have the same units as in the surface-to-surface contact elements (CONTA171 through CONTA174).

When the force-based model is specified, the units of these quantities have a factor of AREA with respect to those used in the traction-based model. For example, 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. See the CONTA176 element description for detailed information on the units for these quantities.

5.3.1.3. Real Constant TOLS

For CONTA176, the contact normal is uniquely defined and is perpendicular to both the contact and the target surfaces (two contacting beams). 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).

5.3.1.4. KEYOPT(4)

You can use CONTA176 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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