3. Surface-to-Surface Contact
3.8. Set the Real Constants and Element KEYOPTS
3.8.11. Selecting Surface Interaction Models
The surface-to-surface contact elements support normal unilateral contact models as well as other mechan-ical surface interaction models.
3.8.11.2. Using KEYOPT(12)
Use KEYOPT(12) to model different contact surface behaviors.
• KEYOPT(12) = 0 models standard unilateral contact; that is, normal pressure equals zero if separation occurs.
• KEYOPT(12) = 1 models perfectly rough frictional contact where there is no sliding. This case corresponds to an infinite friction coefficient and ignores the material property MU.
• KEYOPT(12) = 2 models no separation contact, in which the target and contact surfaces are tied (although sliding is permitted) for the remainder of the analysis once contact is established.
• KEYOPT(12) = 3 models "bonded" contact, in which the target and contact surfaces are bonded in all directions (once contact is established) for the remainder of the analysis.
• KEYOPT(12) = 4 models no separation contact, in which contact detection points that are either initially inside the pinball region or that once involve contact always attach to the target surface along the normal direction to the contact surface (sliding is permitted).
• KEYOPT(12) = 5 models bonded contact, in which contact detection points that are either initially inside the pinball region or that once involve contact always attach to the target surface along the normal and tangent directions to the contact surface (fully bonded).
• KEYOPT(12) = 6 models bonded contact, in which the contact detection points that are initially in a closed state will remain attached to the target surface and the contact detection points that are initially in an open state will remain open throughout the analysis.
For all types of bonded contact (KEYOPT(12) = 2, 3, 4, 5, and 6), separation of contact can be modeled using the debonding feature. For more information, see Chapter 12, Debonding (p. 131).
For the no-separation option (KEYOPT(12) = 4) and the bonded-always option (KEYOPT(12) = 5), a relatively small PINB value (pinball region) may be used to prevent any false contact. For these KEYOPT(12) settings, the default for PINB is 0.25 (25% of the contact depth) for small deformation analysis (NLGEOM,OFF) and 0.5 (50% of the contact depth) for large deformation analysis (NLGEOM,ON). (The default PINB value may differ from what is described here if CNOF is input. See Using PINB (p. 49) for more information.)
For the bonded-initial option (KEYOPT(12) = 6), a relatively large ICONT value (initial contact closure) may be used to capture the contact. For this KEYOPT(12) setting, the default for ICONT is 0.05 (5% of the contact depth) when KEYOPT(5) = 0 or 4.
See Positive and Negative Real Constants for more information on the real constants mentioned above.
Note
For bonded contact definitions (KYEOPT(12) = 5 or 6), if the contact is not in a “just touching”
position, you may find that no zero modes appear for free vibration. To avoid this issue, use the MPC approach instead of other contact algorithms.
3.8.11.3. Using FKOP
The FKOP real constant can be used in two different ways, depending on the surface interaction model used.
For no separation or bonded contact (KEYOPT(12) = 2 through 6), FKOP is the stiffness factor applied when contact opens. For standard or rough contact (KEYOPT(12) = 0 or 1), FKOP represents a contact damping coefficient.
When modeling either no-separation or bonded contact, you may need to set a value for FKOP to apply a stiffness factor when contact opens. If FKOP is a scaling factor (positive value for command input), the true contact opening stiffness equals FKOP times the contact stiffness applied when contact closes. If FKOP is an absolute value (negative value for command input), the value is applied as an absolute contact opening stiffness. The default FKOP value is 1.
No separation and bonded contact generate a "pull-back" force when contact opening occurs, and that force may not completely prevent separation. To reduce separation, define a larger value for FKOP. Also, in some cases separation is expected while connection between the contacting surfaces is required to prevent rigid body motion. In such instances, you can specify a small value for FKOP to maintain the connection between the contact surfaces (this is a "weak spring" effect).
For standard contact (KEYOPT(12) = 0) or rough contact (KEYOPT(12) = 1), you can use FKOP to define a contact damping coefficient. This option is primarily used to damp relative motions between the contact
3.8.11. Selecting Surface Interaction Models
and target surfaces for open contact. It provides certain resistance to reduce the risk of rigid body motion.
The damping force is calculated by Fd=∫FKOP V⋅ reldAc
where Vrel is the slip rate and Ac is the area domain of the contact surface. The units of the damping coefficient are FORCE/(AREA*VELOCITY). For the contact force-based model, the units are FORCE/VELOCITY. To specify the contact damping coefficient, enter a negative number for FKOP. Positive input will be ignored.
3.8.11.4. Bonded Contact for Shell-Shell Assemblies
The bonded contact options (KEYOPT(12) = 5 or 6) can be used with the MPC approach (KEYOPT(2) = 2) to model various types of assemblies (see Chapter 9, Multipoint Constraints and Assemblies (p. 99)). When this method is used to model shell-shell assemblies, there may be cases where the MPC approach causes the model to be overconstrained. To alleviate this problem, you can use a penalty-based method for shell-shell assemblies. Using the penalty-based method constrains rotational DOFs in addition to translational DOFs.
This capability is available for contact elements CONTA173,CONTA174, and CONTA175 in conjunction with TARGE170.
To use this method, first set KEYOPT(2) = 0 or 1 (augmented Lagrangian or penalty function) and KEYOPT(12)
= 5 or 6 (bonded always or bonded initial contact) in the contact elements. Setting KEYOPT(5) = 2 (shell-shell constraint) for the target elements will cause this penalty-based method to be used.
The penalty stiffness used for rotational DOFs is equal to (contact stiffness used for translational DOFs) * (contact length). The contact stiffness for translational DOFs is input by real constant FKN, or defaults to an internal value. The contact length is always calculated internally and it is printed in the output file. The figure below shows the difference in using a conventional penalty-based shell-shell assembly and this method.
Figure 3.23: Penalty-Based Shell-Shell Assembly
Note
In the case of a penalty-based shell-shell assembly, spurious rotational energy exists if there are gaps or penetrations between the contact and target surfaces. This can affect the accuracy of the solution. In this case it is recommended that you use a shell-solid constraint type by setting KEYOPT(5) = 3, 4, or 5 on the target element, which requires use of the MPC algorithm (KEYOPT(2)
= 2 on the contact element).