The majority of contact implementation within OptiStruct
involves the penalty-based process.
This can be activated using the CONTACT (PCONT, if
necessary) and TIE Bulk Data Entries. The TIE
entry supports both Penalty-based and Lagrange
Multipliers-based formulations. Both Node-to-Surface (N2S) and
Surface-to-Surface (S2S) discretization methods utilize this process to implement
contact between parts. The discretized contact elements are assigned variable stiffness
values (penalties) depending on the proximity of the contact surfaces to each other and
the type of contact forces on the element. The primary purpose of the variable stiffness
values is to minimize penetration of the contact surfaces.
The stiffness of each contact element typically depends on the type of analysis, linear or
nonlinear, that is being performed. For Linear Analysis, the contact stiffness is
constant throughout the solution. The contact stiffness depends on the initial state of
the contact. If the contact is initially open, it remains open for the duration of the
run. The same goes for closed contacts. The contact status or the contact stiffness
values are not updated during the course of the solution for linear runs. The initial
contact stiffness of each contact element depends on the initial contact gap opening
distance (U0), which is calculated based on the positions of the
Slave and Master (including contributions from the GPAD field, if
any). For open contact elements, a very small stiffness value of KB=10-14 * STIFF is
used to avoid numerical singularities. Figure 1. Penalty-based Contact Element Force-Displacement Curves for Linear
Analysis
