Contact controls

Contact stabilization can be used to control rigid body motions that may exist in a model before contact is fully developed. The option adds viscous damping in both the normal and tangential directions. By default, the damping is calculated automatically, but it is possible to modify the damping coefficient, the variation of the damping coefficient over the step, the range over which the damping works, and the ratio between normal and tangential damping. The controls specified with this option remain in effect until they are either changed by another contact controls procedure or reset to their default values. Contact stabilization can be defined for a specific contact pair or for the entire model. Further description of the stabilization controls can be found in Adjusting Contact Controls in Abaqus/Standard. In these tests various combinations of stabilization controls are tested in multistep analyses with multiple contact pairs.

The first group of analyses consists of six pairs of blocks that are pushed together in Step 1, subjected to tangential sliding in Step 2, and pulled apart in Step 3. The blocks are elastic, and the motion of the blocks is controlled with boundary conditions. Contact stabilization parameters are specified for the whole model and are overridden by different parameters for several individual contact pairs. The stabilization parameters vary from step to step. A restart file is written, and some restarts are made to test the restart functionality.

The second group of analyses consists of three blocks that are pushed together in Step 1, subjected to tangential sliding in Step 2, and pulled apart in Step 3. The blocks are elastic; and the top and bottom blocks are controlled with boundary conditions, whereas the middle block is completely free and held in place by contact stabilization. Different contact stabilization parameters are used for each contact pair. In addition, frictional properties are prescribed for one contact pair. This group contains two-dimensional and three-dimensional static analyses as well as a dynamic analysis.

During linear perturbation steps, all points in contact (i.e., with a “closed” status) are assumed to be sticking if friction is present. However, stick conditions are not enforced for contact nodes for which a velocity differential is imposed by the motion of the reference frame or the transport velocity. Stick conditions are enforced with a penalty method by default, and the perturbation tangent scale factor can be used to scale the penalty stiffness. For example, setting this parameter to zero will result in zero penalty stiffness, such that the stick conditions are not enforced during the perturbation step. Setting this parameter to a value greater than unity results in a larger-than-default penalty stiffness and, thus, stricter enforcement of stick conditions during the perturbation step.

The model consists of two blocks of different sizes in contact, with a nonzero friction coefficient in effect. In the first and second general steps we establish contact and apply a tangential displacement boundary condition such that the small block slips along the larger block. Natural frequencies are computed in subsequent perturbation steps:

By default, during linear perturbation steps contact constraints are usually fully enforced for surfaces where contact is active independent of local normal pressure in the base state. This treatment of contact constraints may excessively stiffen the structure because certain areas of the model where the contact pressure is close to zero may actually come in and out of contact during vibrations. The nature of linear perturbation procedures does not allow such contact chattering. However, you can use contact pressure-dependent constraint enforcement to relax or even completely remove the constraints that have low pressure.

You can control the behavior through two user-specified contact pressure coefficients, $p_0$ and $p_1$; by default, $p_0$ is equal to $p_1$. For pressures greater than $p_1$, the constraints are fully enforced, as in the default contact constraint treatment. For pressures between $p_0$ and $p_1$ (if $p_0$ is less than $p_1$), the constraints are gradually weakened. For pressures less than $p_0$, the constraints are totally removed.

The model consists of a plane stress disk placed between two anvils. A sequence of linear perturbation frequency steps and general steps is applied. During the general steps the disk is compressed between the anvils, and the linear perturbation frequency steps compute eigenpairs with different parameter settings as shown in Table 1.