Products: ABAQUS/Explicit ABAQUS/CAE
The contact formulation used with the general contact algorithm in ABAQUS/Explicit:
includes the constraint enforcement method, the contact surface weighting, and the sliding formulation; and
can be applied selectively to particular regions within a general contact domain.
Currently you can specify only the contact surface weighting for the general contact algorithm. The contact formulation propagates through all analysis steps in which the general contact interaction is active.
The surface names used to specify the regions where a nondefault contact formulation should be assigned do not have to correspond to the surface names used to specify the general contact domain. In many cases the contact interaction will be defined for a large domain, while a nondefault contact formulation will be assigned to a subset of this domain. Any contact formulation assignments for regions that fall outside the general contact domain will be ignored. The last assignment will take precedence if the specified regions overlap.
| Input File Usage: | *CONTACT FORMULATION |
| This option must be used in conjunction with the *CONTACT option. It should appear at most once per step; the data line can be repeated as often as necessary to assign contact formulations to different regions. |
| ABAQUS/CAE Usage: | Interaction module: Create Interaction: General contact (Explicit): Contact Formulation |
For general contact ABAQUS/Explicit enforces contact constraints using a penalty contact method, which searches for node-into-face and edge-into-edge penetrations in the current configuration. For node-to-face contact, forces that are a function of the penetration distance are applied to the slave nodes to oppose the penetration, while equal and opposite forces act on the master surface at the penetration point. The master surface contact forces are distributed to the nodes of the master faces being penetrated. For edge-to-edge contact, the opposing contact forces are distributed to the nodes of the two contacting edges.
The penalty contact method is well suited for very general contact modeling, including the following situations:
multiple contacts per node,
contact between rigid bodies, and
contact of surfaces also involved in other types of constraints (such as MPCs).
The “spring” stiffness that relates the contact force to the penetration distance is chosen automatically by ABAQUS/Explicit, such that the effect on the time increment is minimal yet the allowed penetration is not significant in most analyses. The penetration distance will typically be an order of magnitude greater than the parent elements' elastic deformation normal to the contact interface. In purely elastic problems this penetration can affect the stress solution significantly, as demonstrated in “The Hertz contact problem,” Section 1.1.11 of the ABAQUS Benchmarks Manual. You can specify a factor by which to scale the default penalty stiffnesses (see “Scaling default penalty stiffnesses” in “Contact controls for general contact,” Section 21.3.6). This scaling may affect the automatic time incrementation. Use of a large scale factor is likely to increase the computational time required for an analysis because of the reduction in the time increment that is necessary to maintain numerical stability.
Generally, contact constraints in a finite element model are applied in a discrete manner, meaning that for hard contact a node on one surface is constrained to not penetrate the other surface. In pure master-slave contact the node with the constraint is part of the slave surface and the surface with which it interacts is called the master surface. For balanced master-slave contact ABAQUS/Explicit calculates the contact constraints twice for each set of surfaces in contact, in the form of penalty forces: once with the first surface acting as the master surface and once with the second surface acting as the master surface. The weighted average of the two corrections (or forces) is applied to the contact interaction.
Balanced master-slave contact minimizes the penetration of the contacting bodies and, thus, provides better enforcement of contact constraints and more accurate results in most cases. In pure master-slave contact the nodes on the master surface can, in principle, penetrate the slave surface unhindered (see Figure 21.3.4–1).
Figure 21.3.4–1 Master surface penetrations into the slave surface in pure master-slave contact due to coarse discretization.
The general contact algorithm in ABAQUS/Explicit uses balanced master-slave weighting whenever possible; pure master-slave weighting is used for contact interactions involving node-based surfaces, which can act only as pure slave surfaces. However, you can choose to specify a pure master-slave weighting for other interactions as well.
There is no master-slave relationship for edge-to-edge contact; both contacting edges are given equal weighting.
You can specify that a general contact interaction should use pure master-slave weighting for node-to-face contact. This specification has no effect on edge-to-edge contact and cannot be used to make a node-based surface act as a master surface. When two originally flat surfaces contact one another, a more uniform penetration distance distribution may result with pure master-slave weighting where the more refined surface acts as the slave surface as compared to balanced master-slave weighting. This can be particularly evident if the mesh densities of the contacting surfaces differ significantly—with balanced weighting the contact penetrations will be smaller near the nodes of the coarsely meshed surface.
ABAQUS/Explicit will automatically generate contact exclusions for the master-slave orientation opposite to that specified; therefore, node-to-face contact will be excluded for any regions of the two surfaces that overlap. For example, specifying that the general contact interaction between surf_A and surf_B should use pure master-slave weighting with surf_A considered to be the slave surface would result in exclusions being generated internally for faces of surf_A contacting nodes of surf_B; node-to-face contact would be excluded completely for regions of overlap between surf_A and surf_B. A warning message will be issued if the second surface name is omitted or is the same as the first surface name since this input would result in the exclusion of all node–face contact interactions for the surface.
| Input File Usage: | Use the following option to indicate that the first surface should be considered the slave surface (default): |
*CONTACT FORMULATION, TYPE=PURE MASTER-SLAVE surf_1, surf_2, SLAVE Use the following option to indicate that the first surface should be considered the master surface: *CONTACT FORMULATION, TYPE=PURE MASTER-SLAVE surf_1, surf_2, MASTER If the first surface name is omitted, a default surface that encompasses the entire general contact domain is assumed. The second surface name must be specified. |
| ABAQUS/CAE Usage: | Interaction module: Create Interaction: General contact (Explicit): Contact Formulation: Pure master-slave assignments: Edit: select the surfaces in the columns on the left, and click the arrows in the middle to transfer them to the list of master-slave assignments. |
In the First Surface Type column, enter SLAVE to indicate that the first surface should be considered the slave surface, and enter MASTER to indicate that the first surface should be considered the master surface. |
Currently only the finite-sliding formulation is available for general contact in ABAQUS/Explicit. This formulation allows for arbitrary separation, sliding, and rotation of the surfaces in contact. For cases in which small-sliding or infinitesimal-sliding assumptions would be preferred, the contact pair algorithm should be used (see “Contact formulation for ABAQUS/Explicit contact pairs,” Section 21.4.4).
ABAQUS/Explicit is designed to simulate highly nonlinear events or processes. Because it is possible for a node on one surface to contact any of the facets on the opposite surface, ABAQUS/Explicit must use sophisticated search algorithms for tracking the motions of the surfaces. The finite-sliding contact search algorithm is designed to be robust, yet computationally efficient. This algorithm assumes that the incremental relative tangential motion between surfaces does not significantly exceed the dimensions of the master surface facets, but there is no limit to the overall relative motion between surfaces. It is rare for the incremental motion to exceed the facet size because of the small time increment used in explicit dynamic analyses. In cases involving relative surface velocities that exceed material wave speeds it may be necessary to reduce the time increment.
The contact search algorithm uses a global search when a contact interaction is first introduced, and a hierarchical global/local search algorithm is used thereafter. No user control of the search algorithm is needed.