Contact Formulation for General Contact in Abaqus/Explicit

Currently you can specify only the contact surface weighting and polarity 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.

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 main-secondary contact the node with the constraint is part of the secondary surface and the surface with which it interacts is called the main surface. For balanced main-secondary 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 main surface and once with the second surface acting as the main surface. The weighted average of the two corrections (or forces) is applied to the contact interaction.

Balanced main-secondary 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 main-secondary contact the nodes on the main surface can, in principle, penetrate the secondary surface unhindered (see Figure 1).

Main surface penetrations into the secondary surface in pure main-secondary contact due to coarse discretization.

The general contact algorithm in Abaqus/Explicit uses balanced main-secondary weighting whenever possible; pure main-secondary weighting is used for contact interactions involving node-based surfaces, which can act only as pure secondary surfaces and for contact interactions involving analytical rigid surfaces, which can act only as pure main surfaces. Surface-based cohesive behavior also always uses a pure main-secondary algorithm. However, you can choose to specify a pure main-secondary weighting for other interactions as well.

There is no main-secondary relationship for edge-to-edge contact; both contacting edges are given equal weighting.

By default, general contact considers both sides of all double-sided elements in surfaces specified to be included for contact purposes (side labels of double-sided elements are ignored). This default can be overridden for node-to-face and Eulerian-Lagrangian contact and in some cases results in more accurate enforcement of contact.

Surface polarity is not considered for edge-to-edge contact, including edges activated on faces of solid elements.

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 Formulations for Contact Pairs in Abaqus/Explicit).

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 main 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.

Local tangent directions for contact provide a reference frame for select general contact output variables in Abaqus/Explicit (see About General Contact in Abaqus/Explicit). These local tangent directions are separate from local coordinate systems associated with user subroutines VFRICTION and VUINTERACTION. Abaqus/Explicit establishes and updates the orientation of the first local contact tangent direction, ${\mathbf{t}}_1$ , at secondary nodes and edge nodes according to the logic described below for different contact formulation types within general contact. The orientation of the second local tangent direction, ${\mathbf{t}}_2$ , is found as the cross product of the contact normal direction, $\mathbf{n}$ with ${\mathbf{t}}_1$ . A change in the predominant contact formulation type that is active at a node may lead to a sudden change in the local tangent directions.

Finite-sliding, node-to-surface formulation for non-analytical surfaces

The ${\mathbf{t}}_1$ -direction is initialized at a secondary node upon first contact using the standard convention for calculating a first local surface tangent direction (see Conventions) or using the user orientation specified through the keyword SURFACE PROPERTY ASSIGNMENT, PROPERTY=ORIENTATION (applicable only for nodes belonging to an element-based surface). In subsequent increments, if the secondary node belongs to an element-based surface, the ${\mathbf{t}}_1$ -direction rotates with the secondary surface for geometrically nonlinear analyses; otherwise, the standard convention is used.

Finite-sliding, node-to-surface formulation with an analytical surface

The ${\mathbf{t}}_1$ -direction for contact is initialized at a secondary node upon first contact to be aligned with the convention for the ${\mathbf{t}}_1$ -direction of the analytical rigid surface discussed in Analytical Rigid Surface Definition at the point of contact. In subsequent increments, the ${\mathbf{t}}_1$ -direction for contact at a secondary node evolves such that it continues to be aligned with the ${\mathbf{t}}_1$ -direction of the analytical rigid surface at the current point of contact.

Finite-sliding, edge-to-edge formulation

The ${\mathbf{t}}_1$-direction for an edge-to-edge contact constraint is initialized upon first contact to be in the axial direction of one of the edges involved in the contact and will evolve to remain aligned with the axial direction of this edge until a local transition to another edge occurs, and then the axial direction of that edge will be adopted as the ${\mathbf{t}}_1$-direction.

Local surface tangent directions will often differ across a contact interface. For example, respective local ${\mathbf{t}}_1$ -directions (CTANDIR1) on opposite sides of an interface will evolve differently if surface rotations across the interface are not the same. The respective local ${\mathbf{t}}_2$ -directions (CTANDIR2) on opposite sides of an interface are typically in opposing directions initially, due to secondary nodes on opposite sides of an interface having opposing contact normal directions.