Defining contact in Abaqus/Explicit

Abaqus/Explicit provides two algorithms for modeling contact interactions. The general (“automatic”) contact algorithm allows very simple definitions of contact with very few restrictions on the types of surfaces involved (see About General Contact in Abaqus/Explicit). The contact pair algorithm has more restrictions on the types of surfaces involved and often requires more careful definition of contact; however, it allows for some interaction behaviors that currently are not available with the general contact algorithm (see About Contact Pairs in Abaqus/Explicit). General contact interactions typically are defined by specifying self-contact for a default, element-based surface defined automatically by Abaqus/Explicit that includes all bodies in the model. To refine the contact domain, you can include or exclude specific surface pairs. Contact pair interactions are defined by specifying each of the individual surface pairs that can interact with each other.

The contact formulation in Abaqus/Explicit includes the constraint enforcement method, the contact surface weighting, and the sliding formulation.

Constraint enforcement method

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. The penalty stiffness that relates the contact force to the penetration distance is chosen automatically by Abaqus/Explicit so that the effect on the time increment is minimal yet the penetration is not significant.

By default, the contact pair algorithm uses a kinematic contact formulation that achieves precise compliance with the contact conditions using a predictor/corrector method. The increment at first proceeds under the assumption that contact does not occur. If at the end of the increment there is an overclosure, the acceleration is modified to obtain a corrected configuration in which the contact constraints are enforced. The predictor/corrector method used for kinematic contact is discussed in more detail in Contact Constraint Enforcement Methods in Abaqus/Explicit; some limitations of this method are discussed in Common Difficulties Associated with Contact Modeling Using Contact Pairs in Abaqus/Explicit.

The normal contact constraint for contact pairs can optionally be enforced with the penalty contact method, which can model some types of contact that the kinematic method cannot. For example, the penalty method allows modeling of contact between two rigid surfaces (except when both surfaces are analytical rigid surfaces). When the penalty contact formulation is used, equal and opposite contact forces with magnitudes equal to the penalty stiffness times the penetration distance are applied to the main and secondary nodes at the penetration points. The penalty stiffness is chosen automatically by Abaqus/Explicit and is similar to that used by the general contact algorithm. The penalty stiffness can be overridden for surface-to-surface contact interactions by specifying a penalty scale factor or a “softened” contact relationship.

Contact surface weighting

In the pure main-secondary approach one of the surfaces is the main surface and the other is the secondary surface. As the two bodies come into contact, the penetrations are detected and the contact constraints are applied according to the constraint enforcement method (kinematic or penalty). Pure main-secondary weighting (regardless of the constraint enforcement method) will resist only penetrations of secondary nodes into main facets. Penetrations of main nodes into the secondary surface can go undetected, as shown in Figure 1, unless the mesh on the secondary surface is adequately refined.

Figure 1. Penetration of main nodes into secondary surface with pure main-secondary contact.

Balanced main-secondary contact simply applies the pure main-secondary approach twice, reversing the surfaces on the second pass. One set of contact constraints is obtained with surface 1 as the secondary, and another set of constraints is obtained with surface 2 as the secondary. The acceleration corrections or forces are obtained by taking a weighted average of the two calculations. For kinematic balanced main-secondary contact a second correction is made to resolve any remaining penetrations, as described in Contact Formulations for Contact Pairs in Abaqus/Explicit. The balanced main-secondary contact constraint when kinematic compliance is used is illustrated in Figure 2.

Figure 2. Balanced main-secondary contact constraint with kinematic compliance.

The balanced approach minimizes the penetration of the contacting bodies and, thus, provides more accurate results in most cases.

The general contact algorithm uses balanced main-secondary weighting whenever possible; pure main-secondary weighting is used for general contact interactions involving node-based surfaces, which can act only as pure secondary surfaces. For the contact pair algorithm Abaqus/Explicit will decide which type of weighting to use for a given contact pair based on the nature of the two surfaces involved and the constraint enforcement method used.

Sliding formulation

When you define a surface-to-surface contact interaction, you must decide whether the magnitude of the relative sliding will be small or finite. The default (and only option for general contact interactions) is the more general finite-sliding formulation. The small-sliding formulation is appropriate if the relative motion of the two surfaces is less than a small proportion of the characteristic length of an element face. Using the small-sliding formulation when applicable results in a more efficient analysis.