21.2.7 Contact modeling if substructures are present

Products: ABAQUS/Standard ABAQUS/CAE

References

Overview

In ABAQUS/Standard substructures:

can be used to model contact between linear elastic bodies undergoing small deformations;
can be used to model either one or both of the contacting bodies;
need to retain only the nodes on the surfaces of the bodies for the contact analysis;
can greatly reduce the number of degrees of freedom involved in the contact iterations; and
do not provide any information regarding the surface geometry of the underlying body.

Defining the surface geometry of the substructure model

Since a substructure consists only of a group of retained nodal degrees of freedom, it has no surface geometry upon which ABAQUS/Standard can define a contact surface. One of the following methods must be used to define the surface geometry of the substructure:

mesh the surface of the substructure with structural elements,
use a node-based surface, or
use contact elements to define the substructure's surface.

Meshing the surface of the substructure with structural elements provides the most flexibility in defining the contact conditions; the surface can be used as either a master or slave surface in the simulation. Using a node-based surface is probably the easiest method to use, but the limitations inherent to node-based surfaces (such as the need to define nodal contact areas for exact contact stress recovery and the lack of visualization of contact stresses) may limit the usefulness of this approach. Contact elements can be a useful method if the model uses matched meshes.

Meshing the surface of the substructure

The surface geometry of the body being modeled with a substructure can be defined by attaching a mesh of structural elements to the retained surface nodes of the substructure. This mesh can then be used with the element-based contact capability described in this chapter. Use membrane elements in three-dimensional models and axisymmetric models, and use trusses in planar models. The auxiliary surface mesh is used only to define the surface geometry of the substructure. Define the elements to have very small thickness or area and define their material property to have a very small elastic modulus so that their contribution to the stiffness of the model is negligible; the stiffnesses of the underlying bodies are built into the substructures.

Figure 21.2.7–1 shows a simulation where both of the contacting bodies have been modeled with substructures. The nodes retained in the model are indicated in the figure. If this were a planar model, truss elements would be used to reconstruct the appropriate surface geometries of the original mesh. The truss elements would have a very small cross-sectional area and refer to a material property with very low stiffness so that they do not add any significant stiffness to the underlying bodies.

Figure 21.2.7–1 Substructuring in a contact simulation.

This mesh overlay method cannot be used if the substructure consists of second-order, three-dimensional brick elements of type C3D20(R)(H) and the substructure will be used as a slave surface. Normally, ABAQUS/Standard automatically converts C3D20(R)(H) brick elements to elements with midface nodes C3D27(R)(H) because this class of elements performs better in contact simulations (see “Defining element-based surfaces,” Section 2.3.2). ABAQUS/Standard also converts any second-order, three-dimensional structural element that does not have a midface node when it is used in a slave surface. Therefore, if second-order membrane elements (type M3D8) are used to reconstruct the surface topology of a substructure consisting of C3D20 elements, ABAQUS/Standard will convert them to M3D9 elements when the surface is used as a slave surface. The midface nodes that are generated automatically will not correspond to any retained nodes and, thus, will have zero stiffness. The lack of stiffness at these nodes will cause numerical problems during the analysis. The mesh overlay approach can be used if elements of type C3D27(R)(H) have been used on the surface of the substructure.

Using a node-based surface to define the substructure's surface

If the retained nodes of the substructures are associated with the slave surface of a contact pair, the retained nodes can be included in a node-based surface (see “Defining node-based surfaces,” Section 2.3.3). In this case it is not necessary to overlay the surface of the substructure with membrane or truss elements.

Using contact elements to define the substructure's surface

GAP elements (“Gap contact elements,” Section 23.2.1) can be used to define the contact interactions in the model. These elements require that matching nodes be present on the opposite sides of the contact surfaces and allow only for small relative sliding between the surfaces. This latter assumption is usually consistent with the assumption of linear behavior that is built into a substructure.