Element selection

Selection of elements for contact depends heavily on the contact enforcement used. For example, for traditional contact formulations (i.e., the node-to-surface discretization) it is generally better to use first-order elements for those parts of a model that will form a secondary surface. Second-order elements can sometimes cause problems in this case because of the way these elements calculate consistent nodal loads for a constant pressure. The consistent nodal loads for a constant pressure, P, on a second-order, two-dimensional element with area A are shown in Figure 1.

Figure 1. Equivalent nodal loads for a constant pressure on a two-dimensional, second-order element.

The node-to-surface contact formulation bases important decisions on the forces acting on the secondary nodes. It is difficult for the algorithm to tell if the force distribution shown in Figure 1 represents a constant contact pressure or an actual variation across the element. The equivalent nodal forces for a three-dimensional, second-order brick element are even more confusing because they do not even have the same sign for a constant pressure, making it very difficult for the algorithm to work correctly, especially for nonuniform contact. Therefore, to avoid such problems, Abaqus/Standard automatically adds a midface node to any face of a second-order, three-dimensional brick or wedge element that defines a secondary surface when it is used with the node-to-surface formulation. The equivalent nodal forces for a second-order element face with a midface node have the same sign for a constant pressure, although they still differ considerably in magnitude.

The equivalent nodal forces for applied pressures on first-order elements always have a consistent sign and magnitude; therefore, there is no ambiguity about the contact state that a given distribution of nodal forces represents.

If you are using the node-to-surface formulation and your geometry is complicated and requires the use of an automatic mesh generator, the modified second-order tetrahedral elements (C3D10M) in Abaqus/Standard should be used. These elements are designed to be used in complex contact simulations; regular second-order tetrahedral elements (C3D10) have zero contact force at their corner nodes, leading to poor predictions of the contact pressures. The modified second-order tetrahedral elements can calculate the contact pressures accurately.

Regular second-order elements can generally be used without difficulty with the surface-to-surface formulation.