CONTAC52 Element Description

CONTAC52 represents two surfaces which may maintain or break physical contact and may slide relative to each other. The element is capable of supporting only compression in the direction normal to the surfaces and shear (Coulomb friction) in the tangential direction. The element has three degrees of freedom at each node: translations in the nodal x, y, and z directions.

The element may be initially preloaded in the normal direction or it may be given a gap specification. A specified stiffness acts in the normal and tangential directions when the gap is closed and not sliding.

Figure 52.1: CONTAC52 Geometry

CONTAC52 Input Data

The geometry, node locations, and the coordinate system for this element are shown in Figure 52.1: CONTAC52 Geometry. The element is defined by two nodes, two stiffnesses (KN and KS), an initial gap or interference (GAP), and an initial element status (START). The orientation of the interface is defined by the node locations, or by a user-specified gap direction. The interface is assumed to be perpendicular to the I-J line or to the specified gap direction. The element coordinate system has its origin at node I and the x-axis is directed toward node J or in the user-specified gap direction. The interface is parallel to the element y-z plane.

The normal stiffness, KN, should be based upon the stiffness of the surfaces in contact. See Nonlinear Structural Analysis in the Structural Analysis Guide for guidelines on choosing a value for KN. In some cases (such as initial interference analyses, nonconvergence, or over penetration), it may be useful to change the KN value between load steps or in a restart in order to obtain an accurate, converged solution. The sticking stiffness, KS, represents the stiffness in the tangential direction when elastic Coulomb friction is selected (μ > 0.0 and KEYOPT(1) = 0). The coefficient of friction μ is input as material property MU and is evaluated at the average of the two node temperatures. Stiffnesses may also be computed from the maximum expected force divided by the maximum allowable surface displacement. KS defaults to KN.

The initial gap defines the gap size (if positive) or the displacement interference (if negative). This input is the opposite of that used for CONTAC12 (described in the Feature Archive). If you do not specify the gap direction (by means of real constants NX, NY, and NZ), an interference causes the nodes to separate. The gap size may be input as a real constant (GAP) or automatically calculated from the input node locations (as the distance between node I and node J) if KEYOPT(4) = 1. Interference must be input as a real constant. Stiffness is associated with a zero or negative gap. The initial element status (START) is used to define the "previous" condition of the interface to be used at the start of the first substep. This input is used to override the condition implied by the interference specification and is useful in anticipating the final interface configuration and in reducing the number of iterations required for convergence.

You can specify the gap direction by means of real constants NX, NY, and NZ (the global Cartesian X, Y, and Z components of the gap direction vector). If you do not specify the gap direction, the program will calculate the direction based on the initial positions of the I and J nodes, such that a positive normal displacement (in the element coordinate system) of node J relative to node I tends to open the gap. You should always specify the gap direction if nodes I and J have the same initial coordinates, if the model has an initial interference condition in which the underlying elements' geometry overlaps, or if the initial open gap distance is very small. If the gap is initially geometrically open, the correct normal (NX, NY, NZ) usually points from node I toward node J.

The only material property used is the interface coefficient of friction  μ. A zero value should be used for frictionless surfaces. Temperatures may be specified at the element nodes (for material property evaluation only). The node I temperature T(I) defaults to TUNIF. The node J temperature defaults to T(I).

The force deflection relationships for the interface element can be separated into the normal and tangential (sliding) directions as shown in Figure 52.2: CONTAC52 Force-Deflection Relationship. The element condition at the beginning of the first substep is determined from the START parameter. If the interface is closed and sticking, KN is used in the gap resistance and KS is used for sticking resistance. If the interface is closed but sliding, KN is used in the gap resistance and the constant friction force  μFN is used for the sliding resistance.

In the normal direction, when the normal force (FN) is negative, the interface remains in contact and responds as a linear spring. As the normal force becomes positive, contact is broken and no force is transmitted.

KEYOPT(3) can be used to specify a "weak spring" across an open interface, which is useful for preventing rigid body motion that could occur in a static analysis. The weak spring stiffness is computed by multiplying the normal stiffness KN by a reduction factor. The default reduction factor of 1E-6 can be overridden with real constant REDFACT.

This "weak spring" capability is not analogous to overlaying an actual spring element (such as COMBIN14) with a low stiffness value. The REDFACT capability will not limit gap separation when a tensile force is applied.

In the tangential direction, for FN < 0 and the absolute value of the tangential force (FS) less than μ|FN|, the interface sticks and responds as a linear spring. For FN < 0 and FS = μ|FN|, sliding occurs. If contact is broken, FS = 0.

If KEYOPT(1) = 1, rigid Coulomb friction is selected, KS is not used, and the elastic sticking capability is removed. This option is useful for displacement controlled problems or for certain dynamic problems where sliding dominates.

For analyses involving friction, using NROPT,UNSYM is useful (and, in fact, sometimes required) for problems where the normal and tangential (sliding) motions are strongly coupled, such as in a wedge insertion problem.

A summary of the element input is given in "CONTAC52 Input Summary". A general description of element input is given in Element Input.

CONTAC52 Output Data

The solution output associated with the element is in two forms:

Force-deflection curves are illustrated in Figure 52.2: CONTAC52 Force-Deflection Relationship.

The value of USEP is determined from the normal displacement (u_n) (in the element x-direction) between the interface nodes at the end of a substep, that is: USEP = (u_n)_J - (u_n)_I + GAP. This value is used in determining the normal force, FN. The values represented by UT(Y, Z) are the total translational displacements in the element y and z directions. The maximum value printed for the sliding force, FS, is μ|FN|. Sliding may occur in both the element y and z directions. STAT describes the status of the element at the end of a substep. If STAT = 1, the gap is closed and no sliding occurs. If STAT = 3, the gap is open. A value of STAT = 2 indicates the node J slides relative to node I. For a frictionless surface (μ = 0.0), the converged element status is either STAT = 2 or 3.

The element coordinate system orientation angles  α and β (shown in Figure 52.1: CONTAC52 Geometry) are computed by the program from the node locations. These values are printed as ALPHA and BETA respectively. α ranges from 0° to 360° and  β from -90° to +90°. Elements lying along the Z-axis are assigned values of α = 0°,  β = ± 90°, respectively. Elements lying off the Z-axis have their coordinate system oriented as shown for the general α,  β position. Note, for α = 90°,  β → 90°, the element coordinate system flips 90° about the Z-axis. The value of ANGLE represents the principal angle of the friction force in the element y-z plane. A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

Figure 52.2: CONTAC52 Force-Deflection Relationship

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file jobname.out. The R column indicates the availability of the items in the results file.

In either the O or R columns, “Y” indicates that the item is always available, a letter or number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

Table 52.3: CONTAC52 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table of this manual for more information. The following notation is used in Table 52.3: CONTAC52 Item and Sequence Numbers:

CONTAC52 Assumptions and Restrictions

CONTAC52 Product Restrictions

When used in the product(s) listed below, the stated product-specific restrictions apply to this element in addition to the general assumptions and restrictions given in the previous section.

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