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COMBIN40 Element Description

COMBIN40 is a combination of a spring-slider and damper in parallel, coupled to a gap in series. A mass can be associated with one or both nodal points. The element has one degree of freedom at each node, either a nodal translation, rotation, pressure, or temperature. The mass, springs, slider, damper, and/or the gap may be removed from the element. The element may be used in any analysis. See COMBIN40 in the Mechanical APDL Theory Reference for more details about this element. Other elements having damper, slider, or gap capabilities are COMBIN14, MATRIX27, COMBIN37, and COMBIN39.

Figure 40.1:  COMBIN40 Geometry

COMBIN40 Geometry

COMBIN40 Input Data

The combination element is shown in Figure 40.1: COMBIN40 Geometry. The element is defined by two nodes, two spring constants K1 and K2 (Force/Length), a damping coefficient C (Force*Time/Length), a mass M (Force*Time2/Length), a gap size GAP (Length), and a limiting sliding force FSLIDE (Force). (Units listed here apply only to KEYOPT(3) = 0, 1, 2, or 3.)

If the element is used in an axisymmetric analysis, these values (except GAP) should be on a full 360° basis. A spring constant of 0.0 (for either K1 or K2, but not both) or a damping coefficient of 0.0 will remove these capabilities from the element. The mass, if any, may be applied at node I or node J or it may be equally distributed between the nodes.

The gap size is defined by the fourth element real constant. If positive, a gap of this size exists. If negative, an initial interference of this amount exists. If GAP = 0.0, the gap capability is removed from the element. The FSLIDE value represents the absolute value of the spring force that must be exceeded before sliding occurs. If FSLIDE is 0.0, the sliding capability of the element is removed, that is, a rigid connection is assumed.

A "breakaway" feature is available to allow the element stiffness (K1) to drop to zero once a limiting force |FSLIDE| has been reached. The limit is input as -|FSLIDE| and is applicable to both tensile breaking and compressive crushing. A "lockup" feature may be selected with KEYOPT(1). This feature removes the gap opening capability once the gap has closed.

The force-deflection relationship for the combination element is as shown in Figure 40.2: COMBIN40 Behavior (for no damping). If the initial gap is identically zero, the element responds as a spring-damper-slider element having both tension and compression capability. If the gap is not initially zero, the element responds as follows: when the spring force (F1+F2) is negative (compression), the gap remains closed and the element responds as a spring-damper parallel combination. As the spring force (F1) increases beyond the FSLIDE value, the element slides and the F1 component of the spring force remains constant. If FSLIDE is input with a negative sign, the stiffness drops to zero and the element moves with no resisting F1 spring force. If the spring force becomes positive (tension), the gap opens and no force is transmitted. In a thermal analysis, the temperature or pressure degree of freedom acts in a manner analogous to the displacement.

The element has only the degrees of freedom selected with KEYOPT(3). The KEYOPT(3) = 7 and 8 options (pressure and temperature DOFs) allow the element to be used in a thermal analysis (with thermal equivalent real constants).

A summary of the element input is given in "COMBIN40 Input Summary". A general description of element input is given in Element Input.

COMBIN40 Input Summary


I, J

Degrees of Freedom

UX, UY, UZ, ROTX, ROTY, ROTZ, PRES, or TEMP (depending on KEYOPT(3) below)

Real Constants

Units for real constants will depend on the KEYOPT(3) setting.

K1 - Spring constant
C - Damping coefficient
M - Mass
GAP - Gap size
FSLIDE - Limiting sliding force
K2 - Spring constant (par to slide)

Note:  If GAP is exactly zero, the interface cannot open. If GAP is negative, there is an initial interference. If FSLIDE is exactly zero, the sliding capability is removed. If FSLIDE is negative, the "breakaway" feature is used.

Material Properties

MP command: ALPD, BETD, DMPR

Surface Loads


Body Loads


Special Features
Adaptive descent
Nonlinearity (unless both GAP and FSLIDE equal zero)

Gap behavior:

0 -- 

Standard gap capability

1 -- 

Gap remains closed after initial contact ("lockup")


Element degrees of freedom:

0, 1 -- 

UX (Displacement along nodal X axes)

2 -- 

UY (Displacement along nodal Y axes)

3 -- 

UZ (Displacement along nodal Z axes)

4 -- 

ROTX (Rotation about nodal X axes)

5 -- 

ROTY (Rotation about nodal Y axes)

6 -- 

ROTZ (Rotation about nodal Z axes)

7 -- 


8 -- 



Element output:

0 -- 

Produce element printout for all status conditions

1 -- 

Suppress element printout if gap is open (STAT = 3)


Mass location:

0 -- 

Mass at node I

1 -- 

Mass equally distributed between nodes I and J

2 -- 

Mass at node J


Element level time increment control:

0 -- 

No control

1 -- 

Predictions are made to achieve a reasonable time (or load) increment

COMBIN40 Output Data

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

Several items are illustrated in Figure 40.2: COMBIN40 Behavior. The displacement direction corresponds to the nodal coordinate direction selected with KEYOPT(3). The value STR is the spring displacement at the end of this substep, STR = U(J)-U(I)+GAP-SLIDE. This value is used in determining the spring force. For an axisymmetric analysis, the element forces are expressed on a full 360° basis. The value SLIDE is the accumulated amount of sliding at the end of this substep relative to the starting location.

STAT describes the status of the element at the end of this substep for use in the next substep. If STAT = 1, the gap is closed and no sliding occurs. If STAT = 3, the gap is open. If STAT = 3 at the end of a substep, an element stiffness of zero is being used. A value of STAT = +2 indicates that node J moves to the right of node I. STAT = -2 indicates a negative slide. A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

Figure 40.2:  COMBIN40 Behavior

COMBIN40 Behavior

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 number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

Table 40.1:  COMBIN40 Element Output Definitions

ELElement NumberYY
XC, YC, ZCLocation where results are reportedY2
SLIDEAmount of slidingYY
F1Force in spring 1YY
STR1Relative displacement of spring 1YY
STATElement status11
OLDSTSTAT value of the previous time step11
UIDisplacement of node IYY
UJDisplacement of node JYY
F2Force in spring 2YY
STR2Relative displacement of spring 2YY

  1. If the value of STAT is:

    1 - Gap closed (no sliding)

    2 - Sliding right (node J moving to right of node I)

    -2 - Sliding left (node J moving to left of node I)

    3 - Gap open

  2. Available only at centroid as a *GET item.

Table 40.2: COMBIN40 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 in this reference for more information. The following notation is used in Table 40.2: COMBIN40 Item and Sequence Numbers:


output quantity as defined in the Table 40.1: COMBIN40 Element Output Definitions


predetermined Item label for ETABLE command


sequence number for single-valued or constant element data

Table 40.2:  COMBIN40 Item and Sequence Numbers

Output Quantity NameETABLE and ESOL Command Input

COMBIN40 Assumptions and Restrictions

  • The element has only one degree of freedom per node which is specified in the nodal coordinate system (see Elements That Operate in the Nodal Coordinate System).

  • The element assumes only a 1-D action.

  • Nodes I and J may be anywhere in space (preferably coincident).

  • The element is defined such that a positive displacement of node J relative to node I tends to open the gap. If, for a given set of conditions, nodes I and J are interchanged, the gap element acts as a hook element, i.e., the gap closes as the nodes separate.

  • The real constants for this element can not be changed from their initial values.

  • The element can not be deactivated with the EKILL command.

  • The nonlinear options of the element operate only in static and nonlinear transient dynamic (TRNOPT,FULL) analyses.

  • If used in other analysis types, the element maintains its initial status throughout the analysis.

  • A 0.0 value for GAP or FSLIDE removes the gap or sliding capability, respectively, from the element.

  • The mass, if any, is 1-D.

  • The element requires an iterative solution if GAP and/or FSLIDE are nonzero.

  • A stiffness (K1 or K2) must be defined if the gap capability is used. Unreasonably high stiffness values should be avoided.

  • The rate of convergence may decrease as the stiffness increases. If FSLIDE is not equal to zero, the element is nonconservative as well as nonlinear. Nonconservative elements require that the load be applied very gradually, along the actual load history path, and in the proper sequence (if multiple loadings exist).

  • Only the lumped mass matrix is available.

COMBIN40 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.

ANSYS Mechanical Premium 

  • The PRES DOF (KEYOPT(3) = 7) is not available.

Release 18.2 - © ANSYS, Inc. All rights reserved.