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 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 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

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 40.2: 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:

COMBIN40 Assumptions and Restrictions

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.

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