The element has large displacement capability for which there can be two or three degrees of freedom at each node.
See Section 14.39 of the ANSYS Theory Reference for more details about this element. The element has no mass or thermal capacitance. These may be added by using the appropriate elements (see MASS21 and MASS71). A bilinear force-deflection element with damping and gaps is also available (COMBIN40).
Figure 4.39-1 COMBIN39 Nonlinear Spring
The force-deflection curve should be input such that deflections are increasing from the third (compression) to the first (tension) quadrants. Adjacent deflections should not be nearer than 1E-7 times total input deflection range. The last input deflection must be positive. Segments tending towards vertical should be avoided. If the force-deflection curve is exceeded, the last defined slope is maintained, and the status remains equal to the last segment number. If the compressive region of the force-deflection curve is explicitly defined (and not reflected), then at least one point should also be at the origin (0,0) and one point in the first (tension) quadrant. If KEYOPT(2)=1 (no compressive resistance), the force-deflection curve should not extend into the third quadrant. Note that this tension-only behavior can cause convergence difficulties similar to those that can be experienced by contact elements. See Chapter 8 of the ANSYS Structural Analysis Guide, as well as various contact element descriptions, for guidelines on overcoming convergence difficulties. Note that the number of points defining the loading curve can be effectively doubled by using the reflective option.
Slopes of segments may be either positive or negative, except that the slopes at the origin must be positive and, if KEYOPT(1)=1, slopes at the ends may not be negative. Also, if KEYOPT(1)=1, force-deflection points may not be defined in the second or fourth quadrants and the slope of any segment may not be greater than the slope of the segment at the origin in that quadrant.
The KEYOPT(1) option allows either unloading along the same loading curve or unloading along the line parallel to the slope at the origin of the curve. This second option allows modeling of hysteretic effects. As illustrated in Figure 4.39-2, the KEYOPT(2) option provides several loading curve capabilities.
The KEYOPT(3) option selects one degree of freedom. This may be a translation, a rotation, a pressure or a temperature.
Alternately, the element may have more than one type of degree of freedom (KEYOPT(4)>0). The two nodes defining the element should not be coincident, since the load direction is collinear with the line joining the nodes. The longitudinal option (KEYOPT(4)=1 or 3) creates a uniaxial tension-compression element with two or three translational degrees of freedom at each node. No bending or torsion is considered. The torsional option (KEYOPT(4)=2) creates a purely rotational element with three rotational degrees of freedom at each node. No bending or axial loads are considered. The stress stiffening capability is applicable when forces are applied, but not when torsional loads are applied.
The element has large displacement capability with two or three degrees of freedom for each node when you use KEYOPT(4)=1 or 3 in combination with NLGEOM,ON.
The element has only one degree of freedom selected with the KEYOPT(3) option. The KEYOPT(1) option allows either unloading along the same loading curve or unloading along the line parallel to the slope at the origin of the curve. As illustrated in Figure 4.39-2, the KEYOPT(2) option provides several loading curve capabilities.
A summary of the element input is given in Table 4.39-1. A general description of element input is given in Section 2.1.
Table 4.39-1 COMBIN39 Input Summary
| Element Name
|
COMBIN39
|
| Nodes
|
I, J
|
| Degrees of Freedom
|
UX, UY, UZ, ROTX, ROTY, ROTZ, PRES, or TEMP. Make 1-D
choices with KEYOPT(3). Make limited 2- or 3-D choices with
KEYOPT(4).
|
| Real Constants
|
D1, F1, D2, F2, D3, F3,D4, F4, ...D20, F20
|
| Material Properties
|
None
|
| Surface Loads
|
None
|
| Body Loads
|
None
|
| Special Features
|
Nonlinear, Stress stiffening, Large displacement
|
| KEYOPT(1)
|
0 - Unload along same loading curve 1 - Unload along line parallel to slope at origin of loading curve
|
| KEYOPT(2)
|
0 - Compressive loading follows defined compressive curve (or
reflected tensile curve if not defined) 1 - Element offers no resistance to compressive loading 2 - Loading initially follows tensile curve then follows compressive curve after buckling (zero or negative stiffness)
|
| KEYOPT(3)
|
(KEYOPT(4) overrides KEYOPT(3)) 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 - PRES 8 - TEMP
|
| KEYOPT(4)
|
0 - Use any KEYOPT(3) option 1 - 3-D longitudinal element (UX, UY and UZ) 2 - 3-D torsional element (ROTX, ROTY and ROTZ) 3 - 2-D longitudinal element. (UX and UY) Element must lie in an X-Y plane
|
| KEYOPT(6)
|
0 - Basic element printout 1 - Also print force-deflection table for each element (only at first iteration of problem)
|
Figure 4.39-2 COMBIN39 Force Deflection Curves for KEYOPT(1) and KEYOPT(2)
The following notation is used in Table 4.39-2:
A colon (:) in the Name column indicates the item can be accessed by the Component Name method [ETABLE, ESOL] (see Section 2.2.2). The O and R columns indicate the availability of the items in the file Jobname.OUT (O) or in the results file (R), a Y indicates that the item is always available, a number refers to a table footnote which describes when the item is conditionally available, and a - indicates that the item is not available.
Table 4.39-2 COMBIN39 Element Output Definitions
| Name
|
Definition
|
O
|
R
|
| EL
|
Element number
|
Y | Y |
| NODES
|
Nodes - I, J
|
Y | Y |
| CENT: X, Y, Z
|
Center location XC, YC, ZC
|
- | Y |
| UORIG
|
Origin shift upon reversed loading
|
1 | 1 |
| FORCE
|
Force in element
|
Y | Y |
| STRETCH
|
Relative displacement (includes origin shift)
|
Y | Y |
| STAT
|
Status at end of this time step
|
2 | 2 |
| OLDST
|
Same as STAT except status assumed at beginning of this time step
|
2 | 2 |
| UI
|
Displacement of node I
|
Y | Y |
| UJ
|
Displacement of node J
|
Y | Y |
| CRUSH
|
Status of the force deflection curve after buckling
|
3 | - |
| SLOPE
|
Current slope
|
Y | - |
2. If the value of STAT is:
0 - Indicates nonconservative unloading
1-20 - Curve segment number at end of time step
99 - Beyond last segment (last segment is extrapolated) (negative STAT values
indicate compressive segments)
3. If KEYOPT(2)=2 and if the value of CRUSH is:
0 - Use defined tensile curve
1 - Use reflected compressive curve in tension (element has been compressed)
Table 4.39-3 lists output available through the ETABLE command using the Sequence Number method. See Chapter 5 of the ANSYS Basic Analysis Procedures Guide and Section 2.2.2.2 of this manual for more information. The following notation is used in Table 4.39-3:
| Name
|
Item
|
E
|
| FORCE
|
SMISC
|
1 |
| STRETCH
|
NMISC
|
1 |
| UI
|
NMISC
|
2 |
| UJ
|
NMISC
|
3 |
| UORIG
|
NMISC
|
4 |
| STAT
|
NMISC
|
5 |
| OLDST
|
NMISC
|
6 |
If you specify KEYOPT(4)
0, the element has two or three displacement
degrees of freedom per node. Nodes I and J should not be coincident, since
the line joining the nodes defines the direction of the force.
The element is nonlinear and requires an iterative solution. The nonlinear behavior of the element operates only in the static and nonlinear transient dynamic analyses. As with most nonlinear elements, loading and unloading should occur gradually. When the element is also nonconservative, loads should be applied along the actual load history path and in the proper sequence. The element may not be deactivated with the EKILL command. Also, the real constants for this element are not allowed to be changed from their initial values.
Whenever the force that the element carries changes sign, UORIG is reset, and the origin of the force-deflection curve effectively shifts over to the point where the force changed sign. If KEYOPT(2)=1 and the force tends to become negative, the element "breaks" and no force is transmitted until the force tends to become positive again. When KEYOPT(1)=1, the element is both nonlinear and nonconservative. In a thermal analysis, the temperature or pressure degree of freedom acts in a manner analogous to the displacement.
8