LINK180
3D Spar
(or Truss)
LINK180 Element Description
LINK180 is a 3D spar useful in a variety of engineering applications. The element can be used to model trusses, sagging cables, links, springs, and so on. The element is a uniaxial tension-compression element with three degrees of freedom at each node: translations in the nodal x, y, and z directions. Tension-only (cable) and compression-only (gap) options are supported. As in a pin-jointed structure, no bending of the element is considered. Plasticity, creep, rotation, large deflection, and large strain capabilities are included.
By default, LINK180 includes stress-stiffness terms in any analysis that includes large-deflection effects. Elasticity, isotropic hardening plasticity, kinematic hardening plasticity, Hill anisotropic plasticity, Chaboche nonlinear hardening plasticity, and creep are supported. To simulate the tension-/compression-only options, a nonlinear iterative solution approach is necessary. Added mass, hydrodynamic added mass and loading, and buoyant loading are available.
See LINK180 for more information about this element.
LINK180 Input Data
The geometry, node locations, and the coordinate system for
this element are shown in Figure 180.1: LINK180 Geometry. The
element is defined by two nodes, the cross-sectional area (A) input via the SECTYPE and SECDATA commands, added mass per unit length (ADDMAS) input via the SECCONTROL command, and the material properties.
The element x axis is oriented along the length of the element from node I toward node J. If
ocean loading is present, the
global origin is normally at the mean sea level, with the global Z axis pointing away from the
center of the earth; however, the vertical location can be adjusted via
Zmsl (Val6) on the
OCDATA command (following the OCTYPE,BASIC command).
Element loads are described in Element Loading. Temperatures can be input as element body loads at the nodes. The node I temperature T(I) defaults to TUNIF. The node J temperature T(J) defaults to T(I).
By default, the element allows the cross-sectional area to change as a function of axial elongation; therefore, the volume of the element is preserved even after deformation. The default behavior is suitable for elastoplastic applications. (You can also maintain a constant or rigid cross-section via KEYOPT(2) = 1.)
LINK180 offers compression-and-tension, tension-only, and compression-only options (SECCONTROL).
Damping
The damping portion of the element contributes only damping coefficients to the structural
damping matrix. The damping force 
is given by:
where 
is the damping coefficient given by 
, where 
is the velocity calculated in the previous substep.
The second damping coefficient 
is available to produce a nonlinear damping effect characteristic of some
fluid environments. The damping coefficient units are Force *
Time / Length.
Input damping coefficients via
SECCONTROL,,,CV1,CV2.
Ocean Loading
For ocean loading, hydrodynamic added mass and loading, and buoyant loading, are available via the OCDATA and OCTABLE commands.
When ocean loading is applied, the loading is nonlinear (that is, based on the square of the relative velocity between the structure and the water). Accordingly, the full Newton-Raphson option (NROPT,FULL) may be necessary to achieve optimal results. (Full Newton-Raphson is applied automatically in an analysis involving large-deflection effects (NLGEOM,ON).)
Initial State
You can apply an initial stress state to this element (INISTATE). For more information, see Initial State in the Advanced Analysis Guide.
LINK180 Input Summary
For a general description of element input, see Element Input.
- Nodes
I, J
- Degrees of Freedom
UX, UY, UZ
- Material Properties
TB command: See Element Support for Material Models for this element.
MP command: EX, (PRXY or NUXY), ALPX (or CTEX or THSX), DENS, GXY, ALPD, BETD, DMPR, DMPS
- Surface Loads
None
- Body Loads
- Temperatures --
T(I), T(J)
- Special Features
- KEYOPT(2)
Cross-section scaling (applies only when large-deflection effects (NLGEOM,ON) are specified):
- 0 --
Enforce incompressibility; cross section is scaled as a function of axial stretch (default).
- 1 --
Section is assumed to be rigid.
- KEYOPT(12)
Hydrodynamic output (not available in harmonic analyses that include ocean wave effects (HROCEAN)):
- 0 --
None (default)
- 1 --
Additional hydrodynamic printout
LINK180 Output Data
The solution output associated with the element is in two forms:
Nodal displacements included in the overall nodal solution
Additional element output as shown in Table 180.1: LINK180 Element Output Definitions.
Several items are illustrated in Figure 180.2: LINK180 Stress Output. A general description of solution output is given in Solution Output. Element results can be viewed in POST1 via PRESOL,ELEM.
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 180.1: LINK180 Element Output Definitions
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element number | Y | Y |
| NODES | Nodes - I, J | Y | Y |
| MAT | Material number | Y | Y |
| SECID | Section number | Y | - |
| XC, YC, ZC | Center location | Y | 1 |
| TEMP | Temperatures T(I), T(J) | Y | Y |
| AREA | Cross-sectional area | Y | Y |
| FORCE | Member force in the element coordinate system | Y | Y |
| Sxx | Axial stress | Y | Y |
| EPELxx | Axial elastic strain | Y | Y |
| EPTOxx | Total strain | Y | Y |
| EPEQ | Plastic equivalent strain | 2 | 2 |
| Cur.Yld.Flag | Current yield flag | 2 | 2 |
| Plwk | Plastic strain energy density | 2 | 2 |
| Pressure | Hydrostatic pressure | 2 | 2 |
| Creq | Creep equivalent strain | 2 | 2 |
| Crwk_Creep | Creep strain energy density | 2 | 2 |
| EPPLxx | Axial plastic strain | 2 | 2 |
| EPCRxx | Axial creep strain | 2 | 2 |
| EPTHxx | Axial thermal strain | 3 | 3 |
| NL:SEPL | Plastic yield stress | - | 4 |
| NL:SRAT | Plastic yielding (1 = actively yielding, 0 = not yeilding) | - | 4 |
| NL:HPRES | Hydrostatic pressure | - | 4 |
| NL:EPEQ | Accumulated equivalent plastic strain | - | 4 |
| NL:CREQ | Accumulated equivalent creep strain | - | 4 |
| NL:PLWK | Plastic work/volume | - | 4 |
| EXT PRESS | External pressure at integration point | 5 | 5 |
| EFFECTIVE TENS | Effective tension on link | 5 | 5 |
| The following values apply to ocean loading only: [5] | |||
| GLOBAL COORD | Element centroid location | 6 | Y |
| VR, VZ | Radial and vertical fluid particle velocities (VR is always > 0) | 6 | Y |
| AR, AZ | Radial and vertical fluid particle accelerations | 6 | Y |
| PHDYN | Dynamic fluid pressure head | 6 | Y |
| ETA | Wave amplitude over integration point | 6 | Y |
| TFLUID | Fluid temperature (printed if VISC is nonzero) | 6 | Y |
| VISC | Viscosity (output if VISC is nonzero) | 6 | Y |
| REN, RET | Normal and tangential Reynolds numbers (if VISC is nonzero) | 6 | Y |
| CT | Input tangential drag coefficients evaluated at Reynolds numbers | 6 | Y |
| CDY, CDZ | Input normal drag coefficients evaluated at Reynolds numbers | 6 | Y |
| CMY, CMZ | Input inertia coefficients evaluated at Reynolds numbers | 6 | Y |
| URT, URN | Tangential (parallel to element axis) and normal relative velocities | 6 | Y |
| ABURN | Vector sum of normal (URN) velocities | 6 | Y |
| AN | Accelerations normal to element | 6 | Y |
| FX, FY, FZ | Hydrodynamic tangential and normal forces in element coordinates | 6 | Y |
| ARGU | Effective position of wave (radians) | 6 | Y |
Available only at the centroid as a *GET item.
Available only if the element has an appropriate nonlinear material.
Available only if the element temperatures differ from the reference temperature.
Values are given as the average of the hydrodynamic integration points, which are distributed along the wetted portion of the element.
The element printout also includes 'INT, SEC PTS' (which are always '1, Y Z' where Y and Z both have values of 0.0). These values are printed to maintain formatting consistency with the output printouts of the BEAM188, BEAM189, PIPE288, and PIPE289 elements.
Table 180.2: LINK180 Item and Sequence Numbers lists output available through ETABLE using the Sequence Number method. See The General Postprocessor (POST1) and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 180.2: LINK180 Item and Sequence Numbers:
- Name
output quantity as defined in Table 180.1: LINK180 Element Output Definitions
- Item
predetermined Item label for ETABLE and
- E
sequence number for single-valued or constant element data
- I,J
sequence number for data at nodes I and J
Table 180.2: LINK180 Item and Sequence Numbers
| Output Quantity Name | ETABLE and ESOL Command Input | |||
|---|---|---|---|---|
| Item | E | I | J | |
| Sxx | LS | - | 1 | 2 |
| EPELxx | LEPEL | - | 1 | 2 |
| EPTOxx | LEPTO [1] | - | 1 | 2 |
| EPTHxx | LEPTH | - | 1 | 2 |
| EPPLxx | LEPPL | - | 1 | 2 |
| EPCRxx | LEPCR | - | 1 | 2 |
| NL:SEPL | NLIN | 1 | - | - |
| NL:SRAT | NLIN | 2 | - | - |
| NL:HPRES | NLIN | 3 | - | - |
| NL:EPEQ | NLIN | 4 | - | - |
| NL:CREQ | NLIN | 5 | - | - |
| NL:PLWK | NLIN | 6 | - | - |
| FORCE | SMISC | 1 | - | - |
| EXT PRESS [2] | SMISC | 3 | - | - |
| EFFECTIVE TENS [2] | SMISC | 4 | - | - |
| TEMP | LBFE | - | 1 | 2 |
| AREA | NMISC | 29 | - | - |
| The following output quantities are valid for ocean loading only and are averaged values for the element: [3] | ||||
| GLOBAL COORD | NMISC | 1, 2, 3 | -- | -- |
| VR, VZ | NMISC | 4, 5 | -- | -- |
| AR, AZ | NMISC | 6, 7 [4] | -- | -- |
| PHDYN | NMISC | 8 [4] | -- | -- |
| ETA | NMISC | 9 [4] | -- | -- |
| TFLUID | NMISC | 10 | -- | -- |
| VISC | NMISC | 11 | -- | -- |
| REN, RET | NMISC | 12, 13 [5] | -- | -- |
| CT | NMISC | 14 | -- | -- |
| CDY, CDZ | NMISC | 15, 16 | -- | -- |
| CMY, CMZ | NMISC | 17, 18 [4] | -- | -- |
| URT, URN | NMISC | 19, 20, 21 | -- | -- |
| ABURN | NMISC | 22 [4] | -- | -- |
| AN | NMISC | 23, 24 [4] | -- | -- |
| FX, FY, FZ | NMISC | 25, 26, 27 | -- | -- |
| ARGU | NMISC | 28 [4] | -- | -- |
This item is not available via the ESOL command.
External pressure (EXT PRESS) and effective tension (EFFECTIVE TENS) occur at mid-length.
Values are given as the average of the hydrodynamic integration points, which are distributed along the wetted portion of the element.
These quantities are output only if a Reynold's number dependency is used.
LINK180 Assumptions and Restrictions
The spar element assumes a straight bar, axially loaded at its ends, and of uniform properties from end to end.
The length of the spar must be greater than zero, so nodes I and J must not be coincident.
The cross-sectional area must be greater than zero.
The temperature is assumed to vary linearly along the length of the spar.
The displacement shape function implies a uniform stress in the spar.
Stress stiffening is always included in geometrically nonlinear analyses (NLGEOM,ON). Prestress effects can be activated by the PSTRES command.
To simulate the tension-/compression-only options, a nonlinear iterative solution approach is necessary.
When the link works as a rigid constraint, for example in the case of a free swinging pendulum, the rigid cross section option is recommended (KEYOPT(2) = 1).
When the element is used in an ocean environment:
Hydrodynamic output via KEYOPT(12) is not available in harmonic analyses that include ocean wave effects (HROCEAN).
The three-dimensional effect of water pressure on the element is adjusted, as the element has only one direct stress. For more information, see Hydrostatic Loads in the Mechanical APDL Theory Reference.
The output axial force may not be exact when using ocean loading with nonlinear materials.
LINK180 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 Pro —
Birth and death is not available.
Initial state is not available.
Ocean loading is not available.