Explicit Spring-Damper

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COMBI165 Element Description

COMBI165 allows you to model simple spring or damper systems, as well as the response of more complicated mechanisms such as the energy absorbers used in passenger vehicle bumpers. These mechanisms are often experimentally characterized in terms of force-displacement curves. This element provides a variety of discrete element formulations that can be used individually or in combination to model complex force-displacement relations.

COMBI165 is a two-node, 1-D element. You cannot define both spring and damper properties for the same element. Separate spring and damper elements are required, but they may use the same nodes (that is, you can overlay two different COMBI165 elements). A COMBI165 element can be attached to any of the other explicit elements.

This element is used in explicit dynamic analyses only. Refer to the LS-DYNA Theoretical Manual for more information.

Figure 165.1:  COMBI165 Geometry

COMBI165 Geometry

Both figures above show two COMBI165 elements (a spring and a damper) attached to the same two nodes.

COMBI165 Input Data

The real constants Kd to TDL are optional and do not need to be defined.

For example, if Kd, the dynamic magnification factor, is nonzero, the forces computed from the spring elements are assumed to be the static values and are scaled by an amplification factor to obtain the dynamic value:

For example, if it is known that a component shows a dynamic crush force at 15m/s equal to 2.5 times the static crush force, use Kd = 1.5 and V0 = 15, where V0 is the test velocity.

Here, clearance (CL) defines a compressive displacement which the spring sustains before beginning the force-displacement relation given by the load curve. If a nonzero clearance is defined, the spring is compressive only.

The deflection limit in compression (CDL) and tension (TDL) is restricted in its application to no more than one spring per node subject to this limit, and to deformable bodies only. For example, in the former case, if three springs are in series, either the center spring or the two end springs may be subject to a limit, but not all three. When the limiting deflection (FD) is reached, momentum conservation calculations are performed and a common acceleration is computed in the appropriate direction. An error termination will occur if a rigid body node is used in a spring definition where compression is limited.

For this element, you can choose from the following materials:

  • Linear Elastic Spring

  • Linear Viscous Damper

  • Elastoplastic Spring

  • Nonlinear Elastic Spring

  • Nonlinear Viscous Damper

  • General Nonlinear Spring

  • Maxwell Viscoelastic Spring

  • Inelastic Tension or Compression-Only Spring

A summary of the element input is given in "COMBI165 Input Summary". A general description of element input is given in Element Input.

COMBI165 Input Summary


I, J

Degrees of Freedom
UX, UY, UZ, VX, VY, VZ, AX, AY, AZ (KEYOPT(1) = 0)

Note:  For explicit dynamic analyses, V(X, Y, Z) refers to nodal velocity, and A(X, Y, Z) refers to nodal acceleration. Although V(X, Y, Z) and A(X, Y, Z) appear as DOFs, they are not actually physical DOFs. However, these quantities are computed as DOF solutions and stored for postprocessing.

Real Constants
Kd - Dynamic magnification factor,
Vo - Test velocity,
CL - Clearance,
FD - Failure deflection,
CDL - Deflection limit (compression),
TDL - Deflection limit (tension)
Material Properties

TB command: See Element Support for Material Models for this element.

MP command: BETD, DMPR

Surface Loads


Body Loads


Special Features

All nonlinear features allowed for an explicit dynamic analysis.


Spring/damper type (translational or torsional):

0 -- 

The material describes a translational spring/damper

1 -- 

The material describes a torsional spring/damper

COMBI165 Output Data

Output data for COMBI165 consists of the following:

Table 165.1:  COMBI165 Item and Sequence Numbers

Output Quantity Name[1] ETABLE and ESOL Command Input
DescriptionItemSeq. No.
MFORX/MMOMXX-component of member force/momentSMISC1
MFORY/MMOMYY-component of member force/momentSMISC2
MFORZ/MMOMZZ-component of member force/momentSMISC3
MFORSUM/MMOMSUMVector sum of X, Y, and Z components of member force/momentSMISC4

  1. You must specify either force or moment via KEYOPT(1). Note that you cannot specify both force and moment.

    MFOR: KEYOPT(1) = 0

    MMOM: KEYOPT(1) = 1

To output the element data in POST1, you must use the ETABLE command. Then, you can use the PRETAB command to print the output data. The RSYS command has no effect when postprocessing output for this element.

In POST26, you can postprocess the element data using the ESOL command only when postprocessing the Jobname.RST file. The element results are not available on the Jobname.HIS file.

COMBI165 Assumptions and Restrictions

  • The time step size calculation is approximated by using the instantaneous stiffness and one-half the nodal mass of the nodes joined by the spring. If the global time step size is controlled by an explicit spring-damper element, instabilities can develop with the default time step size due to the approximations in the step size calculation.

  • When used to interconnect under-integrated elements, the explicit spring-damper can sometimes excite the zero-energy hourglass modes.

  • To ensure that parts are uniquely defined when using COMBI165, specify a unique set of real constants (R), the element type (ET), and the material properties (TB) for each part. Defining a unique material number (MAT) alone is insufficient.

COMBI165 Product Restrictions

There are no product-specific restrictions for this element.

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