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Radioss Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
Starter Input
This manual provides a list of all the model definition keywords and options available in Radioss.
Materials
Viscosity Option
/VISC/PLAS
Block Format Keyword This is an isotropic elasto-plastic damping option. Applied for solid elements only.

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View new features for Radioss 2026.
Radioss Overview
Radioss is a leading explicit finite element solver for crash and impact simulation.
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Discover Radioss functionality with interactive tutorials.
Radioss User Guide
This manual provides details on the features, functionality, and simulation methods available in Altair Radioss.
Radioss Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
- File Extensions and Formats
  Current Radioss format uses the 12x extension format.
- Single File Input
  This format allows running either Starter or Engine with the same file.
- New Keywords in 2026
  New and modified features in Radioss.
- Starter Input
  This manual provides a list of all the model definition keywords and options available in Radioss.
  - 2D and 3D Analysis Options
  - Starter Keywords Syntax
    Block Format Keyword
  - Smooth Particle Hydrodynamics (SPH)
    The Smooth Particle Hydrodynamics method formulation is used to solve the equations of mechanics, when particles are free from a meshing grid.
  - Computational Fluid Dynamics (CFD)
  - General Controls
    Block Format Keyword In this group, keywords are used to set default value, global parameter, analysis type, input/output print, damping and ALE and CFD treatment for the whole model. For default value, it is still possible to overwrite in each specific keywords.
  - Nodes
    Block Format Keyword In Radioss, two node types are available. They use Cartesian coordinate to describe the position of each node.
  - Elements
    Block Format Keyword
  - Component and Parts Organization
    Block Format Keyword In this group, keywords are used to combine material and property information (/PART), assemble model (/SUBSET) or define a separate model (//SUBMODEL).
  - Interfaces
    Block Format Keyword Interfaces solve the contact and impact conditions between two parts of a model. Several interface types are available in Radioss and use different contact treatments.
  - Materials
    - Material Compatibility
      Block Format Keyword The following tables list the compatibility options for each of the material laws.
    - Elasto-plastic Materials
      These materials can be used to represent elasto-plastic materials.
    - Hyper and Visco-elastic Materials
      These materials can be used to represent hyper and visco-elastic materials.
    - Composite and Fabric Materials
      These materials can be used to represent composite and fabric materials.
    - Multiscale Materials
    - Concrete and Rock Materials
      In Radioss these materials can be used to represent rock or concrete materials.
    - Honeycomb Materials
      These materials can be used to represent honeycomb materials.
    - Connection Materials
      These materials can be used to represent connection materials.
    - Other Materials
      These materials can be used to represent other materials.
    - Multi-Material, Fluid and Explosive Materials
      These materials can be used to represent multi-material, fluid and explosive materials.
    - ALE and CFD Options
      Block Format Keyword ALE and CFD options used to activate ALE formulation or Eulerian formulation for a given material law.
    - Viscosity Option
      - /VISC/LPRONY
        Block Format Keyword This model describes an isotropic visco-elastic Maxwell model that can be used to add visco-elasticity to solid element with total strain formulation (I_smstr=10 or 12).
      - /VISC/PLAS
        Block Format Keyword This is an isotropic elasto-plastic damping option. Applied for solid elements only.
      - /VISC/PRONY
        Block Format Keyword This is an isotropic visco-elastic Maxwell model that can be used to add visco-elasticity to certain shell and solid element material models. The visco-elasticity is input using a Prony series.
    - Leakage Option
    - Thermal Option
    - Equation of State
      Used by Radioss to compute the hydrodynamic pressure.
    - Failure Models
      The supported failure models in Radioss.
  - Properties
    Block Format Keyword
  - Monitored Volumes (Airbags)
    Block Format Keyword Monitored volumes can be used to model an airbag, tire, tank, or any closed volume.
  - Constraints
    Block Format Keyword Radioss supports several different kinematic constraints, which are mainly used to impose acceleration, velocity, displacement or temperature in structure or constraint the moving of structure. They are mutually exclusive for each degree-of-freedom (DOF). Two kinematic conditions applied to the same node may be incompatible.
  - Load Cases
    Block Format Keyword In Radioss the following load cases are available. Stress/strain as initial state could be considered by modeling, as well as pressure, gravity, and thermal load.
  - Tools
    Block Format Keyword In this group, tools like transformation, frames, skews, and curve are introduced.
  - Groups(Sets)
    Block Format Keyword Keywords in this group are used to define how to select a group of node, part, elements, lines, or surfaces.
  - Adaptive Meshings
    Block Format Keyword Adaptive Meshing is used in metal forming to divide the element to better describe the geometry. /ADMESH/GLOBAL and /ADMESH/SET are not available for SPMD computation.
  - Output Database
    Block Format Keyword Time histories output for different element groups, section output, or gauge output are described in this group.
- Engine Input
  This manual provides a list of all the solution definition keywords and options available in Radioss.
- LS-DYNA Input
  This manual provides a list of LS-DYNA input files available in Radioss.
- Multi-Domain
  This manual contains the description of the keywords for the Radioss Multi-Domain.
- Obsolete Keywords
  Keywords that are still supported, but no longer maintained (*) are considered obsolete.
- Other Files
- Definition
Radioss Example Guide
This manual presents examples solved using Radioss with regard to common problem types.
Radioss Verification Problems
This manual presents solved verification models.
Radioss Frequently Asked Questions
This section provides quick responses to typical and frequently asked questions regarding Radioss.
Radioss Theory Manual
This manual provides detailed information about the theory used in the Altair Radioss Solver.
Radioss User Subroutines
This manual describes the interface between Altair Radioss and user subroutines.

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/VISC/PLAS

Block Format Keyword This is an isotropic elasto-plastic damping option. Applied for solid elements only.

Format


(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
/VISC/PLAS/`mat_ID`/`unit_ID`
$G$		$σ_{y}$

Definition


Field	Contents	SI Unit Example
`mat_ID`	Material identifier which refers to the viscosity card. (Integer, maximum 10 digits)
`unit_ID`	(Optional) Unit identifier. (Integer, maximum 10 digits)
$G$	Elastic shear modulus for the elasto-plastic damping. (Real)	$[Pa]$
$σ_{y}$	Yield stress for elasto-plastic damping. (Real)	$[Pa]$

Comments

Elasto-plastic damping is based on a spring and slider in series for solid elements only.
Figure 1.
Elastic-plastic stress is added to stress tensors coming from the material law. This stress is deviatoric and obtained using shear modulus $G$ and yield stress $σ_{y}$ . The trial stress is computed incrementally as:
${\bar{σ}}_{n + 1}^{d} = σ_{n}^{d} + 2 G \cdot Δ ε$
Where,

$Δ ε$

Strain increment.

$σ^{d}$

Deviatoric stress.

This trial stress is scaled using radial projection when the effective von Mises stress $σ_{e f f}$ is bigger than the yield stress ( $σ_{y}$ ) as follows:
$σ_{n + 1}^{d} = {\bar{σ}}_{n + 1}^{d} \cdot \min (1, \frac{σ_{y}}{σ_{e f f}})$
It can be used to apply additional damping for rubbers and foams to avoid numerical instability.
High elasto-plastic damping may artificially stiffen material.