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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
Hyper and Visco-elastic Materials
These materials can be used to represent hyper and visco-elastic materials.
/MAT/LAW134 (VISCOUS_FOAM)
Block Format Keyword This material law represents the open-celled urethane foam used for the ribs of a Euro-SID side impact dummy. Only available for solid.

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What's New
View new features for Radioss 2026.
Radioss Overview
Radioss is a leading explicit finite element solver for crash and impact simulation.
Tutorials
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.
      - /MAT/LAW33 (FOAM_PLAS)
        Block Format Keyword This law models a viscous-elastic foam material with unloading/reloading like plastic behavior. This law is applicable only for solid elements and is typically used to model low density, closed cell polyurethane foams such as impact limiters.
      - /MAT/LAW34 (BOLTZMAN)
        Block Format Keyword This law describes the Boltzmann (visco-elastic) material. This law is applicable only for solid, shell, truss and integrated beam elements and can be used to model polymers and elastomers.
      - /MAT/LAW35 (FOAM_VISC)
        Block Format Keyword This law describes a visco-elastic foam material using Generalized Maxwell-Kelvin-Voigt model where viscosity is based on Navier equations.
      - /MAT/LAW38 (VISC_TAB)
        Block Format Keyword This law describes the visco-elastic foam tabulated material and can only be used with solid elements.
      - /MAT/LAW40 (KELVINMAX)
        Block Format Keyword This law describes the generalized Maxwell-Kelvin material. This law can only be used with solid elements.
      - /MAT/LAW42 (OGDEN)
        Block Format Keyword This keyword defines a hyperelastic, viscous, and incompressible material specified using the Ogden, Mooney-Rivlin material models.
      - /MAT/LAW62 (VISC_HYP)
        Block Format Keyword This law describes the hyper visco-elastic material. This law is compatible with solid and shell elements. In general it is used to model polymers and elastomers.
      - /MAT/LAW69
        Block Format Keyword This law is an extension of /MAT/LAW42 (OGDEN) and defines a hyperelastic and incompressible material specified using the Ogden, Mooney-Rivlin material models.
      - /MAT/LAW70 (FOAM_TAB)
        Block Format Keyword This law describes the visco-elastic foam tabulated material. This material law can be used only with solid elements.
      - /MAT/LAW77
        Block Format Keyword This open cell foam material law is a generalization of LAW70. It accounts for a non-viscous compressible ideal gas flow inside of the foam and its interaction with the foam structure.
      - /MAT/LAW82
        Block Format Keyword This keyword defines the Ogden material. This law is compatible with solid and shell elements. In general it is used to model polymers and elastomers.
      - /MAT/LAW88
        Block Format Keyword This law represents the behavior of a hyperelastic material with strain rate effects. This law is generally used to model incompressible rubbers, polymers, foams, and elastomers. It is defined by a family of stress versus strain curves at different strain rates.
      - /MAT/LAW90
        Block Format Keyword This law describes the visco-elastic foam tabulated material. This material law can be used only with solid elements.
      - /MAT/LAW92
        Block Format Keyword This law describes the Arruda-Boyce material model, which can be used to model hyperelastic behavior.
      - /MAT/LAW94 (YEOH)
        Block Format Keyword This law describes the YEOH material model, which can be used to model incompressible hyperelastic behavior. This law is only compatible with solid elements.
      - /MAT/LAW95 (BERGSTROM_BOYCE)
        Block Format Keyword This law is a constitutive model for predicting the nonlinear time dependency of elastomer like materials.
      - /MAT/LAW100 (MNF)
        Block Format Keyword The multi network framework or MNF is used to model polymers and elastomers with nonlinear viscous behavior.
      - /MAT/LAW101
        Block Format Keyword This law is a time and temperature dependent material model for thermoplastic polymers using a thermodynamic approach with physically-based multiscale internal state variables. This law is only available for solid elements.
      - /MAT/LAW111
        Block Format Keyword Describes the Marlow material model, which can be used to model hyper elastic behavior. This law is only compatible with solid elements.
      - /MAT/LAW134 (VISCOUS_FOAM)
        Block Format Keyword This material law represents the open-celled urethane foam used for the ribs of a Euro-SID side impact dummy. Only available for solid.
    - 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
    - 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.

View All Altair HyperWorks Help

/MAT/LAW134 (VISCOUS_FOAM)

Block Format Keyword This material law represents the open-celled urethane foam used for the ribs of a Euro-SID side impact dummy. Only available for solid.

Format


(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
/MAT/LAW134/`mat_ID`/`unit_ID` or /MAT/VISCOUS_FOAM/`mat_ID`/`unit_ID`
`mat_title`
$ρ_{i}$
`E`₁		`N`₁		$ν$
`E`₂		`V`₂		`N`₂

Definition


Field	Contents	SI Unit Example
`mat_ID`	Material identifier. (Integer, maximum 10 digits)
`unit_ID`	(Optional) Unit identifier. (Integer, maximum 10 digits)
`mat_title`	Material title. (Character, maximum 100 characters)
$ρ_{i}$	Initial density. (Real)	$[\frac{kg}{m^{3}}]$
`E`₁	Young’s modulus. (Real)	$[Pa]$
`N`₁	Exponent in power law for Young’s modulus. Default = 1.0 (Real)
$ν$	Poisson ratio. (Real)
`E`₂	Elastic modulus for viscosity. (Real)	$[Pa]$
`V`₂	Viscous coefficient. (Real)	$[Pa \cdot s]$
`N`₂	Exponent in power law for viscosity. (Real)

Example (Foam)

#RADIOSS STARTER
#---1----|----2----|----3----|----4----|----5----|----6----|----7----|----8----|----9----|---10----|
/UNIT/1
Unit for mat
                  Mg                  mm                   s
#---1----|----2----|----3----|----4----|----5----|----6----|----7----|----8----|----9----|---10----|
/MAT/VISCOUS_FOAM/1
Foam
#              Rho_I
1.00000000000000E-09
#                 E1                  N1                  PR
                 1.0                 4.0                0.05
#                 E2                  V2                  N2
              1000.0               0.015                 0.2
#---1----|----2----|----3----|----4----|----5----|----6----|----7----|----8----|----9----|---10----|
#ENDDATA
/END
#---1----|----2----|----3----|----4----|----5----|----6----|----7----|----8----|----9----|---10----|

Comments

The model consists of nonlinear elastic stiffness and viscous damper in parallel (Figure 1). Elastic stiffness is intended to limit total deformation while the viscosity absorbs energy.
Figure 1. Model scheme
Both $E_{1}^{t}$ and $V_{2}^{t}$ give the nonlinear material behavior with:
$E_{1}^{t} = E_{1} \cdot R^{- N_{1}}$
$V_{2}^{t} = 2 V_{2} \cdot {|1 - R|}^{N_{2}}$
Where, $R = \frac{V}{V_{0}}$ is the volume ratio.

$V$

Actual volume

$V_{0}$

Initial volume