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Theory Manual
This manual provides detailed information about the theory used in the Altair Radioss Solver.
Large Displacement Finite Element Analysis Theory Manual
Monitored Volume
An airbag is defined as a monitored volume. A monitored volume is defined as having one or more 3 or 4 node shell property sets.
AIRBAG Type
The airbag simulation used by Radioss uses a special uniform pressure airbag. Hence, regardless of state of inflation or shape, the pressure remains uniform.
Initial Conditions

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What's New
View new features for Radioss 2024.1.
Overview
Radioss^® is a leading explicit finite element solver for crash and impact simulation.
Tutorials
Discover Radioss functionality with interactive tutorials.
User Guide
This manual provides details on the features, functionality, and simulation methods available in Altair Radioss.
Reference Guide
This manual provides a detailed list of all the input keywords and options available in Radioss.
Example Guide
This manual presents examples solved using Radioss with regard to common problem types.
Verification Problems
This manual presents solved verification models.
Frequently Asked Questions
This section provides quick responses to typical and frequently asked questions regarding Radioss.
Theory Manual
This manual provides detailed information about the theory used in the Altair Radioss Solver.
- Large Displacement Finite Element Analysis Theory Manual
  - Introduction
    Nonlinear finite element analyses confront users with many choices. An understanding of the fundamental concepts of nonlinear finite element analysis is necessary if you do not want to use the finite element program as a black box. The purpose of this manual is to describe the numerical methods included in Radioss.
  - Basic Equations
  - Finite Element Formulation
  - Dynamic Analysis
  - Element Library
    Radioss element library contains elements for one, two or three dimensional problems.
  - Kinematic Constraints
    Kinematic constraints are boundary conditions that are placed on nodal velocities. They are mutually exclusive for each degree of freedom (DOF), and there can only be one constraint per DOF.
  - Linear Stability
    The stability of solution concerns the evolution of a process subjected to small perturbations. A process is considered to be stable if small perturbations of initial data result in small changes in the solution. The theory of stability can be applied to a variety of computational problems.
  - Interfaces
    Interfaces solve the contact and impact conditions between two parts of a model.
  - Material Laws
    A large variety of materials is used in the structural components and must be modeled in stress analysis problems. For any kind of these materials a range of constitutive laws is available to describe by a mathematical approach the behavior of the material.
  - Monitored Volume
    An airbag is defined as a monitored volume. A monitored volume is defined as having one or more 3 or 4 node shell property sets.
    - AREA Type
      This type is only a post processing option. It allows plotting of the Time History of volume and area of a closed surface.
    - PRES Type
    - GAS Type
    - AIRBAG Type
      The airbag simulation used by Radioss uses a special uniform pressure airbag. Hence, regardless of state of inflation or shape, the pressure remains uniform.
      - Thermodynamical Equations
      - Energy Variation Within a Time Step
      - Mass Injection
        The amount of mass injected into the airbag needs to be defined with respect to time. This is required as a function.
      - Venting Outgoing Mass Determination
      - Porosity
      - Initial Conditions
      - Jetting Effect
      - Reference Metric
        This option can be used to inflate an airbag instead of simulating the real unfolding which is difficult numerically. A jetting effect can be added in order to set a preferential direction for the unfolding.
      - Tank Experiment
    - COMMU1 Type
  - Static
    Explicit scheme is generally used for time integration in Radioss, in which velocities and displacements are obtained by direct integration of nodal accelerations.
  - Radioss Parallelization
    The performance criterion in the computation was always an essential point in the architectural conception of Radioss. At first, the program has been largely optimized for the vectored super-calculators like CRAY. Then, a first parallel version SMP made possible the exploration of shared memory on processors.
- ALE, CFD and SPH Theory Manual
- Appendix A: Basic Relations of Elasticity
User Subroutines
This manual describes the interface between Altair Radioss and user subroutines.

Initial Conditions

To avoid initial disequilibrium and mathematical discontinuity for zero mass or zero volume, following initial conditions are set:

$P_{i n i} = P_{e x t}$
$T_{i n i}$

If the initial volume is less than

10^{- 4} A^{\frac{3}{2}}

a constant small volume is added to obtain an initial volume:

$V_{i n i} = 10^{- 4} A^{\frac{3}{2}}$

Initial mass energy density is defined from the above values.

There is no need to define an injected mass at time zero.