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User Guide
This manual provides details on the features, functionality, and simulation methods available in Altair Radioss.
Explicit Structural Finite Element Analysis
In this section, available explicit features for different explicit analyses are presented.
Finite Elements

Welcome
What's New
View new features for Radioss 2024.
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.
- Run Radioss
  The Radioss solver can be executed using different methods described here.
- Explicit Structural Finite Element Analysis
  In this section, available explicit features for different explicit analyses are presented.
  - Time Step
    An explicit is solved by calculating results in small time increments or time steps. The size of the time step depends on many factors but is automatically calculated by Radioss.
  - Finite Elements
    - Solid Elements (/PROP/SOLID)
      Solids hexahedron and tetrahedron with linear and quadratic interpolation functions are available in Radioss.
    - Solid-Shell Elements (/PROP/TSHELL)
      The elements HA8, HEPH and BRICK20 can be transformed to solid-shell elements by setting constant the normal stress through the thickness.
    - Shell Elements
    - Beam Elements (/PROP/BEAM, /PROP/INT_BEAM)
      The two beam elements available in Radioss are used on one-dimensional structures and frames. It carries axial loads, shear forces, bending and torsion moments (contrary to the truss that supports only axial loads).
    - Springs
    - Mesh Recommendations
      Mesh recommendations in crash worthiness and in Implicit Analysis are covered.
    - Hourglass Formulations
      Under-integrated elements are very familiar in crash worthiness. In these elements, a reduced number of integration points are used to decrease the computation time. This simplification generates zero energy deformation modes, called hourglass modes.
    - Stress-Strain Computation Options (/PROP)
    - Radioss Coordinate System
  - Modeling Tools
  - Materials
    Different material tests could result in different material mechanic character.
  - Failure
  - Composites
    Composite materials consist of two or more materials combined each other. Most composites consist of two materials, binder (matrix) and reinforcement. Reinforcements come in three forms, particulate, discontinuous fiber, and continuous fiber.
  - Connections
  - Kinematic Constraints
    In Radioss, a kinematic condition is a nodal constraint applied to a set of nodes.
  - Interfaces
    Several interfaces are available in Radioss, this section deals with contact interfaces only. Each interface is distinguished with a type number.
  - Loads
  - Airbag Modeling
    Airbags are modeled as monitored volumes /MONVOL in several different ways.
  - Debugging Guidelines
  - Appendix
- Implicit Structural Finite Element Analysis
- Fluid and Fluid-Structure Simulation
  In this section, fluid and fluid-structure simulation is presented.
- 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.
- Multi-Domain Technique
  The objective of the Multi-Domain technique (also referred to as RAD2RAD) is to optimize the computing performances of large scale Radioss models.
- Design Optimization
  Optimization in Radioss was introduced in version 13.0. It is implemented by invoking the optimization capabilities of OptiStruct and simultaneously using the Radioss solver for analysis.
- Error Message Database
  This section is comprised of error messages in ascending numerical order.
- Engine Error Messages
- Warning Message Database
  This section is comprised of warning messages in ascending numerical order.
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.
User Subroutines
This manual describes the interface between Altair Radioss and user subroutines.

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Finite Elements

Based on mechanics of continuum, a three dimensional solid can be classified in a given category. For a solid with dimensions R, L and t, with t being the thickness, L is the working direction and R is the curvature radius, write:

If L/t > 20 and R/t > 20: Thin shell assumptions are valid (Kirchhoff or Mindlin shell models)
If 10 < L/t < 20 and 10 < R/t < 20: Moderately thick shell (Mindlin model)
If 4 < L/t < 10 and 4 < R/t < 1: Thick shell
If L/t < 4 or R/t < 4: Solids

An adapted finite element mesh is then applied to each case:

Shell theory: 2D mesh of the mid-surface
Thick shell: 3D mesh with constant normal stress assumption
Solids: 3D mesh without constant normal stress assumption

In Radioss all of these formulations are available. For a given mesh, several kinds of formulations may be available based on:

The number of integration points
The stress updating method
Large or small strain assumption
Plasticity options for necking and material law integration
Type of element and hourglass formulation