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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.
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.
Nodal Time Step
The nodal time step calculates the time step based on the nodal mass and nodal stiffness in the model.

Welcome
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.
- 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.
    - Element Time Step
      The default time step calculation used by Radioss is the element time step.
    - Nodal Time Step
      The nodal time step calculates the time step based on the nodal mass and nodal stiffness in the model.
    - Global Time Step
      The global time step (GTS) method can be used to calculate the time step of a model based on the natural frequency of the model.
    - Contact Interface Time Step
      The contact interface time step is calculated in two different ways. First, based on stiffness and second, based on the velocity of the secondary nodes.
    - Time Step Output from a Model
      The time step of the initial model is output to the Starter output file. Whereas the time step of a running model can be output to the animation files.
    - Time Step Control Methods
      The time step can often be increased using some of these time step control methods.
    - Time Step Scale Factor
      The theoretical stable time step for both elements and nodes is an approximation and may change during the following time increment.
    - General Recommendations
      Every time step control method has advantages and limitations.
  - Finite Elements
  - 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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Nodal Time Step

The nodal time step calculates the time step based on the nodal mass and nodal stiffness in the model.

The nodal time step is calculated after the computation of all the internal forces at each node using:

Δ t_{n o d a l} = \sqrt{\frac{2 m}{k}}

Where,

$m$: Nodal mass
$k$: Equivalent nodal stiffness

The equivalent nodal stiffness is calculated using one half of the eigenvalue from each attached element stiffness matrix. If the node is also involved in contact, the contact stiffness is also included in the equivalent nodal stiffness calculation. The stiffness is calculated during internal force computation. To ensure stability, the simulation's cycle time step is found by multiplying the minimum time step of all the nodes by a scale factor (default is 0.9).

For a high quality finite element mesh, the element time step and nodal time step conditions are nearly identical. Most typical finite element meshes include some elements with poor element quality and for these situations the nodal time step calculation is usually higher than the element time step calculation.

The nodal time step method is applied for the entire model using:

/DT/NODA
$Δ T_{s c a}$ $Δ T_{\min}$

Where,

$Δ T_{s c a}$: Scale factor for the element time step
$Δ T_{\min}$: Not used and is entered as zero