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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.
Kinematic Constraints
In Radioss, a kinematic condition is a nodal constraint applied to a set of nodes.
Rigid Link (/RLINK)
The rigid link option imposes the same velocity on all secondary nodes for one or more directions. Directions are defined to a skew or a global frame, velocity is computed with momentum conservation.

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.
  - 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.
    - Rigid Body (/RBODY)
      A rigid body is defined by a set of secondary nodes and a main node. It can be compared to a part with an infinite stiffness. No relative displacement is allowed between secondary nodes, and the general motion of the rigid body manages the main node.
    - Rigid Wall (/RWALL)
      A rigid wall is a nodal constraint applied to a set of secondary nodes in order to avoid the node penetration to the wall. If contact is detected, then the secondary node acceleration and velocity are modified.
    - Tied Contact (/INTER/TYPE2)
      Interface TYPE2, also called tied interface is a nodal constraint to rigidly connect a set of secondary nodes to a main surface. The secondary nodes forces and moments are transferred to the main nodes, and then secondary nodes are positioned kinematically according to the motion of the main nodes.
    - Cylindrical Joint (/CYL_JOINT)
      A cylindrical joint is like a rigid body, except that one specific direction is defined with the first two secondary nodes. All nodes are free to move along this direction and to rotate around it.
    - Rigid Link (/RLINK)
      The rigid link option imposes the same velocity on all secondary nodes for one or more directions. Directions are defined to a skew or a global frame, velocity is computed with momentum conservation.
    - Multi-Point Constraints (/MPC)
      Gear type joints are more complex than other kinematic joints. They use the Lagrange Multiplier method and are compatible with all other Lagrange Multiplier kinematic conditions and incompatible with all classical kinematic conditions.
    - Methods to Apply Kinematic Conditions
    - Incompatible Kinematic Conditions
      As nodal constraints are based on kinematic conditions applied on nodal DOF, therefore it is not allowed to apply two nodal constraints to the same set of nodes, unless the induced kinematic conditions are perfectly orthogonal (for example: boundary condition in the X-direction and rigid link in the Y-direction).
  - 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.

Rigid Link (/RLINK)

The rigid link option imposes the same velocity on all secondary nodes for one or more directions. Directions are defined to a skew or a global frame, velocity is computed with momentum conservation.

No global moment equilibrium is respected. A rigid link is equivalent to an infinite stiff spring TYPE8.

Note: A rigid link is a Radioss Engine input. A rigid link is alive only during the current restart run. For permanent rigid bodies, the input has to be repeated in each Engine input deck.