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User Guide
This manual provides details on the features, functionality, and simulation methods available in Altair Radioss.
Implicit Structural Finite Element Analysis
Quasi-static Analysis Notes

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Overview
Radioss^® is a leading explicit finite element solver for crash and impact simulation.
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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.
- Implicit Structural Finite Element Analysis
  - Implicit Features and Compatibility
    Not all explicit features are available for implicit analyses; actual available features are presented in this section.
  - Implicit Analysis Activation
    The keywords required to launch a Radioss Implicit Analysis are defined in the Engine file and begin with /IMPL.
  - Implicit Solvers
  - Quasi-static Analysis Notes
  - Nonlinear Implicit Analysis Notes
  - Output Messages
  - Troubleshooting
  - Appendix
- 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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Quasi-static Analysis Notes

When running a Quasi-static Analysis (/IMPL/QSTAT), the positive definite property of stiffness matrix will be reinforced by including the inertia matrix. The extra stiffness is a function of masses, inertia and the time step. Smaller time steps will add more stiffness. In addition, a scaling factor can be defined for the added matrix through the keyword /IMPL/QSTAT/DTSCAL (this factor is inversely proportional to this added matrix just like time step). For Nonlinear Analysis, this will only modify the convergence speed; whereas for Linear Analysis, time step (one step) should be chosen carefully (neither too small, as this will change the result significantly; nor too big, as this might lead to a non-positive definite matrix).

This option is quite suitable for a model that consists of parts connected only by contact interfaces and has failed with other analysis types. A Linear Quasi-static Analysis can also be used for model checking of high level (even for Explicit Analysis). When using a time step that is not too large, results can always be obtained, whether the model is well constrained or not.

As mentioned in Implicit Analysis Activation, when /IMPL/QSTAT/DTSCAL is used with /IMPL/LINEAR/INTER (two steps), the scaling factor is only applied in the second step. Be careful choosing this scaling factor and the termination time, the correct contact in the first step can be found and minimizes the error (due to added stiffness) on the final step.

A typical example with this methodology is the initial state simulation under gravity of a full car with dummies, in which dummies link the car only by contact. A short stop time is defined, so that a large displacement between the parts does not occur during the contact research on the first step, and a large quasi-static scaling factor is applied for the second step to get the static solution.