# Implicit Analysis Activation

The keywords required to launch a Radioss Implicit Analysis are defined in the Engine file and begin with /IMPL.

The only exception is /EIG, which is defined in Radioss Starter for running Modal Analysis. Most Radioss output terms have the same meaning for Implicit and Explicit Analysis.

For static problems, a time step is used. This has no physical meaning and only serves to define the increments for loading and displacements; that is the cycle number here will correspond to the increment number.

## Linear Analysis

- Linear Static:
*/IMPL/LINEAR: Static linear computation

/IMPL/SOLVER/n: Linear solver method, where n indicates the type of solver

/IMPL/PRINT/LINEAR/Nprint: Printout frequency for linear resolution

The printout frequency keyword is used mainly for iterative solver (or mixed). When used with a direct solver, the relative residual will be printed.

/IMPL/LINEAR/INTER: Contacts can be taken into account in a Linear Analysis through a simple two-step linear approach: A linear first step that evaluates penetrations and generates interface reaction springs followed by a linear second step.

- Linear Quasi-static:
*/IMPL/QSTAT: Quasi-static Analysis

/IMPL/QSTAT/DTSCAL: Scaling factor of adding inertial stiffness

When /IMPL/QSTAT/DTSCAL is used with /IMPL/LINEAR/INTER, the scaling factor is only applied to the second step.

- Euler Buckling:
*/IMPL/BUCKL: Euler buckling modes

*/IMPL/LINEAR: Static linear computation

It becomes /IMPL/BUCKL/1 and Lanczos solver (BCSLIB-EXT) is used for buckling mode extraction. Buckling from restart or with pre-stresses is also available (/IMPL/BUCKL/2).

Animation output files (*Annn) are special in the case of a Buckling Analysis. Each file contains one buckling mode (where the critical loading is presented by the time value). If a linear computation precedes a Buckling Analysis, then the first two animation files (*A001, *A002) contain results of linear structure computation and the buckling modes are available starting from the third animation file (*A003).

## Nonlinear Analysis

Iterations are needed in an implicit Nonlinear Analysis. The simulation is typically divided into several increments (steps) to manage the nonlinearity that will be represented by the time step control.

A nonlinear solver (Newton-based method) is used to find the balanced forces on the updated geometry (large displacement, large strain) at each step.

- Nonlinear solver (using /IMPL/NONLIN/n)
- Steps (using /IMPL/DT/n, /IMPL/DTINI and /IMPL/DT/STOP)

The basic keywords for Nonlinear Analysis are:

*/IMPL/NONLIN/n: Nonlinear solver method, where, n indicates the type of nonlinear solver

/IMPL/SOLVER/n: Linear solver method, where, n indicates the type of linear solver

/IMPL/PRINT/NONLIN/Nprint: Printout frequency for nonlinear iterations

*/IMPL/DT/n: Time step control, where, n indicates the time step control method

*/IMPL/DTINI: Initial time step

/IMPL/DT/STOP: Min-max time step values

/IMPL/QSTAT: Quasi-static implicit

/IMPL/SPRBACK: Spring-back computation