# Time Step Control Limitations

It is worthwhile to note that the minimum time step computed by Radioss is based on theoretical demonstrations. The use of options side-stepping the problem changes the definition of the physical problem; therefore, it is not recommended (change in mass, in the stiffness, in the energy balance, etc.).

## Comments

- All options with /DEL change the model.
- /DT/NODA/CST and /DT/INTER/CST will add some mass to the model. Increasing the mass will also increase kinetic energy. You have to check to see if these mass and energy increases are small enough and that they do not affect the solution. With option /ANIM/DMASS, relative mass increase can be visualized in ModAnim with contour plot. With /ANIM/DMASS and /ANIM/MASS, total mass increase can be visualized in ModAnim with the balance option. With /TH, program energy and mass, time history can also be plotted.
- /DT/INTER/CST will provide a constant time step. This option will not correct bad interface type 7 behavior. If the time step decreases drastically (factor 10000 or more), without time step control, the option /DT/INTER/CST will generate a near infinite mass and the computation will diverge.
- /DT/BRICK/CST, /DT/SHELL/CST and /DT/SH_3N/CST switch to a small strain option. See Finite Elements for limitations concerning this option.
- To verify if the stability of time discretization is ensured, it is recommended to plot the total energy variation after the computation, to detect any unexpected changes.