***automatic_time#

Description#

This command is used to give parameters controlling the time stepping based on the convergence and gross change in material variables. Variables which control the time step may be taken from the FLUX or VINT data members.

Note#

This automatic time-stepping controls the global convergence stepping, which means that if there is a local divergence or violation of a limit variable, the current increment will be thrown away and the global solution time step reduced. One can use the auto_step beahvior modifier to control local-only automatic time stepping (see auto_step. This command depends on the capabilities of the FEA solver used. ABAQUS allows the material to control the next time step to be used, while Cosmos and Ansys only allow the material to sub-cut the time step by 2.

Syntax#

The syntax for the automatic time stepping control is the following:

***automatic_time \(~\,~\,\) [ *security factor ] \(~\,~\,\) [ *divergence div ] \(~\,~\,\) *limit var1  val1 [ … * varN  valN* ]

where the following parameters are used:

factor

real value giving the maximum time increase factor for a well converged time step. The value must be greater than one.

div

dividing factor for a diverging increment. The global solution step will be re-run with a time step smaller than the rejected one by this factor.

var1

a character name of a variable of the problem. This can be from the FLUX, or VINT variables. If the name is the base name of a tensorial or vector set of variables, the limit will be placed on all the components of that variable (e.g. if sig is given all the stress components will be used).

val1

real value for the limit of the last-given variable name. Limit names/values must be given in pairs.

Example#

Here is a small typical use for viscoplasticity type problems. The time step will be limited both in the increment of viscoplastic strain (evcum to changes less than \(0.1\%\) and stress component changes to less that 15 MPa. All components of the stress tensor will be checked to be within the limits.

***automatic_time
  *limit
     evcum   1.e-3
     sig     15.
  *divergence 2.0
  *security   1.2