cutcontrol

Mapdl.cutcontrol(lab='', value='', option='', **kwargs)

Controls time-step cutback during a nonlinear solution.

APDL Command: CUTCONTROL

Parameters:

lab

Specifies the criteria for causing a cutback. Valid labels are:

PLSLIMIT

Maximum equivalent plastic strain allowed within a time-step (substep). If the calculated value exceeds the VALUE, the program performs a cutback (bisection). VALUE defaults to 0.15 (15%).

CRPLIMIT

Set values for calculating the maximum equivalent creep ratio allowed within a time step. If the calculated maximum creep ratio exceeds the defined creep ratio limit, the program performs a cutback.

DSPLIMIT

Maximum incremental displacement within the solution field in a time step (substep). If the maximum calculated value exceeds VALUE, the program performs a cutback (bisection). VALUE defaults to 1.0 x 107.

NPOINT

Number of points in a cycle for a second order dynamic equation, used to control automatic time stepping. If the number of solution points per cycle is less than VALUE, the program performs a cutback in time step size. VALUE defaults to 13 for linear analysis, 5 for nonlinear analysis. A larger number of points yields a more accurate solution but also increases the solution run time.

This option works well for linear problems. For nonlinear analyses, other factors such as contact status changes and solution convergence rate can overwrite NPOINT. See Automatic Time Stepping in the Mechanical APDL Theory Reference for more information on automatic time stepping.

NOITERPREDICT

If VALUE is 0 (default), an internal auto time step scheme will predict the number of iterations for nonlinear convergence and perform a cutback earlier than the number of iterations specified by the NEQIT command. This is the recommended option.

If VALUE is 1, the solution will iterate (if nonconvergent) to NEQIT number of iterations before a cutback is invoked. It is sometimes useful for poorly-convergent problems, but rarely needed in general.

Bisection is also controlled by contact status change, plasticity or creep strain limit, and other factors. If any of these factors occur, bisection will still take place, regardless of the NOITERPREDICT setting.

CUTBACKFACTOR

Changes the cutback value for bisection. Default is 0.5. VALUE must be greater than 0.0 and less than 1.0. This option is active only if AUTOTS,ON is set.

value

Numeric value for the specified cutback criterion. For Lab = CRPLIMIT, VALUE is the creep criteria for the creep ratio limit.

option

Type of creep analysis. Valid for Lab = CRPLIMIT only.

IMPRATIO

Set the maximum creep ratio value for implicit creep. The default is 0.0 (i.e., no creep limit control) and any positive value is valid. (See Implicit Creep Procedure in the Structural Analysis Guide for information on how to define implicit creep.)

EXPRATIO

Set the maximum creep ratio value for explicit creep. The default value is 0.1 and any positive value up to 0.25 is allowed. (See Explicit Creep Procedure in the Structural Analysis Guide for information on how to define explicit creep.)

STSLIMIT

Stress threshold for calculating the creep ratio. For integration points with effective stress below this threshold, the creep ratio does not cause cutback. The default value is 0.0 and any positive value is valid.

STNLIMIT

Elastic strain threshold for calculating the creep ratio. For integration points with effective elastic strain below this threshold, the creep ratio does not cause cutback. The default value is 0.0 and any positive value is valid.

Notes

A cutback is a method for automatically reducing the step size when either the solution error is too large or the solution encounters convergence difficulties during a nonlinear analysis.

Should a convergence failure occur, the program reduces the time step interval to a fraction of its previous size and automatically continues the solution from the last successfully converged time step. If the reduced time step again fails to converge, the program again reduces the time step size and proceeds with the solution. This process continues until convergence is achieved or the minimum specified time step value is reached.

For creep analysis, the cutback procedure is similar; the process continues until the minimum specified time step size is reached. However, if the creep ratio limit is exceeded, the program issues a warning but continues the substep until the analysis is complete. In this case, convergence is achieved but the creep ratio criteria is not satisfied.

The CRPLIM command is functionally equivalent to Lab = CRPLIMIT with options IMPRATIO and EXPRATIO