CUTCONTROL

CUTCONTROL, Lab, VALUE, Option
Controls time-step cutback during a nonlinear solution.

Valid Products: Pro | Premium | Enterprise | PrepPost | Solver | AS add-on

Lab

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

ALIMIT

 — 

Set the maximum edge-flux degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 1025. This cutback trigger criterion is part of the physics limit set.

CONCLIMIT

 — 

Set the maximum concentration degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 1025. This cutback trigger criterion is part of the physics limit set.

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. For the first substep or the rebalance substeps after remeshing in nonlinear mesh adaptivity or rezoning, however, the user-defined maximum plastic-strain limit is ignored.

CUTBACKFACTOR 

 — 

Key to set the cutback factor, a multiplier for reducing the time step interval to the fraction specified in VALUE, where 0 < VALUE < 1 (default is 0.5 if VALUE is not specified.) The option to change the cutback factor is valid only when automatic time stepping is on (AUTOTS,ON), which is the default.

DMGLIMIT 

 — 

For damage models (such as generalized damage and anisotropic damage), the maximum allowable damage increment in a time step. If the calculated value exceeds VALUE, the program performs a cutback (bisection). If VALUE is unspecified, the program does not check the allowable damage increment.

DPPLMT 

 — 

Set the maximum pore-pressure increment allowed within a time step. If the calculated maximum increment exceeds the specified limit, the program performs a cutback. This option has no default and is valid for coupled structural-pore-fluid-diffusion analysis only.

DSPLIMIT

 — 

Set the maximum degree-of-freedom increment allowed within a time step. Considering all degrees of freedom at every node, if the absolute value of the maximum incremental degree-of-freedom solution within in a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 107.This cutback trigger criterion is the sole member of the DSPLIMIT set.

EMFLIMIT

 — 

Set the maximum electromotive force degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 1025. This cutback trigger criterion is part of the physics limit set.

MAGLIMIT

 — 

Set the maximum scalar magnetic potential degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 1025. This cutback trigger criterion is part of the physics limit set.

MDMG 

 — 

For regularized microplane damage models, the maximum allowable microplane homogenized damage increment in a time step. If the calculated value exceeds VALUE, the program performs a cutback (bisection). If VALUE is unspecified, the program does not check the allowable microplane homogenized damage increment.

PLSLIMIT 

 — 

Maximum equivalent plastic strain allowed within a time-step (substep). If the calculated value exceeds VALUE, the program performs a cutback (bisection). Default: VALUE = 0.15 (15 percent)

If CUTCONTROL with Lab = PLSLIMIT is not issued, the minimum time step specified is reached, and the maximum plastic limit calculated from the solution exceeds 15 percent, the program generates a warning and continues the Newton iterations.

If CUTCONTROL with Lab = PLSLIMIT is issued, the minimum time step specified is reached, and the maximum plastic limit calculated exceeds the specified limit, the program generates an error and stops the Newton iterations. For the first substep or the rebalance substeps after remeshing in nonlinear mesh adaptivity or rezoning, however, the user-defined maximum plastic-strain limit is ignored.

PRESLIMIT

 — 

Set the maximum pressure degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 1025. This cutback trigger criterion is part of the physics limit set.

ROTLIMIT

 — 

Set the maximum rotation degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 107. This cutback trigger criterion is part of the physics limit set.

TEMPLIMIT

 — 

Set the maximum temperature degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 1025. This cutback trigger criterion is part of the physics limit set and is also valid for coupled structural-pore-fluid-diffusion-thermal analysis.

ULIMIT

 — 

Set the maximum translation degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 107. This cutback trigger criterion is part of the physics limit set.

VOLTLIMIT

 — 

Set the maximum volt degree-of-freedom increment allowed within a time step. If the absolute value of the calculated increment within a time step exceeds VALUE, the program performs a cutback. If not specified, default VALUE = 1.0 x 1025. This cutback trigger criterion is part of the physics limit set.

VSLIMIT 

 — 

For viscoelastic materials, the maximum equivalent viscous strain increment allowed within a time step. If the calculated value exceeds VALUE (default = 0.01), the program performs a cutback (bisection). If VALUE = 0, the program does not check the equivalent viscous strain increment.

Other Valid Labels:

 — 

NOITERPREDICT 

 — 

If VALUE = 0 (default), the program predicts the number of iterations for nonlinear convergence and performs a cutback earlier than the number of iterations specified (NEQIT). This is the recommended option.

If VALUE = 1, the solution iterates (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 still occurs, regardless of the NOITERPREDICT setting.

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. Default: VALUE = 13 (linear analysis) or 5 (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 Theory Reference for more information about automatic time-stepping.)

VALUE

Numeric value for the specified cutback criterion. For Lab = CRPLIMIT only, 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. Default = 0.0 (no creep limit control). Any positive value is valid.

STSLIMIT  

 — 

Stress threshold for calculating the creep ratio. For integration points with effective stress below this threshold, the creep ratio does not cause cutback. Default = 0.0. 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. Default = 0.0. 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.

If a convergence failure occurs, 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.

A cutback occurs when a trigger criteria is encountered. The magnitude of the time-step reduction is determined by the cutback factor. Both the trigger and the cutback factor can be specified via CUTCONTROL:

  • The trigger criteria can be specified for individual physics as detailed in Lab descriptions above.

  • The cutback factor is 0.5 by default, and can be specified via the CUTBACKFACTOR label described above.

CUTCONTROL can be issued multiple times.

DSPLIMIT Set

DSPLIMIT is the sole label in the DSPLIMIT Set. It can be used to specify the cutback trigger criterion as a limit on the maximum degree-of-freedom increment within a time step, considering all degrees of freedom in an analysis (see DSPLIMIT label description above).

Physics Limit Set

The physics limit set comprises the following labels: ULIMIT, ROTLIMIT, TEMPLIMIT, PRESLIMIT, VOLTLIMIT, EMFLIMIT, MAGLIMIT, ALIMIT, CONCLIMIT. They can be used to specify distinct degree-of-freedom limits as cutback triggers for individual physics in a multiphysics analysis. Whenever one of these distinct limits is encountered, a cutback is triggered.

Degree-of-freedom limits as cutback trigger criteria are specified via CUTCONTROL with labels from either the DSPLIMIT set or the physics limit set. If multiple CUTCONTROL commands include labels from both of these sets, the last command issued determines whether the cutback trigger criteria is set by the DSPLIMIT or the physics limit set (see example command listing snippets below).

Command Default: If no CUTCONTROL commands are issued with labels from the DSPLIMIT set or the physics limit set, the default cutback trigger criterion is DSPLIMIT with VALUE = 1.0 x 107.

Creep and Viscoelastic Analyses

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

CUTCONTROL enables a step size reduction for analyses experiencing convergence difficulties; OPNCONTROL is an analogous but opposite command that increases the step size to speed up converging analyses.

Example Usage

Multiple CUTCONTROL Commands with labels from both the DSPLIMIT and the Physics Limit Sets

When multiple CUTCONTROL commands include labels from both the DSPLIMIT and the physics limit sets, DSPLIMIT is ignored unless Lab = DSPLIMIT on the last CUTCONTROL command issued as illustrated in the examples below.

  • Label on the last CUTCONTROL command issued is from the physics limit set: The cutback trigger criteria are those specified by the physics limit set, and the earlier CUTCONTROL,DSPLIMIT command is ignored.

    CUTCONTROL,TEMPLIMIT,10 
CUTCONTROL,DSPLIMIT,0.7
CUTCONTROL,ULIMIT,0.2   !trigger criteria is specified by physics limit set (and DSPLIMIT is ignored)
!both ULIMIT and TEMPLIMIT contribute to the physics limit set of cutback trigger criteria

  • Lab = DSPLIMIT on the last CUTCONTROL command issued: DSPLIMIT sets the cutback trigger criterion and earlier CUTCONTROL command(s) issued with Lab = labels from the physics limit set are ignored.

    CUTCONTROL,TEMPLIMIT,10
CUTCONTROL,ULIMIT,0.2 
CUTCONTROL,DSPLIMIT,0.7 !trigger criteria is specified by DSPLIMIT (and physics limit set labels are ignored)

Menu Paths

Main Menu>Preprocessor>Loads>Analysis Type>Sol'n Controls>Nonlinear
Main Menu>Preprocessor>Loads>Load Step Opts>Nonlinear>Cutback Control
Main Menu>Solution>Analysis Type>Sol'n Controls>Nonlinear
Main Menu>Solution>Load Step Opts>Nonlinear>Cutback Control