Nonlinear Setup

To setup Nonlinear Setup for a Nonlinear Static or Transient Response analysis, right-click on Nonlinear Setup under your Subcase in the tree and select Edit.

Nonlinear Static or Transient Response Analysis

Name: Used to label a nonlinear setup.

ID: Used to identify a nonlinear setup numerically.

Nonlinear Settings:

• Number of Increments: The number of increments for a nonlinear analysis is input here. The solver must break up the total load into this number of increments. This should not be confused with an iteration. The nonlinear solution algorithm may take many iterations to solve an increment.
• Intermediate Output:
  • On: Specifies that intermediate output (output to be saved at each converged increment) be saved.
  • Off: No intermediate output will be saved.
  • All: When a nonlinear analysis bisects the current load increment, the bisected load increment may complete and will therefore save an output file at that bisected increment.
• Arc Length: This is an On / Off toggle that specifies the use of arc length methods used in analyzing snap-through nonlinear problems.
• Advanced Settings: This button brings up the Advanced Settings for nonlinear analysis. This applies to both nonlinear static and nonlinear transient analysis.

Nonlinear Static Parameters: These are advanced overrides and typically do not need to be changed unless a model has trouble converging, and you are comfortable with the changes that are being made.

Nonlinear Static Analysis

• Nonlinear setup:
  • Stiffness update method: Three options, AUTO, SEMI, and ITER.
    • AUTO: The program automatically selects the most efficient strategy based on convergence rates. At each step the number of iterations required to converge is estimated. Stiffness is updated, if (i) the estimated number of iterations to converge exceeds MAXITER or (ii) the solution diverges.
    • SEMI: The program for each load increment (i) performs a single iteration based upon the new load, (ii) updates the stiffness matrix, and (iii) resumes the normal AUTO options.
    • ITER: If the ITER option is selected, the program updates the stiffness matrix at every KSTEP iterations and on convergence if KSTEP ≤ MAXITER. However, if KSTEP > MAXITER, the stiffness matrix is never updated. Note that the Newton-Raphson iteration strategy is obtained by selecting the ITER option and KSTEP = 1, while the Modified Newton-Raphson iteration strategy is obtained by selecting the ITER option and KSTEP = MAXITER.
  • Iterations before stiffness update: Number of iterations before the stiffness matrix is updated.
  • Maximum iterations for an increment: Maximum number of iterations allowed for an increment.
  • Maximum incremental rotation (Degree): Maximum rotation tolerance. If exceeded, the solution will issue a load bisection.
• Termination control:
  • Terminate on displacement value: This value is used to determine displacement based analysis termination.
  • Terminate on displacement of Node: This is the node ID used for displacement based analysis termination.
  • Terminate on displacement component: This is the component direction of displacement, in the output coordinate system, of the node.
• Convergence-Divergence:
  • Convergence criteria and Error tolerances: This is the criteria used in determining the convergence of a particular load increment.
    • Displacement: Designates displacement based convergence criteria.
    • Load: Designates load based convergence criteria.
    • Work: Designates work based convergence criteria.
  • Maximum divergence conditions per iteration: Controls the criteria for a diverged solution.
  • Maximum iterations for unbisecting load increment: The maximum number of iterations for unbisecting a load increment.
  • Maximum line searches for an iteration: Controls the number of line searches used in scaling the displacement in order to minimize energy error. This only works in conjunction with a Line search tolerance.
  • Line search tolerance: Specifies a tolerance for line search procedure. If the absolute value of relative energy is less than this value, then the line search procedure is skipped.
  • Maximum bisections for each increment: Specifies the number maximum load bisections allowed before a fatal error is issued and the solution terminates.
  • Fraction of effective stress: This is a fraction of effective stress used to limit the subincrement size for nonlinear materials.
  • Initial/Minimum/Maximum load increment: It is used in the adaptive load increment/convergence method to define the overall upper and lower bounds on the load increment in the subcase. The initial load increment replaces the value determined using Number of increments. These load increments are not applicable when arc-length methods are specified via the NLPCI Bulk Data entry.

Nonlinear Transient Parameters: These are advanced overrides and typically do not need to be changed unless a model has trouble converging, and you are comfortable with the changes that are being made.

Nonlinear Transient Response Analysis

• Nonlinear setup:
  • Stiffness update method: Three options, AUTO, SEMI, and ITER.
    • AUTO: The program automatically selects the most efficient strategy based on convergence rates. At each step the number of iterations required to converge is estimated. Stiffness is updated, if (i) the estimated number of iterations to converge exceeds MAXITER or (ii) the solution diverges.
    • SEMI: The program for each load increment (i) performs a single iteration based upon the new load, (ii) updates the stiffness matrix, and (iii) resumes the normal AUTO options.
    • ITER: If the ITER option is selected, the program updates the stiffness matrix at every KSTEP iterations and on convergence if KSTEP ≤ MAXITER. However, if KSTEP > MAXITER, the stiffness matrix is never updated. Note that the Newton-Raphson iteration strategy is obtained by selecting the ITER option and KSTEP = 1, while the Modified Newton-Raphson iteration strategy is obtained by selecting the ITER option and KSTEP = MAXITER.
  • Time steps before stiffness update: Number of time steps before the stiffness matrix is updated.
  • Maximum iterations for an increment: Maximum number of iterations allowed for an increment.
  • Maximum incremental rotation (Degree): Maximum rotation tolerance. If exceeded, the solution will issue a load bisection.
• Termination control:
  • Terminate on displacement value: This value is used to determine displacement based analysis termination.
  • Terminate on displacement of Node: This is the node ID used for displacement based analysis termination.
  • Terminate on displacement component: This is the component direction of displacement, in the output coordinate system, of the node.
• Dynamic Options:
  • Skip factor for time step adjustment: This is a time step skip factor used for adaptive time stepping.
  • Steps to obtain dominant period: Used in determining the dominate period of a model for proper automatic adaptive time stepping.
  • Tolerance on displacement to filter time step: If the ratio of velocity and displacement is less than this value, no time step adjustment is performed.
  • Bounds for stepping function: Used in conjunction with steps to determine the adjusted time step.
  • Maximum ratio of incremental time to time step: Used to define the upper and lower bounds for the adjusted time step size.
• Convergence-Divergence:
  • Convergence criteria and Error tolerances: This is the criteria used in determining the convergence of a particular load increment.
    • Displacement: Designates displacement based convergence criteria.
    • Load: Designates load based convergence criteria.
    • Work: Designates work based convergence criteria.
  • Maximum divergence conditions per iteration: Controls the criteria for a diverged solution.
  • Maximum iterations for unbisecting load increment: The maximum number of iterations for unbisecting a load increment.
  • Maximum line searches for an iteration: Controls the number of line searches used in scaling the displacement in order to minimize energy error. This only works in conjunction with a Line search tolerance.
  • Line search tolerance: Specifies a tolerance for line search procedure. If the absolute value of relative energy is less than this value, then the line search procedure is skipped.
  • Maximum bisections for each increment: Specifies the number maximum load bisections allowed before a fatal error is issued and the solution terminates.
  • Fraction of effective stress: This is a faction of effective stress used to limit the subincrement size for nonlinear materials.