# /IMPL/NONLIN

Engine Keyword Nonlinear implicit methods.

## Format

/IMPL/NONLIN/N

L_A Itol Toli Tolj Tolk

## Definitions

Field Contents SI Unit Example
N Nonlinear solver method.
= 0
Set to 1.
= 1
Modified Newton method.
= 2
BFGS Quasi-Newton method.

L_A Maximum iteration number of stiffness matrix updates.
= 0
Set to 3, if iterative solver and set to 6 if direct solver.

Itol Termination criteria.
= 0
Set to 2.
= 1
Relative residual in energy.
= 2
Relative residual in force.
= 3
Relative residual in displacement.
=12
Relative residual in energy and in force.
=13
Relative residual in energy and in displacement.
=23
Relative residual in force and in displacement.
=123
Relative residual in force, energy and in displacement.

Toli ... Tolj ... Tolk When (Itol = 1, 2, 3), tolerance for termination.
Toli
= 0 (Default)
Set to 1.0e-3 for energy or displacement (Itol = 1, 3).
Set to 5.0e-3 for force (Itol = 2).

When (Itol = 12, 13, 23) two tolerances for termination.

Toli Tolj
=0.0 0.0 (Default)
Set to 1.0e-3 for energy; to 1.0e-2 for force and 1.0e-2 for displacement.

When (Itol = 123) three tolerances for termination.

Toli Tolj Tolk
=0.0 0.0 0.0 (Default):
Set to 1.0e-3 for energy; to 1.0e-2 for force and 1.0e-2 for displacement.

## Comments

1. For static implicit computation, the loading should be defined as monotonous, increasing time function to better manage the convergence.
2. Order of input tolerance is according to the Itol value with 1: energy; 2: force; and 3: displacement.

Example: Itol = 23, the order is Tolerance in force and Tolerance in displacement.