This section examines the Nonlinear Controls as they apply to the following analysis types:
Go to a property description:
Newton-Raphson Option
For nonlinear Static Structural and Full Transient Structural analysis types, the Newton-Raphson Option property is available. This property allows you to specify how often the stiffness matrix is updated during the solution process. Newton-Raphson Option property options include:
(default setting)
The option allows the program to select the Newton-Raphson Option setting based on the nonlinearities present in your model. For more information about the additional options, see the Newton-Raphson Option section in the Mechanical APDL Structural Analysis Guide. If you experience convergence difficulties, switching to an solver may aid in Convergence.
Convergence Criterion
When solving nonlinear steady-state, static, or transient analyses, an iterative procedure (equilibrium iterations) is carried out at each substep. Successful solution is indicated when the out-of-balance loads are less than the specified convergence criteria. Criteria appropriate for the analysis type and physics are displayed in this grouping. Convergence controls are "step aware". This means that the setting can be different for each step.
The following convergence criteria properties are available:
Coupled Field Static and Coupled Field Transient:
If Structural physics is enabled: Force Convergence, Moment Convergence, Displacement Convergence, and Rotation Convergence. If Thermal physics is enabled: Heat Convergence and Temperature Convergence. If Electric physics (Charge) is enabled: Voltage Convergence and Charge Convergence. If Electric physics (Conduction) is enabled Voltage Convergence and Current Convergence. Electric analysis: Voltage Convergence and Current Convergence.
Magnetostatic analysis: CSG Convergence and AMP Convergence.
Static Structural analysis and Transient Structural analysis: Force Convergence, Moment Convergence, Displacement Convergence, and Rotation Convergence.
Steady-State Thermal analysis: Heat Convergence and Temperature Convergence.
Thermal-Electric analysis: Heat Convergence, Temperature Convergence, Voltage Convergence, and Current Convergence.
The following convergence controls are available for each of these properties:
Program Controlled (default setting): The application sets the convergence criteria. See the STABILIZE command reference for application defaults settings for Unstable structures as well as Semi-Implicit Solution Controls.
On: You specify that a convergence criterion is activated. Once activated, additional properties become available and include:
Value: This is the reference value that the solver uses to establish convergence. The recommended and program controlled setting, , automatically calculates the value based on external forces, including reactions, or you can input a constant value.
When Temperature Convergence is set to , the Value field provides a drop-down menu with the options or User Input. Selecting User Input displays an Input Value field you use to enter a value.
When any other convergence property is set to , selecting the field enables you to manually enter a value.
Tolerance times Value determines the convergence criterion
Minimum Reference: This is useful for analyses where the external forces tend to zero. This can happen, for example, with free thermal expansion where rigid body motion is prevented. In these cases the larger of Value or Minimum Reference will be used as the reference value.
Note: If you do not want any convergence options to be turned on, then you may try setting the solution controls to off, using a Commands (ADPL) object.
Line Search
Line search can be useful for enhancing convergence, but it can be expensive (especially with plasticity). You might consider setting Line Search on in the following cases:
When your structure is force-loaded (as opposed to displacement-controlled).
If you are analyzing a "flimsy" structure which exhibits increasing stiffness (such as a fishing pole).
If you notice (from the program output messages) oscillatory convergence patterns.
Note: The Line Search control is "step aware" and can be different for each step.
Stabilization
Convergence difficulty due to an unstable problem is usually the result of a large displacement for small load increments. Nonlinear stabilization technique can help achieve convergence. Nonlinear stabilization can be thought of as adding artificial dampers to all of the nodes in the system. Any degree of freedom that tends to be unstable has a large displacement causing a large damping/stabilization force. This force reduces displacements at the degree of freedom so stabilization can be achieved.
There are three Keys for controlling nonlinear stabilization:
(default): The application does not issue any request to the solver to activate stabilization.
: Deactivate stabilization.
: Activate stabilization. The energy dissipation ratio or damping factor remains constant during the load step.
: Activate stabilization. The energy dissipation ratio or damping factor is reduced linearly to zero at the end of the load step from the specified or calculated value.
There are two options for the Method property for stabilization control:
Energy: Use the energy dissipation ratio as the control (default setting).
Damping: Use the damping factor as the control.
When Energy is specified, an Energy Dissipation Ratio must be entered. The energy dissipation ratio is the ratio of work done by stabilization forces to element potential energy. This value is usually a number between 0 and 1. The default value is 1.0e-4.
When Damping is specified, a Damping Factor value needs to be entered. The damping factor is the value that the Mechanical APDL solver uses to calculate stabilization forces for all subsequent substeps. This value is greater than 0.
Note: The Damping Factor value is dependent on the active unit system and may influence the results if unit systems are changed. You may wish to use an initial trial value from a previous run for this entry (such as a run with the Energy Dissipation Ratio as input). See the Controlling the Stabilization Force section of the Mechanical APDL Structural Analysis Guide for additional information.
There are three options for Activation For First Substep control:
No: Stabilization is not activated for the first substep even when it does not converge after the minimal allowed time increment is reached (default setting).
On Nonconvergence: Stabilization is activated for the first substep if it still does not converge after the minimal allowed time increment is reached. Use this option for the first load step only.
Yes: Stabilization is activated for the first substep. Use this option if stabilization was active for the previous load step Key = Constant.
For Stabilization Force Limit, a number between 0 and 1 should be specified. The default value is 0.2. To omit a stabilization force check, set this value to 0.
Refer to Unstable Structures in the Structural Analysis Guide for assistance with using the stabilization options listed above.