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different component names so that the same type of criteria with the same options but different values (NLADAPTIVE) can be applied. The components must be defined before issuing the first SOLVE command.

5.2.1.2. Defining Nonlinear Adaptivity Criteria

During solution, the mesh nonlinear adaptivity criteria are checked for each component to determine if elements need to be split or remeshed. Based on the criteria, the program splits and remeshes solid elements. Surface, target, and contact elements are regenerated when solid elements underneath are split and remeshed. Nonlinear adaptivity criteria remain active until overwritten by a new command.

The criteria can be modified between load steps and upon restart. Defined criteria can also be deleted or listed. For more information, see the NLADAPTIVE command.

5.2.1.3. Defining Criteria-Checking Frequency

This part of the mesh nonlinear adaptivity process defines the frequency at which mesh nonlinear adaptivity criteria are checked during solution.

The frequency is problem-dependent. Too-frequent checking can slow the simulation, but checking too infrequently can cause local deformations to be missed during solution.

For more information, see Frequency of Criteria Checking (p. 145).

5.3. Mesh Nonlinear Adaptivity Criteria

Three nonlinear adaptivity criteria can be defined to control the mesh during nonlinear solutions:

5.3.1.Energy-Based

5.3.2.Position-Based

5.3.3.Contact-Based

5.3.4.Frequency of Criteria Checking

5.3.1. Energy-Based

This criterion is defined for current-technology structural 2-D and 3-D solid elements, and is based on the magnitude of strain energy applied to the element compared to the mean strain energy of components to which the element belongs. During the substep at which mesh nonlinear adaptivity criteria are checked, if the element’s strain energy is greater than or equal to the mean strain energy of its components times the user-defined VAL1 (set via the NLADAPTIVE,,ADD command), then the element is split and remeshed.

This criterion is used to refine the mesh to achieve high-accuracy simulation in regions where a high concentration of stress exists and elements are too large. It can also be used to refine the mesh at certain intervals of substeps if a very small value or 0 is input for VAL1.

5.3.2. Position-Based

This criterion is defined for current-technology structural 2-D and 3-D solid elements. A region is defined in the global Cartesian coordinates. During the substep at which mesh nonlinear adaptivity criteria are checked, if all nodes of an element are within the defined region, the element is split and remeshed.

The criterion is used to refine the mesh in regions where it is difficult to predict which elements of the model will be present, or move in to. It is commonly used in rubber seal analysis where small cavities are filled by the deformation of a compressed seal.

If some elements could potentially move into two different regions, define two different components for the same elements so that one component can be associated with each region.

5.3.3. Contact-Based

This criterion is defined for target elements only (as the component that bears the criterion consists of target elements), but defines how solid elements underneath corresponding contact elements should be split and remeshed. This criterion is defined by the number of contact elements which should be in contact with each target element of a component. During the substep at which mesh nonlinear criteria are checked, if the number of contact elements in contact with the target element is less than the defined value, the solid element underneath the contact element is split and remeshed. Because the solid elements are split, contact elements on the faces of the solid elements are regenerated as well.

This criterion is used to allow a contact region to follow the geometry of targets more accurately. If a target has a large number of small fillets or round angles, or if it forms small cavities, splitting and mesh refinement could make convergence more difficult. The desired number of elements may not be reached when the solution is complete if the checking frequency is set too low. If the program detects that a contact element is detaching from a target surface, the underlying solid elements will no longer be

split and refined, even if the desired number of elements has not been reached. Target elements with this criterion applied are taken as reference elements and therefore cannot be split or refined. Exercise caution when applying contact-based criteria on flexible-flexible boundaries and self-contact boundaries.

Should a particular target element have multiple contact-based criteria defined through different components, the program uses the most strict criterion. For example, if the desired number of elements is defined as 5 in component A and 7 in component B, the program uses 7 as the criterion for a target element present in both components.

5.3.4. Frequency of Criteria Checking

Define the frequency at which criteria for each component are checked during solution via the NLADAPTIVE,,ON command.

The VAL1 parameter controls the checking frequency. If VAL1 is set to a positive value, the program checks the mesh nonlinear adaptivity criteria every VAL1 substeps. If VAL1 is set to a negative value, the program checks VAL1 number of times distributed evenly throughout the load step or defined time interval.

To modify the time range at which criteria checking occurs, use the VAL2 and VAL3 arguments.

Criteria being checked does not guarantee that splitting and remeshing will occur in that substep. A new mesh is generated only if any element meets the criteria assigned to it. When splitting and remeshing occurs, the new mesh is used in the subsequent substep.

Splitting Points

Because nonlinear adaptivity does not control the substep length (solution points), it cannot accurately control the number of splitting points.

When issuing defining frequency checking via an NLADAPTIVE,,ON,,,-n,StartTime,EndTime command, the program may not split between StartTime and EndTime if no solution point falls between them. The program may perform fewer splits, even fewer than n times, if the number of solution points is less or much less than n.