ANSYS Mechanical Introduction to Structural Nonlinearities€¦ · · 2015-04-14ANSYS Mechanical Introduction to Structural Nonlinearities . ... Refer also to Chapter 5 of MAPDL

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Appendix A: Mesh Nonlinear Adaptivity

ANSYS Mechanical Introduction to Structural Nonlinearities


Mesh Nonlinear Adaptivity

• Introduction to Mesh Nonlinear Adaptivity

• Understanding the Criterion

─ Energy Based

─ Position Based

─ Contact Based

─ Mesh –Quality Based

• Generating the new Mesh

Refer also to Chapter 5 of MAPDL Advanced Analysis Guide

• Procedure

• Example Demo

• Limitations


Background on Rezoning

Many applications can benefit from the ability to strategically modify a mesh during solution, based on certain criteria to:

• Simulate challenging large deflection/distortion which otherwise cannot be solved

• Improve the accuracy of simulation results.


• Unlike manual rezoning, mesh nonlinear adaptivity is completely automatic, requiring no user input during solution.

Introduction to Mesh Nonlinear Adaptivity

Automatic During Solution


• Supported in MAPDL:

─ NLADAPTIVE

─ NLMESH

• Supported in WB-Mechanical GUI

… Introduction to Mesh Nonlinear Adaptivity


Insert ‘Nonlinear Adaptive Region’ prior to executing the SOLVE:

• Scope the Geometric features to be included in adaptive remeshing

• Define the Criterion which triggers mesh modification

• Define the checking frequency and range of application – Equally Spaced Points

– Specified Recurrence Rate

… Introduction to Mesh Nonlinear Adaptivity


Understanding the Criterion

• To use Nonlinear Adaptivity most effectively, it is very important to understand the Criterion that will trigger mesh modification

─ Energy Based

─ Position Based

─ Mesh –Quality Based (Skewness)

─ Contact Based (available via MAPDL only)


… Understanding the Criterion

Energy-Based Criterion:

During a substep, if an element’s strain energy is greater than or equal to the mean strain energy of components to which the element belongs, times a user-defined factor, then the element is split and remeshed.

• Refines the mesh to achieve high-accuracy simulation in regions where a high concentration of stress exists and elements are too large.

• Applies to current technology 2D elements and 3-D linear tetrahedral elements (SOLID285).



Position-Based Criterion:

During a substep, if all nodes of an element are within a user defined region, the element is split and remeshed.

• Refines the mesh in regions where it is difficult to predict which elements of the model will be present, or move in to. For example, a small cavity filled by the deformation of a compressed seal

• Applies to current technology 2D elements and 3-D linear tetrahedral elements (SOLID285).

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Mesh-Quality-Based Criterion

Mesh quality is defined by element skewness:

Where

– Vel is the volume of the element under calculation

– Vreg is the volume of the standard tetrahedral linear element inscribed in the same sphere as the element under calculation.

– Ideal Shape: Skewness = 0 because Vel = Vreg (standard tetrahedral element)

– Worst shape: Skewness = 1 because Vel=0 (Flat element)

When an element’s skewness is >= the user defined threshold, that element is identified as a seed element to be remeshed.

Only applies to 3-D linear tetrahedral elements (SOLID285).

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… Understanding the Criterion Contact-Based Criterion (MAPDL only) : Number of Contacting Elements

• If the number of contact elements in contact with the target element is less than the defined value, the underlying element is split. Contact elements on the faces of the solid elements are regenerated as well.

• If contact elements are moving away from target, no splitting occurs.

• This criterion is used to allow a contact region to follow the geometry of targets more accurately.

• Component that bears the criterion consists of target elements only

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Contact-Based Criterion : (cont’d)

Contact Surface Wear (MAPDL only) :

During a substep, when the amount of wear at a contact element exceeds a user defined critical value, the mesh is morphed at the interface to improve the quality of the mesh.

• Only applies to contact elements having surface wear, specified via the TB,WEAR command.

• User defines critical ratio of magnitude of wear at the contact element to the average depth of the solid element underlying the contact element.

Wear distorting elements at interface

Adaptive Mesh Morphing improves mesh

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• During splitting, the current elements are divided into elements having a half-edge length.

• When the defined criteria are met, mesh modification occurs either by a combination of splitting followed by morphing or by general remeshing:

Generating the New Mesh

– Some transition layers are created automatically to connect the refined regions to the unrefined regions.

– For 3-D tetrahedral element, some topology changes and morphing occur during splitting to improve the quality of the new mesh.

– For 2-D meshes, morphing is only done after the element-splitting operation.

• Splitting applies to Energy-based, Position-based and Contact-based criterion.

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In WB-Mechanical:

Insert ’Nonlinear Adaptive Region’ in Structural Environment branch:

• Scope the Geometric features to be included in adaptive remeshing

• Define the Criterion which triggers mesh modification

• Define the Checking frequency and range of application – Equally Spaced Points

– Specified Recurrence Rate

WB-Mechanical Procedure- Nonlinear Adaptivity

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…WB-Mechanical Procedure- Nonlinear Adaptivity

For 3D structural analysis, Criterion options available are:

• Energy (Strain Energy)

• Box (Position) • Skewness (Mesh)

For 2D structural analysis Criterion options are limited to:

• Energy (Strain Energy)

• Box (Position)

* Note: Equivalent to ‘NLAD, , , Criterion’ in MAPDL

*

*

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When Criterion is defined as‘skewness’, additional mesh control options become available in the ‘Details Window of Analysis Settings’ Branch

• Refer to NLMESH command for details


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Checking Frequency can be strategically activated/deactivated at user defined time intervals

* Equivalent to ‘NLAD,,ON ,,,1’

*


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After solving, a special column ‘Changed Mesh’ will appear in Tabular Data indicating when Mesh modifications occurred during the solve


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Create User Defined Results (using the PNUMELEM Expression) to view the new elements that have relatively larger element identities than the original element identities.


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All results are automatically saved to one jobname.rst file in working directory.

A separate file is saved for each modified mesh.

• With Energy, Position and Contact based Criterion, these are Binary database files with rdxx extensions.

• With Mesh-Quality-Based (skewness) Criterion, these are ASCII CDB files. – *000x.cdb contains complete mesh

– *000x_gmx.cdb contains only newly created elements


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For each mesh modification, mesh quality matrics stats are recorded in the Solver output comparing previous mesh with new mesh

• Source Column represents previous mesh

• Target Column represents modified mesh

• In general, lower Skewness and Aspect Ratio values for the Target mesh, indicate improved quality

• If these stats indicate a deterioration of the mesh, try adjusting NLMESH options.


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3D Nonlinear Adaptivity Example

Rigid body engaging with flexible nonlinear body with large displacement fails to converge with element distortion errors at about 60% of the total load

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... 3D Nonlinear Adaptivity Example

Set Nonlinear Adaptivity and related Analysis Setting specifications as follows:

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• With these changes, model will solve after several automatic mesh modifications and restarts


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Post Processing procedure is same as for any conventional Structural Analysis with multiple results sets


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Nonlinear Adaptivity Limitations

• Cannot be used in combination with the following features/conditions on the same part:

– Cyclic Symmetry

– Contact Formulations: Normal Lagrange (3D), MPC, and Beam

– Contact Behaviors: Auto Asymmetric

– Point Mass, Beam Connection, Joints, Spring, and Bearing

– Remote Force, Remote Displacement, Moment, Thermal Condition, and Remote Point

– Spatially varying boundary conditions

• Cannot be used in combination with the following boundary conditions:

– Coupling

– Constraint Equation

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… Nonlinear Adaptivity Limitations

• The following materials properties are not supported:

– Cast Iron

– Concrete

– Cohesive Zone

– Damage

– Microplane

– Shape Memory Alloy

– Swelling

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• Mesh nonlinear adaptivity via splitting or refinement, in general cannot repair an already distorted mesh.

– In some cases, it can exacerbate mesh distortion by creating smaller elements.

– To reduce the effect, the code uses built-in morphing and topology repair after splitting; even so, mesh quality improvement is slight.

– Avoid splitting or refinement in highly distorted regions, therefore, as doing so may lead to convergence problems.


Original Mesh After 1st Mesh Refinement

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• Combinations of criteria might be required for applications involving complex geometry under complex loading (i.e. local deformations in buckling or rubber seal problems)

– The contact-based criterion should be used only when solid elements touch target elements and more elements/nodes are necessary to simulate details of contact boundaries and filling.

– The position-based criterion should be used for refining elements moving in to particular regions (for example, small cavities).

– Without a fine enough mesh and a sufficient number of degrees of freedom, the simulation might not accurately predict the behavior of material moving into such regions.

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• Mesh nonlinear adaptivity via the general remeshing method does not support self contact robustly. – Self-contact penetration means that volume intersections

exist in an atomic remeshing region. Remeshing failure occurs if this conditions exists at any time during the nonlinear adaptivity simulation process.

– To prevent self-contact penetration, split the original region

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… Nonlinear Adaptivity Limitations • Unstable Material

– Most nonlinear material models, especially those employing hyperelastic materials, have their own applicable ranges.

– When a deformation is too large or a stress state exceeds the applicable range, the material may become unstable. The instability can manifest itself as a mesh distortion, but nonlinear adaptive region cannot help in such cases.

– While it is sometimes difficult to determine when material is unstable, you can check the strain values, stress states, and convergence patterns. A sudden convergence difficulty could mean that material is no longer stable.

– The program also issues a warning at the beginning of the solution indicating when hyperelastic material could be unstable, although such a warning is very preliminary and applies only to cases involving simple stress states.

•

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• Unstable Structures

– For some geometries and loads, a deformation may cause a "snap-through," or local buckling.


KT=0

A C

B

D

O

– Such behavior can also manifest itself as a mesh distortion, but one that nonlinear adaptive region cannot repair.

– The effect can be detected by closely checking the deformed region or the load-versus- time (displacement) curve.

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• Nonlinear Adaptivity cannot resolve numerical Instabilities

– The constraints can include kinematic constraints such as applied displacements, couplings, and constraint equations, and volumetric constraints introduced by fully incompressible material in mixed u-P elements.

– In many cases, numerical instability is apparent even in the early stages of an analysis.

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• Nonlinear Adaptivity does not supported :

– Distributed ANSYS Solver

– WB-Mechanical Convergence Tools

• When used during a restart, the Nonlinear Adaptive Region object does not support Named Selections if your model contains a mesh change prior to the restart point.

• If analysis failed to converge and you are adding a new Nonlinear Adaptive Region object, it is necessary that the contact object property, Behavior, was set to either Symmetric or Asymmetric for the initial solution that was processed.

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ANSYS Mechanical Introduction to Structural Nonlinearities€¦ · · 2015-04-14ANSYS Mechanical Introduction to Structural Nonlinearities . ... Refer also to Chapter 5 of MAPDL