Best Practices for Contact Modeling - ANSYS Practices for... · 2 © 2014 ANSYS, Inc. May 14, ......
Transcript of Best Practices for Contact Modeling - ANSYS Practices for... · 2 © 2014 ANSYS, Inc. May 14, ......
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1 © 2014 ANSYS, Inc. May 14, 2014 ANSYS Confidential
Best Practices for Contact Modeling
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2 © 2014 ANSYS, Inc. May 14, 2014 ANSYS Confidential
• With typical structural problems, the presence of nonlinear contact can often be the biggest reason for increased solution times.
• Poorly defined contact may lead to unstable contact conditions. These conditions usually mean bad convergence and lost time.
• With properly defined contact conditions and meshes, contact results converge much more quickly and the results tend to be smoother.
Why are these best practices important?
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3 © 2014 ANSYS, Inc. May 14, 2014 ANSYS Confidential
• Section 1: Contact Setup and Verification:
– Contact Generation and Management Tips
• Section 2: Getting Ready for the Solver:
– Mesh Quality & Mesh Sizing
– Advantages of MPC Contact
– Overlapping Contact and Boundary Conditions
– Understanding the Effect of Contact Stiffness
• Section 3: Dealing With Non Convergence
– Diagnostic Tools
– Contact Results Tool
– Procedure for Overcoming Convergence Difficulties
• Section 4: What’s new at R15 in Contact
What this presentation will cover
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4 © 2014 ANSYS, Inc. May 14, 2014 ANSYS Confidential
Section 1: Contact Generation and Management Tips
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5 © 2014 ANSYS, Inc. May 14, 2014 ANSYS Confidential
Automatic Contact Generation • On import, or when requested, Contact Pairs will automatically be
detected and generated in Mechanical. Searching options can be set on the Contact Folder such as:
• Scoping (Search Entire assembly or selected bodies)
• Auto Detection Tolerance.
• Topology types (e.g. Face/Face)
• Grouping Options
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Adding Manual Contact Pairs
• If a required contact has a gap which is outside of the search tolerance, it can be added manually.
Gap > Tolerance
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Using Multiple Contact Folders
Multiple Connection Folders Can Be Used • Allows for different automatic detection settings • Better organize and track large numbers of contact regions
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Contact Search/Select
RMB “Go To” options to find Connections acting on a Geometric Selection
Use Tags To Keep Track of Certain Pairs Of Interest
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Using the Worksheet
The worksheet view is a great way to review all of the contact settings
Column Visibility can be controlled via RMB
Rows can be sorted by clicking on column header
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Body Views • A quick way to spatially identify contact regions especially when
contact occurs interior to model
• Activated via Button on Contact Toolbar
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Shell Contact Normal Directions
To model the self-contact when pinching this shell, the normal direction of the contacts must be correct.
Undesirable Normals
Desired Normals
Use the ‘Shell Face’ properties to flip the normal directions
Contact desired to occur on the underside of the shell
F
F
F F
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Using the Initial Contact Tool
• Use the Initial Contact Tool to quickly learn about contact status before solving.
• Inserted under Connection Folder
• Information reported which pairs are open/closed, how much initial penetration, inactive pairs, etc.
• Color coding to help user identify possible issues
• Contour results such as status and Penetration can be calculated
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Section 2: Getting Ready for the Solver
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• Poor mesh quality in solid elements can cause convergence problems.
• A difficult contact problem may be diverging simply because of the mesh
• Tip: Use aggressive shape checking
for nonlinear contact problems.
Mesh Quality
Poor Mesh Quality on Contact Surface
Better Mesh Quality
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• On curved surfaces, or surfaces which deform to a curve, having sufficient contact elements to closely follow the curvature is essential for smooth results.
• This is especially true for nonlinear contact
• Use similar element sizes for the source and target sides.
Mesh Sizing
Too few elements
Better set elements with similar mesh density
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• MPC contact prevents artificial stiffness when gaps exist between curved surfaces. (Another alternative is to use joints)
• Ideal for shell-solid, shell-shell, and beam-shell contacts
• A caveat is that MPC is the most sensitive contact type to over-constraint, so avoid it when there are other contacts or boundary conditions that overlap
Advantages of MPC Contact
Gap between bonded parts
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• Care should be taken when contact pair overlap with constraint type boundary conditions
– MPC should be avoided and a warning may be issued
– Contact Trimming, Pinball on Remote BC’s, and nodal based BC’s can be used to avoid this situation
Overlapping Contact and Boundary Conditions
Contact shares an edge with a Fixed Support
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Viewing the MPC Equations
After the solution is done MPC equations and other “FE Connections” can be graphically viewed
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• High stiffness leads to reduced penetration and increasing accuracy.
• But higher contact stiffness can also lead to ill-conditioning and divergence.
Contact Stiffness
Poor Convergence; many bisections, 122 iterations
Good Convergence due to stiffness being reduced by user; no bisections, 30 iterations
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• For bulk-dominated problems, start with the default of 1.
• For bending (thin structures) problems, start with 0.01 – 0.1.
• For contacts with difficulty converging, lower the stiffness
• For pretension problems, use a stiffness factor greater than one, because penetration can strongly influence the pretension forces.
• Set “Update Stiffness” to a frequency of “Each Iteration” (Default in WB Mechanical)
• When there is difficulty converging due to high penetration, increase the stiffness.
Contact Stiffness Factor: Tips
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21 © 2014 ANSYS, Inc. May 14, 2014 ANSYS Confidential
Section 3: Dealing With Non Convergence
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• Provides contact information during solution.
• The trends observed can help diagnose problems.
Diagnostic Tool: Contact Result Tracker
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The contact status is a useful sanity check for understanding the global behavior and finding problem areas.
Making Use of Partial Solutions
When a Solution fails to fully converge, reviewing results at the converged substeps can still be post-processed. This can be very helpful to diagnose the issue.
Failed Partial Solution
Converged Steps able to be post-processed
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• Ensure the penetration is small relative to local displacements.
• Ways to reduce penetration:
– Increase the stiffness
– Reduce the penetration tolerance
Contact Results Tool: Penetration
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For a failed solution, requesting Newton-Raphson residuals helps identify possible areas where changes are needed.
If Solution Fails to Converge, NR Residual Force Objects are Populated in the Tree
Diagnostic Tool: NR Residuals
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• Identify the problematic contact region(s) using:
– Contact tracking
– Contact results (e.g. status, penetration)
– Force convergence plots
– NR residuals
• Once identified, possible remedies:
– Check Mesh Quality.
– Make sure model units are on an appropriate scale
– Adjust the Contact Stiffness
– Check for proper initial conditions/pinball.
– Change to nodal detection if the problem is at a corner.
– Reduce the time step size before and during the onset of the divergence.
– If large friction coefficient is defined(>.25) consider using unsymmetric solver
– If immediately diverges based on poor initial contact conditions(not well engaged or rigid body motion) try using aggressive contact stiffness update
– Last resort: Add stabilization(contact or global)
Checklist for Overcoming Convergence Difficulties
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Section 4: What’s new in R15 Contact Technology
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• Produces bolt thread stress profile without meshing the threads
– Contact normals computed internally based on user-specified bolt parameters
– Supports 3D and 2D-axisymmetric models
– All contact types except bonded
– Applicable for standard straight threads
– Small strain & small rotation
• Simple set-up
• Improved efficiency
– In this example:
• True thread solves in 22167 seconds
• Simplified thread solves in 9142 seconds
New: Simplified Bolt Thread Modeling
True thread Virtual Thread
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• Build conventional surface-to-surface asymmetric contact between cylindrical faces.
• Define thread parameters in the details window:
– Mean Pitch diameter
– Thread Pitch distance
– Thread angle
– Starting/ending orientation axis points – defaults to the center of the top and bottom of scoped cylindrical bolt body.
…Bolt Thread Modeling
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• With sufficient mesh refinement, stress profiles match very closely
…Bolt Thread Modeling
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Archard wear model:
Simulates the progressive loss of material from the contact surface.
Assumes rate of volume loss, 𝒘 , due to wear is proportional to the contact surface pressure and relative sliding velocity at the contact surface. Wear is in direction opposite to contact element normal.
Contact nodes are moved to new positions. Contact variables (for example, contact pressure) change. The underlying continuum elements also experience a loss in material (and volume), thus simulating the wear.
MADPL: Contact Surface Wear
𝑲 = wear coefficient
𝑯 = material hardness
𝒎 = pressure exponent
𝒏 = velocity exponent
where:
𝒘 =𝑲
𝑯𝑷𝒎𝒗𝒓𝒆𝒍
𝒏
𝑷 = contact pressure
𝒗𝒓𝒆𝒍 = the relative sliding velocity
𝑷
𝒗𝒓𝒆𝒍
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Defined by TB and TBDATA commands:
• Applicable to nonlinear contact only (frictional, frictionless and rough)
• Asymmetric behavior recommended
• Penalty based formulation recommended for convergence
• Nodal detection necessary
… Contact Surface Wear
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No Wear Archard Wear
… Contact Surface Wear
Sliding block with frictional contact
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Brake Pad Model:
… Contact Surface Wear
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Q and A
Best Practices for Contact Modeling
E-mail : [email protected] Cell : 010-4001-4308 Direct : 02-3441-5057