best practices for contact modeling - ansys practices for... · 2 © 2014 ansys, inc. may 14, ......

1 © 2014 ANSYS, Inc. May 14, 2014 ANSYS Confidential

Best Practices for Contact Modeling


• 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?


• 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


Section 1: Contact Generation and Management Tips


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


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


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


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


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


Body Views • A quick way to spatially identify contact regions especially when

contact occurs interior to model

• Activated via Button on Contact Toolbar


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


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


Section 2: Getting Ready for the Solver


• 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


• 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


• 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


• 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


Viewing the MPC Equations

After the solution is done MPC equations and other “FE Connections” can be graphically viewed


• 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


• 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


Section 3: Dealing With Non Convergence


• Provides contact information during solution.

• The trends observed can help diagnose problems.

Diagnostic Tool: Contact Result Tracker


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


• Ensure the penetration is small relative to local displacements.

• Ways to reduce penetration:

– Increase the stiffness

– Reduce the penetration tolerance

Contact Results Tool: Penetration


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


• 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


Section 4: What’s new in R15 Contact Technology


• 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


• 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


• With sufficient mesh refinement, stress profiles match very closely

…Bolt Thread Modeling


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


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


No Wear Archard Wear


Sliding block with frictional contact


Brake Pad Model:



Q and A

Best Practices for Contact Modeling

E-mail : [email protected] Cell : 010-4001-4308 Direct : 02-3441-5057

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best practices for contact modeling - ansys practices for... · 2 © 2014 ansys, inc. may 14, ......

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