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1 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
Workbench Mechanical Contact Best Practices
John Lin
2 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• Section 1: Contact Setup and Verification:
– Contact Management Tips for Large Models
• Section 2: Getting Ready for the Solver:
– Mesh Quality & Mesh Sizing
– Setting the Contact Formulation
– Advantages of MPC Contact
– Initial Gaps and Rigid Body Motion
• Section 3: Dealing With Non Convergence
– Diagnostic Tools
– Contact Results Tool
– Procedure for Overcoming Convergence Difficulties
What this presentation will cover
3 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
Section 1: Contact Management Tips for Large Models
4 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
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
5 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
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
6 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
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
7 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
Using the Initial Contact Tool
• Use the Initial Contact Tool to quickly learn about contact
status before solving.
• Color coding to help user identify possible issues
• Contour results such as status and Penetration can
be calculated
8 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
Section 2: Getting Ready for the Solver
9 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• 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
10 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• 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
11 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• Augmented Lagrange (Default): Suitable
for most problems.
• Pure Penalty:
– Contact occurring only on Edge or Corner
• MPC (Multi-Point Constraint): Ideal for all
linear contact when there is no over-
constraint
• Normal Lagrange:
– Highest accuracy
– Contact with material nonlinearities
– Between shells or thin layers
– Interference fit
– Large Sliding
Understanding Contact Formulation
12 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• MPC contact prevents artificial
stiffness when gaps exist between
curved surfaces.
• 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
13 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
Viewing the MPC Equations
After the solution is done
MPC equations and other
“FE Connections” can be
graphically viewed
14 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
Initial Gaps and Rigid Body Motion
Mathematical adjustment to close
gap causes rigid region to existNo Gap
When nonlinear contact is present, small gaps that are initial open can lead to rigid
body motion. Changing the interface treatment to “Adjust to Touch” or using
Contact Stabilization can be effective means to handle this situation
15 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
Section 3: Dealing With Non Convergence
16 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
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
17 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• 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
18 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• 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
19 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• Provides contact information
during solution.
• The trends observed can help
diagnose problems.
• For instance, a decreasing
number of contact points
indicate a loss of contact
Diagnostic Tool: Contact Result Tracker
20 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
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 sub-steps can still be post-processed. This can be very helpful
to diagnose the issue.
Failed Partial
Solution
Converged Steps
able to be post-
processed
21 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
• 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.
– 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 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
22 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential
Good modelling starts at the beginning
• Use contact grouping for efficient bookkeeping
• Inspect contact before solving; solving >> pre-processing
• Understand your contact settings, use linear contact when you can
• Start with a simplified model
• Use residual plots to pinpoint location of instability
Summary