CONF2014 John Lin MechanicalContactBestPractices

1 © 2014 ANSYS, Inc. June 20, 2014 ANSYS Confidential

Workbench Mechanical Contact Best Practices

John Lin


• 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


Section 1: Contact Management Tips for Large Models


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


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


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


• 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


• 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


Viewing the MPC Equations

After the solution is done

MPC equations and other

“FE Connections” can be

graphically viewed


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


Section 3: Dealing With Non Convergence


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


• 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


• 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


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


• 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


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

Documents

CONF2014 John Lin MechanicalContactBestPractices