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2004 IMPACT Engineerin g Solutions, Inc.
Techniques for
Successfully UsingANSYS ContactElements
Presented by:
Rich BothmannIMPACT Engineering Solutions, Inc.
Brookfield, WI
Tel: 847-265-4140www.impactengsol.com
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Analysis Services Group
Advanced FE Project Support and
Education
Local and International Recognition for
Educational Offerings
Vince Adams & Rich Bothmann
Over 25 Years Experience in Use,
Training, and Support of FEA
Active in Leadership of Local, National andInternational Users Groups
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ANSYS
Extensive experience in linear, nonlinear ,and dynamics Certified instructor for local reseller
In conjunction wi th ANSYS, founding Midwest ANSYS UG
LS-DYNA Drop and Crash Testing
MSC.NASTRAN
ComosWorks&CosmosMotion
FE-Fatigue (nCode) Durability Analysis
FEMAP Pre & Post-Processing
Pro/MECHANICAExtensive combined engineering analysis experience in a variety of
materials and industries
Analysis Capabilities and Expertise
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What is Nonlinear Behavior
A Structure Is Nonlinear If the Loading Causes
Significant Change in Stiffness
Strains Beyond Elastic Limit
Large Deflections (Fishing Pole) Contact Between Two Bodies
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Types of Nonlinearities
Nonlinear Analysis
Material Geometric Boundary
Nonlinear Elastic
Plastic
Hyperelastic
Contact
Follower Forces
Large Displacement Small Rotation/Small Strain
Large Rotation/Small Strain
Large Strain
Geometrically Instable
or Multi-Stable
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Linear vs. Nonlinear Behavior Linear structure obeys this linear relationship
Many problems do not have a linear relationshipand instead the displacement varies with force
Force
Displacement
K=constant
Force
Displacement
Kt=TangentStiffness
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Solving Nonlinear Problems
Newton-Raphson method iterates to a solution
using the equation:
Displacement
Load
u
Fnr
F[ ]{ } { } { }
[ ]{ }
{ }
{ }stresses)elementof(sum
VectorForceInternal
VectorForceApplied
IncrementntDisplaceme
MatrixStiffnessTangent
:
=
=
=
=
=
nr
T
nrT
F
F
u
K
where
FFuK
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170 Series Contact Element
Conta171/172 2D/3D 4 Node (surface to surface)
Conta173/174 2D/3D 4 Node (surface to surface)
Conta175 Node to Surface (line/point to surface)
Conta178 Node to Node
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Primary Contact Algorithms (Keyopt 2)
Penalty: penalty springs
Augmented Lagrangian: penalty springs + pressure
dofs
Lagrangian: zero penetration enforced Penalty/Lagrangian: zero penetration normal /
penalty spring tangential
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Contact Nonlinearities & the PenaltyApproach
Contact Problems are Usually Hardening Structures More Difficult to Analyze
Prone to Slow or
Unsuccessful Convergence
Force
Displacement
No Force is
Required to
Move the
Sphere
Some Force is
Required to Move
the Sphere
More Force is
Required to Move
the Sphere
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Contact Nonlinearities & the PenaltyApproach
Initial
Stiffness
is Low
Results in
SignificantInterpenetration
and Large
Internal Forces
TangentModulus wil l
Decrease
Displacements
Force
Displacement
K initial
K tangent
1
2
3
Iterations Continue until
Force Imbalance or
Residual is within
Tolerance
P
P
P
1
2
3
P
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Penalty vs. Augmented Lagrangian,
Penalty Method74 Iterations
0.00233 in of penetration
Augmented LagrangianMethod
88 Iterations
0.00064 in of penetration
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Penalty vs. Augmented Lagrangian
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Pure Lagrange
Pure Lagrange Never Converged >230 iterations,
Distorted Element Errors
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Dealing with Convergence Issues
Penalty Stiffness (FKN)
Default=1, Start with something small
Stiffness Update (KEYOPT 10) 0, each load step
1, each substep
2, each iteration
Penetration Tolerance (FTOLN)
If penalty stiffness decreased, consider increasingFTOLN
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Stress vs. FKN
FKN Iterations Max Stress Contact Pressure
0.001 426 19217 4160
0.01 74 19173 3880
0.1 213 19527 4886
1 Didn't Converge
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Stiffness Update vs. Iterations
Keyopt 10 Description Iterations
0 Update Each Loadstep (old default) 155
1 Update Each Substep (new default) 74
2 Update Each Iteration 75
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Other Convergence Issues
Too much initial penetration
KEYOPT 9 = 1, excludes initial penetration= 2, ramps initial penetration
Rigid Motion
Parts not initially touching
Relying on contact to hold parts together usually resultsin convergence problems
Many Techniques to deal with this issue, sometimesproblems requires more than one method.
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Interference Problems
Set Keyopt 9 = 2
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Eliminating Rigid Motion
Build Geometry so it is Just Touching
Adjust Initial Contact Conditions
Displacement Control
Weak (or not so weak) Springs (or possibly FKOP) Include Friction
Dynamics CNCHECK/ADJUST (New)
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Automated Adjustment of InitialContact Conditions
KEYOPT 5 = 1-3, Close Gap/Reduce Penetration
(Auto CNOF)
= 4, Offset individual nodes to target surface(ICONT)
CNCHECK/ADJUST: Use CNCHECK to determine if
gap or penetration exists.
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Checking Contact Conditions
Issue CNCHECK to Determine Contact Status
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Using Auto CNOF
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Displacement Control
Very Robust and Most Used Method
Use Alone or with Subsequent Force Control
Apply displacement control and obtain solution
Replace displacement with reaction load and obtainsolution
Apply final load and obtain solution
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Displacement Control Examples
Displacement Control Used for Crush Tests
Click to Animate
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Displacement Control Examples
Displacement Control Used inConjunction with Force Control
1. Apply UY
2. Replace UYwith Reaction
3. Apply FinalLoad
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Displacement Control Examples
Displacement Control Used inConjunction with Force Control
1. Apply RY
2. Replace RYwith Reaction
3. Apply FinalMoment
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Weak Springs
Small amount of stiffness to prevent parts from
flying off into space.
Springs can attach parts to one another or to
ground. Adjust stiffness values so that springs dont impact
results.
For complicated assemblies, consider using multiplereal sets for different springs.
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Weak Spring ExamplesWeak Springs Added at2 Locations to PreventFishcord from Spinning
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Weak Spring Examples
Weak springs attachhousing and strap to
ground
Weak springs attach lock
bolt to housing
Different spring
constants for all. Starting
value 10N/mm
Friction was also used
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Friction
In some instances a small amount of friction
(mu=0.1) can be used to control lateral sliding
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Dynamics Solving F=ma eliminates rigid
motion issues.
Use of slow dynamics to solvestatic problems can overcome rigidmotion issues. Include density and
damping and verify system comesto rest
When performing true dynamicproblems with contact, use
Element Time Increment Control(keyopt 7=2, maintain reasonable).This keeps time step a fraction ofthe systems apparent frequency
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Dynamics Examples
Click to Animate
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Dynamics Examples
Click to Animate
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Mesh Issues
Finer Mesh on Contact surface/ Coarser on Target
Faceting negatively affects curved contact surfaces
Use fine mesh
Exclude penetration (keyopt 9 = 1) Auto CNOF/ICONT (keyopt 5)
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Mesh Issues
Refined Mesh + keyopt 9 = 1
Eliminated Convergence Problems
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Nonlinear Diagnostics
Version 9.0 added contact pair based diagnostics
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Plotting Residual Norms can also help locate
problem contacts
Nonlinear Diagnostics
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Other Methods of Dealing withConvergence
Increase Convergence Tolerance Note this maysimply allow you to converge to a wrong solution
Specify a min ref convergence value Adjust Pinball Region
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Summary Try to converge using easy settings first then tighten
settings to determine sensitivity Small FKN
Large FTOLN
Use displacement control whenever you can
Get to know ANSYS automatic methods for dealing withrigid motion
In situations where auto methods dont work consider theapplication and apply springs, friction, etc.
Setup NL Diagnostics and look at contact penetration tosee how various contacts are behaving