2014 Sd Mechanical Contact Best Practices

8/17/2019 2014 Sd Mechanical Contact Best Practices

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1 © 2014 ANSYS, Inc. December 16, 2014 ANSYS Confidential

Workbench Mechanical Contact BestPractices


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• With typical structural problems, the presence ofnonlinear contact can often be the biggest reason for

increased solution times.

• Poorly defined contact may lead to unstable contact

conditions. These conditions usually mean badconvergence 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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• Section 1: Contact Setup and Verification: – Contact Generation and Management Tips

• Section 2: Getting Ready for the Solver: – Mesh Quality & Mesh Sizing

– Setting the Contact Formulation

–

Advantages of MPC Contact – Understanding the Effect of Contact Stiffness

– Overlapping Contact and Boundary Conditions

• 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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Section 1: Contact Generation andManagement Tips


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Automatic Contact Generation

• On import, or when requested, Contact Pairs will automatically bedetected and generated in Mechanical. Searching options can be set on

the Contact Folder such as:

• Scoping (SearchEntire 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 Connectionsacting 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



Body Views

• A quick way to spatially identify contact regions especially whencontact occurs interior to model

• Activated via Button on Contact Toolbar



Shell Contact Normal Directions

To model the self-contactwhen 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

FF



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



The Goal: Robust Defaults

Robust defaults that solve

the largest variety ofcontact situations is our

goal


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• On curved surfaces, or surfaceswhich deform to a curve, having

sufficient contact elements to

closely follow the curvature is

essential for smooth results.

• This is especially true for nonlinearcontact

• Use similar element sizes for thesource and target sides.

Mesh Sizing

Too few elements

Better set elements with similar

mesh density



• Augmented Lagrange (Default): Suitablefor 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 is a true linear contact for small-deflectionapplications

• When convergence is difficult switching to MPC isan attractive alternative to changing the contact

stiffness.

• MPC contact prevents artificial stiffness when gapsexist between curved surfaces. (Another

alternative is to use joints)

• Ideal for shell-solid, shell-shell, and beam-shellcontacts

• A caveat is that MPC is the most sensitive contacttype to overconstraint, so avoid it when there are

other contacts or boundary conditions with shared

topology!

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



• 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 defaultof 1.

• For bending (thin structures) problems, start with0.01 – 0.1.

• For contacts with difficulty converging, lower thestiffness

• For pretension problems, use a stiffness factorgreater 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



• Care should be taken whencontact 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 BoundaryConditions

Contact shares an edge with a

Fixed Support



Section 3: : Dealing With Non Convergence



• Provides contact informationduring solution.

• The trends observed can helpdiagnose problems.

• For instance, a decreasingnumber 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 theconverged 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 localdisplacements.

• Ways to reducepenetration:

–Increase the stiffness

– Reduce the penetration

tolerance

– Try the Pure Lagrange

formulation.

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


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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 ConvergenceDifficulties


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• Mechanical

– New: Simplified Bolt Thread Modeling

– Trim Contact enhancement

– Bolt Pretension enhancement

• MAPDL – New: Interface Wear Model

• Miscellaneous

Section 4: What’s new in R15 ContactTechnology


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• Produces bolt thread stress profile withoutmeshing 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 9142seconds

New: Simplified Bolt Thread Modeling

True thread Virtual Thread


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• With sufficient mesh refinement, stress profiles match very closely

…Bolt Thread Modeling


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• ‘Increment’ - applies user defined offset as an

incremental sequential adjustment for challengingbolted assemblies

─ Value gets added to the solved deformation from

the previous step

─ Is ramped on from previous step

─

Can follow ‘Load’, ‘Adjustment’ or ‘Open’ after LS1 ─ Can be applied multiple times in sequence

Mechanical: Bolt Pretension Incremental load

– Supports Restarts

• If a solution restart is performed from a substep of a load step including an‘Increment’, the increment value gets added to the solved deformation

value at the beginning of the selected restart sub-step.


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Archard wear model:

Simulates the progressive loss of material from the contact surface.

Assumes rate of volume loss, ̇, 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, contactpressure) change. The underlying continuum elements also experience a loss in

material (and volume), thus simulating the wear.

MADPL: Contact Surface Wear

= wear coefficient

= material hardness

= pressure exponent

= velocity exponent

where:

̇ =

= contact pressure

= the relative sliding velocity


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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


Sliding block with frictional contact


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Brake Pad Model:



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Contact Problems – Model setup and

troubleshooting

- Contact modeling

- Tips on contact setup

- Dealing With Non Convergence


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Modeling Contacts: Formulations andDetection Method

Detect nodes which penetrateand remove penetration – But

how?

Contact Formulations1. Penalty Method

2. Lagrange Method

3. Augmented Lagrange

4. MPC – bonded contacts only Detection Method

1. Integration points

2. Nodes

3. Projection Based


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Modeling Contacts: Penalty Method

Penalize the nodes whichpenetrate

Attach a stiff spring

to the nodes

Penalty traction

pushes back the nodes to

reduce penetration

PenaltyF

δ ×= PenaltyPenalty K F

PenaltyK

Stretch the spring by the

penetration distance

PenaltyF

δ

δ


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Contacts using Penalty method always have residual

penetration after convergence - Residual Penetration canbe decreased by increasing the penalty stiffness.

ANSYS solver internally computes appropriate penalty

stiffness based on initial stiffness, stress - User can scale

up/down penalty stiffness.

High penalty stiffness factor leads to reduced penetration

and increasing accuracy but at a cost of

increasing number of equilibrium iterations – contact chattering

divergence – ill conditioning of the system equation

Tip: Update the penalty stiffness to allow the solver smartly

calculate the penalty stiffness – must for nonlinear

problems

Modeling Contacts: Penalty Method Notes


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100x increase in penalty results in small change in pressure/stressd is result from FKN and equilibrium equation

Pressure=d *FKN => Contact stress on contact surface

100-times Difference with FKN leads to about 60-times Difference with d

but only leads to about 0.7% Difference with contact pressure and stress.

Penetration d=0,26e-3,P=43642 Penetration d=0,45e-5,P=43927

60545.0

326.0=

−

−=∆

e

ed 007.1

43642

43927==∆ p

Penetration VS. Contact Pressure

Modeling Contacts: Penalty Method Notes


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Modeling Contacts: Normal LagrangeMethod

Goal: Reduce contactpenetration to zero

(contact constraint)

Detect penetrating nodes

p

Apply contact constraint to

the nodes

δ

δ

ANSYS Solver calculates the

surface traction at the

penetrating nodes to satisfy

contact constraint.

p0=δ

contact

constraint


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Advantages:

• No need to define contact stiffness

• Accuracy - constraint is satisfied exactly, there are no matrix conditioning problems

Disadvantages:

• Extra DOFs for each contact constraint

• Zero diagonal terms- iterative solvers are not applicable• Chattering- sensitivity to the variation of contact status

• Over constraints may appear – Symmetric contact, self contact, boundary conditions, MPC

F

Modeling Contacts: Normal LagrangeMethod

0T

K G u F

G λ δ =


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Use Lagrange multiplier method if:Penetration critical applications (accuracy)

Contact with predominant material nonlinearity

Contact between shells or thin layers

Large sliding problem: the convergence behavior is very good and stable

Contact at corners, edges

Suitable for solving Threaded connectors, press fit joints, seals, etc. In

which underlying stresses vary with contact stiffness

Convergence is still not achieved after several attempts by adjusting

contact stiffness

Modeling Contacts: Normal LagrangeMethod Notes


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Pair-1

Pair-2

Single pair

Modeling Contacts: Normal LagrangeMethod Notes

Target

ContactContact

Target

Contact

Target

Contact Target

If the normal Lagrange method is used:

• Always use asymmetric or auto asymmetric contact.

• Do not use CP/CE in on contact surfaces

• Do not define the multiple contact pairs, which share the common interfaces.

• Do not define self-contact pair or symmetric pairs. Split contact surfaces anddefine asymmetric contact pairs


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Modeling Contacts: Augmented LagrangeMethod

pF Penalty +

pK Penalty +×= δ traction

δ

Why not take advantage of

easiness of Penalty method

and accuracy of Normal

Lagrange method?

Use a penalty surface

traction augmented with

the surface traction

computed to resolve

contact constraint


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• Robust and default method in ANSYS WB

• Able to solve complex contact problems, producing a certainlevel of penetration which is generally so small that the effect is

negligible and can be controlled

• Residual penetration still present but lower than Penaltymethod – less sensitive to the penalty stiffness

• Usually requires fewer number of equilibrium iterations relativeto the Normal Lagrange method, but more iterations than the

Penalty method

• User still has the option to scale and update the penalty stiffness

Augmented Lagrange Method Notes


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Modeling Contacts: Augmented LagrangeMethod Notes

• Using default contact stiffness for most contact applications, unless: – 0.01-0.1 for bending deformations

– 10 for critical bolt joint which is pre-tensioned

– 0.01x-0.1x when the calculated convergence norm is almost parallel to the

criterion, or the calculated convergence norm is oscillating.

–10x, 100x if you have seen large penetration warnings many time

• Aggressive options: remedy chattering or preventing rigid body motionwhen initial contact conditions are not well posted.


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Tip:

Use Augmented Lagrange if:

• Symmetric contact or self-contact is used.

• Multiple parts share the same contact zone

• Mesh is relative coarse

• 3D large model(> 300.000 DOFs), use PCG solver.

Use Penalty method if:

• Tough contact conditions: pairs overlap, non smoothing interfaces, multiple thin

layers, model with potential over-constraints

• A contact pair used to apply fluid penetration pressure overlaps with other pairs

• You want to get fast solution and are not interested in local contact results

Modeling Contacts: Augmented LagrangeMethod Notes


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Modeling Contacts: MPC Method

In Bonded/No-Separation

between nodes are

prevented in one or both

directions

Penalty based formulationsattach a very stiff spring to

prevent this (residual

relative motion is still

present)

Advantage: Requires one

equilibrium iteration (linear)

MPC formulation ties the

nodes at two surfaces

preventing no relative motion

in one or both directions


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• MPC means: Mult ipoint Constraint

•To connect dissimilar mesh regions

•To connect the different element types

_

| |T

T T constrained reduced = −u u u

_ .

r rr rc

cr cc c

∆ ∆ =

∆

u K K F

K K 0u

Modeling Contacts: MPC Method


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• “Target Normal, Couple U to ROT” (default in WB-

Mechanical) is sufficient for most applications

• “Inside Pinball, Couple U to ROT” allows contact

detection regardless of element normal direction

- This option is especially helpful for unique applications

of connecting a line body edge with a shell or solid face

or edge

“ Inside Pinball, Couple U to ROT” optionproperly creates CEs using nodes around

entire perimeter of shell edge, regardless of

beam element normal direction

MPCs created w ith Default based on beam

element normal di rection

Modeling Contacts: MPC Method Notes

Shell elements

B e a m N o

r m a l D i r e c t i o n

B e a m N

o r m a l D

i r e c t i o n

Shell elements


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• Use MPC contact for bonded and no-separation contact as much as possibleunless overconstraints exist.

• Run Over-constraint check via WB contact tool

• Interface with small gap/penetration

– It avoids spurious frequencies for modal analysis

• Gaps are frequently encountered in CAD models – Seam Weld preparation

– Suppressed auxiliary parts (e.g. seals)

– Offsets of shell surfaces

• Rather than modify the geometry to fill the gap, they can be accuratelyignored when using MPC Contact or a fixed joint.

– Joints are recommended if convergence difficulties arise for large rotation

problems.

Modeling Contacts: MPC Method Notes


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All contact formulations require the solver to identify the

elements where surface penetration has occurred

Detection Methods

1. Surface integration points (Gauss points)

2. Nodes

3. Surface Projection

Contact Detection Methods


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• Surface integration point method allow for additional points to detect

penetration between surfaces - Default method for Penalty andAugmented Lagrange method.

– Best for assemblies, true surface-to surface contact.

– However it is poor when contact occurs at corners or edges.

• Nodal based detection methods are default for MPC and Normal Lagrange

method. – For contacts at corners (such as interference fit problems, threaded connector

models), best results are obtained when either Nodal – Normal to Target (or)

Normal for Contact.

• Nodal – Projected Normal From Contact

–For true surface-surface contacts, it provides good results with minimal contactpressure spikes at nodes

– Convergence behavior is also better if mesh is adequately discretized at contact

surfaces.

– For best performance, use similar mesh size for both contact and target surfaces.

Contact Detection Methods: Notes


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• Nodal – Projected Normal From Contact

–Always use Projection method for contact involving gasket layers so that stressand strain distribution near contacting edges is more smooth.

– Same rule is applied for FSI interface.

– Avoid use the projected contact in conjunction with MPC bonded contact. It

causes increased bandwidth for the global equation set and leads to poor

performance.

– For 3D higher order elements, use Normal Lagrange in conjunction with

projection based option for the best accuracy.

Contact Detection Methods: Notes

Unexpected spiky results

Using Gauss detectionUsing Projected contact


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Example of when to use Normal to target surface

Double beams in contact.

Normal to targetGauss detection

Contact Detection Methods: Example


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Asymmetric and Symmetric ContactBehavior

Penalty based methods apply

penalty traction only to the

contact surface (Auto

asymmetric contact - default)

PenaltyF

δ

PenaltyF

In Symmetric contacts, anaddition step in each equilibrium

iteration is used to compute the

penalty tractions on the

formerly target surface

Contact

Target

Target

Contact


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• Symmetric contacts

– Lower residual penetration

– Lower number of equilibrium iterations but require more computational

time to calculate surface tractions on both surfaces

–

Less sensitive to mesh sizing at contact interface• Choose asymmetric contacts when the difference in stiffness (elastic modulus)

between both surfaces are > 10.

• Stiff surface – Target, Compliant surface – Contact

• Normal Lagrange and MPC methods requires Asymmetric contact where

contact traction is calculated only on the contact surface• Tip: In most cases, use auto-asymmetric contact – program makes decision

• Projected contact is not sensitive to the designation. However pick the refinersurface as the contact for the best performance.

Asymmetric and Symmetric ContactBehavior - Guidelines


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Some times, It is also important to classify the possible contact configuration,which surface or point will be in contact with which surface. The way to

define the contact or target can influence the convergence behavior.

Contact

Contact

Target

Target

Asymmetric and Symmetric ContactBehavior

b ll


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ANSYS solver internally categorizes

the state of contact as: Far field,

Near field, Touching

Category is based on if the distance

between contact and target surface

is lesser (or) greater than pinball

radius For interference problems, ensure

that the pinball radius is greater

than the maximum interference

In bonded contact and no-separation contacts, any region

between the surfaces which

touches or lies within pinball radius

will be assumed to be in contact

Contact Pinball

i b ll


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Contact Pinball - Notes

Pinball radius• This is one of the most important parameters to get the desired contactresult

– Effective PINB : measure the largest gap in your model via Initial Contact

Tool, then specify PINB a little bit larger than the gap, e.g if the largest gap

is X, use PINB=X+X/10.

– Usually, you can use large PINB to run the analysis. The contact searchtime will increase.

• If you introduce a large pinball, you will have risks to introduce the spuriousregion.

FE model Contact status Deformation

I f Adj h


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Interface Treatment: Adjust to touch

Mesh discretization can create

artificial gaps between surfaceseven though the CAD surfaces

touch each other

Adjust to touch option will

remove the gap and assume the

surfaces touch each other

gap

Physically moving contact nodes

towards target surface is notdirectly exposed in Mechanical,

but can be manually defined via

CNCH,ADJU in a command

object.

Interface Treatment: Add offset


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Interface Treatment: Add offsetwith/without ramped effects

• Press-fit and Threaded

connector problems have theirinitial surfaces penetrating

• Challenge – resolve the initialinterference

• Use Add offset options

– Ramped effects – for large

interferences (hint : do not apply

any other load)

–No Ramped effects – for smallinterferences

• An Alternative: defined offsetin tabular format via time

Interference Fit Notes


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Interference Fit - Notes

Use nodal detection methods

• Normal to target: if the target surface is smoother

• Normal from contact: if the contact surface is smoother

• User defined normal: if both sides are not smooth

Use Normal Lagrange method

Enough Pinball radius to capture initial over-closure

Death-Birth of contact elements whose surface normal is nearly

perpendicular to the direction of the press fit

Define contact elements only at crests and roots

Not at sides. Including all sides may cause undesirable locking.

C S bili i D i


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Contact Stabilization Damping• What about applications where the gap is non-negligible (it’s too large to be

ignored).

- For such situations, Contact Stabilization Damping can be used to introduce a

viscous damping traction proportional to but opposite to the relative pseudo

velocities between the two surfaces along contact normal and/or tangential

directions.

Where: = damping coefficient in normal direction

= damping coefficient in tangential direction

= pseudo velocity

Fn

Target

Contact

Ft

22

11

ud P

ud P

ud P

t d

t d

nndn

=

=

=

Pdn

Pd1,d2

u

d

d

t

n

C t t St bili ti D i


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… Contact Stabilization Damping

• Stabilization Damping Factor is applied in the

contact normal and it is valid only for nonlinearcontact (frictionless, rough and frictional

contacts).

– If its value is greater than 0, the damping is

activated for all load steps.

– Additional controls are available via KEYOPT(15) ina command object.

– Tangential damping factor is not directly exposed

in Mechanical, but can be manually defined via

RMODIF in a command object.


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…Contact Stabilization DampingExample: Consider a fixed pin interfacing with a hole in plate with initial radial

clearance and under a force based load

– Stabilization captures localized stress distribution more accurately then ‘Adjust to Touch’Conventional ‘Adjust to Touch’ Contact Stabilization Damping

M d li C t t R b t D f lt


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Modeling Contacts: Robust Defaults

• Robust defaults that solve the

largest variety of contact

situations is our goal

• In general the “Program

Controlled” settings inMechanical match the MAPDL

solver defaults.

• Exceptions include Behavior(auto-asymmetric) and Update

Stiffness (update each iteration)

S l S tti D d d


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Solver Settings Decoded

1) Weak Springs

•Always turn it off , unless it is really

needed for prevent rigid body motion.

Adding displacement constraints is the

right way.

• When weak springs are used, carefullyverify the reaction forces

2) Large Deflection

• Always turn it on, unless the model istruly for small strains, small

deformations, small rotations, small

sliding.

3) Stabilization

• Required for unstable problems

• Perform stringent checks on the resultswhen stabilization is used

F

Tips on Model Set Up


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ANSYS provides many tools in helping users to monitor nonlinear analyses aswell as diagnose any problems.

• Oftentimes, it may be better to start simple and add complexity as you go,so that sources of problems can be isolated more readily. Adding lots of

complexity to the first analysis can result in wasted time down the road.

• Do not randomly change settings. Use the program control settings first,then change contact or solver settings only if there is clear reason to do so,

as illustrated in the Solver Output, Results Tracker, or Newton-Raphson

Residuals.

• When your FE model is large, you do not have means to try “error and trial”many times. You should always use initial contact tools and your expertise to

setup and query the model before starting.



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• Poor mesh quality in solid elementscan cause convergence problems.

• A difficult contact problem may bediverging simply because of the mesh

• Tip: Use aggressive shape checkingfor nonlinear contact problems.


Poor

Mesh

Quality

on

Contact

Surface

Better

MeshQuality

Check mesh quality



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Check modeling Unites

• Always verify the model units. Bad units can results in inaccurate solutionand bad convergence due to tolerance, precision, and numerical round offissues.

• When the tolerance on residual force is not satisfied because the contactlength is too small you will see this warning in output:

• Or if tolerance on residual force is not satisfied because the elastic moduli or force/mass quantities too big you will see this warning in output:

Tip: Select mm-N units for the most contact models.


–Warning: Min. contact depth 5.376e-6 is too small which can cause

accuracy problem, you may scale the length unit in the model.

Warning: Max. contact stiffness 1.21e16 is too big which can cause

accuracy problem, you may scale the force unit in the model.



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Check geometry

• Verify contact and target surfaces assignment. Flip contact and targetsurface if necessary.

• Ensure that the target and contact surface definitions extend far enough tocover the full expected range of motion for the analysis.

• Be sure to adequately discretize underlying elements. Coarse discretizationcan cause convergence difficulties.

• Use similar mesh sizes for contact and target surfaces to gain the bestaccuracy and convergence.


Tips on Model Set Up: Initial Contact Tool


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Tips on Model Set Up: Initial Contact Tool

• Use the Initial Contact Tool to quickly learn about contactstatus 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

Tips on Model Set Up: Contact Tool


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Tips on Model Set Up: Contact Tool

The contact tool

can also provide

visual contour

plots of the initial

contact status of

the various bodies

in an assembly

Contact Tool:

Tips on Model Set Up: Worksheet


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Tips on Model Set Up: 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 sortedby clicking on column

header



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Check contact settings via worksheet and contact tool

• Always verify all Contact/Target surfaces and contact settings to ensure that ContactRegions are defined as expected. Review the detailed initial contact results in the

contact tool to verify the initial contact status of Contact Regions and what value of

the penetration or gap is, if present.

• Ensure that the appropriate surface pairs are initially in contact via the contact tool.

• If the initial penetration is too large, use Ramped option. You can solve initialinterference over several sub-steps.

• Verify pinball. If the resulting penetration/gap is large, use a larger pinball. If theresulting penetration is unreal, use smaller pinball to prevent spurious contact.

• If necessary use “just in touch” to close small gap or apply contact damping to

prevent rigid body motions.• Use MPC option for bonded and no-separation contact pair if possible.

• Identify potential overconstraints . Modify model to manually removeoverconstraints.




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Set proper Sub-step size

• Linear elastic and small deflection NLGEO,OFF

– Generally speaking, in this case you do not need more than one substep to run the

analysis. It is usually better to put the 100% load in one substep, because, the less

the load, the smaller will be the contact area, and the small contact area usually

make the problem unstable, so one can see the oscillating in convergence norm.

• Linear elastic and large deflection NLGEO,ON

– For bulky problem, you should try to use only one substep to run the analysis. The

unstable solution during the equilibrium iterations can be stabilized via aggressive

contact stiffness updating or contact damping.

– For thin structure, you should be careful with the substeps, because the stressstiffening effect can largely change the structure behavior, and the contact region

also changes rapidly during the large rotation. Use multiple substeps to do the

analysis. To prevent large deformation due to spring-back, use time step control

option (based on contact status change ) or global stabilization.




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Set proper Sub-step size

• Always use AUTO time stepping scheme, let program think

• Friction is present

– Since friction has path dependent nature, you should use multiple substep

(e.g.>5) to run the analysis.

• Plasticity is present

– Plasticity also has path dependent nature, you should use multiple substep

say (e.g.5) to run the analysis.

• Large rotation and finite deformation – You should use small rotation increment e.g. < 10 degree for substep

• Large rotation for rigid body

– You can use large increment in general.


Debug strategies: Results Tracker


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Debug strategies: Results Tracker

Use solution tracking to identify

contact chattering• Identify when and how contact occurs.

• Determine the regions where contact isunstable and then focus on the specific

settings of those particular pairs instead of

having to deal with all of them.

Debug strategies: Study Output


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When troubleshooting contact problems, notes, warnings,errors in the output file often provide valuable feedback:

• Warning: The initial penetration/gap is relatively large. Using bonded/noseparation option may cause an accuracy issue. You may use the

CNCHECK,ADJUST command to move the contact nodes towards the targetsurface.

• Warning: The closed gap/penetration may be too large. Increase pinball ifit is a true closed gap/penetration. Decrease pinball if it is a false one.

• Warning: Convergence has been achieved in spite of large penetration. Ifthis message is repeated frequently, we recommend either increasingpenalty stiffness(FKN), or enlarge penetration tolerance(FTOLN).

Debug strategies: Study Output

Debug strategies


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Debug strategies

Failure to achieve force equilibr ium• Friction in the model

• Unconstrained rigid body motion indicated by the presence ofvery large displacement corrections.

• Overconstraints usually indicated by zero pivot/negative warning

messages.• Buckling indicated by the load-displacement curve reaching a load

maximum

• Use the output to determine which problem you have!! Therecommended corrective actions are different for each problem.

Debug strategies


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Debug strategies

Friction in the model• Use frictionless contact or rough contact to see

any help

• If the difficulty is indeed due to friction, then consider thefollowing:

– Use the unsymmetric solver (NROP,unsym ) for curved target

surface even if the coefficient of friction is small.

– Re-examine the choice of the friction coefficient.

– Loose slip tolerance (better than to use a smaller tangent stiffness).

– Refine the mesh so that more points come into contact at the sametime.

Debug strategies: Rigid-Body Motion


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F

b

F

a

F

F

< F

c

Sometime, the parts in a contact model is not uniquely constrained, It means some parts

can move without producing any elastic deformation- Rigid Body Motion. This rigid bodymotion is unfortunately not allowed in a static analysis. Usually, there are 3 types of rigid

body motion

–The parts are not in contact(gap) -- Fig. a)

–The initial penetration is too high, this will leads to a very high contact force -- Fig. b

– Friction force is too small comparing the external force -- Fig. c

Debug strategies: Rigid Body Motion



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• Add a small friction to avoid the numeric instability

How to avoid the rigid body motion?

•

Use specified displacement, if possible, in stead of force.

FE Model Without friction


Tip: Constrain rotational DOFs for structure elements

F

Real problem

U

FE model

This technique uses imposed

displacements to move the body

and as result you get the reaction

force



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• Use contact damping to avoid the numeric noise.

•

Do a slow dynamic analysis to introduce the inertial force.

F The damping effects should be negligible compared to the stiffness ofthe system.

F

FE Model

F

Static

•You will need to add mass and damping

in order to convert the solution from a

static to a dynamic solution. This is

known as a quasi-static solution.

Debug strategies: Rigid Body Motion

Debug strategies: Overconstraints


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Overconstrained model• Overconstraints are indicated by the presence of zero pivotwarnings. It often results in very large residual force (orders ofmagnitude larger than a typically applied force) followed by veryeasy convergence.

• First check potential overconstraints via Contact Tool

Tips for manually removing overconstraints

• Remove overlapped pairs

•Merge pairs

• Flip contact and target surfaces

• Add multiple layers



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The assembly is connected with MPC contacts

In these regions Parts arenot correctly connected

Contact status

2 elements in sweep direction

Contact status


90

Procedure for Overcoming Convergence


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• Identify the problematiccontact region(s) using:

– Contact tracking

– Contact results (e.g.

status, penetration)

– Force convergence plots

– NR residuals

– Outputs

• 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(>.20)consider using unsymmetric solver

– Last resort: Add stabilization(contact or

global)

Procedure for Overcoming ConvergenceDifficulties

Debug strategies: Check non contact-


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Debug strategies: Check non contactrelated issues

• Unrealistic physical model• Unreasonable loading and boundary conditions. – Insufficient constraints, missing rotation constraints

– Overconstraints

– Loading condition: Step vs. ramped (KBC)

• Element formulation: Hourglass and locking, U/P, joints

• Material constitutive Large plastic deformation, creep,incompressible or near incompressible

• Unreasonable or incorrect material properties and inconsistentunits

• Local & global instabilities

• Follower loads, Link Elements

Debug strategies: Check non contact-


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Debug strategies: Check non contactrelated issues

If above steps still don’t provide enough information on whatthe problem is, there are other things that can be done:

• Turn on large deflection.

• Turn off weak springs.

• Use SPARE direct solver instead of PCG iterative solver.• Turn off prediction

• Use single process instead of multiple processes.

• Solve problem using transient option.

If none of above tricks works, call ANSYS experts for technical

supports.

What’s New at R16 in Contact


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at s e at 6 Co tact

• General Contact• Robustness improvement for contact modeling

• Wear modeling with automatic adaptivity

•CZM unification

Documents

2014 Sd Mechanical Contact Best Practices