108 Contact

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The COSMOS CompanionModeling Contact in COSMOSWorks

Volume 108Sponsored by:

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What is the COSMOS Companion?

• The COSMOS Companion is a series of short subjects tohelp design engineers build better products withSolidWorks Analysis

• Video presentations and accompanying exercises

• A tool for Continuous Learning on your schedule

• Pre-recorded videos are accompanied by a more detailedwebcast with Q & A – Download videos and review webcast schedule at:

http://www.cosmosm.com/pages/news/COSMOS_Companion.html

• It is not an alternative to instructor-led introductory training – We highly recommend you take a course with your local reseller to build a

solid knowledge base

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Why Contact Analysis?

• Contact conditions allow parts to interact with each othermore naturally

• It is good practice is to use a contact interaction between

parts of similar stiffness instead of trying to model thatinteraction with Loads/Restraints

• Without a contact condition defined, two meshes, or twoelements within the same mesh, that aren’t joined at acommon node, don’t know the other exists.

• Contact, in COSMOSWorks, also allows a user to Bondparts together where they touch

• In COSMOSWorks, using contact is so straightforward, it isto every users advantage to take advantage of it whereapplicable

• There is still some information that will aid you in the useof contact for simulating your designs…

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Contact Definition Options

Set Global Contact

• Three options – Bonded (Default)

– Free/No Interaction – No Penetration

• Compatible vs. Incompatible

– Available for Bonded only – When two surfaces are touching:• Using Compatible, COSMOSWorks will imprint each face

with the intersection of the two and force a matchingmesh on both – More accurate local solution but can

cause meshing problems• Using Incompatible, both parts are meshed without anyconsideration of the other and internal constraintequations tie the intersecting faces together.

– If assemblies with bonded contact fail to mesh, tryusing the Incompatible Mesh option

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• Always set Global to Free

if there isn’t a reason toleave it Bonded

• Will improve meshing

and forces you toconsider each contactcondition

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• When multiple contact conditions are specified foran entity, they are enforced as follows:

– Local contact conditions override global and componentcontact conditions.

– Component contact conditions override global contact

conditions. – Global contact condition will be used for all touching

faces for which no component or local contact conditionhas been specified

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Define Contact Set…

• Manually choose Source and Target sets

• Select Contact Options

• Choose “Friction” if this is desired

– Don’t necessarily expect that a Coeff. Of Friction in

FEA will correlate exactly with test data – Someiterative adjustment may be required if this is critical

– Adds complexity to the solution and should be leftoff unless needed

– Remember that real-world friction is difficult tomeasure and even more difficult to control – Usethis cautiously

Source

Target

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Find Contact Sets

• Choose 2 or more components

• When you select “Choose Touching Faces,COSMOSWorks will identify potential contactpairs

• Choose “No Penetration”, “Bonded”, or “Free”• For “No Penetration” choose:

– Node-to-Node

– Node-to-Surface

– Surface-to-Surface

• This is the fastest way to find and set contactpairs that will not use the Global Contact

setting

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Source and Target

• COSMOSWorks differentiates thetwo entity selections for a contactpair as the “Source” and the

“Target”

• One way of looking at them is toassume the “Source” acts on the

“Target” – COSMOSWorks attempts to project

Source entities onto the Target

• It can make a difference whichentities get assigned to each!

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Source and Target

• Choice of Source & Target is unimportant in these cases:

– Free (No Interaction)

– Bonded Contact with Compatible Mesh Option

– No Penetration with Node-to-Node Option

• Choice of Source & Target can be critical to bothconvergence and solution time for all other contactconditions, including Shrink Fit

• An entity can be defined as a Source in one contact setand a Target in another

• Both Source & Target selections can include multipleentities

– All entities in either Source or Target, should belong to the same

body, respectively

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Source and Target

Target

Source

Target

Source

Acceptable Contact Set Unacceptable Contact Set

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Source and Target

Choosing the Target Entities

• In general, the Target should be the stiffer of two parts in a

Contact Set• While the Source set can be Vertices, Edges, and Faces,

the Target must only be Faces

– Vertices and Edges are only allowed in Node-to-Surface (Surface)

• Target should be flatter and larger than Source• Target can be meshed more coarse than Source

• Target should have no sharp corners or small fillets in the

area of contact

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Source and Target

• Ideally, Source and Target would be switched• Small round on Target is better

• If Source and Target can’t be switched, the best solution

would be to create 2-3 contact sets

Target

Source

Bad Better

Best

Best

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Contact Types

• No Penetration – Primary reason to use contact

– Defines regions in the model that can’t pass thru eachother

• Bonded – “Welds” or “Glues” entities together

• Shrink Fit

– Allows press fit or interference fit interactions

• Free (No Interaction)

• Virtual Wall

– Useful when a part contacts ‘ground’ or a large flat rigidsurface

– Available for a single part

– User selects a Reference Plane

– Local compliance, or Virtual Wall stiffness, can be

specified

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Contact Options

Node-to-Node

• Gap Elements

• Contacting surfaces must touchinitially

• Assumes direction of normalcontact force and points of contactare known in advance and remain

unchanged throughout analysis

• Applied force should be parallel tothe contact normal force orpressure

• Shouldn’t be used if sliding isexpected

• Contacting surfaces should berelatively flat and parallel

• COSMOSWorks requires aCompatible mesh and willattempt to line up the nodesbetween the Target and Source

• Use with care as this option islimited

F F F

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Contact Options

Node-to-Surface

• Gap elements attached to the nodes onthe Source and element faces on the

Target• Advantages over Node-to-Node Gap

Contact: – The user does not need to know the exact

location of the point of contact a priori. Theprogram internally will determine that locationand apply the contact forces accordingly.

– The direction of the contact forces isdetermined by the program based on the

deformed shape of the entities in contact. – The nodal points on the contacting entities do

not need to match each other.

• Shrink Fit conditions use this type of

contact

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Contact Options

Surface-to-Surface

• More general than node to node andnode to surface contacts

• Suitable for complex contacts withgeneral loading

• Most resource intensive contactalgorithm

• Not recommended when the area ofcontact between a source face and

target face becomes too small orreduces to a line or point duringdeformation. In such cases, use theNode-to-Surface option

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Contact Options

Poor

Fair

Good Best

Fair

Fair

Fair

Good Good

BestBest

Best

Summary of Options:

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Bonded Contact in v2006

• Overlapping Surfaces or Faces – Clearance allowed

– COSMOS will provide a warning if clearance is largecompared to global element size

– If the gap is too large, COSMOS will provide a messagethat bonding did not occur and give user chance to stop

analysis

• Edges and Vertices (Source) to Face (Target) – Clearance Allowed

• Interfering Edges or Faces – COSMOS will provide a warning if clearance is large

compared to global element size

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Bonded Contact in v2006

• Works with Compatible & Incompatible Meshes

• General Guidelines:

– Bonding internally uses constraint equations to match thedisplacements and rotations of the two parts.

– This command is currently applicable to linear and nonlinear static,frequency, buckling and heat transfer analyses.

– When bonding solids and shells, it is advisable to use shells as theSource and solids as the Target

– In problems where the stress concentration at the bondedintersection is critical, both parts should have a fine mesh in this

region. – Result local to the bonded contact deteriorate as the difference in

stiffness between parts increases. Local mesh refinement helps.

– It is advisable for the mesh on the Source set to have more nodes

than the Target set if the meshes are incompatible

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Compatible vs. Incompatible Mesh

• If Split Lines are used to force coincidencebetween edges & faces, the meshes will be

Compatible, regardless of the option setting

Compatible Incompatible

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Study Properties – Gap/Contact

Include Global Friction – Pre-sets the Coeff. Of Friction for all applicable

contact studies to the defined value

– If Global Contact is set to No Penetration, all

contact sets defined automatically will have frictionactivated at the specified value

– DOES NOT automatically activate friction onmanually defined contact sets. Option must still be

selected per contact set

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Study Properties – Gap/Contact

Ignore Clearance for Surface Contact

• Used with Node-to-Surface contact only

• When Off: – Initial contact between a node and the associated face is assumed if the initial

distance is less than or equal to 0.5% of the global element size. If the initialdistance exceeds the threshold, no initial contact is assumed.

• When On: – Initial contact (between each node and its associated face) is assumed regardless

of the initial distance between them. The initial gaps will be ignored during contact

iterations.• This option should be turned Off (default) for most applications. It is specifically

designed to take care of a specific situation when, due to tolerances, a smallclearance exists between faces that should have been initially in contact.

• Incompatible meshes are likely when there is a small clearance between faces

of similar curvature. If mesh positioning causes numerical penetration (Notphysical penetration), it should be within the tolerance stated above. However,if convergence fails, re-try with this option On

• The option should be Off when the two contact sets have large curvaturedifferences.

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Linear vs. Nonlinear Contact

• Basic contact in a Static Study defaults to a “Linear”contact algorithm where certain assumptions are made toimprove solution speed

– Very small or no gaps between contacting parts

– Little or no sliding

– External load is parallel to contact element

– Faces that touch stay in contactF

OK

F

Not OK

ContactPressure

ContactPressure

Li N li C

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• If any of the “linear” conditionsare violated, choose LargeDisplacement in Study

Properties – Note that if Large Displacement isnot turned on, the “linear” solutionmay complete without any warningmessages…this doesn’t mean thesolution is correct

• If Large Displacement isrequired, Node-to-Node should

be avoided• Be advised that a solution using

Large Displacement will takelonger to solve

F

ContactPressure

Li N li C t t

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43000.00.0039Standard Contact

53000.00.0042Large Displacement

Max.Von Mises Stress

Max.Displacement

Initial ContactFinalContact

Li N li C t t

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• Standard Static studies, even with LargeDisplacement, only report the final solution

• An animation shows a linear path from theinitial position of the assembly to the finalposition

• If your results are path dependent, you maynot have all the data you need to properlyunderstand the results

• One benefit of a nonlinear analysis is theability calculate and plot results atintermediate steps

– Advanced Professional package required

Oth C t t Ti

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Other Contact Tips

• Split lines to reduce Source area can improvesolution efficiency

• Split lines to reduce Target area will have noeffect on solution

• In general, a more refined mesh on contact faceswill result in better and faster converging contact

• Too fine a mesh on a Target surface could cause

convergence issues

Presentation Summary

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Presentation Summary

• In this COSMOS Companion unit, we explored the“Ins & Outs” of contact set definition

• The capabilities & limitations of each option werereviewed

• Recommendations for contact set choices wereoutlined

• The benefits of Large Displacement & Nonlinear

options were reviewed

Conclusion

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Conclusion

• For more information…

– Contact your local reseller for more in-depth training orsupport on using contact conditions in your design work

– Review the on-line help for a more detailed descriptionof the features discussed

– Attend, or better yet, present at a local COSMOS orSolidWorks user group.

• See http://www.swugn.org/ for a user group near you

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