Contact Mechanic

8/6/2019 Contact Mechanic

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Training Manual

7. Introduction to Contact


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Training Manual7. Introduction to contact

What is contact?

When two separate surfaces touch each

other such that they become mutuallytangent, they are said to be in contact.

In the common physical sense, surfacesthat are in contact have thesecharacteristics:

They do not interpenetrate. They can transmit compressive normal

forces and tangential friction forces. They often do not transmit tensile normal

forces. They are therefore free to separate and

move away from each other.


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Training Manual Introduction to contact

In this chapter, we will present an introduction to nonlinearcontact analysis via the following topics:

A. Basic concepts

B. Using the contact wizard

C. Obtaining the solution

D. Postprocessing

E. Workshop

F. Assembly contact

The purpose is to introduce you to contact analysisprocedures for simple (easily converged) models. Other,more advanced aspects of contact analysis that are used tohandle more difficult models are discussed in the Advanced Contact and Bolt Pretension training manual.


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Training Manual

Introduction to contact

A. Basic Concepts

Contact is a changing-status nonlinearity. That is, thestiffness of the system depends on the contact status:

Status = Open Status = Closedand Sticking

Status = Closedand Sliding


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Training Manual


... Basic Concepts

Contact is a strong nonlinearity, because both the normaland tangential stiffness at contact surfaces changesignificantly with changing contact status.

Large, sudden changes in stiffness often cause severeconvergence difficulties.

u

F

Opencontact

Closedcontact


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Training Manual


... Basic Concepts

Other factors that make contact analysis complicatedinclude:

The region of contact is typically unknown at the start of the

analysis. Most contact problems include friction.

Friction is a path-dependent (energy-dissipating)phenomenon that requires an accurate load history, withsmall time steps.

Frictional response can be chaotic, making solutionconvergence difficult.

Parts might be unconstrained except for contact with otherparts.

Prior to establishment of contact, such parts are initiallyunconstrained free bodies, with zero overall stiffness.

In a static analysis, unconstrained free bodies aremathematically unstable. The solution blows up.


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Training Manual


... Basic Concepts

Fortunately, current ANSYS contact technology enables youto analyze a broad class of contact models fairly easily.

Many models can be solved successfully with very little userintervention.

For those models that exhibit more difficult convergencebehavior, a rich library of contact element options are

available to help you overcome such difficulties. These more advanced features are discussed in the Advanced

Contact and Bolt Pretension training manual.


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Training Manual


... Basic Concepts

Contact classification

Contact problems fall into two general classes:rigid-to-flexible and flexible-to-flexible.

Rigid-to-flexible One or more contacting surfaces are treated as rigid. (One

surface has a significantly higher stiffness than the other.)Many metal forming problems fall into this category.

Stresses within the rigid body are not calculated.

Flexible-to-flexible Both or all contacting bodies are deformable. (All surfaces

have similar stiffnesses.) A bolted flange connection wouldbe an example of flexible-to-flexible contact.


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Training Manual


... Basic Concepts

Rigid-to-flexible contact

Rigid Surface

DeformableBody


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Training Manual


... Basic Concepts

Flexible-to-flexible contact

Splined shaft interference

fit, both parts are flexible.


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Training Manual


... Basic Concepts

Contact compatibility

Physical contacting bodies do not interpenetrate. Therefore,the program must establish a relationship between the twosurfaces to prevent them from passing through each other inthe finite-element analysis.

When the program prevents interpenetration, we say that it

enforces contact compatibility.F

TargetContact

Penetration occurs when contact compatibility is not

enforced. F


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Training Manual


... Basic Concepts

Contact compatibility penalty method

The penalty method of enforcing contact compatibility uses acontact spring to establish a relationship between the twosurfaces.

The spring stiffness is called the penalty parameter or, morecommonly, the contact stiffness .

The spring is inactive when the surfaces are apart (open status),and becomes active when the surfaces begin to interpenetrate(closed).

The spring will deflect anamount , such that equilibriumis satisfied:

F = k where k is the contact stiffness.

F


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Training Manual


... Basic Concepts

contact compatibility penalty method

Some finite amount of penetration, , is requiredmathematically to generate a contact force at the interface.

This contact force is needed for equilibrium. Thus, must be greater than zero for equilibrium.

However, physical contacting bodies do not interpenetrate. Therefore, for best accuracy, the goal is to minimize the

amount of penetration that occurs at the contact interface.

Minimum penetration gives maximum accuracy. This implies that, ideally, the contact stiffness should have a

very great value. However, too high of a value can lead to convergence

difficulties.


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Training Manual


... Basic Concepts

If the contact stiffness is too great, a slight penetration willgenerate an excessive contact force, potentially throwing thecontacting surfaces apart in the next iteration.

Using too great a contact stiffness usually leads to oscillatingconvergence, and often to outright divergence.

Iteration n Iteration n+1

F

F

F contactF

Iteration n+2


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Training Manual


... Basic Concepts

Contact compatibility Lagrange multiplier method

An alternative method, the Lagrange Multiplier method, adds

an extra degree of freedom (contact pressure), to satisfy theimpenetrability condition.

F


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Training Manual


... Basic Concepts

Contact compatibility augmented Lagrangian method The most modern ANSYS contact elements can combine both

the penalty method and the Lagrange multiplier to enforcecontact compatibility. This is called the augmented Lagrangian method.

In the first series of iterations, contact compatibility isdetermined based on the penalty stiffness. Once equilibriumis achieved, the penetration tolerance is checked. At thispoint, if necessary, the contact pressure is augmented andthe iterations continue.

F

PenetrationTolerance


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Training Manual


... Basic Concepts

contact compatibility augmented Lagrangian method

Penetration

correctiondue to equilibrium.

Oscillation occursduring correctionstage.

Augmentingcontact stresses toreduce

penetration.


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Training Manual


... Basic Concepts

Surface-to-surface elements

The most modern contact elements available in ANSYS arethe surface-to-surface elements.

Targe169 and 170 Rigid or deformable target surfaces

Conta171 to 174 Deformable contact surfaces

These elements are well suited for surface-to-surface contactapplications such as interference fit assembly contact, entry

contact, forging, and deep drawing.


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Training Manual


... Basic Concepts

surface-to-surface elements

These contact elements use the concept of a contact pair,which is composed of a target surface and a contact surface.

The surface-to-surface contact elements overlie the underlyingfinite element model like a skin.

Separate element types define the target and contact surfaces. The contact pair is identified through a shared real constant set.

Contact elements (REAL = N )

on the contact surface

Target elements (REAL = N )

on the target surface


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Training Manual


... Basic Concepts


ANSYS supports both rigid-to-flexible and flexible-to-flexible surface-to-surface contact models.

Flexible-to-flexible models have a deformable target surface ,

which is created whenever target elements overliedeformable elements.

Rigid-to-flexible models have a rigid target surface , whichdoes not overlie any deformable elements.

Rigid target element

Contact elements

Underlyingmesh


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Training Manual


... Basic Concepts


The surface-to-surface contact elements use the augmented Lagrangian to enforce contact compatibility (default).

Augmented Lagrangian uses both a contact stiffness and apenetration tolerance.

For simple contact models, you will normally need tospecify just three characteristics of the contact pair:

A value for the contact stiffness A value for the penetration tolerance.

You will also need to determine which surface in the pair shouldbe the target, and which should be the contact.


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Training Manual


... Basic Concepts

Contact stiffness

The contact stiffness is the most important parameteraffecting both accuracy and convergence behavior.

Recall that greater stiffness gives better accuracy, but moredifficult convergence.

You must carefully determine an appropriate value forcontact stiffness.

The best value is often problem-dependent. Program-supplied default value will often not be appropriate.

Some experimentation may be required to determine anappropriate value that generates a converged solution with anacceptable level of accuracy.

d


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Training Manual


... Basic Concepts

contact stiffness For the surface-to-surface elements, you specify the contact

stiffness as a factor (FKN). That is, the program determinesthe contact stiffness by multiplying the stiffness of theunderlying element times a factor that you specify.

kcontact = FKN x f(k underlying )

For the surface-to-surface elements, the contact stiffness hasunits of stiffness per unit area, or (F/L)/(L 2). A good value for contact stiffness is often obtained by

making the contact stiffness equal to the stiffness of the

underlying elements. As a starting estimate, try:

FKN = 1.0 for bulky solids in contact.

FKN = 0.1 for more flexible (bending-dominated) parts.

I d i


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Training Manual


... Basic Concepts

contact stiffness

Sometimes, different regions of a model may need to havedifferent contact stiffnesses.

Bulky contact; try FKN = 1.0

Flexible contact; try FKN = 0.1

I t d ti t t t


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Training Manual


... Basic Concepts

contact stiffness Determining a good stiffness value may require some

experimentation. The following procedure may be used as a

guideline: Use a low value of FKN to start. Run the analysis to a fraction of the final load.

Check the penetration and number of equilibrium iterations usedin each substep. If convergence is being driven by the penetration tolerance,

FKN may be underestimated (or the penetration tolerancemay simply be too tight).

If no force convergence, or if many iterations are required toobtain force convergence, FKN may be overestimated.

Adjust FKN and run the full analysis.



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Training Manual


... Basic Concepts

contact stiffness

Because results can be sensitive to the value used for FKN,you should always verify the validity of your results.

If you have any doubts about the validity, perform a sensitivitystudy.

Vary the contact stiffness and re-run.

Monitor some item of significance to your analysis, suchas:

Contact pressure. Maximum stress.

Look for a value of FKN beyond which results stabilize to afairly consistent value.



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Training Manual


... Basic Concepts

contact stiffness

As an example, FKN was varied for an interference-fitanalysis of a collar on a shaft. Maximum equivalent vonMises stress was monitored.

In this particular case, a value of FKN = 1.0 proved to giveadequate accuracy.

FKN Max. SEQV

0.001 4,000

0.01 20,0000.1 65,0001 91,000

10 92,900100 93,000



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Training Manual


A. Contact stiffness Workshop

Please refer to your Workshop Supplement for instructions on:

W14. Introduction to Contact - Contact Stiffness Study (FKN)



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Training Manual


... Basic Concepts

Penetration tolerance

The penetration tolerance also affects convergence andaccuracy, although to a lesser extent than the contactstiffness.

As the penetration tolerance is tightened, the accuracy mayimprove, but at the expense of more difficult convergence.

As with the stiffness, you specify thepenetration tolerance by means of a factor (FTOLN).

The program determines the penetrationtolerance by multiplying the depth of theunderlying element (h) times a factor that youspecify.

Tolerance = FTOLN x h



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Training Manual


... Basic Concepts

penetration tolerance

Dont use a soft FKN and a tight FTOLN.

Best convergence behavior is usually obtained if the contactstiffness does most of the work of enforcing compatibility.

Use a reasonably stiff value for FKN.

Fine-tune the penetration with a reasonable value ofFTOLN.

Too small a value for FTOLN will lead to convergencedifficulty. Never use too small of a tolerance! Increasing the

penalty stiffness (FKN) will reduce the penetration. Although increasing FKN by 100 times will usually reduce the

penetration accordingly, other items of significance, such asthe contact pressure, might change by as little 5%.



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Training Manual


A. Penetration tolerance Workshop


W15. Introduction to Contact - Penetration Tolerance Study(FTOLN)



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Training Manual... Basic Concepts

Designating contact and target surfaces The program defines the contact surface by a set of discrete

contact points (the element Gauss points). The program defines the target surface as a continuous

surface. The two surfaces can interpenetrate between the Gauss

points, without contact being recognized. This causesinaccuracies.

Target Surface

Contact Surface

The target can penetrate the contact surface in between the Gauss points.



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designating contact and target surfaces

You can reduce the amount of unrecognized interpenetrationby careful designation of the target and contact surfaces.

However, for rigid-to-flexible contact, the choice of target andcontact is unchangeable. The rigid surface must always bethe target.

Steel shaft (rigid)must be the targetsurface

Rubber boot (flexible)must be the contactsurface



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Training Manual


For flexible-to-flexible contact, best accuracy is achieved bymaximizing the number of contact points. Follow theseguidelines when designating flexible-to-flexible contact andtarget surfaces:

If one surface has a coarse mesh and the other a fine mesh, thesurface with the coarse mesh should be the target surface.

Consider what would happen if the designations werereversed:

... Basic Concepts

xxxxIf the finer mesh were the

target, it would droopacross the Gauss points,and penetrate as shown.



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Other guidelines for designating contact and target surfaces: If a convex surface comes into contact with a flat or concave

surface, the flat or concave surface should be the target surface.

If one surface is stiffer than the other, the stiffer surface shouldbe the target surface.

If one surface is higher order and the other is lower order, thelower order surface should be the target surface.

If one surface is larger than the other, the larger surface shouldbe the target surface.



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designating contact and target surfaces When all contact elements are on one surface and all target

elements are on another surface, the model is said to have

asymmetric contact. Asymmetric contact is generally the most efficient way to model

surface-to-surface contact. However, under some circumstances asymmetric contact will

not perform satisfactorily. No clear distinction exists between target and contact

surfaces.

Both target and contact surface have coarse meshes.

Sometimes the target-contactdistinction is not clear



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An alternative approach is to create a double set of contactpairs. This is known as symmetric contact.

After creating one contact pair, simply create a secondcontact pair having a reversed target-contact designation, forthe same surfaces.

Contact

Target

Target

Contact

Step 1 - Create 1 st

Contact Pair

Step 2 - Create 2 nd

(Symmetric) Contact Pair



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Although some models may require symmetric contact foraccuracy, be aware that it is less efficient than asymmetriccontact.

Therefore, dont use symmetric contact unless it is required.

Note also, that when using symmetric contact,postprocessing is more difficult.

The contact pressure is the average value from both sets ofcontact elements.

This average value is not automatically available as a standardresults item. You will need to calculate it using APDL orETABLE.



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Training ManualA. Contact & target Workshop


W16. Introduction to Contact - Contact & Target Designation



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Rigid target surfaces pilot node By default, the program will automatically constrain a rigid

target surface. That is, it will automatically ground the target

with zero-value displacements and rotations. To model more complicated behavior of the rigid target, you

can create a special one-node target element, called a pilot node.

This element is associated with the target surface by means ofhaving the same REAL attribute.

The pilot node acts as a handle for the entire rigid surface.

You can specify nonzero displacements, rotations, forces,and/or moments at the pilot node to model rigid-body motion ofthe target surface.

Note that if a pilot node exists, the program will not auto-

constrain the rigid surface.



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pilot node

The pilot node can be specifiedat any location. This allows forgeneral rotations of the rigidtarget surface.

Only the pilot node can connect

to other elements. Forexample, to account for themass of a rigid body, define amass element (MASS21) at the

pilot node. Each target surface can have

only one pilot node.

Rigid surface rotated

Pilot node

(at centerof rotation)


i h i d


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Training ManualB. Using the contact wizard

Surface-to-surface contact elements can be created usingstandard element-generation procedures: establish elementTYPE, REAL, and MAT data, set element attributes, designate

target and contact surfaces, and then either mesh or useESURF operations.

This procedure forces you to carry a fairly heavy mentaloverhead as you create your contact pairs.

Fortunately, there is a better way ...


U i h i d


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Training Manual... Using the contact wizard

The contact wizard provides a simple way to construct acontact pair for most contact problems. The contact wizardwill guide you through the process of creating a contact pair.

Preprocessor > Create > Contact Pair > Contact Wizard


U i th t t i d


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Benefits of the Contact Wizard: Automatically defines element types and real constant sets Quickly accesses the contact options and parameters Contact pair viewing tools Quickly displays and reverses contact normals

The Contact Wizard is not available until you have meshed some portion of your model.

Before launching the wizard to create a flexible-to-flexible model,mesh all parts of the model that will be used as contact surfaces.

To create a rigid-to-flexible model, mesh only the parts of themodel that will be used as flexible contact surfaces.


U i g th t t i d


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Lets see how we could add a contact pair to the connectormodel from the Chapter 6 workshops.

First, designate rigid or flexible target type (if a choice isapplicable), and pick the target surface. You can pick nodes, nodecomponents, lines, or areas, depending on what entities are presentin the model.


Using the contact wizard


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Next, pick the contact surface. Again, you can pick nodes, nodecomponents, lines, or areas, depending on what entities are presentin the model.




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If you need to include friction effects, specify the coefficient of friction.(More on friction on the next slide.)

Click on Optional settings tospecify the contact stiffness andpenetration tolerance. (Otheroptions will be discussed in theAdvanced Contact and Bolt

Pretension training course.)




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A note about friction The surface-to-surface elements provide a basic Coulomb

friction model that defines the equivalent friction stress atwhich sliding occurs: = x p

is the coefficient of friction, defined as a material property, asshown in the contact wizard on the previous slide.

If equals zero (default), no shear stress will be transmitted. p is the contact pressure.

When the shear stress between the two surfaces exceeds x p, the two surfaces will slide relative to each other.

More advanced friction options are also available. They arediscussed in the Advanced Contact and Bolt Pretension training manual.

T i i M l




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Last, click on Create to create the contact and target elementsthat comprise this contact pair.

The REAL constant

set used for thispair is identified.

The elements in the pair areautomatically plotted, with their ESYSsymbols ON to indicate outwardnormal directions. If any normals arepointing in the wrong direction, youcan flip them now.

T i i g M l




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Training Manual

At any time after a contact pair is created, you can also usethe wizard to view and list the elements.

... Using the contact wizard

Training Manual




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You can also use the wizard to delete a contact pair.

Training Manual




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Pilot node The contact wizard does not enable you to create a pilot node

for a rigid target surface. To create a pilot node, you will have

to use meshing or direct generation: First, set the element attributes (MAT, REAL, TYPE) for the target

elements. Remember to use the same REAL set as for the rest of the

contact pair. Then mesh a keypoint that is located at the desired position for

the pilot node. Recall that the pilot node can be at any location it is not

required to be physically attached to the other targetelements.

Alternatively, for direct generation, set an additional attribute forthe target-element shape (TSHAP, PILO ) and then create the

element at the desired node.

Training Manual


C Obtaining the solution


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Training ManualC. Obtaining the solution

Automatic solution control, along with the default optionsettings for surface-to-surface elements, generally leads tofairly robust solutions for a broad class of contact analyses.

If convergence difficulties are encountered, they generallyarise due to one or more of three causes: Too great a value for contact stiffness. Too tight a value for penetration tolerance. Too large a value for minimum time step size.

To improve convergence, try these modifications to yourmodel, in the following order of implementation:

Use a smaller FKN. Use a larger FTOLN. Use a smaller minimum time step size (or larger maximum

number of substeps).

Training Manual


... Obtaining the solution


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Training Manual... Obtaining the solution

If your model still wont converge with modified FKN, FTOLN,and time step size, then the advanced contact options willprobably be needed.

These are discussed in the Advanced Contact and Bolt Pretension training manual.

If your model includes friction, small time step sizes arerequired for accuracy, because friction is a path-dependentphenomenon.

Unlike plasticity, there is no cutback control that will triggerbisection if a friction time step is too large.

Note that in a contact solution, all equilibrium iterations arecarried out before bisection for the first substep, to helpestablish initial contact conditions.

Training Manual


D. Postprocessing


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gD. Postprocessing

Results from contact models will contain many additionalitems related to contact. Some of the more commonly useditems include:

STAT Contact StatusPENE PenetrationGAP GapPRES Contact PressureSFRI Friction Stress

These items are associated with the contact element (not thetarget element), and can be readily accessed in the GUI.

Using the General Postprocessor, you can display them in Nodal(averaged) or Element (unaveraged) contour plots.

Animated plots are especially helpful for contact analyses. Using the Time-History Postprocessor, you can plot them as

time-history variables.

Training Manual


... Postprocessing


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p g

The contact status is an integer number that designates thecurrent status of the contact element:

STAT = 0: Open and not near contact.

STAT = 1: Open, but near contact. STAT = 2: Closed and sliding. STAT = 3: Closed and sticking.

Training Manual


... Postprocessing


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p g

Note that a Nodal (averaged) contour plot of status will exhibit non-integer contour values, due to the averaging. Element (unaveraged)contour plots are usually more appropriate for contour plots ofelement status.

Similarly, anomalous Nodal contour plots can occur for othercontact results items. If a Nodal (averaged) contour plot doesntseem to make sense, try an Element (unaveraged) plot instead.

Contact Region Status >1 innodal

contour plot

Status = 1 inelement

contour plot

Training Manual


... Postprocessing


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p g

The penetration or gap distance represents the actual amountof penetration (for STAT = 2 or 3) or open gap (for STAT = 1),in consistent units of length.

In listings, penetration distance is positive, and gap distance isnegative. Because contour plots display either penetration or gap (not

both simultaneously), the contours always display as positive

values for both items. The contact pressure (PRES) and friction stress values (SFRI)

represent the current values for the contact element, copiedfrom the Gauss points to the nodes.

Training Manual


E. Workshop


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p


W17. Introduction to Contact - Snap-fit

Training Manual


F. Assembly contact


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Many analysts now import solid models from CAD packages,and then mesh and analyze the imported models in ANSYS.

For most CAD programs, a single volume, however complex,

represents a single part. Multiple-volume models will haveseveral parts associated together in an assembly.

Part B

Part A

Training Manual


... Assembly contact


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The boundaries between parts within an assembly usuallyrepresent mathematical discontinuities within the CAD model.

When meshed in ANSYS, there will be a discontinuity in the

mesh at these part boundaries. Nodes on one side of theboundary will not talk with nodes on the other side of theboundary.

Surface-to-surface contact elements can be used to connect

the mesh across part boundaries, using a concept known asassembly contact.

Glue parts A & B

together usingassembly contact

Training Manual




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Although assembly contact uses advanced contact featuresto glue parts together, the procedure is straightforward andvery consistent for different models.

In fact, ANSYS Inc.s designer-level product, DesignSpace,includes automated assembly-contact procedures. It has beenused successfully by designers who have no concept ofnonlinear analysis procedures.

Because assembly contact is widely applicable, and isusually simple (i.e., having robust convergence behavior),we will present the procedure here, with little explanation ofthe advanced features being used.

Training Manual




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The CAD assembly must have these characteristics: The parts must touch each other along a boundary. The boundary surfaces need not match perfectly; some

mathematical noise in the geometry is tolerable. The two adjacent parts must be meshed before an assembly

contact pair can be generated.

Training Manual




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As a tool for gluing interfacing boundaries together,assembly contact is more generally applicable thanNUMMRG, EINTF, CPINTF, CEINTF, and other such tools thathave been used in the past.

It is valid in large-displacement analyses (unlike coupling andconstraint equations).

It can connect mismatched meshes (unlike NUMMRG and

EINTF). Assembly contact takes advantage of the bonded contact

feature of the surface-to-surface contact elements to gluedisconnected parts together.

To model a stronger bond, a very stiff value of FKN is oftenused.

It is not unusual to use FKN = 10.

Training Manual




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The key to assembly contact is the creation of an initiallyperfect contacting surface having no initial forces actingacross the contact interface.

This is achieved by using the Exclude everything initial-penetration option, along with the Bonded (always) contactsurface behavior:

Training Manual




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If the only nonlinearity in your model is assembly contact, youcan sometimes turn nonlinear iterations off in your analysis.

Solution > Unabridged Menu > -Load Step Opts- Solution Ctrl

Solution >Unabridged Menu >-Load Step Opts- Nonlinear>Equilibrium Iter

However, realize that turning off nonlinear iterations couldproduce a model that is not in perfect equilibrium.

Turn automaticsolution controlOFF

Specify ONEequilibriumiteration

Training Manual




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A model that has parts glued together with assembly contactcan be used in many types of analyses, including:

Nonlinear static.

Nonlinear transient dynamic. Linear modal. Linear eigenvalue buckling.

The initial status of the contact elements is frozen in alinear analysis (such as modal or eigenvalue bucklinganalysis).

Training Manual


F. Assembly contact Workshop


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W18. Introduction to Contact - Assembly Contact

Documents

Contact Mechanic