aws90 structural nonlin ch03 contact

8/17/2019 AWS90 Structural Nonlin Ch03 Contact

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Advanced Contact

Chapter 3


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Advanced Solid Body Contact Options

Chapter Overview

• The various advanced solid body contact options will bediscussed in detail in this chapter:

– Most of these advanced options are only applicable to contactinvolving solid body faces, not surface bodies.

– It is assumed that the user has already covered Chapter NonlinearStructural prior to this chapter.

• The following will be covered in this Chapter: – Contact !ormulations

– Contact "s.Target, #ymmetric$Asymmetric %ehaviors

– &eviewing results

– 'inball &egion, #tatus

– Interface Treatment , offset, ad(ust to touch

– !riction

• The capabilities described in this Chapter are generally applicableto ANSYS Structural licenses and above.

– )*ceptions will be noted accordingly


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A. Contact

Description of Contact:• +hen two separate surfaces touch each other such that

they become mutually tangent, they are said to be in

contact.

• In the common physical sense, surfaces that are in contact

have these characteristics: – 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.

• Contact is a changingstatus nonlinearity. That is, the

stiffness of the system depends on the contact status,

whether parts are touching or separated.


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! "nforcing #$penetrability Condition

%ow co$patibility is enforced in a contact region:• 'hysical contacting bodies do not interpenetrate.

Therefore, the program must establish a relationship

between the two surfaces to prevent them from passing

through each other in the analysis.

– +hen the program prevents interpenetration, we say that itenforces contact co$patibility.

– #imulation offers several different contact algorith$s to

enforce compatibility at the contact interface.!

TargetContact

&enetration occurs when contact

co$patibility is not enforced.

!


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! Contact Algorith$: &enaltybased

• !or nonlinear solid body contact of faces, &ure &enalty or Aug$ented 'agrange formulations can be used:

– %oth of these are penaltybased contact formulations:

-ere, for a finite contact force ( nor$al , there is a concept of

contact stiffness ) nor$al . The higher the contact stiffness, the

lower the penetration * penetration, as shown in the figure below

• Ideally, for an infinite ) nor$al , one would get ero penetration. This is

not numerically possible with penaltybased methods, but as long

as * penetration is small or negligible, the solution results will be

accurate.

!n

*p

n penetrationormal normal xk F =


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! Contact Algorith$: &enaltybased

• The main difference between &ure &enalty and Aug$ented'agrange methods is that the latter augments the contact

force /pressure0 calculations:

• %ecause of the e*tra term , the augmented 1agrange

method is less sensitive to the magnitude of the contact

stiffness ) nor$al .

n penetrationormal normal xk F =

λ += n penetrationormal normal xk F

'ure 'enalty:

Augmented 1agrange:


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! Contact Stiffness

• The Nor$al Contact Stiffness ) nor$al is the most importantparameter affecting both accuracy and convergence

behavior.

– A large value of stiffness gives better accuracy, but the

problem may become more difficult to convergence.

• If the contact stiffness is too large, the model may oscillate, with

contacting surfaces bouncing off of each other

Iteration n Iteration n+,

!

!

contact!

Iteration n+-


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! Contact Stiffness

• The default 2ormal #tiffness is automatically determined by#imulation. The user may enter a usersupplied value

manually

– The user may input a 2ormal #tiffness !actor4 with 5.64

being the default. The lower the factor, the lower the contact

stiffness.

• #ome general guidelines on selection of 2ormal #tiffness

for contact problems:

– !or bul7dominated problems: 8se 'rogram Controlled4 or

manually enter a 2ormal #tiffness !actor4 of 54

– !or bendingdominated problems: Manually enter a 2ormal#tiffness !actor4 of 6.654 to 6.54

• The user may also have #imulation update the contact

stiffness between each e9uilibrium iteration or substep.


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! Contact Stiffness

• )*ample showing effect of contact stiffness:

• As is apparent from the abovetable, the lower the contact

stiffness factor, the higher the

penetration. -owever, it also

often ma7es the solution

faster$easier to converge/fewer iterations0

• The 2ormal 1agrange4 method

will be discussed ne*t.

!ormulation 2ormal #tiffness Ma* 'enetration Iterations

Augmented Lagrage 0.01 2.84E-03 1% 26.102 1% 0.979 36% 2.70E-04 2

Augmented Lagrage 0.1 2.80E-03 0% 25.802 0% 1.228 20% 3.38E-05 2

Augmented Lagrage 1 2.80E-03 0% 25.679 0% 1.568 2% 4.32E-06 3

Augmented Lagrage 10 2.80E-03 0% 25.765 0% 1.599 4% 4.41E-07 4

Normal Lagrange - 2.80E-03 0% 25.768 0% 1.535 0% 3.17E-10 2

Ma* eform Ma* )9v #tress Ma* Contact 'ressure


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! Contact Algorith$: 'agrangebased

•Another available option is 'agrange $ultiplier algorithm: – The 2ormal 1agrange algorithm adds an e*tra degree of

freedom /contact pressure0 to satisfy contact compatibility.

Conse9uently, instead of resolving contact force as contact

stiffness and penetration, contact force /contact

pressure0 is solved for e*plicitly as an e*tra ;!.

• )nforces ero$nearlyero penetration with pressure ;!

• oes not re9uire a normal contact stiffness /ero elastic slip0

• &e9uires irect #olver, which can be more computationally

e*pensive

!

DOF F normal =


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! Contact Chattering

•Chattering is an issue which often occurs with 2ormal1agrange method

– If no penetration is allowed /left0, then the contact status is

either open or closed /a step function0. This can sometimes

ma7e convergence more difficult because contact points may

oscillate between open$closed status. This is called chattering

– If some slight penetration is allowed /right0, it can ma7e it

easier to converge since contact is no longer a step change.

Closed

;pen


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! Contact Algorith$: &Cbased

•!or the specific case of %onded4 type of contact, the &C formulation is available.

– M'C, or Multi'oint Constraint, internally adds constraint

e9uations to tie4 the displacements between contacting

surfaces

– This approach is not penaltybased or 1agrange multiplier

based. It is a direct, efficient way of relating surfaces of

contact regions which are bonded.

– 1argedeformation effects also are supported with M'Cbased

bonded contact


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! /angential Behavior

•The aforementioned options relate contact in the nor$al direction. If friction or rough$bonded contact is defined, a

similar situation e*ists in the tangential direction.

– #imilar to the i$penetrability condition, in the tangential

direction, the two bodies should not slide relative to each other

if they are stic7ing4

– A penalty algorithm is always used in the tangential direction

– Tangential contact stiffness and sliding distance are the

analogous parameters:

where * sliding ideally is ero for stic7ing, although some slip is

allowed in the penaltybased method.

– 8nli7e the 2ormal Contact #tiffness, the Tangential Contact

#tiffness cannot directly be changed by the user.

sliding tangential tangential xk F =If stic7ing4:


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! Contact Algorith$ Su$$ary

•A summary of the contact algorithms available in#imulation is listed below:

5 Tangential stiffness is not directly input by user

– The 2ormal 1agrange4 method is named as such because

1agrange multiplier formulation is used in the Nor$al

direction while penaltybased method is used in the tangential

direction.

!ormulation 2ormal Tangential 2ormal #tiffness Tangential #tiffness Type

Augmented Lagrange Augmented Lagrange enalt! "e# "e#1 An!

ure enalt! enalt! enalt! "e# "e#1 An!

$ $ $ - - &onded 'nl!

Normal Lagrange Lagrange $ult()l(er enalt! - "e#1

An!


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! Solid Body Contact Options

• Although 'ure 'enalty4 is the default in #imulation, Augmented

1agrange4 is recommended for general frictionless or frictional

contact in largedeformation problems.

– Augmented 1agrange formulation adds an additional control of the

automatically reducing the amount of penetration, so that is why it is

preferred in general nonlinear problems

• The 2ormal #tiffness4 is the contact stiffness) nor$al e*plained earlier, used only for 'ure

'enalty4 or Augmented 1agrange4

– This is a relative factor. The use of 5.6 is

recommended for general bul7 deformation

dominated problems. !or bendingdominated

situations, a smaller value of 6.5 may be useful

if convergence difficulties are encountered.

– The contact stiffness can also be automatically

ad(usted during the solution. If difficulties arise,

the stiffness will be reduced automatically. AN*"* L(+en#e A,a(la(l(t!

esign#pace )ntra

esign#pace

'rofessional #tructural

Mechanical$Multiphysics


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! Co$parison of (or$ulations

• The table below summaries some pros /=0 and cons /0 with

different contact formulations:

– 2ote that some topics, such as symmetric contact or contact detection, will be

discussed shortly

/

0ood +on,ergen+e

ea,(or 23e4 e5u(l(r(um

(terat(on#6

-

$a! re5u(re add(t(onal

e5u(l(r(um (terat(on# (3

)enetrat(on (# too large

-

$a! re5u(re add(t(onal

e5u(l(r(um (terat(on# (3

+atter(ng (# )re#ent

/

0ood +on,ergen+e

ea,(or 23e4 e5u(l(r(um

(terat(on#6

-

*en#(t(,e to #ele+t(on o3

normal +onta+t #t(33ne##

Le## #en#(t(,e to

#ele+t(on o3 normal

+onta+t #t(33ne##

/

No normal +onta+t

#t(33ne## (# re5u(red /

No normal +onta+t

#t(33ne## (# re5u(red

-

%onta+t )enetrat(on (#

)re#ent and

un+ontrolled

%onta+t )enetrat(on (#

)re#ent ut +ontrolled to

#ome degree

/

7#uall!8 )enetrat(on (#

near-9ero /

No )enetrat(on

/7#e3ul 3or an! t!)e o3

+onta+t ea,(or /

7#e3ul 3or an! t!)e o3

+onta+t ea,(or /

7#e3ul 3or an! t!)e o3

+onta+t ea,(or -

'nl! onded +onta+t

ea,(or (# allo4ed

/E(ter :terat(,e or ;(re+t

*ol,er# +an e u#ed/

E(ter :terat(,e or ;(re+t

*ol,er# +an e u#ed-

'nl! ;(re+t *ol,er +an

e u#ed/

E(ter :terat(,e or ;(re+t

*ol,er# +an e u#ed

/

*!mmetr(+ or

a#!mmetr(+ +onta+t

a,a(lale

/

*!mmetr(+ or

a#!mmetr(+ +onta+t

a,a(lale

A#!mmetr(+ +onta+t

onl!

A#!mmetr(+ +onta+t

onl!

/%onta+t dete+t(on at

(ntegrat(on )o(nt#/

%onta+t dete+t(on at

(ntegrat(on )o(nt#


node#


node#

M'C2ormal 1agrangeAugmented 1agrange'ure 'enalty


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! Co$parison of (or$ulations

• !or bonded contact , #imulation uses 'ure 'enalty

formulation with large 2ormal #tiffness by default.

– This provides good results since the contact stiffness is high,

resulting in small$negligible penetration.

– M'C formulation is a good alternative for bonded contact

because of its many nice features.

• !or frictionless or frictional contact , consider using either

Augmented 1agrange or 2ormal 1agrange methods.

– The Augmented 1agrange method is recommended, as noted

previously, because of its attractive features and fle*ibility.

– The 2ormal 1agrange method can be used if the user does notwant to bother with 2ormal #tiffness value and wants ero

penetration. -owever, note that the irect #olver must be

used, which may limit the sie of the models solved.


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B. Contact vs. /arget

• Internally, the designation of Contact and /arget surfaces

can be very important

– In #imulation, under each Contact &egion,4 the Contact and

/arget surfaces are shown. The normals of the Contact

surfaces are displayed in red while those of the /arget

surfaces are shown in blue.

– The Contact and /arget

surfaces designate which

two pairs of surfaces

can come into contact

with one another.

AN*"* L(+en#e A,a(la(l(t!

esign#pace )ntra

esign#pace




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! Sy$$etric0Asy$$etric Behavior

• %y default, A2#># uses Sy$$etric Behavior .

– This means that the Contact surfaces are constrained from

penetrating the Target surfaces and the Target surfaces are

constrained from penetrating the Contact surfaces.

• If the user wishes, Asy$$etric Behavior can be used

– !or Asy$$etric or AutoAsy$$etric

Behavior , only the Contact surfaces are

constrained from penetrating the Target

surfaces.

– In AutoAsy$$etric Behavior , the Contact

and /arget surface designation may be

reversed internally

– Although it is noted that surfaces are

constrained from penetrating each other,

recall that with 'enaltybased methods,

some small penetration may occur. AN*"* L(+en#e A,a(la(l(t!

esign#pace )ntra

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! Sy$$etric0Asy$$etric Behavior

• !or Asy$$etric Behavior , the nodes of the Contact surface

cannot penetrate the /arget surface. This is a very

important rule to remember. Consider the following:

– ;n the left, the top red mesh is the mesh on the Contact side.

The nodes cannot penetrate the /arget surface, so contact is

established correctly

– ;n the right, the bottom red mesh is the Contact surface

whereas the top is the /arget . %ecause the nodes of the

Contact cannot penetrate the /arget , too much actual

penetration occurs.

Target #urface Contact #urface

Target #urfaceContact #urface


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! Contact vs. /arget Designation

• %ecause of the fact that, for Asy$$etric Behavior , the

Contact surface cannot penetrate the /arget surface but the

inverse is not necessarily true, there are some guidelines in

proper selection of contact surfaces:

– If a conve* surface comes into contact with a flat or concave

surface, the flat or concave surface should be the /arget

surface.

– If one surface has a coarse mesh and the other a fine mesh, the

surface with the coarse mesh should be the /arget surface.

– If one surface is stiffer than the other, the stiffer surface should

be the /arget surface.

– If one surface is higher order and the other is lower order, thelower order surface should be the /arget surface.

– If one surface is larger than the other, the larger surface should

be the /arget surface.


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! Sy$$etric0Asy$$etric Su$$ary

• There are some important things to note:

– ;nly &ure &enalty and Aug$ented 'agrange formulations

actually support #ymmetric %ehavior.

– Nor$al 'agrange and &C re9uire Asymmetric %ehavior.

• %ecause of the nature of the e9uations, #ymmetric %ehavior would

be overconstraining the model mathematically, so Auto

Asymmetric %ehavior is used when #ymmetric %ehavior selected.

– It is always good for the user to follow the general rules of

thumb in selecting Contact and Target surfaces noted on the

previous slide for any situation below where Asymmetric

%ehavior is used.#pecified ;ption 'ure 'enalty Augmented 1agrange 2ormal 1agrange M'C

#ymmetric %ehavior *!mmetr(+ *!mmetr(+ Auto-Asymmetric Auto-Asymmetric

Asymmetric %ehavior Asymmetric Asymmetric Asymmetric Asymmetric

Auto,Asymmetric %ehavior Auto-Asymmetric Auto-Asymmetric Auto-Asymmetric Auto-Asymmetric

#ymmetric %ehavior Results on Both Results on Both Results on Either Results on Either

Asymmetric %ehavior


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! 1eviewing 1esults

• The table on the previous slide alluded to an important

factor in reviewing Contact /ool results

– !or #ymmetric %ehavior, results are reported for both Contact

and Target surfaces.

– !or any resulting Asymmetric %ehavior, results are only

available on Contact surfaces.

• +hen viewing the Contact Tool

wor7sheet, the user may select

Contact or Target surfaces to

review results.

– !or AutoAsymmetric %ehavior,the results may be reported on

either the Contact or Target

– !or Asymmetric %ehavior, ero

results are reported for Target AN*"* L(+en#e A,a(la(l(t!

esign#pace )ntra

esign#pace




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! 1eviewing 1esults2 "*a$ple ,

• !or e*ample, consider the case below of 2ormal 1agrange

!ormulation with #ymmetric %ehavior specified.

– This results in autoasymmetric behavior. #ince it is

automatic, #imulation may reverse the Contact and Target

specification.

– +hen reviewing Contact Tool results, one can see that the

Contact side reports no /ero0 results while the Target side

reports true Contact 'ressure.



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! 1eviewing 1esults2 "*a$ple -

• In another situation, Augmented 1agrange !ormulation with

#ymmetric %ehavior is used

– This results in true symmetric behavior, so both set of

surfaces are constrained from penetrating each other

– -owever, results are reported on both Contact and Target

surfaces. This means that the true4 contact pressure is an

average of both results.



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! 1eviewing 1esults

• #ymmetric %ehavior:

– )asier to set up /efault in #imulation0

– More computationally e*pensive.

– Interpreting data such as actual contact pressure can be more

difficult

• &esults are reported on both sets of surfaces

• Asymmetric %ehavior:

– #imulation can automatically perform this designation /Auto

Asymmetric0 or?

– 8ser can designate the appropriate surface/s0 for contact and

target manually .

• #election of inappropriate Contact vs.Target may affect results.

– &eviewing results is easy and straightforward. All data is on

the contact side.


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! Contact Detection &oints

• ;ne additional note worth mentioning is that contact is

detected differently, depending on the formulation used:

– 'ure 'enalty and Augmented 1agrange !ormulations use

integration point detection. This results in more detection

points /56 in this e*ample on left0

– 2ormal 1agrange and M'C !ormulation use nodal detection

/normal direction from Target0. This results in fewer detection

points /@ in the e*ample on right0

– 2odal detection may handle contact at edges slightly better,

but a localied, finer mesh will alleviate this situation with

integration point detection.

Integration 'oint etection 2odal etection


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! Contact Detection &oints

• !or Asymmetric %ehavior, the integration point detection

may allow some penetration at edges because of the

location of contact detection points.

– The figure on the bottom illustrates this case:

• ;n the other hand, there are more contact detection pointsif integration points are used, so each contact detection

method has its pros and cons.

Target #urface

Contact #urface

/he target can penetrate

the contact

surface.


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C. &inball 1egion

• The 'inball &egion is a very useful concept to understand.

There are several uses for the 'inball &egion:

– 'rovides computational efficiency in contact calculations. The

'inball &egion differentiates near4 and far4 open contact

when searching for which possible elements can contact each

other in a given Contact &egion.

– etermines the amount of allowable gap for bonded contact.

If M'C !ormulation is active, it also affects how many nodes

will be included in the M'C e9uations.

– etermines the depth at which initial penetration will be

resolved if present


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! &inball 1egion

• The 'inball &egion can be thought of as a sphere

surrounding each contact detection point

– If a node on a Target surface is within this sphere, #imulation

considers it to be in near4 contact and will monitor its

relationship to the contact detection point more closely /i.e.,

when and whether contact is established0. 2odes on target

surfaces outside of this sphere will not be monitored asclosely for that particular contact detection point.

– If %onded %ehavior is specified within a gap smaller than the

'inball &adius, #imulation will still treat that region as bonded

'inball radius


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! &inball 1egion

• The sie of the 'inball &egion for each contact detection

point is determined automatically by default.

– The user can change the 'inball &adius directly in the etails

"iew of any Contact branch

– The 'inball sphere will be visualied on the Contact &egion

label. 8se the 1abel icon to move the annotation


esign#pace )ntra

esign#pace



%y specifying a 'inball

&adius, one can visually

confirm whether or not a gap

will be ignored in %onded

%ehavior.The 'inball &egion can also

be important in initial

interference problems or

largedeformation problems.


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D. #nterface /reat$ent

• In the previous section, it was noted that for %onded

%ehavior, a large enough 'inball &adius may allow any gap

between Contact and Target surfaces to be ignored

• !or !rictional or !rictionless %ehavior, bodies can come in

and out of contact with one another. Conse9uently, an

initial gap is not automatically ignored since that mayrepresent the geometry.

• -owever, the finite element method does not allow for rigid

body motion in a static structural analysis. If an initial gap

is present and a force loading is applied, initial contact may

not be established, and one part may fly away4 relative toanother part.


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! Contact Offset

• To alleviate situations where a clearance or gap is modeled

but needs to be ignored to establish initial contact for

!rictional or !rictionless %ehavior, the Interface Treatment

can internally offset the Contact surfaces by a specified

amount.

– ;n the left is the original model /mesh0. The top red mesh is

the body associated with the Contact surfaces

– The Contact surface can be offset by a certain amount, as

shown on the right in light green. This will allow for initial

contact to be established.


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! Contact Offset

• 2ote that using this method will actually change the

geometry since a rigid4 region will e*ist between the

actual mesh and the offset Contact surface.

• This slight modification may be an allowable appro*imation

in some cases, so it is a useful tool to establish initial

contact in static analyses without having to modify the CAgeometry.


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! #nterface /reat$ent

• In the etails view, the user can select Ad(usted to Touch4

or Add ;ffset4

– Ad(usted to Touch4 will let #imulation determine what

contact offset amount is needed to close the gap and establish

initial contact. 2ote that the sie of the 'inball &egion will

affect this automatic method, so ensure that the 'inball

&adius is greater than the smallest gap distance. – Add ;ffset4 allows the user to specify a

positive or negative distance to offset

the contact surface. A positive value will

tend to close a gap while a negative value

will tend to open a gap.

• This can be used to model initial interference

fits without modifying the geometry. Model

the geometry in (usttouching position and

change the positive distance value to the

interference value. AN*"* L(+en#e A,a(la(l(t!

esign#pace )ntra

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%olted oint Assembly

+or7shop 3A

Contact #tiffness


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•


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• &eview the contents of the model

-ighlight each item in the


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• &eview the contents of the model /contDd0:

2ote especially Contact

&egion @. It will be used to

evaluate the pressure profile at

the bushingbrac7et interface

after the bolt preload closes

this gap.

&egion @ is initially set up asan asymmetric frictionless pair

using the 'ure 'enalty

method. &ecall that this

algorithm depends on a

contact stiffness and a very

small penetration to generate

forces at the interface toprevent penetration once

contact is established.

!3or)shop 4A Contact Stiffness


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• &eview the #olution Information %ranch

The red lighting bolts in the #olutionbranch indicates an incomplete #olution

run.

%y highlighting the #olution Information

branch and scrolling down to near the

end of the output, we can see that there

was an unconverged solution with the

current specifications.



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In the etails of #olution

Information4 +indow, switch #olver

;utput to !orce Convergence. This

displays the same convergence

data in graphical form.

2ote, the force convergence value

oscillates up and down betweeniterations well above the acceptable

convergence criteria. After two

automatic bisections, substep 5

converges. -owever, substep

ultimately fails to converge.

• &eview the #olution Information %ranch /contDd0



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• &eview the #olution Information %ranch /contDd0:

Again, scroll further up the

solution information wor7sheet

/near the top of the file0 to find

the contact specifications and

calculations.

The contact pair associated with

the warning /real constant set

5E0 is the manually generated

asymmetric pair for Contact

&egion @. 2ote the large

default contact stiffness

/6.F3e@0 being used. This is an

order of magnitude larger then

the elastic modulus of theunderlying geometry.


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• +e will attempt to achieve a successful

convergence by ad(usting the normalstiffness of Contact &egion @ downwardbased on the feedbac7 reviewed in theunconverged output.

• +ithout changing any specifications inthe current tree, duplicate the Modelbranch as follows:

– In te e'!st!n" *ro+ect tree, !"$!"tBo$te(.o!nt/s03 o(e$

– B – u)$!cate

• &ename the new model branch to reflectthe change that will be made

– Bo$te(.o!nt/s03,or t!&& Factor 1e-3

• This will enable us to run a modifiedanalysis without losing the e*istinginformation.


Ad d S lid B d C O i


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• 8nder the newly created model branch,

highlight Contact &egion @4

• In the etails +indow: – an"e te nora$ st!&&ness s)ec!&!cat!on &ro

*ro"ra ontro$$e( to anua$

– an"e te nora$ st!&&ness &actor &ro te(e&au$t 1: to 1e-3;

• -ighlight the #olution branch for thismodel and &M% to e*ecute a new #olve


Ad d S lid B d C t t O ti


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• The solution now converges successfully in 55 iterations and no bisections. This is ideal.

%isections are a helpful automatic ad(ustment to achieve a converged solution, but theyare not efficient as all the C'8 time from the last successfully converged solution leading

up to the bisection is wasted.




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• &eview the #olution Information of

the successful run. "erify that themodified contact stiffness was used

as e*pected.

• 2ote: A second loadstep with one

iteration was run automatically. This

is because of the presence of bolt

pretension. The first load step

calculates the necessary assemblyinterference needed to generate the

prescribed preload. The interference

used in the analysis is reported as an

Ad(ustment4 value in etails of

'retension %olt 1oad4 window,

along with the resulting reaction

force. The second load step loc7s the

bolt pretension element into this

calculated ad(ustment to achieve the

bolt pretension load. The calculated

reaction force should match the

initially applied preload.




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• &eview Contact &esults at the bushingbrac7et interface /nut

side0:• ;pen the Contact Tool !older and #elect Contact &egion @

–

– e)eat &or ontact *enetrat!on

Is 5e34 an acceptable normal stiffness factor for this modelG

The best way to ensure an accurate result with a standard contact pair is to perform a

sensitivity study with different stiffness values, stiffness updating schemes and algorithms

until results converge to the same correct4 answer. Too high a stiffness can produce

divergence, too low a stiffness can produce convergence but possible over penetration, an

e*cessive bolt pretension ad(ustment and ultimately an inaccurate prediction of surface

contact pressure profile.



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Training Manual


(. (rictional Contact Options

• In addition to the above, frictional contact is available with

ANSYS Structural licenses and above.

• In general, the tangential or sliding behavior of two

contacting bodies may be frictionless or involve friction.

– !rictionless behavior allows the bodies to slide relative to one

another without any resistance. – +hen friction is included, shear forces can develop between

the two bodies.

• !rictional contact may be used with smalldeflection or

largedeflection analyses



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! (rictional Contact Options

• !riction is accounted for with CoulombDs 1aw:

where is the coefficient of static friction

– ;nce the tangential force ( tangential e*ceeds the above value,

sliding will occur

normal tangential F F ⋅≤ µ

=n

=t

µ =n



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! (rictional Contact Options

• In addition to the above, frictional contact is available with

ANSYS Structural licenses and above.

• !or frictional contact, a friction coefficient4 must be input

– A !riction Coefficient of 6.6 results in

the same behavior as frictionless4

contact – The contact formulation, as noted

earlier, is recommended to be set

to Augmented 1agrange4


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esign#pace

'rofessional

#tructural




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! 1eviewing 1esults

• If frictional contact is present, additional contact output is

available

– Contact !rictional #tress and Contact #liding istance can be

reviewed to get a better understanding of frictional effects

– !or Contact #tatus, #tic7ing4 vs. #liding4 results

differentiate which contacting areas are moving


esign#pace )ntra

esign#pace

'rofessional

#tructural




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! Su$$ary of Contact in Si$ulation

• In #imulation, the user can solve contact problems:

–


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%olted oint Assembly

with !riction

+or7shop 3%

Contact !riction



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•


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#teps to !ollow:

• #tart an A2#># +or7bench session. %rowse for and open%oltedointws63%.wbdb4 pro(ect file.

– This pro(ect contains a esign Modeler /M0 geometry file %oltedointws63.agdb4 and a#imulation /#0 file %oltedointws63%.dsdb4.

• -ighlight the %oltedointws63%, frictionless4 file and open a #imulation #ession.

!3or)shop 4B Contact (riction


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• &eview %oltedointws63%, frictionlessbranch /contDd0

The green chec7 mar7s in the #olution

branch indicate a successful4

solution.

2otice the two load steps were solve

within one substep. This was the

default initial specification as indicated

in the solution output wor7sheet.




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• &eview %oltedointws63%, frictionless branch /contDd0

espite the clean solution output,

with no errors, a review of the

displacement results show that

the first run is not correct.

The addition of the bearing load

pushes the bushing thru thebrac7et at the nut side of the

assembly without resistance.

Also the bonded contact pairs at

the bolt head end prevent free

sliding of the bushing

perpendicular to the bolt.




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• &eview %oltedointws63%, friction

A number of contact related changeswere made for the second run to

correctly simulate the relative

displacement of the parts under load.

Carefully review the following changes

in the second branch.

• Contact &egions 5, , 3, B and @:

Change behavior to (rictional 4 with acoefficient of friction of 6..

• Contact &egion E represents the bolt

to nut interface and will remain bonded.

• Create a new frictionless contact

region /PJ0 between the bolt and hole inbrac7et ad(acent to the nut.

• Add !rictional #tress to #olution

Information branch for each of the

regions e*cept PE /bonded0 and PJ

/frictionless0. !or region J we add

2umber /of elements0 Contacting4 to

help monitor solution progress.




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d a ced So d ody Co tact Opt o s

• &eview %oltedointws63%, friction /contDd0

The second simulation convergessuccessfully, this time with multiple

substeps between load steps.

This is by design. !riction is path

dependent and result accuracy is

inversely proportional to time increment

sie.

2otice in the solution output that a

nominal E substeps with a ma*imum of

6 is specified by default. -ad this

model e*perienced convergence trouble,

autotime stepping would have ad(usted

the timestep sie down to a minimum of

5$6 as necessary to resolve theproblem.




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y p

• &eview %oltedointws63%, friction branch /contDd0

+ith the addition of contact friction, the results now reflect the correct response to the

bearing load applied to the bushing perpendicular to the bolt a*is.




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y p


!rom the solution information

branch, plot the number of elements

in contact for &egion PJ. 2otice that

none of these elements come in

contact indicating that the frictional

resistance generated between

brac7et and bushing under the bolt

preload is enough to resist the bear

load.

This pair is still useful, however, for

evaluating the gap between the bolt

and hole after the bearing load isapplied.




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y p


'lot the contact frictional stresses saved to the #olution Information %ranch

These results loo7 9ualitatively correct, but how accurate are theyG

Two basic but important aspects to consider when modeling friction is 9uality of mesh

/especially on the curved surfaces0 and time increment sie /substeps0 used.




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y p


2ow that the model is producing 9ualitatively correct answers, consider the effect of

mesh refinement in critical areas.

&eturn to the 'ro(ect page, highlight the %oltedointws63%,friction4 simulation and

enter the !) )nvironment to evaluate more closely the mesh 9uality of this model.




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y


!) Modeler opens with an Import #ummary

page listing all the !) statistics associated with

this model.

8nder "iews4, highlight the Contacts4

option.




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#elect the region representing the

boltbushing interface /region PB0.

After ooming in on a plot of the

>Q plane, /loo7ing in negative R

direction0 notice how poorly the

curved surfaces are represented

by elements on these surfaces.




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• &eturning to #imulation, review %oltedointws63%, friction 4 branch

2ote the strategic mesh refinement

and siing that has been added to

improve 9uality of results




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• &eview %oltedointws63%, friction /contDd0

The third simulation convergessuccessfully in a very similar pattern to

previous run, e*cept with considerably

longer C'8 time /in seconds0 reflective of

the larger ;! count /previous runs were

in the order of 5E66S0




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• &eview %oltedointws63%, friction /contDd0.

'lot relevant results and compare 9ualitatively and 9uantitatively and with previous run.




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The last simulation is the same model as %oltedoint63%, friction only using smallertime step sie.

This will force the solver to use at least a time step sie of 5$56 with a minimum of 5$66

if necessary.

Another useful but e*pensive option available to help with unconverged solutions

involving friction is activation of full 2ewton&aphson with unsymmetric matrices of

elements. 2&;'T,82#>MM4 This offers a more robust formulation of the stiffness

matri* but should only be used to overcome convergence trouble.

• &eview %oltedointws63%, friction 3 /contDd0




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• Compare contact frictional stresses from the last three

runs

Course mesh, 3 substeps &efined mesh, 56 substeps&efined mesh, 3 substeps

C'8 Time H 5,BE C'8 Time H 56,FJB C'8 Time H @,666



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aws90 structural nonlin ch03 contact

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