Hints for Obtaining a Converged Contact Solution in Abaqus

8/19/2019 Hints for Obtaining a Converged Contact Solution in Abaqus

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Hints for obtaining a convergedcontact solution in Abaqus/Standard

Question My Abaqus/Standard contact analysis is having convergence difficulties. What should I check?

(he follo!ing a""lies to all versions.#

$equest detailed out"ut of the contact state%

Abaqus/&A'% Ste" Module% Output Diagnostic Print ... Contact

ey!ord% *PRINT, CONTACT=YES

&ontact state infor)ation !ill be !ritten in the )essage (.msg# file. *ou should also !rite the necessary contact

out"ut variables to the data (.dat# and the out"ut database (.odb# files frequently enough so that you candiagnose the "roble).

&onsider the follo!ing ite)s%

&heck that the contact surfaces are "ro"erly defined+ and correct any errors. he surfaces canbe vie!ed in Abaqus/,ie!er. If the surface has been created in the in"ut file by auto)atically using thefree surface of an ele)ent set+ check !hether the $IM "ara)eter should have been used.

&heck the contact direction defined for each surface. he nor)als to a surface can be vie!edin Abaqus/,ie!er. If the nor)al directions are !rong+ frequently you !ill get large overclosures that )aylead to convergence difficulties.

-rictional contact "roble)s are generally )ore difficult to solve than frictionless contact"roble)s. ry solving the "roble) !ithout friction to see if the difficulty is due to friction or if it is due toso)ething else. If the difficulty is indeed due to friction+ consider the follo!ing%

. $eea)ine the choice of the friction coefficient0larger coefficients of friction aregenerally )ore difficult to use.

1. 'a)ine the allo!able elastic sli" (lo! values can cause convergence "roble)s+although too high a value can give "hysically incorrect solutions#.

2. $efine the )esh so that )ore "oints co)e into contact at the sa)e ti)e.

3. 4se i))ediate onset of friction if only a fe! "oints are co)ing into in contact at thesa)e ti)e and sli""ing occurs. In ,ersions 5. and higher this is the default behavior. In earlierversions you can use

*CONTACT CONTROLS, FRICTION ONSET=IMMEDIATE

$ough friction !ith inter)ittent contact generally gives convergence "roble)s. If a""ro"riate for the analysis+ do not allo! the contacting surfaces to se"arate after contact is established%

. Abaqus/&A'% Interaction Module% Interaction Property Create Contact

Mechanical Normal Behavior deselect Allow separation after contact

1. ey!ord% *SURFACE BEHAIOR, NO SEPARATION

o hree6di)ensional finite6sliding contact !ith highly curved faceted )aster surfaces

generates a highly nonsy))etric tangent stiffness )atri. 4se the unsy))etric solver even if thecoefficient of friction is less than 7.1.

o If you have shar" corners on the contact surfaces+ the co))ents in 8 a""ly. In

addition+ try the follo!ing%


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. S)ooth the surface. 9odes on the slave surface can be caught in folds in the )astersurface+ causing convergence difficulties !hen the surrounding ele)ents defor) to take thisinto account. he ele)ents )aking u" the slave surface should be s)all enough to be able toresolve the geo)etry. A rough guideline is to use 7 ele)ents around a :7; corner< you )ustuse your =udg)ent to decide if this is adequate or too fine.

1. If the "hysical "roble) has a shar" concave fold+ use t!o se"arate surface definitions.

2. Shar" conve folds cannot be )odeled !ith a reasonable finite ele)ent )esh. S)ooththe fold !ith a radius larger than the ele)ent si>e on the slave surfaces. A rough guideline is touse 7 ele)ents around a :7 degree corner< obviously+ you )ust use your =udg)ent to decide if this is adequate or too fine.

1. &ontact chattering+ !hich is indicated by the contact state for a grou" of slave nodescyclically changing fro) o"en to closed until the )ai)u) nu)ber of severe discontinuity iterations isreached. Abaqus/Standard can auto)atically co)"ute an overclosure tolerance and a se"aration"ressure tolerance to "revent chattering%

. Abaqus/&A'% Interaction Module% Interaction Contact Controls Create...

Automatic overclosure tolerances

1. ey!ord% *CONTACT CONTROLS, AUTOMATIC TOLERANCES

o If you get an error )essage in the )essage (.msg# file that &$I should be

increased+ check that it )akes sense to do so< if it does )ake sense+ increase the value for the relevantcontact "air. he current value of &$I is "rinted to the data (.dat# file.

-or )ore infor)ation see%

o @&ontact "ressure6overclosure relationshi"s@

. Section 25..1 of the Abaqus 5.1 Analysis 4ser@s uide

1. Section 2B..1 of the Abaqus 5.2 Analysis 4ser@s uide

o @-rictional behavior@

. Section 25..8 of the Abaqus 5.1 Analysis 4ser@s uide

1. Section 2B..8 of the Abaqus 5.2 Analysis 4ser@s uide

o @&ontact interaction analysis% overvie!@

. Section 28.. of the Abaqus 5.1 Analysis 4ser@s uide

1. Section 25.. of the Abaqus 5.2 Analysis 4ser@s uide

o @&o))on difficulties associated !ith contact )odeling in Abaqus/Standard@

. Section 2C..1 of the Abaqus 5.1 Analysis 4ser@s uide

1. Section 2:..1 of the Abaqus 5.2 Analysis 4ser@s uide

o @Defining contact "airs in Abaqus/Standard@

. Section 28.2. of the Abaqus 5.1 Analysis 4ser@s uide

1. Section 25.2. of the Abaqus 5.2 Analysis 4ser@s uide


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o Modeling contact !ith tetrahedral ele)ents in Abaqus/Standard

o 4sing aug)ented Eagrangian contact in Abaqus/Standard

o &auses of contact chatter

o &ause of s"urious overconstraint and >ero "ivot !arnings for Abaqus/Standard contact"roble)s

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Modeling contact with second ordertetrahedral elements in

Abaqus/StandardQuestion What is the best !ay to )odel contact surfaces using second6order tetrahedral ele)ents in

Abaqus/Standard?

(he follo!ing a""lies to releases 5.36 and higher.#

Flease consider the follo!ing guidelines !hen )odeling contact !ith second6order tetrahedral ele)ents%

. In $eleases 5.B and higher+ if the finite6sliding+ surface6to6surface contact for)ulation !ithdefault settings is used+ contact surfaces on &2D7 ele)ents generally "rovide better accuracy thancontact surfaces on &2D7M ele)ents 6 !ithout degraded contact robustness. 9ote that the "enalty

constraint enforce)ent )ethod !ith node6centered constraint "ositioning is the default behavior.1. In $eleases 5.: and higher+ &2D7 is "referable !hen the s)all6sliding+ surface6to6surface

contact for)ulation is being used.

2. If the $elease 5.B (or higher# finite6sliding+ surface6to6surface default for)ulation is not used+having &2D7 ele)ents underlay the slave surface can degrade convergence and cause contact stressnoise.

o It can be difficult to obtain converged solutions !hen &2D7 ele)ents are used for the

slave surfaces if GhardG contact constraints are strictly enforced. In these cases contactconstraints associated !ith nodes on corners of &2D7 ele)ents are likely to chatter as a resultof uneven force distributions for these ele)ents. If the analysis does converge+ the contact"ressures are likely to be noisy even though the underlying ele)ent stresses are "robably quiteaccurate.

o &onvergence !ith slave surfaces based on &2D7 ele)ents is i)"roved if aHsoftened "ressure6overclosure behavior is s"ecified (note ho!ever that slave surfaces basedon &2D7M ele)ents are "referred !hen the $elease 5.B (or higher# finite6sliding+ surface6to6surface default for)ulation is not used#%

JS4$-A&' K'A,IL$+ F'9AE*

JS4$-A&' K'A,IL$+ A4M'9'D EA$A9'

JS4$-A&' K'A,IL$+ F$'SS4$'6L,'$&ELS4$' N'OFL9'9IAE P

EI9'A$ P AK4EA$

-or these cases+ Abaqus uses su""le)entary constraints to i)"rove the distribution of contact

forces and reduce contact "ressure noise. While the su""le)entary constraints re)ove onesource of convergence degradation (uneven contact force distribution# they cause another"otential source of convergence degradation in that the total nu)ber of constraints eceeds thenu)ber of slave nodes.

3. 9ote that if the finite sliding surface6to6surface default for)ulation is used !ith &2D7ele)ents+ but the for)ulation "ara)eters (such as "enalty stiffness# are changed+ the effect onconvergence !ill be "roble)6de"endent. -or ea)"le+ in so)e cases raising the "enalty stiffness )ayhar) the convergence behavior.

8. &2D7 ele)ents can )ore accurately re"resent surface curvature than &2D7M ele)ents.

o In )any cases this factor is "redo)inant in the choice of ele)ent ty"e underlying the

)aster surface.

5. -or a H)atched )esh across a contact interface+ having different ele)ent ty"es underlying the

)aster and slave surfaces !ill cause noise in the contact stress solution 6 the ece"tion being the caseof flat surfaces using the surface6to6surface for)ulation.


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o -or ea)"le+ if a )atched )esh is used and other factors i)"ly that &2D7M

ele)ents should underlay the slave surface+ then choosing &2D7M ele)ents (rather than&2D7# for the )aster surface !ill "rovide a better contact stress solution.

B. he "enalty )ethod (available beginning !ith $elease 5.5# is reco))ended over strictly6enforced hard contact+ regardless of !hich contact for)ulation is used. In general+ the "enalty )ethodincreases the robustness and "erfor)ance !ith insignificant degradation in accuracy.

C. If the underlying )aterials are inco)"ressible or nearly inco)"ressible+ such as the case !ith)any hy"erelastic or elastic6"lastic )aterials+ &2D7 and &2D7M ele)ents )ay have volu)etriclocking "roble)s (!ith &2D7 being so)e!hat )ore susce"tible#. &2D7 and &2D7M ele)entscan be used to avoid these "roble)s+ at so)e added e"ense.

:. -or "roble)s !ith contact stress singularities or highly6locali>ed contact stress concentrationsrelative to the )esh refine)ent+ &2D7M ele)ents are )ore likely to Gs)ooth overG a locali>ed effect.-or ea)"le+ &2D7M ele)ents tend to under6"redict the )ai)u) contact stress for a highly locali>edcontact stress concentration< !hereas+ !ith the sa)e )esh+ &2D7 ele)ents )ay tend to over6"redictthe )ai)u) contact stress.

7. If the "roble) "osed has a stress singularity near the edge of the contact region (!here acorner of one surface )eets a s)ooth "art of another surface# 6 !ith a fairly coarse )esh &2D7Mele)ents )ay "redict that the )ai)u) contact stress is a bit high at these locations but the &2D7ele)ents !ith the sa)e )esh !ill tend to sho! significantly higher "eaks. With either ele)ent ty"e+ the

"eak value !ith continue to increase !ith )ore and )ore )esh refine)ent.


. @Defining contact "airs in Abaqus/Standard@

o Section 28.2. of the Abaqus 5.1 Analysis 4ser@s uide

o Section 25.2. of the Abaqus 5.2 Analysis 4ser@s uide

1. @&ontact "ressure6overclosure relationshi"s@

o Section 25..1 of the Abaqus 5.1 Analysis 4ser@s uide

o Section 2B..1 of the Abaqus 5.2 Analysis 4ser@s uide

2. @Ad=usting contact controls in Abaqus/Standard@

o Section 28.2.5 of the Abaqus 5.1 Analysis 4ser@s uide

o Section 25.2.5 of the Abaqus 5.2 Analysis 4ser@s uide

3. What are the causes and resolutions of contact chattering in Abaqus/Standard?

8. &ontact enhance)ents in ,ersion 5.B of Abaqus/Standard

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6/8

Question What are the causes an resolutions of contact chattering!

(he follo!ing a""lies to all versions.#

When Abaqus has difficulty resolving a contact constraint+ the contact status of a "articular slave node )ay

so)eti)es re"eatedly change fro) o"en to closed bet!een each severe discontinuity iteration< this is kno!n ascontact chattering. It can lead to cutbacks in the ti)e incre)ent if the contact state cannot be resolved !ithin acertain nu)ber of severe discontinuity iterations. If several cutbacks are required+ contact chattering )ayulti)ately lead to loss of convergence.

As outlined belo!+ there are )any "ossible causes of contact chattering. Invoking the auto)atic overclosuretolerances very often alleviates the difficulty. Introduce the) as follo!s%

Abaqus/&A'% Interaction Moule" Interaction Contact Controls Create Continue

check Automatic overclosure tolerances

ey!ord% *CONTACT CONTROLS, AUTOMATIC TOLERANCES

With the auto)atic tolerances+ Abaqus calculates an alternative set of tolerances that are intended for those"roble)s !here the standard controls do not "rovide cost6effective solutions. hese "roble)s often requireseveral iterations at the start of the analysis to establish the correct contact state. he auto)ated tolerances%

increase the allo!able "enetration of a slave node to t!ice the )ai)u) dis"lace)entcorrection+ and

allo!+ during the first t!o iterations+ a tensile contact "ressure equal to 7 ti)es the )ai)u)allo!able force residual divided by the contact area of a node.

If convergence should occur in the first t!o iterations !ith these )odified tolerances+ at least one )ore additionaliteration is )ade !ith the se"aration tolerance set equal to the largest allo!able residual. -or additionalinfor)ation on the use of auto)atic contact tolerances+ see

@&o))on difficulties associated !ith contact )odeling in Abaqus/Standard@

o Section 2C..1 of the Abaqus 5.1 Analysis 4ser@s uide

o Section 2:..1 of the Abaqus 5.2 Analysis 4ser@s uide

If the use of the auto)atic contact controls does not resolve the chattering+ so)e additional causes andsuggested resolutions are%

A slave node is sliding off the )aster surface (!hen using finite sliding#% check and etend the)aster surface if necessary.

Lnly a fe! nodes are in contact% refine the underlying )esh of the slave surface or usesoftened contact (e"onential+ tabular or linear "ressure6overclosure relationshi"# to distribute thecontact over )ore nodes. -or )ore infor)ation see


Section 25..1 of the Abaqus 5.1 Analysis 4ser@s uide

Section 2B..1 of the Abaqus 5.2 Analysis 4ser@s uide

he si>e of the region in contact is changing ra"idly% )ake sure that the a""lication of friction isnot being delayed to the incre)ent after contact occurs (i.e. )ake sure that *CONTACT CONTROLS,

FRICTION ONSET=DELAYED is not being used#. I))ediate onset of friction is the default behavior

for ,ersions 5.1 and higher.


7/8

he )odel has long+ fleible "arts !ith s)all contact "ressures% use softened contact.

he )aster surface is not sufficiently s)ooth and kinks in the )aster surface get caughtbet!een t!o slave surface nodes% s)ooth the )aster surface by refining the underlying )esh or definingthe contact directions !ith

*NORMAL, TYPE=CONTACT SURFACE

If "ossible+ use analytical rigid surfaces instead of rigid surfaces defined !ith ele)ents.

If rigid ele)ents )ust be used+ s)ooth the rigid surface !ith

*CONTACT PAIR, SMOOTH


o @&ontact for)ulations in Abaqus/Standard@

Section 2B.. of the Abaqus 5.1 Analysis 4ser@s uide

Section 2C.. of the Abaqus 5.2 Analysis 4ser@s uide

Make sure analytical rigid surfaces are s)ooth bet!een seg)ents% use the

*SURFACE, FILLET RADIUS

o"tion. It is the user@s res"onsibility to )ake sure rigid surfaces are sufficiently s)ooth. -or )oreinfor)ation see%

o @Analytical rigid surface definition+@ Section 1.2.3 of the Abaqus 5.1 or 5.2 Analysis

4ser@s uide

If contact can be established but Abaqus/Standard has difficulty (changing the contact status

fro) closed to o"en#+ try viscous da)"ing to control chattering%

o Abaqus/&A'% Interaction Moule" Interaction Contact Controls Create

#ta$ili%ation tab &lick #ta$ili%ation coefficient S"ecify da)"ing "ara)eters

o ey!ord% *CONTACT DAMPIN!, DEFINITION=DAMPIN! COEFFICIENT





If none of the above cases seem to apply, try using auto)atic stabili>ation in a static+ cou"ledte)"erature6dis"lace)ent+ soils or quasi6static ste"%

o Abaqus/&A'% #tep Moule" #tep Create... #elect step type #elect &se

#ta$ili%ation with"

o ey!ord% Include the STABILI"E "ara)eter on the *STATIC, *COUPLED

TEMPERATURE#DISPLACEMENT, *SOILS o$ *ISCO o"tions.

Auto)atic stabili>ation is nor)ally used to stabili>e globally unstable "roble)s but can so)eti)es hel""revent contact chatter "roble)s fro) occurring. -or )ore infor)ation see%

o @eneral and linear "erturbation "rocedures+@ Section 5..2 of the Abaqus 5.1 or 5.2

Analysis 4ser@s uide


8/8

In so)e cases+ using dash"ots on ele)ents on and near the contact region can hel" stabili>e thecontact. If you use this technique+ a""ly dash"ots to all translation degrees of freedo) until e"erienceindicates you can a""ly the dash"ots only in a "articular direction or at a li)ited nu)ber of nodes.

For extremely difficult situations, and only as a last resort+ you can allo! so)e "oints to violatecontact conditions by setting the )ai)u) nu)ber of "oints "er)itted to violate contact and the)ai)u) value of tensile stress allo!ed to be trans)itted at a contact "oint%

o Abaqus/&A'% Interaction Moule" Interaction Contact Controls Create

'eneral tab

o ey!ord% *CONTACT CONTROLS, %MA&CHP ' PERRM&(

hese "ara)eters can be reset in a subsequent ste".

WA(NIN'" )hese controls are intene for e*perience analysts an shoul $e use with greatcare.





9L'% he *CONTACT CONTROLS, APPROACH and *CONTACT CONTROLS, STABILI"E o"tions are

not )eant to solve contact chatter "roble)s+ so they !ill "robably not hel" in these cases. hese o"tions are)eant to "revent ecessive overclosures due to unconstrained rigid body )otion.

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Hints for Obtaining a Converged Contact Solution in Abaqus