Contact Modelling in LSDYNA

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Contact modeling in LS-DYNA

6.1 Introduction

Contact treatment forms an integral part of many large-deformation problems. Accurate modeling of

contact interfaces between bodies is crucial to the prediction capability of the finite element simulations.

LS-D!A o ers a large number of contact types. Some types are for specific applications" and others are

suitable for more general use. #any of the older contact types are rarely used but are still retained to enable

older models to run as they did in the past. $sers are faced with numerous choices in modeling contact.

%his document is designed to pro&ide an o&er&iew of contact treatment in LS-D!A and to ser&e as a

guide for choosing appropriate contact types and parameters.

6.' (ow Contact )or*s

In LS-D!A" a contact is defined by identifying +&ia parts" part sets" segment sets" and,or node

sets what locations are to be chec*ed for potential penetration of a sla&e node through a master segment. A

search for penetrations" using any of a number of di erent algorithms" is made e&ery time step. In the case of

a penalty-based contact" when a penetration is found a force proportional to the penetration depth is applied

to resist" and ultimately eliminate" the penetration. $nless otherwise stated" the contacts discussed here are penalty-based contacts as opposed to constraint-based contacts. igid bodies may be included in any

penalty-based contact but in order that contact force is realistically distributed" it is recommended that the

mesh defining any rigid body be as fine as that of a deformable body.

%hough sometimes it is con&enient and e ecti&e to define a single contact that will handle any

potential contact situation in a model" it is permissible to define any number of contacts in a single model. It

is generally recommended that redundant contact" i.e." two or more contacts producing forces due to the

same penetration" be a&oided by the user as this can lead to numerical instabilities.

%o enable fle/ibility for the user in modeling contact" LS-D!A presents a number of contact types

and a number of parameters that control &arious aspects of the contact treatment. In the following sections"

contact types are first discussed with recommendations regarding their application. A description of the

contact parameters is then presented.

6.0 Contact %ypes

%ype 1 2C3!%AC%4SLIDI!543!L

%ype ' 2C3!%AC%4%ID4S$7AC4%34S$7AC

%ype 0 2C3!%AC%4S$7AC4%34S$7AC



%ype 8 2C3!%AC%4SI!5L4S$7AC

%ype 10 2C3!%AC%4A$%3#A%IC4SI!5L4S$7AC

%ype a10 2C3!%AC%4AI9A54SI!5L4S$7AC

%ype '6 2C3!%AC%4A$%3#A%IC45!AL

%ype i'6 2C3!%AC%4A$%3#A%IC45!AL4I!%I3

%ype : 2C3!%AC%4!3DS4%34S$7AC

%ype 6 2C3!%AC%4%ID4!3DS4%34S$7AC



%ype ; 2C3!%AC%4%ID4S(LL4D54%34S$7AC

%ype < 2C3!%AC%4%I9A=4!3DS4%34S$7AC

%ype i< 2C3!%AC%4%I9A=4!3DS43!L



%ype > 2C3!%AC%4%I9A=4S$7AC4%34S$7AC

%ype 1? 2C3!%AC%43!4)A4S$7AC4%34S$7AC

%ype '' 2C3!%AC% SI!5L D5

2C3!%AC%4A$%3#A%IC4SI!5L4S$7AC +with S37%@'" S93%@0 and D%(@:

2C3!%AC%4A$%3#A%IC45!AL4I!%I3



In crash analysis" the deformations can be &ery large and predetermination of where and how

contact will ta*e place may be di cult or impossible. 7or this reason" the automatic contact options are

recommended as these contacts are non-oriented" meaning they can detect penetration coming from either

side of a shell element. Automatic contact types in LS-D!A are identifiable by the occurrence of the word

A$%3#A%IC in the 2C3!%AC% command. %he contact search algorithms employed by automaticcontacts ma*e them better suited than older contact types to handling disBoint meshes. In the case of shell

elements" automatic contact types determine the contact surfaces by proBecting normally from the shell mid-

plane a distance eual to one-half the contact thic*ness . 7urther" at the e/terior edge of a shell surface" the

contact surface wraps around the shell edge with a radius eual to one-half the contact thic*ness thus

forming a continuous contact surface. )e sometimes refer to this o setting of the contact surfaces from shell

mid-planes as considering shell thic*ness o sets. %he contact thic*ness can be specified directly or scaled by

the user using optional parameters in the contact definition. If the contact thic*ness is not specified by the

user" the contact thic*ness is eual to the shell thic*ness +or" in the case of single surface contacts" the

minimum of the shell thic*ness and element edge length. In li*e fashion" the contact surface for beam

elements +where beam contact is considered is o set from the beam centerline by the eui&alent radius of

the beam cross-section. 9ecause contact surfaces are o set from shell midplanes and from beam centerlines"

it is e/tremely important that appropriate gaps between shell and beam parts be modeled in the finite

element geometry in order to account for shell thic*ness and beam cross-section dimensions. !ot doing so

will result in initial penetrations in the contact surfaces. LS-D!A will ma*e one pass to eliminate any

detected initial penetrations by mo&ing the penetrating sla&e nodes to the master surface. !ot all initial

penetrations will necessarily be remo&ed and this can lead to nonphysical contact beha&ior. %ime ta*en in

setting up an accurate initial geometry is always time well spent.

#ost contact types in LS-D!A place a limit on the ma/imum penetration depth that is allowed

before the sla&e node is released and its contact forces are set to ero. %his is done mainly in automatic

contact types to pre&ent large contact forces from de&eloping in the opposite sense should the sla&e node pass through a shell mid-plane. %his ma/imum penetration depth is tabulated for &arious contact types in

%able 6.1 of the Eersion >6? $ser s #anual. Sometimes automatic contact interfaces appear not to wor*

because this contact threshold is reached early in the simulation. %his often occurs if e/tremely thin shell

elements are included in the contact surface. In these cases" contact failure can usually be pre&ented by

scaling up the default contact thic*ness or setting the contact thic*ness to a &alue larger than the shell

thic*ness. Alternately" setting S37%@1 +discussed later will often correct the problem.

6.3.1 One-Way Treatment of Contact

3ne-way contact types allow for compression loads to be transferred between the sla&e nodes and

the master segments. %angential loads are also transmitted if relati&e sliding occurs when contact friction isacti&e. A Coulomb friction formulation is used with an e/ponential interpolation function to transition from

static to dynamic friction. %his transition reuires that a decay coe cient be defined and that the static

friction coe cient be larger than the dynamic friction coe cient. %he one-way term in oneway contact is used

to indicate that only the user-specified sla&e nodes are chec*ed for penetration of the master segments. 3ne-



way contacts may be appropriate when the master side is a rigid body" e.g." a punch or die in a metal

stamping simulation. A situation where one-way contact may be appropriate for deformable bodies is where

a relati&ely fine mesh +sla&e encounters a relati&ely smooth" coarse mesh +master. 3ther common

applications are beam-to-surface or shell-edge-to-surface scenarios where the beam nodes or the shell edge

nodes" respecti&ely" are gi&en as the sla&e node set. %here are a number of *eyword options that acti&ate

one-way contact.

7or contact between an airbag +sla&e and segmented rigid dummy model +master" one of the following

two contact types are often employed2C3!%AC%4A$%3#A%IC4!3DS4%34S$7AC +a:

2C3!%AC%4A$%3#A%IC43!4)A4S$7AC4%34S$7AC +a1?

7or metal stamping" special one-way forming contacts are recommended with the wor*piece defined on the

sla&e side

2C3!%AC%473#I!54!3DS4%34S$7AC +m:

2C3!%AC%473#I!543!4)A4S$7AC4%34S$7AC +m1?

3rientation is automatic with forming contacts. %he rigid tooling surface can be constructed from

disBoint element patches where contiguous nodal points are sometimes merged out" but not always. %hese

patches are not assumed to be consistently orientedF conseuently" during initialiation" the reorientation of

these disBoint element patches is performed. 7orming contact trac*s the nodal points of the blan* as they

mo&e between the disBoint element patches of the tooling surface. enalty forces are used to limit

penetrations. 5enerally the 3!4)A4S$7AC4%34S$7AC option is recommended since the

penetration of master nodes through the sla&e surface is considered in adapti&e remeshing. )ithout this

feature" adapti&e remeshing may fail to adeuately refine the mesh of the blan* to capture sharp details in

the master surface" and the master surface will protrude through the blan*.

)hen the surface orientations are *nown throughout the analysis" the following nonautomatic contact types

may be e ecti&e

2C3!%AC%4!3DS4%34S$7AC +:

2C3!%AC%43!4)A4S$7AC4%34S$7AC +1?2C3!%AC%4C3!S%AI!%4!3DS4%34S$7AC +1<

2C3!%AC%43DI!54!3DS4%34S$7AC +16

If there is a possibility that the nodes of the sla&e surface can physically end up behind the master

surface" these contact types should be a&oided. Shell thic*ness o sets may or may not be considered with

these non-automatic contact types +see S(L%(= in 2C3!%3L4C3!%AC%. If shell thic*ness o sets are

inacti&e +default" then the old node-to-surface contact treatment from public domain D!A0D is used for

contact types : and 1? abo&e where incremental searching is used to locate potential master segments for

any gi&en sla&e node. %his searching techniue uses segment connecti&ityF therefore" the master surface

must not be disBoint. If the geometry of the surfaces ha&e sharp angles or if the segments are &ery badly

shaped" the searching algorithm can fail to find the proper master segment. If the shell thic*ness o sets areacti&e" S(L%(= G ?" the master surface is proBected based on nodal normal &ectors" and the location of the

sla&e node on a master segment is determined by using global segment-based buc*et sortingF therefore" the

master surface can be disBoint and sharp edges and bad element shapes do not create significant problems in

the searching. %he use of nodal normal &ectors to proBect the master surface is uite e/pensi&e in C$

costs" but has an ad&antage that the proBected master surface is continuous e&en for con&e/ surfaces. $ntil

the 73#I!5 contact types were de&eloped" types : and 1? contacts with shell thic*ness o sets were often

the contact of choice for sheet metal stamping.

%he contact type

2C3!%AC%4C3!S%AI!%4!3DS4%34S$7AC +1<

is similar in treatment to2C3!%AC%4!3DS4%34S$7AC with shell thic*ness o sets.

9eing constraint-based rather than penalty-based" type 1< contact cannot be used with rigid bodies.

%he forces are computed to *eep the sla&e nodes e/actly on the master surface +ero penetration. In



general" this contact has ne&er been as stable as the penalty-based contacts and is" therefore" not

recommended.

roding contact types are recommended whene&er solid elements in&ol&ed in the contact definition

are subBect to erosion +element deletion due to material failure criteria. %hese eroding contacts contain

logic which allow the contact surface to be updated as e/terior elements are deleted. In

2C3!%AC%43DI!54!3DS4%34S$7AC" the sla&e side of the contact should be defined using a

node set containing all the nodes +not Bust nodes on the outer suface of the sla&e side part+s.

6.3.2 Two-Way Treatment of Contact

%his contact wor*s essentially the same way as the corresponding one-way treatments described abo&e"

e/cept that the subroutines which chec* the sla&es nodes for penetration" are called a second time to chec*

the master nodes for penetration through the sla&e segments. In other words" the treatment is symmetric and

the definition of the sla&e surface and master surface is arbitrary since the results will be the same. %here is

an increased cost of appro/imately a factor of two due to the e/tra subroutine calls.

In crash analysis" the contact type

2C3!%AC%4A$%3#A%IC4S$7AC4%34S$7AC +a0

is a recommended contact type since" in crash simulations" the orientation of parts relati&e to each other

cannot always be anticipated as the model undergoes large deformations. As mentioned before" automatic

contacts chec* for penetration on either side of a shell element.

7or metal forming simulations" the contact type

2C3!%AC%473#I!54S$7AC4%34S$7AC +m0

is a&ailable but is generally not used in fa&or of the one-way forming contacts.

%he two-way +symmetric counterparts to the pre&iously discussed contact types :" 1<" and 16 are

2C3!%AC%4S$7AC4%34S$7AC +0

2C3!%AC%4C3!S%AI!%4S$7AC4%34S$7AC +1;

2C3!%AC%43DI!54S$7AC4%34S$7AC +18.

6.3.3 Tied Contact (Translational DOF only, No Failre, No O set!

I n tied contact types" the sla&e nodes are constrained to mo&e with the master surface. At the

beginning of the simulation" the nearest master segment for each sla&e node is located based on an

orthogonal proBection of the sla&e node to the master segment. If the sla&e node is deemed close to the

master segment based on established criteria" the sla&e node is mo&ed to the master surface. In this way" the

initial geometry may be slightly altered without in&o*ing any stresses. It is always recommended that tied

contacts !3% be defined by part Ids but rather by node,segment sets. In this way" the user has more direct

control o&er what gets tied to what and thus can pre&ent unintended constraints. As the simulation

progresses" the isoparametric position of the sla&e node with respect to its master segment is held fi/ed

using *inematic constraint euations. /amples of this contact type are

2C3!%AC%4%ID4!3DS4%34S$7AC +62C3!%AC%4%ID4S$7AC4%34S$7AC +'

%hese contact types should generally only be used with solid elements since rotational degrees-o

reedom of the sla&e node are not constrained. %he use of this contact type for shell elements may produce

unrealistically soft beha&ior. Contact types ' and 6 di er only in the input format +sla&e segments &s. sla&e

nodesF the numerical treatment is the same.

In general" when using tied interfaces between similar materials" the master surface should be the

more coarsely meshed side since these constraints are not applied symmetrically. (owe&er" if one material

is significantly softer" the master side should be the sti est material.

Constraint-based tied contacts such as types ' and 6 cannot be used to tie a rigid body to a

deformable body or to another rigid body. !odes of deformable bodies that the user wishes to be tied to a

rigid body can be included as e/tra nodes for the rigid body using the 2C3!S%AI!D4H%A4!3DS

command. Alternately" the 377S% option can be used for tied contacts in&ol&ing rigid bodies +see below.



6.3." Tied Contact (Translational DOF only, No Failre, Wit# O set!

%his contact types wor*s the same as abo&e but an o set distance between the master segment and

the sla&e node is permitted. 3 set tied contacts use a penalty-based formulation and thus can be used to tie

rigid bodies. /amples of this contact type are

2C3!%AC%4%ID4!3DS4%34S$7AC4377S% +o6

2C3!%AC%4%ID4S$7AC4%34S$7AC4377S% +o'

%his contact type wor*s best if the surfaces are &ery close" since moments that de&elop due to the o

set are not ta*en into account. !ot accounting for the moment transmission due to o sets can impose

rotational constraints on the structure. )ith the penalty approach this is not too much of a problem"

howe&er" with the constraint method" the results can be completely wrong.

%o account for the moment transmission between the o set surfaces" two methods are a&ailable. %he

first" based on a penalty formulation" uses beam-li*e spring elements to transmit the moments

2C3!%AC%4%ID4!3DS4%34S$7AC49A#4377S% +b6

2C3!%AC%4%ID4S$7AC4%34S$7AC49A#4377S% +b'

and the second uses constraint euations

2C3!%AC%4%ID4!3DS4%34S$7AC4C3!S%AI!D4377S% +c6

2C3!%AC%4%ID4S$7AC4%34S$7AC4C3!S%AI!D4377S% +c'

%he o sets can be reasonably large with the 9A# and C3!S%AI!D options. (owe&er" since

rotational degrees-of-freedom are not a ected" the o set contacts should not be used with structural elements

li*e beams and shells. %he o set contacts that transmit moments were added to the first release of &ersion

>6? after the manual was published.

6.3.$ Tied Contact (Translational DOF and %otational DOF, Wit# Failre, No O set!

%his contact interface uses a *inematic type constraint method to tie the sla&e nodes to the master

segments and treats both translational and the rotational degrees-of-freedom. Additionally" failure can be

specified when combined with beam elements of material type" 2#A% S3%)LD" when modeling spot

welds. /amples of this contact type are

2C3!%AC%4%ID4S(LL4D54%34S$7AC +;

2C3!%AC%4S3%)LD +;

2C3!%AC%4S3%)LD4)I%(4%3SI3! +s;

)ith the abo&e types the nodes are proBected to lie on the master segment. %his is uite important

for 2C3!%AC%4S3%)LD" since the beams that model the spot welds need to be as long as possible to

minimie the mass scaling that is necessary to allow the calculation to ha&e a reasonable time step sie.

)ith the %3SI3! option" the torsional forces in the beam" which models the spot weld" are

transmitted as eui&alent forces to the surrounding nodes of the master surface. %he rotational constraint

about the a/is of the beam is then enforced. %he nonlinear shell elements in LS-D!A ha&e a ero sti ness

drilling degree-of-freedom at each node" so it is necessary to carry the torsional forces through the

membrane beha&ior of the shell.

6.3.6 Tied Contact (Translational DOF and %otational DOF, Wit# O set!

%hese contact interface options uses either a *inematic or penalty type constraint method to tie o set

sla&e nodes to the master segments2C3!%AC%4%ID4S(LL4D54%34S$7AC4377S% +o;

2C3!%AC%4%ID4S(LL4D54%34S$7AC49A#4377S% +b;

2C3!%AC%4%ID4S(LL4D54%34S$7AC4C3!S%AI!D4377S% +c;



)ith the 9A# and C3!S%AI!D option" the moments that de&elop from the o sets are

computed and used in the update of the master surface. %he nodes in&ol&ed should belong to deformable

elements. %he C3!S%AI!D option cannot be used with rigid bodies. %he di culty with using e&en the

penalty option with rigid bodies is related to the nodal masses of the rigid body. If the nodal masses are

accurate then the penalty method is o*ay. If the masses are nonsense" as is often the case if the rigid body

geometry is accurate but the inertial properties are defined independently of the mesh" then the penalty

method may brea* down since the nodal masses of the rigid body are used to set the penalties that are used

in the rotational constraints.

6.3.& Tied Contact (Translational DOF, Wit# Failre!

%he following penalty based contact types allow for the definition of failure parameters. It is

e/tremely important to ha&e the contact segment orientation aligned appropriately as it determines the

tensile and compression direction. 7ailure can be based on the forces or stress along the normal +tensile and

shear directions. /amples of this contact type are

2C3!%AC%4%I9A=4!3DS4%34S$7AC +<

2C3!%AC%4%I9A=4!3DS43!L +i<

2C3!%AC%4%I9A=4S$7AC4%34S$7AC +>

6.3.' in)le rface

%hese contact types are the most widely used contact options in LS-D!A" especially for

crashworthiness applications. )ith these types" the sla&e surface is typically defined as a list of part ID s.

!o master surface is defined. Contact is considered between all the parts in the sla&e list" including self-

contact of each part. If the model is accurately defined" these contact types are &ery reliable and accurate.

(owe&er" if there is a lot of interpenetrations in the initial configuration" energy balances may show either a

growth or decay of energy as the calculation proceeds.

7or crash analysis" the contact type

2C3!%AC%4A$%3#A%IC4SI!5L4S$7AC +10

is recommended. %his contact has impro&ed from &ersion to &ersion of LS-D!A and is the most popular contact option.

%he older single surface contact type

2C3!%AC%4SI!5L4S$7AC +8

should be a&oided since it has not undergone impro&ement. It e&entually will be remo&ed or recoded.

%he differences between 2C3!%AC%4SI!5L4S$7AC and

2C3!%AC%4A$%3#A%IC4SI!5L4S$7AC are twofold.

7irst" the older method uses nodal based buc*et sorting where closest nodes are found that do not

share common segments. %his nodal based searching can brea* down if the segments &ary appreciably in

sie and shape" especially" if aspect ratios are large. Secondly" the older method uses segment proBection to

determine the contact surface. %his reuires the calculation of nodal normal &ectors that are area weighted by the segments that share the node" which in turns creates further diffculties for %-intersections and other

geometric complications. %he calculation of the &ectors can reuire ': of the total C$ reuired.

7or modeling the deployment of airbags the following contact option is recommended

2C3!%AC%4AI9A54SI!5L4S$7AC +a10

)ith 2AI9A54SI!5L4S$7AC" contact between nodes and multiple segments is considered.

#uch more searching is done than in the normal contact option and" conseuently" this contact option is

much more e/pensi&e. During the past se&eral years" the soft constraint option" on optional card A" in the

contact definition" set to ' has pro&ed to deploy airbags &ery accurately. )e current recommend this option

for airbag deployment. %he latter option is currently being implemented for # usage.

%he final contact is

2C3!%AC%4A$%3#A%IC45!AL +'6



%he contact treatment with this option was similar to type 10 through the >:?c release of LS-

D!A. %he main di erence was that three possible contact segments" rather than Bust two" were stored for

each sla&e node. )ith >:?d and later &ersions" type 10 was substantially impro&ed and now type 10 is

freuently more accurate. %he main feature of the 5!AL option is that shell edge-to-edge and beam-

to-beam contact is treated automatically. All free edges of the shells and all beam elements are chec*ed for

contact with other free edges and beams. $nli*e type 10 contact" type '6 contact chec*s for contact along

the entire length of beams and e/terior shell edges" not Bust at the nodes. %here is a new option in >6? to

also chec* internal shell edges +I!%I3 option. %his is uite e/pensi&e" howe&er" and is not usually

needed. )e plan to update this contact type in &ersion >;? of LS-D!A to include all the recentimpro&ement in the 2A$%3#A%IC4SI!5L4S$7AC contact.

6.3.* Contact +ntity

%his contact type is used for treating deformable nodes against rigid geometric surfaces. %he analytical

euations defining the geometry of the surface are used in the contact calculations. %his is an impro&ement

o&er the usual segmented surface as represented by a mesh. A penalty-based approach is used in calculating

the forces that resist penetration. %his contact type is widely used to couple LSD!A with rigid body

dummies" which ha&e surfaces appro/imated by nice geometric shapes such as ellipsoids. An automatic

mesh generator is used to mesh the rigid surfaces to aid &isualiing the results. %he mesh is not used in the

contact calculations. %he analytical rigid surfaces can be of the following types

• 7lat lanes +infinite and finite

• Sphere

• Cylinder

• (yper-ellipsoid

• %orus

• Load cur&e defining the line

• CAL0D,#AD#3 plane

• CAL0D,#AD#3 ellipsoid

• EDA surface +read from a file I5S surface +read from a fileJ



6.8 Contact Stiffness Calculation

Contact treatment is internally represented by linear springs between the sla&e nodes and the nearest

master segments. %he sti ness of these springs determines the force that will be applied to the sla&e nodes

and the master nodes. %here are currently two methods of calculating the contact spring sti ness and they

are briefly discussed below.

6.".1 enalty-ased aroac# (S37%@? in Otional Card / in K2C3!%AC%4K!

%he formula for the stiffness of a contact segment is as follows



%his method is the default method and uses the sie of the contact segment and its material

properties to determine the contact spring sti ness. As this method depends on the material constants and the

sie of the segments" it wor*s e ecti&ely when the material sti ness parameters between the contacting

surfaces are of the same order-of-magnitude. In cases where dissimilar materials come into contact" the

contact might brea* down" as the sti ness" which is roughly the minimum of the sla&e and master stiffness"

maybe too small. %his freuently happens with soft dense foams contact metal materials. Conseuently" for

crash analysis we do not recommend the option" S37% @ ?" unless prior e/perience shows that no problems

occur.

6.".2 oft Constraint-ased aroac# (S37%@1 ' on Otional Card / in 2C3!%AC%4 !

%his non-default method calculates the sti ness of the linear contact springs based on the nodal

masses that come into contact and the global time step sie. %he resulting contact sti ness is independent of

the material constants and is well suited for treating contact between bodies of dissimilar materials. %he sti

ness is found by ta*ing the nodal mass di&ided by the suare of the time step sie with a scale factor to

ensure stability.

5enerally" for the case of metals contacting metals the resulting penalty sti ness for S37% @ ? or S37% @ 1

is similar. 7or the case where soft dense foams contact metal" the option" S37% @ 1 often gi&es interface

stiffness that are one or two orders-of-magnitude greater. %he S37% @ 1 option is recommended for impact

analysis where dissimilar materials come into contact.

%he S37% @ ' option uses mass and time step based penalty sti ness as in S37% @ 1. S37% @ ' in&o*es a

segment-based contact algorithm which has it origins in inball contact de&eloped by 9elytsch*o and his

co-wor*ers. )ith this contact algorithm" contact between segments is treated rather than using the usual

node-to-segment treatment. )hen two 8-noded segments come into contact" forces are applied to eightnodes to resist segment penetration. %his treatment has the e ect of distributing forces more realistically and

sometimes is uite e ecti&e for &ery stubborn contact problems. %he S37% @ ' option is currently being

ported for # calculations. 9eam contact is not handled by S37% @ ' type contact. 7urther" S37% @ ' is

a&ailable only for surface-to-surface and single surface contacts and not for nodes-to-surface contacts. %he



optional parameter D5 on 3ptional Card A should be used cautiously when segment-edge-to-segment-

edge contact is anticipated and S37% is set to '.

6.: Contact 3utput

%here are numerous output files pertaining to contact which can be written by LS-D!A. LS-3S%

can read these output files and plot the results.

%he most common contact-related output file" C73C" is produced by including a2DA%A9AS4C73C command in the input dec*. C73C is an ASCII file containing resultant

contact forces for the sla&e and master sides of each contact interface. %he forces are written in the global

coordinate system. !ote that C73C data is not written for single surface contacts as all the contact

forces from such a contact come from the sla&e side +there is no master side and thus the net contact forces

are ero. %o obtain C73C data when single surface contacts are used" one or more force transducers

should be added &ia the 2C3!%AC%473C4%A!SD$C4!AL% command. A force transducer

does not produce any contact forces and thus does not a ect the results of the simulation. A force transducer

simply measures contact forces produced by other contact interfaces defined in the model. 3ne would

typically assign a subset of the parts defined in a single surface contact to the sla&e side of a force

transducer. !o master side is defined. %he C73C file would then report the resultant contact forces on

that subset of parts.

%he ASCII output file !C73C reports contact forces at each node. %he command

2DA%A9AS4!C73C is reuired in the input dec* to produce such a file. 7urther" one or more contact

print flags must be set +see S and # on Card 1 of 2C3!%AC%4. 3nly those surfaces whose print

flag is set to a &alue of 1 will ha&e their nodal contact force output to the !C73C file.

9y including a 2DA%A9AS4SL3$% command" contact interface energies are written to the ASCII

ouput file SL3$%. In cases where there are two or more contact interfaces in a model and the global

statistics file +5LS%A% indicates a problem with contact energy" such as a large negati&e &alue" the

SL3$% file is useful for isolating which contact interfaces are responsible. 7or general information on

interpreting contact energies" see the LS-D!A %heory #anual" Section '0.<.8.In some cases" it can be &ery useful to &isualie contact surfaces and produce fringe plots of contact stress

both in directions normal and tangential to the contact surface. %o do this" a binary interface file must be

written by

1. including a 2DA%A9AS49I!A4I!%73 command in the input dec*"

'. setting one or more contact print flags as detailed abo&e"

0. and including the option s@filename on the LS-D!A e/ecution line where filename is the intended

name of the binary database. %he database can be postprocessed using LS-3S%.

6.6 Contact arameters

%here are se&eral contact-related parameters in LS-D!A that can be used to modify or" in manycases" impro&e contact beha&ior. %he default settings for these parameters should be used as a starting point"

but often non-default &alues are appropriate depending on the beha&ior of the contact. %he following

sections describe the most common contact parameters and ma*e general recommendations regarding their

use.

Contact parameters may be set using the commands 2C3!%3L4C3!%AC%" KCONTACT_ " and

2A%4C3!%AC%. Certain parameters may be set using more than one command and so a command

hierarchy must e/ist. arameters set with 2C3!%3L4C3!%AC% redefine default settings for all contacts

in the model. Contact parameters set in 2C3!%AC%4 ... will o&erride default settings for indi&idual

contacts. Contact parameters set in 2A%4C3!%AC% supercede settings in 2C3!%AC%4 for contactin&ol&ing a specific part.

6.6.1 T#ic0ness offsets



arameter SL%(= +card 1" 2C3!%3L4C3!%AC% and 3ptional Card A in 2C3!%AC%4option

A$%3#A%IC +2C3!%AC%4option

In crashworthiness analysis" sheet metal components are represented using shell elements with the

nodal points at the mid-plane surface. ach shell has a thic*ness" ts" that by default is eual to the thic*ness

of the sheet metal. )hen these components are included in the contact treatment" shell thic*ness o sets are

used to proBect the mid-surface of the shell to create the surface for contact. %he choice of the contact type

determines whether shell thic*ness o sets are considered.

In LS-D!A the non-automatic contact types

2C3!%AC%4S$7AC4%34S$7AC

2C3!%AC%4!3DS4%34S$7AC

2C3!%AC%43!4)A4S$7AC4%34S$7AC

use two di erent treatments depending on the parameter S(L%(=. %his parameter can be specified globally

on the 2C3!%3L4C3!%AC% card and locally for a gi&en contact definition on optional card 9 of the

2C3!%AC% input. If S(L%(= @ ?" an incremental search techniue is used to determine the closest master

segment and shell thic*ness o sets are not included. If S(L%(= @ 1" LS-D!A considers the shell

thic*ness o sets for deformable nodes but ignores the o sets for the nodes of rigid bodies. If S(L%(= @ '"

then LS-D!A considers the thic*ness for both deformable and rigid nodes. 7or S(L%(= set to 1 or ' a

global buc*et search is used to identify contact pairs. After contact is established" incremental searching is

used to trac* the position of the sla&e nodes on the master surface. An ad&antage of global buc*et searching

is that the master and sla&e surfaces can be disBoint. %his is impossible if incremental searching is used

since incremental searching assumes that the contact surfaces are fully connected. In these contact types" it

is important to orient the contact segment normals" based on the right-hand-rule" towards the contacting

surface before the calculation begins. %his is called oriented contact. An optional automatic orientation

feature may be in&o*ed using the parameter 3I! on the 2C3!%3L4C3!%AC% cardF howe&er" for

this option to wor* a gap must e/ist between opposing shell mid-plane surfaces.

A$%3#A%IC and single surface contact types always consider shell thic*ness o sets as shown in

7igure MM. %hese contact types use both global buc*et sorting and local incremental searching indetermining the contact pairs. A$%3#A%IC contacts are generally more robust than their non-automatic

counterparts since this contact type has no orientation reuirement" i.e." contiguous segments do not obey

the right-hand-rule. %his is important in crash analysis since metal part can fold o&er and change the

orientation. %he contact search algorithm chec*s for penetration from either side of the shell mid-plane.

7igure 6.1 Automatic Contact Segment 9ased roBection

#ell T#ic0ness Offset %ecommendations

%he A$%3#A%IC contact types" which consider shell thic*ness o sets" are recommended for

impact and crash analysis. If it is desired that shell thic*ness o sets of rigid components be disregarded" a

non-automatic contact type may be used with the parameter S(L%(= set to 1 in either

2C3!%3L4C3!%AC% or on 3ptional Card 9 of 2C3!%AC%. Additionally" it is important to ensure that

the finite element mesh is constructed so that the shell mid-plane surfaces of the opposing parts are set apart

by at least +tsNtm,' with meshes of similar density around sharp changes in cur&ature. If this condition is

not satisfied" LS-D!A will issue warning messages to indicate that penetrations were detected and that the

penetrating nodes were mo&ed to eliminate the penetrations. Sometimes the modification of the geometry

can change the results. In &ersion >6? of LS-D!A" an option e/ists whereby penetrating nodes are not

mo&ed but rather the initial penetrations become the baseline from which additional penetration is

measured. %his option of trac*ing initial penetrations is in&o*ed by setting the parameter I5!3 eual to



1 on Card 8 of 2C3!%3L4C3!%AC% or on optional card C of 2C3!%AC%. )e recommend that this

option be used in most calculations.

See Sections 6.8 and 6.: for more on shell thic*ness o sets. In those sections" the term contact thic*ness

refers to the magnitude of the shell thic*ness o sets.

6.6.2 Contact lidin) Friction

arameters 7S and 7D +card '" 2C3!%AC% optionContact sliding friction in LSD!A is based on a Coulomb formulation and uses the eui&alent of an

elastic-plastic spring. 7riction is in&o*ed by gi&ing non-ero &alues for the static and dynamic friction coe

cients" 7S and 7D" respecti&ely" in the 2C3!%AC% or 2A%4C3!%AC% input. 7or a detailed description

of the frictional contact algorithm" please refer to Section '0.<.6 in the LS-D!A %heory #anual.

Contact lidin) Friction %ecommendations

)hen setting the frictional coe cients" physical &alues ta*en from a handboo* such as #ar*s"

pro&ide a starting point. !ote that to di erentiate static and dynamic friction" 7D should be less than 7S and

the decay coe cient DC must be nonero. 7or numerically noisy problems such as crash" the static and

dynamic coe cients are freuently set eual to a&oid the creation of additional noise. %he decay coe cient

determines the manner in which the instantaneous net friction coe cient is transitioned from 7S to 7D. %he

parameter" EC" pro&ides a means to limit the frictional contact stress based on the strength of the material.

%he suggested &alue for EC is SI5,srt+0 where SI5 is the minimum yield stress of the materials in

contact. In LS-D!A" &ersion >6?" the optional parameter 7C!5 on card 8 of

2C3!%3L4C3!%AC% may be set to write the frictional contact energy to the binary interface database

+2DA%A9AS49I!A4I!%73.

outinely" one automatic" single-surface contact with numerous dissimilar materials" are used in full

&ehicle simulations. In these cases" using a uniform &alue for 7S and 7D may be inappropriate. In such

instances" it is recommended that the frictional parameters be specified part by part using the contact optionin the part definition" 2A%4C3!%AC%.

It is helpful in understanding the sensiti&ity contact friction in a calculation by ma*ing two runs utiliing

lower-bound and upper-bound friction coeffcients.

6.6.3 enalty cale Factors

arameters S7S and S7# +card 0" 2C3!%AC%4option So-called penalty scale factors pro&ide a

means of increasing or decreasing the contact sti ness. SLS7AC in 2C3!%3L4C3!%AC% scales the sti

ness of all penalty-based contacts" which ha&e the parameter S37% set eual to ? or '. SLS7AC is applied

cumulati&ely with S7S" i.e." the actual scale factor is the product of S7S and SLS7AC" the sla&e penaltyscale factor" or S7#" the master penalty scale factor" defined on card 0 of the 2C3!%AC%K input. SS7"

when defined in 2A%4C3!%AC%" is cumulati&e with the aforementioned penalty scale factors. 7or

contacts with S37% @ 1" the aforementioned penalty scale factors ha&e no a ectF rather S37SCL on optional

card A is used to scale the contact sti ness when S37% @ 1. +S37% is the first parameter specified on

optional card A of 2C3!%AC%.

enalty cale Factors %ecommendations

%he default &alues +SFS = SFM = 1.0F SLSFAC = 0.1 generally wor* well for contact between

similarly refined meshes of comparably sti materials. 7or contacts in&ol&ing dissimilar mesh sies and

dissimilar material constants" non-default &alues penalty scale factors may be necessary to a&oid the brea*down of contact if S37% @ ?. 5enerally" a better alternati&e than setting scale factors is to set S37% @

1 and lea&e all penalty scale factors at their default &alues.

6.6." Contact T#ic0ness



arameters SS% and #S% +card 0" 2C3!%AC%4option SS% and #S% on card 0 of 2C3!%AC% allow

users to directly specify the desired contact thic*ness. )hen the default &alue of SS% @ #S% @ ?" is used"

the contact thic*ness is eual to the element thic*ness specified in the 2SC%I3!4S(LL card.

Contact T#ic0ness %ecommendations

!onero &alues of SS% and #S% are sometimes used to decrease the contact thic*ness and thus

eliminate initial penetrations. %his is a poor substitute for accurate mesh generation. )hen using nonero&alues of SS% and #S%" it is highly recommended to use reasonable &alues. Specifying a &ery small

thic*ness &alue" such as ?.1mm" will result in contact brea*down owing to the fact that contact thic*ness

goes into determining the ma/imum penetration allowed before the contact releases a penetrating node.

3ften" by increasing the contact thic*ness" brea*down of contact in&ol&ing &ery thin materials can be

a&erted. 9ased on e/perience" SS% and #S% should not be less than ?.6 - ?.; millimeters.

Since nonero &alues of SS% and #S% are applied to all the parts defined in the contact" it may be

more prudent to use the 3%% or S7% parameter in 2A%4C3!%AC% to control the contact thic*ness for

indi&idual parts in cases where many parts of widely ranging thic*ness are included in a single contact.

6.6.$ Contact T#ic0ness calin)

arameters S7S% and S7#%" card 0" 2C3!%AC%4option As an alternati&e to directly specifying

the contact thic*ness as described abo&e" S7S% and,or S7#% may be defined to ser&e as contact thic*ness

scale factors. %hese factors are applied to the shell thic*ness specified in 2SC%I3!4S(LL in order to

obtain a contact thic*ness. %he default &alues of S7S% and S7#% are 1.?.

Contact T#ic0ness calin) %ecommendations

%he same concepts discussed in Contact %hic*ness ecommendations apply here. Care must be

ta*en though not to assign contact thic*ness scale factors so small as to result in a contact thic*ness that isless than ?.6 ?.;mm.

6.6.6 iscos Damin)

arameter E DC +Card '"2C3!%AC%4option

%he &iscous contact damping parameter" E DC" on card ' of 2C3!%AC% is ero by default. 3riginally"

contact damping was implemented to damp out the oscillations that e/isted normal to the contact surfaces in

sheet metal forming simulations. It has been found that contact damping is often beneficial in reducing

high-freuency oscillation of contact forces in crash or impact simulations.

iscos Damin) %ecommendations

In contacts in&ol&ing soft materials such as foams and honeycombs" freuent instabilities e/ist due

to contact oscillations. $sing a &alue of E DC between 8? - 6? +corresponding to 8?to6? of critical

damping" it is found that the model stability impro&esF howe&er" it may be necessary to reduce the scale

factor for the time step sie. 5enerally" a smaller &alue of '? is recommended when metals" which ha&e

similar material constants" interact.

6.6.& Contact e)ment +tension

arameter #AHA +3ptional Card A " 2C3!%AC% option

#AHA on 3ptional Card A of 2C3!%AC% controls the enlargement of each contact segment that isneeded to combat an inherent flaw in segment-based proBection.

T#is arameter is no lon)er sed in t#e /TO4/T5C contact otions, ecet for



K2A$%3#A%IC45!ALK, startin) wit# ersion *$7d of 8D9N/.

7igure MM shows the contact surface that is proBected from the shell mid-plane when using the

segment-based proBection scheme. It can be seen that at corners of con&e/ surfaces"an open space or gap is

present in the contact surface through which a sla&e node could freely enter without any contact detection.

%his can result in contact instability" negati&e contact energy" etc. due to a sudden" large penetration of a

node that has entered through a gap. %o combat this problem" the contact surface is automatically e/tended

a slight distance parallel to the plane of the contact segment +as well as proBected normally from the

segment. %his slight e/tention ser&es to close the gap in the contact surface. In &ersions starting with >:?d"a cylindrical surface is created in the &alley which is used as the contact surface with the forces acting

normal to the surface.

7igure 6.' Segment e/tension using #AHA. %his option is now obsolete in the A$%3#A%IC contact

types

6.6.' e)ment +tension %ecommendations

%he default &alue of #AHA+1.?': wor*s well for most analyses" as most sheet metal

components are not much greater than 0 - 8mm. (owe&er" contact instabilities may de&elop when a part

with a &ery large thic*ness +G : - 1?mm or ha&ing an angular surface is present in the contact definition.

Such an instability may be corrected by reducing the contact thic*ness +discussed in earlier sections or by

increasing the segment enlargement parameter MAXPAR +to as high as" but no greater than" a &alue of 1.'.efining the mesh to reduce sharp angles in the contact surface will also help. A certain cost penalty is paid

for MAXPAR &alues greater than default.

6.6.* :c0et-ort Fre;ency

arameters 9S3% +3ptional Card A " 2C3!%AC% " !S9CS" +Card '" 2C3!%3L4C3!%AC%

9uc*et sorting refers to a &ery e ecti&e method of contact searching to identify potential master

contact segments for any gi&en sla&e node. %his sorting is an e/pensi&e part of the contact algorithm so the

number of buc*et sorts should be *ept to a minimum to reduce runtime. If thic*ness o sets are considered"

then all contact types use the buc*et sort approach to trac* the most probable contacting segments. 9S3%specifies the number of time steps between buc*et sorts. Depending on the contact type" the default buc*et

sort inter&al is between 1? and 1?? cycles. /cept for high speed impact" this inter&al is almost always

adeuate. %he contact buc*et searching freuency should increase" i.e." 9S3% should be reduced" if nodes

mo&e from one disconnected surface to another in short time inter&als or if the surface is folding onto itself.

If two relati&ely smooth simply-connected surfaces are mo&ing across each other without folds" the buc*et

sorting can be done at larger inter&als. !ote that if the surfaces are more than se&eral segment widths away

from each other" no information is stored related to future contact" and later buc*et searching is reuired to

pic* up future contacts. 3nce a sla&e node is in contact" local searching trac*s the motion" and buc*et

sorting for the nodes" which are in contact" is not necessary.

:c0et-ort Fre;ency %ecommendations

In certain contact scenarios where contacting parts are mo&ing relati&e to each other in a rapid

fashion" such as airbag deployment" more freuent +than default buc*et sorting inter&als may impro&e the

contact beha&ior. A tell-tale sign inadeuate buc*et sorting is the appearance of certain penetrating nodes



ine/plicably being bypassed in the contact treatment. In such cases" using the 9S3% parameter in

2C3!%AC% or !S9CS in 2C3!%3L4C3!%AC%" the user can decrease the cycle inter&al between

buc*et sorts. arely will a &alue of less than 1? be reuired.

6.6.17 4aimm enetration

arameters !#AH +3ptional card 9 " 2C3!%3L4C3!%AC%" H! +Card '"

2C3!%3L4C3!%AC%

%o a&oid instability in models" sla&e nodes that penetrate too far are eliminated from the contact

algorithmF howe&er" they remain in other calculations. %his is done so that &ery high forces" which are

proportional to large penetration &alues" are not applied to the penetrating nodes that might lead to

instabilities. It s also necessary for contacts that consider shell thic*ness o sets to pre&ent a sudden re&ersal

in direction of contact force as a penetrating node passes through the shell midplane.

In non-automatic types and S(L%(= @ ?" the default ma/imum penetration is set to 1e N '?. In other

words" no nodes are released at all. )hen S(L%(= @ 1 or '" the H! parameter determines the nodal

release criteria and is gi&en as follows

• #a/ Distance +Solids @ H! +default@8.?2+thic*ness of the solid element" S(L%(= @ 1

• #a/ Distance +Solids @ ?.?: 2 +thic*ness of the solid element" S(L%(= @ '

• #a/ Distance +Shells @ H! +default@8.? 2 +thic*ness of shell element" S(L%(= @ 1

• #a/ Distance +Shells @ ?.?: 2 +minimum diagonal length" S(L%(= @ '

In A$%3#A%IC types and single surface" e/cluding A$%3#A%IC 5!AL" the ma/imum allowable

penetration is a function of !#AH that is set to a default &alue of ?.8+8?. %he ma/imum allowable

penetration in these cases are shown below 2

• #a/ Distance @ !#AH 2 +thic*ness of the solid

• #a/ Distance @ !#AH 2 +sla&e thic*ness N master thic*ness

7or A$%3#A%IC 5!AL only" the default &alue of !#AH is set to '?? and pro&ides an almost nonodal release criteria.

4aimm enetration %ecommendations

It is generally recommended that parameters a ecting ma/imum penetration not be changed from the default

&alues. If nodes penetrate too far and are released" the preferred solution is to increase the contact sti ness"

change the penalty formulation +S37%" or increase the contact thic*ness.

6.; #odeling 5uidelines 7or 7ull Eehicle Contact

Crash analysis in&ol&ing a full &ehicle incorporates contact interactions between all free surfaces. %his is

uite e/pensi&e since '?-0? percent of the total calculation C$ time is used by the contact treatment. 3ne

of the challenging aspects of contact modeling in crash analysis is the handling of interactions between

structural metallic parts and nonstructural components typically made from foam and plastic. %his is

especially important when occupants are included in the model. Another challenge is handling contact at

corners or edges of geometrically comple/ parts. 5uidelines should be followed to achie&e stability in

contact as well as reasonable contact beha&ior. Some of the modeling practices based on e/perience are

discussed below.

6.&.1 <loal or 8ocal Contact

(istorically" many indi&idual contact definitions were used for the treatment of contact. %he de&elopment

and implementation of a robust single surface type of contact has changed the way engineers model the

contact today. 7rom the standpoints of simplicity in preprocessing" numerical robustness" and computational



effciency" it is now usually ad&antageous to forsa*e the use of numerous contact definitions in fa&or of

3! singlesurface- type contact that includes all parts which may interact during the crash e&ent. )e often

casually refer to this single contact approach as a global contact approach.

%his" howe&er" does not mean that one should always a&oid local contact definitions. 7reuently" there e/ist

certain areas of the &ehicle that reuire special contact considerations where the global contact definition is

obser&ed to fail. In such instances the user is encouraged to define local contact interfaces with non-default

parameters that would best suit the contact condition.

6.&.2 A$%3#A%IC4SI!5L4S$7AC or A$%3#A%IC45!AL

%hough both contact algorithms belong to the single surface contact type" se&eral *ey parameters

distinguish these two contact types. %able MM highlights the important differences.

%able 6.1 Difference 9etween 2A$%3#A%IC4SI!5L4S$7AC +10 and 2A$%3#A%IC45!AL

+'6

3f the two single surface contact types listed in %able MM" 2A$%3#A%IC45!AL is computationally

more e/pensi&e owing to its additional capabilities and its more freuent and thorough contact search. %he

2A$%3#A%IC4SI!5L4S$7AC contact option is recommended for global contact. %o treat special

contact conditions where shell edge-to-edge or beam-to-beam contact is anticipated" the additional use of

the 2A$%3#A%IC45!AL contact in localied regions is recommended. 2A$%3#A%IC45!AL

contact should be used sparingly and only where conditions dictate its use. 3ne ad&antage of the

2A$%3#A%IC4SI!5L4S$7AC contact starting with LSD!A &ersion >:?d is in its more rigorous

treatment of interior sharp corners within the finite element mesh and in the handling of triangular contact

segmentsF conseuently" the A$%3#A%IC4SI!5L4S$7AC contact is usually superior for parts

meshed from triangular and tetrahedron elements. In future &ersion of LS-D!A" the

2A$%3#A%IC45!AL option will also include these impro&ements.

6.&.3 tandard enalty-:ased or oft Constraint tiffness 4et#od

)hen se&eral parts of dissimilar mesh sies and,or dissimilar material properties are included into oneglobal sla&e set for 2A$%3#A%IC4SI!5L4S$7AC" the soft constraint sti ness method +S37%@1 is

recommended. %he soft constraint method see*s to ma/imie contact sti ness while also maintaining stable

contact beha&ior. %he interacting nodal masses and the global time step are used in formulating the contact

sti ness. %he segment-based contact method" in&o*ed by setting S37%@'" calculates contact sti ness much

li*e the soft constraint method but otherwise is uite di erent. Segment-based contact can often be uite e

ecti&e where other methods fail at treating contact at sharp corners of parts.

In contrast to a soft constraint approach" the standard penalty-based contact sti ness +S37%@? is based on

material elastic constants and element dimensions. In foam and plastic materials" the contact sti ness gi&en

by the two methods can di er by one or more orders of magnitude. %he primary disad&antage of choosing

the soft constraint method is its dependence on the global time step. 3ccasionally" the global time step must be scaled down using the %SS7AC parameter in 2C3!%3L4%I#S% to a&oid numerical instabilities

in the contact beha&ior. %his results in an increased run time for the entire simulation. As an alternati&e to

reducing the global time step the soft constraint scale factor" S37SCL" in the 2C3!%AC% definition can be

reduced from the default &alue of ?.1 to ?.?8-?.?;. If the standard penalty-based approach in used in a



global contact definition" the soft constraint approach can be used locally to handle dissimilar materials in

contact. %he following are e/amples where contact beha&ior may benefit from use of the soft constraint

method

• Airbag to Steering )heel

• Airbag to 3ccupant

• 7ront %ire to SIL

• Spare tire to neighboring components

• 7oam to structural components

$sing a combination of both contact sti ness methods may promote good contact beha&ior without ha&ing

to reduce the global time step.

6.&." Definition of lae et

%here are se&eral ways to define the sla&e set for the global contact definition. %hese include all parts +this

is the default" a set of included parts" a set of e/cluded parts" or a set of segments. %he default" which

includes all parts" can sometimes result in ob&ious instabilities at the beginning of a simulation unless great

care is ta*en in setting up the model to a&oid such things as initial penetrations and nonphysical

intersections of parts. %he option to ignore penetrations on the 2C3!%3L4C3!%AC% *eyword +setI5!3 eual to 1 is recommended if care is not ta*en to eliminate initial penetrations.

#any models run perfectly with Bust one interface definitionF others" howe&er" will not run until changes are

made to the input" usually by e/cluding parts or by modifying the finite element mesh to more accurately

reflect the physical model. %o reiterate" the following methods can be used for defining the global contact

definition

• All parts +default

• Included parts by 2S% A%

• /cluded parts by 2S% A%. !on-/cluded parts will be considered for contact

• Segments by 2S% S5#!%

In addition to the abo&e sla&e sets" a three-dimensional bo/" defined using 2D7I!493H" may be used to

restrict the contact to the parts or segments that lie within the bo/ at the start of the calculation. %his will

reduce the e/tent of the contact definition leading to a reduction in contact-associated cpu time.

6.&.$ Friction

)hen using one global contact that includes se&eral components of the &ehicle" a uniform friction coe cient

+possibly ero may be acceptable for initial analyses. (owe&er" the use of 2A%4C3!%AC% *eyword to

specify friction coe cients on a part-by-part basis is recommended when friction is e/pected to play a

significant role. 7riction coe cients specified in 2A%4C3!%AC% will o&erride friction coe cients

specifed elsewhere if and only if 7S in 2C3!%AC% is set to -1.?. lease note that the dynamic friction coecient 7D will ha&e no e ect unless a nonero decay coe cient DC is pro&ided.

6.&.6 Contact T#ic0ness

%o reduce the number of initial penetrations" the contact thic*ness can changed from the default element

thic*ness by using the global SS% and #S% parameters in 2C3!%AC%. %he 3%% parameter in

2A%4C3!%AC% can be used to o&erride SS% and #S% on a part-by-part basis. %he user is cautioned

against setting the contact thic*ness to an e/tremely small &alue as this practice will often cause contact

failure. In fact" for treating contact of &ery thin shells" e.g." less than 1 mm" it may be necessary to increase

the contact thic*ness to pre&ent contact failure.

If a contact surface is comprised of tapered shell elements" then a uniform contact thic*ness should always

be specified. %he contact assumes that the segment thic*ness is constant" which can result in thic*ness

discontinuities between adBacent segments. As a node mo&es between segments of di ering thic*ness" the



interface force will either suddenly drop or increase as a result of the discontinuous change in the

penetration distance. %his can result in negati&e contact interface energies.

6.< Airbag Contact

Simulation of airbag deployment and interaction of an airbag with other components may reuire special

contact treatment. Some of the challenges associated with airbag contact are as follows

• (igh Airbag !odal Eelocity +G 1??m,s• Soft %issue roperties + O :?#pa

• Small %issue %hic*ness +O ?.:mm

• 7reuent Initial enetrations in 7olded 9ag

• %reatment of Airbag 7abric Layers

%o promote stability and accuracy in simulating airbag contact" the following contact types and contact

parameters are recommended.

6.'.1 /ira) elf-Contact

)hen treating airbag self-contact +fabric-to-fabric contact" the use of

2C3!%AC%4AI9A54SI!5L4S$7AC is highly recommended. %his contact type is based on

2C3!%AC%4A$%3#A%IC4SI!5L4S$7AC but has significant modifications to account for the di

culties associated with deployment of a folded airbag.

S37% @ ' is generally recommended +S# only to better deal with the many initial penetrations present in

a folded airbag and to in&o*e a segment-to-segment contact search which is often ad&antageous in dealing

with the comple/ geometry of a folded or partially unfolded airbag. Airbag contact with S37% @ ' is

e/pensi&e relati&e to other contact options so to impro&e cpu performance when using S37% @ '" an

additional contact with S37% @ ? or 1 can be implemented as shown in 7igure MM. 9y defining two separate

contacts and employing contact birthtime and deathtime to switch from the S37% @ ' contact to the S37%@ 1 contact when the bag has unfolded" a good combination of contact reliability and e ciency can be

achei&ed.

7igure 6.0 Airbag Self Contact Algorithm Switch

If the airbag simulation is run using an # e/ecutable" note that S37% @ ' is not yet a&ailable and so

S37% @ ? or 1 must be used. 7or a folded airbag" this will li*ely mean that a load cur&e defining the fabric

contact thic*ness &ersus time will be necessary to transition from a &ery small thic*ness in the folded state

to a larger thic*ness as the bag unfolds. %his is done to pre&ent initial penetrations in the folded state and

still ha&e good contact beha&ior during the unfolding process. %he contact thic*ness &s. time cur&e is

identified by LCIDA9 on 3ptional Card A of 2C3!%AC%. As a possible alternati&e to a time-dependent

contact thic*ness" the user may try in&o*ing the option for trac*ing of initial penetrations by settingI5!3 @ 1 on 3ptional Card C. %his latter option is new in &ersion >6? and has not been thoroughly

chec*ed out for airbag applications.

6.'.2 /ira)-to-trctre Contact



During and after airbag deployment" the airbag fabric comes into contact with other parts of the

model such as the steering wheel" occupant" instrument panel" door trim components and" in the case of side

curtain deployment" the seat. 7or these contact conditions" a two-way contact such as

2C3!%AC%4A$%3#A%IC4S$7AC4 %34S$7AC is generally recommended. In instances when

the airbag nodes comprise the sla&e side in a one-way type contact such as

2C3!%AC%4A$%3#A%IC4!3DS4%34S$7AC" the structural nodes are not chec*ed for penetration

through the airbag segments. %his may result in noticeable penetration of finely-meshed structural

components into airbag segments. Single surface contacts such as2C3!%AC%4A$%3#A%IC4SI!5L4S$7AC for airbag-to-structure interaction may be ill-ad&ised as

this would result in duplication of self-contact treatment for the fabric.

Di culties in airbag-to-structure contact are largely associated with significant di erences in material bul*

moduli +up to 1???/ and &ery low thic*ness of the fabric. %o a&oid premature nodal release triggered by a

small fabric thic*ness" it is recommended that the contact thic*ness of the fabric be set to a minimum &alue

of 1.? mm. Since a wide range of materials are in&ol&ed" the use of S37% @ 1 is highly recommended as it

eliminates the need to fine-tune penalty scale factors. An e/ample of the o&erall setup for airbag-related

contact is shown in 7igure 6.8.

7igure 6.8 Airbag Contact Definition

6.> dge-to-dge Contact

#ost contact types do not chec* for edge-to-edge penetrations as the search entails only nodal penetration

through a segment. %his may be adeuate in many casesF howe&er" in some uniue shell contact conditions"

the treatment of edge-to-edge contact becomes &ery important. %here are se&eral ways to handle edge-to-

edge contactF the merits,demerits of each one of these methods are discussed below.

6.*.1 2C3!%AC%4A$%3#A%IC45!AL ecldin) 5nterior +d)es

9y default" 2C3!%AC%4A$%3#A%IC45!AL considers only e/terior edges in its edge-to-edge

treatment as indicated by 7igure MM. An e/terior edge is defined as belonging to only a single element or

segment whereas interior edges are shared by two or more elements or segments. %he entire length of each

e/terior edge" as opposed to only the nodes along the edge" is chec*ed for contact. As with other penalty-

based contact types" S37%@1 can be acti&ated to e ecti&ely treat contact of dissimilar materials.



7igure 6.: Interior and /terior Shell dges

6.*.2 2C3!%AC%4A$%3#A%IC45!AL incldin) 5nterior +d)es

dge-to-edge contact which includes consideration of interior edges may be in&o*ed in one of two

ways. 3ne method ta*es ad&antage of the beam-to-beam contact capability of

2C3!%AC%4A$%3#A%IC45!AL. %his labor-intensi&e approach in&ol&es creating null beam

elements +2L#!%49A#" 2#A%4!$LL appro/imately 1mm in diameter +elform @ 1" ts1 @ ts' @

1.'mm" tt1 @ tt' @ ? in 2SC%I3!49A# along e&ery interior edge wished to be considered for edge-to-

edge contact and including these null beams in a separate A$%3#A%IC 5!AL contact. %his is

illustrated in 7igure MM. %he elastic constants in 2#A%4!$LL are used in determining the contact stiffness

so reasonable &alues should be gi&en. !ull beams do not pro&ide any structural stiffness.

7igure 6.6 !ull 9eams to treat edge-to-edge treatment

A preferred alternati&e to the null beam approach" a&ailable in &ersion >6?" is to in&o*e the interior edge

option by using 2C3!%AC%4A$%3#A%IC45!AL4I!%I3. A certain cost penalty is associated

with this option.

6.*.3 2C3!%AC%4SI!5L4D5

%his contact type treats edge-to-edge contact but" unli*e the other options abo&e" it treats only edge-toedgecontact. %his contact type is defined &ia a part ID" part set ID" or a node set on the sla&e side. %he master

side is omitted.



6.17 %i)id :ody Contact Components for which deformation is negligible and stress is unimportant may

be modeled as rigid bodies using 2#A%4I5ID or 2C3!S%AI!D4!3DAL4I5ID493D. %he

elastic constants defined in 2#A%4I5ID are used for contact sti ness calculations. %hus the constants

should be reasonable +properties of steel are often used. %hough there are se&eral contact types in LS-

D!A which are applicable specifically to rigid bodies +I5ID appears in the contact name" these types

are seldom used. Any of the penalty-based contacts applicable to deformable bodies may also be used with

rigid bodies" and in fact" are generally preferred o&er the I5ID contact types. igid bodies and deformable

materials may be included in the same penalty-based contact definition. Constraints and constraint-based

contacts may not be used for rigid bodies.

igid bodies should ha&e a reasonably fine mesh so as to capture the true geometry of the rigid part. An

o&erly coarse mesh may result in contact instability. Another meshing guideline is that the node spacing on

the contact surface of a rigid body should be no coarser than the mesh of any deformable part which comes

into contact with the rigid body. %his promotes proper distribution of contact forces. As there are no stress

or strain calculations for a rigid body" mesh refinement of a rigid body has little effect on cpu reuirements.

In short" the user should not try to economie in the meshing of rigid bodies.

2C3!%AC%4!%I% is an altogether different way of defining an analytic" rigid contact surface which

interacts with nodes of deformable bodies. 7or more information

6.11 Summary %able

%he contact types can be grouped as follows

<ro /= %ypes 0" :" 1? +S(L%(= @ ?

<ro := %ypes 0" :" 1? +S(L%(= @ 1

<ro C= %ypes :" 10" 18" 1:" 16" a0" a:" a1?" '6

<ro D= %ypes 1>" '?" '1

RIGID WALL CONTACT

1. Fleile :ody - %i)id Wall



2. %i)id :ody - %i)id Wall epsilonr . . . penaltyfactor ri)id ody

epsilon s . . . penaltyfactor rigid wall

Spring forces 7i @ epsilon s P deltai

Otions of t#e stonewalls=

• initial mass and &elocity

• fi/ed in space

• &elocity or displacement specified by a load cur&e

A stonewall may e/tent to infinity or the e/tent may be finite.

Friction=

• frictionless sliding after contact

• no sliding after contact

• coulomb friction 7c @ Q P 7 N

• orthotropic frictional coe cients by defining fi/ed &ectors

• orthotropic frictional coe cients by defining nodes



7igure ;.1 5eneralied Stonewalls

CONSTRAINTS AND SPOTWELDS

Constraint nodes

1. Common translational de)rees of freedom in common

• /" y" or translational D37• /-y" y-" or /- translational D37

• /-y- translational D37

2. %i)id massless trss (riet!

7orce &ector always in direction of the rigid truss.

3. %i)id massless eam (sotweld!

%ransmission of moments" shear and normal forces



:rittle failre of t#e sotweld occr w#en

otweld failre de to lastic strainin) occrs w#en t#e e ectie nodal lastic strain eceeds t#e

int ale

%his option can model the tearing out of a spotweld from the sheet metal due to plasticity in the material

surrounding the spotweld.

Constraints between nodes and surfaces

". Contact tye '= nodes sotwelded to srface

Sla&e nodes are tied to the masters until a failure criterion is reached. %hereafter they can slide on or

separate from the masters as in a type : contact surfaces. %his type of surface can be used to represent spot-

welded or bolted connections.

7ailure criterion

Constraints between surfaces

$. Contact tye 1= slidin)

Sliding only" no separation. 3nly sliding along the contact surfaces" no separation.

6. Contact tye 2= tied

%ying surfaces with translational degrees of freedom. !odes of one surface are tied to the opposite surface

and &ice &ersa.

&. Contact tye *= tierea0 interface

%his is similar to type < e/cept that failure is based on stress rather than force at indi&idual nodes.

UNITS

%here is no way of telling LS-D!A what units the model uses so units must be compatible. 3ne way of

testing whether a set of units is compatible is to chec* that

and that

/amples of sets of compatible units are



ELEMENTS

1?.1 Solids

%he <-node solid element by default uses one point integration plus &iscous hourglass control. 7ully

integrated bric* elements are also a&ailable - see 2SC%I3!4S3LID +#aterial Card 1" Column <? for

IA9@?" roperty Set Card 1" Column ': for IA9@1 and Section 0.0 of the %heory #anual- F these

perform better where element distortions are large but are about four times more costly. !o hourglass

control is needed as there are no ero-energy modes. )edges and tetrahedra are simply degenerate bric*s

+i.e some of the nodes are repeatedF they cause problems in some situations and are best a&oided.

1?.' Shells

All shell elements include membrane" bending and shear deformation. %he default 9elytsch*o-%say

formulation is the most economical and should be used unless features particular to other formulations are

reuired e.g.

>)#es 8i= can o set the mid-plane of the element away from the nodes.

?% co-rotational >)#es-8i= fully integrated" so hourglass deformations do not occur +but much more

costly.

:elytsc#0o-Tsay memrane= +and fully integrated membrane appropriate for fabrics etc. where bendingstiffness is negligible.

As degenerate uadrilateral shell elements are prone to loc* under trans&erse shear" triangular shell

elements ha&e now been implemented" based on wor* by 9elytsch*o and co-wor*ers. %riangular shells can

be mi/ed with uadrilateral shells within the same material property set" pro&ided that the element sorting

flag I%IS% on 2C3!%3L4S(LL +Control Card 1'" Column '? is set to 1.

%hree-dimensional plane stress constituti&e subroutines are implemented for the shell elements which

update the stress tensor such that the stress component normal to the shell mid surface is ero. %he

integration points are stac*ed &ertically at the centroid of the element" as shown in 7igure MM.

%hrough-thic*ness directions at each node are initially normal to the element surface but rotate with thenodes. Strains are linear through the thic*ness. %wo integration

olid +lements



#ell +lements

:eam and Trss +lements

Discrete +lements

7igure 1?.?1 Integration oints



points are su cient for linear elastic material" while more points are reuired for nonlinear material. Stress

output gi&es stresses at the outermost integration points" not at the surfaces +despite the nomenclature of

post-processors" which refer to top and bottom surfaces" so care is needed in interpretation of results. 7or

elastic materials" stresses can be e/trapolated to the surfaces. 7or nonlinear materials the usual policy is to

choose four or fi&e integration points through the thic*ness and to ignore the error +i.e. the di erence in

stress between the surface and the outermost integration point. %he location of the outermost integration

points for 5auss uadrature are gi&en in the following table

17.2.1 Newton-Cotes Formlas

In the !ewton-Cotes method r pairs of weights wi and regularly spaced coordinates /i i integrate a

polynomal of degree r - 1 e/actly.

%rapeoidal ule +r@'" domain di&ided into n subdomains

Simpson ule +r@0" domain di&ided into n subdomains



17.2.2 <ass 5nte)ration

In the 5auss method r pairs of weights wi and coordinates /ii integrate a polynomal of degree m O@ 'r - 1

e/actly.

17.2.3 <ass 8oatto 5nte)ration



7igure 1?.' shows the displacement of a cantile&er beam subBected to a load at the free end.

%his figure shows the poor beha&iour of the %rapeoidal integration rule +order r@' using up to 1>

subdomains +'? integration points compared with other !ewton-Cotes formulas without using

subdomains. $sing order r@0 +Simpson points already gi&es the correct result.

7igure 1?.' oor beha&iour of trapeoidal rule



1?.0 Location of Integration oints %hrough %hic*ness

In LS-D!A the location of integration points through thic*ness of shell elements for LS-3S% database

depends on

• database +d0plot or ASCII database elout

• number of shell integration points written to the d0plot database" #AHI!% on

2DA%A9AS4H%!%49I!A" +Control Card '1" Column '?

• uadrature rule +5auss" trapeoidal" user defined

Assume a shell element with fi&e through thic*ness integration points. %hen the location of these

integration points for LS-3S% database is as shown in figures 1?.0 and 1?.8.

7igure 1?.0 Location of integration points for !I@:" #AHI!%@:

7igure 1?.8 Location of integration points for !I@:" #AHI!%@0

Energy data

%he energy data which is printed in the d0hsp and glstat files forms a useful chec* on an analysis. %he

following euation should hold at all times during an analysis.

)here



Internal energy includes elastic strain energy and wor* done in permanent deformation. /ternal

wor* includes wor* done by applied forces and pressures as well as wor* done by &elocity" displacement or

acceleration boundary conditions.

nergy associated with hourglassing is e/cluded by default" but can be included by setting (5! to ' on

2C3!%3L4!5 +Control Card 1>" Column :.

%he computation of ayleigh damping energy dissipation can be acti&ated by setting L! to ' on2C3!%3L4!5 +Control Card 1>" Column '?. )hen acti&ated" this energy dissipation is added to

the internal energy.

%he terms in the euation can all be plotted using LS-3S% and ASCII database glstat. If the euation does

not hold the user should suspect an error. If the left hand side of the euation rises abo&e the right hand side"

energy is being introduced artificially - for e/ample" by numerical instability" or the sudden detection of

artificial penetration through a contact surface +see Section MM. %he latter condition is often shown by

sudden Bumps in the total energy. If the left hand side falls below the right hand side" energy is being

absorbed artificially" perhaps by e/cessi&e hourglassing or by stonewalls or o&er-compliant contact

surfaces.

%he energy in each material can also be plotted using LS-3S%. If the total energy indicates an error"

plotting by material can sometimes indicate where the problem is occurring.

%he energy ratio is defined by

%his energy ratio may be used as a criterium for termination of calculation by defining !D!5 on

2C3!%3L4%#I!A%I3! +Control Card <" Columns 01-8?.



Documents

Contact Modelling in LSDYNA