contact modelling in lsdyna
TRANSCRIPT
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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
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%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
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%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
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%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
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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? $ser s #anual. Sometimes automatic contact interfaces appear not to wor*
because this contact threshold is reached early in the simulation. %his often occurs if e/tremely thin shell
elements are included in the contact surface. In these cases" contact failure can usually be pre&ented by
scaling up the default contact thic*ness or setting the contact thic*ness to a &alue larger than the shell
thic*ness. Alternately" setting S37%@1 +discussed later will often correct the problem.
6.3.1 One-Way Treatment of Contact
3ne-way contact types allow for compression loads to be transferred between the sla&e nodes and
the master segments. %angential loads are also transmitted if relati&e sliding occurs when contact friction isacti&e. A Coulomb friction formulation is used with an e/ponential interpolation function to transition from
static to dynamic friction. %his transition reuires that a decay coe cient be defined and that the static
friction coe cient be larger than the dynamic friction coe cient. %he one-way term in oneway contact is used
to indicate that only the user-specified sla&e nodes are chec*ed for penetration of the master segments. 3ne-
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way contacts may be appropriate when the master side is a rigid body" e.g." a punch or die in a metal
stamping simulation. A situation where one-way contact may be appropriate for deformable bodies is where
a relati&ely fine mesh +sla&e encounters a relati&ely smooth" coarse mesh +master. 3ther common
applications are beam-to-surface or shell-edge-to-surface scenarios where the beam nodes or the shell edge
nodes" respecti&ely" are gi&en as the sla&e node set. %here are a number of *eyword options that acti&ate
one-way contact.
7or contact between an airbag +sla&e and segmented rigid dummy model +master" one of the following
two contact types are often employed2C3!%AC%4A$%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
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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.
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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;
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)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
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%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
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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
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%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
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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$% file is useful for isolating which contact interfaces are responsible. 7or general information on
interpreting contact energies" see the LS-D!A %heory #anual" Section '0.<.8.In some cases" it can be &ery useful to &isualie contact surfaces and produce fringe plots of contact stress
both in directions normal and tangential to the contact surface. %o do this" a binary interface file must be
written by
1. including a 2DA%A9AS49I!A4I!%73 command in the input dec*"
'. setting one or more contact print flags as detailed abo&e"
0. and including the option s@filename on the LS-D!A e/ecution line where filename is the intended
name of the binary database. %he database can be postprocessed using LS-3S%.
6.6 Contact arameters
%here are se&eral contact-related parameters in LS-D!A that can be used to modify or" in manycases" impro&e contact beha&ior. %he default settings for these parameters should be used as a starting point"
but often non-default &alues are appropriate depending on the beha&ior of the contact. %he following
sections describe the most common contact parameters and ma*e general recommendations regarding their
use.
Contact parameters may be set using the commands 2C3!%3L4C3!%AC%" KCONTACT_ " and
2A%4C3!%AC%. Certain parameters may be set using more than one command and so a command
hierarchy must e/ist. arameters set with 2C3!%3L4C3!%AC% redefine default settings for all contacts
in the model. Contact parameters set in 2C3!%AC%4 ... will o&erride default settings for indi&idual
contacts. Contact parameters set in 2A%4C3!%AC% supercede settings in 2C3!%AC%4 for contactin&ol&ing a specific part.
6.6.1 T#ic0ness offsets
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arameter SL%(= +card 1" 2C3!%3L4C3!%AC% and 3ptional Card A in 2C3!%AC%4option
A$%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
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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
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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. $sing a &alue of E DC between 8? - 6? +corresponding to 8?to6? of critical
damping" it is found that the model stability impro&esF howe&er" it may be necessary to reduce the scale
factor for the time step sie. 5enerally" a smaller &alue of '? is recommended when metals" which ha&e
similar material constants" interact.
6.6.& Contact e)ment +tension
arameter #AHA +3ptional Card A " 2C3!%AC% option
#AHA on 3ptional Card A of 2C3!%AC% controls the enlargement of each contact segment that isneeded to combat an inherent flaw in segment-based proBection.
T#is arameter is no lon)er sed in t#e /TO4/T5C contact otions, ecet for
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K2A$%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
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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
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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
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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
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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
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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$%3#A%IC45!AL considers only e/terior edges in its edge-to-edge
treatment as indicated by 7igure MM. An e/terior edge is defined as belonging to only a single element or
segment whereas interior edges are shared by two or more elements or segments. %he entire length of each
e/terior edge" as opposed to only the nodes along the edge" is chec*ed for contact. As with other penalty-
based contact types" S37%@1 can be acti&ated to e ecti&ely treat contact of dissimilar materials.
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7igure 6.: Interior and /terior Shell dges
6.*.2 2C3!%AC%4A$%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.
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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
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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
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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
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: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
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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
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#ell +lements
:eam and Trss +lements
Discrete +lements
7igure 1?.?1 Integration oints
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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
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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
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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
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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
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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?.
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