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Nonlinear Contact
Analysis Techniques
using ANSYS
Mechanics Development Group
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Outl ine of Presentat ion
• General considerations
• Contact applications
• Contact kinematics• Node-to-node element CONTA178
• Node-to-surface element CONTA175
• Surface-to-surface elements CONTA171-174• Multi-physics contact
• Bolt Pretension Element PRETS179
• Troubleshooting• Conclusions
• Future developments
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General Cons iderat ions in Con tact
Analys is • The general goal for contact analysis is to
determine
– Contact stresses transmitted across contacting interface – Contacting area
• Contact problems present significant difficulties
– Unknown contacting zone prior to the analysis – Contact constraint is either active or inactive
– Friction introduces another kind of nonlinearities
• A small amount of positive or negative relative sliding can
change the sign of frictional forces/stresses completely.
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General Cons iderat ions in Con tact
Analys is • Contact finite element development covers
– Kinematics, discretization, Inequality
– Accuracy, robustness and computational overhead
• The numerical treatment of contact problems
involves
– Formulation of geometry – Integration of interface laws
– Variational formulation
– Development of algorithms
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Contact Kinemat ics
• Contact problem involves a variety of geometric andkinematic situations
• Contact surface discretization – Contact surface must be discretized because the
underlying bodies are discretized.
– Node-node, node-surface, surface-surface
– Smoothing provides a significant improvement inconvergence behavior.
• Contact detection and searching
– Global search, local search
• Penetration/gap calculation
– Numerical iterations for higher order
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General Considerat ions
• The resulting elements should be able to
– Pass patch test
• Mesh discretization effects
– Satisfy Ladyshenskaja-Babuska-Brezzi (LBB) condition
• Overconstraint criterion
– Support contact with quadratic order element
– Solve multi-field contact problems
undeformeddeformed
Uniform pressure
Constant pressure over one face fails LBB
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Solv ing Larger Assembly Models
• In the early days of FE technology only the separate
structural components of a structure were analyzed.
• It is now recognized that the interaction betweenstructural parts can have a great influence on the
results.
• Increased computer performance in combinationwith efficient solver technology and parallel
computing techniques has resulted in FE models
which may exceed 1,000,000 elements not only for
linear but also for nonlinear analysis.
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App l icat ion : 2D Gear Model
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App l icat ion : Wir ing
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App l icat ion: 3D Rope Form ing
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App l icat ion : 3D Gear Model
Contact status
SOLID45
PCG Solver
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Appl icat ion: Suspension Model
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App l icat ion: Rubber Boo t Seal
HYPER58
Self-contact
3D rubber material
Steering
shaft
Rubber boot
Self contact
Self contact
Rigid-deformable contact
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Pressure stress ongasket material
gasket
material
∆ (mm)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
P r e s s u r
e
( P a )
0
2e+7
4e+7
6e+7
8e+7
1e+8
1e+8
1e+8
2e+8
2e+8
2e+8
FEA prediction
Gasket Element/Material Simu lat ion
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Electr ical Connecto r
14 symmetric contact pairs
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App l icat ion: Impact of Two Cyl inders
SHELL181
PlasticityLarge strain
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ANSYS Contact Background
CONTAC12CONTAC52
CONTA178 CONTAC48CONTAC49
CONTA175 CONTA171-CONTA174
Type Node-Node Node-Node Node-Surface
Node-Surface
Surf-Surf
Sliding Small Small Large Large Large
High order Yes
AugmentedLagrange
Yes Yes Yes Yes
PureLagrange
Yes Development Development
Contactstiffness
Userdefined
Semi-automatic
Userdefined
Semi-automatic
Semi-automatic
ThermalElectric
Yes YesYes YesYes
Mesh tool EINTF EINTF GCGEN ESURF ESURF
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Contact26Contact26
PointPoint--SurfaceSurface
Contact48Contact48
PointPoint--SurfaceSurface
Contact48Contact48
PointPoint--SurfaceSurfaceRigidRigid
FlexibleFlexible
FlexibleFlexible
GroundGroundContact12Contact12PointPoint--PointPoint
ANSYS Contact Background
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ANSYS Contact Background
• CONTA12,52,26,48,49 exist only because of the historical reason• These elements are not under continue development
• All the new nonlinear tools, like solution control options are not applied to
these elements.
• If you use these elements and at the same time uses SOLCON,ON, thesolution could be very inefficient.
• If you would like to use elements mentioned above, the following options
might help:
SOLCON,OFF !uses the old nonlinear tools
CNVT,U !Node-to-Node contact converges much fast with U
MP,MU,1,0 !Friction make the convergence difficult
R,1,1E6 !for E-Modules 2.1e5
For the efficient reason, the above elements should be replaced with
Contact171-175,178, Target169,170
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ANSYS Contact Background
• Node-node contact element 178
– Model point-to-point contact
• Pipe whip - contact point is always located between the pipe
tip and the restraint
• Node-surface contact element 175
– Model point/edge contact
• Snap-fit - contact can occur around corners
• Surface-surface contact elements 171-174
– Model surface contact
• interference fit
• metal forming
• Mixed contact element types
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ANSYS Contact Background
• If you have a problem which includes contact at a
sharp corner, the surface-to-surface elements,
which use the Gauss point as the contact detection
point, can experience an over penetration at thecorner.
• In such cases, you can mix the surface-to-surface
contact elements with node-to-surface elements.
Surface-to-Surface
Elements
Node-to-Surface Elements
used to model the contact
at the corner
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Patch Test
• A uniform pressure is applied on top of surface
• Uniform stress state should be obtained
irrespective of the mesh
undeformed
deformed
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Patch Test
• Node-node contact pass the patch test
• Node-surface contact fails the patch test
• Surface-surface contact conditionally pass the patch test
• Weak enforcement of contact constrains (penalty method),
rather than strong enforcement (Lagrange multiplier method),
is the recipe by which patch tests of this type may be passed.
• Local penetration violates compatibility condition of
displacement based FEA
– Cross penetration into coarsely discretized contact surfaces can
lead to inaccurate solution
– Refinement of contact surface leads to global accuracy, although
local contact stress oscillation may still be observed
– Only matching meshes completely eliminate these problem
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2D/3D Node-Node Contact
Element CONTA178
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Node-to -Node Element 178
• It is the simplest and least expensive (in terms of
solution CPU) contact element available. When
modeling conditions warrant their use, it can be an
effective tool for modeling a variety of contactsituations.
CONTA178 - 3D Gap (with damping)
I
J
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Node-node CONTA178: Ob jec tives
• Replacement of old gap elements 12,52,40
• Alternative contact algorithms
• Exact compatibility of contact constraint
• More functionality
• Easy of use and intelligent default settings – Chattering & penetration control, contact stiffness
• More efficient and more stable than
– Node-surf, surf-surf contact elements
• Request from ANSYS critical customers
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Lim itat ions of Old Gap Elements
• CONTAC52
– Contact normal based on initial locations of two nodes
• two nodes must be separated initially• may provides wrong contact normal
– Issues with real geometry model
• two node are initially coincident or overlap
• initial interference fit where initial penetration is not constant
– No penetration control
• contact stiffness must be an input
– Lack of cylindrical gap
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Lim itat ions of Old Gap Elements
• CONTAC12
– Contact normal based on real constant
• each contact element may have its own real constant set
– Circular gap
• 2D frictionless
• element stiffness is not updated
• COMBIN40
– Gap normal based on nodal coordinate
• only support one dimension
• accelerations operate, mass and inertia relief calculations are
not correct
– Not a true contact element
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Node-Node CONTA178: Overv iew
• The contact nodal forces are assumed to be directly
conjugate to the nodal gaps.
• The approach can not support 3D higher orderelement contact
• The approaches is capable of passing so-called
contact patch tests and LBB condition due tomatching mesh pattern.
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Node-Node CONT175: Overview
– Pure Lagrange, augmented Lagrange, or pure penalty
– algorithms
• Pure Lagrange permits tiny penetrations (best accuracy)
– User-definable contact normal direction (several options)
– Advanced surface behavior options (bonded, no-
separation, ramped interference, etc.)
– User-definable initial gap or interference – Element damper available (for closed gap status)
– Semi-automatic real constants (factors)
– Frictional circular & cylindrical gap types
– Weak spring option for open gap
– Unsymmetric solver option (NROPT)
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Con tact Element Generation
– EINTF enhancements
• User-specified DX, DY, DZ offset values
– Useful for either coincident or offset (noncoincident)nodes
- User control over
node number
ordering
- EINTF,,,REVE flips
normal direction
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Node Ordering
• Node ordering is critical for contact normal
– First node on one side, second on other side
• Ordering control: EINTF,,,low/high
• Order display: /PSYM, esys
• Order verification
– NSEL,s,pos,1 + ESLN + NSLE + /PSYS,esys + EPLOT
• Ordering correction: EINTF,,,reve
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Contact Normal
• Support real geometry model
– Nodes may be coincident or overlap
• KEYOP(5)=0 – Real constants (NX,NY,NZ) if defined
– Node locations (initially separate position)
• KEYOP(5)=4,5,6 – X (Y,Z) axis of element coor. sys.
– ESYS must be a Cartesian system
– Each element can has its own
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Contact Normal
• KEYOP(5)=1,2,3
– X (Y,Z) axis of nodal coor. sys.
• averaging on both nodes
– Each element has its own
– Easy to build if using solid model
command
• NORA,area,ndir • NORL,line,area,ndir
– Acceleration, mass and inertia relief
are correct
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A lternat ive con tact algo r i thms
• Pure Lagrange multipliers method
– Near zero penetration and slip, no contact stiffness
– More DOF, over constraint, chattering
– Solver issue: PCG, impact, eigenvalue buckling
• Augmented Lagrange method
– Less expensive, more robust – Contact elements superposition
– Less accurate, ill condition if too big contact stiffness
• Lagrange multiplier on normal & penalty on tangent – Behaviors between above two
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A lternat ive con tact algo r i thms
Lagrange
unsymmetry
Lagrange
symmetry
Lagrange
Penalty
Augmented
Lagrange
DOF 5722 5722 5466 5338
Iteration 15 17 13 14
Penetration
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A lternat ive con tact algo r i thms
• Pure Lagrange multiplies method
– KEYOP(2)=0
– Contact length: average distance from contact node to
center of underlying element
– Chattering control parameters
• TOLN: Max. overlap contact remains open
– 0.1*Max. Displacement convergence tolerance – 1.e-3*contact length
• FTOL: Max. tension force contact remains close
– 0.1*Max. residual force
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Lagrange Mu lt ip l ier Method
Contact status
adjustment andCompatibility check
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A lternat ive con tact algo r i thms
• Lagrange multiplier on normal and Penalty on
frictional plane
– KEYOP(2)=1
– Max. slip control SLTOL
• 5.e-3*contact length
• used to determine tangent contact stiffness KS
– KS = max (mu*fn/SLTOL, rvr(4)) – Chattering control parameters
• TOLN, FTOL
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A lternat ive con tact algo r i thms
• Augmented Lagrange method
– KEYOP(2)=2
– Normal contact stiffness KN
• Contact length*Elastic Modulus (EX) of underlying elements
(contact length*1.e9 if EX=0)
– Penetration tolerance TOLN
• 5.e-3*contact length
• degenerate to penalty method if using big TOLN
– Max. slip control SLTOL and KS
• Pure Penalty method
– KEYOP(2)=3
– Contact stiffness KN, KS
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In it ial Gap s ize
• Real constant GAP + initial node locations
(KEYOPT(4)=0)
– Depends on contact normal
– Not a distance
– Supports varying initial interference
• Real constant GAP (KEYOP(4)=1)
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Cyl ind r ical Gap
• Set KEYOPT(1)=1 & ignore KEYOPT(4,5)
• Cylindrical axis direction cosines
– Real constant NX, NY, NZ. (0,0,1 as default)
• Cases
Case 1: GAP = r2-r1 > 0
un = GAP - |x2-x1| >= 0
Case 2: GAP = - (r1+r2) < 0
un = |x2-x1| + GAP >= 0
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Weak Spr ing Opt ions
• KEYOP(3)=0 does not use
• KEYOP(3)=1 acts across an open gap
• KEYOP(3)=2 open gap and/or sliding plane
– Only contributes to contact stiffness
• KEYOP(3)=3 acts across an open gap
• KEYOP(3)=4 open gap and/or sliding plane
– Contributes to contact stiffness and force
• Real constant REDFACT (default=1.e-6)
• Do not combine with no-separate & bonded
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App l icat ion: 2D Cyc l ic Secto r Contact
Contact normal based on
nodal coordinate system
Equivalent stress
after spin
Contact results
A l i i
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Appl icat ion: Bar Impact on a Rigid Wall
Initial shape
Contact element results Equivalent stress
A l i t i A t i f i i l Hi J i t
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App l icat ion: A rt i f ic ial Hip Jo int
Stress
Contact
force
Contact
results
Femoral head
Acetabular
component
Socket
N d N d CONTA178 L i i t t i
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Node-Node CONTA178: L im itat ion
• Requires matching mesh pattern on both side of
contact face
• Requires 8 nodes hexahedron• Only supports small sliding and rotation
• Does not support multi-physics contact
• Contact results can not be visualized
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2D/3D Node-Surface Contact
Element CONTA175
Node Surface Con tact Element 175
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Node-Surface Con tact Element 175
I
IX
Y
X
Y
Z
2D associated target
surface (TARGE169) 3D associated target
surface (TARGE170)
target normal
target normal
CONTA175CONTA175
T t S t
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Target Segments
Line
Parabola
Arc
CircleSphere
Cone
Cylinder
Pilot node
Quad
Quad8
Tri
Tri6
TARGE169 TARGE170
Node Sur f CONT175: Ob jectives
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Node-Sur f CONT175: Ob jectives
• Supports dissimilar mesh pattern on both sides ofcontacting surface that CONTA178 can not handle.
• Solve contact problems when surface-surface contact
elements CONTA171-174 have difficulties.• Point-surface contact & edge-surface contact
• Replace CONTAC26,48,49.
– Much less number of elements
– Better way to treat large frictional sliding
– 2d/3d rigid-deformable & deformable-deformable contact
– Midside node for 2d/3d target, 2d contact surface
• This element is typically used only in highly specializedniche applications:
– In 3D, contact between a line (or sharp edge) and a surface.
– In 2D, contact between a node (or sharp corner) and a surface.
Doub le Beams Con tact
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Doub le Beams Con tact
14 symmetric contact pairs
P i t C t t
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Poin t Contact
14 symmetric contact pairs
Electr ical Connecto r
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Electr ical Connecto r
14 symmetric contact pairs
Less Numbers of elements
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Less Numbers of elements
12 CONTAC48
C*T
3 CONTA175 + 4 TARGE169
(C+T)
Lim itat ions of Node-Based Con tact
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Lim itat ions of Node Based Con tact
Problem with 3D
higher-order contact
element
Questionable for
unique penetrationContact node slips off the
edge of target surface
Lim itat ion of Node-Based Con tact
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Node-surface handles
20 nodes brick element
3D Upsett ing
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3D Upsett ing
Node-surfacehandles
8 nodes brick
element
CONTA175: Overv iew
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• The contact nodal forces are assumed to be directly conjugate
to the nodal gaps. The contact traction can be recovered by
trace the area on contact node.
• Smoothing is done by averaging normal at the target nodes.The normal direction inside of target segment is obtained by
interpolation of shape function
• ANSYS automatically extends the target surface to prevent
“slip-off”.
• The approach can not support 3D higher order element
contact
• The approaches is incapable of passing so-called contactpatch tests. It satisfies LBB condition.
CONTA175: Overv iew
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• 2D/3D contact
• Ridge-flexible, flexible-flexible contact
• 2D lower/higher order contact & target
• 3D lower order contact, 3D lower/higher target
• Static, transient, model, linear buckling,harmonic, sub-structuring analysis
• Thermal-electric contact ?
Con tact Pair Concept
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• Target (master) surface - a continuous surface
• Contact (slave) surface
– A set of discrete contact nodes – Contact constraint equation
• Symmetric (two pass) contact
– Difficulty of contact pressure interpretation – Overconstraint of the model
The target nodes can pass
through the contact
su rface in between thecontact poin ts .
The contact detect ion
po ints can no t penetrate
the target su rface.
CONTA175 Mesh Too l: ESURF
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ESURF
GCGEN
for
48,49
ESURF
for
175
CONTA175 Mesh Too l: ESURF
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Only exterior nodes
are re-selected
CONTA175: Con tact Models
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• Contact force based model KEYOP(3) = 0
– Gives contact forces as CONTAC48,49
– FKN, FKT unit: Force/length – TCC, ECC depend on element size
– Pressure contact force in etable and PLES/PLNS
• Contact traction based model KEYOP(3)= 1 – Gives contact pressure as CONTA171-173
– FKN, FKT unit: Force/length**3
– Thermal-electric contact is available (TCC, ECC) – Point contact is not supported (auto switching).
Con tact Normal Direct ion
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• Perpendicular to target surface KEYOP(4)=0
– Target surface smoothing must be done
– Used if target surface is smoother than contact surface
• Perpendicular to target surface KEYOP(4)=1
– Contact surface smoothing must be done
• Perpendicular to target surface KEYOP(4)=2
– Used for shell/beam bottom surface contact
– Contact surface smoothing must be done
• Smoothing is performed by averaging surface
normals connected to the node.
Upsett ing
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KEYOP(4)=0
Normal to
target surface
O-ring Prob lem
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KEYOP(4)=1 Normal from
contact surface
Doub le Beams Prob lem
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Different normal
Definition gives
Different anwsers
CONTA175: Target Edge Extens ion
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• Auto extension of target surface (TOLS)
– Prevent from slipping of the target edge
– Defaults 2 (2%) for nlgeom,on; 10 for nlgeom,off; 20 for
interference fit
Target Edge extens ion facto r
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Interference fit
TOLS=20 TOLS=2
Miss contact
detection
CONTA175: Pos tpro cess
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• Contact quantities can not be clearly visualized (asopposed to CONTA171-173).
• Etable items and Sequence Numbers are similar toCONTA171-173
– Easily switch element types
– Waste spare space
• Items in PLES, PLNS for contact force based model
– PRES: contact nodal force
– SFRI: contact frictional force
Name Item I J
PRES SMISC 5 1 2
SFRIC SMISC - 3 4
STAT1 NMISC 19 1 2
OLDST NMISC - 3 4
PENE2 NMISC - 5 6
Node-Surf CONTA175: L im itat ion
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• Does not support 3D higher order element contact
• Requires 8 nodes hexahedron
• Contact results can not be visualized
• The approaches is incapable of passing so-called
contact patch tests.
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Surface-Surface Contact
Element CONTA171-174
A New Revolution Technology
Con tact Elemen ts
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CONTA171 CONTA172
CONTA173 CONTA174
CONTA175
Target Segments
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Line
Parabola
Arc
CircleSphere
Cone
Cylinder
Pilot node
Quad
Quad8
Tri
Tri6
TARGE169 TARGE170
Con tact/Target Degenerated Shape
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• Fully support 3D 2nd order of contact and target
elements with any kind of dropping mid-side
nodes.
Surface-to -su rface elements
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• The surface-to-surface contact elements are the
most versatile contact elements in the ANSYS
program.
• Because they are robust, feature-rich, and user-friendly, they have become the contact element of
choice for most ANSYS users.
Surface-to -su rface elements : Overview
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• The surface-to-surface elements are the most widely usedcontact elements in ANSYS, due to the many advantages thatthey have over the other contact elements:
– Compatible with both lower order and higher order elements.
– Support large deformations with significant amounts of slidingand friction efficiently.
– Provide better contact results (easier to postprocess contactpressure and frictional stresses).
– Can account for shell and beam thickness, as well as shellthickness changes.
– Semi-automatic contact stiffness calculation.
– “Pilot node” control of rigid surface.
– Intelligent default settings, Contact Wizard (easy to use).
– Multiphysics contact capability.
– Numerous advanced options for overcoming difficult problems.
Surf -Sur f Contact - Object ives
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• Extend family of ANSYS contact elements
– Surf-surf instead of node-node, node-surf contact
• Provide higher-order contact/target elements
– Mesh generation of 10 nodes tetrahedral is not an issue
– Represent curve surfaces (non-faceted)
• Provide general thermal/electric contact analysis
capability
• Support unmatched mesh pattern on contactingbodies
• Easy of use and intelligent default settings
• Solve real world problems
ANSYS Con tact - Overv iew
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•• RigidRigid--flexflex
– – Bodies of vastly different stiffnessBodies of vastly different stiffness
– – Steel against rubber sealsSteel against rubber seals
•• FlexFlex--flexflex – – Bodies of comparable stiffnessBodies of comparable stiffness
– – Metal contacting metalMetal contacting metal
•• Self contactSelf contact – – Body folds over itself Body folds over itself
– – Column bucklingColumn buckling
•• Large sliding with friction for allLarge sliding with friction for all
Surface-Surface Con tact: Overv iew
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• The formulation is given in terms of contact tractions
• Transmission of pressure across contact element surface is
basic to contact problem
• Pressure is applied to element faces by using element shapefunctions to calculate the equivalent consistent nodal forces
• The use of surface traction approach is recommended as the
influence of element size is eliminated which supports well for
thermal/electric conductance analysis.
• Patch tests cannot be expected to be passed for arbitrary
discretization and for arbitrary element orders.
Surf -Sur f Con tact - Overv iew
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Consistent nodal forces for higher-order elements
No contact node“slips” off the edgeUnique penetration, No smoothing of target surface
Gauss point
Consistent thermal interface definition
Surf -Sur f Contact - Overv iew
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• Quadratic order contact/target elements
– Comparatively stiff behavior associated with 4 nodes tet
– Fully Automated mesh generation
• 10 nodes tetrahedron is not an issue
• 8 nodes Hexahedron is challenging
– Represent curved surfaces (non-faceted)
8-Nodes
Hex
10-nodes
Tets
20-Nodes
Hex
Surf -Sur f Con tact - Overv iew
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Surf -Sur f Con tact - Overv iew
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Surface-surface handles
20 nodes brick element
Con tact Pair Concept
T t ( t ) f ti f
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• Target (master) surface - a continuous surface
• Contact (slave) surface
– A set of discrete contact points (element quadrature points)
– Contact constraint equation
• Symmetric (two pass) contact
– Difficulty of contact pressure interpretation – Overconstraint of the model
The target nodes can pass
through the contact
su rface in between thecontact poin ts .
The contact detect ion
po ints can no t penetrate
the target su rface.
Con tact Pair Concept
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Target
Target
Contact
Contact
Convex surfaceConvex surface
Fine mesh surfaceFine mesh surface
Softer surfaceSofter surface Higher order surfaceHigher order surface
Smaller surfaceSmaller surface
Concave/flat surfaceConcave/flat surface
Coarse mesh surfaceCoarse mesh surface
Stiffer surfaceStiffer surface Lower order surfaceLower order surface
Larger surfaceLarger surface
ContactContact surface:surface: TargetTarget surface:surface:
Pilot Node
Go erns the motion of
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• Governs the motion oftarget surface
– Force/displacement
– Rotation/moment – Temperature/voltage
• Characterizes heat flow
– -: net heat loss
– +: net heat gain
• Can be at any location
• Connects to other element – Mass element
Rigid surface rotated
Pilot node
(at center of
rotation)
App l icat ion: Wire Bend ing
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SOLID92
Plasticity
Pilot node
App l icat ion: Deep Draw ing
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4 node shell
PlasticityBlank-holder force
applied on pilot node
Pilot node
punch
die
Blank holder
F
Blank
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Pilot node:KEYOPT(2)=1
Pilot Node
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Pilot node:KEYOPT(2) 1
Bonded (Initial contact)
Pilot Node(before)
TargetSegment
Pilot Node(after)
F
Pilot Node
F
Pilot node can be used together
with the bonded contact option
to introduce the rigid regionbetween the loading point and
the structure, as shown here.
Pilot node:KEYOPT(2)=1
Pilot Node
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Pilot node:KEYOPT(2) 1
Force FY on Pilot node
Pilot node
Pilot node
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Con tact Wizard
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Con tact Wizard
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signSpace ® Graph ical User In terface
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•Auto pair definition
Design Space Model
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Quadrature Rules
• Gauss full integration rule leads tot i ti f d l (f il LBB diti )
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goverconstrainting of model (fails LBB condition)
– Penalty or augmented Lagrange method may alleviate the
problem.
– Smoothing is not required. It passes patch test.
PressureVon Mises stress
Existence of checkerboard mode
Quadrature Rules
• Nodal quadrature rule
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q
– Smoothing is always required
– Overconstraint still exists for higher order contact element
– Add a midface node to the center of contact element to getconsistent nodal force
– Use mixed U-P formation
• The order of interpolation function for pressure is lower than
that for displacement
Quadrature Rules
• Nodal integration rule (satisfies the LBB condition)
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g ( ) – Smoothing is always required which is performed by
averaging surface normals connected to the node
– Overconstraint exists for quadratic order contact element.
– Use mixed U-P formation
Surface-surface handles
20 nodes brick element
Highly distorted
Interconnected Rai ls
14 symmetric contact pairs
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Contact Normal Direct ion
• KEYOPT(4)=0, Guass point, perpendicular to contact surface
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• KEYOPT(4)=1, Nodal point, perpendicular to contact surface
– New, smoothing on contact surface is performed which ismore expensive than Gauss point Scheme
– When contact surface is smoother then target surface
• KEYOPT(4)=2, Nodal point, perpendicular to target surface
– New, smoothing on target surface is performed which is
much more expensive than option (KEYOPT(4)=1) – When target surface is smoother then contact surface
• All schemes support low/high order contact elements
• Smoothing is always required which is performed by averagingsurface normals connected to the node
O-ring Prob lem
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KEYOP(4)=1
Normal from
contact surface
Doub le Beams Prob lem
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KEYOP(4)=2
Normal to
target surface
Doub le Beams Prob lem
KEYOP(4)= 1
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( )
Normal from contact
Different contact
element pattern givesdifferent normal
eventually gives
different anwsers
Electr ical Connecto r
14 symmetric contact pairs
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Auto Spurious Prevent ion : KEYOP(8)=1 is no long needed
/batch,list rmod,1,6,10
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/batch,list
/prep7
et,1,182,2
et,2,169
et,3,175
keyop,3,4,1
keyopt,3,8,0
mp,ex,1,2e5
rect,0,3,0,1
kmod,1,0.5
esiz,0.3
ames,all
type,3
esurf
type,2
esurf
Spurious contact
most likely occurs
rmod,1,6,10
/solu
nsel,s,loc,x,3
d,all,all
nsel,s,loc,y,0
sf,all,pres,1
alls
solv
Ansys70
ansys61
Automatic Asymmetr ic Detect ion KEYOP(8)=2
• Ansys70 can automatically detect asymmetric contact
i h i i d fi d
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pair when symmetric pairs are defined
– Many pairs exists in the model
• Be difficult to pick
– Model came from DS using auto contact detection
– The distinction between the contact and target surfaces is not
clear.
– The contact results are very hard to be interpreted.
– Overstrained model
– Decision is made by many factors
• Stiffness, numb of element, element size, area, curvature
Believe or notBelieve or not
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It has 209It has 209unsuppressed partsunsuppressed parts
and 450 contactand 450 contact
pairs.pairs. The modelThe model
has 385,000 nodeshas 385,000 nodesand 1.15million DOFand 1.15million DOF
Au tomat ic Asymmetr ic Detect ion
*** NOTE *** CP= 19.490 TIME= 18:22:37Symmetric Deformable- deformable contact pair identified by real
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constant set 8 and contact element type 9 has been set up. The
companion pair has real constant set ID 9. Both pairs should have the
same behavior.
ANSYS will keep the current pair and remove its companion pair,
resulting in asymmetric contact.
*** NOTE *** CP= 19.490 TIME= 18:22:37
Symmetric Deformable- deformable contact pair identified by real
constant set 9 and contact element type 9 has been set up. The
companion pair has real constant set ID 8. Both pairs should have the
same behavior.
ANSYS will remove the current pair and keep its companion pair,
resulting in asymmetric contact.
In i t ial Con tact Adjustment
• PSOLVE, ELFORMCheck initial contact status db -> rdb
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– Check initial contact status, db -> rdb
– Physically moves contact nodes to the target surface
• Only for nodal detection option or CONTA175• Initial contact nodes inside ICONT zone
• Initial penetrated nodes with KEYOP(9)=1
– After save, all the setting can be modified
Before PSOLVE After PSOLVE
Output (if CONCNTR,PRINT,>0)
Node 22 moved to target surface
Node 25 moved to target surface
Contact Algor i thms
• Penalty method (KEYOP(2)=1)
• Augmented Lagrange method (KEYOP(2)=0)
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• Augmented Lagrange method (KEYOP(2)=0)
• Lagrange multiplier method (KEYOP(2)=3)
• Lagrange multiplier on normal & penalty on tangent
(KEYOP(2)=2)
• Automatic adaptation of boundary conditions andconstraints
• Each method has advantages and disadvantages
depending on the particular problem considered
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Penalty Method
• The linearized form :fCBBftKfKpBpBfKu δδδδδδ T T T ld p ++=++=− ),(
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• The system equation in New-Raphson iteration form
• Advantages: It is simple and displacement based FE
framework remains
• Disadvantages: It suffers from ill-conditioning as
penalty stiffnesses are increased.
( ) old T
T T N N g uf CBBgK K −=++ δ ε ε σ ,
f CBBf tK f K pBpBf K u δ δ δ δ δ δ σ old p ++++ ),(
Augmented Lagrange Method
• Ill-conditioning can be alleviated by augmentingLagrange multipliers
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g g p
• The total potential energy (virtual work)
– It can be considered as a generalization of the Lagrange
multiplier method where an additional term involving thecontact tractions is added to the variational equations.
• Augmented Lagrange method at element level
– If the Lagrange multipliers are retained as variables in theactive contact elements and they remain as element level
variables and do not enter the global structural solution.
( ) ( )[ ]dA g g T T T T N N N N ∫Γ +++=Ψ gg δ ε λ δ ε λ δ
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Augmented Lagrange Method
• Penalty stiffness is the most important parameteraffecting accuracy and convergence behavior
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– High stiffness
• Less penetration (better accuracy)
• ill-conditioning (more difficult convergence)
– Low stiffness
• May help convergence, but lead to more penetration
• Need more iterations if using too small penetration tolerance
• What’s default for normal contact stiffness
– Bulk modulus & material behaviors of underlying elements
– Size of element (depth), structural flexibilities, self-contact
– Total DOF
– Cover 80-90 % applications
Con tact Stif fness
• Normal penalty coefficient FKN – 1.0 (default) for bulk solid in contact
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( )
– 0.1 for more flexible (bending-dominated) parts
• Default tangent penalty coefficient FKT
– Can be overwritten
Bulky contact; try FKN = 1.0
Flexible contact; try FKN = 0.1
d d is result from FKN and equilibrium equationis result from FKN and equilibrium equation
Pressure=Pressure=dd *FKN => Contact stress on contact surface*FKN => Contact stress on contact surface
Penetrat ion VS. Con tact Pressure
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100100--times Difference with FKN leads to 100times Difference with FKN leads to 100--times Difference withtimes Difference with d d
but only leads to about 5% Difference with contact pressure andbut only leads to about 5% Difference with contact pressure and stress.stress.
Penetration d=0,26e-3,P=43642 Penetration d=0,45e-5,P=43927
60545.0326.0 =
−−=∆
eed 007.1
4364243927 ==∆ p
KN=0.001 und TOLN=0.1KN=0.001 und TOLN=0.1 KN=0.001 und TOLN=0.01KN=0.001 und TOLN=0.01
Local Accu racy VS. Global Equ i libr ium
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Penetration Penetration Penetration Penetration
Bending stress Bendin g stress Bendin g stress Bending stress
0
.002202
.004404
.006606
.008808
.01101
.013212
.015414
.017616
0
.432E-03
.864E-03
.001296
.001728
.002159
.002591
.003023
.003455
-123.031
-88.59
-54.148
-19.70714.735
49.176
83.618
118.059
152.501
-121.92
-87.795
-53.671
-19.547
14.578
48.702
82.826
116.951
151.075
Contact St i ffness Update
• KEYOP(10)=0 – update contact stiffnesses at eachload step if FKN, FKT are redefined
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• KEYOP(10)=1 – update contact stiffnesses at each
sub-step based on current geometry• KEYOP(10)=2 – update contact stiffnesses at each
iteration – adaptive scheme
Contact St i f fness Update
• Adaptive scheme for normal contact stiffness
– First iteration bases on previous convergence pattern
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– The subsequent iterations depend on penetration
≤
>•
=
10
102.0
0
0
1
k if
divergeor k if
N
N
N ε
ε
ε
≤
>•
=+
+
+
K
N
K
N
K
N
K
N
K
N
K
N K
N
g g if
g g if
411
411
15
ε
ε ε
Contact St i f fness Update
• Adaptive scheme for tangential stiffness – Based on current normal pressure
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– Allowable Max. elastic slip distance (SLTOL)
– Positive: as a factor SLTOL*(contact length)
– Negative: as a true vale
– Default to 1.d0
crit T l
p
ε =
crit l Allowable Max. elastic slip = l *01.0
Frict io n Overview
• Friction is a complex physical phenomena thatinvolves the characteristics of the surface
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– Surface roughness, temperature, pressure, and relative
velocity
• Friction is a path dependent process, certain history
variables must be tracked and accounted for
traction determination.
• The definition of reference system is a key point for
integrations of the derivatives of contact tractions.
Fr ict ion Law
• Coulomb model with additional cohesion b andshear behavior maxt
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– Sticking contact is reversible
– Sliding contact is irreversible
• Dynamic friction model
p
•
−−+= g d
d sd
c )e µ(µ µ µt t
maxt
bSticking
Static, dynamic, decay
friction coefficients
Fr ict ion Law
• If the static and dynamic coefficients of friction atleast one data point (µ1 ; Vrel1) are know
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• The decay coefficient:
−
−−=
k s
k
rel
cV
d µ µ
µ µ 1
1
ln*1
s µ
eqcd
k sk e γ
µ µ µ µ &−
−+= )(
eqγ &
k µ
µ
Sliding
Sticking
Dynamic Fr ict ion
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Dynamic Fr ict ion
Heat generated by friction
2.4
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1
1.2
1.4
1.6
1.8
2
2.2
0 200 400 600 800 1000
Velocity
T e m p e r a t u r e
Fr ic t ion Condi t ions
• Kuhn-Tucker conditions for Coulomb friction0
≤−−≡Φ µ pbT t Coulomb friction condition
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0
0
1
=Φ
≥
−=Φ
∂−
•••
ξ ξ
ε ξ
λ T T T T T
tu t Non-associated flow rule
Tangential traction is applied in the sliding direction
Sliding only occurs when Φ=0
If Φ
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k T T
k T
k
T T
k
T
tr T
tr
T k tr
T
k
T
k
k
T
k
T T
k
T k T
t
b pt
t
gt
gt
t
t
nn
ngtt
∆+
∆+===
+=
∆+
=
++
+
+
+++
ε
ε
µ
ε
1,1
max
1
1k
crit
11k
crit
1
1+≤−∆+
k k
T T
n
T pb µ ε gtif
otherwise
max
1 t b pk
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– The matrix is unsymmetric which corresponds to the non-
associativity of Coulomb’s frictional law
– The combination of the frictional interface law with return
mapping algorithm leads to a consistent matrix.
( )
dpl
g dg dt
dpn p
pdg nnt
dt
crit
iiT i
i
k
j jiijtr T
k
crit
T i
µ ε
µ µ δ ε
∆+=
∂
∂++−=
+
+
11
t b pt k
+=
++ 11k
crit µ if
If sticking
Consis ten t Sti f fness Matr ix
• Consistent behavior – It makes system equation converge quadratically
U t i b h i
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• Unsymmetric behavior
– Frictional sliding contact – Stress stiffness matrix due to curved contact/target
surfaces
– Adaptive tangential contact stiffness
– Use the unsymmetric solver even friction coefficient issmall
• Stiffness matrix symmetrization
– Maintain limit pressure if sliding – Efficient for most contact problems
Surface Interact ion Models
• Standard contact (unilateral contact)
• Rough contact - no sliding permitted (infinite MU)
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• No separation can not open but can slide
• Bonded contact no opening/sliding permitted
– Assembly contact takes advantage of the bonded contact
to glue multiple parts together
• Debonding for modeling crack (Beta)
Glue parts A & B
together usingassembly contact
/prep7
et,1,45
mp,ex,1,2e5
mp,dens,1,7.8e-9
cylin,0,5,0,60,0,90 CEINT is valid only if the relations
Bonded Contact
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cylin,0,5,0,60,0,90
block,0,10,0,10,25,35
vovl,all
esiz,2vsweep,all
vsym,x,all
vsym,y,all
numm,node
numm,kp
/solu
nlgeo,on
nsel,s,loc,z,0
d,all,all
nsel,s,loc,y,10
sf,all,pres,200
alls
solv
Large deflection
Traditional way Bonded contact CEINT
between the CE DOFs are linear
App l icat ion: Hel icop ter Roto r Shaft
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Appl icat ion : Grapp l ing Hook
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Bonded con tact Based MPC
• A powerful tool to connect incompatible meshregions
– Solid-solid, shell-shell, shell-solid assembly
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• Multi-point constraints are built automatically
– Based on surface normal and shape function of targetelement
– Updated during each iteration
– Working properly for larger deformation – Solving purely linear contact problems without iteration
– DOF of the constrained contact nodes is eliminated
• Translational, rotational, temperature, voltage, andmagnetic potential can be constrained
3D Assemb ly Stress Analys is
14 two pass contact pairs
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MPC
Bonding
surface
Shell-Shel l Assembly
14 symmetric contact pairs
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Translational, rotational DOFsare constrained
Shel l Edge Assembly
14 symmetric contact pairs
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Multi-Physical Contact Element
CONTA171-175
A Cutting Edge Technology
Mult i -Phys ics Contact
• Heat/electric conduction between contacting surfaces
• Heat convection and/or radiation
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• Heat generation due to
– Frictional dissipated energy
– Electric current
• Electric charge across the contacting interface
– Piezo-electric analysis
– Electrostatic analysis
• Magnetic flux across the contacting interface
App l icat ion: Upsett ing o f a B i l let
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• Heat generation due to plastic work
• Heat convection and radiation on free surface
• Heat conduction and frictional heating on contact
surface
Free Surface
Rigid die
Contact
Surface
Temperature
App l icat ion: Wheel Brake
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Rotational velocity v. timeStress
Temperature
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Piezo-electr ic Env ironment
VOLT DOF VM Stress
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App l icat ion: Electr ic Welding
Voltage
ImposedVoltage &
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Bulk Temp=20 C
Transient analysis (Time: 0-0.1s)
Grounded
Voltage &
Displ.
Temperature
App l icat ion: Electrostat ic Micro Mirror
• Using sequential electrostatic-structural coupling procedure• Voltage difference between mirror and pad creates unbalanced
electrostatic forces
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100µmpad
mirror
App l icat ion: Electrostat ic Micro Mirror
Mesh for air
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Voltage
•Simulation with iterative process•Electrostatic solution to get force
•Structural solution with contact
•Morphing of electrostatic mesh
Micro Mirror
Stress
Ax isymmetr ic DC Ac tuator
Air
Magnetic
potential
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Armature
Model is disconnected
along this line
Building bonded contact
Stator
Coil
Magnetic
flux
Fluid -Struc tu re In terat ion
• Fully-automated multiphysics solution for
– Fluid-structure interaction, fluid-thermal-electric interaction
– Full support for all nonlinearities: Geometric, material, contact
– Fully-implicit time-stepping scheme:
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Fully-implicit time-stepping scheme:
– FLOTRAN Element Birth and Death:
• Suitable for FSI problems involving contact between immersed,
moving structures
• Fluid elements may be automatically deactivated as surfaces
come into contact (e.g., valve closes), or reactivated as theyseparate (e.g., valve opens)
• Extremely wide set of applications
Deformable Flow Contro l Device
CONTA172’sUnder
P
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FLUID141’s
PLANE183’s
TARGE169’s
•Incompressible, turbulent water flow
•Prescribed inlet-to-outlet P = 45 PSI
•Hyperelastic, high strain
(>100%) materials
•Mooney-Rivlin model
•Determine steady-state shape of solid and accompanying
steady-state fluid flow rate
Deformable Flow Contro l Device
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Bolt Pretension Element
PRETS179
A State of Art
Bolt Pretens ion
• Whenever you model a bolted structure, it might be important
to include the pretension (or preload) in the bolt caused by the
tightening of the bolt.
• ANSYS provides a convenient way to simulate bolt pretension:
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– Pretension elements PRETS179
– Automatic pretension mesh generation
– Load management for pretension sequence of multiple bolts
Stresses due
to specified
pretension in
bolt
Bolt Pretens ion : Tradit ional Ways
• There was no automated wayto prescribe the pre-load. It
was modeled through
– Contact initial interference
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– Thermal expansion
• Manually iteration process is
required
• It is difficult to control and
monitor the variation of the
bolt force during whole
modeling
Bolt Pretens ion : Overview
• An automated way tospecify bolt pretension
• Useful for creating,
managing and loading
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managing, and loading
structures having multiple
pretensioned bolts (no limit
on the number of bolts)
• Replaces previous trial-
and-error techniques
• Automated generation
capability
Stresses Due to
Specified Pretension
in Bolt
Surrounding Structure
PRETS179 Element
• Pretension elements (PRETS179) apply thespecified preload across a pretens ion sect ion .
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g
Preloaded Bolt
Pretension Node
Pretension Section
(brick or tet meshes)
PRETS179 Element
• Features of the pretension element:
– A set of pretension elements is
identified as a “section”.
– 2-D or 3-D line element that acts like a
"hook" connecting two halves of a Preload
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g
bolt.
– Nodes I, J are the end nodes, usually
coincident.
– Node K is the pretens ion node :
• Location is arbitrary.
• Has one DOF: UX.
• Used to define the preload, as an FX
force or UX displacement.
• Actual line of action is in pretensionload direction
direction
J
K
Node I
Pretension
section
Bolt — solids,shells, or beams
PRETS179 Element
• Features of the pretension element (continued): – Preload direction is constant – it does not update for
rotations. It can be re-defined during load steps.
– No material properties or key options
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– Underlying bolt elements may be solids, shells, or beams,
lower or higher order.
– The DOF: translations, rotations, temperature, voltage
which are detected internally based the DOF of underlying
elements.
– Elements created automatically using GUI-based
procedure.
Typ ical Procedure
• Five main steps:1. Import or create the geometry, including the bolt(s) and
the surrounding structure.
2. Mesh all parts.
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3. Create the pretension elements.
4. Apply loads and solve using multiple load steps:
• Load step 1 for the bolt preload
• Load step 2 to “fix” the bolt length• Load step 3 for other loads on the structure
5. Review results.
• We will expand on steps 3 and 4 next.
Typ ical Procedure
Creating the Pretension Elements
• After all parts of the structure, including the bolt(s), havebeen meshed, the next step is to create the pretensionelements.
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• Two options:
– Using PSMESH
• Fastener must be meshed as one piece.
• PSMESH will cut the fastener in two and generate the pretension sectiontogether with the pretension elements.
– Elements at coincident nodes, EINTF (not discussed here)
• The fastener must be meshed in two separate pieces.
• Requires a matching node pattern.
Typ ical Procedure
• The menus provide a wide variety of methods to
create the pretension mesh.
– Preprocessor > Create > Elements > - Pretension –
Pretensn Mesh >
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• We will illustrate the procedure using the WithOptions > pick.
ANSYS splits the bolt shown
and inserts the necessarypretension elements.
ANSYS splits the bolt shown
and inserts the necessarypretension elements.
Typ ical Procedure
• Select the option to – Divide at Node – Picked
Elements +
– Follow the menu prompts
Window the
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Pick a node to indicatewhere to section the bolt.
A pretension node will be
created at this location.
elements where the
bolt should be split.
Typ ical Procedure
– Next, fill in the dialog box Assign a sectionnumber and name
Specify the preload direction
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– Pressing OK will separate the elements of the bolt into
two unconnected groups, tied together with pretension
elements.
(Y-axis points along the boltaxis in this case)
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Typ ical Procedure
Pretension Load Application• When a physical bolt is pretensioned:
– Turning the nut reduces the unstretched grip length of the bolt,
thereby inducing pretension
Wh th d i d t i i hi d d th h i
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– When the desired pretension is achieved and the wrench isremoved, the new unstretched grip length becomes ‘locked’
• Typical ANSYS pretension loading procedure represents
this same sequence
– First, apply the specified pretension (usually a specified force)
in one load step
– Then, lock the pretension section displacement (lock the
shortened grip length) in a subsequent load step.
– Once all bolts are pretensioned and locked, apply externalloads in the final load step
Typ ical Procedure
Pretensioning and locking a large number of bolts in a
Stiff plate
Soft gasket
bolt1 bolt2 bolt3
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• Pretensioning and locking a large number of bolts in aspecified sequence is made easy with the load-management
tool.
– Solution > Loads > Apply > Pretnsn Sectn …
Section to be preloaded first
Semi-automated ‘locking’ of
pretension displacement in
the next load step
Pretension load to be applied
to this bolt
Typ ical Procedure
• Continue the loading sequence for the other two sections:
Incrementing the loadstep number
controls the pretensioning
sequence
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Semi-automatic restraint of the
free-body bolt pieces
Typ ical Procedure
• In solution, ANSYS automatically applies and locks the
loads in the specified load step every time you issue a
SOLVE.
– For the three bolts shown in the previous example, you would
need to solve four times to apply the specified pretension
sequence
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sequence.• SOLVE (LS1, applies the preload to bolt1)
• SOLVE (LS2, fixes the preload displacement for bolt 1 and tightens
bolt2)
• SOLVE (LS3, locks bolt 2 and tightens bolt3)
• SOLVE (LS4, locks bolt3)
• With the preload applied, apply external loads acting on the
overall structure.
Typ ical Procedure
• Deformed geometry plot and bolt forces for the previous
example
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8 node brick 20 node brick
Appl icat ion: 3D bo l t Joint s imulat ion
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10 node Tet
Pre-tension section
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Appl icat ion: Bracket Assemb ly
Static Analysis: Loading
First load step: Resolve bolt pre-tension (Fi=5,000 lb)
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Appl icat ion: Bracket Assemb ly
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Results for First Load Step
Appl icat ion: Bracket Assemb ly
Second Load StepApplied pressure on bracket
(1,500 lb total load)
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Modal analysis
Contact status maintained
App l icat ion: Bo l ted Jo int Thermal
Contact
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App l icat ion: Bo l ted Jo int Thermal
Contact
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App l icat ion: Bo l ted Jo int Thermal
Contact
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Solver Issue for Contact
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Solver Issue for ContactAnalysis
Solver issue
• Sparse solver is the default solver
– Best for medium contact models (under 1,000,000 DOF)
– Good for slender/thin structure
– Direct solver - more robust than PCG solver
– Unsymmetric system is available but expensive
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• PCG solver as an option
– Enhanced for handling indefinite matrix
– Large contact models (over 1,000,000 DOF) – Good for bulky solid
– Iteration solver - performance bases on element shape andcontact condition
– Not applicable for pure Lagrange algorithm, unsymmetricsolver
Solut ion Opt ions
• Sparse solver is the default solver
Total DOF = 502851
# Sparse in ansys57
Memory=480, CPU=2123
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# Sparse in ansys60:
Memory=312, CPU=1146
# PCGMemory = 294, CPU=1014
App licat ion : Sp lined Shaft
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SOLID92
PCG Solver
App licat ion : Sp lined Shaft
Number of equations = 497380, Maximum wavefront = 383
EQUIL ITER 1 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= -0.4047E-01FORCE CONVERGENCE VALUE = 0.1941E+05 CRITERION= 18.79
EQUIL ITER 2 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= -0.7171E-03
FORCE CONVERGENCE VALUE = 0.3028E+05 CRITERION= 16.13
EQUIL ITER 3 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= -0.3307E-03
FORCE CONVERGENCE VALUE = 108 6 CRITERION= 16 56
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FORCE CONVERGENCE VALUE = 108.6 CRITERION= 16.56
EQUIL ITER 4 COMPLETED. NEW TRIANG MATRIX. MAX DOF INC= -0.5784E-04
FORCE CONVERGENCE VALUE = 9.115 CRITERION= 16.88
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3D Assembly
Finite Element Model
119,000 Elements
590,000 DOFs
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AMG shows superior convergence and scaling for this problem
300
722
Mbytes
Memory
5301
200
Iter
10-6
8679
5265
7745
3831
6636
2638
6909
1884
NP=1 NP=2 NP=3 NP=4
Solver Elapsed Time (sec)
PCG
AMG
Method
Tips and Troubleshooting
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p gfor Contact Analysis -
A Diagnostic tools
Non contact-related issues
• Unrealistic physical model
• Unreasonable loading and boundary conditions
• Poorly/coarsely discretized mesh, sharp corner, location of
mid-side nodes, element distortion
• Hourglass and locking
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• Unreasonable or incorrect material properties and bad input
unit
• Large plastic deformation
• Incompressible or near incompressible
• Local & global instabilities
• Follower loads
What You Should Do in the Begin
• Read Chapter 10 of Ansys structural analysis guide
• Verify outward normal of contact/target surfaces
• Verify pinball region
• Verify constraints
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• Verify initial contact status through output
– Issue: PSOLVE, elform
• Use CONCNTR, PRINT, level, to get detailed printout
of the contact state in the output file so that you
can diagnose the problem.
• Rigid-to-Flexible and Flexible-to-Flexible contact can be
defined in the same model. Be sure to use separate realconstants to define the contact pairs. The output file
information can be helpful to check your contact pairs.
• Be sure to check the outward normal direction of the contact
and target elements (via Main Menu > Preprocessor > Create >Contact Pair > View and Edit…) Contact occurs on the
What You Shou ld Do in the Begin: Tip
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Contact Pair View and Edit…) Contact occurs on the
positive outward normal side of the contact and target
elements.
Improper
Proper
What You Shou ld Do in the Begin: Tip
• Defining duplicate nodes contact/target surface
– On contact surface
• Overconstraints if using pure Lagrange mutiplier
– On target surface
• Contact node may stuck there
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• Using poorly discretized surfaces
– Unrealistic penetration occurs with coaresly discretized
contact surface. Use refiner mesh to define contact
accurately.
– Coarsely discretized, curved target surfaces can lead tounaccepatble solution accuracy. Using a more refined
mesh or higher order mesh will improve the overall
What You Should Do in the Begin: Tip
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mesh or higher order mesh will improve the overall
accuracy of the solution.
• Ensure that the target and contact surfacedefinitions extend far enough to cover the full
expected range of motion for the analysis.
What You Shou ld Do in the Begin: Tip
• Sharp corners on the contact/target surfaces:
– Smooth the surfaces.
– Use refined mesh around corner.
– Use unsymmetric solver.
– If the physical problem has a sharp concave fold, use two
separate contact pair definitions
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separate contact pair definitions.
Outward NormalSmoothing Radius
Rigid Target Surface
What You Shou ld Do in the Begin: Tip
• Interference fit problems with widely varying
contact normal directions can result in non-physicalbehavior. In this case try switching the target and
contact surfaces. Or, try to use contact normal
being perpendicular to target surface (KEYOP(4)=2)Target
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Contact
surface
Target
surface This can result innon-physical
behavior
• Summary of contact status for each contact pair
– CONCNTR,print, -1 – No contact pair information
– CONCNTR,print, 0 - Limited information
• At beginning of solution
• At the stage of solution divergence
– CONCNTR, print, 1
Output Fi le Info rmation
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CONCNTR, print, 1
• At the end of each sub-step
– CONCNTR, print, 2• At the end of each iteration
– CONCNTR, print, 3
• For individual contact point
Output: CONTACT INFORMATION PRINTOUT LEVEL 3
More
Info
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*** NOTE *** CP= 0.000 TIME= 00:00:00Rigid-deformable contact pair identified by real constant set 2 and
contact element type 3 has been set up. Please verify constraints on
target nodes which may be automatically fixed by ANSYS.
Contact algorithm: Augmented Lagrange method
Contact detection at: nodal point (normal from contact nodes)
Default contact stiffness factor FKN 1.0000
The resulting contact stiffness 2579.8Default penetration tolerance factor FTOLN 0.10000
The resulting penetration tolerance 0.22786E-01
Max. initial friction coefficient MU 2.1000
Default tangent stiffness factor FKT 1.0000
Default Max. friction stress TAUMAX 0.10000E+21
Average contact surface length 0.34860 Average contact pair depth 0.22786
Default pinball region factor PINB 3.0000
h l i i b ll i 0 68358
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The resulting pinball region 0.68358
Default target edge extension factor TOLS 2.0000
Initial contact closure factor ICONT 0.10000E-07
The resulting initial contact closure 0.22786E-08
*WARNING*: Initial penetration is included.
*** NOTE *** CP= 0.000 TIME= 00:00:00
Max. Initial penetration 0 was detected between contact element 454
and target element 435.
****************************************
*** WARNING *** CP= 0.000 TIME= 00:00:00
Max. Friction coef. 2.1 has defined in the model. Switch the
unsymmetric solver (NROP,UNSYM) instead if convergence difficulty is
encountered.
Too big penetrat ion at the Beginning o f
the Analysis
• Read output to see the contact penetration pair
based.
• Real initial interfence
– Use Ramped option KEYOP(9)=2,4 solving initial
interference over several sub-steps.
– For reasonable small one use KEYOP(9)=1
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For reasonable small one use KEYOP(9) 1
– Use smaller FKN and increase FKN in subsequent load
steps
• Spurious contact:
– Redefine PINB
– Check surface normal & flip the surface normal
Troubleshoot ing
• Very small displacement correction, but the
tolerance on residual force is not satisfied due to:
– Contact length too small
• Warning: Min. contact depth 5.376d-6 is too small which may causeaccuracy problem, you may scale the length unit in the model.
– The Elastic moduli or force/mass quantities too big
W i M t t tiff 1 21d16 i t bi hi h
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• Warning: Max. contact stiffness 1.21d16 is too big which may causeaccuracy problem, you may scale the force unit in the model
– Contact surface offset using CNOF or Bonded always• Introduce rotation energy
– Contact stiffnesses are too large
– Solution: scale units or modify CNVTOL or reduceFKN, FKT, or use PSOLVE,
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*** NOTE *** CP= 0.000 TIME= 00:00:00
6 contact points have abrupt change in contact status.
Load step size is too big: use auto time prediction keyopt(7)>0
Pinball is too small, increase PINB
The contact node is sliding off the target edge: increase TOLS
*** NOTE *** CP= 0.000 TIME= 00:00:00
Contact element 454 has the highest chattering level 7.
Troub leshoo t ing (Pair Based)
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g g
Contact stiffness is too big: reduce FKN, FKT
Load step size is too bigSharp corner or coarse mesh: refine mesh
Stick-sliding: use NROP,unsym?
The contact node is sliding off the target edge: increase TOLS
Divergence of Solut ion
• Too Many Cutbacks in the Last Time Increment
– Restart the analysis
– Reset solution options, number of equilibrium iterations
– Increase the number of allowable increments on the NSUB
option and redo the analysis.
• Failure to determine the contact state change
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Failure to determine the contact state change
occurs
– in first increment: Possible causes include too big thepenetartion
– Occurs after the first increment: Possible causes are
contact chattering
• Failure to achieve equilibrium: force residuals
Fai lure to ach ieve equ i l ibr ium : force
residuals
• Unconstrained rigid body motion indicated by the
presence of very large displacement corrections.
• Overconstraints usually indicated by zero
pivot/negative warning messages.
• Buckling indicated by the load-displacement curve
reaching a load maximum: use arc-length
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reaching a load maximum: use arc-length
algorithm.
• Friction in the model
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•Add chattering measurement CNOS to PLES, PLNS, PRES, PRNS
PLNS,CONT,CNOS
Con tact Chatter ing
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Con tact Chatter ing
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Causes o f Con tact Chatter ing
• Contact stiffnesses are too higher: reduce the initial stiffness
or redefine during load step. – Often occur when the model has long, flexible parts with small
contact pressures.
– FKN and FTOLN need to be set appropriately: FKN usually willbe between 0.01 and 10. Use a value of 1.0 (default) for bulk
deformation problems, and 0.1 for bending dominated problems.Do NOT set FTOLN too small, too tight a penetration tolerancecan result in divergence.
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• Only a few nodes are in contact: refine the underlying mesh
of the contact surface or reduce the contact stiffnesses todistribute the contact over more nodes.
• The size of the region in contact is rapidly changing: usekeyopt(7) to control time increment.
• The target surface is not sufficiently smooth : smooth thetarget surface by refining the underlying mesh.
Causes o f Con tact Chatter ing
• A contact node is sliding off the target surface:
– Ensure that the target and contact surface definitions extend farenough to cover the full expected range of motion for the
analysis.
– When modeling contact, ensure that the target surface edge is
extended far enough for all expected motions of the contactingparts of the model. Use CONCNTR,print,3 option to monitor the
history of the contact node that might slide off the target surface
and find where the target surface needs to be extended more
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and find where the target surface needs to be extended more.
• For extremely dif f icu l t si tuation s , you can allow some points
to violate contact conditions by setting the numb on the
CONCNTR, MAXP option. These parameters can be reset in a
subsequent step.
• If none of the above cases seem to app ly, try using thetransient option
Troubleshoo t ing (Solut ion)
Allow some points to violate contact conditions (too much
penetration) by setting
–CONCNTR, MAXP, numb (default to 0)
And/or change default number of iterations resultingconvergence inspire of penetration:
–CONCNTR, CNIT, numb (default to 4)
e.g.
CONCNTR,MAXP,3
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CONCNTR,CNIT,5
MAX. NUMBER OF VIOLATIONS POINT. FOR OVER PENETRA. 3
NUMBER OF ITERATIONS IN SPITE OF PENETRATION 5
The control parameters can be reset in a subsequent step. WARNING: These controls are intended for experienced analystsand should be used with great care.
Ax isymmetr ic con tact
• Inaccurate contact stresses when using
axisymmetric elements at the symmetry axis.
– For axisymmetric elements the contact area is zero at the
nodes lying on the symmetry axis when:
• CONTA175
• CONTA171,172 with KEYOP(4)=1,2
– To avoid numerical singularity problems caused by a zero
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contact area, ANSYS provides a small area at that nodes.
This may result in inaccurate local contact stresses for the
node located on the symmetry axis.
F
Sometime, the parts in a contact model is not uniquely constrained, It
means some parts can move without producing any elastic deformation-
Rigid Body Motion. This rigid body motion is unfortunately not allowed in
a static analysis. Usually, there are 3 types of rigid body motion
–The parts are not in contact
–The initial penetration is too high, this will leads to a very high contact force
– Friction force is too small comparing the external force
Rigid Body Mot ion
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F
b
F
a
F
F
< F
c
• Add a small friction to avoid the numeric instability
• Use reasonable boundary condition, e.g. use 2 load steps, load step 1
to move the body together and then apply the load.
F
FE Model
U
LS-1
F
LS-2
Rigid Body Mot ion Prevent ion
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• Add a small friction to avoid the numeric instability
FE Model Without friction
• Do a slow dynamic analysis to introduce the inertial force.
• Use specified displacement, if possible, in stead of force.
F
FE Model
F
Static
F U
•You will need to add mass anddamping in order to convert the
solution from a static to a dynamic
solution. This is known as a slow
dynamic solution.
This technique uses imposed
displacements to move the
b d d lt t th
Rigid Body Mot ion Prevent ion
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Real problem FE model
• Use weak spring to avoid the numeric noise.
F The spring stiffnesses should be negligible compared to thestiffness of the system. By connecting the springs to ground,
the reactions at the grounded nodes can be compared to the
total reaction forces to ensure that the springs have no effect
on the solution.The open-spring contact option could also be
used.
body and as result you get the
reaction force
• Use artificial initial penetration to close the contact.
FE model with initial gap FE model with
geometrical penetration
FE model with artificial
penetration
Rigid Body Mot ion Prevent ion
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• Use no-separation Option KEYOPT(12)
geometrical penetration penetration
Never try to solve a static problem with rigid body motion!
When possible, one should always use a very small geometric penetration to
prevent the rigid body motion, at the same time, the geometric penetration
establishes the initial contact region, which hold the structure together todistribute the load.
The initial penetration can be modeled via:
TIP
F
Rigid Body Mot ion Prevent ion
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p
a) Overlap the geometry slightly, be careful in plastic analysis, because even the
small penetration can produce plasticity.
b) Use CNOF to close the geometric gap, it can be very tedious, because you
must know the gap size to adjust the CNOF.
c) Use KEYOPT(5) to close the geometric gap automatically.
d) Use Contact178, instead of surface contact, Contact178 can ignore the all the
gaps
In i t ial Con tact Cond i t ion Adjus tment
• User defined contact surface offset CNOF
• Automatic adjustment using KEYOPT(5)
• Adjustment band ICONT - making in contact
– Always check default value – Turn off for other strategies to be effective
KEYOPT(9) t dj t i iti l t ti
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• KEYOPT(9) to adjust initial penetration
– Include, exclude, ramp
• Initial allowable penetration range (PMIN,PMAX)
– Auto move target along unconstrained DOF
• Combination of above techniques
Default - parallel to globale
11
X
Y
1
2
3
45
Rotated to local cylindrical
11
X
Y
Rotated nodal coord inate system Rotated nodal coo rdinate system
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p g
coordinate systemy
coordinate system
X
X n2
Yn
1
Y
30o
Caution: for large rotation analysis, the nodal coordinate system is not rotated to the
deformed configuration. e.g. If node 1 is rotated into CSYS-1, the original nodal
coordinate direction will not be changed.
Possible position
UX=0
Wanted position
Rotated nodal coord inate system Rotated nodal coo rdinate system
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All nodes keep the original direction
This kind of problem can be nicely solved with contact element with Option:
Sliding only (KEY(12)=4).With Rigid-Target, all the nodes can only move in tangential direction.
Rigid-Target
Allowed movement
Rotated nodal coord inate system Rotated nodal coo rdinate system
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The