ANSYS Mechanical ANSYS Mechanical Structural - Inside Mines

74 Pages·2012·1.99 MB·English

Customer Training Material
Lecture 3
Introduction to Contact
AANNSSYYSS MMeecchhaanniiccaall
Structural Nonlinearities
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ANSYS Mechanical – Introduction to Contact
Chapter Overview
Customer Training Material
• An introduction to solid body contact will be presented in
this chapter:
– It is assumed that the user has alreadyy covered Chappter 2
Nonlinear Structural prior to this chapter.
• The Specific topics introduced are:
A. Basic conceppt of contact
B. Contact Formulations
C. Stiffness and Penetration
D. Workshopp 3A
E. Pinball Region
F. Symmetric vs. Asymmetric
G. Postpprocessingg Contact Results
H. Workshop 3B
• The capabilities described in this Chapter are generally
apppplicable to ANSYS Structural licenses and above.
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ANSYS Mechanical – Introduction to Contact
A. Basic Concepts
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Contact:
• When two separate surfaces touch each other such that they become
mutuallyy tanggent, theyy are said to be in contact.
• In the common physical sense, surfaces that are in contact have
these characteristics:
– TThheeyy ddoo nnoott iinntteerrppeenneettrraattee..
– They can transmit compressive normal forces and tangential friction
forces.
– Theyy often do not transmit tensile normal forces.
• They are therefore free to separate and move away from each other.
• Contact is a changing-status nonlinearity. That is, the stiffness of the
syystem deppends on the contact status,, whether pparts are touchingg or
separated.
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ANSYS Mechanical – Introduction to Contact
... Basic Concepts
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How compatibility is enforced in a contact region:
• Physical contacting bodies do not interpenetrate. Therefore, the
pproggram must establish a relationshipp between the two surfaces to
prevent them from passing through each other in the analysis.
– When the program prevents interpenetration, we say that it enforces
contact compatibility.
– Workbench Mechanical offers several different contact formulations to
enforce compatibility at the contact interface.
F
Penetration occurs when contact
compatibility is not enforced.
F
Target
Contact
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ANSYS Mechanical – Introduction to Contact
B. Contact Formulations
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• For nonlinear solid body contact of faces, Pure Penalty or
Augmented Lagrange formulations can be used:
– Both of these are penalty-based contact formulations:
F = k x
normal normal penetration
– HHeerree, ffoorr aa ffiinniittee ccoonnttaacctt ffoorrccee FF , tthheerree iiss aa ccoonncceepptt ooff ccoonnttaacctt
normal
stiffness k . The higher the contact stiffness, the lower the
normal
penetration x , as shown in the figure below
penetration
– Ideally, for an infinite k , one would get zero penetration. This is not
normal
numerically possible with penalty-based methods, but as long as
x is small or negligible, the solution results will be accurate.
penetration
FF
n
x
p
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ANSYS Mechanical – Introduction to Contact
... Contact Formulations
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• The main difference between Pure Penalty and Augmented Lagrange
methods is that the latter augments the contact force (pressure)
calculations:
F = k x
Pure Penalty:
normal normal penetration
FF == kk xx ++λλ
AAuuggmmeenntteedd LLaaggrraannggee::
normal normal penetration
• Because of the extra term λ, the augmented Lagrange method is less
sensitive to the magnitude of the contact stiffness k .
nnoorrmmaall
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ANSYS Mechanical – Introduction to Contact
... Contact Formulations
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• Another available option is Lagrange multiplier Formulation:
– The Normal Lagrange Formulation adds an extra degree of freedom
(contact pressure) to satisfy contact compatibility. Consequently,
iinstteadd off resollviing conttactt fforce as conttactt sttiiffffness andd penettrattiion,
contact force (contact
F = DOF
pressure) is solved for explicitly as an extra DOF.
normal
• EEnnffoorrcceess zzeerroo//nneeaarrllyy-zzeerroo ppeenneettrraattiioonn wwiitthh pprreessssuurree DDOOFF
• Does not require a normal contact stiffness (zero elastic slip)
• Requires Direct Solver, which can be more computationally expensive
F
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ANSYS Mechanical – Introduction to Contact
... Contact Formulations
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• Chattering is an issue which often occurs with Normal Lagrange
method
– If no penetration is allowed (left), then the contact status is either open or
closed (a step function). This can sometimes make convergence more
difficult because contact points may oscillate between open/closed
status. This is called chattering
– IIff ssoommee sslliigghhtt ppeenneettrraattiioonn iiss aalllloowweedd ((rriigghhtt)), iitt ccaann mmaakkee iitt eeaassiieerr ttoo
converge since contact is no longer a step change.
Contact Status Contact Status
Open Open
PPeenneettrraattiioonn GGaapp PPeenneettrraattiioonn GGaapp
CClloosseedd CClloosseedd PPeenneettrraattiioonn
Normal Lagrange Method Penalty-Based Method
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ANSYS Mechanical – Introduction to Contact
... Contact Formulations
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• For the specific case of “Bonded” and “No Separation” type of
contact between two faces, a multi-point constraint (MPC)
formulation is available.
– MPC internally adds constraint equations to “tie” the displacements
between contacting surfaces
– This approach is not penalty-based or Lagrange multiplier-based. It is a
ddiirreecctt, eeffffiicciieenntt wwaayy ooff rreellaattiinngg ssuurrffaacceess ooff ccoonnttaacctt rreeggiioonnss wwhhiicchh aarree
bonded.
– Large-deformation effects also are supported with MPC-based bonded
contact
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ANSYS Mechanical – Introduction to Contact
… MPC Formulation
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– To illustrate, consider the connection between a shell edge and a
solid face. The constraint equation that would transfer action
between ROTZ at node 2 and UY at nodes 1 and 3 has this form:
0 = UY - UY - 10*ROTZ
3 1 2
SHELL Element
SOLID element
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