© 2011 ANSYS, Inc. September 8, 2011 1
ANSYS Mechanical Tips & Tricks
Harish Radhakrishnan
© 2011 ANSYS, Inc. September 8, 2011 2
Material Library
Elastic
Linear /
Multilinear
Hyperelastic
Viscoelastic
Mullins effect
Bergstrom-Boyce model
Plasticity
Isotropic / Kinematic
hardening
Drucker Prager
Cap Model
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Plasticity
Hardening Necking
Yield
UTS Failure
2
6222222
zxyzxyxxzzzzyyyyxx
eq
Equivalent stress calculates
the distortional energy in the
material
Splits the stress to two components –
hydrostatic stress + deviatoric stress
Causes plastic
yielding
No role!!!
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Hardening
Kinematic hardening
Isotropic hardening
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Plasticity….Multilinear hardening
Hardening Necking
Yield
UTS Failure
Required for
modeling
Available from
experiments
current
true
A
F
0A
FS
eS 1true
e 1ln
NOTE: Using Engineering stress
instead of true stress will cause
instabilities!!
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Modeling Contact in ANSYS
Penetration occurs when contact
compatibility is not enforced. Contact definition requires three
components
1. Normal behavior &Tangential
(sliding) behavior
2. Contact detection behavior
3. Formulation
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Penalty Method npenetratiocontactnormal
xKF
Integration Point Detection
• As , penetration goes
to zero
• Large values of lead to
contact chattering – change the
Normal stiffness factor
• For highly Nonlinear materials
use update stiffness
• Consider Asymmetric contact
for dissimilar materials - ratio of
Young’s modulus is large
(>100)
contact
K
contactK
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• In the Lagrange Method, we seek to solve for the constraint – penetration is zero.
• Lagrange multipliers are used to enforce the constraint – result in extra degrees of freedom (DOF)
F
• Zero penetration
• Can be computationally
expensive
• Augmented Lagrange Method uses the best of both worlds
• Less sensitive to the value of contact stiffness
• Multipoint Constraints (MPC) is for bonded contacts – ties the nodes between the two surfaces
pressurepentrationcontactnormal
xKF
Other Contact formulations
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• A plateau in the force convergence plot could be attributed to large contact stiffness
• MPC method is preferred over the Penalty method for bonded contacts
• Plot the Newton-Raphson residuals to identify problem contact regions
• For contact between dissimilar materials, consider using
– Asymmetric contact
– Modeling the stiff body as rigid
General Tips on Modeling Contact
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Contact Tool
• Keep track of contacts with large penetration -
Consider refining mesh
• If penetration is intended consider using Add
Offset with / without ramped effects
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209 parts with 450 symmetric contact pairs
For Large models…
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Element Library in ANSYS
Continuum
Elements
2d elements
3d elements
Solid Shells Structural
Elements
Beam, Link
Shell
Pipe, elbow • Choices
– Elements with / without mid-side nodes
– Reduced / Full integration
• When do you use SOLID SHELL / SHELL elements?
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Lagrange and Serendipity elements
Lagrange Elements
• Computationally cheap
• Mesh convergence is slow
• Avoid using CST elements – overly stiff
cybxau dxycybxau
Serendipity elements
• Computationally more expensive
• Mesh convergence is faster
• Ideal for curved bodies
22 fyexdxy
cybxau
2222 hxyygxfyex
dxycybxau
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Full vs Reduced Integration
vol
T BdVEBK
Element Stiffness matrix
Integration is carried out numerically using Gauss-Legendre quadrature
• Value of integral is calculated at specific Gauss points and summed
• Number of Gauss points depend on order of equation
1
2
3 4
1 2
3 4
Mapped
4 points – full
integration
1 point – Reduced
integration
Element Full Reduced
4 noded rectangle 2 x 2 1
8 noded rectangle 3 x 3 2 x 2
8 noded brick 2 x 2 x 2 1
20 noded brick 14 2 x 2 x 2
• Accuracy in reduced integration is seldom compromised
• Mesh convergence is rapid
• Susceptible to hour-glassing
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• In FEM we seek to satisfy
– Material Constitutive law
– Compatibility
– Equilibrium
Mesh Convergence
Satisfied at all points
Identically satisfied at nodes
Approximate at the element edges
Satisfaction of equilibrium conditions can be
verified by the ability to capture stress
gradients – the variation of the stress in
adjacent elements should be small – time
consuming!!!
110000
112000
114000
116000
118000
120000
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Max
Eq
uiv
ale
nt
Stre
ss (
psi
)
Element Size (mm)
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Use Post Processing Tools…
Averaged
Von Mises
Stress
Nodal
difference
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• Resist the temptation to use solid element for geometries with large aspect ratios undergoing bending
• Solid elements perform poorly when bending is present
• Serendipity elements give good results but a good mesh requires a very fine mesh
• Use SOLID SHELL or SHELL elements
• Solid Shells can use the existing CAD geometry (no mid surfaces)
• Can connect with other SOLID elements with no special treatment
Solid Elements Limitations
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• In static problems
• Nonlinearities arise from
– Materials
– Contact
– Geometric
• What is Large Deflection, and when do you need it?
Nonlinearities
0internalexternal
FF externalF
internal
1F
internal
2F
1u 2u 3u
displacement
Force
y
x
F displacements
u, v
du/dx du/dy
dv/dx dv/dy
Deformation gradients
Finite strain – no longer
small
• Use large deflection when in
doubt
• Nonlinear materials
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• Newton’s Method requires a stable system – Positive definite Stiffness matrix
• For unstable problems use Riks method (or) Stabilization
Instabilites
externalF
internal
1F
internal
2F
1u 2u 3u
displacement
Force
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Numerical Damping
Stable
Damped energy
0dampinternalexternal FFF
dt
dxF
dampdamp
Force
displacement
• Damping force is numerical – not physical
• Damping is intended for system instabilities – not for rigid body motions
Unstable
Static solution
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• Perform initial runs without damping. Rule out other causes for non-convergence
– coarse meshes
– contact chattering
– material instability
Some guidelines
• Stabilization applies damping to the entire model – Contact instabilities require damping applied to a specific region – Possible in R14
• Damping cannot resolve rigid body motion issues. Stiffness matrix is ill-conditioned and cannot be inverted
• Avoid using the constant option in multi-step analysis – causes divergence if stabilization is removed in subsequent steps
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• Critically analyze results when stabilization is used
• Compare the ratio of stabilization energy to the strain energy. There is no magic number – The smallest value of damping coefficient to obtain a converged solution is desired.
• Use the Energy dissipation ratio or the stabilization force limit to keep track of the magnitude of stabilization needed.
Solution Check
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Enter and view your APDL code here
IMPORTANT: Understand where your
APDL snippet is inserted!!!
Command Snippets
To utilize the full breadth and scope of ANSYS use APDL command Snippets
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/PREP7
Assign materials,
sections, element type
Contact algorithm,
details
Solution parameters
/SOLU
/POST Post processing
Inserted Command Snippet Location
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•Can associate an external text file,
macro with snippet *.txt, *.mac.
• Import places the text from external
file – click refresh
•Export creates a file from the text you
enter
•File name and status are shown in
detail
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• Snippets work on FE model – KP, Lines, Areas, Volumes don’t exist in Mechanical
• You can move between modules – make sure you know where you are!!!
• Not interactive – must batch solve before status of APDL snippet is realized
IMPORTANT
• Plotting and listing is to files only
• If you create items (nodes,
elements etc), you cannot post
process in WB
• Arguments can be WB parameters
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User Defined Results
Can be any valid
mathematical
expression
Highlight solution
in tree outline
and use to
worksheet to
view the
variables
• Can be calculated on the
fly… (unlike APDL snippets)
• Can access element tables
• Can export easily to excel,
txt files..]
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THANK YOU