ANSYS Mechanical Tips & Tricks · PDF file2 © 2011 ANSYS, Inc. September 8, 2011 Material...

© 2011 ANSYS, Inc. September 8, 2011 1

ANSYS Mechanical Tips & Tricks

Harish Radhakrishnan


Material Library

Elastic

Linear /

Multilinear

Hyperelastic

Viscoelastic

Mullins effect

Bergstrom-Boyce model

Plasticity

Isotropic / Kinematic

hardening

Drucker Prager

Cap Model


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!!!


Hardening

Kinematic hardening

Isotropic hardening


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!!


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


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


• 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


• 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


Contact Tool

• Keep track of contacts with large penetration -

Consider refining mesh

• If penetration is intended consider using Add

Offset with / without ramped effects


209 parts with 450 symmetric contact pairs

For Large models…


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?


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


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


• 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)


Use Post Processing Tools…

Averaged

Von Mises

Stress

Nodal

difference


• 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


• 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


• 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


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


• 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


• 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


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


/PREP7

Assign materials,

sections, element type

Contact algorithm,

details

Solution parameters

/SOLU

/POST Post processing

Inserted Command Snippet Location


•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


• 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


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..]


THANK YOU

ANSYS Mechanical Tips & Tricks · PDF file2 © 2011 ANSYS, Inc. September 8, 2011 Material...

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Transcript of ANSYS Mechanical Tips & Tricks · PDF file2 © 2011 ANSYS, Inc. September 8, 2011 Material...