MCEN 4173/5173
Chapter 2
Building An FEA Model
Fall, 2006
FEA Software Packages
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ALGOR
ANSYS
COSMOS/M
STARDYNE
NASTRAN
SAP90
ADINA
ABAQUS
MARC
DYTRAN
…….
FEA Software Packages
Licenses: floating license, first-come-first-serve
For ANSYS in this class:
Education version
Limited function. Total number of nodes < 32000
Two license server
flexlm.colorado.edu
For labs. For computers managed by ITS. 30 seats.
magellan.colorado.edu
For research. For individual computers. 25 seats.
Ingredients in an FEA model
P
P
deformed
undeformed
X2
X2
X1
X1
Basic information that needs to be provided:
1.
2.
3.
4.
Geometry.
Material properties: Young’s modulus, Poisson’s ratio, bending stiffness…
Boundary conditions, prescribed forces or displacements.
Type of analysis: static, transient, modal, buckling, ….
Ingredients in an FEA model
Additional ingredients:
Elements or mesh:
ƒ
ƒ
Strategy to create a good mesh
Element types
Time functions:
• A time function defines how the prescribed boundary conditions
change with time, such as ramp, or sinusoidal.
• In FEA, “Time” means a physical time that you count by a clock
(such as in dynamics), or it means that one thing happens before
another thing (such as in statics).
• In some FEA packages, the default time function for static
analysis is a ramp function.
Ingredients in an FEA model
Six basic ingredients of an FEA software package
1.
2.
3.
4.
5.
6.
Type of analysis
Geometry (defined through nodes)
Elements
Material properties
Boundary conditions
Time functions
As long as these six components are defined, an analysis can be conducted.
It usually does not matter in what sequences these components are given.
Physical model vs FEA model
Developing a physical model is a process that simplifies a real-world
problem into a problem that is suitable by FEA.
Considerations in building a physical model
1.
2.
The nature of real-world problem
The cost of conducting FEA
In many cases, the cost of conducting FEA is a major barrier,
which imposes a great challenge. Sometimes, we have to
sacrifice accuracy in order to reduce the cost.
Physical model vs FEA model
Example:
3
1
2
If the loads applied are uniform along 1-direction, then
3
2
2D
Physical model vs FEA model
If the two layers of this laminate are glued together, do we
need to consider the small layer of glue?
If the two layers are glued together perfectly, and will not separate
under working conditions,
3
Ignore the interface
2
2D
If the strength of interface is a major concern in your design,
3
Consider the interface
2
2D
Physical model vs FEA model
If you want to consider the initiation and propagation of damages of interface
(delamination)
3
Consider the interface
2
2D
3
2
2D
Physical model vs FEA model
After a physical model is decided, the next step is to build an FEA model
Considerations in building an FEA model
1.
2.
The accuracy of the FEA model
The cost of conducting FEA
FEA model
Factors that affect the cost:
1. Degree of Freedoms (DOF): p
2. Number of Nodes : N
3. How the nodes are numbered
4. Number of Integration Points in Each Elements
5. Nonlinear Analysis
FEA model
Factors that affect the cost:
In general,
The type of analysis (static, dynamic,…, linear or nonlinear …) is
decided at physical model level.
The number of nodes and number of integration points are decided at
FEA model level. But a good physical model can reduce the number of
nodes by one or two order of magnitude.
Rule of Thumb
FEA model
Example: Abrasive Wear in Seals-Background
20
18
1st cycle
2nd cycle
4th cycle
-Ture Stress (MPa)
16
Seals in a joint
14
12
10
8
6
4
2
0
0
0.25
0.5
0.75
1
-True Strain
Stress-strain behavior of the
material for the seal
Abrasive particle ~10-60μm
FEA model
Example: Abrasive Wear in Seals-Physical Model
Scratching
simplification
B
A
Elastomers
Abrasive particle
100μm~1mm
Cross-section of the 3D case
FEA model
Example: Abrasive Wear in Seals-Physical Model
6mm
B
C
3mm
Scratching
3mm
A
D
y
Knife
x
2D
AB~6mm
Tip Radius ~6μm
FEA model
Example: Abrasive Wear in Seals-FEA Model
1mm
100μm
FEA model only has ~8000 nodes
Cost: on a Compaq (HP) alpha workstation, one week for one simulation