finite element method -...
TRANSCRIPT
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Finite Element Method
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Finite Element Method(ENGC 6321)
Syllabus
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Objectives
• Understand the basic theory of the FEM
• Know the behaviour and usage of each type of
elements covered in this course one dimensional
(line elements) and two dimensional (planar
elements)
• Have some hand on experiences in solving
various simple engineering problems by FEM
• Interpret and evaluate the quality of the results
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TopicChapter
Introduction 1, 2
Development of Truss, Beam, Grid and Plane Frame Equations 3-5
Computer programs for truss beam and frame elements (using Matlab)--
Review of the Basic Theory in 2-D Elasticity.Appendix A
Development of Plane Stress and Plane Strain Stiffness Equations6
Practical Considerations in Modeling; 7
Computer Program for Plane Stress/Strain Analysis--
Development of the Linear-Stain Triangle Equations8
Axisymmetric Elements 9
Isoparametric Formulation10
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Grading
• Homework and Projects 25%
• Midterm Exam 25%
• Final Exam 50%
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Text book
• A First Course in the Finite Element Method, Fifth
Edition (2012) by Daryl L. Logan.
Reference
• Any Finite Element Book
• Finite Element Procedures, K. J. Bathe, Prentice Hall
• First Course in Finite Elements, J. Fish and T. Belytschko
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What is Finite Element Method (FEM)
It is a numerical method for solving problems of engineering
and mathematical physics.
It is based on the idea of dividing a complicated object into
small and manageable pieces
Brief history
The modern development of the finite element method began in
the 1940s in the field of structural engineering.
From the early 1940s to the present, enormous advances have
been made in the application of the FEM to solve complicated
engineering problems.
At present FEM/FEA is the most widely applied computer
simulation method in engineering
Introduction
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Introduction
Applications of FEM in Engineering
Mechanical/Aerospace/Automobile Engineering
Structure Analysis (static/dynamic, linear/nonlinear)
Thermal/Fluid flows
Electromagnetics
Geo-Mechanics
Bio-Mechanics
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Advantages of the Finite Element Method
Handle general loading conditions.
Model bodies composed of several different materials (because the
element equations are evaluated individually).
Handle unlimited numbers and kinds of boundary conditions.
Vary the size of the elements to make it possible to use small
elements where necessary
Change the finite element model easily and cheaply
Include dynamic effects
Handle nonlinear behavior existing with large deformations and
nonlinear materials.
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Introduction
Computer Implementations
Pre-Processing (build FE model, loads and constraints)
FEA solver (assemble and solve the system of equations)
Post-Processing (sort and display the results)
Available Commercial FEM Software Packages
ANSYS, , ALGOR, ADINA, ABAQUS, SAPPRO, ROBOT
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Suspension Bridge
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San Francisco Oakland Bay Bridge
Before the 1989 Loma Prieta earthquake
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San Francisco Oakland Bay Bridge
After the earthquake
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San Francisco Oakland Bay Bridge
A finite element model to analyze the bridge under
seismic loads
Courtesy: ADINA R&D
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Building
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Curved Beam
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Picnic Table
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Drag Force Analysis of Aircraft
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Crush Analysis of Ford Windstar
• Question
– What is the load-deformation relation?
• Solve
– Partial Differential Equations of Continuum Mechanics
• Recent Developments
– Meshless Methods, Iterative methods, Automatic Error Control
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Radiation Therapy of Lung Cancer
http://www.simulia.com/academics/research_lung.html
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Computer Simulation of 9/11 Attack
Structural engineers need to know from a scientific
perspective what happened to the buildings during
the terrorist attacks in order to prevent future
failures.
The search for answers continues with the help of a
state-of-the-art animated visualization created by
researchers at Purdue University.
http://www.youtube.com/watch?v=gH02Eh44yUg
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Elements and Nodes Finite
elements resemble fragments of
the structures. Nodes appear on
element boundaries and serve as
connectors that fasten elements
together. All Elements that share
a node have the same
displacement components at that
node.
Elements and Nodes.
for elastic continuum, such as a deep beam or a plate /shell structure, such a
natural subdivision does not exist and we have to artificially divide the
continuum into a number of elements.
for frame and truss structures, elements
and nodes are more or less natural.
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Elements and Nodes.
The finite elements, are usually either triangular or rectangular in
shape as shown below:
It appears that a FE structure can be produced by sawing the
actual structure apart and then pinning it back together at nodes.
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Engineering design
Physical Problem
Mathematical model
Governed by differential
equations
Assumptions regarding
Geometry
Kinematics
Material law
Loading
Boundary conditions
Etc.
General scenario..
Question regarding the problem
...how large are the deformations?
...how much is the heat transfer?
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Engineering design
Example: A bracket
Physical problem
We consider here a simple example of a bracket supporting a
vertical load. We need to choose a mathematical model. The
choice of this model clearly depends on what phenomena are to
be predicted and on the geometry, material properties, loading
and support conditions of the bracket.
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Engineering design
Example: A bracket
Physical problem
Questions:
1. What is the bending moment at section AA?
2. What is the deflection at the pin?
Finite Element Procedures, K J Bathe
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Engineering designExample: A bracket
Physical problem
We notice that
The bracket has been fastened to a “very thick steel column”.
The term “very thick” is relative to the thickness “t” and
height “h” of the bracket. We translate this statement into the
assumption that that the bracket is fastened to a (practically)
rigid column.
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Engineering designExample: A bracket
Physical problem
We notice that We also assume that the load is applied very slowly. The
condition of time “very slowly” is relative to the largest
natural period of the bracket: i.e., the time span over which the
load W is increased from 0 to its full value is much longer
than the fundamental period of the bracket. We translate this
statement into meaning that we require static analysis (as
opposed to a dynamic analysis).
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Engineering design
Example: A bracket
Mathematical model 1:
beam
How reliable is this model?
How effective is this model?
3
Moment at section AA
Deflection at
27,500
( ) ( )1
53
6
0.
l
0
oad
53
N Nat load W
M WL N cm
W L r W L r
EIAG
cm
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Engineering design
Example: A bracket
Mathematical model 2:
plane stress
Difficult to solve by hand!
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Engineering design
Physical Problem
Mathematical model
Governed by differential
equations
..General scenario..
Numerical model
e.g., finite element
model
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Engineering design
..General scenario..
Finite element analysis
Finite element modelSolid model
PREPROCESSING
1. Create a geometric model
2. Develop the finite element model
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Engineering design
..General scenario..
Finite element analysis
FEM analysis scheme
Step 1: Divide the problem domain into non
overlapping regions (“elements”) connected to
each other through special points (“nodes”)
Finite element model
Element
Node
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Engineering design
..General scenario..
Finite element analysis
FEM analysis scheme
Step 2: Describe the behavior of each element
Step 3: Describe the behavior of the entire body by
putting together the behavior of each of the
elements (this is a process known as “assembly”)
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Engineering design
..General scenario..
Finite element analysis
FEM analysis scheme
Step 4: Solve the system of equations involving
unknown quantities at the nodes (e.g., displacements)
Step 5: Calculate desired quantities (e.g., strains and
stresses) at selected elements
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Engineering design
..General scenario..
Finite element analysis
POSTPROCESSING
Compute moment at section AA
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Engineering design
..General scenario..
Finite element analysis
Preprocessing
Analysis
Postprocessing
Step 1
Step 2
Step 3
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Engineering design
Example: A bracket
Mathematical model 2:
plane stress
FEM solution to mathematical model 2 (plane stress)
Moment at section AA
c m064.0
c mN500,27M
Wloadat
Deflection at load
Conclusion: With respect to the questions we posed, the
beam model is reliable if the required bending moment is to
be predicted within 1% and the deflection is to be predicted
within 20%. The beam model is also highly effective since it
can be solved easily (by hand).
What if we asked: what is the maximum stress in the bracket?
would the beam model be of any use?
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Engineering design
Example: A bracket
Summary
1. The selection of the mathematical model
depends on the response to be predicted.
2. The most effective mathematical model is the
one that delivers the answers to the questions
in reliable manner with least effort.
3. The numerical solution is only as accurate
as the mathematical model.
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Example: A bracketModeling a physical problem
...General scenario
Physical Problem
Mathematical
Model
Numerical model
Does answer
make sense?Refine analysis
Happy
YES!
No!
Improve
mathematical
model
Design improvements
Structural optimization
Change
physical
problem
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Example: A bracketModeling a physical problem
Verification and validation
Physical Problem
Mathematical
Model
Numerical model
Verification
Validation
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General steps of the FEM in Structural Analysis
Step 1: Discretize and Select Element Types
This step involves dividing the body into an equivalent system
of finite elements with associated nodes (lines or surfaces)
and choosing the most appropriate element type. Small
elements (and possibly higher-order elements) are desirable
where results are changing rapidly, such as changes of
geometry.
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Introduction to the Stiffness Method
Types of Finite Elements
1-D (Line) Element
2-D (Plane) Element
3-D (Solid) Element
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Step 2: Select a Displacement Function
Here we chose a displacement function within each element. This
function is defined using the nodal values of the element. Linear,
quadratic, and cubic polynomials are frequently used functions
because they are simple to work with in finite element
formulation. However, trigonometric series could also be used.
General steps of the FEM in Structural Analysis
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Step 3: Define the Strain / Displacement and
Stress / Strain Relationships
These relationships are necessary for deriving the equations for
each finite element.
For example, in the case of 1-D
General steps of the FEM in Structural Analysis
strains smallfor ,dx
dux
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Step 3: Define the Strain / Displacement and
Stress / Strain Relationships
In addition, the stresses must be related to the strains through the
stress/strain low (constitutive law).
For example, for I-D stress analysis, Hooke’s law has the form
where stress in the x direction and E = modulus of elasticity.
General steps of the FEM in Structural Analysis
xx E
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Step 4: Derive the Element Stiffness Matrix and
Equations
a) Direct Equilibrium Method:
b) Work or Energy Methods:
c) Methods of Weighted Residuals
General steps of the FEM in Structural Analysis
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n
n
n
n d
d
d
d
kkkk
kkkk
kkkk
kkkk
f
f
f
f
3
2
1
321
3333231
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1131211
3
2
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Step 5: Assemble the Element Equations to Obtain
the System (or Total) Equations
General steps of the FEM in Structural Analysis
F K d
Where,
Vector of global nodal forces including applied known loads,
Structureglobal or total stiffness matrix, and
Vectorof known and unknown structure nodal degrees of fre
:
edom: .
:
F
K
d
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Step 5: Introduce Boundary Conditions & Solve
for the Unknown Displacements (DOF)
Step 6: Solve for the Element Strains and Stresses
Step 7: Interpret the Results
General steps of the FEM in Structural Analysis