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FINITE ELEMENT METHOD
INTRODUCTION
What is finite element analysis, FEM?
A Brief history of FEM
What is FEM used for?
1D Rod Elements, 2D Trusses
FINITE ELEMENT METHOD – WHAT IS IT?
The Finite Element Method (FEM) is a numerical method of solving systems of partial differential equations (PDEs)
It reduces a PDE system to a system of algebraic equations that can be solved using traditional linear algebra techniques.
In simple terms, FEM is a method for dividing up a very complicated problem into small elements that can be solved in relation to each other.
MGWS
Overview of the Finite Element Method
Strong
form
Weak
form
Galerkin
approx.
Matrix
form
1. Lord John William Strutt Rayleigh (late 1800s), developed a
method for predicting the first natural frequency of simple
structures. It assumed a deformed shape for a structure and
then quantified this shape by minimizing the distributed energy
in the structure.
2. Ritz then exp Walter ended this into a method, now known as
the Rayleigh-Ritz method, for predicting the stress and
displacement behavior of structures.
6
3. Dr. Ray Clough coined the term “finite element” in 1960. The 1960s saw
the true beginning of commercial FEA as digital computers replaced analog
ones with the capability of thousands of operations per second.
4. In the 1950s, a team form Boeing demonstrated that complex surfaces
could be analyzed with a matrix of triangular shapes.
5. In 1943, Richard Courant proposed breaking a continuous system into
triangular segments. (The unveiling of ENIAC at the University of
Pennsylvania.)6. In the early 1960s, the MacNeal-Schwendle Corporation (MSC) develop a
general purpose FEA code. This original code had a limit of 68,000
degrees of freedom. When the NASA contract was complete, MSC
continued development of its own version called MSC/NASTRAN, while the
original NASTRAN become available to the public and formed the basis of
dozens of the FEA packages available today. Around the time
MSC/NASTRAN was released, ANSYS, MARC, and SAP were introduced.
7
8. standards such as IGES and DXF. Permitted limited geometry transfer
between the systems.
9. In the 1980s,CAD progressed from a 2D drafting tool to a 3D surfacing tool,
and then to a 3D sIn the 1980s, the use of FEA and CAD on the same
workstation with developing geometry olid modeling system. Design
engineers began to seriously consider incorporating FEA into the general
product design process.
10. As the 1990s draw to a place, the PC platform has become a major force in
high end analysis. The technology has become to accessible that it is
actually being “hidden” inside CAD packages.
7. By the 1970s, Computer-aided design, or CAD, was introduced later in the decade.
BASIC CONCEPTS
Loads
Equilibrium
Boundary conditions
fT
iP
0~
, ijji f
DEVELOPMENT OF THEORY
Rayleigh-Ritz Method Total potential energy equation
Galerkin’s Method
1D ROD ELEMENTS
To understand and solve 2D and 3D problems we must understand basic of 1D problems.
Analysis of 1D rod elements can be done using Rayleigh-Ritz and Galerkin’s method.
To solve FEA problems same are modified in the
Potential-Energy approach and Galerkin’s approach
1D ROD ELEMENTS
Loading consists of three types : body force f , traction force T, point load Pi
Body force: distributed force , acting on every elemental volume of body i.e. self weight of body.
Traction force: distributed force , acting on surface of body i.e. frictional resistance, viscous drag and surface shear
Point load: a force acting on any single point of element
1D ROD ELEMENTS
Element strain energy
Element stiffness matrix
Load vectors Element body load vector Element traction-force vector
qkqU eTe
][
2
1
11
11][
e
eee
l
AEk
1
1
2
flAf eee
1
1
2ee Tl
T
Element -1 Element-2
2D TRUSS
2 DOF
Transformations
Modified Stiffness Matrix
Methods of Solving
2D TRUSS
Transformation Matrix Direction Cosines
ml
mlL
00
00][
212
212 yyxxle
el
xxl 12cos
el
yym 12sin
2D TRUSS
Element Stiffness Matrix
22
22
22
22
][
mlmmlm
lmllml
mlmmlm
lmllml
l
AEk
e
eee
METHODS OF SOLVING
Elimination Approach Eliminate Constraints
Penalty Approach
ELIMINATION METHOD
Set defection at the constraint to equal zero
ELIMINATION METHOD
Modified Equation DOF’s 1,2,4,7,8 equal to zero
2D TRUSS
Element Stresses
Element Reaction Forces
qmlmll
E
e
e
QKR
2D TRUSS
Development of Tables
Coordinate Table Connectivity Table Direction Cosines Table
2D TRUSS
Coordinate Table E.g;
2D TRUSS
Connectivity Table E.g;
2D TRUSS
212
212 yyxxle
el
xxl 12cos
el
yym 12sin
3D TRUSS STIFFNESS MATRIX
3D Transformation Matrix Direction Cosines
nml
nmlL
000
000][
212
212
212 zzyyxxle
el
xxl 12cos
el
yym 12cos
el
zzn 12cos
3D TRUSS STIFFNESS MATRIX
3D Stiffness Matrix
22
22
22
22
22
22
][
nmnlnnmnln
mnmlmmnmlm
lnlmllnlml
nmnlnnmnln
mnmlmmnmlm
lnlmllnlml
l
AEk
e
eee
EXAMPLE 1D ROD ELEMENTSExample 1Problem statement: (Problem 3.1 from Chandrupatla and Belegunda’s book)Consider the bar in Fig.1, determine the following by hand calculation: 1) Displacement at point P 2) Strain and stress
3) Element stiffness matrix 4) strain energy in element
21.2eA in
630 10E psi 1 0.02q in
2 0.025q in
Given:
Solution:
1) Displacement (q) at point P
We have
12 1
2( ) 1
( )
2(20 15) 1 0.25
(23 15)
x xx x
Now linear shape functions N1( ) and N2( ) are given by
1
1( ) 0.375
2N
And 2
1( ) 0.625
2N
EXAMPLE 2D TRUSS
CONCLUSION
Good at Hand Calculations, Powerful when applied to computers
Only limitations are the computer limitations
MATLAB PROGRAM TRUSS2D.M
REFERENCES