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2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Introduction to the Finite
Element Method
2nd order DE’s in 1-D
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Objectives
• Understand the basic steps of the finite
element analysis
• Apply the finite element method to second
order differential equations in 1-D
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
The Mathematical Model
• Solve:
• Subject to:
Lx
fcu
dx
du
a
dx
d








0
0
  00 ,0 Q
dx
du
auu
Lx








2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Step #1: Discretization
• At this step, we divide
the domain into
elements.
• The elements are
connected at nodes.
• All properties of the
domain are defined at
those nodes.
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Step #2: Element Equations
• Let’s concentrate our
attention to a single
element.
• The same DE applies
on the element level,
hence, we may follow
the procedure for
weighted residual
methods on the
element level!
21
0
xxx
fcu
dx
du
a
dx
d








   
21
2211
21
,
,,
Q
dx
du
aQ
dx
du
a
uxuuxu
xxxx














2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Polynomial Approximation
• Now, we may propose an approximate
solution for the primary variable, u(x),
within that element.
• The simplest proposition would be a
polynomial!
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Polynomial Approximation
• Interpolating the values
of displacement
knowing the nodal
displacements, we may
write:
  01 bxbxu 
  01111 bxbuxu 
  2
12
1
1
12
2
u
xx
xx
u
xx
xx
xu 
















  02122 bxbuxu 
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Polynomial Approximation
 
     e
ux
u
u
uu
u
xx
xx
u
xx
xx
xu
 
























2
1
212211
2
12
1
1
12
2
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Step #2: Element Equations
(cont’d)
• Assuming constant
domain properties:
• Applying the
Galerkin method:
21
2
2
0
xxx
fcu
dx
ud
a


          02
2






Domain
jiiji
i
j dxfxuxxcu
dx
xd
xa 


2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Step #2: Element Equations
(cont’d)
• Note that:
• And:
   
ee hdx
xd
hdx
xd 1
,
1 21


   
       










Domain
ij
x
x
i
j
Domain
i
j
dx
dx
xd
dx
xd
a
dx
xd
xa
dx
dx
xd
xa




2
1
2
2
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Step #2: Element Equations
(cont’d)
• For i=j=1: (and ignoring boundary terms)
• Which gives:
0
12
1
2
1
2
2
2














 














 

x
x eee
dx
h
xx
fu
h
xx
c
h
a
0
23
1 





 ee
e
fh
u
ch
h
a
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Step #2: Element Equations
(cont’d)
• Repeating for all terms:
• The above equation is called the element
equation.



































1
1
221
12
611
11
2
1 ee
e
fh
u
uch
h
a
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
What happens for adjacent
elements?
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Objectives
• Learn how the finite element model for the
whole domain is assembled
• Learn how to apply boundary conditions
• Solving the system of linear equations
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Recall
• In the previous lecture, we obtained the
element equation that relates the element
degrees of freedom to the externally
applied fields
• Which maybe written:



































1
1
221
12
611
11
2
1 ee
e
fh
u
uch
h
a



















2
1
2
1
43
21
f
f
u
u
kk
kk
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Two–Element example



















1
2
1
1
1
2
1
1
1
4
1
3
1
2
1
1
f
f
u
u
kk
kk



















2
2
2
1
2
2
2
1
2
4
2
3
2
2
2
1
f
f
u
u
kk
kk











































3
2
1
3
2
1
3
2
1
2
4
2
3
2
2
2
1
1
4
1
3
1
2
1
1
0
0
Q
Q
Q
f
f
f
u
u
u
kk
kkkk
kk
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Illustration: Bar application
1. Discretization: Divide the bar into N number of
elements. The length of each element will be
(L/N)
2. Derive the element equation from the differential
equation for constant properties an externally
applied force:
  02
2



xF
x
u
EA
0
2
1
2


















x
x
ij
ij
e
dxfu
dx
d
dx
d
h
EA


2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Performing Integration:





















1
1
211
11
2
1 e
e
e
e
fh
u
u
h
EA
Note that if the integration is evaluated from 0 to he,
where he is the element length, the same results
will be obtained.
0
2
1
2


















x
x
ij
ij
e
dxfu
dx
d
dx
d
h
EA


2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Two–Element bar example





















1
2
1
1
1
2
1
1
11
11
f
f
u
u
h
EA
e





















2
2
2
1
2
2
2
1
11
11
f
f
u
u
h
EA
e













































0
0
1
2
1
2
110
121
011
3
2
1 R
fh
u
u
u
h
EA e
e
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Applying Boundary Conditions
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Applying BC’s
• For the bar with fixed left side and free
right side, we may force the value of the
left-displacement to be equal to zero:













































0
0
1
2
1
2
0
110
121
011
3
2
R
fh
u
u
h
EA e
e
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Solving
• Removing the first row and column of the
system of equations:
• Solving:





















1
2
211
12
3
2 e
e
fh
u
u
h
EA













4
3
2
2
3
2
EA
fh
u
u e
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Secondary Variables
• Using the values of the displacements
obtained, we may get the value of the
reaction force:



















































0
0
1
2
1
2
2
4
2
3
0
110
121
011 R
fh
fh
fh e
e
e
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Secondary Variables
• Using the first equation, we get:
• Which is the exact value of the reaction
force.
R
fhfh ee

22
3
efhR 2
2nd order DE’s in 1-D
Mohammad Tawfik
#WikiCourses
http://WikiCourses.WikiSpaces.com
Summary
• In this lecture, we learned how to
assemble the global matrices of the finite
element model; how to apply the boundary
conditions, and solve the system of
equations obtained.
• And finally, how to obtain the secondary
variables.

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FEM: Element Equations

  • 1. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Introduction to the Finite Element Method 2nd order DE’s in 1-D
  • 2. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Objectives • Understand the basic steps of the finite element analysis • Apply the finite element method to second order differential equations in 1-D
  • 3. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com The Mathematical Model • Solve: • Subject to: Lx fcu dx du a dx d         0 0   00 ,0 Q dx du auu Lx        
  • 4. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Step #1: Discretization • At this step, we divide the domain into elements. • The elements are connected at nodes. • All properties of the domain are defined at those nodes.
  • 5. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Step #2: Element Equations • Let’s concentrate our attention to a single element. • The same DE applies on the element level, hence, we may follow the procedure for weighted residual methods on the element level! 21 0 xxx fcu dx du a dx d             21 2211 21 , ,, Q dx du aQ dx du a uxuuxu xxxx              
  • 6. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Polynomial Approximation • Now, we may propose an approximate solution for the primary variable, u(x), within that element. • The simplest proposition would be a polynomial!
  • 7. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Polynomial Approximation • Interpolating the values of displacement knowing the nodal displacements, we may write:   01 bxbxu    01111 bxbuxu    2 12 1 1 12 2 u xx xx u xx xx xu                    02122 bxbuxu 
  • 8. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Polynomial Approximation        e ux u u uu u xx xx u xx xx xu                           2 1 212211 2 12 1 1 12 2
  • 9. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Step #2: Element Equations (cont’d) • Assuming constant domain properties: • Applying the Galerkin method: 21 2 2 0 xxx fcu dx ud a             02 2       Domain jiiji i j dxfxuxxcu dx xd xa   
  • 10. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Step #2: Element Equations (cont’d) • Note that: • And:     ee hdx xd hdx xd 1 , 1 21                         Domain ij x x i j Domain i j dx dx xd dx xd a dx xd xa dx dx xd xa     2 1 2 2
  • 11. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Step #2: Element Equations (cont’d) • For i=j=1: (and ignoring boundary terms) • Which gives: 0 12 1 2 1 2 2 2                                  x x eee dx h xx fu h xx c h a 0 23 1        ee e fh u ch h a
  • 12. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Step #2: Element Equations (cont’d) • Repeating for all terms: • The above equation is called the element equation.                                    1 1 221 12 611 11 2 1 ee e fh u uch h a
  • 13. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com What happens for adjacent elements?
  • 14. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Objectives • Learn how the finite element model for the whole domain is assembled • Learn how to apply boundary conditions • Solving the system of linear equations
  • 15. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Recall • In the previous lecture, we obtained the element equation that relates the element degrees of freedom to the externally applied fields • Which maybe written:                                    1 1 221 12 611 11 2 1 ee e fh u uch h a                    2 1 2 1 43 21 f f u u kk kk
  • 16. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Two–Element example                    1 2 1 1 1 2 1 1 1 4 1 3 1 2 1 1 f f u u kk kk                    2 2 2 1 2 2 2 1 2 4 2 3 2 2 2 1 f f u u kk kk                                            3 2 1 3 2 1 3 2 1 2 4 2 3 2 2 2 1 1 4 1 3 1 2 1 1 0 0 Q Q Q f f f u u u kk kkkk kk
  • 17. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Illustration: Bar application 1. Discretization: Divide the bar into N number of elements. The length of each element will be (L/N) 2. Derive the element equation from the differential equation for constant properties an externally applied force:   02 2    xF x u EA 0 2 1 2                   x x ij ij e dxfu dx d dx d h EA  
  • 18. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Performing Integration:                      1 1 211 11 2 1 e e e e fh u u h EA Note that if the integration is evaluated from 0 to he, where he is the element length, the same results will be obtained. 0 2 1 2                   x x ij ij e dxfu dx d dx d h EA  
  • 19. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Two–Element bar example                      1 2 1 1 1 2 1 1 11 11 f f u u h EA e                      2 2 2 1 2 2 2 1 11 11 f f u u h EA e                                              0 0 1 2 1 2 110 121 011 3 2 1 R fh u u u h EA e e
  • 20. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Applying Boundary Conditions
  • 21. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Applying BC’s • For the bar with fixed left side and free right side, we may force the value of the left-displacement to be equal to zero:                                              0 0 1 2 1 2 0 110 121 011 3 2 R fh u u h EA e e
  • 22. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Solving • Removing the first row and column of the system of equations: • Solving:                      1 2 211 12 3 2 e e fh u u h EA              4 3 2 2 3 2 EA fh u u e
  • 23. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Secondary Variables • Using the values of the displacements obtained, we may get the value of the reaction force:                                                    0 0 1 2 1 2 2 4 2 3 0 110 121 011 R fh fh fh e e e
  • 24. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Secondary Variables • Using the first equation, we get: • Which is the exact value of the reaction force. R fhfh ee  22 3 efhR 2
  • 25. 2nd order DE’s in 1-D Mohammad Tawfik #WikiCourses http://WikiCourses.WikiSpaces.com Summary • In this lecture, we learned how to assemble the global matrices of the finite element model; how to apply the boundary conditions, and solve the system of equations obtained. • And finally, how to obtain the secondary variables.