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Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Introduction to Finite Element Method
Georges CAILLETAUD & Saber EL AREM
Centre des Materiaux, MINES ParisTech, UMR CNRS 7633
WEMESURF course, Paris 21-25 juin
1/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
2/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Introduction 3/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Numerical methods for PDE solving
Many physical phenomena in engineering andscience can be described in terms of partialdifferential equations (PDE) .In general, solving these equations by classicalanalytical methods for arbitrary shapes is almostimpossible.The finite element method (FEM) is a numericalapproach by which these PDE can be solvedapproximately.The FEM is a function/basis-based approach tosolve PDE.FE are widely used in diverse fields to solve staticand dynamic problems − Solid or fluid mechanics,electromagnetics, biomechanics, etc.
Introduction 4/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Google search results for FEM
Introduction 5/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Google search results for FE+course
Introduction 6/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
FE problem solving steps
Two key words: Discretization & Interpolation1 Definition of the physical problem: development of
the model.2 Formulation of the governing equations.
Systems of PDE, ODE, algebraic equations,define initial conditions and/or boundary conditions to get awell-posed problem.
3 Discretization of the equations.4 Solution of the discrete system of equations.5 Interpretation of the obtained results.6 Errors analysis.
Introduction 7/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Examples 8/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
AUTOMOBILE
Abaqus et l’automobile
* Engine durability * Transmission performance * Seal integrity * Component design *Powertrain bending * Noise & vibration * Gasket analysis * Thermal cycling * Sheet metalforming * Forging analysis * Mechanisms analysis * Assembly (bolt loading)(d’apres http://www.abaqus.com)
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Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
AERONAUTICS
Crash-test, Boeing 737 par la NASA* Static, dynamic, and coupled acoustic-structural analysis of aircraft frames *Simulations of large deployable space structures such as solar sails, space radars andreflector antennas * Simulating the performance of various aircraft components, such asbulkhead under pressurization, wing panel buckling, and crack propagation in the fuselage *Blade containment evaluations and bird strike simulations * Thermomechanical simulationof aircraft engines and rocket motors under different operating conditions * Verification ofturbine blade designs *Simulation of various aircraft mechanisms such as landing gears, wingflaps, and cargo doors
Examples 10/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Power generation equipment
Examples 11/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Bibliography on finite element 12/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Bibliography on finite element
Bathe, K. (1982).
Finite element procedures in engineering analysis.Prentice Hall, Inc.
Batoz, J. and Dhatt, G. (1991).
Modelisation des structures par elements finis, I—III.Hermes.
Belytschko, T., Liu, W., and Moran, B. (2000).
Nonlinear Finite Elements for Continua and Structures.
Besson, J., Cailletaud, G., Chaboche, J.-L., and Forest, S. (2001).
Mecanique non–lineaire des materiaux.Hermes.
Buchanan, G. (1995).
Finite element analysis.Schaum’s outlines.
Ciarlet, P. and Lions, J. (1995).
Handbook of Numerical Analysis : Finite Element Methods (P.1), Numerical Methods forSolids (P.2).North Holland.
Crisfield, M. (1991).
Nonlinear Finite Element Analysis of Solids and Structures.
Bibliography on finite element 12/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Wiley.
Dhatt, G. and Touzot, G. (1981).
Une presentation de la methode des elements finis.Maloine.
Hughes, T. (1987).
The finite element method: Linear static and dynamic finite element analysis.Prentice–Hall Inc.
Kardestuncer, H., editor (1987).
Finite Element Handbook.Mc Graw Hill.
Mc Neal, R. (1993).
Finite Element: their design and performance.Marcel Dekker.
Simo, J. and Hughes, T. (1997).
Computational Inelasticity.Springer Verlag.
Zienkiewicz, O. and Taylor, R. (2000).
The finite element method, Vol. I-III (Vol.1: The Basis, Vol.2: Solid Mechanics, Vol. 3:Fluid dynamics).Butterworth–Heinemann.
Discrete versus continuous 13/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Discrete versus continuous 13/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Continuous→ Discrete→Continuous
How much rain ?
Discrete versus continuous 14/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Continuous→ Discrete→Continuous
Geometry discretization
Discrete versus continuous 15/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Continuous→ Discrete→Continuous
Unknown field discretization
Discrete versus continuous 16/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Continuous→ Discrete→Continuous
Use elements
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Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Finite Element Discretization
Replace continuum formulation by a discrete representation for unknownsand geometry
Unknown field:ue(M) =
∑i
Nei (M)qe
i
Geometry:
x(M) =∑
i
N∗ei (M)x(Pi )
Interpolation functions Nei and shape functions N∗ei such as:
∀M,∑
i
Nei (M) = 1 and Ne
i (Pj) = δij
Isoparametric elements iff Nei ≡ N∗ei
Discrete versus continuous 18/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Element 19/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Element 20/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
2D-mapping
Subparametric Superparametric Isoparametricelement element element
Geometry
Unknown field
Geometry
Unknown field
Geometry
Unknown field
more field nodes more geometrical nodes same number ofthan geometrical nodes than field nodes geom and field nodes
Rigid body displacement not represented for superparametric element that has nonlinear edges !
The location of the node at the middle of the edge is critical for quadratic edges
Element 21/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Shape function matrix, [N] – Deformation matrix, [B]
Field u, T , C
Gradient ε∼, grad(T ),. . .
Constitutive equations σ∼ = Λ∼∼: ε∼, q = −kgrad(T )
Conservation div(σ∼) + f = 0, . . .
First step: express the continuous field and its gradient wrt thediscretized vector
Element 22/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Deformation matrix [B] (1)
Knowing:
ue(M) =∑
i
Nei (M)qe
i
Deformation can be obtained from the nodal displacements, forinstance in 2D, small strain:
εxx =∂ux
∂x=
∂N1(M)
∂xqe
1x +∂N2(M)
∂xqe
2x + . . .
εyy =∂uy
∂y=
∂N1(M)
∂yqe
1y +∂N2(M)
∂yqe
2y + . . .
2εxy =∂ux
∂y+
∂uy
∂x=
∂N1(M)
∂yqe
1x +∂N2(M)
∂xqe
1y + . . .
Element 23/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Deformation matrix [B] (2)
Matrix form, 4-node quadrilateral
ue = [N]T qe =
(N1 0 N2 0 N3 0 N4 00 N1 0 N2 0 N3 0 N4
)qe
1x
qe1y
...qe
4y
εe = [B]T qe
=
N1,x 0 N2,x 0 N3,x 0 N4,x 00 N1,y 0 N2,y 0 N3,y 0 N4,y
N1,y N1,x N2,y N2,x N3,y N3,x N4,y N4,x
qe1x
qe1y
...qe
4y
Shear term taken as γ = 2ε12
Element 24/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Reference element
x
y
ξ
η
Actual geometryPhysical space (x , y)
η
ξ
1
−1
−1 1
Reference elementParent space (ξ, η)
∫Ω
f (x , y)dxdy =
∫ +1
−1
∫ +1
−1
f∗(ξ, η)Jdξdη
J is the determinant of the partial derivatives ∂x/∂ξ. . . matrixElement 25/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Remarks on geometrical mapping
The values on an edge depends only on the nodal values on the sameedge (linear interpolation equal to zero on each side for 2-node lines,parabolic interpolation equal to zero for 3 points for 3-node lines)
Continuity...
The mid node is used to allow non linear geometries
Limits in the admissible mapping for avoiding singularities
Element 26/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Mapping of a 3-node line
-1
-0.5
0
0.5
1
1.5
-1 -0.5 0 0.5 1
x
ξ
x2=0
x2=1/2
x2=1
Physical segment: x1=-1 x3=1 −1 6 x2 6 1Parent segment: ξ1=-1 ξ3=1 ξ2=0
x = ξ +“1− ξ2
”x2
Element 27/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Jacobian and inverse jacobian matrix
(dxdy
)=
∂x
∂ξ
∂x
∂η∂y
∂ξ
∂y
∂η
(dξdη
)= [J]
(dξdη
)
(dξdη
)=
∂ξ
∂x
∂ξ
∂y∂η
∂x
∂η
∂y
(dxdy
)= [J]−1
(dxdy
)
Since (x , y) are known from Ni (ξ, η) and xi ,
[J]−1 is computed from the known quantities in [J], using also:
J = Det ([J]) =∂x
∂ξ
∂y
∂η− ∂y
∂ξ
∂x
∂η
Element 28/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Expression of the inverse jacobian matrix
[J]−1 =1
J
∂y
∂η−∂y
∂ξ
−∂x
∂η
∂x
∂ξ
For a rectangle [±a,±b] in the ”real world”, the mapping function isthe same for any point inside the rectangle. The jacobian is adiagonal matrix, with ∂x/∂ξ = a, ∂y/∂η = b, and the determinantvalue is abFor any other shape, the ”mapping” changes according to thelocation in the elementFor computing [B], one has to consider ∂Ni/∂x and ∂Ni/∂y :
∂Ni
∂x=
∂Ni
∂ξ
∂ξ
∂x+
∂Ni
∂η
∂η
∂x
∂Ni
∂y=
∂Ni
∂ξ
∂ξ
∂y+
∂Ni
∂η
∂η
∂y
then(
∂Ni/∂x∂Ni/∂y
)= [J]−1
(∂Ni/∂ξ∂Ni/∂η
)
Element 29/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Element 30/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
2D solid elements
Type shape interpol # of polynomof disp nodes terms
C2D3 tri lin 3 1, ξ, ηC2D4 quad lin 4 1, ξ, η, ξηC2D6 tri quad 6 1, ξ, η, ξ2, ξη, η2
C2D8 quad quad 8 1, ξ, η, ξ2, ξη, η2, ξ2η, ξη2
C2D9 quad quad 9 1, ξ, η, ξ2, ξη, η2, ξ2η, ξη2, ξ2η2
Element 31/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
3D solid elements
Type shape interpol # of polynomof disp nodes terms
C3D4 tetra lin 4 1, ξ, η, ζC3D6 tri prism lin 6 1, ξ, η, ζ, ξη, ηζC3D8 hexa lin 8 1, ξ, η, ζ, ξη, ηζ, ζξ, ξηζC3D10 tetra quad 10 1, ξ, η, ζ, ξ2, ξη, η2, ηζ, ζ2, ζξC3D15 tri prism quad 15 1, ξ, η, ζ, ξη, ηζ, ξ2ζ, ξηζ, η2ζ, ζ2,
ξζ2, ηζ2, ξ2ζ2, ξηζ2, η2ζ2
C3D20 hexa quad 20 1, ξ, η, ζ, ξ2, ξη, η2, ηζ, ζ2, ζξ,ξ2η, ξη2, η2ζ, ηζ2, ξζ2, ξ2ζ, ξηζ,ξ2ηζ, ξη2ζ, ξηζ2
C3D27 hexa quad 27 ξiηjζk , (i , j , k) ∈ 0, 1, 2
Element 32/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Isoparametric representation
Example: 2D plane stress elements with n nodes
Element geometry
1 =n∑
i=1
Ni x =n∑
i=1
Nixi y =n∑
i=1
Niyi
Displacement interpolation
ux =n∑
i=1
Niuxi uy =n∑
i=1
Niuy i
Matrix form1xyux
uy
=
1 1 1 ... 1x1 x2 x3 ... xn
y1 y2 y3 ... yn
ux1 ux2 ux3 ... uxn
uy1 uy2 uy3 ... uyn
N1
N2
N3
.
.Nn
Element 33/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The linear triangle
IFEM
–Fel
ippa
Terms in 1, ξ, ηElement 34/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The bilinear quad
IFEM
–Fel
ippa
Terms in 1, ξ, η, ξη
Element 35/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The quadratic triangle
IFEM
–Fel
ippa
Terms in 1, ξ, η, ξ2, ξη, η2
Element 36/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The biquadratic quad
IFEM
–Fel
ippa
Terms in 1, ξ, η, ξ2, ξη, η2, ξ2η, ξη2, ξ2η2
Element 37/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The 8-node quad
IFEM
–Fel
ippa
Corner nodes: Ni =1
4(1 + ξξi )(1 + ηηi )(ξξi + ηηi − 1)
Mid nodes, ξi = 0: Ni =1
2(1− ξ2)(1 + ηηi )
Mid nodes, ηi = 0: nI =1
2(1− η2)(1 + ξξi )
Terms in 1, ξ, η, ξ2, ξη, η2, ξ2η, ξη2
Element 38/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Approximated field
Polynomial basis
1ξ η
ξ2 ξη η2
ξ3 ξ2η ξη2 η3
Examples :
C2D4 (1 + ξiξ)(1 + ηiη)C2D8, corner 0.25(−1 + ξiξ + ηiη)(1 + ξiξ)(1 + ηiη)C2D8 middle 0.25(1.− ξ2)(1. + ηiη)
Element 39/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The 2-node infinite element
Displacement is assumed to be q1 at node 1 and q2 = 0 at node 2
x ξ
x1 x2 →∞ 1 2
Interpolation
N1 =1− ξ
2N2 =
1 + ξ
2
Geometry
N∗1 such as x = x1 + α1 + ξ
1− ξN∗2 = 0
ξ =?
Resulting displacement interpolation
u(x) =??
Element 40/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The 2-node infinite element
Displacement is assumed to be q1 at node 1 and q2 = 0 at node 2
x ξ
x1 x2 →∞ 1 2
Interpolation
N1 =1− ξ
2N2 =
1 + ξ
2Geometry
N∗1 such as x = x1 + α1 + ξ
1− ξN∗2 = 0
ξ =x − x1 − α
x − x1 + α
Resulting displacement interpolation
u(x) =?
Element 41/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The 2-node infinite element
Displacement is assumed to be q1 at node 1 and q2 = 0 at node 2
x ξ
x1 x2 →∞ 1 2
Interpolation
N1 =1− ξ
2N2 =
1 + ξ
2Geometry
N∗1 such as x = x1 + α1 + ξ
1− ξN∗2 = 0
ξ =x − x1 − α
x − x1 + α
Resulting displacement interpolation
u(x) = N1(x) q1 = N1(ξ(x)) q1 =αq1
x − x1 + αElement 42/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Connecting element
1 2 3
7 6 5
4
lin. quad.
Connection betweena linear and aquadratic quad
Quadratic interpolation with node number 8 in the middle of 1–7:
u(M) = N1q1 + N8q8 + N7q7
On edge 1–7, in the linear element, the displacement should verify:
q8 =?
Overloaded shape function in nodes 1 and 7 after suppressing node8:
u(M) =??
Element 43/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Connecting element
1 2 3
7 6 5
4
lin. quad.
Connection betweena linear and aquadratic quad
Quadratic interpolation with node number 8 in the middle of 1–7:
u(M) = N1q1 + N8q8 + N7q7
On edge 1–7, in the linear element, the displacement should verify:
q8 = (q1 + q7)/2
Overloaded shape function in nodes 1 and 7 after suppressing node8:
u(M) =??
Element 44/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Connecting element
1 2 3
7 6 5
4
lin. quad.
Connection betweena linear and aquadratic quad
Quadratic interpolation with node number 8 in the middle of 1–7:
u(M) = N1q1 + N8q8 + N7q7
On edge 1–7, in the linear element, the displacement should verify:
q8 = (q1 + q7)/2
Overloaded shape function in nodes 1 and 7 after suppressing node8:
u(M) =
(N1 +
N8
2
)q1 +
(N7 +
N8
2
)q7
Element 45/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Global problem 46/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Global problem 47/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Thermal conduction
Strong form:”GIVEN r : Ω → R, a volumetric flux,
φd : Γf → R, a surface flux,T d : Γu → R, a prescribed temperature,
FIND T : Ω → R, the temperature, such as:”
in Ω φi,i = ron Γu T = T d
on ΓF −φini = Φd
Constitutive equation (Fourier, flux (W /m2) proportional to thetemperature gradient)
φi = −κijT ,j conductivity matrix: [κ] (W /m.K )
Global problem 48/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Thermal conduction (2)
Weak form:S, trial solution space, such as T = T d on Γu
V, variation space, such as δT = 0 on Γu
”GIVEN r : Ω → R, a volumetric flux,Φd : Γf → R, a surface flux,T d : Γu → R, a prescribed temperature,
FIND T ∈ S such as ∀δT ∈ V
−∫
Ω
φiδT ,i dΩ =
∫Ω
δTrdΩ +
∫ΓF
δTΦddΓ
”For any temperature variation compatible with prescribed temperaturefield around a state which respects equilibrium, the internal power
variation is equal to the external power variation: δT ,i φi is in W /m3”T is present in φi = −κijT ,j
Global problem 49/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Elastostatic
Strong form:
volume Ω with prescribed volume forces fd : σij,j + fi = 0
surface ΓF with prescribed forces Fd : F di = σijnj
surface Γu with prescribed displacements ud : ui = udi
Constitutive equation: σij = Λijklεkl = Λijkluk,l
So that: Λijkluk,lj + fi = 0
Global problem 50/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Principle of virtual power
Weak form:
volume V with prescribed volume forces : fd
surface ΓF with prescribed forces : Fd
surface Γu with prescribed displacements : ud
Virtual displacement rate u kinematically admissible (u = ud on Γu)
The variation u is such as: u = 0 on Γu. Galerkin form writes, ∀u:∫Ω
σ∼ : ε∼dΩ =
∫Ω
fd u dΩ +
∫ΓF
Fd u dS
Global problem 51/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Discrete form of virtual power
Application of Galerkin approach for continuum mechanics:
virtual displacement rate u ≡ wh ; σ∼ ≡ uh,x
ue, nodal displacements allow us to compute u and ε∼:
u = [N]ue ; ε∼ = [B]ue
Galerkin form writes, ∀ue:∑elt
(∫Ω
σ(ue).[B].ue dΩ
)=∑elt
(∫
Ω
fd .[N].ue dΩ
+
∫ΓF
Fd .[N].ue dS)
Global problem 52/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Internal and external forces
In each element e:Internal forces:
F eint =
∫Ω
σ(ue).[B] dΩ =
∫Ω
[B]T σ(ue) dΩ
External forces:
F eext =
∫Ω
fd .[N]dΩ +
∫ΓF
Fd .[N]dS
The solution of the problem: F eint(ue) = F e
ext with Newtoniterative algorithm will use the jacobian matrix :
[K e ] =∂F e
int∂ue
=
∫Ω
[B]T .∂σ∂ε
.∂ε∂ue
dΩ
=
∫Ω
[B]T .∂σ∂ε
.[B] dΩ
Global problem 53/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Linear and non linear behavior
Applying the principle of virtual power ≡ Stationnary point ofPotential Energy
For elastic behavior
[K e ] =
∫Ω
[B]T .[Λ∼∼] .[B] dΩ
is symmetric, positive definite (true since σ∼ and ε∼ are conjugated)
For non linear behavior, one has to examine [Lc ] =
[∂σ∂ε
]. Note
that [Lc ] can be approached (quasi-Newton).
F eext may depend on ue (large displacements).
Global problem 54/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Elastostatic, strong and weak form, asummary
Displacementsu
Body forces f
Strains Stressesε σ
BCu
BCs
Kinematics
Constitutiveequations
Equilibrium
STRONG
BCu: u = ud on Γu
Kinematics: ε = [B] u in Ω
Constitutive equation:σ = Λε
Equilibrium: [B] σ + f = 0
BCs: σn = F on ΓF
WEAK
BCu: uh = ud on Γu
Kinematics: ε = [B] uh in Ω
Constitutive equation:σ = Λε
Equilibrium: δΠ = 0
BCs: δΠ = 0
Global problem 55/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Global problem 56/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Matrix–vectors formulation of the weak formof the problem
[K ] q = F
Thermal conduction:
[K ] =
∫Ω
[B]T [κ] [B] dΩ F =
∫Ω
[N] rdΩ +
∫∂Ω
[N] ΦddΓ
Elasticity:
[K ] =
∫Ω
[B]T [Λ] [B] dΩ F =
∫Ω
[N] fddΩ +
∫∂Ω
[N]FddΓ
In each element e:Internal forces:
F eint =
∫Ω
σ(ue).[B] dΩ =
∫Ω
[B]T σ(ue) dΩ
External forces:
F eext =
∫Ω
fd .[N]dΩ +
∫ΓF
Fd .[N]dSGlobal problem 57/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
The stiffness matrix
Example of a 4-node quad and of a 20-node hexahedron ()
[B]T [D] [B] [K ]
=
3 (6) 3 (6) 8 (60) 8 (60)
8 3 3 8(60) (6) (6) (60)
The element stiffness matrix is a square matrix, symmetric, with no zeroinside.
Its size is equal to the number of dof of the element.
Global problem 58/79
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Nodal forces (1)
F eext =
∫ΓF
[N]T F ddS
Fn
1
5
7F t
8
4
23
6
ξ
η
η
ξ
1
−1
−1 1
Fxds = Ftdx − Fndy
Fyds = Fndx + Ftdywith
(dxdy
)= [J]
(dξdη
)
Global problem 59/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Nodal forces (2)
Integration on edge 5–7: dx =∂x
∂ξdξ dy =
∂y
∂ξdξ
Components 9, 10, for the nodes 5; 11, 12 for nodes 6; 13, 14 for nodes 7
F eext(2i − 1) = e
∫ 1
−1
Ni
(Ft
∂x
∂ξ− Fn
∂y
∂ξ
)dξ
F eext(2i) = e
∫ 1
−1
Ni
(Fn
∂x
∂ξ+ Ft
∂y
∂ξ
)dξ
Example, for a pressure Fn = p, and no shear (Ft = 0) on the 5–7 edgeof a 8-node rectangle
−a 6 x 6 a y = b represented by − 1 6 ξ 6 1 η = 1
∂x
∂ξ= a
∂y
∂ξ= 0
N5 = ξ(1 + ξ)/2 N6 = 1− ξ2 N7 = −ξ(1− ξ)/2
Global problem 60/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Nodal forces (3)
7 6 5 7 6 5
(1)
(4)
(1)
F10 = F5y = e
∫ 1
−1
1
2ξ(1 + ξ)padξ =
ap
3
F12 = F6y = e
∫ 1
−1
(1− ξ2)padξ =4ap
3
The nodal forces at the middle node are 4 times the nodal forces atcorner nodes for an uniform pressure (distribution 1–2–1–2–1... after
adding the contribution of each element)
Global problem 61/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Nodal forces (4)
Axisymmetric 8-node quad
7 6 57 6 5
(2)
(1)
z
Face of a 20-node hexahedron(4)
(4)
(4)
(4)
(−1)
(−1)
(−1)
(−1)
Global problem 62/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Nodal forces (5)
Face of a 27-node hexahedron
who knows ?
Face of a 15-node hexahedron
(3)
(3)
(3)
Global problem 63/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
2 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Local versus global numbering
Global problem 64/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
0BBBBBBBB@
F1 = FA1
F2 = FA2 +FB
1F3 = +FB
2F4 = FA
3 +FB4 +FC
1F5 = +FB
3 +FC2
F6 = +FC4
F7 = +FC3
1CCCCCCCCA
0BBBBBBBB@
q1 = qA1
q2 = qA2 = qB
1q3 = = qB
2q4 = qA
3 = qB4 = qC
1q5 = = qB
3 = qC2
q6 = = qC4
q7 = = qC3
1CCCCCCCCA
2 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 65/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
11
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 66/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
21
11 12
22
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 67/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
21
11 12 13
22 23
3231
33
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 68/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
21
11 12 13
1122 23
3231
33
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 69/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
21
11 12 13
1122 23
3231
33
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 70/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
21
11 12 13
1122
1223
2221
3231
33
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 71/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
33
21
11 12 13
1122
1223
13
232221
3231
31 32
33
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 72/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
44
33
1133
43
34
21
11 12 13
1122
1214
2313
23242221
4132
31
31 32
42
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 73/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
44
33
1133
22
12
21
43
34
21
11 12 13
1122
1214
2313
23242221
4132
31
31 32
42
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 74/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
44
33
1133
22
12
21
43
34 23
13
31
21
11 12 13
1122
1214
2313
23242221
4132
31
31 32
42
32 33
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 75/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Assembling the global matrix
44
33
1133
22
12
21
43
34 23
13
31
21
11 12 13
1122
1214
2313
23242221
4132
31
31 32
42 14
43444241
3432 33
24
12 3
4 5
7
1
6
AB
1 1
1
2 2
3 3
3
4
4
C
2
Global problem 76/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Contents
1 Introduction
2 Examples
3 Bibliography on finite element
4 Discrete versus continuous
5 ElementInterpolationElement list
6 Global problemFormulationMatrix formulationAlgorithm
Global problem 77/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Global algorithm
For each loading increment, do while ‖Riter‖ > EPSI :iter = 0; iter < ITERMAX ; iter + +
1 Update displacements: ∆uiter+1 = ∆uiter + δuiter
2 Compute ∆ε = [B].∆uiter+1 then ∆ε∼ for each Gauss point
3 Integrate the constitutive equation: ∆ε∼→ ∆σ∼, ∆αI ,∆σ∼∆ε∼
4 Compute int and ext forces: Fint(ut + ∆uiter+1) , Fe5 Compute the residual force: Riter+1 = Fint − Fe6 New displacement increment: δuiter+1 = −[K ]−1.Riter+1
Global problem 78/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Convergence
Value of the residual forces < Rε, e.g.
||R||n =
(∑i
Rni
)1/n
; ||R||∞ = maxi|Ri |
Relative values:
||Ri − Re ||||Re ||
< ε
Displacements ∣∣∣∣Uk+1 − Uk
∣∣∣∣n
< Uε
Energy [Uk+1 − Uk
]T. Rk < Wε
Global problem 79/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Bathe, K. (1982).Finite element procedures in engineering analysis.Prentice Hall, Inc.
Batoz, J. and Dhatt, G. (1991).Modelisation des structures par elements finis, I—III.Hermes.
Belytschko, T., Liu, W., and Moran, B. (2000).Nonlinear Finite Elements for Continua and Structures.
Besson, J., Cailletaud, G., Chaboche, J.-L., and Forest, S. (2001).Mecanique non–lineaire des materiaux.Hermes.
Buchanan, G. (1995).Finite element analysis.Schaum’s outlines.
Ciarlet, P. and Lions, J. (1995).
Global problem 79/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Handbook of Numerical Analysis : Finite Element Methods (P.1),Numerical Methods for Solids (P.2).North Holland.
Crisfield, M. (1991).Nonlinear Finite Element Analysis of Solids and Structures.Wiley.
Dhatt, G. and Touzot, G. (1981).Une presentation de la methode des elements finis.Maloine.
Hughes, T. (1987).The finite element method: Linear static and dynamic finite elementanalysis.Prentice–Hall Inc.
Kardestuncer, H., editor (1987).Finite Element Handbook.Mc Graw Hill.
Global problem 79/79
Introduction Examples Bibliography on finite element Discrete versus continuous Element Global problem
Mc Neal, R. (1993).Finite Element: their design and performance.Marcel Dekker.
Simo, J. and Hughes, T. (1997).Computational Inelasticity.Springer Verlag.
Zienkiewicz, O. and Taylor, R. (2000).The finite element method, Vol. I-III (Vol.1: The Basis, Vol.2: SolidMechanics, Vol. 3: Fluid dynamics).Butterworth–Heinemann.
Global problem 79/79