finite element formulation for beams - handout 2 · pdf filefinite element formulation for...
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Finite Element Formulation for Beams
- Handout 2 -
Dr Fehmi Cirak (fc286@)
Completed Version
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Review of Euler-Bernoulli Beam
■ Physical beam model
■ Beam domain in three-dimensions
■ Midline, also called the neutral axis, has the coordinate■ Key assumptions: beam axis is in its unloaded configuration straight■ Loads are normal to the beam axis
midline
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Kinematics of Euler-Bernoulli Beam -1-
■ Assumed displacements during loading
■ Kinematic assumption: Material points on the normal to the midline remain on the normal duringthe deformation
■ Slope of midline:
■ The kinematic assumption determines the axial displacement of the material points acrossthickness
■ Note this is valid only for small deflections, else
reference configuration
deformed configuration
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Kinematics of Euler-Bernoulli Beam -2-
■ Introducing the displacements into the strain equations of three-dimensional elasticity leads to■ Axial strains
■ Axial strains vary linearly across thickness
■ All other strain components are zero
■ Shear strain in the
■ Through-the-thickness strain (no stretching of the midline normal during deformation)
■ No deformations in and planes so that the corresponding strains are zero
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Weak Form of Euler-Bernoulli Beam
■ The beam strains introduced into the internal virtual work expressionof three-dimensional elasticity
■ with the standard definition of bending moment:
■ External virtual work
■ Weak work of beam equation
■ Boundary terms only present if force/moment boundary conditions present
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Stress-Strain Law
■ The only non-zero stress component is given by Hooke’s law
■ This leads to the usual relationship between the moment and curvature
■ with the second moment of area
■ Weak form work as will be used for FE discretization
■ EI assumed to be constant
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■ Beam is represented as a (disjoint) collection of finite elements
■ On each element displacements and the test function are interpolated usingshape functions and the corresponding nodal values
■ Number of nodes per element
■ Shape function of node K
■ Nodal values of displacements
■ Nodal values of test functions
■ To obtain the FE equations the preceding interpolation equations areintroduced into the weak form■ Note that the integrals in the weak form depend on the second order derivatives of u3 and v
Finite Element Method
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■ A function f: Ω→ℜ is of class Ck=Ck(Ω) if its derivatives of order j, where0 ≤ j ≤ k, exist and are continuous functions ■ For example, a C0 function is simply a continuous function■ For example, a C∝ function is a function with all the derivatives continuous
■ The shape functions for the Euler-Bernoulli beam have to be C1-continuousso that their second order derivatives in the weak form can be integrated
Aside: Smoothness of Functions
C1-continuous functionC0-continuous function
diffe
rent
iatio
n
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■ To achieve C1-smoothness Hermite shape functions can be used■ Hermite shape functions for an element of length
■ Shape functions of node 1
■ with
Hermite Interpolation -1-
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■ Shape functions of Node 2
■ with
Hermite Interpolation -2-
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■ According to Hermite interpolation the degrees of freedom for each element are thedisplacements and slopes at the two nodes
■ Interpolation of the displacements
■ Test functions are interpolated in the same way like displacements
■ Introducing the displacement and test functions interpolations into weak form gives the element stiffness matris
Element Stiffness Matrix
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■ Load vector computation analogous to the stiffness matrix derivation
■ The global stiffness matrix and the global load vector are obtained by assembling theindividual element contributions
■ The assembly procedure is identical to usual finite elements
■ Global stiffness matrix
■ Global load vector
■ All nodal displacements and rotations
Element Load Vector
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■ Element stiffness matrix of an element with length le
Stiffness Matrix of Euler-Bernoulli Beam
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■ Assumed displacements during loading
■ Kinematic assumption: a plane section originally normal to the centroid remains plane, but inaddition also shear deformations occur
■ Rotation angle of the normal:■ Angle of shearing:■ Slope of midline:
■ The kinematic assumption determines the axial displacement of the material points acrossthickness
■ Note that this is only valid for small rotations, else
Kinematics of Timoshenko Beam -1-
reference configuration
deformed configuration
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■ Introducing the displacements into the strain equations of three-dimensional elasticity leads to■ Axial strain
■ Axial strain varies linearly across thickness
■ Shear strain
■ Shear strain is constant across thickness
■ All the other strain components are zero
Kinematics of Timoshenko Beam -2-
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■ The beam strains introduced into the internal virtual work expressionof three-dimensional elasticity give
■ Hookes’s law
■ Introducing the expressions for strain and Hooke’s law into the weak form gives
■ virtual displacements and rotations:
■ shear correction factor necessary because across thickness shear stresses are parabolicaccording to elasticity theory but constant according to Timoshenko beam theory
■ shear correction factor for a rectangular cross section
■ shear modulus
■ External virtual work similar to Euler-Bernoulli beam
Weak Form of Timoshenko Beam
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■ Comparison of the displacements of a cantilever beam analyticallycomputed with the Euler-Bernoulli and Timoshenko beam theories
■ Bernoulli beam■ Governing equation:
■ Boundary conditions:
■ Timoshenko beam■ Governing equations:
■ Boundary conditions:
Euler-Bernoulli vs. Timoshenko -1-
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■ Maximum tip deflection computed by integrating the differential equations
■ Bernoulli beam
■ Timoshenko beam
■ Ratio
■ For slender beams (L/t > 20) both theories give the same result■ For stocky beams (Lt < 10) Timoshenko beam is physically more realistic because it includes the shear
deformations
Euler-Bernoulli vs. Timoshenko -2-
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■ The weak form essentially contains and the correspondingtest functions■ C0 interpolation appears to be sufficient, e.g. linear interpolation
■ Interpolation of displacements and rotation angle
Finite Element Discretization
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■ Shear angle
■ Curvature
■ Test functions are interpolated in the same way like displacements and rotations■ Introducing the interpolations into the weak form leads to the element stiffness matrices
■ Shear component of the stiffness matrix
■ Bending component of the stiffness matrix
Element Stiffness Matrix
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■ Gaussian Quadrature■ The locations of the quadrature points and weights are determined for maximum accuracy
■ nint=1
■ nint=2
■ nint=3
■ Note that polynomials with order (2nint-1) or less are exactly integrated
■ The element domain is usually different from [-1,+1) and an isoparametricmapping can be used
Review: Numerical Integration
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■ Necessary number of quadrature points for linear shape functions■ Bending stiffness: one integration point sufficient because is constant■ Shear stiffness: two integration points necessary because is linear
■ Element bending stiffness matrix of an element with length le and one integrationpoint
■ Element shear stiffness matrix of an element with length le and two integration points
Stiffness Matrix of the Timoshenko Beam -1-
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Limitations of the Timoshenko Beam FE
■ Recap: Degrees of freedom for the Timoshenko beam
■ Physics dictates that for t→0 (so-called Euler-Bernoulli limit) the shear anglehas to go to zero ( )■ If linear shape functions are used for u3 and β
■ Adding a constant and a linear function will never give zero!■ Hence, since the shear strains cannot be arbitrarily small everywhere, an erroneous shear strain
energy will be included in the energy balance■ In practice, the computed finite element displacements will be much smaller than the exact solution
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Shear Locking: Example -1-
■ Displacements of a cantilever beam
■ Influence of the beam thickness on the normalized tip displacement
2 point
2
4
1
# elem.
0.0416
8
0.445
0.762
0.927
Thick beam
0.00021
2
4
8
0.0008
0.0003
0.0013
# elem. 2 point
Thin beam
from TJR Hughes, The finite element method.
TWO integration points
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■ The beam element with only linear shape functions appears not to be ideal for verythin beams
■ The problem is caused by non-matching u3 and β interpolation■ For very thin beams it is not possible to reproduce
■ How can we fix this problem?■ Lets try with using only one integration point for integrating the element shear stiffness matrix■ Element shear stiffness matrix of an element with length le and one integration points
Stiffness Matrix of the Timoshenko Beam -2-
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Shear Locking: Example -2-
■ Displacements of a cantilever beam
■ Influence of the beam thickness on the normalized displacement
ONE integration point
2
4
1
# elem.
0.762
0.940
0.985
0.9968
1 point
Thick beam
0.750
0.938
0.984
0.996
1
2
4
8
# elem. 1 point
Thin beam
from TJR Hughes, The finite element method.
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■ If the displacements and rotations are interpolated with the same shapefunctions, there is tendency to lock (too stiff numerical behavior)
■ Reduced integration is the most basic “engineering” approach to resolvethis problem
■ Mathematically more rigorous approaches: Mixed variational principlesbased e.g. on the Hellinger-Reissner functional
Reduced Integration Beam Elements
CubicShape functionorder
Quadrature rule
Linear
One-point
Quadratic
Two-point Three-point